of north-central New Mexico, are described and referred to Seymouria ... specimens known from New Mexico and provide a more extensive definition of the ...
Seymouria sanjuanensis (Amphibia, Batrachosauria) from the Lower Permian Cutler Formation of north-central New Mexico and the occurrence of sexual dimorphism in that genus questioned DAVIDS BERMAN Carnegie Museum of Natural History, Section of Vertebrate Fossils, 4400 Forbes Avenue, Pittsburgh, PA 15213, U.S.A.
ROBERTR. REISZ Department of Biology, Universiw of Toronto, Erindale Campus, Mississauga, Ont., Canada L5L l C 6 AND
DAVIDA. EBERTH Tyrrell Museum of Palaeontology, P.O. Box 7500, Drumheller, Alta., Canada TOJ OYO Received November 25, 1986 Revision accepted February 16, 1987 Six specimens of the amphibian Seymouria, preserved in a single block of matrix from the Lower Permian Cutler Formation of north-central New Mexico, are described and referred to Seymouria sanjuanensis Vaughn. They are the only Seymouria specimens known from New Mexico and provide a more extensive definition of the species. It is interpreted that the specimens from New Mexico were collected from an early to middle Wolfcampian horizon and therefore represent the earliest known members of the genus. Evidence is presented that challenges previous explanations for the variability of several features of the skull and axial skeleton in specimens of Seymouria baylorensis and S. sanjuanensis as an indication of sexual dimorphism. Differences in the number of maxillary teeth, depth of the maxilla, and development of the maxillary dentition, particularly in the "canine" region, are interpreted as closely related morphological trends in Seymouria. Although no satisfactory explanation is offered for differences in the serial position of the first haemal arch and in the interorbital breadth, sexual dimorphism is considered very unlikely. La description de six spkcimens de l'amphibien Seymouria, pkservks dans un bloc de matrice individuel appartenant 21 la formation de Cutler, d'lge permien infkrieur, dans le centre-nord du Nouveau-Mexique, permet d'assigner ces spkcimens a Seymouria sanjuanensis Vaughn. 11s reprksentent les seuls spkcimens de Seymouria reconnus dans le Nouveau-Mexique et ils contribuent a mieux dkfinir l'espkce. Ces spkcimens du Nouveau-Mexique sont interprktks comme ayant Ctk collectionnks dans les strates du Wolfcampien infkrieur et moyen, et par conskquent ils reprksentent les plus anciens membres reconnus de ce genre. Leur description remet en cause les explications antkrieures concernant les dissemblances dans les particularitks du cfine et du squelette axial des spkcimens de Seymouria baylorensis et de S. sanjuanensis qui servaient a dkfinir le dimorphisme sexuel. Les diffkrences dans le nombre de dents maxillaires, de la profondeur de la maxille, et du dkveloppement de la dentition maxillaire, particulikrement dans la kgion des N canines >>, sont interpktkes comme Ctant des tendances morphologiques t&s voisines dans les Seymouria. M&mesi aucune explication satisfaisante n'est offerte des differences dans la position sCriCe du premier arc hCmal et de la largeur interorbitale, il est plus probable que ce dimorphisme sexuel n'existe pas. [Traduit par la revue] Can. J. Earth
Sci. 24, 1769-1784
(1987)
Introduction The occurrence of the well-known amphibian Seymouria from the Lower Permian Cutler Formation of north-central New Mexico (Fig. 1) was briefly reported recently by Eberth and Berman (1983). Subsequent preparation of the block containing the specimens has revealed six articulated skeletons, five of which are well preserved and nearly complete (Fig. 2). These specimens, the first to be reported from the state, are referable to Seymouria sanjuanensis Vaughn (Vaughn 1966a), known otherwise only from beds of late Wolfcampian age in Utah, and provide a more extensive definition of the species. Seymouria also occurs in the middle Wolfcampian to early Guadalupian Permian classic collecting beds of Texas and Oklahoma (see Fig. 5). Here, in addition to the relatively common, well-known S. baylorensis known mainly from the Leonardian Arroyo Formation of Texas, two other poorly known species, S. grandis and S. agilis, have been described from the late Leonardian and early Guadalupian of Texas and Oklahoma (Olson 1979, 1980). The earliest known occurrences of Seymouria specimens in the Texas-Oklahoma region are from the late Wolfcampian Putnam and Admiral Printed m Canada 1 lmprimt au Canada
formations of Texas; these specimens are, unfortunately, too incomplete to safely assign to species (Olson 1980; Vaughn 1 9 6 6 ~ ) .The New Mexico specimens of S. sanjuanensis described here are early to middle Wolfcampian age and therefore represent the earliest known members of this genus. The preservation of the New Mexico specimens as a lot in a single block (Fig. 2) provides an excellent opportunity to test previous hypotheses that explain the variability of certain osteological features in specimens of Seymouria baylorensis and S. sanjuanensis as attributable to sexual dimorphism. The presence of Seymouria in the Lower Permian of north-central New Mexico also has a significant bearing on hypotheses that attempt to explain Permo-Pennsylvanian faunal distribution patterns in New Mexico, as well as in the southwestern United States. Further studies of the anatomy and relationships of the New Mexico specimens are planned pending extensive, detailed preparation. In particular, because Seymouria is still often viewed as intermediate between amphibians and reptiles, it is hoped that the New Mexico specimens will provide new information on the important question of the evolution of the reptiliomorph grade of organization. In addition, new data are
CAN. J. EARTH SCI. VOL. 24, 1987
FIG. 1. (A) Locality map showing distribution of outcrops of Permo-Pennsylvanian Cutler Formation (here including the Abo Formation of San Diego Canyon) of north-central New Mexico (stippled) and location of stratigraphic sections (numbers) shown in Fig. 3. (B) Detailed map of the cross-hatched inset of (A) showing locality from which the Seymouria sanjuanensis specimens described here were collected and localities of stratigraphic sections in Fig. 4.
anticipated that will help to reassess the still problematic relationships of the seymouriamorphs with the other batrachosaurs. Abbreviations CM, CNHM, MCZ, NTM VP, TMM, and
USNM are used to refer to collections of the Camegie Museum of Natural History, Field Museum of Natural History (Chicago), Museum of Comparative Zoology (Harvard University), Navajo Tribal Museum (Window Rock, Arizona),
BERMAN ET AL.
177 1
FIG. 2. Photograph showing six New Mexico specimens of Seymouria sanjuanensis as p r e s e ~ e din block from the Cutler Formation.
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Texas Memorial Museum (University of Texas), and United States Museum of Natural History, respectively. Stratigraphy and sedimentology The Cutler Formation occurs throughout north-central New Mexico as a thick, southwestward-thinningclastic wedge. The sediments of the Cutler Formation are derived from the ancient Uncompahgre highland to the north and northeast that extended from Colorado into New Mexico during PermoPennsylvanian time. The Cutler Formation is here interpreted to include the Abo Formation of San Diego Canyon (Fig. 1A) (Eberth 1986). Although there is no doubt that Cutler Formation deposition took place across the Pennsylvanian- Permian boundary (Smith et al. 1961; Fracasso 1980; Hunt 1983; Eberth 1986), the exposures in the Rio Puerco valley from which the referred specimens described here were collected are interpreted as Wolfcampian in age. This interpretation is based on a study of stratigraphic variation in average grain sizes and the application of data from many separate biostratigraphic studies (Eberth 1986). Eberth (1986) demonstrated that portions of three megasequences are present in the exposures of the Cutler Formation in three separate erosional valleys in north-central New Mexico that include not only the Rio Puerco valley but also El Cobre Canyon and San Diego Canyon. By utilizing a combination of litho-, tecto-, and bio-stratigraphic data, the megasequence portions were fitted together to produce a stratigraphic model (Fig. 3). The Seymouria specimens described here were collected from the middle of the finingupward portion of megasequence 2. The ages of the megasequences were determined (with varying degrees of certainty) by assessing previous studies and new collections of both invertebrate and megaplant fossils. Based upon the assumption that rates of sediment input, aggradation, and subsidence remained relatively constant during the deposition of megasequence 2, we hypothesize that the New Mexico Seymouria specimens are no younger than middle Wolfcampian. The locality of the specimens described here occurs in the Rio Puerco valley near the Abiquiu Reservoir (Fig. 1). The sedimentology of the Cutler Formation sediments has been studied in detail by Eberth (1986). In this area and at this horizon (megasequence 2) the sediments comprise a complexly interbedded sequence of laterally limited lithofacies that include ribbon and sheet sandstone bodies, minor sandstone bodies (less than 1 m thick), U-shaped mixed-fill sandstone - siltstone-mudstone bodies, laterally persistent mudstone - siltstone units, and pedogenic and lacustrine carbonates (Fig. 4). The dominant sedimentary structures preserved in the major sandstone ribbons and sheet bodies include large-scale trough cross-bedding, very low-angle cross-bedding, and horizontal lamination with parting lineation. The megasequence 2 sediments were deposited in a high-energy fluvial system that was dominated by ephemeral flow conditions (high-energy, seasonal flooding). Low-sinuosity channels, rarely measuring more than 200 m wide, aggraded vertically and changed their position on the floodplain largely through avulsion. The climate was semi-arid, as indicated by the presence of pedogenic carbonates (calcretes or caliches). The Seymouria specimens described here were collected from a 5 -7 m thick, massive siltstone-mudstone unit rich in iron oxide (hematite) (Fig. 4). The matrix surrounding the specimens is a massive siltstone. No sedimentary structures are preserved in this horizon, and on the basis of grain size and the lack of a channel-like geometry we assume that the environ-
ment of deposition was a floodplain. Outcrop limitations prevent any reliable calculation of the distance from the locality to the source of flooding. However, the presence of nodular caliches at discrete horizons within the 5 -7 m thick unit both above and below the fossiliferous horizon indicates that this region of the floodplain may have remained unaffected by flooding for periods of as much as 3000-6000 years (Gile et al. 1966; Leeder 1975; Netterburg 1980), indicating a relatively distal position for the active channel. Systematic paleontology ORDER Batrachosauria SUBORDER Seymouriamorpha FAMILY Seymouriidae GENUS Seymouria Broili, 1904
Seymouria sanjuanensis Vaughn, 1966 Revised diagnosis Two derived cranial features (identified using primitive temnospondyls and batrachosaurs for outgroup comparison) distinguish Seymouria sanjuanensis from the most closely related species Seymouria baylorensis: (1) postorbital is narrower and chevron-shaped with the limbs of the chevron giving a greater dorsoventral contribution to the posterior margin of the orbit and the apex of the chevron projecting posteriorly and separating partially the intertemporal and squamosal and (2) maxilla and quadratojugal narrowly separated rather than in contact. The following primitive features separate S. sanjuanensis from S. baylorensis: (1) percent ratio of the anteroposterior length of the postparietal to total skull length is greater, ranging from 10.2 to 11.1; (2) process formed by downtumed postemlateral comer of tabular far less developed; (3) greater number of maxillary teeth, ranging from 25 to 30; (4) lesser development of maxillary dentition, with percent ratio of the average unit length of the maxilla per tooth to total skull length equal to 2.3 or less; and (5) lesser development of "canine" region of maxillary dentition, with a ratio of the diameter of the largest maxillary tooth to total skull length less than 0.025. Holotype NTM VP 1024, described by Vaughn (1966a, p. 604) as "a dorsoventrally flattened skull lacking the anteriormost part of snout and portions of the dermal skull roof in the interorbital region, but well preserved otherwise . . . " Paratypes Specimens listed by Vaughn (1966~): NTM VP 1023, slightly distorted, partial skull with parts of shoulder girdle; NTM VP 1025, skull roof represented as impression, articulated with anterior vertebrae, parts of shoulder girdle, and parts of front limbs; NTM VP 1026, skull roof represented mainly as impression, articulated with anterior vertebrae and parts of front limbs. Referred specimens Six specimens scattered throughout the same block as the holotype and paratypes but that could not be positively associated with any of the skulls were described as follows by Vaughn (1966a, pp. 604-605): "NTM VP 1027 consists of an innominate bone and attached proximal part of femur; 1028 is a manus, with impressions of epipodials; 1029 includes posteriormost dorsal, sacral, and anteriormost caudal vertebrae, and a femur, tibia and fibula; 1030 is a string of about a half dozen caudal vertebrae; 1031 is a partial shoulder girdle
BERMAN ET AL.
SW
NE
Mega-
"'/ 3
2 Mega1
Exposed Stratigraphic
brown, grey
01 11
Sections
X - Collecting Horizon
TestWell cuttings
A - Anderson q u a r r y
---. --
B - B a l d w i n bonebed
---
+
J - J o h n s o n 1 ocal it y C
Marker Horizons: 000000 F i r s t o c c u r r e n c e o f extraformational pebbles; mM a s s i v e c a l i c h e horizon
'1-
- C l a s s i c c o l l e c t i n g area i n El Cobre Canyon
FIG.3. Stratigraphic model illustrating relationships of Permo-Pennsylvanian deposits of north-central New Mexico. Sections 1 , 2 , 3 , 4 , and 7 are composites indicating maximum thicknesses of exposures. Sections 5 and 6 are based on test-well cuttings (for details see Ebelth 1986). Section localities are indicated in Fig. 1A. Vertical scale is not attached to any datum and shows only relative thicknesses of sections. The stlatigraphic position of Seymouria sanjuanensis specimens described here is indicated by asterisk of section 4. The stratigraphic positions of wellknown fossil localities are indicated by letters (for detailed information see Langston 1953; Fracasso 1980; Eberth 1986).
CAN. 1. EARTH SCI. VOL. 24, 1987
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I
Masswe Cal~cheHorizon
I
1.
r!~!
3 57:;.
05
Massive Sandstone Bodies Low-Angle t o Horizontal Laminated Sandstone Cross-Bedded Sandstone lntraformational
Pebble
C ] ~ u d s t o n e s , Siltstones Caliche
*
Collecting
Horizon
Lags
FIG.4 . Measured stratigraphic sections from vicinity of collecting area of the New Mexico Seymouria specimens. Locations of sections are given in Fig. 1B. As shown in Fig. 1B, collecting locality occurs between sections 6110182 and 7/9/8212. Abbreviations: Si, siltstone-mudstone; F, fine sandstone; M, medium sandstone; C, coarse sandstone; V, very coarse sandstone.
and attached proximal part of humerus; and 1032 consists of a partial innominate bone and attached femur." Vaughn also lists six partial skeletons preserved in a single block from a second nearby site and consisting of the greater parts of their vertebral columns. Of these specimens, NTM VP 1033, 1034,
and 1035 also include small portions of the skull and partial pectoral and pelvic girdles and limbs, whereas 1036, 1037, and 1038 are less complete and include only fragments of limb girdles and free limbs. Referred specimens described here for the first time from the
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BERMAN ET AL.
Cutler Formation of New Mexico were preserved in a single block (Fig. 2) and include CM 38022, nearly complete, articulated skeleton missing parts of fore- and hind-limbs, and tail; CM 28596, nearly complete, articulated skeleton missing forelimbs and most of hindlimbs. CM 28597, nearly complete, articulated skeleton missing most of the fore- and hind-limbs, and tail; CM 28598, most of the skull, shoulder girdles, and presacral vertebral column in articulation; CM 28599, essentially complete, articulated vertebral column, and portion of pelvic girdles; CM 34900, small portion of vertebral column that includes the last four presacrals, sacrals, and first three caudals, and parts of pelvic girdles.
that species here. Also, the S. baylorensis specimen USNM 9140,available to this study as a cast, was recognized as Conodectes favosus Cope by White (1939), who presented only meager evidence to support the recognition of this genus. More recent opinion (Romer 1947; Olson 1980) recognizes C. favosus as generically identical with Seymouria. Though C.favosus was described prior to S. baylorensis, it seems best to follow Olson's (1980) recommendation to retain Seymouria and drop Conodectes because Cope's (1896) original description of the latter is brief, lacks illustrations, and is based on only a poorly preserved skull fragment. It is important to note that the different states of preservation, of not only the New Mexico skulls but also those used Horizon and locality here for comparison with them, have undoubtedly resulted in The locality of the holotype, paratypes, and referred specisome inaccuracies in the measurements of Table 1. For mens NTM VP 1027, 1028, 1029, 1030, 1031, and 1032 and example, the deep, box-like structure of the skull is preserved that of the referred specimens NTM VP 1033, 1034, 1035, in only a couple of specimens, the others exhibit a wide range 1036, 1037, and 1038 are separated by about 2.8 krn in San of dorsoventral flattening. In addition, the Utah skulls of Juan County of the southeastern comer of Utah (see Vaughn Seymouria sanjuanensis are incomplete and Vaughn (1966~) 1966a for locality data). Both localities are reported as being at estimated their overall lengths. Such inaccuracies would approximately the same horizon in the lowermost Organ Rock account for at least some of the small differences in the cranial Shale, Cutler Group, and equivalent to some level in the upper proportions between the Utah and New Mexico specimens of part of the Wichita Group of Texas. Although Vaughn (1966~) S. sanjuanensis. More importantly, although the percentage suggested an early Leonardian age for these sediments, recent ratios of cranial measurements presented in Table 1 appear to studies support a late Wolfcampian assignment (Blakey 1980; exhibit consistent differences for distinguishing between Vaughn 1970). The referred specimens from New Mexico S. baylorensis and S. sanjuanensis, in some instances only were collected from a lower portion of the exposed section very small inaccuracies in the measurements of the maxillae or (Fig. 4) of the Early Permian Cutler Formation, estimated to postparietals of a few tenths of a millimetre would alter their be early to middle Wolfcampian in age, about 5.9 krn north of values enough to suggest an alternative assignment. We interYoungsville, Rio Arriba County, in the north-central part pret the closeness in the ratio values of the two species as of the state at approximately lat. 36°14'30f 'N and long. reflecting a closeness in grades of morphological trends, and 106O32'7.5"W (Fig. 1). therefore even some overlap in values would not be unexpected. Except where otherwise stated, we have accepted the Description and comparison cranial measurements given by Vaughn (1966~)for the Utah specimens of S. sanjuanensis. The anatomy of Seymouria sanjuanensis requires only a brief account because of the detailed description by White All the skulls of the New Mexico specimens of Seymouria (1939) of the closely related species S. baylorensis. The close sanjuanensis exhibit moderate dorsoventral crushing (Fig. 6). The sculpturing, where preserved, is moderately developed, similarity between these two species is brought out in Vaughn's (1966~)description of S. sanjuanensis in which he but lateral lines are not detectable. Most sutures are not closed recognized only one distinguishing feature. However, features and are traced easily. A reconstruction of the skull is given in that he attributed to sexual dimorphism are reinterpreted below Fig. 7. The only diagnostic feature recognized with certainty by as probably representing evolutionary trends and utilized to h of Seymouria Vaughn (1.966~)to separate the ~ t a specimens distinguish between the two species. Two other species of sanjuanensis from S. baylorensis was the greater anteroposSeymouria have been described: S. grandis from the Lower tenor extent of the postparietals, in which the postparietals Permian Leonardian of Texas and Oklahoma (Olson 1979) and occupy about 11% of the total skull length. In the New Mexico S. agilis from the Upper Permian Guadalupian of Oklahoma specimens of S. sanjuanensis the percentage is less, (Olson 1980) (Fig. 5). Though relatively poorly known, they 10.2-10.7%, but still greater than the 8.9-9.6% of the exhibit features that allow them to be easily distinguished from S. baylorensis specimens of Texas (Table 1). As Vaughn coneither S. sanjuanensis or S. baylorensis, and they need not be vincingly argued, the greater extent of the postparietals in considered here. The description of S. sanjuanensis that S. sanjuanensis is undoubtedly a primitive feature. The outline follows will, therefore, emphasize mainly those features that of the postorbital can also be used to separate the two species are useful in distinguishing S. sanjuanensis from S. baylorof Seymouria. In S. sanjuanensis the postorbital is a very ensis or add to the description of the genus. Cranial measurenarrow, chevron-shaped element in which the limbs of the ments in Table 1 are for the S. sanjuanensis specimens from chevron form the posterior border of the orbit and the apex proNew Mexico and Utah, and specimens of S. baylorensis from Texas. Among the Texas specimens of S. baylorensis conjects posteriorly t o separate partially the intertemporal and squamosal. In S. baylorensis, on the other hand, the postsidered here, the skull TMM 31189-1 (mistakenly referred to as UT BEG 31 187-1 by Vaughn (1966a) ) is the only one not orbital is subrectangular with a slightly convex posterior border and does not extend as far dorsally and ventrally along from the Arroyo Formation, Clear Fork Group; it is from a considerably lower horizon, probably close to the boundary the orbital rim or project strongly between the intertemporal and squamosal. A postorbital shaped like that of S. baybetween the Admiral and Belle Plains formations, Wichita lorensis, particularly its lesser dorsoventral extent along the Group (Fig. 5). Though intermediate in several characters posterior margin of the orbit, is typical of temnospondyls and between S. sanjuanensis and S. baylorensis, it is closer to the batrachosaurs. The narrow, chevron-shaped postorbital of latter in overall form and is, therefore, tentatively referred to -
-
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CAN. J. EARTH SCI. VOL. 24. 1987
1
New Mexico
I r-l
Flowerpot S. agilis
...............................
............................
San Angelo
Arroyo
S. baylorensis
Lueders Clyde
1
(
.........................
............................
sanjuanensis
types &
Plains Admiral Frn.
iM-3
referred specimens referred specimens
I
I
31189-1) .,,..........................
Seymouria sp.
Seymouria sp.
3 sanjuanensis
PTMM
Moran
Pueblo
........................... Cisco
Fm.
7
FIG.5. Stratigraphic occurrences of Seymouria. Where tick marks indicating stratigraphic positions of specimens are absent, level within given formation is uncertain. Stratigraphic correlation chart of selected Permian and Pennsylvanian formations in southwestern United States compiled from a variety of sources (see Eberth 1986). Stratigraphic positions of the Oklahoma specimens S. agilis and S. grandis (in part) are shown according to their Texas equivalents.
S. sanjuanensis is, therefore, considered a derived state. The maxilla and quadratojugal narrowly contact each other on the lateral skull surface, and the laterally downturned, posterolateral comer of the tabular forms a well-developed process or horn in Seymouria baylorensis, whereas in TMM 31189-1 and S. sanjuanensis the maxilla and quadratojugal are narrowly separated, and the process formed by the laterally downturned posterolateral comer of the tabular horn is very weakly developed. A maxillary -quadratojugal contact is typical of most primitive temnospondyls and batrachosaurs, suggesting that the narrow separation of these two elements in S. sanjuanensis is a derived feature. On the other hand, the laterally downturning of the tabular horn is unique among temnospondyls and batrachosaurs, and therefore its less pronounced state in S. sanjuanensis is judged as primitive. Only the upper marginal dentitions are available for descrip-
tion in Seymouria sanjuanensis; the lower jaws are either absent or their dentitions are not accessible. As in S. baylorensis, the premaxilla holds six teeth of subequal size, but they are noticeably smaller, measuring up to about 3.5 4.0 mm in length and about 1.0 - 1.2 mm in basal diameter. As indicated in Table 1, the number of maxillary teeth in the New Mexico specimens of S. sanjuunensis, 28-30, is slightly greater than that of the Utah specimens, about 25, which in turn is considerably greater than the 16 or 17 possessed by the Clear Fork Group specimens of S. baylorensis. The 21 maxillary teeth of TMM 31189-1 from the lower Wichita Group place it in an intermediate position between the S. sanjuunensis specimens and the other Texas specimens of S. baylorensis. In S. sanjuanensis the maxillary teeth reach their maximum size at tooth positions 5 -7, measuring as much as about 3.8 mm in length and about 1.5 mm in basal diameter. Posterior to this
FIG.6. Photographs of Seymouria sanjuunensis skulls CM 38022 and CM 28596 in dorsal view, and CM 28597 in dorsal and ventral views.
CAN. J.
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EARTH SCI. VOL. 24, 1987
TABLE1. Skull measurements (in rnm) of specimens of Seymouria baylorensis and S. sanjuanensis
EE!L Specimen Seymouria sanjuanensis CM 38022 CM 28596 CM 28597 NTM VP 1023 NTM VP 1024 NTM VP 1025 NTM VP 1034 Seymouria baylorensis TMM 31 189-1 CNHM 663 MCZ 1081 MCZ 1082 MCZ 1083 MCZ 1085 USNM 15553 USNM 15554 USNM 9140
SkL
PpL
SkL
IoB
IoB SkL
93.0 86.0 88.0 90.0 90.0 c87.0
10.0 9.0 9.0 10.0 10.0
10.7 10.5 10.2 11.1 11.1
33.0 32.0 31.0 35.0 35.0 32.0
35 36 35 39 39 37
99.0 114.0 115.0 103.5 ?110.0 114.0 100.0 124.0
9.3
9.4
11.0 9.2
9.6 8.9
MxD
MxL
MxD SkL
MxT
MxLIMxT SkL
7.3 8.0 10.0
51 51 51 51
8 9 11
30 29 28 25
1.8 2.0 2.1 2.3
10.0
51
9.8 15.0 15.0 14.0 15.0 16.0
58 57 62
-.
38.5 ?40.0 33.0 31.0 36.0 33.0 43.0
39 35 29 30
62 60
?24 10 13 13 14 14
21 17 16 17 17 16
2.8 2.9 3.4 3.3
32 33 35
NOTES: (1) All proportions given as percentages. (2) Abbreviations: c, calculated; IoB, interorbital breadth; MxD, maxillary depth; MxL, maxillary length; MxT, number of maxillary teeth; PpL, postparietal length; SkL, skull length.
FIG.7. Reconstruction of skull of Seymouria sanjuanensis in (A) dorsal and (B) lateral views. Based on New Mexico specimens of Fig. 6.
level the teeth vary randomly but not greatly in size, sometimes reaching a length of about 3.0 mrn and a basal diameter of about 1.3 mm. The last five or six teeth exhibit a marked decrease in size. In none of the S. baylorensis skulls available to us were the maxillary teeth complete. Their size distribution pattern, however, appears to be about the same as that of S. sanjuanensis, except that the "postcanines" tend to show, as White (1939) illustrated, a more or less gradual decrease in overall size posteriorly. Both tooth size distribution patterns
are suggested by the graph of Fig. 8, even though overall size is based on basal diameter. The graph of Fig. 8 depicts the relative development of the maxillary dentitions for specimens of both species. Relative size of the maxillary teeth is plotted as the ratio of their basal diameters to skull length; tooth diameter is used as an index to overall size because the teeth are seldom complete. The graph ) for the is identical to that presented by Vaughn ( 1 9 6 6 ~except additional plots of Seymouria sanjuanensis specimens C M
BERMAN ET AL. 0.040
ysOT,, I
I I
0.035
I
,
(
-NTM VP 1034
-
!
I 9
-------
't
!?
I -.,
C M 28596 31189-1 MCZ 1082
......,......... T~~
L* 0
5
POSITION
10
OF
RIGHT
15
MAXILLARY
20
25
,,
TOOTH
FIG. 8. Graph depicting relative development of maxillary dentitions in Seymoun'a baylorensis specimens MCZ 1082 and TMM 31189-1and S. sanjuanensis specimens CM 28596 and NTM VP 1034 by comparison of diameters of right maxillary teeth with total skull lengths. Numbering of tooth positions from anterior end of maxilla. Graph modified from Vaughn (1966a).
28596 and NTM VP 1034. It should be emphasized that where measurements of other specimens can be substituted the comparisons remain essentially the same. The well-preserved, disarticulated maxilla of NTM VP 1035 was used despite the fact that the skull is too incomplete to measure its total length. However, since the length of the maxilla is the same as that for NTM VP 1023, the calculations were made using the skull length of the latter. Depicted clearly in the graph is a marked difference between the two species in the developments of the maxillary dentitions. The Wichita Group skull TMM 3 1 189-1, though again occupying a position between the plots representing S. sanjuanensis and the Clear Fork Group specimen of S. baylorensis, is much closer to the latter. As an additional way of expressing the difference in the relative, overall developments of the maxillary dentitions between the species, the percent ratios of the average unit length of the maxilla per tooth to overall skull length are calculated in Table 1; this reveals a distinct gap between the range of values for S. sunjuanensis, 1.8-2.3, and that for S. baylorensis, 2.8 - 3.4. The graph of Fig. 8 also depicts an accentuated development of the "canine" region of the tooth row in S. baylorensis compared with that of S. sanjuanensis. The selection in the Revised diagnosis of the ratio 0.025 for the largest "canine" tooth to skull length as the value for separating the two species was arbitrary. With an increase in the development of the maxillary dentition there is a corresponding increase in the height of the maxilla; this is most noticeable at the level of the "canine" region, where the maxilla attains its greatest height. Increase in the relative size of the maxilla is calculated in Table 1 as the percent ratio of the maximum height of the maxilla to total skull
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length. With one exception there is a small but distinct jump between the range of values for that of S. sanjuanensis specimens and that of S. baylorensis specimens; the ratio for TMM 31189-1 falls within the range of the S. sanjuanensis specimens. The exact number of 24 presacral vertebrae can be determined in only the New Mexico specimens CM 28596 and 28597. In only one of the Utah specimens (NTM VP 1033) of Seymouria sanjuanensis was the presacral column complete enough for Vaughn (1966~)to estimate a presacral count of at least 24, or perhaps 25, vertebrae. White (1939) described 24 presacral vertebrae in one specimen of S. baylorensis, but noted Williston's (1911) and Watson's (1918) counts of 23 as probably the normal number for that species. In none of the New Mexico specimens of S. sanjuanensis is there evidence of more than one sacral vertebra. In CM 28596 the rib of the vertebra following the sacral vertebra, though very short, curves strongly posteriorly and could not have contacted the ilium or joined the sacral rib to form a second functional sacral vertebra as White (1939) described in two specimens of S. baylorensis. A complete caudal series is not present in any of the New Mexico specimens. The first six caudal vertebrae possess ribs, and the first haemal arch appears on the sixth, or last, caudal that has a rib. White noted that the serial position of the first haemal arch varies in specimens of S. baylorensis by being either on the third or the sixth caudal. It should be remembered, however, that in numbering the caudal vertebrae of S. baylorensis specimens White recognized a second sacral vertebra, so that the first caudal in the S. sanjuanensis specimens from New Mexico is equivalent to the second sacral in his numbering scheme. As discussed in the following section (Sexual dimorphism), White suggested that the varying position of the first haemal arch was sex-linked. The atlas-axis complex is preserved, at least in part, in all the specimens except CM 34900; however, only those of CM 38022 and 28599 were removed from the skeletons and fully prepared (Fig. 9), and a reconstruction (Fig. 10) is based on these specimens. The complexes of both specimens are only slightly disturbed; that of CM 38022 is complete, whereas that of CM 28599 is missing almost all of the atlantal neural arch. The atlantal intercentrum and the axial intercentrum and pleurocentrum of CM 38022 were separated in articulation from the rest of the complex. The atlantal neural arch consists of paired halves. Each arch half has a long, prominent spine that projects dorsally across the anterolateral surface of the axial spine, terminating slightly below the top of the axial spine. Though the spine of the left arch of CM 38022 is complete, only the distally incomplete spine of the right arch is illustrated. The anterior zygapophysis is greatly reduced to a lip-like process on the anterior edge of the neural arch; its anterodorsal-facing articular surface presumably supported the proatlas. In none of the specimens, however, has it been possible to identify any element as a proatlas. The posterior zygapophysis is only slightly smaller than those of succeeding cervicals, and its articular surface faces posteroventrally. The neural arch does not possess a transverse process, and there is no sign of an atlantal rib. Ventrally the pedicel of each archhalf curves strongly medially just above the facet for the pleurocentrum as a bluntly rounded process that probably contacted, but did not fuse with, its mate. In lateral view the atlantal intercentrum is a very low, triangular wedge, and in end view has the form of a thin-walled, low crescent. In ventral view the anterior margin is expanded into a strongly convex arc, whereas the posterior margin is slightly concave.
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FIG.9. Atlas-axis complexes of Seymouria sanjuanensis. Complex of CM 38022 in (A) lateral and (B) ventral views with atlantal intercentrum and axial intercentrum and pleurocentrum detached and shown in (C) ventral and (D) dorsal views. Complex of CM 28599 in (E) lateral view with atlantal and axial intercentra detached and shown in (F) ventral view. Abbreviations: ati, atlantal intercentrum; atn, atlantal neural arch; axi, axial intercentrum; axn, axial neural arch; axp, axial pleurocentrum; ic, intercentrum; n, neural arch; p, pleurocentrum.
There are no facets for a rib. In neither CM 38022 nor 28599 was an atlantal pleurocentrum found, but there is reason to suspect that this element remained cartilaginous. In the Seymouria baylorensis specimen CNHM 6138, which includes the first 12 articulated vertebrae, a left atlantal pleurocentrum is clearly visible despite the generally poor quality preservation of the atlas-axis complex. The pleurocentrum is a very small kidney-shaped element, measuring about 6 mm in vertical height and about 1.8 mm in horizontal width. Though the pleurocentrum is displaced a short distance ventrally, its ventral end undoubtedly wedged between the dorsal apices of the lateral ascending processes of the atlantal and axial intercentra but did not reach the ventral surface of the column. CNHM 6138 is considerably larger than CM 28559 or 38022, and if the transverse width of the 10th vertebra is used for size comparisons, then it is about 87 and 95 % larger respectively. This suggests that the atlantal pleurocentrum in Seymouria may not have ossified until a late adult stage of development.
The axial neural arch, best exemplified in CM 38022, is the most massive element of the cervical series. The neural spine is considerably higher than those of succeeding vertebrae and has a large anterior expansion that extends between the paired spines of the atlantal neural arch. A prominent lateral ridge lies close to and parallels the posterior margin of the spine. The anterior zygapophysis is weakly developed, and its articular face slopes anteroventrally. In contrast, the posterior zygapophysis is noticeably larger than those of immediately succeeding vertebrae, but, as in all the postaxial trunk vertebrae, the plane of its articulation with the anterior zygapophysis of the following vertebra is horizontal. The transverse process extends laterally and very slightly posteriorly. It is suboval in cross section and ends in a rib facet of the same outline shape. The processes of the more posterior cervicals increase in diameter through an anteroventrally oriented axis and therefore have a very oblong outline in cross section. The axial intercentrum is like that of the atlas in appearing as a low wedge in
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men. With the chevrons beginning with the third vertebra on the specimens formerly studied and with the sixth in the material at my disposal it seems probable that the same skeletal differences existed between the sexes in Seymouria as we find in the Testudinata and Loricata. Also, the males possessed a single intromittent organ because the paired hemipenes of lizards and snakes do not alter the disposition of the chevrons between sexes. In view of this, the skeletons studied by Profs. Williston and Watson were females and those collected by Dr. Olson were males.
FIG. 10. Restoration of atlas-axis complex of Seymouria sanjuanensis based on CM 38022 and 28599.
lateral view and as a crescent in end view. As best exhibited in CM 38022, in ventral view its slightly convex anterior margin abutts smoothly against the entire posterior margin of the ventral surface of the atlantal intercentrum. A facet for a rib is seen at the posterolateral margins of the ventral surface of CM 28599. The axial pleurocentrum has the shape of an inverted wedge in lateral view, its flattened dorsal surface forming the floor of the neural canal. The axial pleurocentra of CM 38022 and 28599 are unequally developed. In the less developed CM 38022 the ventral apices of the lateral descending processes curve medially, but are widely separated from contact on the ventral midline of the column. In the much more fully developed pleurocentrum of CM 28599, on the other hand, the thicker descending processes form a complete ring to enclose the notochordal canal, project between the lateral ascending processes of the intercentra of the axis and third vertebra, and probably narrowly entered the ventral surface of the column.
Sexual dimorphism White (1939) noted an interesting variation in the serial position of the first caudal haernal arch of Seymouria, which he interpreted as possibly representing a sexually dimorphic character. Because evidence is presented here that challenges this interpretation, it is important to note precisely the data on which this hypothesis is based. This is most expediently done by quoting at length from White's discussion (pp. 356-357): In the . . . specimen of Conodectes favosus Cope ( U .S.N .M. No. 9140) [= Seymouria; see Romer 1947; Olson 19801 the intercentrum of the sixth vertebra bears two short backwardly directed processes on the ventral surface which represent the first haemal arch. The sixth vertebra, as in the specimens studied by Profs. Williston and Watson, is the last vertebra to bear a rib. In the material which Dr. Olson collected (W.M., Nos. 166 and 167) the haemal arches begin with the last vertebra to bear a rib. Williston states (191 1, p. 55) that a haemal arch is present on the intercentrum of the fourth caudal vertebra. If we subtract the vestigial second sacral which he considered the first caudal, the haemal arches begin with the third intercentrum. Watson (1918) does not state which intercentrum carries the first haemal arch, but in his restoration he places it on the fourth. Correspondingly the chevrons would begin with the third as in Williston's speci-
Several points related to the above statement require clarification or comment. The axial materials associated with the skulls described by White were completely disarticulated and therefore of no value in determining the position of the first haemal arch. Of the above materials mentioned by White, those described by Williston (1911) and Watson (1918) were not available to him. Though there is no reason to doubt White's observation that in the material collected by Olson the haernal arches begin with the last vertebra to bear a rib, no direct evidence is given to demonstrate that in this instance the last caudal to bear a rib is the sixth. This is probably because only short series of caudals were present in this material. In none of the several specimens of Seymouria sanjuanensis described by Vaughn (1966~)were the caudal materials well enough preserved to demonstrate the serial position of the first haernal arch, and as he points out, adequate caudal materials of Seymouria are unfortunately so rare as to prevent statistical confirmation of White's theory. Evidence most critical to the theory that the serial position of the first haemal arch is sex-determined is seen in the Seymouria specimens described here from New Mexico and referred to S. sanjuunensis. Only in CM 28596 and 28599 are the anterior caudals well enough preserved to determine without doubt the serial position of the first haemal arch. In both cases the haemal arch series begins on the sixth caudal intercentrum. This corresponds, however, to the fifth caudal intercentrum and not the third or sixth when using White's (1939) numbering scheme, which recognizes a second sacral vertebra. ~ o w e v e r , a White s described in the supposed male specimens of S. baylorensis, the first haemal arch appears on the last caudal to possess a rib and consists of two short, posteriorly directed p~ocesses.The above observations suggest that differences in the serial position of the first haemal arch and the last caudal rib more likely reflect some other factor, perhaps (1) random variations between populations, (2) different stages of structural trends of the genus, or (3) different independent evolutionary histories or lineages. The plausible but less likely explanation of sexual dimorphism remains; if accepted, however, the two New Mexico specimens of S. sanjuanensis (CM 28596, 28599) in which the first haemal arch and the last rib both occur on the fifth caudal vertebra of White's numbering scheme would not only have to be considered males but be considered as possessing a somewhat smaller intromittent organ, or penis, than the male specimens of S. baylorensis. Taken one step further, it could then be suggested that within the genus there was selection for increased size of the intromittent organ. Vaughn (1966~)recognized three variable cranial features of Seymouria as possibly being sexually dimorphic: (1) interorbital breadth, (2) depth of the maxillary bone, and (3) development of the maxillary dentition, particularly in the "canine" region. His initial approach in determining how the values of these features varied with sex was by correlation with a specimen whose sex was predetermined using White's
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(1939) method of the serial position of the first haemal arch. Among the Seymouria specimens he studied, Vaughn could correlate the expressions of the above cranial features with the sex of the individual, using the position of the first haemal arch, in only the specimen CMNH 663 as described by Williston (191 1). The sex of the other specimens was then determined by comparison of their skulls with that of CMNH 663. Thus, Vaughn notes that in calculating the ratios of interorbital breadth to total skull length and greatest depth of the maxillary bone to total skull length the values for the skulls NTM VP 1023 and 1024, TMM 31 189-1 (not UT BEG 31 187-1 as in Vaughn 1966a), and CMNH 663 express a near-linear relationship into which the value for the skull MCZ 1082 cannot be fitted. From this comparison, Vaughn identified the former group as females (because it includes the presumed female CMNH 663) with the presumed sexually dimorphic features of relatively wider spaced orbits and shallower maxillae, and the latter specimen as a male with relatively narrower spaced orbits and deeper maxillae. To demonstrate that Seymouria may be sexually dimorphic in the relative development of the maxillary dentition, Vaughn plotted the ratios of the tooth diameters to total skull length for various tooth positions in the presumed male and female skulls MCZ 1082 and TMM 3 1189-1, respectively; if tooth diameter is accepted as a rough index to overall tooth size, then MCZ 1082 does appear to exhibit, as Vaughn described, a greater degree of development of the maxillary dentition in general and of the "canine" region in particular than TMM 31 189-1. We believe, however, that a stronger argument can be made for reinterpreting the variability of all but one of the cranial features believed by Vaughn (1966~)to be sexually dimorphic as probably representing evolutionary trends; though Vaughn's suggestion that the variability in interorbital width is sex-linked is considered very unlikely, it also cannot be shown that it represents a morphological trend. Four closely related tendencies of the maxilla and its dentition can be noted as probably representing trends using a series that includes specimens of Seymouria baylorensis and S. sanjuanensis: (1) overall increased size of the teeth, (2) increased heterodonty of the "caniniform" region, (3) increased height of the maxilla, and (4) reduction in the number of teeth. Of the above characters, all but the last was recognized by Vaughn as possibly being sex-linked. Our recognition of these structural differences as possibly representing trends is based on several observations of their measurements shown in Table 1: (1) the values can be arranged into a series of gradual changes, rather than sorting out into two distinct sets, and forms in general a chronologically ordered sequence; (2) the values that reflect a more advanced evolutionary stage of development are in general seen in the later occumng specimens and vice versa; and (3) the ranges of values are noticeably narrower for specimens found as lots either in the same block, closely associated, or in the same immediate area. The series of Seymouria specimens illustrating the above possible trends, and whose cranial measurements are given in Table 1, includes, in order of increasing degree of advancement and general stratigraphic sequence from lowest occurrence, (1) S. sanjuanensis specimens CM 38022, 28596, and 28597 described here from the Cutler Formation of New Mexico; (2) S. sanjuanensis specimens NTM VP 1023, 1024, 1025, and 1034 from the Organ Rock Shale of Utah (Vaughn 1 9 6 6 ~ ) ;(3) the skull TMM 31 189-1 from probably close to the boundary between the Admiral and Belle Plains formations, Wichita Group, of
Texas; and (4) S. baylorensis specimens CMNH 663 (Williston 191l), MCZ 1081, 1082, 1083, and 1085 (White 1939), and USNM 9140 (= "Conodectes favosus" in White 1939), 15553, and 15554 from the Arroyo Formation, Clear Fork Group, Texas. The relative ages of the members of this series can be determined (Fig. 5). The S. baylorensis specimens from the Arroyo Formation are middle Leonardian (later Early Permian) in age and occur substantially later than the other members of the series, which are closely grouped chronologically. The stratigraphic position of TMM 31 189-1 relative to the S. baylorensis from the Arroyo Formation presents no problems, since it is from the upper part of the Wichita Group of the same section, which is earliest Leonardian age. As for the age of the Utah specimens of S. sanjuanensis, on the basis of biostratigraphic evidence Vaughn (1964, 1966a) correlated the level from which they were found, the Organ Rock Shale, with some horizon in the upper part of the Wichita Group of Texas, thereby giving them an earliest Leonardian age. This level of the Organ Rock Shale is now widely accepted as late Wolfcampian (earliest Permian) age (Vaughn 1970; Blakey 1980), whereas we have assigned an early to middle Wolfcampian age to the S. sanjuanensis specimens from New Mexico. The S. sanjuanensis specimens from the Cutler Formation of New Mexico, the S. sanjuanensis specimens from the Organ Rock Shale of Utah, the S. baylorensis specimens MCZ 1081, 1082, 1083, and 1085, and the S. baylorensis specimens USNM 15553 and 15554 were found either preserved in the same block, closely associated, or in the same immediate area, and the members of each group exhibit relatively close morphological conformity. The graph of Fig. 8 illustrates that within the above series of specimens of the closely related Seymouria sanjuanensis and S. baylorensis not only can a tendency toward increased size of the entire dental row of the maxilla be suggested, but as well accentuated growth in the "canine" region. The plots for the S. sanjuanensis specimens of New Mexico and Utah suggest identical, or nearly identical, stages of development. However, calculation of the percent ratios of the average unit length of maxilla per tooth to total skull length (Table 1) suggests a greater development of the maxillary dentition in the Utah specimen NTM VP 1023, as well as in consecutively younger members of the series, though conceivably the comparison could reflect only a difference in spacing. It is expected that with the relative increase in the size of the maxillary teeth there would be a concomitant increase in the size of the maxilla to accommodate them; this is clearly suggested by the values for the percent ratios of the maxillary height to skull length (Table 1). These closely related trends of the maxilla and its dentition are not unusual or unexpected, since they are common among Paleozoic vertebrates (i.e., captorhinid and pelycosaur reptiles and dissorophid amphibians). If, as can be inferred from Table 1 and as already discussed, there was a general tendency toward reduction in the relative anteroposterior length of the postparietals in Seymouria, then the percent ratios of the postparietal length to skull length of the S. sanjuanensis specimens of Utah on the one hand, and those of New Mexico specimens on the other, exhibit an anachronistic relationship. Although evidence presented by Vaughn (1966~)that the variability of the relative spacing of the orbits in Seymouria is sex-linked is inconclusive, there are also no grounds for interpreting this variable as a morphological trend. The cranial measurements given by Vaughn (1966~)for CNHM 663 were
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taken from a cast, and he was apparently unaware that the orbital margins and the occipital portion of the skull table had been restored. Examination of the actual specimen revealed that the medial margins of the both orbits are extensively eroded away, and the breadth of the remaining interorbital bone is only about 2.5 cm. Vaughn's measurement of 48 mm for the interorbital breadth is apparently much too high, and we believe a reasonable but rough estimate for this dimension would be about 40 mm or even slightly less. Using this lower figure, the ratio of this dimension to skull length is now closely comparable to those of most of the specimens considered here. In addition to MCZ 1082, Vaughn also identified NTM VP 1025 as a male with narrowly separated orbits. The skull of NTM VP 1025, however, is incomplete anteriorly and represented mainly by impression, and Vaughn was forced to use the distance from the occiput to the shortest line that can be drawn between the orbits as an index to skull length. In calculating the ratios of interorbital breadth to this dimension, Vaughn again noted that the values for NTM VP 1023 and 1024, TMM 31189-1, and CMNH 663 express a nearly linear progression into which that for NTM 1025 does not fit. At first glance this comparison seems quite convincing, until one takes into account the poor state of preservation of the specimens, the unlikelihood of getting consistent and accurate measurements for the sort of dimension used as an index to skull length, and that even a small error of only a few millimetres in measuring will account for much of the deviation of the NTM VP 1025 ratio. If, on the other hand, the breadth across the tabulars is used as an index to skull size, and if the ratio of interorbital breadth to this dimension in NTM VP 1025 is compared with only those of specimens of the same lot, then no significant difference is seen. The breadth across the tabulars is 58 mm in NTM VP 1025 and 60 mm in both NTM VP 1023 and 1024, and if it is assumed that the ratio of this measurement to skull length is the same for all three specimens, then the skull length of the former should be about 87 mm. This is slightly less than that for NTM VP 1023 and 1024, rather than slightly greater as Vaughn calculated. Recalculation of the ratio of interorbital breadth to skull length of NTM VP 1025 now yields a value that fits into the linear relationship of the values for all the other skulls of this study for which these data are available except MCZ 1081 and 1082. The ratios for the latter two skulls are nearly identical, however, despite a significant difference in their skull lengths. This example, as well as other examples in Table 1, underscores that specimens of the same lot exhibit great conformity in cranial proportions. It further argues against the explanation of sexual dimorphism in Seymouria for the variations in cranial features noted above, unless it is accepted as coincidental that all the specimen lots are unisexual.
Paleogeographic distributions Vaughn (1966b, 1969, 1970) has attempted to explain the distribution patterns of certain Permo-Pennsylvanian vertebrates throughout the southwestern United States. By plotting faunal distributions on paleogeographic maps Vaughn has identified what he refers to as "truly deltaic" and "somewhat more upland" faunas. These relative terms simply refer to degree of proximity to either shorelines of persistent seaways or margins of positive elements. Whereas his "truly deltaic" fauna includes rhipidistian fishes, the amphibians Diplocaulus and Seymouria, and the pelycosaur ~imetrodon,the "somewhat more upland" fauna is identified by the absence of these
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"marker" faunal elements. The faunal associations of the Cutler and Abo formations of north-central New Mexico and Colorado have been included by Vaughn in his "somewhat more upland" category. The recent discoveries (Berman 1977; Berman and Reisz 1980) of Dimetrodon and Diplocaulus in the Cutler Formation of the San Diego Canyon area about 45 km south of the Cutler Formation site from which the specimens described here were found have cast doubt on Vaughn's hypothesis. Berman and Reisz (1980) recommended amending the hypothesis by extending the range of the "truly deltaic" fauna well inland across a coastal plain to the margins of the highlands to include the San Diego Canyon area. It was also recommended that the term "truly deltaic" be abandoned in favor of the more inclusive "coastal plain." With the discovery of the New Mexico Seymouria an even farther extention of the coastal plain "inland" and "upland" would be required, thus casting serious doubt on the utility of Vaughn's hypothesis.
Acknowledgments We are indebted to the Field Museum of Natural History, Museum of Comparative Zoology, Harvard University, United States Museum of Natural History, University of California, Los Angeles, and University of Texas for the loan of specimens. Thanks are due Ms. Diane Scott, Erindale Campus, University of Toronto, and Ms. Amy Henrice, Carnegie Museum of Natural History, for the difficult task of preparing the specimens from New Mexico. Ms. Diane Scott also drafted the illustrations. Support for this research was provided by grants from the New Mexico Bureau of Mines and Mineral Resources (to DSB and DAE), the Natural Sciences and Engineering Research Council of Canada (to RRR), and the M. Graham Netting Research Fund through a grant from the Cordelia Scaife May Charitable Trust (to DSB). BERMAN, D. S. 1977. A new species of Dimetrodon (Reptilia, Pelycosauria) from the Lower Permian of north-central New Mexico. Journal of Paleontology, 51: 108- 115. BERMAN, D. S, and REISZ,R. R. 1980. A new species of Trimerorhachis (Amphibia, Temnospondyli) from the Lower Permian Abo Formation of New Mexico, with discussion of Permian faunal distributions in the state. Annals of Carnegie Museum, 49: 455 -485. BLAKEY, R. C. 1980. Pennsylvanian and Early Permian paleogeography, southern Colorado Plateau and vicinity. In Paleozoic paleogeography of west-central United States: west-central United States paleogeography Symposium 1. Society of Economic Paleontologists and Mineralogists, Rocky Mountain Section, pp. 239-258. COPE,E. D. 1896. The reptilian order Cotylosauria. American Philosophical Society Proceedings, 34: 436-457. EBERTH, D. A. 1986. Stratigraphy, sedimentology, and paleoecology of Cutler Formation redbeds (Permo-Pennsylvanian) in northcentral New Mexico. Ph.D. thesis, University of Toronto, Toronto, Ont. EBERTH, D. A., and BERMAN, D. S. 1983. Sedimentology and paleontology of Lower Permian fluvial redbeds of north-central New Mexico-preliminary report. New Mexico Geology, 5: 21 -25. FRACASSO, M. J. 1980. Age of the Permo-Carboniferous Cutler Formation vertebrate fauna from El Cobre Canyon, New Mexico. Journal of Vertebrate Paleontology, 54: 1237- 1244. GILE,L. M., PETERSON, F. F., and GROSSMAN, R. B. 1966. Morphological and genetic sequences of carbonate accumulation in desert soils. Soil Science, 101: 347-360. HUNT,A. 1983. Plant fossils and lithostratigraphy of the Abo Formation (Lower Permian) in the Socorn, area and plant biostratigraphy of Abo red beds in New Mexico. New Mexico Geological Society
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Guidebook, 34th Field Conference, Socorro Region 11, pp. 157- 163. LANGSTON, W., JR. 1953. Amphibians from New Mexico. University of California Publications in Geological Sciences, 29: 349-416. LEEDER,M. R. 1975. Pedogenic carbonate and flood sediment accretion rates: a quantitative model for alluvial, arid-zone lithofacies. Geological Magazine, 112: 257 -270. NETTERBURG, F. 1980. Geology of southern African calcretes: 1 . Terminology, description, macrofeatures, and classification. Transactions of the Geological Society of South Africa, 83: 255-283. OLSON, E. C. 1979. Seymouria grandis n.sp. (Batrachosauria: Amphibia) from the Middle Clear Fork (Permian) of Oklahoma and Texas. Journal of Paleontology, 53: 720 -728. 1980. The North American Seymouriidae. In Aspects of vertebrate history; essays in honor of Edwin Harris Colbert. Edited by L. Jacobs. Museum of Northern Arizona Press, Flagstaff, AZ, pp. 137-152. ROMER,A. S. 1947. Review of the Labyrinthodontia. Bulletin of the Museum of Comparative Zoology, 99: 1-368. SMITH,C. T., BUDDING, A. J., and PITRAT,C. W. 1961. Geology of the southeastern part of the Chama Basin. State Bureau of Mines and Mineral Resources, New Mexico Institute of Mining and Technology, Bulletin 7 5 .
VAUGHN,P. P. 1964. Vertebrates from the Organ Rock Shale of the Cutler Group, Permian of Monument Valley and vicinity, Utah and Arizona. Journal of Paleontology, 38: 567 -583. 1966a. Seymouria from the Lower Permian of southeastern Utah, and possible sexual dimorphism in that genus. Journal of Paleontology, 40: 603 -612. 19666. Comparison of the Early Permian vertebrate faunas of the Four Corners region and north-central Texas. Los Angeles County Museum of Natural History, Contributions in Science, 105: 1-13. 1969. Early Permian vertebrates from southern New Mexico and their paleozoogeographic significance. Los Angeles County Museum of Natural History, Contributions in Science, 166: 1-22. 1970. Lower Permian vertebrates of the Four Corners and the midcontinent as indices of climatic differences. Proceedings of the North American Paleontological Convention, Chicago, IL, 1969, Part D, pp. 388-408. WATSON,D. M. S. 1918. On Seymouria, the most primitive known reptile. Proceedings of the Zoological Society of London, pp. 267-301. WHITE, T. E. 1939. Osteology of Seymouria baylorensis Broili. Bulletin of the Museum of Comparative Zoology, 85: 325-409. WILLISTON, S. W . 191 1 . American Permian vertebrates. University of Chicago Press, Chicago, IL.
