Diademodon is a distant extinct relative of mammals that lies along the Cynodontia lineage. This lineage includes extinct taxa such as Thrinaxodon, Probainognathus, Probelesodon, Exaeretodon, and Ecteninion, as well as all mammals, extinct and extant. Members of Cynodontia, including mammals, are known as cynodonts.
Fossils of Diademodon are found mainly in the Lower Triassic Cynognathus zone of the Karroo Basin of South Africa. Cranial remains are fairly common for Diademodon but relatively little is known about its biology except that it was large (up to the size of a small cow) and herbivorous.
The skull is characterized by a long, narrow snout; large temporal fenestrae; high sagittal crest; and robust zygomatic arches (Kemp, 1982). The dentition includes four upper and three lower incisiform teeth, well-developed upper and lower caniniforms, and up to sixteen multicuspate postcaniniforms (Ziegler, 1969). The postcaniniforms increase in transverse width posteriorly. The teeth apparently undergo two waves of replacement, the second of which only includes the anterior dentition up to the fourth or sixth postcaniniform (Ziegler, 1969).
The postcranial skeleton of Diademodon is poorly known and virtually indistinguishable from Cynognathus in the absence of cranial material (Jenkins, 1971). However, a histological study of the long bones revealed differences between the two taxa (Botha and Chinsamy, 2000).
Diademodon possesses a number of derived cranial characters that are diagnostic of “advanced” cynodonts (or eucynodonts sensu Rowe, 1993). These include an enlarged dentary with a well developed coronoid process and corresponding reduction of the postdentary elements; the participation of the surangular and squamosal in the jaw joint; and presence of occlusion between upper and lower dentition (Kemp, 1982).
Many phylogenetic analyses place Diademodon as sister taxon to a clade that includes Probainognathus, Exaeretodon, tritylodonts, Morganucodon, and mammals (Wible, 1991; Rowe, 1993; Luo, 1994). An alternative placement of Diademodon is in a clade of extinct cynodonts known as Cynognathia that includes Exaeretodon and tritylodonts but excludes Morganucodon and mammals (Hopson and Kitching, 2001).
About the Species
This specimen was collected by Charles Camp in 1948 from the Cynognathus Zone (early Triassic), Karoo Group, South Africa. It was made available to the University of Texas High-Resolution X-ray CT Facility for scanning by Dr. Timothy Rowe of The University of Texas at Austin. Funding for image processing was provided by a National Science Foundation Digital Libraries Initiative grant to Dr. Rowe.
About this Specimen
The specimen was scanned by Matthew Colbert and Richard Ketcham on 18 February 2003. It was scanned along the coronal axis for a total of 410 slices, each slice 1.0 mm thick with an interslice spacing of 0.8 mm (for a slice overlap of 0.2 mm).
Botha, J., and A. Chinsamy. 2000. Growth patterns deduced from the bone histology of the cynodonts Diademodon and Cynognathus. Journal of Vertebrate Paleontology 20:705-711.
Brink, A. S. 1963a. Notes on some new Diademodon specimens in the collection of the Bernard Price Institute. Palaeontologica Africana 8:97-111.
Brink, A. S. 1963b. Two cynodonts from the Ntawere Formation in the Luangwa Valley of northern Rhodesia. Palaeontologica Africana 8:77-96.
Hopson, J. A., and J. W. Kitching. 2001. A probainognathian cynodont from South Africa and the phylogeny of nonmammalian cynodonts. Bulletin of the Museum of Comparative Zoology 156:5-35.
Jenkins, F. A. 1971. The postcranial skeleton of African cynodonts. Peabody Museum of Natural History Bulletin 36:1-216.
Kemp, T. S. 1982. Mammal-like Reptiles and the Origin of Mammals. Academic Press, New York, 363 pp.
Luo, Z. 1994. Sister-group relationships of mammals and transformations of diagnostic mammalian characters; pp. 98-128 in N. C. Fraser and H. D. Sues (eds.), In the Shadow of the Dinosaurs: Early Mesozoic Tetrapods. Cambridge University Press, New York.
Rowe, T. 1993. Phylogenetic systematics and the early history of mammals; pp. 129-145 in F. S. Szalay, M. J. Novacek, and M. C. McKenna (eds.), Mammalian Phylogeny. Volume 1: Mesozoic Differentiation, Multituberculates, Monotremes, Early Therians, and Marsupials. Springer-Verlag, New York.
Watson, D. M. S. 1911. The skull of Diademodon, with notes on those of some other cynodonts. Annals and Magazine of Natural History vol. 8, series 8, pp. 293-331.
Watson, D. M. S. 1913. Further notes on the skull, brain, and organs of special sense of Diademodon. Annals and Magazine of Natural History vol. 12, series 8, pp. 217-228.
Wible, J. R. 1991. Origin of Mammalia: the craniodental evidence reexamined. Journal of Vertebrate Paleontology 11:1-28.
Ziegler, A. C. 1969. A theoretical determination of tooth succession in the therapsid Diademodon. Journal of Paleontology 43:771-778.