LATE WASATCHIAN AND EARLY BRIDGERIAN EOCENE DERMATEMYDID TURTLES FROM THE WEST-CENTRAL AND NORTHERN GREEN RIVER BASIN, WYOMING

Dermatemydids are a small group of aquatic turtles that dates to the Late Cretaceous but has been characterized by very low diversity throughout their history. They are represented today only by the Central American River Turtle (Dermatemys mawaii), a large nocturnal herbivorous form that is critically endangered. This study describes Lostcabinian (Wa7), Gardnerbuttean (Br1a), and early Blacksforkian (Br1b) specimens collected from the alluvial fan to meandering stream deposits of the La Barge and Cathedral Bluffs members of the Wasatch Formation and meandering stream and lake-margin deposits of the overlying Bridger Formation.

Early workers described several late Wasatchian to Bridgerian dermatemydids from western North America. We agree with recent workers that only two species are represented. Baptemys wyomingensis (including B. fluviatilis) is characterized by the absence or poor posterior development of lateral costal carinae and a gulo-humeral sulcus crossing the entoplastron near its midpoint. Baptemys tricarinata (“Notomorpha garmanii”) has well developed lateral carinae that extend to the 1st costal and a gulo-humeral sulcus well posterior to the entoplastron midpoint. Both taxa range from the Lostcabinian (Wa7) through the early Bridgerian (Br1a-Br1b) in our study area.

Many shell characteristics are more common or are better developed in one species or the other, but all are too variable to be reliable diagnostic tools for separating these two very similar and closely related forms. Notomorpha gravis, N. garmanii, Kallistira costilata have all been identified as representing the same taxon as B. tricarinata based heavily on their Wasatchian age. The type of N. gravis was undiagnostic and has been lost, the type of N. garmanii is very fragmentary and lacks diagnostic characteristics, and one isolated costal referred to K. costilata possesses a lateral carina. We considered all these nomina dubia, and retain Baptemys tricarinata as the appropriate name for this taxon.

We speculate that B. tricarinata was established in the earlier Wasatchian, overlapped in Basin-Margin areas with B. wyomingensis during the Wasatchian-Bridgerian transition, and continued in basin centers of the Blacksforkian but was eventually replaced by B. wyomingensis by the late Blacksforkian middle Bridgerian (Br2).