Exaeretodon frenguelli is a member of Eucynodontia, a lineage that includes mammals and their closest extinct relatives. As with the other eucynodonts (Probainognathus, Probelesodon, and Ecteninion) and the oldest-known dinosaurs (Eoraptor and Herrerasaurus) featured on this site, Exaeretodon was collected from the late Triassic sediments of Argentina.   

Exaeretodon is at the heart of a controversy as to which eucynodonts are the closest extinct relatives of mammals. One group argues that even though Exaeretodon shows many seemingly unique resemblances with mammals, it lies out on a distant side branch with other extinct herbivorous eucynodonts. Most researchers accept an alternative view, that Exaeretodon is a close relative of mammals. We scanned this specimen to look for new evidence in the braincase, snout, and pattern of tooth replacement that might have bearing on this controversy.

This skull was collected from the Ischigualasto Formation (Upper Triassic) of Argentina, and was made available to the University of Texas High-Resolution X-ray CT Facility for scanning by Dr. Oscar Alcober and Dr. Riccardo Martinez of the Museo de Ciencias Naturales, and Dr. William Sill of the Universidad Nacional de San Juan, Argentina. Funding for scanning was provided by a National Science Foundation Digital Libraries Initiative grant to Dr. Timothy Rowe of The University of Texas at Austin.

The specimen was scanned by Timothy Rowe on 11 April 1998 along the horizontal axis for a total of 176 slices, each slice 0.5 mm thick, with an interslice spacing of 0.45 mm (for a slice overlap of 0.05 mm). The dataset displayed was reduced for optimal Web delivery from the original, much higher-resolution CT data.

Literature

Bonaparte, J. F. 1962. Descripción del cráneo y mandíbula de Exaeretodon frenguelli, Cabrera. Publicaciones del Museo Municipal de Ciencias Naturales y Tradicional de Mar del Plata 1:135-202.

Bonaparte, J. F. 1966. Sobre las cavidades cerebral nasal y otras estructuras del craneo de Exaeretodon sp. (Cynodontia, Traversodontidae). Acta Geologica Lilloana 8:5-29.

Chatterjee, S. 1982. A new cynodont reptile from the Triassic of India. Journal of Paleontology 56:203-214.

Flynn, J. J., J. M. Parrish, B. Rakotosamimanana, L. Ranivoharimanana, W. F. Simpson, and A. R. Wyss. 2000. New traversodontids (Synapsida: Eucynodontia) from the Triassic of Madagascar. Journal of Vertebrate Paleontology 20:422-427.

Gauthier, J., A. G. Kluge, and T. Rowe. 1988. Amniote phylogeny and the importance of fossils. Cladistics 4:105-209.

Kemp, T. S. 1982. Mammal-like Reptiles and the Origin of Mammals. Academic Press, London.

Martinez, R. N., C. L. May, and C. A. Forster. 1996. A new carnivorous cynodont from the Ichigualasto Formation (Late Triassic, Argentina), with comments on eucynodont phylogeny. Journal of Vertebrate Paleontology 16:271-284.

Rogers, R. R., C. C. Swisher III, P. C. Sereno, A. M. Monetta, C. A. Forster, and R. N. Martinez. 1993. The Ischigualasto tetrapod assemblage (Late Triassic, Argentina) and ⁴⁰Ar/³⁹Ar dating of dinosaur origins. Science 260:794-797.

Rowe, T. 1988. Definition, diagnosis and origin of Mammalia. Journal of Vertebrate Paleontology 8:241-264.

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. Springer-Verlag, New York.

Rowe, T., W. Carlson, and W. Bottorff. 1995. Thrinaxodon:Digital Atlas of the Skull. CD-ROM (Second Edition, for Windows and Macintosh platforms), University of Texas Press, 547 megabytes.

Sues, H. D. 1985. The relationships of the Tritylodontidae (Synapsida). Zoological Journal of the Linnean Society 85:205-218.

None available.