PALEOENVIRONMENTAL PREFERENCES AND DISTRIBUTION OF LATE CRETACEOUS TURTLES IN THE FRUITLAND AND KIRTLAND FORMATIONS SAN JUAN BASIN, NEW MEXICO

The Upper Cretaceous (upper Campanian) Fruitland and Kirtland formations in the San Juan Basin contain an extensive fossil record of turtles (Sullivan et al., 2012). These formations represent a transect from coastal plain and swamp (Fruitland) to inland flood plain facies (upper Kirtland), and thus allow analysis of how their fossil turtles have varied with respect to paleoenvironments. We compared carapace maximum width to plastron width behind the bridge, finding that a ratio ≤1.45 is indicative of a terrestrial turtle.

Based on these analyses, Basilemys is terrestrial based on the low bridge height and the wide plastron. Denazinemys and Scabremys are highly aquatic given the wide low plastron and resemblance to modern river turtles, particularly the podocnemids. Neurankylus bauri shows some resemblance to modern map turtles, and resided in a fluvial environment. Boremys and Thescelus appear to fall in the middle-- neither fully aquatic nor terrestrial-- possibly similar to modern pond turtles (i.e. Psuedemys)given their midrange in shape. Aspideretoides are soft shell turtles with habits similar to modern trionychids in being aquatic and using their soft shell to burrow in the substrate.

These turtles show a variation in relative abundances in fossil finds across the Fruitland and Kirtland formations. The abundance of trionychids is highest in the De-na-zin Member of the Kirtland Formation (and Naashoibito Members of the Ojo Alamo Formation). Adocus and Denazinemysdominate the Fossil Forest and Hunter Wash members of the Fruitland and Kirtland formations.

The diversity of turtles appears higher in the Fruitland and Hunter Wash Member of the Kirtland Formation than in the upper members of the Kirtland Formation. This likely resulted from a higher diversity of environments near the coast with swamp channels in addition to the flood plain deposits also seen in the stratigraphically higher units. Basilemys is a prime example, being known from the Fossil Forest Member of the Fruitland Formation and the Naashoibito Member of the Ojo Alamo Formation. Presence of a swamp environment during Fruitland deposition may have resulted in the increase in the abundance of Basilemys by creating an area where the terrestrial species came into proximity with channels and floods, which increased the possibility of preservation.