Affinities of ?Scalenodontoides plemmyridon Hopson, 1984 (Synapsida: Cynodontia) from the Upper Triassic of Nova Scotia Author(s): Hans-Dieter Sues, James A. Hopson, Neil H. Shubin Source: Journal of Vertebrate Paleontology, Vol. 12, No. 2 (Jun. 10, 1992), pp. 168-171 Published by: The Society of Vertebrate Paleontology Stable URL: http://www.jstor.org/stable/4523438 Accessed: 04/06/2010 18:16 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=vertpaleo. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact
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SUES ET AL.-NO VASCOTIAN TRAVERSODONTCYNODONT
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FIGURE 1. Arctotraversodonplemmyridon (Hopson, 1984). Incomplete upper postcanine tooth (NSM 990GF89.1). A, occlusal,B, posterior,C, anterior,D, lingual,and E, buccalviews. Abbreviations:a.ci, anteriorcingulum;b.a.c, buccalaccessory cusp; b.c, buccal cusp; c.c, central cusp; 1.c, lingual cusp.
and thus the North Pole or, more generally, the North; in allusion to both its northern occurrence and the informal reference to the holotype as the "bear jaw." Traversodon Huene, 1936 is the nominal genus of the family-level taxon "Traversodontidae." Diagnosis - Dentary with greatly enlarged posterior mental foramen. Very broad symphyseal region, widest at the level of the canines. Lower incisors with large mesial and distal marginal cuspules. Crowns of postcanine teeth distinctly compressed anteroposteriorly. Upper postcanines with prominent central cusp. Anterior cusps of lower postcanines anteroposteriorly narrow; posterior heel of lower postcanines without raised rim and buccal accessory cusp placed high on buccal ridge, rather than on margin of heel. PLEMMYRIDON ARCTOTRA (Hopson, 1984) vERSODON ?Scalenodontoides plemmyridon, sp. nov. Hopson, 1984:188. 19190, horizontal ramus of the Holotype-YPM-PU right dentary with conjoined symphyseal portion of the left dentary and partially erupted right third incisor and unerupted right first incisor. Figured by Hopson (1984:figs. Id-f and 4-7). Wolfville Formation (Fundy Group, Newark Supergroup). Age: Late Triassic (middle to late Carnian). Burntcoat shore, 2.4 km (1.5 miles) northwest of Noel, Hants County, Nova Scotia. Burntcoat shore: YPM-PU Hypodigm -From 19190A, isolated ?right canine (probably pertaining to holotype; Hopson, 1984:fig. 8); YPM-PU 21693, small incomplete left dentary without teeth (Hopson, 1984: fig. 9); NSM 983GF2.1, right lower postcanine tooth (Hopson, 1984:fig. 10); NSM 990GF89.1, left upper postcanine tooth (this paper; Fig. 1). From Evangeline Beach, Kings County: YPM-PU 22343, fragment of a small dentary without teeth.
Diagnosis--Type and only known species of genus as diagnosed above. DESCRIPTION OF NSM 990GF89.1 NSM 990GF89.1 is an isolated left upper postcanine tooth (Fig. 1). It has been damaged due to exposure to wave action on the sea cliff. The central portion of its crown, including most of the central cusp, much of the anterior cingulum and basin, and the posterobuccal region of the buccal cusp are damaged or lost. The thick enamel covering of the tooth crown shows coarse vertical wrinkling very similar to that on the crown of the erupting third lower incisor in the holotype dentary (YPM-PU 19190) of A. plemmyridon. The crown is nearly twice as wide buccolingually than long anteroposteriorly (37 mm vs. ca. 20 mm). It is more or less ovate in occlusal view, with a distinctly convex anterior (or mesial) margin and a very slightly convex, almost straight posterior (or distal) margin. Three large cusps surmount a vertical transverse ridge that dominates the crown. A narrow basin is developed anterior to the buccal and central cusps and is bounded anteriorly by a low cingulum bearing a series of beadlike denticles. Only the more lingual portions of the basin and cingulum are preserved, but both presumably extended to the damaged anterobuccal margin of the crown. When the crown is oriented vertically, the anterior basin and ridge form a distinct ledge anterior to the root. The posterior side of crown lacks any trace of a cingulum. The nearly flat posterior surface of the crown is dished in in the middle. The enamel is smooth on the lower portion of the posterior surface, below the central and buccal cusps; we interpret this area as representing a contact facet with the anterior ledge of the succeeding tooth. The thick enamel covering the posterior surface of the crown extends further toward the base of the tooth.
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The buccal cusp is the most complete of the three principal cusps. It is anteroposteriorly compressed with a slightly concave anterior and a gently convex posterior surface. A low ridge extends anteroventrally from its tip down along the buccal aspect and presumably merged with the outer end of the anterior cingulum as in other traversodonts. The ridge bears a low cusp near the midpoint of its inferred length. A broad, worn ridge extends down along the lingual face of the buccal cusp to the notch between the buccal and central cusps. Just lateral to the notch, it bears a cuspule that is similar to an accessory cuspule found in the traversodont cynodonts Luangwa and Traversodon (J.A.H., unpubl. obs.). The central cusp is heavily damaged, and only the lower portion of its anterolingual surface adjacent to the lower part of the lingual cusp is preserved. Enough of its basal outline is available to indicate that it was longer anteroposteriorly than the buccal and lingual cusps. Its anterior surface is separated from that of the lingual cusp by a broad shallow groove, which merges ventrally into the lingual extension of the anterior basin. As preserved, the lingual cusp is a few millimeters lower than the buccal cusp, although they may have been subequal in height on an undamaged and unworn tooth crown. The anterolingual portion of the cusp is missing a narrow vertical strip of enamel but there appears to have been no trace of a descending ridge or accessory cuspules in this region, as usually found in traversodont cynodonts. Viewed from behind, the root of the tooth is only slightly longer than the crown is high although its tip is undoubtedly somewhat abraded. As preserved, it tapers rather smoothly to a broadly rounded truncated end. In side view, the root tapers only slightly and curves forward. COMPARISONS Primarily on the basis of its large size, NSM 990GF89.1 is referred to the same taxon as the dentaries described as ?Scalenodontoides plemmyridon by Hopson (1984). Furthermore, its anteroposteriorly narrow crown is matched by that of the right lower postcanine tooth NSM 983GF2.1 (Hopson, 1984:fig. 10), which, however, represents a much smaller individual. The anteroposterior compression of the tooth crowns is consistent with the anteroposteriorly narrow alveoli in the holotype dentary of?S. plemmyridon and supports reference of the teeth to the same taxon. The postcanine teeth of this form differ considerably from those of all other known traversodont cynodonts, and thus the material warrants formal recognition as a new genus. At present, there exists no evidence to indicate the presence of more than one taxon of gomphodont cynodonts in the Wolfville Formation, and the new combination Arctotraversodon plemmyridon (Hopson, 1984) is applied here to the entire hypodigm.
NSM 990GF89.1 resembles upper postcanine teeth of Exaeretodon Cabrera, 1943 from the Ischigualasto Formation (Carnian) of Argentina (Crompton, 1972: fig. 12; Hopson, 1985:296) in its large size and in the absence of a posterior cingulum. It differs, however, from the latter in lacking the pronounced buccolingual flexure of the crown in occlusal view and in retaining a central cusp. Furthermore, upper postcanines of Exaeretodon have a distinct anterolingual cusp and a tall anterior cingulum ridge; the former is absent and the latter is low in NSM 990GF89.1. The new tooth resembles upper postcanine teeth of Massetognathus Romer, 1967 from the Ischichuca Formation (Ladinian) of Argentina (Crompton, 1972:fig. 11) in the absence of an anterior accessory cusp buccally, the retention of a central cusp, and the development of a low anterior cingulum. The upper postcanines of Massetognathus are transversely oriented in the maxilla, rather than very obliquely as in Exaeretodon (Crompton, 1972), and this was probably also the case in NSM 990GF89.1, as indicated by an apparent interdental contact facet on the posterior surface of the crown. The structure of the postcanine teeth precludes reference of Arctotraversodon to the clade comprising Ex-
aeretodon, GomphodontosuchusHuene, 1928, and
Scalenodontoides as defined and diagnosed by Hopson (1985). Hopson (1984:197) noted that NSM 983GF2.1 resembles lower postcanine teeth of Massetognathus more closely than those of Exaeretodon and Scalenodontoides macrodontes in lacking both the anteroposterior expansion of the anterior cusps and the distinct posterior inclination of the lingual cusp found in the latter taxa. These features are also absent in Boreogomphodon (H.-D.S., unpubl. obs.), with which Arctotraversodon shares the derived presence of three, rather than two, anterior cusps on the lower postcanines. (An as yet undescribed lower postcanine of a traversodont cynodont from the upper Lettenkeuper (Ladinian) of Wiirttemberg, Germany, has three anterior cusps as well.) The latter feature links Arctotraversodon and Boreogomphodon, but a more definitive assessment of the affinities of these genera must await a comprehensive phylogenetic analysis of all "gomphodont" cynodonts currently in preparation by J.A.H. ACKNOWLEDGMENTS Field-work in Nova Scotia was undertaken with the permission of the Province of Nova Scotia and the Government of Canada. We gratefully acknowledge the assistance of the authorities at the Nova Scotia Museum, Halifax, especially R. Ogilvie and R. Grantham. D. Baird generously provided H.-D.S. with copies of his unpublished field-notes and much useful advice. M. A. Parrish skillfully prepared Figure 1. Fieldwork in Nova Scotia was made possible by Grant #4232-89 from the National Geographic Society (to H.-D.S.).
SUES ET AL. -NO VA SCOTIAN TRA VERSODONT CYNODONT
LITERATURECITED Cabrera, A. 1943. El primer hallazago de teraipsidos en la Argentina. Notas del Museo de La Plata 8:317-331. Carroll, R. L., E. S. Belt, D. L. Dineley, D. Baird, and D. C. McGregor. 1972. Vertebrate Paleontology of Eastern Canada. Guidebook for Field Excursion A59. 24th International Geological Congress, Montreal, Quebec, 1972, 113 pp. Crompton, A. W. 1972. Postcanine occlusion in cynodonts and tritylodontids. Bulletin of the British Museum (Natural History), Geology 21:29-71. and F. Ellenberger. 1957. On a new cynodont from the Molteno Beds and the origin of the tritylodontids. Annals of the South African Museum 44:1-14. Hopson, J. A. 1984. Late Triassic traversodont cynodonts from Nova Scotia and southern Africa. Palaeontologia Africana 25:181-201. 1985. Morphology and relationships of Gomphodontosuchus brasiliensis von Huene (Synapsida, Cynodontia, Tritylodontoidea) from the Triassic of Brazil. Neues Jahrbuch fiir Geologie und Palliontologie, Monatshefte 1985:285-299. Huene, F. v. 1928. Ein Cynodontier aus der Trias Brasiliens. Centralblatt fiir Mineralogie, Geologie und Paliiontologie, B, 1928:251-270.
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1936. Die fossilen Reptilien des siidamerikanischen Gondwanalandes. Ergebnisse der Sauriergrabungen in Siidbrasilien 1928-29. Lieferung 2 [pp. 93-159]. Verlag Franz F. Heine, Tiibingen. [Imprint for the entire work: 1935-42.] Kitching, J. W., and M. A. Raath. 1984. Fossils from the Elliot and Clarens formations (Karoo sequence) of the northeastern Cape, Orange Free State and Lesotho, and a suggested biozonation based on tetrapods. Palaeontologia Africana 25:111-125. Olsen, P. E., R. W. Schlische, and P. J. W. Gore (eds.). 1989. Tectonic, Depositional, and Paleoecological History of Early Mesozoic Rift Basins, Eastern North America. Guidebook for Field Trip T351, 28th International Geological Congress. American Geophysical Union, Washington, D.C., X + 174 pp. Romer, A. S. 1967. The Chafiares (Argentina) Triassic reptile fauna. III. Two new gomphodonts, Massetognathus pascuali and M. teruggii. Breviora 264:1-25. Sues, H.-D., and P. E. Olsen. 1990. Triassic tetrapods of Gondwanan aspect from the Richmond basin of Virginia. Science 249:1020-1023. Received 4 June 1991; accepted 23 August 1991. Corresponding editor: R. J. Emry.
