Enigmatic Triassic Taxa - Kraterokheirodon colberti

When you watch 'Walking With Dinosaurs' the Triassic portion (which supposedly represents the Chinle Formation of Arizona) contains numerous scenes featuring a group of cynodonts. In reality this presence of cynodont therapsids in the Chinle Formation was based on two teeth. The first (AMNH 4947) was found at St. Johns Arizona in 1946 by Guy Hazen of the USGS and presented to Ned Colbert that summer and reposited at the American Museum of Natural History. The second tooth (PEFO 9984) was found in 1984 on a geology field trip through Petrified Forest National Park. Preliminarily assigned to traversodont cynodonts (e.g., Long and Murry, 1995) these teeth were under study by Colbert until his passing in 2001.

The location of these specimens was unknown until 2002 when Randall Irmis was helping sort out Colbert's office at the Museum of Northern Arizona and found the original and a cast of PEFO 9984 and a cast of AMNH 4947 (unfortunately the original specimen has yet to be rediscovered).

In 2005 Randy and I published a paper describing these teeth as a new taxon, Kraterokheirodon colberti. The name, created by Randy, means "cupped hand tooth", which describes the morphology of the occlusal surfaces (cusps) of the teeth (six in all), and also honors Ned Colbert. [photo below is of PEFO 9984 in what we believe is a posterior view, the crown is to the left].

The morphology of these teeth is unique "differing from all other known vertebrate teeth in possessing a convexly arched transverse ridge of six cusps." The problem is that they are so unique that determining a taxonomic assignment for this taxon is almost impossible at this time. The thecodont implantation suggests tetrapod affinities, but assignment to a less inclusive clade is ambiguous. [photo below is of PEFO 9984 in occusal view].

One thing is clear, these teeth belong to a very large tetrapod with a long stratigraphic range. The AMNH tooth is from the Monitor Butte Member near the base of the Chinle, whereas the PEFO tooth is from the middle of the Petrified Forest Member. What is even more of a mystery is how such a large animal could escape detection in the well known Chinle faunal assemblage. The largest tetrapods found in these assemblages are phytosaurs and rauisuchians, two groups in which the dentition is very well known and extremely different than that of Kraterokheirodon.

Even though Revueltosaurus was only known from teeth for over 15 years, this was simply due to not finding teeth directly associated with other bones as non-dental material had been collected as early as the 1920s. To date, my colleagues and I have no candidate specimens to associate with the teeth of Kraterokheirodon. No potential postcrania, zip. As with Acallosuchus rectori, our only hope is to keep sampling the horizons in which these teeth occur in hopes of finally solving the mystery as we did with Revueltosaurus (Parker et al., 2005).

REFERENCES

Irmis, R. B., and W. G. Parker. 2005. Unusual tetrapod teeth from the Upper Triassic
Chinle Formation, Arizona, USA. Canadian Journal of Earth Science 42: 1339–1345.
doi: 10.1139/E05-031

Long, R. A., and P. A. Murry. 1995. Late Triassic (Carnian and Norian) tetrapods from the southwestern United States. New Mexico Museum of Natural History and Science Bulletin 4:1-254.

Parker, W.G., Irmis, R.B., Nesbitt, S.J., Martz, J.W., and L.S. Browne. 2005. The Late Triassic pseudosuchian Revueltosaurus callenderi and its implications for the diversity of early ornithischian dinosaurs. Proceedings of the Royal Society B 272:963-969. doi:10.1098/rspb.2004.3047