ENVIROMENTAL PREFERENCES OF CRETACEOUS 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 Kirtland and Fruitland formations. Bordered by the Picture Cliffs sandstone and San José formations this area is viewed as a mix of fluvial and swamp environments. The conditions of this coast appear to provide an excellent environment for turtles. Specimens of four genera Dermatemys, Denazymens, Neurankylus and Trionichids discussed below were examined at the New Mexico Museum of Natural History.

Dermatemys is a rather unique genus. Members have reduced scute margins. The only living member of this genus, Dermatemys mawii, bears a shell similar to Cretaceous species. D. mawii is highly aquatic having difficulty moving on land. It would seem likely the Cretaceous forms were also highly aquatic given the extreme similarity in shell osteology.

The Cretaceous species Denezymens nodosa has an exaggerated widening on the shell toward the rear and a heavily knobbed surface. It is likely from a lower flow régime considering the habits of similar turtles i.e. genus Psuedemys. The Cretaceous Denezymens ordnata bears the same ornamentation lacking the widened posterior costals. Given the roughness of the shell surfaces, low plastron curves and low domed shell, both seem aquatic.

The Cretaceous genus Neurankylus shows some variation in the prominence of the neural ridge. Neurankylus’s similarities to living turtles are closest to the strongly pronounced neural ridges of the North American Wood, Blandings and Asian Leaf turtles. This ridge and the overall depth of the shell lead to the conclusion that the turtles were semi-aquatic spending at least some time on land.

Modern Trionichidae turtles remain structurally similar to their ancestors in the Cretaceous as shown in the fossil specimens. As such it would seem likely they retain a similar life habit. This group is dependent on a soft stream bottom to burrow for defense and hunting.

All of these finds point to the conclusion that there was abundant aquatic vegetation in this area. However few fossils of this flora have been found. Many of these plants are only represented by the interbedded coal fields they formed.