New Fossil Evidence on the Sister-Group of Mammals and Early Mesozoic Faunal Distributions Author(s): Neil H. Shubin, A. W. Crompton, Hans-Dieter Sues and Paul E. Olsen Source: Science, New Series, Vol. 251, No. 4997 (Mar. 1, 1991), pp. 1063-1065 Published by: American Association for the Advancement of Science Stable URL: http://www.jstor.org/stable/2875224 Accessed: 03-04-2017 21:50 UTC 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 [email protected]. Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at http://about.jstor.org/terms American Association for the Advancement of Science is collaborating with JSTOR to digitize, preserve and extend access to Science This content downloaded from 128.103.149.52 on Mon, 03 Apr 2017 21:50:59 UTC All use subject to http://about.jstor.org/terms This content downloaded from 128.103.149.52 on Mon, 03 Apr 2017 21:50:59 UTC All use subject to http://about.jstor.org/terms acteristic feature of undisputed Mesozoic cingulum; (iv) upper postcanines with a suchid crocodyliform archosaurs, reveal fur- mammals is unilateral occlusion. The lower buccal cingulum; (v) postcanine teeth with ther close similarities among continental te- jaw on the active side moves dorsomedially incipiently divided, massive roots. From trapod assemblages of Early Jurassic age. relative to the upper teeth during occlusion. these features, the family Tritheledontidae is The sphenodontian Clevosaurus is known This movement produces wear facets on the hypothesized to include the genera Diar- from Upper Triassic fissure-fillings in En- lingual face of the uppers and the buccal face thrognathus, Pachygenelus, and Tritheledon. gland (16), and closely related forms are of the lowers. In mammals this pattern of The dentition of Tritheledon is poorly known from the Forest Sandstone of Zim- jaw movement produces complex and con- known but dosely resembles that of Diar- babwe and the lower Lufeng Formation of sistent multiple wear facets on the molars. In thrognathus in the buccolingual expansion of Yunnan, China, both of which are Early these taxa the teeth are progressively added the upper postcanine teeth (1, 2, 11). The- Jurassic in age (17). The protosuchid Proto- from front to back and are not replaced. In rioherpeton does not possess any of the fea- suchus is known elsewhere from the Lower contrast to other nonmammalian cyn- tures above and cannot be considered to be Jurassic Glen Canyon Group of the south- odonts, Pachygenelus cf P. monus also pos- a tritheledontid. The phylogenetic status of western United States and the upper Storm- sesses unilateral occlusion but lacks precise, Chaliminia is uncertain because of the poor berg Group of southern Africa (18). Th,e matching wear facets. The matching facets preservation of the only known specimen. dinosaurian material from the McCoy on the teeth of Pachygenelus extend over the The newly discovered tritheledonts main- Brook Formation recovered to date is frag- entire lingual surface of the upper postca- ly support the cynodont phylogeny original- mentary but it includes an anchisaurid pro- nines and the buccal surface of the lower ly proposed by Hopson and Barghusen (1). sauropod and teeth of an ornithischian in- postcanines (Fig. 1A). Occlusal wear facets Dental characteristics such as the develop- distinguishable from Lesothosaurus from are also present on the incisors and canines ment of a buccal cingulum on the upper southern Africa. Both dinosaurian families of Pachygenelus cf. P. monus (Fig. 1F). In teetf, matching ocdusal wear on the buccal appear to have a worldwide distribution Pachygenelus cf. P. monus the postcanine surface of the lower postcanines and the during the Early Jurassic. teeth are replaced alternately and this pro- lingual surface of the upper postcanines, and The homogeneity of tetrapod distribu- cess appears to have continued throughout contact between the dentary and squamosal tions during the Early Jurassic represents the life. As new postcanines erupted, they con- bones apparently evolved only once among continuation of a trend toward increased tacted opposing teeth on the opposite jaw. cynodonts. cosmopolitanism that began in the late Pa- This resulted in the less well-defined facets The occurrence of Pachygenelus cf. P. mo- leozoic (19). Similar cosmopolitan distribu- on the postcanines of Pachygenelus cf. P. nus in Nova Scotia underscores a pattem of tions are also seen in Early Jurassic terrestrial monus. The other putative mammalian sister- wide, in many cases virtually cosmopolitan, floral assemblages that are dominated by group, the Trityledontidae, have postcanine distribution of Early Jurassic taxa of terres- cheirolepidaceous conifers (10). This trend wear facets that lack buccal-lingual postca- trial tetrapods. Other tetrapod taxa from the in terrestrial biotic cosmopolitanism is nine wear and do not suggest patterns of McCoy Brook Formation, particularly the broadly contemporaneous with the initial unilateral occlusion. Consequently, the tri- sphenodontian lepidosaurs and the proto- stages of the breakup of Pangaea. These tylodontid and mammal-tritheledontid oc- dusal patterns cannot be homologized as a single character as has been suggested (15). RPC WF EC In Pachygenelus cf. P. monus, there is a G close coupling of enamel distribution on the teeth to patterns of occlusal wear: enamel that borders on the occluding surfaces is A M~~~~~ thin whereas enamel on nonocduding sur- faces is thick (as seen on the apical portion of the principal postcanine cusp). Once the thin enamel is worn away the apical edge of thick enamel is exposed. This feature pro- vides a resistant cutting edge on the apical surface of the tooth. A :~ WF ;..3 . - A . <R^^t Pachygenelus and Diarthrognathus are also characterized by a contact between the den- I mm tary and the squamosal that is lateral to the EJ A articular-quadrate joint. This new jaw artic- ulation develops on the posterio-lateral as- pect of the articular process of the dentary and is probably homologous to the well- developed condyle on the dentary of mam- mals. Fig. 1. (A) Lingual view of left maxilla of Pachygenelus cf. P. monus (NSM 988GF11.1l) showing wear Tritheledontidae can be distinguished patterns (WF), replacing teeth (RPC), empty aveoli (A), and dentine-enamel junction (EJ). (B) Buccal from other nonmammalian cynodonts by view of NSM 988GF11.1 showing excternal cingulum (EC), infraorbital foramnen (10), and jugal (J). the following dental features: (i) presence of (C) Occdusal view of NSM 988GF11.1 showing gubernaculum for the dental lamiina (G). (D) Lingual only two upper and two lower incisors (1); view of right dentary of Pachygenelus cf. P. monus (MCZ 9139) showing large angle (An) and (ii) transversely expanded upper postcanines incipiently bifurated postcanine roots (R). (E) Right dentary of Pachygenelus cf. P. monus (NSM 988GF10.1) showing trogh for post-dentary bones (T). (F) Dentary of Pachygenelus cf. P. monus (1, 8); (iii) longitudinally ovate postcanines (NSM 988GF12.1) showing enlarged canine with occlusal wear (WF). Numbers 1 through 5 pertain with large anterior cusp followed by one or to the number of individual postcanine teeth. Institutional abbreviations: NSM, Nova Scotia Museum; two posterior accessory cusps and a lingual MCZ, Museum of Comparative Zoology, Harvard University. Scale bar, 1 mm. 1064 SCIENCE, VOL. 251 This content downloaded from 128.103.149.52 on Mon, 03 Apr 2017 21:50:59 UTC All use subject to http://about.jstor.org/terms patterns suggest that the persistence of Pan- Bioestratigr. 1 Congr. Latinoam. Paleonttol. (Buenos 14. D. Stern and A. W. Crompton, J. Dent. Res. 67, Aires) 2, 123 (1980). 231 (1988); D. Stern, thesis, Harvard University gaea throughout the Triassic provided few 5. S. Chatterjee, Science 220, 1151 (1983). (1989). barriers for the migration of terrestrial ver- 6. J. A. Hopson and A. W. Crompton, in preparation. 15. T. Rowe, J. Vertebr. Paleontol. 8, 241 (1988). tebrates. The initial stages of rifling in the 7. F. E. Grine, C. E. Gow, J. W. Kitching, Proc. 16. N. C. Fraser, Philos. Trants. Roy. Soc. London Ser. B Electron Microsc. Soc. S. Afr. 9, 99 (1979). 321, 125 (1988). Late Triassic resulted in no corresponding 8. A. W. Crompton, Proc. Zool. Soc. London Ser. B 17. H.-D. Sues et al., in preparation. provinciality of terrestrial vertebrate distri- 130, 183 (1958). 18. J. M. Clark, thesis, University of Chicago, Chicago, 9. P. E. Olsen, N. H. Shubin, M. H. Anders, Science bution by the Early Jurassic. IL (1986). 237, 1025 (1987). 19. N. H. Shubin and H.-D. Sues, in preparation. REFERENCES AND NOTES 10. P. E. Olsen and P. M. Galton, Palaeonttol. Afr. 25, 21. W. W. Amaral, W. A. Clemens, N. Greenwald, J. A. 87 (1984). Hopson, K. Padian, M. Russell, and M. Seidl pro- 1. J. A. Hopson and H. Barghusen, in The Ecology and 11. C. E. Gow, Proc. R. Soc. London Ser. B 208, 461 vided helpful discussion and comments on the Biology of Mammal-like Reptiles, N. Hotton, P. D. (1980). manuscript. F. A. Jenkins, Jr., and W. W. Amaral MacLean, J. J. Roth, E. C. Roth, Eds. (Smithsonian 12. A. W. Crompton, Annt. S. Afr. Mus. 46, 479 provided facilities and extraordinary expertise for Institution Press, Washington, DC, 1986), pp. 83- (1963); Proc. Zool. Soc. London Ser. B 140, 697 laboratory and field investigation. Field permits and 106; T. S. Kemp, Mammal-like Reptiles and the (1963); ..and F. Ellenberger, Atin. S. Afr. logistical support were provided by the Nova Scotia Origin of Mammals (Academic Press, New York, Mus.