ENVIRONMENTAL PREFERENCES OF FOSSIL TURTLES OF THE SAN JUAN BASIN, NEW MEXICO

The turtles of the Cretaceous of New Mexico’s San Juan Basin show a variety of environmental preferences and habits. The turtles are from the Naciemento, Ojo Alamo, Kirtland and Fruitland formations. The area sampled is interpreted as follows based on Fasset 1971. The swamp facies of the Fruitland Formation are overlain by the flood plain alluvium in the Kirtland Formation. Above this are deposits Interpreted as braided streams the Ojo Alamo Formation. Finally the eastern most of the formations in this study are the meandering stream deposits of the Naciemento Fm. Environmental preferences were interpreted from shell geometry including the ratio of plastron width to carapace width. This coastal setting appears to have provided an excellent environment for turtles. Specimens of the genera Adocus, Boremys, Denazinemys, Neurankylus, Scabemys, Thescelus, Basilemys and Asperetoides were examined.

The genus Adocus is likely a genus of highly aquatic low flow turtles. The genus Boremys represented in the study area by Boremys grandis this large turtle was likely semi-aquatic, and possed a wider plastron than Denazinemys and Scabemys.

The genera Denazinemys and Scabemys both possess wide carapaces relative to their plastrons. This indicates that they were likely aquatic in a higher flow regime. The genus Neurankylus is a group of baenid turtles with a narrow plastron and a spined keel. This turtle appears to be from a higher flow area. Further, it shows adaptations to avoid predation such as spines. The genus Thescelus is something of a mixed bag in the community some seemingly aquatic, but others appear more semi-aquatic.

Basilemys gaffneyi is a broad plastron species resembling modern turtles of the genera Glyptemys, Rhinoclemmys, Geochelone, and Agrinemys (Testudo). This leads to the conclusion that they were likely a similarly semi-aquatic species needing water to live, but easily moving between sources. It may indicate a likely preference for slower, more easily navigable waters.

Aspideretoides is a genus of trionichid turtles that we interpret to be similar to modern trionichids which possess a soft shell over the bone. We believe that it also likely they preferred a soft bottom as do living trionichids.