taxonomic discrepancies between Glossotherium and Paramylodon, principal components ...... less developed in Paramylodon, with a short portion running parallel before curving towards the ...... OsteologÃa de los GravÃgrados o Perezosos.
NORTHERN ILLINOIS UNIVERSITY
REASSESSING THE TAXONOMY AND AFFINITIES OF THE MYLODONTINAE SLOTHS, GLOSSOTHERIUM AND PARAMYLODON (MAMMALIA: XENARTHRA: TARDIGRADA)
A DISSERTATION SUBMITTED TO THE GRADUATE SCHOOL IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE DOCTOR OF PHILOSOPHY
DEPARTMENT OF BIOLOGICAL SCIENCES
BY ROBERT K. MCAFEE ©2007 Robert K. McAfee
DEKALB, ILLINOIS AUGUST 2007
ABSTRACT
Xenarthra is a diverse mammalian order consisting of extant and extinct armadillos, anteaters and sloths, and extinct glyptodonts and pampatheres. The extinct ground sloths show a greater taxonomic richness compared to the living tree sloths and a more expansive range covering the Americas and portions of the Antillean islands. Ground sloths rose to prominence in South America during the Eocene, with the three major families (Mylodontidae, Megalonychidae and Megatheriidae) fully formed and distinct. Despite the successful colonization of North America and the Antilles, the large-bodied forms died out approximately 10,000 years ago near the start of the Holocene. Nomenclatural confusion has existed within the Mylodontinae for several genera since their initial description, especially for Glossotherium and Mylodon from South America and Paramylodon from North America. To sort out some of the taxonomic discrepancies between Glossotherium and Paramylodon, principal components analyses were performed on the crania and major limb elements, using suites of measurements obtained for each bone. Results predominantly showed a size difference, with Paramylodon being the greater, but shape differences are evident in the skulls that are related to length versus width. Further investigation of the measures, various ratios and qualitative features revealed a number of additional differences between the two genera, many relating to potential differences in function and lifestyles
of the animals. Glossotherium and Mylodon are sympatric in their distribution, with many of their fossils being confused for the other because of a lack of postcrania to associate with distinct skulls. There were not enough associated elements from the specimens sampled to provide any separation, although generic differences were found for the astragali. Examination of a Pliocene species, Glossotherium chapadmalense, of South America showed a combination of characters indicative of each genus but exhibited far more in common with Glossotherium. The mix of characters indicates that G. chapadmalense later gave rise to Paramylodon, although when and where the transition took place is still unclear. During the later evolution, Paramylodon crania emphasized an increase in length of the palate, while those of Glossotherium emphasized an increase in cranial width.
ACKNOWLEDGEMENTS
Thanks and appreciation go out to the following people and institutions for making this dissertation a reality:
Dissertation committee: Virginia L. Naples, Advisor J. Michael Parrish Melvin Duvall Reed Scherer Jennifer White
The following museum collections and their staff for the specimens studied: American Museum of Natural History, New York Facultad de Ciencias, Paleontological Collections of Vertebrates, Montevideo Field Museum of Natural History, Chicago Florida Museum of Natural History, Gainesville George C. Page Museum, Los Angeles Harvard Museum of Comparative Zoology, Cambridge Idaho State Museum, Pocatello Kansas University Natural History Museum, Lawrence Los Angeles County Museum, Los Angeles Museo Argentino de Ciencias Naturales, Buenos Aires Museo de La Plata, La Plata Museo Nacional d'Hisoire Naturelle, Paris Natural History Museum (British), London Smithsonian Instituation, Museum of Natural History, Washington, D.C. Texas Memorial Museum, Austin University of Nebraska State Museum, Lincoln Yale Peabody Museum of Natural History, New Haven, CT Zoologisk Museum, Universitat Copenhagen, Copenhagen
iv Support for travel, supplies, and equipment: American Museum of Natural History, Collections Study Grant Elgin Community College Kishwaukee Community College Northern Illinois University, Department of Biological Sciences
For emotional, intellectual, physical and spiritual support along this journey: David J. Allen Christopher McAfee and family Keith McAfee Lindsay Barron William and Jeanette McAfee Scott Foss Greg McDonald Ted Fremd Amy M. Miller Tim Gaudin Matthew Mundwiler Israel Gersten Dan Olson Julie L. Hill Kelly Richards and family Mindy Householder Barbara Shaw Richard Hulbert Muffie Slater Jonathan Jewel Kurt Spearing Chris Johnson Paula Work Brian R. Kenyon Peg Yacobucci Richard Kenyon
With special dedication: To my father, Keith K. McAfee, and in memoriam of my mother, Margaret J. W. McAfee, for their unwavering love and support throughout of all my ventures.
TABLE OF CONTENTS
Page LIST OF TABLES
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LIST OF FIGURES .
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LIST OF APPENDICIES
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Chapter 1. INTRODUCTION TO THE MYLODONTINAE AND THE TAXONOMIC PROBLEMS THEREIN . . . . . . . 1 Tardigrada
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Mylodontidae
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History and Taxonomic Confusion Within the Mylodontinae
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Glossotherium chapadmalense – The Missing Link?
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2. METHODS IN THE FACE OF TAXONOMIC MADNESS .
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That Yet to Come… .
Methodology . Abbreviations
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3. PRINCIPAL COMPONENTS ANALYSIS .
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Multivariate Analyses
Cranium
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Mandible
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Humerus
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Radius .
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Femur .
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Tibia .
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Astragalus
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Discussion
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4. REASSESSMENT OF CRANIAL AND POSTCRANIAL CHARACTERS FOR GLOSSOTHERIUM AND PARAMYLODON . . .
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PCA Variables – Cranium and Mandible
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Additional Cranial Characters.
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Additional Mandibular Characters
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Glossotherium chapadmalense
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Postcrania
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Humerus .
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Radius
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Femur
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Tibia
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Astragalus .
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5. THE FUNCTIONAL IMPLICATIONS OF THE GENERIC CHARACTERS . . . . . . . Food Procurement – Predental Spout .
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Food Processing
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Temporalis and Masseter .
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Pterygoids .
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Locomotor Movements.
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Deltoid Crest Expansion
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Pectoral Ridge/Shaft Angle
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Epicondyle Development
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Humerus .
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Femur
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Vastus Muscle Group .
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Distal Articulations
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6. SYSTEMATICS AND TAXONOMY .
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Order XENARTHRA, Family MYLODONTIDAE .
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Subfamily MYLODONTINAE
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Genus GLOSSOTHERIUM .
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Greater Trochanter
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Genus PARAMYLODON
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Conclusions and Future Work
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LITERATURE CITED.
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APPENDICES
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2. MANOVA values
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3. Extant cranial PCA values and loadings 4. Crania PCA values and loadings.
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5. Mandible PCA values and loadings
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6. Humerus PCA values and loadings
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7. Radius PCA values and loadings
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8. Femur PCA values and loadings .
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9. Tibia PCA values and loadings .
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10. Astragalus PCA values and loadings
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11. Cranial dimensions and ratios for Glossotherium and Paramylodon . . . . . .
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12. Mandible dimensions and ratios for Glossotherium and Paramylodon . . . . . .
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13. Additional cranial ratios for Glossotherium and Paramylodon .
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14. Comparative cranial dimensions and ratios of Glossotherium chapadmalense against those of Glossotherium and Paramylodon . . . . . . .
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15. Humerus dimensions and ratios for Glossotherium and Paramylodon . . . . . .
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17. Femur dimensions and ratios for Glossotherium and Paramylodon . . . . .
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18. Tibia dimensions and ratios for Glossotherium and Paramylodon . . . . .
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19. Astragalus dimensions and ratios for Glossotherium and Paramylodon . . . . . .
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LIST OF FIGURES
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2. Plio-Pleistocene time scale showing the synchronization for the North and South American Land Mammal Ages .
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3. Cranial measurements
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4. Mandible measurements .
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5. Humerus measurements .
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6. Ulna measurements
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7. Radius measurements
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8. Femur measurements
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9. Tibia measurements
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10. Astragalus measurements .
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11. PCA for extant cranial data
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12. Combined PCA for cranial data .
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13. Combined PCA for mandible data
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14. Combined PCA for humerii data .
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15. Combined PCA for radii data
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16. Combined PCA for femora data .
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17. Combined PCA for tibia data
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18. Combined PCA for astragali data .
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xii Figure
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19. Skulls in dorsal view of Glossotherium and Paramylodon.
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20. Ventral views of Paramylodon skull dentition
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21. Right maxillary dentition outlines of Glossotherium, Paramylodon, and Mylodon . . . . . . .
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22. Skulls in lateral view
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23. Skulls in ventral view for Glossotherium and Paramylodon
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24. Lateral mandible views of Glossotherium and Paramylodon
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25. Mandible outlines for Glossotherium, Paramylodon, and Mylodon . . . . . . .
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26. Lateral and ventral views for the type specimen of Glossotherium chapadmalense . . .
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27. Dorsal view of the mandible for Glossotherium chapadmalense .
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28. Anterior humerii views of Glossotherium and Paramylodon.
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29. Posterior radii views of Glossotherium, Paramylodon, and Lestodon . . . . . . .
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30. Anterior femora views of Glossotherium and Paramylodon
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31. Lateral and dorsal astragali views for Glossotherium, Paramylodon, and Mylodon . . . . . . .
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32. Distal and ventral astragali views for Glossotherium, Paramylodon, and Mylodon . . . . . . .
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33. Skulls and mandible for Glossotherium and Paramylodon
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34. Humerii of Glossotherium and Paramylodon in anterior view showing the differences in the deltoid crests, lateral and medial epicondyles, and the angles of the shaft . .
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36. Glossotherium and Paramylodon femora in anterior and posterior views, respectively . . . .
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37. Glossotherium mandibles showing the different shapes for c1 in dorsal view . . . . . .
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38. AMNH jacket containing a Blancan-aged mylodontid from Texas with a cranium and some postcranial elements .
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CHAPTER 1 INTRODUCTION TO THE MYLODONTINAE AND THE TAXONOMIC PROBLEMS THEREIN
The Order Xenarthra represents one of the oldest mammalian groups, diverging with other basal placental mammals groups from the Afrotheria in the Early Cretaceous and, according to molecular clock data, becoming fully established by the Late Cretaceous, with the earliest fossils dated at 58 Mya from the Paleogene (Vizcaíno, 1994; Vizcaíno et al. 1998b; Delsuc et al. 2001, 2002, 2004). The Order consists of two distinct lineages: the Cingulata (glyptodonts, pampatheres, and armadillos) and the Pilosa, which is further divided into the Vermilingua (anteaters) and Tardigrada (ground and tree sloths) (Fig. 1). Overall, the group is diverse with more than 30 extant species in 14 genera, along with more than 150 extinct genera whose high numbers can be attributed to the extensive radiations of the extinct cingulates and ground sloths (Wetzel, 1985; McKenna and Bell, 1997; Gaudin, 2003). Xenarthrans are united by the shared characters of a reduced and modified homodont dentition characterized by a lack of enamel and deciduous teeth, reduced incisors, a scapular fenestra above the coronoid process, development of a secondary scapular spine, fusion between the ischium and the posterior sacral and/or anterior caudal vertebrae, and additional or “xenarthrous”
2 intervertebral articulations, from which the order derives its name (Grassé, 1955; Hoffstetter, 1958, Engelmann, 1985; Novacek and Wyss, 1986; Gaudin 1999, 2003).
Figure 1. Xenarthran phylogeny showing the major divisions. Modified from Gaudin, 2003.
Tardigrada
Extinct sloths represent one of the largest and most diverse of the xenarthran groups and consequently are in need of resolution for a number of taxonomic and phylogenetic issues. The earliest known specimen, a partial upper caniniform, is from the middle to late Eocene, close to the proposed molecular date of 55Mya for the slothanteater divergence at the Paleocene-Eocene transition (Vizcaíno and Scillato-Yané, 1995; Delsuc et al., 2004). Due to its partial nature, this specimen was not possible to place within the context of the three major sloth families of Mylodontidae,
3 Megatheriidae and Megalonychidae, all of which were fully diversified and distinct by the late Oligocene/early Miocene. The sudden appearance of these families and a lack of transitional forms have made it difficult to determination of phylogenetic relationships among the sloths. Like the other xenarthrans, ground sloths were confined to South America until the late Miocene when the continent was reconnected with North America via the Panamanian Isthmus, although members of the Megalonychidae had successfully colonized and diversified on the islands of the Antilles prior to that time (MacPhee and Iturralde-Vincent, 1994). Once the land bridge was established, what followed was a period of faunal exchange and turnover known as the Great American Biotic Interchange (GABI), with the more successful colonizers being those from North America (Webb 1978, 1985, 2006; Morgan, 2005). The most successful mammals were represented by members of the families Camelidae, Canidae, Cervidae, Felidae, Mustelidae, Procyonidae, and Tayassuidae and likely hastened a number of South American taxa towards extinction (Webb 1978, 1985, 2006). Xenarthrans, and in particular the sloths, represent one of the few South American groups to successfully take part in the GABI, with many taxa becoming established in North America and with wide distributions across the open areas of the continent. One taxon (Megalonyx jeffersoni Desmarest 1822) has even been recorded from deposits as for north as Canada and Alaska (Stock and Richards, 1949; McDonald et al., 2000). The tardigrades, along with many of the other elements of the Pleistocene megafauna, went extinct close to 10,000 years ago (Long et al., 1974, 1998; Schubert et al., 2004), although there is some evidence that the sloths of the Antilles survived into
4 the Holocene until as recently as 4,000 years ago (Steadman et al., 2005). The extant sloths are fully arboreal, earning the moniker “tree sloths,” and consist of the three-toed sloths, Bradypus Linnaeus 1758, and the two-toed sloths, Choloepus Illiger 1811. Despite a long history of being assigned to the same family based upon small body size and unique arboreal habits, they are now placed into two separate families, Bradypodidae and Megalonychidae, respectively (Patterson and Pascual 1968, 1972; Englemann, 1985; Webb, 1985; Gaudin 1995, 2004). The remaining two genera of sloths, which are represented by six recognized species, mark a vast reduction in diversity from the more than 30 recognized extinct genera and have a smaller distribution restricted to Central and South America (Wetzel, 1985; Anderson and Handley, 2001). Representatives of the three major sloth families are recorded from North American deposits, with nearly all the genera being novel forms not found in South America. The exception is the megatheriid Eremotherium Spillman 1948, which represents the only known pan-American sloth with remains confirmed from both continents (Morgan, 2005). The possibility of a second pan-American taxon exists among the mylodontids but requires further study as less attention has been given to the taxonomy and distribution of those sloths when compared to that of the megalonychids and megatheriids. The mylodontid sloths are among the more basal groups, with many genera sharing the character of retaining dermal ossicles, which are likely a reduced remnant of the armored cingulate carapace. Yet their early position on the tardigrade tree has caused them phylogenetic problems, further hampered by the lack of specimens to
5 provide character polarities between the first Eocene sloths and their appearance in the Oligocene. As such, a number of subfamilies, genera and species have been named, renamed and synonymized through the years, but there is still a need for revision. The most common call has been for revision and reassessment of the members of the Mylodontinae (McDonald, 1987; Gaudin, 2003; Morgan, 2005). The goal of this dissertation is to alleviate some of the nomenclatural confusion within a subset of the Mylodontinae.
Mylodontidae
Mylodontid sloths are defined by the following characters: dental formula 5/4 or 4/4; last upper and lower molariform bilobed; M2-M4 and m2-m3 typically lobed but lobation secondarily lost in some forms; mandibular condyle within the plane of the teeth in advanced forms but slightly above in earlier forms; limbs generally short and massive; manual unguals dorsoventrally compressed with the width greater than the height; fovea ligamentum teres forms a notch on the femoral head; and astragular odontoid process (Sinclair, 1910; Stock, 1925; Hoffstetter, 1958; Engelmann, 1985; McDonald, 1987). Members of this family are typically placed into one of three subfamilies: Scelidotheriinae, Lestodontinae, and Mylodontinae. Typically, only Mylodontinae and Scelidotheriinae are recognized, with members of the Lestodontinae placed as the Tribe Lestodontini within Mylodontinae (Engelmann, 1985; Gaudin 1995, 2004; McKenna and Bell, 1997). Of interest are the Plio-Pleistocene members of the Mylodontinae that
6 are found in North and South America, which thus represent two separate land mammal ages that are correlated in Figure 2.
Figure 2. – Plio-Pleistocene time scale showing the synchronization for the North and South American Land Mammal Ages (SALMA & NALMA, respectively). Numbers at the base of each epoch and land mammal age represent millions of years (mya).
7 History and Taxonomic Confusion Within the Mylodontinae
Mylodontid sloths were the second sloth family recognized, dating back to the descriptions by Owen of Darwin’s findings while on the H.M.S. Beagle (Owen, 1840). Representatives of the subfamily Mylodontinae are found throughout the Americas, and it is composed of the following genera: Mylodon Owen 1840, Glossotherium Owen 1840, Paramylodon Brown 1903, and Pleurolestodon Roverto 1914 (Gaudin 1995, 2004; McKenna and Bell, 1997). Early assessments of relationships among members of the Mylodontinae placed them with the megatheriids as a result of the lack of comparative studies in the early 1800s. The taxonomic problems that followed are less understandable and affect mylodontids throughout the Americas, specifically Glossotherium and Paramylodon, as well as the question of their relationships with one another. In 1840, Richard Owen established a number of genera from the fossil specimens brought back from South America by Charles Darwin. The first, Glossotherium, was based upon a fragment of the left temporal region of the skull, but Owen failed to designate a species, creating only the genus. Owen attempted to rectify this situation in 1842, stating that the original temporal fragment was synonymous with the genus Mylodon, also established in the 1840 monograph, with the type species M. darwini. Thus, the original Glossotherium element became Mylodon robustus. Also during this time, Owen (1840) established the North American species Mylodon harlani.
8 Some confusion arose in the 1840 prologue when Owen recognized M. harlani from North America as the first Mylodon type and M. darwini as the second. When discussing the new genus in the actual 1840 text, Owen stated, “The species of which the fossil remains [from North America] are described by Dr Harlan may be dedicated to that indefatigable Naturalist… The fossil about to be described represents a second and smaller species of the same genus [Mylodon], and I propose to call it Mylodon Darwinii” (p.68) further confirming the order listed in the prologue. However, it appears Owen revised his own diagnosis, ordering the species of Mylodon in the 1842 memoir as (1) M. darwini, (2) M. harlani, and (3) M. robustus. Further revision took place later in a footnote from the same publication (p. 154), wherein Owen believed the temporal fragment assigned to Glossotherium showed potential species-level differences to the skull of M. robustus and thus placed Glossotherium as a synonym of Mylodon. In the systematic summary, Owen did not make any note of Glossotherium in his discussion of M. robustus, which was odd as he believed a number of taxa to have been erroneously assigned and made note of them. The result of the decision that the two specimens were congeneric effectively changed Mylodon darwini to Glossotherium darwini, as Glossotherium held a few pages of seniority over Mylodon. That change was unofficial until Ameghino used Glossotherium darwini in his 1889 work (Kraglievich, 1928). Subsequent authors (Reinhardt, 1879; Burmeister, 1886; Ameghino 1889, 1898, 1900, 1904; Lydekker, 1894; Kraglievich 1925, 1928) either believed that Owen erred and reversed his synonym because of the seniority Glossotherium had over Mylodon,
9 making G. darwini the actual type species, or that his entire taxonomy was invalid, thus paving the way for a host of new names. What had been a fairly constrained problem in terms of the number of names involved suddenly ballooned with the creation of new taxa, such as Grypotherium Reinhardt 1879, Pseudolestodon Gervais & Ameghino 1880, Neomylodon Ameghino 1898, and Eumylodon Ameghino 1904, making the issue even more convoluted. Many of these names were later recognized to be invalid or superfluous and became junior synonyms of Mylodon or Glossotherium; even Glossotherium has been included as a synonym of Mylodon (McKenna and Bell, 1997). Kraglievich (1928) retraced this nomenclatural minefield and decided the original South American specimens belonged to two distinct genera. This was possible due to the discovery of Reinhardt’s (1879) Grypotherium specimens, consisting of a mandible that mirrored the characters of M. darwini and also a skull with characters that were very distinct from M. robustus. Thus, Mylodon was salvaged and M. darwini placed as the type species, while for Glossotherium it was determined that G. robustum was a valid species, but not suitable as the type. To solve the problem, Kraglievich questionably created G. uruguayensis as the type species. Kraglievich’s designation of G. uruguayensis was a curious conclusion, as there was no comparison of the fragment with respect to characters distinguishing it from the type of G. robustum. A reassessment of the relationship between these two species is no longer possible as the type skeleton of Glossotherium robustum was unfortunately destroyed by WWII bombings of London. However, a review of the species-level taxonomy of Glossotherium was undertaken by Cabrera (1936), who concluded that G.
10 uruguayensis was distinct from G. robustum, but the characters conformed to those of an older species G. lettsomi, under which he placed G. uruguayensis as a junior synonym. The convention of recognizing just the two Pleistocene species does not appear to have been followed in subsequent years, with a large number of species names assigned to Glossotherium flourishing in the literature. However, the consensus is that G. robustum is the type species of Glossotherium and that G. uruguayensis was a superfluous designation as it has not appeared again in the literature. Additionally, Kraglievich and Cabrera determined that the North American mylodontids belonged instead to the genus Paramylodon. Brown (1903) had created P. nebrascensis in the midst of the nomenclatural turmoil and it was considered valid as it lacked the first upper teeth, giving the animal a 4/4 dental formula. However, this character was later found to be inconclusive by Stock (1914a, 1914b, 1917a, 1925), who noted a great variability in the retention or loss of the first upper caniniform of specimens representing “Mylodon harlani” from Rancho La Brea, California. Prior to P. nebrascensis, all North American mylodontids were designated as Mylodon, following the example of Owen (1840) for M. harlani. As such, Kraglievich established P. harlani as the true species in North America, as this specific ephthet had nearly 80 years of priority over P. nebrascensis. In the following years, a trend developed where Glossotherium became a catchall taxon for mylodontid taxa, ignoring many of the conclusions and groupings of Cabrera (1936). Genera of closely related taxa were relegated to a subgeneric status of Glossotherium (Hoffstetter, 1958; Cartelle, 1980; Cartelle & Fonseca, 1981). This was even applied to the northern taxon which was deemed to be synonymous and listed as
11 Glossotherium (Paramylodon) harlani. This idea was further supported by Robertson (1976) and Kurtén and Anderson (1980), creating a level of credibility to the concept that Paramylodon was a subgenus of Glossotherium in North America. The subgeneric status persisted until McDonald (1995) again separated the two genera and, although questioning the validity of the South American genus, retained it to some degree for Blancan-aged mylodontids under the name “Glossotherium” chapadmalense, as the North American specimens bore a similarity to the mid-Pliocene sloth, Glossotherium chapadmalense, from South America. Despite this, many collections with specimens belonging to Irvingtonian and Rancholabrean-aged mylodontids still bear the G. harlani designation. This could be due in part to a lack of characters being given for the separation of the two taxa, leaving uncertainty as to what truly constitutes each genus.
Glossotherium chapadmalense – The Missing Link?
The type specimen of Glossotherium chapadmalense is a cranium and mandible (MACN 8675) from the Buenos Aires Province, Argentina. Kraglievich (1925) initially placed the species into Ameghino’s genus Eumylodon, which is now a junior synonym of Glossotherium. The specific epithet refers to the Chapadmalan sediments (mid-Pliocene) from which the remains were recovered and was established mainly on its early age and smaller size compared to the mylodontids of the Pampean Formation (Pleistocene). For the North American specimens, the quotation marks for “G.” chapadmalense signal the uncertain assignment to that taxon, which is predominantly
12 based upon their small size and occurrence in the late Blancan (late Pliocene), which makes them approximately contemporaneous with the South American specimens (Fig. 2). The uncertain status of “G.” chapadmalense in North America has persisted because of the scant material available for study, especially cranial material. This has resulted in two potential theories regarding the placement of this taxon into either Glossotherium or Paramylodon. Should the North American specimens belong to Glossotherium, the implication is that the genus evolved in South America and later migrated to North America during the late Blancan. This would give further credence to the idea that Paramylodon is a junior synonym of Glossotherium. If the specimens should instead be found to be more closely aligned with Paramylodon, then the Blancan sloths could represent a sister species to P. harlani, increasing the species diversity for the genus, along with the temporal range (McDonald, 2005; Morgan, 2005). In either event, these ideas have persisted without testing, with the closest being the opinion of McDonald (1995) that the conclusions of prior authors in placing the two genera as synonymous were unwarranted.
That Yet to Come…
The following portions of the dissertation are aimed at defining the generic differences between Glossotherium and Paramylodon to provide a revised diagnosis for each genus. While their separation seems obvious with Paramylodon occurring only in North America, revision for Glossotherium is important as many of the postcranial
13 elements are undistinguished from Mylodon. Such a diagnostic revision will also be beneficial to future taxonomic studies by addressing the species-level problems within Glossotherium, ending its catch-all status for South American mylodontid specimens. Revision will also benefit the North American taxon by determining the generic limits such that any new specimens of Blancan age can be used to address the placement issue of “Glossotherium” chapadmalense. Determining the generic status of “G.” chapadmalense will further elucidate the taxonomic relationships between Glossotherium and Paramylodon The framework for the remaining sections of the dissertation is as follows. Chapter Two outlines the methodologies utilized, with a discussion of the principal components analysis (PCA) performed. Chapter Three reviews the PCA results for the cranial and postcranial elements, prior to a more detailed review of the generic characters for Glossotherium and Paramylodon. Chapter Four explores the generic differences between the taxa, highlighting the delimiting characters suggested by the PCA, along with quantitative values from collected measurement data and various qualitative characters unique to each group. Chapter Five builds upon the characters established in the previous chapter, exploring the functional implications of the differences. Chapter Six reiterates the systematic taxonomy of the Mylodontinae and provides an emended diagnosis for Glossotherium and Paramylodon, with discussion on potential species differences within each genus. The final chapter outlines directions for future work necessary to further elucidate the relationship between Glossotherium and Paramylodon.
CHAPTER 2 METHODS IN THE FACE OF TAXONOMIC MADNESS
In seeking to address the questions surrounding the identification of Paramylodon and Glossotherium, a decision had to be made concerning the methodology to follow. At face value, the problem seems to be one of simple phylogeny, and therefore a standard phylogenetic or cladistic approach would be the method to employ. In fact, recent investigations by Gaudin (1995, 2004) concerning the overall phylogeny of the xenarthans and more specifically of the sloths have separated the two based on qualitative cranial characters. Yet there still remains an insistence in his discussion of the results that the other taxonomic issues within the Mylodontinae would “greatly benefit from a thorough revision of Glossotherium and Paramylodon” (Gaudin, 2004, p.281). Most workers agree that the two genera belong as separate and distinct groups, yet previous analyses are somewhat unclear as to how they differ from each other. Phylogenetic results typically delimit taxa based on qualitative differences in the characters selected. The broad range of taxa and characters placed within Glossotherium over the years makes assessment of characters difficult, especially with the issues of assigning polarity between the two genera. Addressing these issues instead requires quantitative approaches that can determine character value ranges. Again, the questions of interest are not just whether Glossotherium and Paramylodon differ, but how they
15 differ, what taxonomic and functional implications arise from those differences, and what role, if any, geographic and temporal distribution have upon their separation. Multivariate analyses were thus chosen as an initial method to help address the issues and to evaluate the differences between Paramylodon and Glossotherium. This provides a method for obtaining quantitative character ranges to complement those of a more qualitative nature. There are many multivariate methodologies currently available and a high number of accompanying computer packages that aid in the often complicated statistics necessary for each study. Methodologies were evaluated for their ability to address the questions of interest for the mylodontids. Some are adept at determining evolutionary changes in morphology, which, while ideal in essence, are often hampered by an a priori need for an established phylogenetic context. Principal components analysis (PCA) was ultimately selected as recent studies have been successful in determining generic and species-level differences in extant mammals where only a few genera or species are in question (Díaz et al., 2002; González et al., 2002; Ventura et al., 2002; Turner and Worthy, 2003). PCA studies have even been utilized in the study of another North American mylodontid sloth, Thinobadistes Hay 1919, whose taxonomy is well known (Webb, 1989). One of the major benefits of PCA is that it enables researchers to take a large number of variables and reduce them to a few primary vectors or axes. The combination of axes and their loadings can provide information about shape, once size differences have been taken into consideration. The loadings for the axes are also useful in determining those variables or measures that have the highest variation and are typically an indication that they are the most influential. Sample sizes can be small and analyses
16 are often adept at handling specimens with some missing data, two issues that frequently occur in paleontological investigations (Strauss et al., 2003). Another benefit is that the samples can help establish the ranges of characters in extinct populations and assist in determining juvenile specimens; such specimens might end up being coded as new species in phylogenetic methods.
Methodology
PCA data sets were created by measuring the crania and major limb bones of the postcrania of North and South American mylodontid sloths. See Figures 3-10. Measurements were obtained using a 300mm digital caliper and also a 600mm for larger specimens. Tailor’s tape was used to measure the dimensions of a few curved surfaces. The measurements taken were designed to capture a three-dimensional representation of the skeletal elements investigated. End points between measures were established on features that were consistent and generally homologous between taxa. For example, the alveoli were always measured for dentition variables to avoid discrepancies in sizes of measuring the actual teeth when they were present. Specimens were selected based upon their level of preserved completeness to avoid pitfalls that occur in analyses in which there are large amounts of missing data. Adults were the preferred specimens, but a few juveniles were measured when available and included in the analyses for the cranium and mandible. Specimen ages were determined using a combination of overall size and the degree of fusion in the elements, either in the cranial sutures or in the epiphyses of the limb bones.
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Figure 3. – Crania measurements. The following correspond to the measurement numbers above and their abbreviations: 1. Total Length (SKL); 2. Anterior Width (AntW); 3. Anterior Height (AntHt); 4. Lacrimal Width (LacW); 5. Lacrimal Height (LacHt); 6. Postorbital Width (PorbW); 7. Postorbital Height (PorbHt); 8. Posterior Width (PostW); 9. Posterior Height (PostHt); 10. Occipital Condyle Width (OCW); 11. Maxillary-Palate Length (Max-PalL); 12. Lacrimal-Squamosal Length (LacSqlL); 13. Toothrow Length (TrowL); 14. Molariform Toothrow Length (M-TrowL); 15. Tooth Width – taken for all teeth (C1 or M#W); 16. Tooth Length – taken for all teeth (C1 or M#L); 17. Post-M4 Length (PostM4L).
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Figure 4. – Mandible measurements. The following correspond to the measurement numbers above and their abbreviations: 1. Toothrow Length (TrowL); 2. Molariform toothrow Length (M-TrowL); 3. Tooth Length – taken for all teeth (c1 or m#L); 4. Tooth Width (c1W, m1W, m2W); 5. m3 Width, anterior lobe (m3W); 6. m3 Width, posterior lobe (2m3W); 7. Mandible Depth at m1 (m1D); 8. Mandible Depth at m3 (m3D); 9. Coronoid process Height (CoroHt); 10. Condyle Height (CndHt); 11. Condyle Width (CndW); 12. Total mandible Length (TL); 13. Diastema Length (DL).
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Figure 5. – Humerus measurements. The following correspond to the measurement numbers above and their abbreviations: 1. Total Humeral Length (TL); 2. Width across the Greater and Lesser Tubercles (TbW); 3. Mediolateral Width of Humeral Head Surface (HdML); 4. Anteroposterior Width of Humeral Head Surface (a and b) (HdAP); 5. Anteroposterior Shaft Width (SftAP); 6. Mediolateral Shaft Width (SftML); 7. Epicondyle Width (EpcndW); 8. Maximum Condyle Width (MaxCndW); 9. Deltoid Tubercle Height (DeltHit); 10. Medial Epicondyle Height (MdEpcndHt).
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Figure 6. - Ulna measurements. The following correspond to the measurement numbers above and their abbreviations: 1. Total Length (TL); 2. Proximal Trochlea Notch Anteroposterior Width (PTN AP W); 3. Coronoid Process Anteroposterior Width (Coro Pr AP W); 4. Maximum Coronoid-Trochlear Width (Max Coro-T W); 5. Radial Notch Width (Rad N W); 6. Radial Notch Height (Rad N Ht); 7. CoronoidTrochlea (Coro-Troc); 8. Lateral Trochlea Height (Lat Troc Ht); 9. Trochlea Height (Troc Ht); 10. Ulnar Tuberosity Anteroposterior Width (Uln Tuber AP W); 11. Coronoid-Radial Notch Height (Coro-Rad N Ht); 12. Distal Anteroposterior Width (Dist AP W); 13. Distal Mediolateral Width (Dist ML W).
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Figure 7. - Radius measurements. The following correspond to the measurement numbers above and their abbreviations: 1. Total Radius Length (TL); 2. Anteroposterior Width of Radial Head (HdAP); 3. Mediolateral Width of Radial Head (HdML); 4. Anteriorposterior Width of Distal End (DisAP); 5. Mediolateral Width of Distal End (DisML); 6. Mediolateral Width of Articular Surface (ArtSfW); 7. Medial Articular Aurface Mediolateral Width (MdArtSfW); 8. Medial Articular Surface Anteroposterior Width (APArtsfW); 9. Styloid Process Anteroposterior Width (StyloidAP).
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Figure 8. - Femur measurements. The following correspond to the measurement numbers above and their abbreviations: 1. Total Femur Length (TL); 2. Anteroposterior Width of Femoral Head Surface (a and b) (HdAP); 3. Mediolateral Width of Femoral Head Surface (HdML); 4. Circumference of Femoral Head/Neck (a and b) (HdCirc); 5. Width across Greater and Lesser Trochanters (GT-LT W); 6. Width across Greater Trochanter and Femoral Head (GT-Hd W); 7. Width across Lesser Trochanter (LTW); 8. Width across Third Trochanter (3TW); 9. Anterior Articular Surface Width (AntCndW); 10. Posterior Width across the Condyles (PostCndW); 11. Lateral Condyle Width (LtCndW); 12. Medial Condyle Width (MdCndW).
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Figure 9. - Tibia measurements. The following correspond to the measurement numbers above and their abbreviations: 1. Total Tibia Length (TL); 2. Medial Condyle Mediolateral width (MdCndML); 3. Medial Condyle Anteroposterior Width (MdCndAP); 4. Lateral Condyle Mediolateral Width (LtCndML); 5. Lateral Condyle Anteroposterior Width (LtCndAP); 6. Maximum Width across the Condyles (MxCndW); 7. Width between the Condyles (IntCndW); 8. Mediolateral Width of Distal Articular Surface (DistML); 9. Anteroposterior Width of Distal Articular Surface (DistAP).
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Figure 10. – Astragalus measurements. The following correspond to the measurement numbers above and their abbreviations: 1. Height of Odontoid Process (OdntHt); 2. Width of Odontoid Process (OdntW); 3. Height of Fibular Articular Surface (FibArtHt); 4. Width of Fibular Articular Surface (FibArtW); 5. Tibial Plateau Width (TibPltW); 6. Tibial Plateau Length (TibPltL); 7. Height of Navicular Articular Surface (NavHt); 8. Width of Navicular Articular Surface (NavW).
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The humerus, radius and ulna were included in the postcranial study because of the inferred functions these elements served in life. The earliest sloth descriptions have questioned the feeding habits and possible mechanics of the forearms for either digging up tree roots or in reaching upward to gather branches bearing vegetation (Cuvier, 1796; Owen 1840, 1842, 1858). Although consensus on the actual uses of the forelimbs is absent, potential for a diversity of functions makes these bones ideal for study, as different environments could have created selection pressures, further affecting their taxonomic interpretation. Various analyses of sloth trackways have categorized the large ground sloths as quadrupeds with the potential for limited bipedalism (Cuvier 1796, 1804; Owen 1840, 1842, 1860; Abel, 1912; Stock 1917b, 1920b, 1936; Cabrera, 1929; Casamiquela, 1974; Aramayo and de Bianco 1987, 1996; Casinos, 1996; Blanco and Czerwonogora, 2003). As such, a similar reasoning applies to including the femur and tibia in the study. The fibula was excluded from study as the fibular shape is rather simple and does not provide as many homologous measurement points as do the other elements. Instead, the astragalus is included, as it is more amenable to measurement, and its morphology has been frequently used to differentiate various sloth families and genera (Owen, 1842; Lydekker, 1886; Hay 1916, 1919; Kraglievich, 1928; Webb, 1989). The following section illustrates all the measurements taken, along with the number of specimens measured for each genus (Table 1). The specimens studied and the institutions in which they are housed are listed in Appendix A – Specimens.
26 Table 1: Number of specimens studied for each genus. Glossotherium Cranium 17 (3 juveniles) Mandible 19 (6 juveniles) Humerus 9 Ulna 4 Radius 5 Femur 10 Tibia 9 Astragalus 11
Paramylodon 16 (2 juveniles) 29 (3 juveniles) 15 15 19 15 34 43
Mylodon 5 (1 juvenile) 9 (5 juveniles) -4 3 --8
In regards to the teeth, xenarthran dentition is unique in that it is not homologous with the teeth of other mammals. In sloths, the anterior tooth is often separated from the others by a diastema or has a sharp, chisel-like appearance. This first tooth resembles a canine but in appearance only, and is thus referred to as caniniform. Similarly, the other teeth occupy a position similar to that of mammalian molars and are referred to as molariform. The number of molariforms adds to the evidence of a lack of homology as there are typically four molariforms in sloths while mammals never have more than three molars. One of the sloth molariforms should then be the equivalent of a premolar, but there is no way to make such a distinction. The standard abbreviations when discussing mammal teeth are to use a capital or lowercase letter, depending on whether the tooth is maxillary or mandibular, followed by a number reflecting its position within the series. This standard is followed here with the understanding that C/c refers to caniniform teeth and M/m refers to molariform teeth, and does not imply any homology to the typical mammalian dentition.
27 Abbreviations
n – number of specimens studied Min. – minimum value for a range of measurements Max. – maximum value for a range of measurements X – mean s – standard deviation cm – centimeters df1, df2 – first and second degrees of freedom F – F statistic p – significance level AMNH – American Museum of Natural History, New York F:AM – Frick Collection at the AMNH BMNH – British Museum of Natural History, London FCDVP – Paleontological Collections of Vertebrates, Facultad de Ciencias, Montevideo FMNH – Field Museum of Natural History, Chicago FlMNH – Florida Museum of Natural History, Gainesville IMNH – Idaho State Museum of Natural History, Pocatello LACM – Los Angeles County Museum & George C. Page Museum, Los Angeles MACN – Museo Argentino de Ciencias Naturales, Buenos Aires MLP – Museo de La Plata, La Plata MNHN – Museum National d’Histoire Naturelle, Paris TMM – Texas Memorial Museum, Austin UNSM – University of Nebraska State Museum, Lincoln ZMUC – Zoologisk Museum Universitat Copenhagen, Copenhagen
Multivariate Analyses
Analysis of the variables was performed using the multivariate paleontological program PAST v. 1.66 (Hammer et al., 2001). Separate analyses were performed for each skeletal element, as there were an insufficient number of individuals with associated material to perform combined studies. A multivariate analysis of variation (MANOVA) was first carried out to establish statistically the separation of the individual skeletal
28 elements for Paramylodon and Glossotherium, using a 1% level of significance (p > 0.01) as the cutoff. Table 2 shows the MANOVA results obtained for eight skeletal elements, with each showing a significant difference between the genera except for the ulna, which at p = 0.045 falls outside of the established level of significance and is therefore not included in the remaining analyses. While separation is already agreed upon among researchers, that separation is dominated by studies involving the crania alone. The MANOVA results indicate that the postcrania also differ in some manner, but it is not useful in determining which of the variables are the most significant for each element. To determine which variables have the most influence for separating taxa, PCA analyses were performed on each of the measured elements, with an initial test of the methodology upon crania of the extant tree sloths as they are known to be distinct from one another. For the overall PCA methods, analyses were run utilizing a correlation matrix, as opposed to a covariance matrix. Typical PCA guidelines suggest using a covariance matrix when all data are recorded using the same unit, which was the case with all measures taken using a millimeter scale (Jackson, 1991; Norman and Streiner, 1994; Jolliffe, 2002). However, other conventions indicate a correlation matrix to be more appropriate in cases where all measurements are of the same unit and when the individual variances of the measurements greatly differ (Jolliffe, 2002). This becomes especially true when one measurement is excessively large in comparison to the others, such as the total length of a bone compared to the width of an articular facet. Under these conditions, the use of the correlation matrix was the preferred method.
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Table 2: MANOVA values
Cranium Mandible Humerus Ulna Radius Femur Tibia Astragalus
df1 18 12 10 11 9 12 8 8
df2 11 35 9 7 13 14 31 44
F 29.72 11.83 25.63 3.75 9.70 21.53 5.60 11.28
p 8.17 x 10-7 5.13 x10-9 2.07 x 10-5 0.045 1.88 x 10-4 5.97 x 10-7 1.98 x 10-4 1.65 x 10-8
Prior to conducting any of the PCA analyses, the correlation matrix data were transformed on a logarithmic scale. A review of comparative PCA studies suggest utilizing such transformations with the correlation matrix because they can produce results similar to those of the correlation matrix alone, but with the added effect of downweighting outliers (Jackson, 1991; Baxter, 1995; Jolliffe, 2002). While outliers can be useful and informative, the data set for Glossotherium is composed of taxa irresepective of species, as the question of speciation in Glossotherium is not currently under investigation. As the potential exists for the data set to widely vary due to potential species differences, it is desirable to reduce those effects to strengthen the generic signal. There is also the extra benefit that the remaining outliers tend to reflect juvenile specimens. As each axis represents a different amount of variation, it is often necessary to consider a different number of axes for each element. A number of criteria exist for deciding how many axes to include in PCA studies, and in this case two methods were
30 used in conjunction to decide on the number of axes for each element. The first method involved totaling the percent of variance represented by each axis until they represented a minimum of 80%. The number of components required to total 80% of the total variance often mirrored the criteria of the Jolliffe cut-off value for the eigenvalues, another method for deciding on the number of axes to include (Jolliffe, 2002). The second method was used as a qualitative assessment of the first, based upon a scree plot of the eigenvalues generated for each analysis. In this method, the point in the plot that constitutes a “sharp break” in the overall curve between its descent and the point where it levels off towards zero represents the last axis to include. For some elements, this point occurs after the first or second axis, but generally three axes are retained and used (Jackson, 1991; Norman and Steiner, 1994). Typically in analyses of principal components, the first axis is thought to reflect the growth of the organism when all the loadings are either positive or negative, whereas the axes thereafter are interpreted as being indicative of shape when the loadings differ in the direction of their signs (Jolicoeur and Mosimann, 1960; Rao, 1960; Jackson, 1991; Baxter, 1995; Jolliffe, 2002). Typically, the variable with the largest absolute value represents the measure with the greatest contribution to that axis (Norman and Steiner, 1994). In some cases, the variable with the greatest loading was also the greatest on a subsequent axis, although this can often be a reflection of its large measurement value. In those axes where the values were not all loaded in the same direction, a second variable was chosen for investigation by selecting the variable with the highest value with a positive or negative sign that was opposite that of the variable with the overall highest absolute variable.
31 For the test involving the tree sloths, the PCA resulted in using the first two axes, following the criteria of choosing the first axes to total 80% of the total variance. The first axis accounted for 82% alone (Table 3), with the second axis included in order to obtain a graphic plot. The plot (Figure 11) exhibits the distinct separation expected for Bradypus and Choloepus given their taxonomic placement in separate families. Even with a log transformation of the data, the separation appears to be based on size, with Choloepus, known to be the larger of the two, plotting at greater values along the first axis. Surprisingly, the first axis does not display the expected pattern of growth, as not all of the loading values are positive, with those for the length of M1 and M4 being negative; the overall length of the skull is the highest loading for PC I. It is those two variables of tooth length that are the primary loaders of the second axis, although they do not appear to have significance in separating the genera along that axis. When the methodology was applied to the extinct genera, the results were divided into three groupings: Combined, Glossotherium, and Paramylodon. Combined examines both genera together, evaluating those variables useful for separating the genera. The results for the individual genera provide a comparison to the Combined results and also to one another. They allow for the investigation of any differences that are unique to each genus, which might have been missed or overlooked in the Combined analysis alone. Graphic plots were generated in Microsoft Excel XP for each of the PCA analyses using the raw eigenvalues generated by PAST. A complete listing of the data, plus initial measurement data can be found on the CD-ROM containing Appendix B – Data. The graphic plots illustrate the axes where separation is occurring, which can then be evaluated with the factor loadings to determine variables deserving closer study.
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Table 3: Extant cranial PCA values and loadings. Extant Tree Sloths Eigenvalue % variance Loadings SKL Ant W Lac W Porb W Post W Ant Ht Lac Ht Porb Ht Post Ht OCW Max-Pal L Lac-Sqm L TrowL M1-M4 L C1 L M1 L M2 L M3 L M4 L
PC I 15.703 82.65
PC II 1.234 6.49
*0.245 0.245 0.247 0.246 0.238 0.238 0.235 0.245 0.245 0.242 0.249 0.247 0.250 0.231 0.240 -0.012 0.224 0.237 ^-0.078
-0.034 -0.040 0.013 -0.006 0.148 0.130 -0.014 -0.009 0.005 ^-0.138 -0.047 -0.127 -0.008 0.236 -0.040 0.642 0.048 0.151 *0.653
(*) indicates variables with the highest absolute value contributing to the factor loading and (^) indicates variables with the second highest value of the opposite sign.
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5 4 3
PC II
2 1 0 -1 -2 -3 -4 -5 -6 0 1.6 3.2 4.8 6.4 8 PC I Figure 11. PCA for extant cranial data. O = Bradypus and X = Choloepus. -6.4 -4.8 -3.2 -1.6
CHAPTER 3 PRINCIPAL COMPONENTS ANALYSIS
The goal of this chapter is not to provide a long discourse on PCA results, but to determine for each element the most significant variables in terms of their factor loadings, using the criteria set forth in Chapter 2. It is the role of the subsequent chapters to evaluate those variables for differentiating the two genera, along with any functional or geographical implications. Only the graphic plots for the combined sets are given in this chapter. All individual plots of Glossotherium and Paramylodon, along with the full array of measurements taken, including those not utilized in the PCA analyses, are available in Appendix B – Data. Given the amount of data this represents, they have been placed on a CD-ROM at the back of the dissertation. The measurement data are in the Microsoft Excel XP file “Measurement Data,” with each element stored on a separate tab sheet. The PCA data are divided into Excel files for each element and are titled “PCA [Element Name],” such that the file name for the radius is “PCA Radius.” Interpretation of the data was performed using the following guidelines. Analyses where the loadings of the first axis are all positive and similar in value are indicative of growth. In the subsequent axes, the largest variable, regardless of positive or negative value, provides the greatest loading for that axis. The largest variable with a
35 sign opposite of the overall highest is also indicated, as it represents a variation loading in the opposing direction, such that where one variable is positive and indicates an increase in that variable, the second negative variable indicates a decrease, i.e., the length of a bone increases while the width of a process decreases. In evaluating the differences between Glossotherium and Paramylodon, 95% confidence ellipses were graphed around each taxon as a means of estimating the total range of variance for that taxon. Distinct and non-overlapping confidence ellipses indicate a distinct separation of the taxa for the components plotted. Plots where the ellipses are overlapping imply a similarity in the morphometric ranges of those taxa. Ellipses also provide an assessment of direction for the axes and their subsequent loadings.
Cranium (Table 4)
The analysis shows a very strong separation for the crania, with almost no overlap in the ellipses (Figure 12). One specimen assigned to Glossotherium appears as an outlier, falling outside of the ellipsis (F:AM 11270) and is viewed as a juvenile specimen of that genus. Its placement outside the range of confidence for that taxon is unexpected as juvenile specimens of Paramylodon remained inside the ellipsis boundary for that genus. However, those Paramylodon species are closer to subadult status while that of F:AM 11270 represents is a very young juvenile based upon the small size and extreme open condition of the cranial sutures. Although graphically it appears as an outlier, it is retained in the analyses due to the polarizing effect it has on
36
Table 4: Crania PCA values and loadings. Combined
Paramylodon
Eigenvalue % variance
PC I 11.415 63.42
PC II 3.363 18.69
PC I 10.818 60.10
PC II 2.103 11.68
Loadings SKL Ant W Lac W Porb W Post W OCW Ant Ht Lac Ht Porb Ht Post Ht Lac-Sqm L Max-Pal L M1-M4 L M1 L M2 L
*0.287 0.187 0.118 0.088 0.270 0.263 0.263 0.236 0.230 0.210 0.275 0.250 0.278 0.228 0.264
-0.005 0.384 0.430 *0.505 0.117 0.056 0.121 0.222 -0.131 0.145 -0.130 -0.176 -0.135 -0.241 -0.146
0.271 0.270 0.226 0.268 0.266 0.264 *0.280 0.250 0.226 0.060 0.262 0.181 0.264 0.250 0.273
-0.205 -0.052 0.180 0.202 0.100 -0.010 0.203 ^0.299 0.268 -0.282 -0.278 *-0.495 -0.062 0.140 -0.135
(continued on following page)
Glossotherium
PC III 1.436 7.98
PC IV 1.119 6.22
0.018 -0.043 0.186 0.130 0.204 0.107 -0.016 0.031 0.077 -0.396 0.117 0.116 -0.246 0.132 -0.170
0.224 -0.181 -0.447 -0.282 -0.070 ^0.259 0.007 0.079 -0.033 *-0.661 -0.035 0.038 0.234 -0.004 0.098
PC I 14.650 81.39
*0.259 0.251 0.195 0.220 0.256 0.247 0.246 0.244 0.215 0.248 0.252 0.246 0.257 0.209 0.228
PC II 1.155 6.42
0.007 -0.025 *-0.515 -0.320 0.062 -0.022 0.051 0.145 -0.289 0.060 -0.130 0.185 0.107 0.240 ^0.290
37
Table 4 (continued) Combined Loadings M3 L M4 L Post-M4 L
Paramylodon
PC I
PC II
PC I
PC II
PC III
0.259 0.258 0.164
-0.157 0.048 ^-0.350
0.202 0.211 0.037
-0.178 0.041 -0.439
^-0.449 -0.252 *0.575
Glossotherium PC IV 0.035 0.231 -0.032
PC I 0.238 0.243 0.166
PC II 0.269 0.100 -0.480
(*) indicates variables with the highest absolute value contributing to the factor loading and (^) indicates variables with the second highest value of the opposite sign.
38
4 3 2
PC II
1 0 -1
F:AM 11270
-2 -3 -4 -5 -20
-16
-12
-8
-4 0 4 8 12 PC I Figure 12. Combined PCA for cranial data. □ = Glossotherium and + = Paramylodon.
the results that reflect the pattern for growth, with all variables on the first axis positively loaded and with nearly the same value. It also falls within the long axis orientation of PCI, as evidenced by the shape of the ellipsis. The pattern of growth disappears when the specimen is excluded, and the amount of variance explained by the axis dramatically decreases and does not contribute any meaningful information to the analyses. Overall, the separation of the taxa is a result of PC II, as both taxa occupy the same range for PC I, which is dominated by the total length of the skull. The component loadings suggest that the differences in PC II are attributed to differential patterns between the skull widths across the postorbital process and the lacrimals.
39 Paramylodon crania show a wide range of variance, as evidenced by the number of axes required for the individual analysis. There is no consistency among the highlighted variables of principal loading. As such, only the largest loadings with an absolute value greater than 0.500 will be further investigated, i.e., maxillary-palatine length, posterior M4 length, and the posterior height of the skull. The loadings for Glossotherium are more similar to the combined data, with the width across the lacrimals possessing a high loading.
Mandible (Table 5)
Although there is a significant amount of overlap in the elliptical ranges, there is a striking difference in their shape and orientation (Fig. 13). PC I exhibits the typical pattern for growth but is codominanted by variables for the total length of the toothrow and the length of the molariform toothrow. The latter should be a subcomponent of the former, but the distribution along the first axis suggests there may be generic differences between those variables as the plots for Glossotherium are widely distributed along the axis, whereas those of Paramylodon possess a much narrower range. This may be further evidenced as the molariform toothrow length factors in the individual analyses but the overall toothrow length only factors in Glossotherium. As PC I is often a reflection of size, there is the implication that Paramylodon has a longer mandible than Glossotherium. Conversely, Paramylodon appears to have a wider range along PC II than does Glossotherium. The loadings for PC II are influenced by the width of c1
40 Table 5: Mandible PCA values and loadings. Combined Eigenvalue % variance Loadings TrowL c1L c1W m1-m3 m1L m2L m3L m3W 2m3W m1D m3D
Paramylodon
Glossotherium
PC I 9.496 86.33
PC II 0.457 4.15
PC I 5.509 50.09
PC II 1.464 13.31
PC III 1.234 11.21
PC IV 0.89 8.09
PC I 10.220 92.91
*0.318 0.287 0.268 *0.318 0.300 0.313 0.314 0.307 0.281 0.298 0.308
0.030 -0.451 *-0.661 0.139 0.297 0.186 0.054 0.228 ^0.339 -0.211 -0.047
0.360 0.191 0.080 *0.393 0.256 0.369 0.366 0.281 0.150 0.336 0.350
0.287 ^0.422 0.337 0.074 -0.366 -0.152 0.170 -0.236 *-0.611 0.064 -0.044
0.075 -0.356 *-0.6864 0.155 -0.430 0.066 0.229 -0.187 -0.176 ^0.231 0.108
0.292 *0.484 -0.422 -0.087 -0.083 -0.111 0.227 0.224 0.286 -0.278 ^-0.464
*0.310 -0.135 0.300 0.088 0.292 0.414 *0.310 -0.209 0.299 ^-0.339 0.306 -0.082 0.307 -0.157 0.302 -0.103 0.278 *0.764 0.307 -0.021 0.305 -0.134
(*) indicates variables with the highest absolute value contributing to the factor loading and (^) indicates variables with the second highest value of the opposite sign.
PC II 0.309 2.81
41 and the width of the posterior lobe of m3, the latter of which is strongly loaded in the individual analyses. Overall, the orientation of the ellipsis for Glossotherium appears to reflect a trend for an increased width of c1 as the toothrow length grows longer, whereas Paramylodon shows no discernible trend.
2.4 1.6
PC II
0.8 0 -0.8 -1.6 -2.4 -16
-12
-8
-4 PC I
0
4
8
Figure 13. Combined PCA for mandible data. □ = Glossotherium and + = Paramylodon.
Humerus (Table 6)
There is a fair amount of overlap in the humerii data (Fig. 14), with Paramylodon showing a very wide distribution that creates an ellipsis that is more circular than ovoid. The loading for PC I suggests Paramylodon has a greater overall
42 Table 6: Humerus PCA values and loadings. Combined Eigenvalue % variance Loadings TotalL TbW HdML HdAP SftAP SftML EpcndW MxCndW DeltHt MdEpcndHt
Paramylodon
PC I 7.812 78.12
PC II 0.662 6.62
PC I 6.134 61.34
PC II 1.472 14.72
*0.340 0.337 0.317 0.332 0.266 0.322 0.319 0.301 0.292 0.328
0.014 0.200 0.229 -0.177 -0.191 0.319 -0.240 ^0.531 *-0.619 -0.124
0.358 *0.363 0.334 0.297 0.325 0.315 0.306 0.310 0.195 0.328
-0.023 0.187 0.156 -0.331 -0.182 0.331 -0.035 ^0.421 *-0.702 -0.129
Glossotherium PC III 0.844 8.44
0.299 0.273 -0.391 -0.341 0.425 0.012 -0.510 -0.145 -0.086 0.311
PC I 4.352 43.52
PC II 2.467 24.67
PC III 1.342 13.42
0.212 0.351 0.191 0.390 0.263 0.306 0.407 0.329 *0.454 0.015
*-0.512 -0.016 -0.393 -0.086 ^0.434 0.133 -0.241 0.353 0.122 -0.416
0.120 ^0.491 *-0.551 -0.255 0.164 -0.401 0.106 -0.004 0.085 0.412
(*) indicates variables with the highest absolute value contributing to the factor loading and (^) indicates variables with the second highest value of the opposite sign.
43
4 3 2
PC II
1 0 -1 -2 -3 -4 -5 -6.4 -4.8 -3.2 -1.6
0 1.6 3.2 4.8 6.4 8 PC I Figure 14. Combined PCA for humeri data. □ = Glossotherium and + = Paramylodon.
humeral length than does Glossotherium, and with a wider range of sizes. The single Paramylodon specimen falling within the Glossotherium range belongs to UF 208520, which is known to be small, following the pattern of specimens from Florida, and further implicates the first axis as being indicative of size. The second axis is dominated by the height of the deltoid tubercle and the maximum width of the epicondyles, with Paramylodon again having a wide range of values. The strong loading for the height of the deltoid tubercle in all the analyses suggests it to be a significant variable that warrants further investigation. Interestingly, there is a separation of Paramylodon specimens along PC II, such that if it were plotted alone the Glossotherium specimens would lie between the upper and lower groupings.
44 The lower valued Paramylodon specimens all belong to humeri from the La Brea tar pits locality in California; the upper grouping hails from localities in Idaho, Nebraska, and Florida. Further investigation is required to determine if there is a geographic difference within the deltoid tubercle heights and epicondylar widths of those sloths, and if there are any significant differences in the ratios between those two variables.
Radius (Table 7)
Analysis of the radii required investigation of three axes in order to meet the basic criteria outlined in Chapter 2. Examination of the loadings indicate the third axis may not have been necessary given it also strongly loads for the width of the medial articular surface, resulting in the very uninformative Fig. 15c where all the Glossotherium specimens fall inside the Paramylodon ellipsis range. The similar loadings could also account for the comparable clustering patterns seen in Fig. 15a and b, although there are differences within the Glossotherium groups as evidenced by the differential direction and shape of the confidence ellipses. Those differences are attributable to the secondary variable loadings for mediolateral width of the head and the distal end for axes two and three, respectively. In either case, neither genus exhibits much variation or spread in their distribution along PC II. The individual analysis of Glossotherium is unusual for PC I as not all of the variables are positively loaded. This result could be an artifact of the small sample size available (n = 4) or it could indicate that some of the specimens are incorrectly
45 Table 7: Radius PCA values and loadings. Combined Eigenvalue % variance Loadings TotalL HdAP HdML DisAP DistML ArtSfW MdArtSfW MdAP StyloidAP
Paramylodon
Glossotherium
PC I 6.042 67.14
PC II 0.977 10.86
PC III 0.827 9.19
PC I 4.489 49.88
PC II 1.232 13.70
PC III 1.023 11.37
PC IV 0.796 8.84
PC I 4.855 53.94
PC II 3.170 35.22
0.336 0.337 0.298 0.356 0.316 *0.381 0.220 0.373 0.353
0.192 -0.142 ^-0.539 -0.096 0.371 -0.026 *0.679 -0.009 -0.211
0.312 -0.405 -0.301 0.258 ^0.418 -0.070 *-0.512 -0.200 0.315
*0.780 0.370 0.034 0.204 0.255 0.181 0.235 0.174 0.172
^0.512 *-0.681 -0.166 -0.377 0.059 0.041 -0.049 -0.211 -0.227
-0.142 ^-0.476 0.006 0.307 0.385 0.102 -0.084 0.031 *0.702
-0.142 ^-0.476 0.006 0.307 0.385 0.102 -0.084 0.031 *0.702
0.223 0.266 0.371 0.370 -0.030 -0.221 -0.443 *-0.447 ^0.400
0.341 -0.326 -0.324 0.321 *0.556 ^-0.459 0.102 -0.095 -0.174
(*) indicates variables with the highest absolute value contributing to the factor loading and (^) indicates variables with the second highest value of the opposite sign.
46 16 12 8 PC II
4 0 -4 -8 -12 -16 -20 -12 -10
-8
-6
-4
a)
2
-2 0 PC I
6
4
15 10
PC III
5 0 -5 -10 -15 -20 -10
-7.5
-5
-2.5 0 PC I
b)
2.5
5
7.5
10
PC III
5 0 -5 -10 -15 -20 -20
-15
-10
-5
0 PC II
5
10
15
c) Figure 15. Combined PCA for radii data. □ = Glossotherium and + = Paramylodon. (a) PCI vs. PCII, (b) PCI vs. PCIII, and (c) PCII vs. PCIII.
47 assigned. The postcrania of Mylodon are poorly known and over the years many postcranial elements have been assigned to Glossotherium and Lestodon Gervais 1855. A larger sample size of known specimens is required to make a more precise determination.
Femur (Table 8)
Femoral data again show a distinct overlap in the ranges of the genera, and Glossotherium has one specimen plotting almost as an outlier from the main group but still within the overall range (Fig. 16). PC II corresponds to the anteroposterior surface of the femoral head and the width of the anterior articular surface, and the position of the outlier (MNHN PAM 127) can only be attributed to the width of the articular surface, as the surface area of the femoral head for that specimen was too damaged to measure. The value for the width of the anterior articular surface is exceptionally low among the glossotheres and could represent a measurement error during the collection phase. The first axis is loaded by the posterior width across the condyles and exhibits a pattern for growth. The group of three Paramylodon specimens plotting on the far left of PC I, deeply within the Glossotherium range, corresponds to specimens from Florida (UF 64361, 65859, 80776). Specimens hailing from the Florida localities have been consistently smaller in size than other Paramylodon specimens of similar age from more western localities. Their continued appearance within the Glossotherium range indicates that most divisions in PCA are reflective of size differences. It seems
48 Table 8: Femur PCA values and loadings Combined Eigenvalue % variance Loadings TL Hd AP Hd ML Hd Circ GT-LT W GT-Hd W LT W 3T W AntCnd W PostCnd W LtCnd W MdCnd W
Paramylodon
PC I 9.485 79.04
PC II 0.691 5.76
PC I 9.781 81.51
PC II 0.766 6.39
0.296 0.282 0.281 0.269 0.298 0.298 0.294 0.296 0.271 0.306 0.272 *0.300
-0.276 *0.497 0.386 0.432 0.115 -0.053 0.000 -0.042 ^-0.383 -0.207 -0.355 -0.087
0.295 0.298 0.297 *0.310 0.297 0.301 0.282 0.282 0.254 0.308 0.231 0.299
-0.022 0.166 0.097 -0.105 0.253 0.288 ^0.368 -0.052 -0.497 0.038 *-0.641 -0.090
Glossotherium PC I 6.848 57.07
PC II 2.115 17.62
PC III 1.439 12.00
0.242 0.244 0.227 0.218 0.326 0.269 0.327 0.322 0.318 0.279 *0.351 0.301
*0.478 -0.340 -0.123 ^-0.476 -0.057 0.159 0.162 -0.228 0.165 0.422 0.057 -0.316
0.148 -0.411 *-0.598 ^0.325 -0.035 -0.432 0.118 0.233 -0.006 0.115 0.102 0.259
(*) indicates variables with the highest absolute value contributing to the factor loading and (^) indicates variables with the second highest value of the opposite sign.
49
3 2
PC II
1 0 -1 -2 -3 -4 -5 -12
-9
-6
-3
0 PC I
3
6
9
Figure 16. Combined PCA data for femora data. □ = Glossotherium and + = Paramylodon.
premature to conclude that the association of those specimens with the glossotheres reflects a taxonomic relationship given the overlap of the confidence ellipses, although such a possibility cannot be ruled out either.
Tibia (Table 9)
PCA for the tibii exhibit a strong overlap (Fig. 17), with Glossotherium showing a more concise and narrow range compared to Paramylodon, whose ellipsis is nearly circular. The distribution along PC I corresponds to the maximum width across the proximal condyles, with Paramylodon being predominantly larger than Glossotherium.
50 Table 9: Tibia PCA values and loadings. Combined Eigenvalue % variance Loadings TotalL MdCndML MdCndAP LtCndML LtCndAP MxCndW DistML DistAP
Paramylodon
Glossotherium
PC I 6.503 81.28
PC II 0.433 5.41
PC I 6.435 80.43
PC II 0.428 5.35
PC I 4.415 55.18
PC II 1.543 19.29
PC III 0.863 10.79
0.365 0.367 0.353 0.350 0.343 *0.380 0.339 0.329
0.176 0.163 0.251 -0.099 -0.348 0.004 ^0.505 *-0.703
0.362 0.360 0.348 0.348 0.355 *0.379 0.343 0.332
0.043 0.211 ^0.566 0.159 -0.347 -0.088 0.116 *-0.683
0.351 0.429 0.409 0.364 0.218 *0.430 ^-0.159 0.368
0.074 -0.045 -0.124 0.209 0.630 0.048 *0.699 ^-0.215
*-0.647 -0.383 0.027 ^0.486 0.274 0.076 -0.296 0.174
(*) indicates variables with the highest absolute value contributing to the factor loading and (^) indicates variables with the second highest value of the opposite sign.
51
1.5 1 0.5 PC II
0 -0.5 -1 -1.5 -2 -2.5 -3 -10
-8
-6
-4
-2
0 2 4 6 8 PC I Figure 17. Combined PCA for tibia data. □ = Glossotherium and + = Paramylodon.
That variable also loads highly in the individual analyses. The solitary outlier of Paramylodon belongs to a Florida specimen (UF 4764), and there are four other specimens within the Glossotherium range that also hail from Florida. Both genera exhibit a wide dispersal along PC II, which is loaded by mediolateral and anteroposterior widths of the distal end. Those variables also load highly in the individual analyses.
52 Astragalus (Table 10)
Figure 18 exhibits overlap between the ellipses but most of specimens appear to plot on opposite sides of zero along the first axis, with Glossotherium on the left and Paramylodon on the right. Paramylodon has a large amount of variation in the astragalus, as evidenced by the wide distribution, and has at least three outliers. The two outliers on the left side of the plot belong to specimens from Florida, as do those Paramylodon specimens within the Glossotherium range along PC I. The distribution along PC I arises from the principal loading for the length of the tibial plateau and is seen in all analyses, while that of PC II is very strongly linked to the width of the tibial plateau. The dispersal along the second axis and very high loading value suggest that the width of the tibial plateau does not differ strongly between the genera and is subject to a high amount of variability.
Discussion
There are a number of plots where the smaller and older specimens of Paramylodon group near or within the same morphospace of Glossotherium. The implications are that a taxonomic connection exists between the two genera, with a greater retention of ancestral characters in the later forms. The differences seen in the Pleistocene specimens of Paramylodon may reflect further evolutionary changes in response to pressures in the North American environs, unlike those experienced by Glossotherium in the southern continent. Also within the plots for Paramylodon,
53
Table 10: Astragalus PCA values and loadings. Combined Eigenvalue % variance Loadings OdntHt OdntW FibArtHt FibArtW TibPltW TibPltL NavHt
Paramylodon
Glossotherium
PC I 5.091 72.74
PC II 0.872 12.45
PC I 4.644 66.34
PC II 1.049 14.98
2.982 42.60
1.821 26.02
1.229 17.55
0.367 0.391 0.406 0.411 0.206 *0.417 0.402
0.065 0.018 0.222 ^0.236 *-0.938 0.038 -0.100
0.384 0.413 0.409 0.417 0.168 *0.419 0.370
0.239 -0.105 ^0.280 0.237 *-0.847 -0.036 -0.282
^-0.165 0.448 0.222 0.463 -0.092 *0.503 0.498
*-0.652 -0.026 0.103 ^0.154 -0.641 -0.192 -0.305
0.174 ^0.456 *-0.765 -0.144 -0.384 -0.038 0.090
(*) indicates variables with the highest absolute value contributing to the factor loading and (^) indicates variables with the second highest value of the opposite sign.
54
3 2
PC II
1 0 -1 -2 -3 -4 -8
-6
-4
-2
0
2
4
PC I Figure 18. Combined PCA for astragali data. □ = Glossotherium and + = Paramylodon.
there are small groupings that could be interpreted as subclusters. The cause of such groupings could reflect a temporal or geographic distribution pattern, as has been seen for a number of specimens from Florida. Many of the analyses had measurement variables with unusually high factor loadings, greater than what would be expected for any one variable. Part of the high variance could be attributed to measurements that may not have been adequate at capturing the three-dimensional representation of the bone or from a difficulty in taking consistent measures. Such possibilities support the need for further analysis of all the skeletal elements studied so far, analyses going beyond the quantitative and incorporating those of a qualitative nature. It is the goal of the following chapters to
55 merge the two types of data into a new and revised assessment of the characters indicative of each genus.
CHAPTER 4 REASSESSMENT OF CRANIAL AND POSTCRANIAL CHARACTERS FOR GLOSSOTHERIUM AND PARAMYLODON
While some of the PCA results from Chapter 3 were mixed, they overall add further support to the separation of Glossotherium and Paramylodon. These results alone are not sufficient to explain the differences between the genera, but the combinations of the components and the variables which most influenced those factors, along with their loadings, provide directions for further investigations. The following discussion is a more detailed assessment of the cranial and postcranial characters unique to each genus, utilizing a combination of quantitative variables highlighted by the PCA and qualitative characters gleaned from the literature and individual observations. The assessments are separated into cranial and postcranial divisions. The characters within each division have differing functional implications for the animals with regard to locomotion, posture, food procurement and food processing. Historically, there has also been a collection bias favoring crania over postcrania. As each element had to be studied individually because of the very small sample sizes for associated elements, it seems fitting to continue the bias, at least in terms of describing characters. Accompanying the bias is the tendency for cranial characters to be better known and documented, while those of the postcrania are often undescribed. The PCA
57 results also revealed cases where the taxonomy of some postcranial elements was questionable because very few were associated with known cranial remains, making the assessment of generic characters more difficult. As expected for members of the Mylodontinae, Glossotherium and Paramylodon share a number of characters: long, rectangular skulls; ovoid caniniforms and simple molariforms; maxillary-palatine suture at M4; lacrimal canal positioned at M3; the last upper and lower molariforms bilobate in shape; diverging toothrows; temporal fossa positioned laterally on skull; basal tubercles enlarged and pneumatic; unguals clawed on digits I-III of manus and II and III of pes; third metatarsal with enlarged lateral process; and dermal ossicles present (Kraglievich, 1928; McDonald 1987, 1995; Bargo, 2001). Many of these are pertinent to the following character assessment, and many new characters have been added.
PCA Variables - Cranium and Mandible
There are significant shape differences evident from PCA, where assorted variables dominated the principal axes for the combined and individual sets. Factors with the greatest loading and influence were the overall skull length, skull widths (lacrimal and postorbital), maxillary-palate length, and the length of the palate posterior to M4. For the mandibles the significant factors were the length of the toothrow (c1m3), length of the molariform toothrow (m1-m3), width of c1, and width of the posterior lobe of m3.
58 Although the first axis showed a prime loading by the overall skull length, its success in separating the taxa was marginal. In Table 11 there is an apparent overlap in the ranges for skull length, where Paramylodon is the longer of the two. A caveat is that the Paramylodon skull lengths only represent the Pleistocene forms, as it was not possible to obtain usable skulls of older sloths from the Irvingtonian and Blancan, which are known to be smaller (McDonald, 1995). Thus, it is possible that a greater degree of overlap than that seen here could exist with the appropriate specimens. Even with a somewhat larger size, Paramylodon is still shorter in its overall skull length than are Mylodon and Lestodon. The various widths of the skull are informative characters for separating the genera, especially when viewed in relation to one another. The differential widths of the skull were originally noted by Stock (1914a), who wrote “The muzzle [of the North American forms] is somewhat inflated at the middle and narrows to a greater or less extent anteriorly. In [Glossotherium] the muzzle widens from the region anterior to the malars to the front end of the Skull” (p.322). Even Kraglievich (1928) cited this difference as part of the characters for his reassessment of the Mylodontinae sloths. Glossotherium has substantially larger values for anterior and postorbital widths, although the widths across the lacrimals and the posterior portion of the skull have a fair amount of overlap.
The more equivocal widths for the lacrimals and postorbitals
between the genera provide a good basis for ratios, which are even more informative than the raw data alone. Ratios of lacrimal to postorbital width illustrate the greater width in Glossotherium with values less than 1:1 (0.80-0.98). The ratios for Paramylodon are typically 1:1 in their average value, although their range overlaps
59
Table 11: Cranial dimensions and ratios for Glossotherium and Paramylodon. * are measurements highlighted by PCA, n is the number of specimens studied, Min is the minimum value, Max is the maximum value, X is the mean value, s is one standard deviation from the mean.
Dimensions Skull Length* Anterior Width Anterior Height Lacrimal Width* Lacrimal Height Postorbital Width* Post-orbital Height Posterior Width Posterior Height Foramen Magnum Width Occipital Condyle Width Lacrimal-Squamosal Length Maxillary-Palatine Length* C1-M4 Length M1-M4 Length C1 Length C1 Width (continued on following page)
Glossotherium
Paramylodon
n Min Max X s 13 396 468 437.5 ±20.2 13 144 177 160.4 ±10.3 13 108 134 122.4 ±8.3 12 122 149 135.3 ±9.1 13 121 151 138.8 ±9.7 12 141 186 153.7 ±12.4 13 120 155 137.8 ±10.7 13 164 195 182.5 ±8.8 12 116 136 122.4 ±5.7 13 53 63 59.0 ±0.8 13 127 149 136.8 ±1.9 13 114 144 130.6 ±8.1 12 164 207 172.3 ±10.9 28 116 163 142.6 ±11.1 28 92 126 110.5 ±8.8 28 21 32 24.1 ±3.0 28 17 26 21.0 ±2.3
n Min Max X s 14 429 498 460.6 ±18.6 14 120 166 139.8 ±13.1 12 119 159 129.6 ±10.5 14 100 147 121.1 ±13.6 14 128 174 143.4 ±13.4 14 103 154 122.3 ±13.3 14 118 159 137.1 ±11.2 14 170 216 187.8 ±14.1 13 126 160 138.4 ±9.5 12 43 65 53.3 ±1.7 14 124 142 131.8 ±1.3 13 131 171 146.4 ±10.3 14 200 231 215.2 ±8.5 30 118 172 142.8 ±15.0 32 107 142 124.7 ±10.5 19 12 28 19.0 ±4.3 18 10 24 17.3 ±4.3
60 Table 11 (continued) Glossotherium Dimensions M1 Length M1 Width M2 Length M2 Width M3 Length M3 Width M4 Length M4 Width Post-M4 Length* Ratios Ant/Lac Width Ant/Porb Width Ant/Post Width Lac/Porb Width Lac/Post Width Porb/Post Width Ant/Lac Height Ant/Porb Height Ant/Post Height Lac/Porb Height Lac/Post Height Porb/Post Height
Paramylodon
n Min Max X s 29 19 35 27.5 ±4.5 19 15 25 20.1 ±3.1 29 20 32 26.4 ±3.2 19 21 28 23.1 ±1.9 29 19 31 24.6 ±3.2 19 22 29 24.0 ±2.3 24 23 35 29.2 ±3.2 19 17 23 20.0 ±1.7 12 16 30 23.7 ±4.4
n Min Max X s 33 31 45 36.3 ±3.4 25 15 28 21.4 ±3.3 33 25 36 30.3 ±3.4 25 19 28 24.0 ±2.5 33 24 34 28.5 ±2.7 25 19 34 25.1 ±3.2 31 25 36 29.6 ±3.2 24 16 24 20.0 ±2.6 11 34 58 43.8 ±7.6
12 12 13 12 12 12 13 13 12 13 12 12
14 14 14 14 14 14 12 12 11 14 13 13
1.04 0.93 0.81 0.80 0.67 0.76 0.95 0.80 0.87 0.95 0.97 0.96
1.39 1.16 0.95 0.98 0.85 1.01 1.20 0.99 1.10 1.20 1.21 1.27
1.19 1.05 0.88 0.88 0.74 0.84 1.01 0.89 1.00 1.01 1.12 1.13
±0.10 ±0.06 ±0.04 ±0.05 ±0.06 ±0.07 ±0.06 ±0.05 ±0.08 ±0.06 ±0.08 ±0.09
1.04 1.04 0.69 0.89 0.57 0.58 0.88 0.89 0.83 0.98 0.91 0.89
1.36 1.35 0.85 1.05 0.72 0.72 0.94 1.09 0.99 1.16 1.14 1.08
1.16 1.15 0.75 0.99 0.64 0.65 0.91 0.95 0.93 1.04 1.03 0.99
±0.10 ±0.09 ±0.05 ±0.04 ±0.04 ±0.04 ±0.02 ±0.05 ±0.06 ±0.04 ±0.07 ±0.06
61 considerably with that of Glossotherium (Table 11). Additionally, the ratio of the anterior skull width to that of the postorbital width supports the raw data in that the two widths are relatively equal in Glossotherium, with the anterior being slightly larger. In Paramylodon, the anterior skull width tends to be much larger than that of the postorbital width. If the ratios are examined in order of occurrence, from anterior to posterior along the length of the skull, they produce a pattern that relates to the outline of the skull. Both taxa have an hourglass-like shape in dorsal view, but each genus differs in the point of constriction prior to the skull widening again (Fig. 19). This shape is well defined in Glossotherium where the constriction occurs more anteriorly at the lacrimals, compared to Paramylodon where the narrowest point is often the postorbital bar. Even the ratio between anterior and postorbital skull widths demonstrates that the initial values are more similar in Paramylodon. Paramylodon tends to decrease steadily in width toward the postorbitals, whereas Glossotherium narrows sharply toward the lacrimals and then re-inflates posteriorly to the postorbitals. It should be noted that while the widths across the postorbital processes are rather different between the genera, there is variation within each genus that reflects the age of the animal. The postorbital process is subjected to forces by the postorbital ligament, which forms the lateral wall of the orbit, and also by the most anterior fibers of m. temporalis (Naples 1982, 1985a, 1987, 1989). Examination of the tree sloths demonstrates an increasing length and robustness of the postorbital base as age progresses. Within Paramylodon, the range of values for the width across the postorbitals is skewed by LACM 1717-06 and IMNH 15273 with measures of
62
Figure 19. Skulls in dorsal view of Glossotherium (a) and Paramylodon (b). Scale bars equal 10cm.
63 146mm and 154mm, respectively. These values are larger than the average measurements obtained and indicate those animals lived to an old age. The factor of age upon the postorbital widths could also account for some of the wide ranges in those ratios using the width across the postorbital (Table 11). Maxillary-palatine length is longer in Paramylodon, with a very small overlap in the lower range with that of the upper range for Glossotherium. This variable is also influenced by the length of the palate posterior to M4, which has been an important character for differentiating other sloth taxa, and was used by Kraglievich (1922a, 1922b) to erect new South American taxa that have since been found to be synonymous with Glossotherium. Thus, it is not surprising that the measure exhibits a strong generic separation, with Paramylodon having the greater posterior extension. Unlike the maxillary-palatine length, there is no overlap in the ranges of the posterior palate, with a cutoff boundary between 30-34mm. Specimens less than 30mm belong to Glossotherium and those greater than 34mm can be attributed to Paramylodon. The lengths of the mandibular toothrows (c1-m3 and m1-m3) are greater overall in Paramylodon but are not significant when the overlap in the value ranges is considered (Table 12). Some of the overlap is attributable to an Irvingtonian specimen of Paramylodon (UF 83335) that sets much lower minimum values (120 and 95mm, respectively) and lowers the ranges. The minimum value for the overall toothrow length among the Rancholabrean forms is 131mm and 103mm for the molariform toothrow length, both of which overlap far less with the upper boundaries of the Glossotherium ranges.
64
Table 12: Mandible dimensions and ratios for Glossotherium and Paramylodon. * are measurements highlighted by PCA, n is the number of specimens studied, Min is the minimum value, Max is the maximum value, X is the mean value, s is one standard deviation from the mean. Glossotherium Dimensions c1-m3 Length* c1 Length c1 Width* m1-m3 Length* m1 Length m1 Width m2 Length m2 Width m3 Length m3 Width (ant) m3 Width (post)* m1 Depth m3 Depth Coronoid Height Condyle Height Total Length Ratios Condyle/Coronoid
n 13 13 13 13 13 6 13 6 13 13 13 13 13 4 4 7
Mn 115 22 18 86 20 18 23 20 42 20 16 65 75 148 113 301
Paramylodon
Mx 138 30 24 109 28 26 32 27 61 28 22 85 96 180 137 370
X 130.5 26.1 20.5 101.4 25.8 22.3 28.7 25.2 51.9 24.1 19.5 76.8 87.7 160.0 125.3 350.5
s ±6.1 ±2.5 ±2.2 ±6.2 ±2.5 ±2.7 ±2.3 ±2.6 ±5.0 ±2.5 ±1.7 ±6.9 ±7.5 ±15.0 ±13.0 ±22.8
n 26 26 25 26 26 21 26 22 25 26 24 25 26 12 18 20
Mn 120 20 16 95 27 22 26 24 45 23 18 59 67 162 93 315
Mx 162 33 25 129 37 28 36 33 65 32 28 90 103 187 159 436
X 142.8 25.7 18.8 114.1 30.5 24.6 31.2 27.5 55.2 26.8 22.8 75.0 91.5 175.0 138.3 405.4
s ±10.0 ±3.1 ±2.0 ±7.7 ±2.8 ±1.6 ±2.5 ±2.4 ±5.3 ±2.4 ±2.0 ±7.2 ±8.1 ±8.5 ±13.2 ±25.2
5 0.70 0.77
0.74
±0.03
12 0.76 0.87
0.80
±0.03
65 Given the differences in the dental series lengths, the individual parts of the mandible might also be expected to show differences contributing to the whole. A review of that variable data shows there is no difference in the length and width of c1 between the two taxa, although m1 is longer in Paramylodon. The lack of difference in c1 is unexpected given its prominence in the PCA factor loadings.
Additional Cranial Characters
Dental characters are common in establishing mammalian taxa given the complexity of the tooth types and cusp patterns. However, in xenarthrans the dentition is reduced and simple, yet it still possesses a number of informative characters. In terms of dental formula, Glossotherium and Paramylodon are distinguished from Mylodon by their nearly similar 5/4 dentition, although this pattern varies at times in the northern taxa as it relates to the upper caniniform (C1). The lack of C1 was used by Brown (1903) as the main character in his diagnosis of Paramylodon. It has since been noted by Stock (1914a, 1914b, 1917a, 1925) that C1 of Paramylodon tends to be lost randomly on the right or left side, or simultaneously from both sides, giving the taxon a variable dental formula of 4-5/4-4, 4-4/4-4, or 5-4/4-4. No cranium belonging to Glossotherium has been encountered to date where the upper caniniform has been lost and the alveolus reabsorbed. However, such an occurrence has been noted in Mylodon (Burmeister, 1886; Reinhardt, 1879; Stock, 1917a, 1925), which has furthered the proposal that Mylodon represents a sister genus of Paramylodon.
66 In individuals where C1 is absent, the faint outline of the alveolus remains but the actual space of the alveolus has been filled by bone, indicating the animal survived quite readily (Stock, 1914, 1917, 1925). This occurrence is evident in the specimens from the large La Brea sample and also in individuals from other North American localities (Fig. 20) but is restricted to specimens that are Rancholabrean in age (McDonald, 1995, 2006; McDonald et al., 2004). It appears that the when tooth alveolus is reabsorbed and closed by bone, the same goes for the root located deep in the maxilla. In some cases, this results in the snout being somewhat decreased in width. Despite the variable formula, those specimens retaining both upper caniniforms have anterior widths markedly narrower than do those of the typical Glossotherium skull. The widths of Paramylodon are also similar in proportion to those of Mylodon, which loses C1 at a very early age. The overall anterior width of the skull can be attributed to the degree of divergence in the anterior portion of the toothrows, which was noted by Stock (1925). Glossotherium has the greater width because the molariform rows have a greater amount of divergence. The length of M1 is noticeably greater in Paramylodon when compared to Glossotherium (Table 11). Including the relatively equal width of that molariform in both taxa, Paramylodon has a more blade-like appearance, with the long dimension being anteroposterior, whereas Glossotherium is slightly squarer in shape. M3 also shows greater values in the northern taxa, although not to the same degree as does M1, with there being greater overlap in the ranges. As such, the length of M3 does not stand out as a strong character for separating the genera.
67
Figure 20. Ventral views of Paramylodon skull dentition. Arrows indicate where C1 alveoli have closed and the bone has been reabsorbed. Different localities are represented with (a-b) from California, (c) from Nebraska, and (d) from Idaho. Scale bars equal 2cm.
68 The anterior width also affects another character of the anterior portion of the skull when combined with the anterior height. Paramylodon has larger values for the anterior height but the means are relatively equal between the genera. As such, anterior height alone is not a useful character, but when compared with the values for width it assists in describing the opening for the nasal passages. With the greater anterior width, Glossotherium has a larger nasal cavity opening than does Paramylodon, with a ratio less than 1:1 for height versus width (Table 13). This ratio is much closer to 1:1 in Paramylodon. Although the anterior widths are equivalent for Paramylodon and Mylodon, the greater height of the skull gives the latter taxon a greater ratio of about 1.4:1. The height to width ratio for the postorbital measures in Table 11 also exhibits a difference between the genera which is attributed to the greater postorbital width values for Glossotherium. Similarly, the greater height of the posterior skull in Paramylodon creates the taxonomic differences for that measurement versus the width. The lengths for the upper toothrows are relatively equal, but there is a difference in the lengths of the molariform rows (M1-M4), where Paramylodon again has the greater values (Table 11). Quantitatively, the length of M1-M4 accounts for less than 80% of the overall dentition length (C1-M4) in Glossotherium, and the amount is greater than that for Paramylodon. Part of the difference can be attributed to the length of M1, which was discussed earlier as being greater for Paramylodon. As for the overall length of the dentition, it is equalized by the differences in the length of C1, where Glossotherium has a longer value (< 20mm) compared to a shorter measure in Paramylodon (> 20mm).
69
Table 13: Additional cranial ratios for Glossotherium and Paramylodon. n is the number of specimens studied, Min is the minimum value, Max is the maximum value, X is the mean value, s is one standard deviation from the mean. Glossotherium Cranial Ratios Anterior Height/Width Lacrimal Height/Width Postorbital Height/Width Posterior Height/Width M1 Length/Toothrow Length PostM4/Max-Palate Length
n 13 12 12 12 30 13
Min 0.70 0.89 0.75 0.61 0.15 0.08
Max 0.80 1.18 1.01 0.76 0.23 0.15
X 0.76 1.02 0.90 0.67 0.19 0.13
Paramylodon s ±0.03 ±0.09 ±0.08 ±0.05 ±0.02 ±0.02
n 12 14 14 13 29 13
Min 0.85 1.07 1.03 0.66 0.22 0.15
Max 1.03 1.50 1.27 0.86 0.31 0.27
X 0.95 1.19 1.23 0.73 0.25 0.20
s ±0.05 ±0.11 ±0.07 ±0.05 ±0.02 ±0.03
70 The nearly equivalent length for the toothrow can be used as a basis for further exploring the differences in the length of M1. Table 13 lists the ratio of these two measures, with Paramylodon having greater values than Glossotherium and each genus falling into distinct ranges with only a small overlap between the upper and lower boundaries. The length of the maxillary-palatine also reflects the toothrow dimensions, given they occupy the same anteroposterior plane, with very little overlap occurring between the lower and upper ranges for Paramylodon and Glossotherium, respectively. The length of the palatines posterior to M4 have been noted as being greater in Paramylodon, and it likely contributes to the generic difference in the maxillarypalatine length, as it is a subset of that whole. The ratio for the palate posterior to M4 versus the total maxillary-palatine length also separates the genera, where Glossotherium has much smaller values than does Paramylodon (Table 13). Remaining differences in the dentition are related to the shape of the teeth and are subtle in nature. As such, the differences presented here represent consistent patterns that are more likely to reflect taxonomic differences and that are not a product of ontogeny or other growth factors. Compared to Mylodon, the teeth shapes of both taxa are relatively more complex, having less rounded features, and appear more akin to one another than they do to those of Mylodon (Fig. 21). It is the overall similarity in the shapes of the dentition that further support the case of a more recent shared, common ancestor between Paramylodon and Glossotherium.
71
Figure 21. Right maxillary dentition outlines of Glossotherium (a), Paramylodon (b), and Mylodon (c).
M2 is triangular in section but more rounded in Glossotherium, while Paramylodon has a more well-defined posteriorly projecting lobe. M4 has the bilobate shape indicative of all mylodontids. When the two lobes are compared, the anterior is more anteroposteriorly compressed in Glossotherium than in Paramylodon, while the posterior in Paramylodon is more equal to the width of the anterior lobe than in the narrower one of Glossotherium. When the crania are viewed in lateral profile (Fig. 22), there is a noticeable difference in the shape of the dorsal surface. Similar to that seen in Mylodon, the skull roof of Paramylodon is rather straight, with a slight angling from the nasals posteriorly to the frontals at the level of the postorbital bars. In Glossotherium, though, the skull
72
Figure 22. Skulls in lateral view. Paramylodon (b) exhibits a more flattened dorsal surface, whereas Glossotherium (a) has a more domed appearance and smaller temporal fossa.
73 has a more dome-like appearance. The anterior incline is akin to that of Paramylodon but there is a more posterior decline from the parietals towards the occipitals. This shape can also be quantified by examining the posterior skull heights, where the comparatively shorter heights give rise to the domed shape. The posterior height is greater in Paramylodon and also more equal to the height at the postorbitals, producing the relatively straight and flat profile for the posterior half of the skull. Neither taxon has an anterior skull height taller than that of the lacrimal or postorbital as seen in Mylodon and Oreomylodon Hoffstetter 1949. The differences in the posterior skull height also affect the temporal fossa, which is the site of origin for the masticatory muscle m. temporalis (Turnbull, 1970; Naples, 1989). With the taller height, Paramylodon has an increased surface area to accommodate a larger temporalis muscle. Additionally, the area of muscle origin is more prominent and rugose in Paramylodon, which is likely attributable to the greater size of the muscle occupying the fossa. This character is also influenced by the width of the parasagittal crest, visible in dorsal view (Fig. 19). The width of the crest is greater in Glossotherium, further decreasing the size of the origin for m. temporalis. Both taxa have a relatively similar appearance for the jugals, in having three malar flanges, but with a few slight differences. Glossotherium has a small knob projecting just above the main portion of the inferior malar, which is not seen in Paramylodon. The superior malar tends to project more posteriorly in Paramylodon, whereas this same flange in Glossotherium extends back just slightly farther than the middle malar flange. It should be noted that the malar processes also play a role in the origin of the masticulatory muscles for the five divisions of m. masseter superficialis,
74 which insert on the mandible. The differences in shape reflect a differential origin pattern for the muscles, which could also have an effect on the direction and force of mastication. Ventrally, the posterior half of the skull exhibits a character related to the pterygoids. Both taxa have the descending pterygoid flanges that serve as the origins for m. pterygoideus lateralis and m. pterygoideus. medius (Turnbull, 1970; Naples, 1989). The difference lies in the amount of inflation in the pterygoid sinuses (Fig. 23). Paramylodon shows very little inflation, and the spacing between the pterygoids is rather wide, whereas in Glossotherium they are strongly inflated and narrowly spaced.
Additional Mandibular Characters
Considering the number of differences encountered in the crania, the lower jaw shows a surprising amount of similarity, As such, fewer measures and characters are available to distinguish the two genera further, despite the high degree of preservation exhibited by this element and the amount of systematic information reportedly available (Perea, 1992). Measurements for the mandibular dentitions were readily available but those in the posterior regions of the bone were typically of poor preservational quality or missing altogether. This is unfortunate as the posterior regions of the mandible seem to contain promising diagnostic characters.
75
Figure 23. Skulls in ventral view for Glossotherium (a) and Paramylodon (b). Arrows indicate the amount of pterygoid inflation and the space between them. Scale bars equal to 5cm.
76 The specimens complete enough to measure for total length provided data showing the lower jaw of Paramylodon to be longer than that of Glossotherium (Table 12). This is not surprising, as the crania and the associated anteroposterior dental values were also shorter in the southern taxon. Associated with the greater overall length are the previously discussed greater values for the toothrows, which are a subset within the total length. This pattern mirrors the one noted previously in the lengths of the occluding maxillary toothrows. Another character with only a few data points available correlates to the height of the mandibular condyle in relation to that of the coronoid process. In both taxa, the condyle projects posteriorly with regard to the coronoid process, and it is elevated just above the toothrow. The ratio between these two variables shows a trend for the condyle to be placed higher toward the top of the coronoid process in Paramylodon, whereas it is lower in Glossotherium. However, there is a degree of uncertainty in this ratio as Table 12 shows the heights of the coronoid process to be nearly equal between the two taxa, but those individuals with the larger measures did not always have a corresponding measure for the height of the condyle to compare against. To get more than two individual ratios for the Glossotherium data set, juvenile specimens were included but were not used in the raw data for the individual measures. While the ratio may show a taxonomic difference, it should be viewed with a bit of caution until more consistent specimens can be studied. Although it was not always possible to measure the coronoid process at its peak, it was generally present and provides a shape character in lateral view. The ascending slope differs between the genera, rising more vertically in Paramylodon while the same
77 feature becomes more rounded as it nears the apex of the process in Glossotherium (Fig. 24). This feature is also visible in juvenile mandibles and cannot be attributed to ontogenetic differences. Another shape difference occurs in the anterior region of the mandible for the predental spout. The spout in Glossotherium is wider at its most anterior point, which is likely due to the greater width for the upper dentition. This widening at the anterior margin creates a pronounced concavity along the incisive margins (Fig. 25a). Because the upper dentition in Paramylodon is narrower, the corresponding spout does not possess the lateral flare at the anterior point or the concavity that would accompany it. With the increased spout width, there is an increase in the divergence of the anterior teeth similar to those of the upper dentition. Corresponding to this is a more prominent lateral bulge from the incisive margin in Glossotherium. The shapes of the teeth were similar between the genera but are more distinct than those of Mylodon. The only notable difference between the taxa is in m1, which had a more prominent posteriorly projecting lobe in Paramylodon specimens (Fig. 25b).
Glossotherium chapadmalense
The cranial characters outlined are for Pleistocene aged specimens and do not reflect characters related to that of Glossotherium chapadmalense from the midPliocene. The specimens related to this species were overlooked during data collection, possibly because the cranium was somewhat incomplete. However, descriptions by Kraglievich (1925) and measurements obtained from other sources (McDonald, pers.
78
Figure 24. Lateral mandible views of Glossotherium (a) and Paramylodon (b). Scale bars equal 10cm.
79
Figure 25. – Mandible outlines for Glossotherium (a), Paramylodon (b), and Mylodon (c). Scale bars equal 10cm.
80 comm.) have made it possible to address the specimens and their relationship to Glossotherium and the characters outlined above. G. chapadmalense is small in size (Table 14), a trend common among older mylodontids (Kraglievich, 1925; Hirschfeld, 1985; McDonald 1995, 1997). The values of the skull widths are closer to the ranges for Paramylodon, but this also could be a product of the overall smaller size of the individual, although the ratios for skull width paint a mixed picture with a general affinity to the ranges established as indicative of Paramylodon. However, those ratios that reflect the length dimensions for the skull, M1/Toothrow Length and Post-M4/Max-Palate Length, have G. chapadmalense falling fully within the Glossotherium ranges, although the first is very near to the lower boundaries of Paramylodon. Despite the mixed ratios and affinities to Paramylodon, a number of characters support the assignment of chapadmalense as an early species of Glossotherium. The length of the palate posterior to M4 is very short, and despite the relatively short anterior skull width, the toothrows are divergent as in Glossotherium (Fig. 26b). The greater degree of divergence in the toothrows is also evidenced by the predental spout, which demonstrates the lateral flare of the anterior end and the associated concave lateral border that are indicative of Glossotherium (Fig. 27). Laterally, the skull from Kraglievich (1925) shows a dome-shaped roof; however, this feature is not as evident in Figure 26a, as the posterior regions of the skull appear to have suffered damage at a later date. As such, it has become impossible to determine whether there was any inflation present in the pterygoid/basilar tubercle region. The damage also prevents further determination of the dome-shape, if there was any expansion of a
81 Table 14: Comparative cranial dimensions and ratios for Glossotherium chapadmalense against those of Glossotherium and Paramylodon. Min is the minimum value, Max is the maximum value, X is the mean value, s is one standard deviation from the mean.
Total length Anterior Width Lacrimal Width Postorbital Width Posterior Width Maxillary-Palatine Length Post-M4 Length Ratios Anterior/Postorbital Width Anterior/Posterior Width Lacrimal/Postorbital Width Postorbital/Posterior Width M1 Length/Toothrow Length PostM4/Max-Palate Length Max-Palate Length/Anterior Width
G. chapadmalense
Glossotherium
Paramylodon
MACN 8065 390 121 107 107 158 140 12
Min Max X s 396 468 437.5 ±20.2 144 177 160.4 ±10.3 122 149 135.3 ±9.1 141 186 153.7 ±12.4 164 195 182.5 ±8.8 164 207 172.3 ±10.9 16 30 23.7 ±4.4
Min Max X s 429 498 460.6 ±18.6 120 166 139.8 ±13.1 100 147 121.1 ±13.6 103 154 122.3 ±13.3 170 216 187.8 ±14.1 200 231 215.2 ±8.5 34 58 43.8 ±7.6
0.93 0.81 0.80 0.76 0.15 0.08 1.05
1.04 0.69 0.89 0.58 0.22 0.15 1.37
1.13 0.77 1.00 0.68 0.21 0.09 1.16
1.16 0.95 0.98 1.01 0.23 0.15 1.31
1.05 0.88 0.88 0.84 0.19 0.13 1.16
±0.06 ±0.04 ±0.05 ±0.07 ±0.02 ±0.02 ±0.07
1.35 0.85 1.05 0.72 0.31 0.27 1.70
1.15 0.75 0.99 0.65 0.25 0.20 1.55
±0.09 ±0.05 ±0.04 ±0.04 ±0.02 ±0.03 ±0.10
82
Figure 26. – Lateral (a) and ventral (b) views for the type specimen of Glossotherium chapadmalense, MACN 8675.
83
Figure 27. – Dorsal view of the mandible for Glossotherium chapadmalense, MACN 8675.
84 parasagittal crest, and has made assessment of the dentition shapes difficult. Of the visible and discernable dentitions, M4 and m1 exhibit generic characters, but not all for Glossotherium; m1 has a rather well-developed lobe that projects posteriorly in a manner similar to that seen in Paramylodon. However, m2 is more reminiscent of that of Glossotherium in that the rectangular shape lacks lobation at the corners. M4 resembles that of Glossotherium in the posterior lobe being quite narrow and not close to being equal in width to that of the anterior lobe. Despite having characters with a shared affinity for those of the North American genus, the Chapadmalan individual has more similarities with Glossotherium, and the species designation of G. chapadmalense is well founded. The shared affinities to Paramylodon, along with a few character ratios that were close to the range for the northern genus, lend further credence to the idea that G. chapadmalense is the most recent common ancestor to the North American forms. That G. chapadmalense could be the ancestor to Paramylodon further befuddles the issue surrounding the late Blancan specimens assigned to “Glossotherium” chapadmalense and increases the need for good specimens to address when the generic separation took place. Assessments for those North American specimens will require a different combination of characters, as those originally outlined pertain to the later Pleistocene forms that have undergone evolutionary changes, especially in those characters related to the skull width ratios. An examination of the relationship between the lengths of the skull versus the width (Max-Palate Length vs. Anterior Width) exhibits a distinct generic separation and places G. chapadmalense within the Glossotherium range (Table 14). The ratio suggests that an increase in widths of the skull became important
85 characters for the Pleistocene forms of Glossotherium, while increases in the palate length were more important for Paramylodon. In Paramylodon, that supposition is further supported by the greater length of the palate posterior to M4.
Postcrania
For the postcranial analyses, PCA was relatively unsuccessful in providing measurements with a taxonomic significance, based on the observed patterns of the confidence ellipses. Most of the differences between the taxa are predominantly related to size, with Paramylodon having the larger elements compared to Glossotherium. Some of the differences are attributable to the need for an equivalent size in the surfaces of the articulating elements, such as where the humerus meets the radius and the femur meets the tibia. There are still characters available for separating the taxa beyond the criteria of size disparity and geographical division, although the posterior limb appears to be more phylogenetically and functionally constrained with few characters unrelated to size. Thus, the lack of more distinct ellipses was a result of both taxa being rather similar due to a recent shared ancestor and lacking functional pressures upon the postcrania that would have caused greater separation.
Humerus
Table 15 illustrates the size differential previously mentioned, with most of the maximum values for Glossotherium being equal to or slightly greater than the minimum
86
Table 15: Humerus dimensions and ratios for Glossotherium and Paramylodon. * are measurements highlighted by PCA, n is the number of specimens studied, Min is the minimum value, Max is the maximum value, X is the mean value, s is one standard deviation from the mean. Glossotherium Dimensions Total Length* Tubercles Width* Head (ant-post) Head (med-lat)* Shaft (ant-post)* Shaft (med-lat) Epicondyle Width Max Condyle Width* Deltoid crest Height* Medial Epicondyle Height Shaft Angle Ratios Deltoid Height/Total Length Max Condyle Width/Deltoid Ht Med. Epicondyle Ht/Total Length
n Min Max 9 9 9 9 9 9 8 9 9 8 7
Paramylodon
X
s
n
Min Max
X
s
359 136 113 95 64 85 189 109 139 113 65°
411 152 160 110 75 106 219 124 154 128 78°
385.4 142.3 140.9 100.6 69.9 96.9 204 117.9 145.7 120.3 69°
±14.7 ±5.8 ±12.5 ±5.0 ±3.9 ±6.1 ±8.6 ±5.2 ±5.9 ±5.8 ±4.8
14 14 14 14 14 14 14 14 14 12 7
417 154 157 99 67 103 196 115 147 141 78°
483 189 202 125 88 137 264 157 223 181 84°
446.6 167.8 179.6 112.4 77.7 120.4 244.3 133.2 188.4 161.0 81.1°
±23.8 ±11.5 ±14.2 ±6.5 ±7.0 ±11.0 ±18.1 ±10.5 ±24.7 ±10.1 ±5.5
9 0.35 9 0.76 8 0.29
0.40 0.85 0.33
0.38 0.81 0.31
±0.02 ±0.03 ±0.01
14 0.34 14 0.56 13 0.32
0.48 0.95 0.38
0.42 0.72 0.36
±0.05 ±0.12 ±0.02
87 values of Paramylodon. Only the anteroposterior width of the humeral shaft appears to be equivalent between both genera, which is unusual given it was a significantly loaded variable in the PCA. However, the mediolateral dimension of the shaft does differ between the genera, with Paramylodon having greater values than Glossotherium. This difference corresponds to a greater lateral flare of the deltoid crest (Fig. 28b), and this lateral process marks the insertion point of m. deltoideus. The height of the deltoid tubercle was also indicated as significant by the PCA and shows greater values for Paramylodon. Many of the large ground sloths possess a prominent ridge along the anterior humeral surface that joins with the deltoid crest and serves as the attachment site of the m. pectoralis. The pectoral ridge is oriented along the long axis of the bone and between the mylodontids provides a landmark for determining the angle of the humeral shaft in relation to the distal condyles when they are aligned perpendicular to a flat surface. Measurements of this angle show that the humerus of Glossotherium is more angled than that of Paramylodon (Table 15). Neither taxon possesses the entepicondylar foramen that is often found in scelidotheres and megalonychids, although there is a significant difference in the medial epicondyle region. The measurement for the height of this feature is greater in Paramylodon as would be expected based on the overall size differences, but a marked difference is present when regressed against the total humeral length (Table 15). This region also differs in the greater development of the process in Paramylodon, which serves as the origin of the flexor muscles, and in its more proximal position in relation to the main portion of the shaft (Fig. 28b). In the North American sloths, its highest
88
Figure 28. – Anterior humeri views of Glossotherium (a) and Paramylodon (b). Scale bars equal to 10cm.
point is above the distal end of the deltoid tubercle in contrast to its position below that landmark in Glossotherium. The Paramylodon pattern is similar to that seen in Scelidotherium Owen 1840, which is a smaller animal than Glossotherium, and indicates that size is not the dominant factor in the orientation of the medial epicondyle in relation to the deltoid tubercle. The lateral epicondyle region, which is the origin site for the extensor muscles, also exhibits generic differences. In Glossotherium, the lateral epicondyle forms more
89 of a distinct crest that runs in a more parallel direction with the main humeral shaft and is equal or just above the level of the medial epicondyle. The lateral epicondyle is less developed in Paramylodon, with a short portion running parallel before curving towards the shaft. The parallel portion of the lateral epicondyle in Paramylodon occurs below the most proximal point of the medial epicondyle. Quantifiable information regarding the lateral epicondyle was not initially collected and additional study with those measures could shed further light on the taxonomic and functional differences. In the PCA interpretation, the relationship between the height of the deltoid tubercle and the maximum width of the condyles was questioned as both were indicated as being prime loaders. The ratio between these variables does not show a marked difference between the genera and with Paramylodon having a very wide range that Glossotherium falls inside of (Table 15). An examination of the individual ratios shows a noticeable difference among the specimens of Paramylodon from different localities. Those specimens from California range from 0.56-0.72 with a mean of 0.64 that falls below the range of Glossotherium. Conversely, those from Florida, Idaho and Nebraska range from 0.78-0.95 with a mean of 0.85 that is closer to that of Glossotherium but also a bit greater. A similar pattern appears in the individual ratio values for the height of the deltoid tubercle versus the total humeral length.
Radius
The difference in overall size is apparent in all the measurements except for mediolateral widths of the distal end and the articular surface, which are strongly
90 correlated (Table 16). Neither taxon possesses a pronounced lateral flange for the pronator muscles of the forearm that are found in scelidotheres and lestodontines (Fig. 29c). The lack of such a flange along with a relatively straight shaft where the distal end is not displaced from the proximal end is indicative of the Mylodontinae. Given the relation of overall length to the distal width, Glossotherium appears to have slightly more of a pronator flange than Paramylodon (Fig. 29). This again serves as more of a subfamily character as the difference is barely perceptible and is not supported by the ratio of the distal width to the total. Such a feature would be thought of as being quantifiable, but the ratio in Table 16 shows it is not upheld, as both taxa have equivalent ranges and means. However, when the ratio is compared to that of sloths with a more pronounced pronator flange, it also turns out to be equivocal, indicating the feature is not quantifiable at the mediolateral point measured. The distal width marks the point where the articulation for the carpus begins, suggesting that the relationship of the width to total length is isometric across all mylodontid taxa. A mediolateral width taken more proximally along the shaft would have a better chance at quantifying the difference but finding a homologous reference point is difficult. What is visually obvious is the difference in muscle scarring along the lateral side of the shaft in posterior view (Fig 29). In Glossotherium, these scars extend distally onto the projection of the styloid process. The extension is shorter in Paramylodon and does not go past the distal articular surface for the ulna, found on the medial side of the bone. These scars correspond to muscles acting on digit I: m. abductor policis longus and m. extensor policis brevis. A similar pattern has been noted
91
Table 16: Radius dimensions and ratios for Glossotherium and Paramylodon. * are measurements highlighted by PCA, n is the number of specimens studied, Min is the minimum value, Max is the maximum value, X is the mean value, s is one standard deviation from the mean. Glossotherium Dimensions Total Length* Head Width (ant-post) Head Width (med-lat)* Distal Width (ant-post) Distal Width (med-lat)* Articular Surface Width* Medial Articular (med-lat)* Medial Articular (ant-post)* Styloid (ant-post) Ratios Distal Width (med-lat)/Total Length
N Min Max
Paramylodon
X
s
n
Min Max
X
s
5 3 4 4 5 5 5 4
256 50 61 62 97 82 43 55
284 51 68 68 103 90 54 60
270.8 50.3 65.0 65.5 100.0 86.6 48.2 57.5
±12.0 ±0.6 ±2.9 ±2.6 ±2.2 ±3.2 ±4.4 ±2.4
19 18 19 19 19 19 19 19
283 54 71 68 97 95 45 62
321 65 81 85 118 110 57 74
300.8 60.1 76.2 76.4 108.1 102.5 50.8 67.2
±10.5 ±3.8 ±2.6 ±4.3 ±6.2 ±4.3 ±3.5 ±3.3
4
25
27
26.3
±1.0
19
29
37
32.8
±2.4
4
0.35
0.39
0.37
±0.02
19 0.34
0.39
0.36
±0.01
92
Figure 29. – Posterior radii views of Glossotherium (a), Paramylodon (b), and Lestodon (c). Striped pattern indicates region of muscle scarring. Scale bars equal 10cm.
in the extant anteater, Tamandua, and also in the scelidotheres (Taylor, 1978; McDonald, 1987).
Femur
The femora of both animals are similar in their proportions, with the southern taxon generally the smaller of the two (Table 17). In terms of appearance, Figure 30 displays some marked differences when the bones are oriented with the distal articulations perpendicular to a surface. The orientation of the greater trochanter is more of a flat shelf with the neck that leads to the femoral head in Glossotherium,
93
Table 17: Femur dimensions and ratios for Glossotherium and Paramylodon. * are measurements highlighted by PCA, n is the number of specimens studied, Min is the minimum value, Max is the maximum value, X is the mean value, s is one standard deviation from the mean. Glossotherium Dimensions Total Length* Head (ant-post)* Head (med-lat)* Head (circum)* Greater Trochanter-Lesser Trochanter Width Great Troch-Head Width Lesser Trochanter Width Third Trochanter Width Ant Condyle Width Post Condyle Width* Lat Condyle Width* Med Condyle Width Ratios Great Troch-Head W/Total L Lesser Troch W/Total L Third Troch W/Total L
n
Min Max
10 8 8 8 10 10 10 10 10 12 12 12
438 175 148 326 194 192
Paramylodon
X
s
n
Min Max
X
s
506 205 196 390 250 251
484.3 187.1 169.6 352.6 219.9 226.0
±20.4 ±11.0 ±15.7 ±20.8 ±15.2 ±19.5
15 15 15 15 15 15
472 160 160 307 204 223
577 232 220 430 264 307
529.6 206.5 192.1 371.3 238.7 271.4
±32.2 ±22.9 ±19.0 ±36.7 ±19.3 ±23.6
188 139 77 141 46 62
241 165 106 167 64 73
207.0 149.0 94.4 156.8 54.4 66.1
±14.2 ±8.1 ±8.1 ±6.7 ±5.6 ±4.0
15 15 15 15 15 15
192 135 89 155 51 67
266 194 126 203 86 98
231.3 165.5 104.8 184.5 63.8 86.1
±23.6 ±15.9 ±9.5 ±14.8 ±8.8 ±9.3
10 0.40 10 0.41 10 0.29
0.51 0.48 0.35
0.47 0.43 0.31
±0.03 ±0.02 ±0.02
15 0.46 15 0.39 15 0.29
0.57 0.49 0.36
0.51 0.44 0.31
±0.02 ±0.03 ±0.02
94
Figure 30. – Anterior femora views of Glossotherium (a) and Paramylodon (b). Striped areas indicate regions of scarring for the three vastus muscles: m. vastus medialis, m. vastus indermedius and m. vastus lateralis, from left to right. Scale bars equal 10cm.
whereas in Paramylodon the greater trochanter rises on an angle (Fig. 30). As in many large xenarthrans, the bone is transversely expanded, with the lateral border nearly straight and the third trochanter barely visible. A few specimens exhibited a lateral shaft where the third trochanter was more visible, giving the femur a somewhat humpshaped appearance along that edge. Distally, Glossotherium shows a greater expansion of the medial condyle, which seems to also correspond with a more visible, distal boundary on the anteromedial surface for m. vastus medialis. The boundary for this muscle in Paramylodon runs
95 more along the medial border of the shaft, as evidenced by the shading in Figure 30. The space medial to the vastus muscle and the expansion of the medial condyle in Glossotherium likely is for the adductor musculature, with a potential greater emphasis on the hamstring portion of m. adductor magnus. This muscle inserts on the medial supracondylar ridge of the femur and appears to have a more anterior placement as opposed to that seen in Paramylodon, where there is little to no space available on the anterior surface. Additional studies are needed to evaluate further the apparent differences noted for the quadriceps and adductor muscles. Torsion of the femoral shaft is a common occurrence among ground sloths, and as such there are means for gauging the degree of distal displacement from that of the proximal end (De Iuliis, 1996). A difference in the degree of distal torsion could account for the more visible adductor shelf seen in Glossotherium. Measurement of the medial condylar expansion would also help to further quantify the differences noted. As noted, the muscle scars for the quadriceps group differ in their placement along the anterior portions of the proximal one-third of the shaft and also in their relative sizes. With the transverse expansion of the shaft, m. vastus lateralis and m. vastus medialis join m. vastus intermedius on the anterior surface instead of wrapping around on the posterior surface as in other mammals (Fig. 30). The space for m. vastus. lateralis is narrower in Glossotherium, whereas that of m. vastus medialis occupies a slightly smaller space in Paramylodon but extends more medially along the shaft. The proximal positioning of the vastus muscles gives the appearance of a narrower neck in
96 Paramylodon, with the muscles originating nearer to the head and more proximally to the greater trochanter than in Glossotherium, where they are displaced distally. The amount of the distal anterior articular surface that is visible also differs. Paramylodon has a more visible surface than does Glossotherium, although this feature could be related to the overall difference in size. At the time of data collection, quantifiable measures of the distal articular surface heights were not part of the data set. Future work will include such measures to determine whether this is a viable character, and also as a means of investigating the range of motion for the knee joint. Similar reassessments are needed to quantify the differences in the expansion of the medial condyle.
Tibia
The tibia is unremarkable in terms of diagnostic characters. Table 18 shows the expected tendency for a larger size in Paramylodon from all the measures taken. There is a potential oddity in the minimum values for Paramylodon being smaller than those in Glossotherium. The smallest measures for Paramylodon are again associated with the Florida specimens and thus cause the downward shift in the overall means and standard deviations for the northern genus. In the discussion for his new genus of Paramylodon, Brown (1903) determined the difference between the tibia of his genus and Glossotherium was in the flatness of the distolateral semi-elliptical articular surface. Brown believed this feature to be convex in Paramylodon when compared to Mylodon (=Glossotherium) robustus, which he
97
Table 18: Tibia dimensions and ratios for Glossotherium and Paramylodon. * are measurements highlighted by PCA, n is the number of specimens studied, Min is the minimum value, Max is the maximum value, X is the mean value, s is one standard deviation from the mean. Glossotherium Dimensions
n Min Max
Total Length* Med Condyle (med-lat) Med Condyle (ant-post)* Lat Condyle (med-lat)* Lat Condyle (ant-post) Max Condyle Width* Distal (med-lat)* Distal (ant-post)*
9 211 9 69 9 89 9 56 9 50 9 140 9 89 9 94
245 80 105 67 62 154 111 113
X 227.6 73.1 96.6 59.7 56.8 145.7 99.1 101.0
Paramylodon s ±9.9 ±3.6 ±5.8 ±3.2 ±4.1 ±4.1 ±8.4 ±5.7
n Min Max 29 198 29 63 29 82 29 46 29 45 29 130 29 97 29 84
289 102 125 77 77 189 151 124
X 255.9 87.5 109.4 66.3 64.3 167.1 127.0 109.0
s ±21.1 ±9.0 ±10.9 ±7.1 ±7.7 ±15.6 ±16.4 ±10.2
98 viewed as flat. Brown’s character is somewhat vague and does not consistently occur when a larger sample of tibii are studied, likely the reason it was not included by Kraglievich (1928) in his reassessment of the genera and their histories. This character would also be influenced by the shape of the tibial plateau on the astragalus and, as discussed later, there is no difference in that astragalar feature. Beyond that of size, this element lacks characters to distinguish it. That the hindlimb is functionally constrained, with few behavioral differences between the taxa, is a likely cause for the lack of characters. A follow-up investigation could potentially produce generic characters for this element, as the initial emphasis of this study was on morphometric differences that would not be as obvious in a constrained element.
Astragalus
The astragalus has held significance among sloths as a keystone for establishing subfamily- and genus-level taxonomy. Given its important role in many systematic descriptions, a longer discussion for this element and its character history is warranted. Referring again to Brown’s character for the shape of the distal tibial articulation, the corresponding surface in the astragalus, the tibial plateau, would have to possess a depression along its long surface to produce a convex pattern in the tibia. Figures 31a and 31b reveal that the plateaus of both genera are relatively flat with only a slight convex hump, which clearly does not match what would be expected for the character Brown proposed. Similarly, the resultant tibial shape seen by Brown could relate instead to the angle of the odontoid process with the tibial plateau. Stock (1917b)
99
Figure 31. – Lateral and dorsal astragali views for Glossotherium (a-b), Paramylodon (c-d) and Mylodon (e-f). Scale bars equal 5cm.
100 noted a number of pedal characters and remarked that astragali of P. harlani had a sharper angle between these two articular surfaces, but such a character has not been sufficiently seen among the specimens studied. The flatness of the plateau does help in distinguishing Glossotherium from the contemporary genus of Mylodon, although the differences are not diagnostic between Paramylodon and Glossotherium. Specifically, Mylodon possess a much more humped surface, and also a notch in the lateral border when viewed dorsally, as opposed to the more continuous arc seen in the other genera (Fig. 31e, f). The lateral border is rather rounded in both Glossotherium and Paramylodon. Another apparent character in Mylodon, but not in the other two genera, is at the base of the odontoid process where there is a posterior projection into the articular surface, creating a notch where the tibia was not in contact with the astragalus. These features turned out to be important diagnostic characters, as a few of the astragali initially included in the Glossotherium set instead turned out to belong to Mylodon (MLP3-131, MLP3-132 and MLP3-771). Incidents such as this are expected, with many of the specimens in the museum catalogues having entries under multiple names. Distally, Brown commented on the shape of the articulation for the cuboid and navicular. Paramylodon was supposed to be possessed of a more deeply excavated surface for the navicular while that of Glossotherium was supposedly more flattened. These facets show a wide variability in the specimens studied and invalidate Brown’s characterization (Fig. 32). Ventrally, the shape of articular surface for the calcaneus agrees with that of other members of the Mylodontinae and differs from the other subfamilies in being
101
Figure 32. – Distal and ventral astragali views for Glossotherium (a-b), Paramylodon (c-d), and Mylodon (e-f). Scale bars equal 5cm.
102 continuous with the articulation for the cuboid (Fig. 32). This feature assisted in weeding out a number of specimens that had been attributed to the genus Mylodon because of their large size but with the separate facets actually belonged to Lestodon. Brown (1903) stated the difference in the surface as being “elongate and triangular to a greater extent than in M. robustus [Glossotherium robustum], with scarcely a perceptible constriction in the middle” (p.581). Among the specimens studied, there do appear to be differences, but they are of an inconsistent nature. The inconsistency likely led Kraglievich (1928) to state in his diagnosis that the features of Paramylodon mirrored those of Glossotherium. The overall diagnosis for the astragalus should be a mixture of those descriptions put forth by Brown and Kraglievich, in that both genera are similar in appearance and character but with Paramylodon being greater in overall size. The size differences are especially apparent in the measures related to the odontoid process and the height of the fibular articulation (Table 19). The length of the tibial plateau should also be included given the difference in the means and standard deviations, especially as the lowest ranges for Paramylodon (165-188mm) belong to specimens from Florida.
103
Table 18: Astragalus dimensions and ratios for Glossotherium and Paramylodon. * are measurements highlighted by PCA, n is the number of specimens studied, Min is the minimum value, Max is the maximum value, X is the mean value, s is one standard deviation from the mean. Glossotherium Dimensions Odontoid Height* Odontoid Width Fibular Height* Fibular Width Tibial Plateau Width* Tibial Plateau Length* Navicular Height
n
Min Max
11 11 9 9 11 11 11
47 70 35 24 31 157 41
68 85 44 37 39 196 50
Paramylodon
X
s
n
Min Max
55.9 74.8 38.8 28.1 34.3 176.5 45.4
±6.0 ±5.5 ±3.3 ±4.0 ±2.1 ±12.4 ±2.8
42 42 42 42 42 42 42
50 60 39 25 26 165 43
90 106 72 56 50 241 62
X
s
68.1 84.8 58.0 43.7 37.5 212.1 55.0
±7.7 ±9.0 ±7.4 ±7.5 ±4.8 ±17.1 ±4.6
104
CHAPTER FIVE THE FUNCTIONAL IMPLICATIONS OF THE GENERIC CHARACTERS
Biomechanical analyses typically utilize osteological features and patterns of muscular origins and insertions, as well as the overall size of muscles and the forces they can generate, to determine overall functions and range of motions for animals (Alexander et al., 1979; Alexander, 1983, 1985). Such determinations are more difficult to reconstruct in fossil forms where soft tissues are absent or bony muscle scars are obliterated by damage or wear. Still, the presence of muscle scars is useful for discerning lines of action, and the size of the scaring area can be used to estimate overall muscle size. However, biomechanical applications toward fossil taxa can be hampered without establishing a basis for muscle homology. Such homology requires dissection data from closely related, extant species, which are often lacking for many taxa, making muscle reconstruction and biomechanical analyses for extinct taxa somewhat less precise than those for extant taxa (Bryant and Seymour, 1990). Dissection data for establishing muscle homologies are lacking for many xenarthran taxa, but many biomechanical functions can still be inferred from the osteological features and a number of muscle reconstructions have been put forth for some of the extinct forms (Naples 1982, 1987, 1989; Vizcaíno et al., 2003; Bargo et al., 2006a, 2006b).
105 Many of the characters illustrated in Chapter Four have functional correlations with the life of the animal. The differences in those characters also translate to different functional abilities between the genera, further separating them and shedding more light on their abilities and potential behaviors. Many of the functions can be divided into three categories: food procurement, food processing and locomotion.
Food Procurement – Predental Spout
Xenarthran dentitions are notable among mammals in that the teeth are reduced in size, number, and type and lack enamel. The simple and reduced structure is reflected by the presence of cheek teeth and a lack of incisors in many members of the order. Incisors serve many functions in mammals, but among herbivores the prime function is to aid in the procurement of food via cropping the source. This poses a problem among the sloths, which branched from the more insectivorous lineage of anteaters and thus secondarily developed an herbivorous lifestyle. As such, there is a need for a cropping mechanism to aid in obtaining food. Most tardigrades have elongate skulls with very large openings for the nasal passages. This, along with the length of the arrow-shaped premaxillae and the presence of a predental spout in many lineages, has led to the idea that the differential space indicated the presence of large, fleshy lips (Naples 1987, 1989). It is not a far stretch to imagine such features, as the extant sloths exhibit strong and mobile lips despite having
106 more truncated skulls compared to their extinct relatives (Naples, 1982). Thus it is likely that Glossotherium and Paramylodon had lips to aid in cropping vegetation. Another soft-tissue structure that could have aided in obtaining food is the tongue. The original diagnosis for Glossotherium (Owen, 1840) determined that the animal had a long and powerful tongue, based upon the opening for the lingual nerve. This is reflected in the etymology of the name, as glosso- is Greek for tongue, making the overall name for the animal “tongue beast.” Features supportive of a long tongue, reminiscent of the mymecophagous ancestry, have been noted by Naples (1985, 1986, 1987, 1989) in many of the fossil sloth taxa, and the presence of a mobile and active tongue has been well noted in the tree sloths. Although fleshy lips, along with a long and adept tongue, can provide a level of dexterity for positioning food in line with the oral cavity, they do not provide a means of cutting or cropping the food. A harder structure is needed to crop food against, almost like a cutting board. The dentitions are unlikely to be extensively involved in cutting given their placement posterior to the oral opening. This leaves the predental spout as the most likely feature to correspond to such a cutting board when utilized with the premaxilla and anterior end of the maxilla. The spout is present in all sloth lineages, but there is diversity in the morphology of the anterior end of the spout, as evidenced by the patterns in Glossotherium and Paramylodon. The anterior width of the predental spout can be correlated with overall divergence of the toothrows. Megalonychid and megatheriid sloths possess spouts, but these structures tend to be long, narrow, and lack the lateral flare seen in some mylodontids. In those sloth families, the molariforms are parallel in their
107 anteroposterior orientation, and the only sloths with widened spouts are those where the molariform toothrows diverge anteriorly. One might be tempted to associate the widening spout with the presence of caniniforms, as the scelidotheres and some megatheriids that lack those teeth possess narrow spouts, but well-developed caniniforms are also found in the megalonychids and a number of megatheriids that possess narrow predental spouts. The difference in the width of the spout, as it relates to taxa with caniniforms, appears also to be somewhat correlated with whether the caniniform falls within the anteroposterior axis of the molariform toothrow or if it is displaced laterally from that axis. Sloths where the caniniform lies outside the long axis of the toothrow have narrower spouts, as seen in the megalonychids. The exception is Lestodon, where the caniniform is outside of the molariform long axis but the predental spout is very wide. In this case, the spout width matches the width of the maxilla but lacks the anterolateral flare seen in Glossotherium and Paramylodon. Sphenotherus Ameghino 1891 from the late Miocene (Huayquerian) is another mylodontid that exhibits a slight anterolateral flare of the spout, similar to that seen in Paramylodon, but with a shorter length. It is unclear how its maxillary width compares to that of other sloths as there are no known crania for Sphenotherus. Recent reconstruction of the muzzles for Pleistocene ground sloths of South America explored the relationship between the widened muzzle and the potential feeding niche (Bargo et al., 2006b). The findings suggested that those sloths with widened muzzles may have had a bulk feeding strategy that lacked selectivity, whereas those with narrower muzzles were mixed or selective feeders. These strategies are so
108 named for the potential amount of food able to pass through the morphological space of the oral cavity, ranging from a vast intake (bulk feeding) to the least intake (selective feeding). Glossotherium, along with Lestodon, was categorized as a bulk feeder given the larger values for the anterior maxillary width. The somewhat narrower values for Mylodon, which are nearly equal to those of Paramylodon, placed it within the mixed feeding category. The palate of Mylodon is uniform in its width from anterior to posterior, whereas that of Paramylodon narrows posteriorly. As such, an inference that Paramylodon was a mixed feeder cannot be made based only on those values, as the study utilized measurements that required multiple palate widths to be averaged to obtain the mean palate width. If the overall anterior skull width and the shape of the predental spout can be used instead as an approximation, it seems likely that the feeding strategy of Paramylodon would have mirrored that of Mylodon given the similarities in the spout width and shape. Additionally, the maxillary widths were smaller for G. chapadmalense than those of the Pleistocene glossotheres and its spout shape similar to that of Paramylodon, which overall suggests that G. chapadmalense was also a mixed feeder and that bulk feeding developed later, whereas Paramylodon retained a mixed feeding strategy during its evolution. As feeding cannot be defined based upon just one character, it seems likely that the lateral concavity of the predental spout could also have corresponded to a browsing feeding style upon vegetation in a wooded habitat. The concavity would provide a backstop for branches, which could allow the posteriorly positioned caniniforms to strip the bark or all of the smaller twigs from the main branch. This would agree with the findings of Bargo et al. (2006a) that Glossotherium had a lower hypsodonty index than
109 does Lestodon, indicating an adaptation to bulk feeding, but not necessarily upon grasses. Similarly, the reduction in the lateral spout concavity in Paramylodon and the loss of the upper caniniform in the late Pleistocene forms would follow the supposition of McDonald (1995) that these animals occupied a more open grassland habitat. Isotope analysis for Paramylodon also suggests a mixed feeding lifestyle, but with values close to those of a grazer, which would further correspond to an open grassland existence (Ruez, 2005). The loss of the caniniforms could thus be from a change in priority of selection pressures if the feeding strategy had switched from branch bark and leafy vegetation and also was not linked to sexual selection pressures, as the caniniforms are lost in both of the sexual morphs of Paramylodon (McDonald, 2006).
Food Processing
After acquisition, the processing of food for digestion is an important step with a group of muscles involved that correspond to osteological features. The muscles of interest are those primarily associated with mastication; i.e., the temporalis, the masseter, and the medial and lateral pterygoids. Differential origin and insertion points for these muscles will result in different lines of action for transmitting the masticatory forces along the skull and mandible. Recent studies regarding bite force in cingulates have indicated a number of functional and dietary differences across taxa, which could be applied with similar success to the tardigrades, and is planned as part of a postdissertation research plan (Smith and Redford, 1990; Vizcaíno and Bargo, 1998;
110 Vizcaíno et al. 1998a, 2004; Bargo, 2001). Lines of actions are reconstructed following the patterns established by Naples (1982, 1985, 1987, 1989).
Temporalis and Masseter
Character differences in posterior skull height, in the extent of the development of the temporal fossa and height of the mandibular condyle relative to the coronoid process, all indicate generic modifications of the feeding mechanism and associated jaw function in sloths. For example, Paramylodon has higher values for posterior skull height resulting in a larger temporal fossa, the surface of which is already more rugose, suggesting a larger temporalis muscle. The insertion for m. temporalis is the coronoid process of the mandible and the fulcrum for the mandibular lever is the condyle articulating with the skull (Turnbull, 1970; Greaves 1974, 1991). The higher pivot point and longer out lever arm, as indicated by the length of the dental series, probably required a stronger muscle action to generate the necessary bite force for mastication in Paramylodon. This arrangement differs from the structure in Glossotherium, where the out lever arm is shorter and can function with the smaller temporal fossa. Comparison of the lines of action for m. temporalis exhibits orientations that are similar for Glossotherium and Paramylodon (Fig. 33). The differences in the shapes of the coronoid process are then a reflection of adaptations maintaining those parallel lines. The shortened temporal fossa, anteroposteriorly and dorsoventrally, in Glossotherium requires a rounded coronoid process directed more posteriorly to meet up with more posterior occurrence of m. temporalis. Paramylodon has the longer and
111
Figure 33. – Skulls and mandible for Glossotherium (a) and Paramylodon (b). Lines of action are shown for the masticatory muscles: masseter (M.m.), medial pterygoid (M.p.m.) and temporalis (M.t.).
112 taller temporal fossa that produces a more anterior occurrence for the median of m. temporalis, which coincides with the straightened shape of the coronoid process. Despite the similar lines of the action, the greater fossa size in Paramylodon was necessary to meet the greater force demands to elevate the longer mandibular lever arm, whereas Glossotherium would have needed less force with a shorter lever arm. The line of action for m. masseter in Glossotherium is oriented in a more vertical direction than in Paramylodon. The line also appears longer in Glossotherium, owing to a deepening of the angular process of the mandible. The overall interpretation is that the masseter muscle in Paramylodon would have generated a greater anterior movement of the mandible. When the action of this muscle is considered with that of m. temporalis, the North American genus produced a stronger stroke elevating the mandible while also bringing it forward.
Pterygoids
Another complex of interest is the degree of inflation in the posterior pterygoids and the length of the palate posterior to M4. Both characters have implications for the feeding apparatus. The pterygoids are indicative of the origins for m. pterygoideus lateralis and m. pterygoideus medius. Naples (1982, 1985, 1986) showed a differential pattern in the lines of action for these muscles in the extant sloths, although that was a result of the lack of an elongated pterygoid flange in Choloepus. The pterygoid flanges in Glossotherium and Paramylodon are more similar to those of Bradypus, projecting ventrally from the base of the skull and also in the orientation of the pterygoid muscle
113 scars (Fig. 33). However, the lines of action are more similar to those of Choloepus (Naples 1985, 1989). Large flanges such as these indicate a great importance is given to the mediolateral movements of the mandible by both animals, which is expected in herbivores where an increase in the grinding surface area is needed to process food. Additionally, the extinct genera show a differential amount of inflation at the pterygoid sinuses, with Glossotherium more inflated as in Bradypus and with Paramylodon having less inflation as does Choloepus (Fig. 23). A closer examination of the extant species, Bradypus torquatus, might have further bearing on the masticatory pattern in Glossotherium as Naples (1982) indicated that species possessed the long, ventral flanges and showed some inflation of the sinuses. Although their function is unclear, the inflated pterygoid sinuses could have functioned as a resonance chamber, either for vocalization or detecting low-frequency sounds. The narrowness of the pterygoid inflation also corresponds closely with the position of the basilar tubercles, marking the posterior origin of m. pharyngeus superioris, an important pharyngeal constrictor for swallowing (Sicher, 1944; Turnbull, 1970; Naples 1985, 1986, 1987, 1989). There is thus the suggestion that because of their role in swallowing, the positioning of the tubercles can assist in indicating an animal’s head posture to keep the pharynx properly aligned for the bolus to pass (Reidenberg and Laitman, 1991). In this case, the narrowness of the pterygoids can be used as a proxy for the basilar tubercles. However, a more thorough investigation including the orientation of the occipital condyles and the features of the atlas is needed before giving a definitive determination of head carriage.
114 Locomotor Movements
Although the structures of the postcrania, such as portions of the forelimbs, are useful in obtaining food, they are also used in locomotion. The extinct sloths are commonly referred to as “ground sloths,” in reference to their greater size and therefore greater terrestrial ability than possessed of the extant tree sloths. Studies by White (1993a, 1993b, 1997) have demonstrated that some of the smaller bodied sloths were capable of some arboreality, making the moniker “ground sloths” inapplicable to all extinct forms, although the practice is still largely upheld. Glossotherium and Paramylodon are unquestionably viewed as terrestrial animals that were predominantly quadrupedal given their large size. To reflect the size and unique pattern of walking upon the lateral edges of the hands and feet, Toledo (1996) termed the locomotor pattern for the “ground sloths” as traviportal. This manner of locomotion was defined for animals with obligate slow-moving habits using both quadrupedal and bipedal stances. The finer details of patterns exhibited by these animals when not traversing the paleolandscapes and whether they were capable of bipedalism are still open for investigation.
Humerus
Although ground sloths are predominantly viewed as quadrupedal, the length ratios for the forelimb versus the hindlimb show the mylodontids have shorter forelimbs than members of the other sloth families (Bargo et al., 2000; Vizcaíno et al., 2001). As
115 such, body mass estimations place the center of gravity more posteriorly, giving the mylodonts an increased chance to engage in bipedal activities that would leave the forelimbs free for activities other than traviportal locomotion. Various activities have been suggested for nonlocomotor behaviors in ground sloth forelimbs such as digging, overhead grasping for vegetation, and stabbing motions, the latter of which could have been defensive or part of a more aggressive behavior (Fariña and Blanco, 1996). While many of those investigations rely on the ulna, the humerus is still a necessary element to transmit the force of activities from the shoulder to the forearm. The following are humeral characters that can be associated with differential activities related to the forelimb motions of Glossotherium and Paramylodon.
Deltoid Crest Expansion
Deltoid musculature affects a wide range of humeral movements. Within Xenarthra, there is a tendency to have a lateral displacement of the deltoid crest or tubercle, with varying differences between taxa as is evidenced by this feature in Glossotherium and Paramylodon. Within the vermilinguas, the collared anteater, Tamandua Linnaeus, possesses a deltoid tubercle showing an extreme lateral displacement upon the humeral shaft. Studies by Taylor (1978, 1985) indicate the position of the deltoid tubercle in Tamandua helps increase the animal’s ability to rotate the humerus laterally, which aids in the ripping motion for opening termite mounds. There is no supposition that the ground sloths were engaging in activities related to ripping apart insect nests given the other specialized features of Tamandua, but the
116 expansion of the deltoid crest in Paramylodon indicates an increased ability to rotate laterally the humerus compared to that of Glossotherium (Fig. 34). The less laterally expanded crest in Glossotherium (Fig. 34a) could be thought to correlate with the smaller sized humerus and therefore did not require the muscle space to generate the forces necessary to move the arm. This idea seems unlikely as the expanded crest is seen in the humerii of Paramylodon specimens from Florida, whose overall sizes are comparable to those of Glossotherium. Thus, the activities engaged in by Glossotherium would have required less lateral rotation of the humerus than was the case for Paramylodon.
Pectoral Ridge/Shaft Angle
A number of functional studies have inferred the mylodontids as digging animals (Cuenca Anaya, 1995; Bargo et al. 2000; Vizcaíno et al. 2001). Part of this supposition is related to the shorter length of the forearm, as compared to the humerus, and the loadings of the mylodontid humerii for strength and resistance to anteroposterior stresses, characters that are most likely associated with digging. The majority of studies focus on the features of the olecranon process of the ulna, which requires additional investigation, as that element was not included in this study for Glossotherium and Paramylodon. As it relates to the humerus for these taxa, the differential character for the angle of the shaft along the pectoral ridge could add to the evidence for digging behaviors.
117
Figure 34. – Humerii of Glossotherium (a) and Paramylodon (b) in anterior view showing the differences in the deltoid crests, lateral and medial epicondyles, and the angles of the shaft. Scale bar equals 10cm.
118 Paramylodon was probably not a digger because its humeral shaft is more perpendicular and would need to be positioned more laterally to move matrix back and away from the digging site, parallel to the body trunk. A motion for moving dirt backward would involve more of a medial rotation of the humerus, whereas Paramylodon was more suited for lateral rotation because of the greater expansion of the deltoid crest. Glossotherium would have been a better digger as the shaft angle places the distal articular ends farther from the plane of the body, preparing soil to be swept backward with a medial rotation of the humerus. It should be noted that the argument for the proposed movements would be strengthened by investigation of the humeral head shape and its articulation with the glenoid fossa of the scapula to more fully determine the angle of the shaft from the body.
Epicondyle Development
Both taxa exhibit development of the medial epicondyle in the form of a proximally located knob, which corresponds to the origins for flexors of the carpus and phalanges. As previously noted, the development is even greater in Paramylodon, suggesting that flexion of the carpus and manus was more important in that taxon. Flexion can be less important in fossorial animals, with a greater emphasis placed on having a broad, shovel-like manus for moving material posteriorly, which the less prominent flexor morphology of Glossotherium would favor. Flexion can be very important in animals engaged in ripping activities or in those for which grasping and pulling some material is advantageous. The more proximal placement of the medial
119 epicondyle in Paramylodon would also have provided a greater mechanical advantage to the flexor muscles, increasing the grasping abilities. The widths of the distal epicondyle in relation to the total humeral lengths have been used as a means of separating megatheriid taxa and also to estimate the functional ability of the forearm (De Iuliis, 1996). A taxon with a higher ratio would indicate an increased area for the flexor and extensor muscles. One drawback to this method is a failure to differentiate between the lateral and medial epicondyle widths, as one side might be larger than the other, indicating either flexion or extension to be more significant. When applied to Glossotherium and Paramylodon, the two taxa cluster separately based on size, but the slopes for the rate of increase within each genus are nearly equal (Fig. 35). A single exception exists in Paramylodon for a specimen from Florida (UF208520), which does not cluster with the rest of the genus and is nearer in proportions to those of Glossotherium.
Radius
Mylodont radii are distinct in having relatively straight shafts and the absence of the laterally expanded pronator flange found in scelidotheres and some lestodontines. The lack of the pronator flange and the distal end laterally displaced from the proximal end results in a shortening of the moment arm for forelimb rotation, suggesting that while the mylodonts were capable of pronation, their range of motion was less than that in the scelidotheres (Taylor 1978, 1985; McDonald, 1987).
120
300 Epicondylar Width (mm)
Paramylodon y = 0.5538x
250 Glossotherium y = 0.5311x
200
UF208520
150
100 300
350
400
450
500
550
Total Length (mm)
Figure 35. – Epicondylar width vs. Total humeral length. Similar slopes for both genera but with the Florida specimen of Paramylodon falling outside of both groups.
The members of the Mylodontinae were noted as having radii with pronator flanges that flare laterally to a lesser degree than do those of the Scelidotherinae. Among the Mylodontinae, Paramylodon had an even smaller flange than Glossotherium. Pleistocene burrows in South America have been attributed to a number of extinct xenarthrans, including Scelidotherium and Glossotherium, based upon remains found in the tunnels and the patterns of tracks and scratches (Frenguelli, 1928; Quintana, 1992; Vizcaíno et al., 2001). Glossotherium was included, despite the absence of remains, based upon functional forelimb studies of the elbow suggesting a greater mechanical advantage for digging over walking (Bargo et al., 2000). Such a supposition seems to ignore the lack of a radial flange and a reduced ability of Glossotherium to pronate the forearm, which would be advantageous for digging or
121 other behaviors, such as overhead grasping, where pronation is required. The other evidence regarding the ulna and the broad, flat shape of the manus further imply a potential for digging, but the capability of Glossotherium to create burrows remains doubtful. However, the slightly greater flange size in Glossotherium implies that if it could dig, its ability to do so would have been greater than that of Paramylodon. The styloid process of the radius has a more prominent distal extension to prevent carpal movement beyond that point. There is no corresponding feature restricting movement on the ulna, where the carpal articulation is more flattened, and facilitating abduction on the ulnar side of the carpus. This contrasts the condition in the extant xenarthran, Tamandua, where both sides of the wrist joint possess bony features to restrict joint movement. The radial stop in the ground sloths is obvious as these animals bore the weight of their forelimbs on the lateral edges of metacarpals IV and V, pushing the wrist toward the styloid process of the radius. The lack of a feature to stop movement on the ulnar side suggests that abduction on that side was an action of use to those animals, whereas Tamandua needed the restriction to channel the tearing, ripping motion of the forelimb (Taylor 1978, 1985). The increased size of the fourth and fifth metacarpals, along with the reduction in size of the accompanying phalanges and unguals, has led to locomotor reconstructions in which these sloths walked with their weight placed on the lateral palms of the manus. This mirrors the pattern seen in extant anteaters and even in the extant sloth Choloepus, animals that are capable of raising their body off the ground when terrestrial (Beebe, 1926; Meritt, 1977; Mendel, 1981). In all of these extant cases, there is no first digit for m. abductor policis longus and m. extensor policis brevis to
122 attach upon, leaving them instead to act upon the carpus (Taylor, 1978). However, the pollex is present in the mylodontids, and the presence of the muscle scars along the radius suggests that it was probably frequently moved. That the pollex could be abducted conversely suggests these sloths might have possessed good grasping abilities while using the pollex, but such a determination would also hinge on evidence to reconstruct the corresponding adductor muscles of the manus to discuss that motion.
Femur
The idea that some ground sloths were bipedal is far from new, with Owen (1842, 1858) being the first to propose it. Through the years, it has been further based upon characters of the pelvis (Coombs, 1983) and body weight distributions, where 60% was carried by the hindlimbs (Casinos, 1996; Bargo et al. 2000). If Glossotherium and/or Paramylodon could engage in a bipedal stance, then some of the femoral features could be a result of dealing with increasing stresses exerted upon the bone. Differences in the proximal region of the femur are related to the relationship of the greater trochanter to the femoral head, which was dissimilar between the two genera. Evaluation of the differential muscle effects on the bone are not as easy to distinguish without knowledge of the pelvic structure. The greater trochanter represents the insertion point for a number of hip muscles that affect extension, rotation and abduction of the thigh but it is difficult to make determinations regarding the lines of action for those muscles without pelvic points of origin. Pelvic features can also
123 indicate what stresses could be tolerated for movements or actions of the hindlimbs (Hildebrand, 1985).
Greater Trochanter
Although the lines of action for the muscles inserting upon the greater trochanter cannot be determined precisely, comparisons of the trochanteric relation to femoral features in other quadrupeds provide evidence of possible movements. The fact that the height of the trochanter relative to the femoral head differs between Glossotherium and Paramylodon further supports the hypothesis of differential use of the hindlimb between the taxa. Studies have shown that animals in which the femoral head projects superior to the greater trochanter have an increased range of thigh movement, whereas those in which the greater trochanter projects more superiorly have a decreased ability to abduct the thigh and the hindlinbs are moved in a parasagittal plane (Fleagle and Meldrum, 1988; Kappelman, 1988; White, 1993a). While the greater trochanter does not project above the femoral head in Paramylodon, its projection is still above that of Glosssotherium, suggesting Paramylodon was more restricted in abduction movements of the thigh (Fig. 36). Such a restriction would not have been as extensive as that in some digging armadillo taxa where the projection is far above the femoral head (White, 1993a).
124
Figure 36. – Glossotherium (a, b) and Paramylodon (c, d) femora in anterior and posterior views, respectively. Shaded area represents the origins of the vastus muscle group. Scale bars equal 10cm.
125 Vastus Muscle Group
Muscle orientation differs slightly but is relatively consistent between both genera, with the direction of the vastus group oriented distally towards the articular surface that articulates with the patella. The greater width for m. vastus lateralis in Paramylodon could be a compensation for the increased size of the animal and to assist in orienting the general line of action that extends the leg toward the distal articular surface (Fig. 36c). A narrower muscle on the lateral side might not generate enough force to keep the patella from sliding medially. Although not part of the vastus musculature itself, the differential placement of m. vastus medialis in Glossotherium creates a surface area along the medial border that likely conforms to the insertion of the adductor muscles. The adductor ridge could have provided some medial rotation, given the more anterior placement on the femur in Glossotherium. The medial supracondylar ridge is typically the insertion site for the hamstring portion of m. adductor magnus, which assists in extension of the thigh, and is often located more posteriorly on the shaft. A combination of thigh extension and medial rotation would be beneficial for a digging animal in times where the hindlimb is needed to help remove matrix from the digging site. Alternatively, the more anterior position of the adductor musculature could reflect a greater degree of distal torsion of the femur, requiring the unique anteromedial muscle placement to enable walking.
126 Distal Articulations
Features of the distal articulations of the femora correlate with the locomotor habits of animals, specifically with regard to posture and the range of flexion at the knee joint. The size and displacement of the posterior femoral condyles (Figs. 36b, d) indicate that both genera frequently held their knees in a bent position and that the tibia articulated at an angle with the femur, creating an outwardly bowed leg. Such a combination would have increased the adduction capabilities for the pes and is commonly found in animals that walk on the outer edges of the feet (Fleagle and Meldrum, 1988; White 1993a, 1993b, 1997). The height of the anterior articular surface can also be used as a proxy for flexion at the knee, with taller and more grooved anterior surfaces indicating an animal with a more erect leg posture. Again, while both genera spent time with the leg bent, the higher anterior articular surface suggests Paramylodon possessed a posture that was less bent and possibly somewhat erect, which further correspond to theories of bipedalism in that genus (Stock 1917b, 1920b, 1925, 1936). A more precise assessment of the knee joint for Glossotherium and Paramylodon requires more detailed measurements for the anterior articular surface and the condyles that were not able to be obtained during initial data collection. As a result, the posture and functional differences presented are currently speculatory.
127 Summation
A common theme for this chapter has been a call for more research to better assess the functional capabilities of these two animals, leaving few concrete conclusions. Given the differences in morphology, there is no question that these sloths exhibited functional and behavioral differences, but the finer details of those behaviors and function are open for debate. Further evaluation of these differences require revised research approaches that can look beyond taxonomy for character sets related to functional morphology. These endeavors will investigate the full functional complexes of the limbs, including the scapula and ulna. Ranges of motion can also be better assessed with the increased understanding of muscular origins and insertions that have been determined for a number of fossil xenarthrans (De Iuliis, 2003; Vizcaíno et al., 2004; Pujos et al., 2007). A differential approach to investigate feeding, and also probably food selection, would also be useful, given the emphasis placed on skull width in Glossotherium versus the increasing length of the oral cavity in Paramylodon. These differences are further implicated by reduction in the degree of lateral concavity of the predental spout in Paramylodon. The concavity is a derived character shared with Glossotherium, but the reduction in its presence suggests an opposing pressure where individuals without the concavity were better adapted to obtaining or processing food. If the spout did aid in stripping vegetation from branches, then the reduction in Paramylodon could be due to an abundance of open, grassy habitats in North America where the flared spout would not be as effective.
128 Differences in digging abilities between taxa are also considered to be present. The presence of such features implies a potential for fossorial behavior, but those morphological characters’ features could be related to alternative behaviors. If Glossotherium were a digger, the mechanics truly differed from those of known diggers, as Bargo et al. (2000) noted in their comparison against Scelidotherium. The lower height of the greater trochanter relative to the femoral head suggests that Glossotherium would have been a forearm digger, with the hindlimbs providing less assistance than is observed in known xenarthran diggers where the trochanter occurs far above the head. Xenarthrans where the trochanter occurs below the head often fill a climbing niche (White, 1993b), but Glossotherium is not being put forth as climber. However, some degree of climbing could have been possible given the proposed preference for wooded vegetation, but only for the smaller bodied forms that would have been closer in size to those ursids that are known to have arboreal climbing abilities (Larivière, 2001). The position of the trochanter in Paramylodon is more on par with that of the giant anteater, Myrmecophaga Linnaeus, which is terrestrial but is also known to engage in a bipedal defensive stance. Such a stance has long been proposed for the ground sloths, with part of the body weight supported by the tail in a tripod-like position with the legs (Owen, 1842). The decrease in abduction accompanying the more superior position of the greater trochanter would have helped to further stabilize Paramylodon in obtaining a bipedal stance. The greater flexion/grasping capabilities could also have been used in conjunction with a rearing posture to obtain food sources located above the animal. However, such a supposition would require a more wooded habitat, which is counter to the open and grassy environment proposed for
129 Paramylodon. Rather than providing answers to the life and ecology of this sloth, the characters seem better suited to creating more questions and lines of investigation.
CHAPTER SIX SYSTEMATICS AND TAXONOMY
Order XENARTHRA Cope, 1889 Family MYLODONTIDAE Ameghino, 1889
Long, rectangular skulls; dental formula 5/4 or 4/4; last upper and lower molariforms bilobed; reduced length of the zygomatic process of the squamosal; palate extends posteriorly and dorsally as a shelf that ends at the midpoint of the descending lamina; M2-M4 and m2-m3 typically lobed but lobation secondarily lost in some forms; m3 s-shaped in cross-section; condyloid process lying at or just above the level of the toothrow; posterior edge of condyloid process nearly vertical; medial ridge along anterior edge of the coronoid process absent; coronoid process short and anteroposteriorly broad; angular process short and deep; mandibular symphysis extends posterior to the first tooth; lateral edges of the predental spout parallel; limbs generally short and massive; manual unguals dorsopalmarly compressed with the width greater than height; fovea for ligamentum teres forms a notch on the femoral head; astragulus possesses an odontoid process.
131 Subfamily MYLODONTINAE Gill, 1872
Dental formula 5/4 or 4/4; nasal region depressed relative to braincase, but both are nearly horizontal in lateral view; anterior maxillary edges with a fossa lateral to the external nares; toothrows parallel in early forms and anteriorly divergent in later forms; lacrimal bone and foramen small; digastric fossa enlarged and opening laterally on skull; temporal fossa laterally positioned on skull; occipital condyles slightly taller than wide in posterior view; m3 elongate and irregular in cross-section; ascending mandibular ramus partially covers posterior teeth in lateral view; anterior edge of mandibular symphysis straight in lateral view and moderately wide at midpoint; humerus with entepicondylar foramen in early members and absent in most genera; unguals clawed on digits I-III of manus and II and III of pes; third metatarsal with enlarged lateral process; calcaneum with expanded tuber calcis; calcaneal facets of astragalus separate in early forms and confluent with the cuboid facet in most later forms; dermal ossicles present in some genera.
132
Genus GLOSSOTHERIUM Owen, 1840
Type species—Glossotherium robustum (Owen, 1842) Other species—Glossotherium chapadmalense (Kraglievich, 1925) Emended diagnosis—This South American mylodontid is distinguished from its closest relatives, Mylodon and Paramylodon, by the following characters: Skull – Dental formula 5/5, with C1 always present; C1 length along the long axis of the toothrow greater than 20mm; M1 length shorter than in Paramylodon and more equal to width of M1 and to length of M2; M2 triangular in section with a welldefined posterior lobe in early forms that disappears in Pleistocene forms; M4 bilobate with posterior lobe narrower than anterior lobe, which is anteroposteriorly compressed; molariform toothrow accounts for less than 80% of the total toothrow length; palatine length posterior to M4 less than 30mm; ratio of palate length posterior to M4 versus total maxillary-palatine length less than 0.15; skull somewhat dome-shaped in lateral profile; posterior skull height less than that at the postorbitals; rostrum narrows posteriorly toward the lacrimals and widens thereafter to the back of the skull; width of nasal cavity greater than the height; ratio of lacrimal to postorbital width less than 1:1, with postorbitals greatly widened in later forms; pterygoid sinuses strongly inflated medially and narrowly spaced; parasagittal crest wide and temporal fossa less developed.
133 Mandible – Shorter than in Paramylodon, and condyle is lower in proportion to height of coronoid; ascending process of coronoid more curved along upward slope; predental spout wide, with lateral flare at anterior margins creating a pronounced concavity along lateral margins. Humerus – Shaft angle 65-78º; deltoid crest with less lateral development relative to Paramylodon; medial epicondyle poorly developed and below distal end of deltopectoral tubercle; lateral epicondyle well developed and parallel to shaft. Radius – Muscle scars for m. abductor policis and m. extensor policis brevis extend onto the styloid process; pronator flange less developed than in Scelidotherium and Lestodon, but somewhat more developed than in Paramylodon. Femur – Greater trochanter not elevated above level of femoral head; vastus muscle scars occur more distally, creating wider femoral neck region; M. vastus lateralis narrow; m. vastus medialis occurs more on the anterior surface; distal anterior articular surface height short and barely visible. Astragalus – Base of odontoid surface complete and tibial plateau relatively flat, as opposed to Mylodon. Distribution—Specimens are recorded from Plio-Pleistocene (ChapadmalanLujanian) localities in central South America (Argentina, Bolivia, Brazil, Ecuador, Paraguay, Peru and Uruguay). Discussion—Similar to other sloth genera, Glossotherium represents a wide range of character variation that is likely attributable to a lack of discernment of distinct genera and species included in the group. With Owen’s improper assignment of a type species for Glossotherium, and it later being synonymized under Mylodon, a case exists
134 for reassessing the genus as invalid. However, if such a step were agreed upon, it would cause more problems than it would possibly solve. Thus, the current reassessment of Glossotherium makes strides towards redefining the generic limits and narrowing the range of variation to create a more stable and acceptable genus. The range of variation within Glossotherium is very high and has been noted for many years with little to show beyond lip service. Cabrera (1936) provided a revision of the species level and whittled the taxa down to two species, G. robustum and G. lettsomi. The emphasis of that revision appears to be on the Pleistocene forms as there is no mention given to G. chapadmalense. Whether Cabrera’s work was simply overlooked or dismissed as incorrect is unclear, as subsequent publications and work utilize multiple species names, leaving a large degree of confusion as to what Pleistocene species are present within Glossotherium. The determinations of the species are unfortunately beyond the scope of this dissertation, but a few potential patterns for further investigation are suggested. When the mandibles are observed in dorsal profile, a pattern that is related to the shape and orientation of the lower caniniform emerges, suggesting the existence of two morphs. In one form, the caniniform is more ovoid in shape and is placed more in line with the orientation of the toothrow, giving the lateral mandibular border a more uniform shape when seen in dorsal view (Fig. 37b). The caniniform in the other morph is more triangular in cross-section and is more laterally displaced, creating more of a bulge in the lateral mandibular border (Fig. 37a). As these character states were recognized after data collection, it is unclear what, if any, additional character differences accompany the caniniform shape and position. It is likely that additional
135
Figure 37. – Glossotherium mandibles showing the different shapes for c1 in dorsal view. Scale bars equal 10cm.
136 characters related to the two morphs would also occur in the cranium. If the lower caniniform differs in shape, it stands to reason there would be a corresponding shape pattern in the upper caniniform. As such, a further review of the characters related to these morphs and their geographic distribution is required prior to a more official determination and declaration of generic or species separation. Reassigned specimens—The following specimens were initially assigned to Glossotherium, but such designations have been found to be erroneous based upon the revised character sets. Species-level determinants are presently unavailable and assignments are only to the generic level. Mylodon sp. MLP 3-131. Right astragalus MLP 3-132. Right astragalus MLP 3-771. Left astragalus
137
Genus PARAMYLODON Brown, 1903
Type species—Paramylodon nebrascensis Brown, 1903 (= Mylodon harlani Owen 1840). Emended diagnosis—This North American mylodontid is distinguished from its closest South American relatives, Mylodon and Glossotherium, by the following characters: Skull – Dental formula varied with the loss of C1: 5/5, 4/5 or 5/4; C1 length along the long axis of the toothrow less than 20mm; M1 rectangular in shape, longer than wide; M2 triangular in section with a more prominent lingual groove, forming distinct posteriorly projecting lobe; M4 bilobate where posterior lobe is not as narrow as anterior lobe and which is not as anteroposteriorly compressed as in Glossotherium; molariform tooth-row accounts for more than 80% of total toothrow length; palatine length posterior to M4 greater than 30mm; ratio of palate length posterior to M4 versus total maxillary-palatine length greater than 0.15; skull relatively flat in lateral view; posterior skull height nearly equal to height at postorbitals; rostrum narrows posteriorly toward postorbitals and widens thereafter to back of skull; width of nasal cavity nearly equal to height; ratio of lacrimal to postorbital width nearly 1:1; pterygoid sinuses weakly inflated and widely spaced; parasagittal crest narrow and temporal fossa well developed and rugose.
138 Mandible – Longer than Glossotherium and condyle is higher in proportion to coronoid process; ascending process of coronoid is straighter in its upward slope; predental spout narrow and more rounded anteriorly with little lateral flare. Humerus – Shaft angle 78-85º; deltoid crest well developed laterally; medial epicondyle well developed and above distal end of deltopectoral tubercle; lateral epicondyle with less proximodistal extension parallel to shaft. Radius – Muscle scars for m. abductor policis and m. extensor policis brevis do not extend onto styloid process; pronator flange much less developed than Scelidotherium and Lestodon and slightly less than Glossotherium. Femur – Greater trochanter angled from being parallel with distal end and nearly equal in height with femoral head; vastus muscle scars located proximally, creating narrow neck region; M. vastus lateralis wide; m. vastus medialis up against the medial border; distal anterior articular surface tall. Distribution—Specimens are known from multiple localities across the southern United States, with the most northerly occurrence in Washington, and are also found in parts of Mexico, making them restricted to North America. Major localities are California (Rancho La Brea), Idaho (American Falls), Nebraska, and Florida (Haile and Ingleside). Temporally, specimens are known from the late Blancan to late Rancholabrean deposits (late Pliocene to late Pleistocene). Discussion—Paramylodon is considered to be restricted to North America, although its origins still hinge on the taxonomic fate of “Glossotherium” chapadmalense. Currently there is one recognized species for the entirety of its temporal and geographic distribution, P. harlani. Should “G.” chapadmalense belong
139 to Paramylodon, it would represent a second species, and the first available name would be P. garbani (McDonald, 1995). Currently the best specimen to answer the taxonomic placement is an unprepared Blancan-aged sloth from Texas at the American Museum of Natural History that consists of a skull, mandible and most of the upper torso and forelimb. Stock (1917a, 1925) noticed a pattern in the many Paramylodon skulls recovered from the La Brea tar pits, which he attributed to being an example of subspeciation. The subspecies difference was based on two skull morphs, robust and gracile, which he called Mylodon [Paramylodon] harlani harlani and M. harlani tenuiceps, respectively. McDonald (2006; McDonald et al., 2004) has instead determined these morphs to be indicative of sexual dimorphism, although there has been no basis for assigning an actual sex to either morph. The differences between the two are based upon the transverse dimensions relative to the length of the skull, the slope of the occiput and the direction of the wear facet on the upper caniniform. In the robust forms, the skull is wider, the occipital condyles are positioned more closely to the base of the cranium, and the caniniform has an oblique wear facet resulting in a pointed end. This is contrasted with the gracile form which has a narrower skull, occipital condyles positioned more posteriorly, and caniniforms with perpendicular wear facets resulting in blunt end. This pattern is not restricted to the populations from La Brea but is observed in Paramylodon specimens from Arizona, Texas, Florida and Idaho and is even seen in specimens as old as the Blancan. In all cases the transverse dimensions of the robust morph are still narrower than those found in Glossotherium.
140 Size must be considered as a potential diagnostic factor with regard to possible speciation within Paramylodon. McDonald (1995) noted a trend for size increase within Paramylodon, occurring from the Blancan to the Rancholabrean, but still considered them to all represent one species. However, there is precedence among members of the Megatheriidae and Megalonychidae for an increase in body size across the Pliocene to the late Pleistocene. Within those families, the different size morphs for Megalonyx and Megatherium have been established as individual species, with the largest forms occurring in the late Pleistocene (McDonald 1977, 1997; Hirschfeld, 1985; De Iuliis 1996, 2006; Saint-André & De Iuliis, 2001; Pujos, 2006). The separation of these taxa into novel species is not based solely on size and temporal disparities. As it pertains to Paramylodon, there is a size disparity along a temporal gradient, although the matter is not so simple. Specimens of Rancholabrean age are the most numerous and are predominantly the basis for the characters defining Paramylodon. If specimens of Irvingtonian age represent a new species, they would be separate from P. harlani. Such a determination is not feasible as there are fewer specimens of that age than are needed to make an adequate comparative study against P. harlani. Also, the majority of Irvingtonian specimens hail from Florida but are of a considerably smaller size than those from other localities. The size of the Florida specimens appears to be a character related to geography and not just to temporal occurrence, as the Rancholabrean specimens from Florida are also very small in comparison to those of La Brea and American Falls. If the temporal variant were true, then the Rancholabrean forms in Florida would be equivalent in size
141 to those of the western localities. The lack of such a variation implies the Florida pattern is outside that seen elsewhere. Thus the size variant combined with the geographic isolation provides an interesting basis for establishing a novel species for sloths from that region. In addition to the overall size differences noted, the Floridian sloths exhibit a modification to the character state of the deltoid crest on the humerus. The ratio for the height of the deltoid versus total humeral length was not a useful character for separating Glossotherium and Paramylodon due to the overlap in values when all specimens were grouped. However, this character shows a geographic disparity, with those from Florida and Idaho having lower values that fell within the Glossotherium range (0.35-0.40) while all other Paramylodon specimens had larger values (< 0.40). If the Florida and Idaho specimens had been removed from the analysis, there would have been no overlap in the value range and the ratio would have been a character of generic separation. Until the emergence of the smaller values, the ratio appeared to be a strong generic character that was unrelated to overall size. Smaller bodied scelidotheres show ratios smaller than do those of Glossotherium, but lestodontids that have much larger humeri than Paramylodon have ratios equivalent to those of Glossotherium. Thus the small size of the Florida specimens does not guarantee the smaller ratio, especially as the size of the Idaho specimens is comparable to the large humeri from La Brea. The occurrence of the smaller humeral ratio in two widely separated populations makes the possibility of it indicating a second species more problematic and suggests instead a difference in the habitats of those sloths versus those in the other North
142 American localities. Given the larger size of the Idaho sloths, speciation seems less likely as the smaller body of the Florida specimens was being used as a possible species indicator. However, this is supposing that there could only be two Paramylodon species and does not consider the possibility that the Idaho sloths could represent a third. In addition to the humeral ratio pattern, the American Falls specimens from Idaho show a lingual groove on the caniniform. This feature is not evident in Paramylodon specimens from other localities, with all of those having the typical elongate, ovoid shape. This groove is most evident in the lower caniniform and often has an accompanying bony projection on the lingual border of the alveolus. The feature is even visible in juvenile sloths, indicating it is not a product of ontogenetic change. A re-examination of the specimens from Florida is needed as individual teeth were not studied, and of the few mandibular specimens it is unclear whether the groove exists using the images taken. An occurrence of this feature in those sloths would necessitate a further evaluation of sloths from the Midwest, especially Nebraska, as they represent a median population and their humeral ratios were at the upper boundary of the Glossotherium and Florida specimens (0.40-0.41). Any potential migration and distribution from an early Florida population would likely have had to cross through the Midwest regions to settle in the American Falls locality. A final postcranial difference exists within the Floridian specimens that is related to the femur. Part of the revised diagnosis for Glossotherium involved the proximal portion of the femur from the head to greater trochanter being parallel to the distal articular surface and the lateral supracondylar ridge being well developed and
143 prominent on the anterior surface. Among the femoral specimens available from Florida, there appears to be a mix of characters with some exhibiting the features mentioned as indicative of Glossotherium. The mix of characters reinforces the need for additional review of specimens from this geographic location, which would also benefit from the inclusion of postcrania from Pliocene mylodontids from both North and South America. If the differences above merit a new species designation, the potential name for a sister species hinges upon the elucidation of “Glossotherium” chapadmalense. As noted, specimens referred to “G.” chapadmalense are from older deposits of Blancan age and indicate two potential taxonomic assignments. The first supposes that the specimens represent a pan-American migration by the mid-Pliocene G. chapadmalense from South America that quickly evolved into the Paramylodon lineage under the differential North American pressures. In this case no name change would be required. The second supposes that these Blancan sloths are truly distinct as Paramylodon and originated in South America from a lineage that gave rise to Glossotherium under the differential selection pressures of the two continents. In such an occurrence, the first available name would be Paramylodon garbani (McDonald, 1995). Most elements assigned to “G.” chapadmalense are postcranial and lack any true characters to differentiate them. They are smaller in size but roughly equal to those of the Florida specimens. What is required is a well-preserved cranial complex, as most of the characters cited above for separating Paramylodon from Glossotherium are related to the skull. With the cranial characters outlined, it would be a simple and quick matter to determine the placement of the Blancan sloths with a well-preserved skull and
144 finally put an end to the debate. However, the crania for “G.” chapadmalense currently known are either crushed beyond any sort of useful recognition (F:AM 116723) or are from private collections, making their validity somewhat questionable. One viable specimen consisting of a skull and the associated elements of the trunk and forelimb exists in the collections of the American Museum of Natural History for a Blancan-aged mylodontid from Texas, but at present it is still unprepped in its field jacket (Fig. 38). A postdissertation project is currently planned to prepare the sloth for the AMNH and use the characters put forth here to finally determine the placement and relationship of “G.” chapadmalense.
Figure 38. – AMNH jacket containing a Blancan-aged mylodontid from Texas with a cranium and some postcranial elements.
145 Conclusions and Future Work
Glossotherium and Paramylodon are shown to be distinct genera with a number of diagnostic characters for the crania and major elements of the post-crania. Although it was not possible to determine generic separators for Glossotherium and Mylodon for postcranial elements other than the astragalus, the difference is still significant and the new generic limits for Glossotherium will aid in finding new characters in the other elements. There are also many characters indicative of a shared common ancestor between Glossotherium and Paramylodon, which is exhibited by the mid-Pliocene specimen of Glossotherium chapadmalense from South America that displays a mixture of characters that are generically distinct to the Pleistocene forms. Whether G. chapadmalense represents the ancestor of Paramylodon or if an even earlier sloth were the ancestor to both genera still remains to be determined. To address this and other issues that still surround Glossotherium and Paramylodon requires a number of additional investigations. Assessment of the ancestor relationship between the two genera first requires evaluation of the late Blancan specimens from North American that have been designated as “Glossotherium” chapadmalense. The suites of characters set forth in this dissertation provide a basis for establishing the taxonomy of those specimens, especially for the cranium, as it had the most characters of any of the elements studied. Cranial material for Blancan-aged sloths is typically crushed or unknown to the scientific community. The unprepped sloth at the American Museum presents the first viable opportunity to study a relatively complete skull, along with some associated
146 postcranial elements. Currently, there are preliminary plans to prepare and study the specimen at the AMNH within the next year. Also, there are a number of additional elements to be evaluated for generic characters, such as the scapula, pelvis, carpals and tarsals. Determination of characters related to those elements will strengthen taxonomic assessments for “G.” chapadmalense, as there are smaller carpal and tarsal elements assigned to that taxon. Investigations of these additional elements will also assist the further studies of species-level differences within the genera. As mentioned, there are patterns within specimens of Paramylodon that could be geographic, temporal or species-level differences. The methodology for this dissertation required nearly complete specimens and resulted in many partial specimens to be overlooked. With the generic characters now established, these partial specimens can be studied for features that might imply a geographic, temporal or species-level difference, such as the height of the deltoid tubercle in the humerus, the occurrence of a lingual groove in the caniniforms of Paramylodon, and the shape of the lower caniniforms in Glossotherium. For Paramylodon, investigations will focus on specimens from Florida, Idaho and Nebraska localities and will result in a larger sample size than has currently been studied. Along with the additional specimens from the localities mentioned, Paramylodon material from Mexico needs to be included, as those specimens were not able to be studied in the dissertation. Specimens from Mexico localities have the potential to exhibit character suites more akin to those of Glossotherium as migration into North America would likely have taken place through Central America. As such,
147 those specimens might provide further insights into the relationship between Glossotherium and Paramylodon. The addition of the shoulder and pelvic girdle elements, along with those of the wrist and ankle, will benefit the functional interpretations made about these animals. Many of the potential functions require a full functional unit, not just a single element, in order to make appropriate determinations of movement capabilities and ranges. There are a few specimens with associated elements of the fore- and hindlimb that can now be studied, especially as some were overlooked due to preservation issues for the taxonomic study. It is hoped that a better assessment can thus be made about the digging and grasping capabilities of these animals that will further our understanding of their paleoecology. Along the line of functional interpretations are those suggestions put forth regarding their feeding capabilities. The assessment of Glossotherium as a bulk feeder and Paramylodon as a somewhat selective browser seem logical given the morphological characters discussed. However, those assessments would benefit from a literature review of the proposed floral and faunal lists for the known specimen localities that would suggest the kinds of habitats utilized. Assessment of the habitats and their components can also indicate the differential pressures each taxon faced on their respective continents, pressures that may have influenced their derived morphologies. However, habitat reconstruction for Glossotherium may be difficult, as most specimens are recorded as coming from the Pampean Formation, which is a large, stratigraphic mess (Tonni and Fidalgo, 1982; Tonni et al., 1992). Also, an examination of the environments for the different North America localities could explain the patterns
148 noted in some of the Paramylodon populations, such as those from Florida with their smaller size. Glossotherium and Paramylodon truly are remarkable animals, with a rich diversity of form and function. Assessing their generic limits has at times yielded more questions than answers, which only adds to my awe for these long-gone beasts. Although the journey of the dissertation is at an end, I am pleased that the road for studying these creatures is still long and currently without an end in sight. Sloth on!
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LIST OF APPENDICES Appendix
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B. DATA
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Measurement Data PCA Crania PCA Mandible PCA Humerus PCA Radius PCA Femur PCA Tibia PCA Astragalus
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