New "Proto-Turtle" from the Late Triassic of New Mexico

Chinlechelys tenertesta ("delicate shelled turtle from the Chinle") is a newly described stem turtle from the Upper Triassic of New Mexico (Joyce et al., 2008). Based on fragmentary but diagnostic material, the implications of this find are the subject of several recent news articles (here and here for examples). Most importantly the find gives clues to the evolution of the turtle shell and demonstrates that early turtles possessed a carapace of rows of dermal armor. A figure from the paper and news reports (courtesy of the Royal Society) shows a hypothesized gradual evolution of the shell.

Furthermore this specimen demonstrates that the ribs are independent ossifications from the osteoderms, contrasting embryological evidence. Thus this specimen is considered by one researcher to be "one of the most important turtle fossils ever found."

There are a couple of other aspects of this paper that I would like to address. First off, I feel that the genus name is misleading as the specimen is from the Bull Canyon Formation which is actually from the Dockum Group and not from the Chinle Formation. Lucas (1993) and Lucas and Anderson (1994) did place the Dockum (as a formation) in an expanded Chinle Group (and later papers by Lucas and colleagues abandon the name completely), however, this is still controversial and not accepted by all Triassic workers as the older name Dockum would have priority under the North American Stratigraphic Code (e.g., Lehman, 1994; Dubiel, 1994; Carpenter, 1997).

The second point which I find interesting is the interpretation of a terrestrial habitat for Chinlechelys based the depositional setting for the locality as interpreted by Newell (1993). Below is the faunal list for this locality as provided by Hunt (2001) which shows a fair mix of aquatic taxa, although this would not necessarily argue against a terrestrial setting, especially with the presence of aetosaurs, "rauisuchians", Revueltosaurus, and poposaurs.

Bull Canyon fauna from NMMNH loc 1 (from Hunt, 2001)

?Colobodontidae indet.
Actinopterygii indet.
Ceratodontoidei indet.
Quayia zideki (Coelocanthidae)
Apachesaurus gregorii (Metoposauridae)
Lepidosauria indet.
Rhynchosauridae indet.
Parasuchia indet.
Typothorax coccinarum (Aetosauria)
Rauisuchidae indet.
Shuvosaurus inexpectatus (Poposauridae)
Revueltosaurus callenderi (Pseudosuchia)
Sphenosuchia indet. ?theropoda indet.

What is striking is that except for the indeterminate rhynchosaur and leidosaur this fauna is extremely similar to sites in Petrified Forest National Park where Revueltosaurus callenderi is common (Parker and Irmis, 2005). Indeed the site that provided Chinlechelys is also the type locality of R. callenderi. Thus, it is these types of units (terrestrial and Revueltosaurus bearing) which may represent the proper environment and age to provide more "proto-turtle" material.

REFERENCES

Carpenter, K. 1997. A giant coelophysoid (Ceratosauria) theropod from the Upper Triassic of New Mexico, USA. Neues Jahrbuch für Geologie und Paläontologie, Monatschefte 205(2):189-208.

Dubiel, R. F. 1994. Triassic deposystems, paleogeography, and paleoclimate of the Western Interior; pp. 133-168 in M. V. Caputo, J. A. Peterson, and K. J. Franczyk (eds.), Mesozoic Systems of the Rocky Mountain Region, USA. Rocky Mountain Section Society of Economic Paleontologists and Mineralogists (Society for Sedimentary Geology), Denver, Colorado.

Hunt, A. P. 2001. The vertebrate fauna, biostratigraphy, and biochronology of the type Revueltian land-vertebrate faunachron, Bull Canyon Formation (Upper Triassic), East-central New Mexico; pp. 123-152 in in S. G. Lucas and D. S. Ulmer-Scholle (eds.), Geology of the Llano Estacado: New Mexico Geological Society Guidebook, 52nd Field Conference.

Joyce, W. G., Lucas, S. G., Scheyer, T. M., Heckert, A. B., and A. P. Hunt. 2008. A thin-shelled reptile from the Late Triassic of North America and the origin of the turtle shell. Proceedings of the Royal Society B, published early on-line, doi: 10.1098/rspb.2008.1196.

Lehman, T. M. 1994a. The saga of the Dockum Group and the case of the Texas/New Mexico boundary fault. New Mexico Bureau of Mines and Mineral Resources Bulletin 150:37-51.

Lucas, S. G. 1993b. The Chinle Group: Revised Stratigraphy and Biochronology of Upper Triassic Nonmarine Strata in the Western United States; pp. 27-50 in M. Morales (ed.), Aspects of Mesozoic Geology and Paleontology of the Colorado Plateau: Museum of Northern Arizona Bulletin 59.

Lucas, S. G., and O.J. Anderson. 1994. The Camp Springs Member, base of the Late Triassic Dockum Formation in West Texas. West Texas Geological Society Bulletin 34(2):5-15.

Newell, A. J. 1993. Depositional environment of the Late Triassic Bull Canyon Formation (New Mexico): Implications for ‘Dockum Formation’ Paleogeography; pp. 359-368 in S. G. Lucas and M. Morales. 1993. The Nonmarine Triassic: New Mexico Museum of Natural History & Science Bulletin 3.

Parker, W. G., and R. B. Irmis. 2005. Advances in Late Triassic vertebrate paleontology based on new material from Petrified Forest National Park, Arizona; pp. 45-58 in A. B. Heckert and S. G. Lucas (eds.), New Mexico Museum of Natural History and Science Bulletin 29.