Software: Adobe Photoshop CC 2015, Adobe Illustrator CC 2015, TNT (Tree analysis using New. Technology). Institutional Abbreviation: CM, Carnegie Museum ...
A SMALL CTENACANTHIFORM SHARK FROM THE LATEST MISSISSIPPIAN (SERPUKHOVIAN) BEAR GULCH LIMESTONE OF MONTANA, USA John-Paul M. Hodnett1, Eileen D. Grogan1,2, Richard Lund1,2 1Department
of Biology, St. Joseph’s University, Philadelphia PA; 2Carnegie Museum of Natural History, Pittsburg PA
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Introduction G
Are ctenacanths a natural group of Paleozoic sharks? The idea of a ctenacanth group (e.g. Dean 1909, Glikman 1964) and the relationship of such to others within the Class Chondrichthyes is still debated. Schaeffer and Williams (1977) advocated a ctenacanth form and proposed them to be closer to the crown elasmobranchs with xenacanths closer to the stem of elasmobranchs. However the features used to define the ctenacanth form (based on fin and dorsal spine morphology ) are now interpreted as plesiomorphic features (Maisey and Melo 2005). Schaeffer (1981) later presented a cladogram based on neurocranial features that unites, albeit in a Hennig’s ladder, Xenacanthus, Tamiobatis, and Cladodoides as a clade closer to the crown elasmobranchs. Ginter et al. (2010) proposed, based on dental similarities, that Ctenacanthiformes were closely related to other chondrichthyans with cladodont dentitions and, so, formed a new group Cladodontomorphi. This includes stethacanthids, cladoselachians, ctenacanths, and Squatinactis. Recent work on the dentition and neurocrania of chondrichthyans have suggested that cladodont sharks and xenacanths represent a stem clade of chondrichthyans (paleoselachian elasmobranchs of Grogan et al. 2012) in a tree with higher crown holocephalans and euselachian sharks (Janvier and Pradel, 2016). Here we present a shark new taxon from the Upper Mississippian (Serpukhovian) Bear Gulch Limestone which adds additional information to traits that are considered “ctenacanth”. New observations are presented on sharks commonly identified as ctenacanths to enrich the discussion on “what is a ctenacanth?”. A phylogenetic analysis of our data proposes that ctenacanths do form a monophyletic clade within elasmobranchs.
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Figure 1. The Bear Gulch ctenacanth, CM 46006. A) the fossil with part and counterpart in place, B) outline of skeletal elements of the Bear Gulch ctenacanth (gray) and phyllocarid crustacean, Dithyrocaris sp; (yellow), C) skeletal reconstruction of the Bear Gulch ctenacanth. Scale bar is10 cm.
Specimen: CM 46006 A+B, partially complete adult male, collected from the Bear Gulch Limestone of the Heath Formation, Fergus County, Montana. Materials: Olympus SZH10 stereomicroscope, Epson Perfection v500 Photo Scanner. Software: Adobe Photoshop CC 2015, Adobe Illustrator CC 2015, TNT (Tree analysis using New Technology) Institutional Abbreviation: CM, Carnegie Museum of Natural History, Pittsburgh, PA; CMNH Cleveland Museum of Natural History, Cleveland, OH; NMMNH P, New Mexico Museum of Natural History and Science, Albuquerque, NM.
The analysis was performed with a matrix modified from Grogan and Lund (2008), designed to discriminate among chondrichthyan terminal taxa. It comprises of 142 characters and 61 taxa which include six outgroups, a placoderm (Coccosteus decipiens), two sarcopterygians (the coelacanth Hadronector donbairdi, an onychodont Onychodus jandemarrai), an actinopterygian, (Kalops monophyrs), and acanthodians (Ptomacanthus anglicus, Acanthodes bronni). All characters were treated as unordered and unweighted. The matrix was run in TNT as a traditional search with no swapping algorithm.
Figure 3. Phylogenetic Trees showing Ctenacanthiformes as a monophyletic group within Elasmobranchii.
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Materials and Methods
Phylogenetic Analysis
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The cladistic analysis yielded two trees with a tree length of 825. Both trees show nine taxa forming a monophyletic clade including forms previously referred to as ctenacanth sharks. This node, the Ctenacanthiformes* occurs within the subclass Elasmobranchii (“X”), within the class Chondrichthyes (“O”). From this analysis Ctenacanthiformes can be defined as elasmobranch sharks with elongated postorbital region, dorsal otic ridges that originate anterior to the posterior margin of the postorbital process, laterally projecting otic processes, anterodorsal process present on the otic process of the palatoquadrate, and with basolabial and orolingual projections wider than the median cusp in teeth which have enameloid connecting between cusps. The trees show two hypotheses for the relationships of Ctenacanthiformes within Elasmobranchii. Tree 0 proposes ctenacanths as the sistergroup to Doliodus + Euselachii and this larger clade to be sister group to Xenacanthimorpha. Tree 1 proposes a sister relationship of ctenacanths with a clade that contains Xenacanthimorpha + [Squatinactis + Cladoselache]. Doliodus is basal to this entire clade. The larger clade of Doliodus + Xenacanthimorpha + Squatinactis + Cladoselache + Ctenacanthiformes shares a sister relationship with Euselachii. Both trees suggest a basal relationship of Symmoriiformes to all other elasmobranchs.
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Figure 2. The jaws, teeth, and dorsal spines of the Bear Gulch Ctenacanth and other ctenacanths. A-B, the dorsal view of Ctenacanthus sp., CMNH 7852, demonstrating the articulation of the anterodorsal process (AOF) of the palatoquadrate; A) photo of specimen, B) Line diagram. C-F, Palatoquadrates and Meckel’s cartilage of ctenacanth sharks (scale 1 cm); C) Bear Gulch ctenacanth, CM 46006; D) Heslerodus divergens; E) Manzano ctenacanth, NMMNH P-68537; F) Ctenacanthus sp., CMNH 9450. G-J, teeth of multicuspid of ctenacanths; G) teeth of Bear Gulch ctenacanth (scale 2 mm); H) teeth of Saivodus striatus (scale 1 cm); I) teeth of Neosaivodus flagstaffensis (scale 1cm); J) teeth of CMNH 9280, “Tamiobatis vestutus”. K-L, dorsal spines of the Bear Gulch ctenacanth; K) anterior dorsal spine (scale in mm); L) posterior dorsal spine (scale 1 cm). Note the denticulated rows at the base of the spines that grade into smooth costae.
The position of Doliodus isolated from ctenacanths supports the notion that superficial resemblance of the ornamentation of spines of Doliodus and ctenacanths are due to convergence (homoplasy).
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Discussion & Conclusions L
Anatomical Data Assessment A male shark with an estimated total body length of 23 to 25 cm. Dentition is cladodont with the median cusp tall and narrow, lateral cusps less than half the height of the median cusp, two to three intermediate cusps, and lacking labial accessory cusplets. Neurocranium with craniocaudally elongate postorbital region, mediolaterally wide otic region, laterally projecting otic processes, and dorsal otic ridge originating just anterior of the posterior margin of the postorbital process. Palatoquadrates with a moderately elongated palatine ramus, otic process with moderately developed anterodorsal rectangular flange followed by a posteriorly directed rectangular flange. Meckel’s cartilage with a dorsoventrally narrow and craniocaudally elongated dental ramus and well-developed, deep retroarticular process. Hyomandibula in lateral view; concave anteriorly and with caudally expanded laminar process on lower 2/3 of posterior margin. The element is dorsally symmetrical but expands toward its ventral extent, exhibiting lateral ridges. Dorsal fin spines relatively straight, with anterior row of rounded or cuboidal denticles along a costal margin which becomes smooth distally; a series of posterior rounded denticles on the proximal lateral surface of spine grade into four (anterior spine) to seven (posterior spine) smooth costae on the distal lateral surface of the spine; and four to five posteriorly directed recurved denticles present on posterior margin of the spine. Scapulocoracoids recurved anteriorly and meeting in ventral symphysis. Scapular blade margin expanded posteriorly from dorsal apex to caudally rounded margin before narrowing and expanding into a concave ridge. More ventrally this narrows to a posteriorly directed glenoid process. Pectoral fin metapterygial axis not elongate. Pelvic girdle triangular and bearing non-elongate metapterygial axis and male clasper. Anal fin present. Caudal fin heterocercal and approximating the plesodic style. Hypochordal lobe principally supported by proximodistally long radials segmented into a single proximal and single distal array, and articulating to vertebral basiventrals via hemal arch cartilages.
The Bear Gulch taxon is a new genus and species of ctenacanth shark differing from other ctenacanth forms in having dorsal spines with denticulated rows near the base of the spine that grade into smooth costae towards the apex. The dentition of the Bear Gulch taxon is similar to tooth taxa such as Saivodus striatus and Neosaivodus flagstaffensis which have tall narrow median cusps, 3 or more intermediate cusps, and divided basolabial projections. The Bear Gulch taxon differs from these tooth taxa in lacking accessory labial cusplets. Cladistic analysis suggests that the Bear Gulch taxon and eight additional taxa, including Ctenacanthus, form a monophyletic group within Elasmobranchii. A synapomorphy for Ctenacanthiformes is the presence of an anterodorsal process on the otic process of the palatoquadrate (Figure 2A-F). This anterodorsal process articulates on the medial posterior margin of the postorbital process of the neurocranium (Figure 2A-B). This feature has been observed in Ctenacanthus, Heslerodus, an undescribed ctenacanth from New Mexico, “Tamiobatis vetustus”, and the Bear Gulch taxon. The Bear Gulch taxon and “Tamiobatis vetustus” share an additional feature of a dorsal rectangular shaped facet on the otic process of the palatoquadrate. Another synapomorphy is the presence of a large laterally directed otic process on the neurocranium in all ctenacanths except Cladodoides. However, the lack of this feature in Cladodoides may be attributed to ontogeny and/or preservational concerns. Contra Janvier and Pradel (2016), this analysis identifies ctenacanths to be more closely related to crown elasmobranchs (Euselachii) and xenacanths than to other “cladodont” sharks such as the stethacanthids. This analysis also suggests that Cladodontomorphi, as proposed by Ginter et al. (2010), is paraphyletic without the inclusion of Xenacanthimorpha or Euselachii.
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Dean, B. 1909. Studies on fossil fishes (sharks, chimaeroids, and arthrodires). Memoirs of the American Museum of Natural History 9(5): 209-287 Ginter M, O, Hampe, and C.J. Duffin. 2010. Chondrichthyes Paleozoic Elasmobranchii: Teeth. In: Schultze H-P editor. Handbook of Paleoichthyology, vol 3D. 168 pp. Glikman 1964. {Paleogene sharks and their stratigraphic meaning.} -228 pp. (Nauka), Moscow. {in Russian} Grogan, E.D., and R., Lund. 2008. A basal elasmobranch, Thrinacoselache gracia n. gen & sp., (Thrinacodontidae, new family) from the Bear Gulch Limestone, Serpukhovian of Montana, USA. Journal of Vertebrate Paleontology 28(4): 970-988 Grogan, E., R. Lund, and E. Greenfest-Allen 2012. The Origin and Relationships of Early Chondrichthyans. In: Carrier, J.C. & Musick, J.A. & Heithaus, M.R. (eds) Biology of Sharks and their Relatives, Edition 2. CRC Press, Boca Raton, Florida: 3-30 Janvier, P. and A. Pradel. 2016. Elasmobranchs and their extinct relatives: diversity, relationships, and adaptations through time. Physiology of Elasmobranch Fishes: Structure and Interaction with Environment: Volume 34A. pgs 1-17 Maisey, J.G. and J.H. Melo 2005. Some middle Devonia (EiFelian-Givetian) Fossil Fish Remains from the Pimenteira Formation of the Parnaiba Basin, Northeast Brazil. Arquivos do Museu Nacional, Rio De Jaeiro 63(3): 495-505 Schaffer, B. 1981. The xenacanth shark neurocranium, with comments on elasmobranch monophyly. Bulletin of the American Museum of Natural History 169(1): 1-66 Schaffer, B. and M.E. Williams. 1977. Relationships of fossil and living elasmobranchs. American Zoologist 17(2): 293-302
Acknowledgement Special thanks to the Bear Gulch field crew, volunteers, and the Montana ranch families for their longstanding support. Thanks also to Amanda McGee, Lee Hall, and Michael Ryan of CMNH who hosted JPH during his visit to work on Cleveland Shale sharks. This study was enriched by discussions between JPH and John Maisey and Wayne Itano. Thanks are extended to them and for materials they kindly shared. JPH would also like to thank Sarah, Molly, Mark, Lena, mom, and dad for their support during this project.
Author contributions Experimental design by JPH, EDG, and RL Manuscript Written by: JPH and EDG Cladistic and other Analyses by: JPH, EDG, and RL Illustrations by: JPH
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