LARGE CROCODYLIANS FROM THE EARLY BRIDGERIAN EOCENE OF THE NORTHERN AND CENTRAL GREEN RIVER BASIN, WYOMING

The large size, diverse landscape and warm climate of the Eocene Green River Basin made it ideal for crocodylians to thrive and diversify. This study describes the large bodied taxa from the early Bridgerian (Gardnerbuttean and early Blacksforkian, Br1a-Br1b) of part of that basin. Fossils are from the La Barge and Cathedral Bluffs members of the Wasatch Formation and the lower part of the overlying Bridger Formation. Most large crocodylians are rare in the alluvial fan to meandering stream deposits of the Wasatch members, usually represented only by fragmentary remains. They are, however, more common and complete in the meandering stream and lake-margin deposits of Bridger Formation.

The basal eusuchian Borealosuchus wilsoni and the crocodylid “Crocodylus” affinis, are the most common taxa represented in the early Bridgerian of the study area. Both were large (4m+) crocodile-like aquatic predators with intermediate width snouts. B. wilsoni is most easily recognized by it wide interorbital plate and short symphysis. “C.” affinis is distinguished by narrower frontals which rise above the posterior nasals and longer symphysis. Both are present in Basin-Margin and upstream and lake margin Basin-Center deposits. The large, long and narrow snouted crocodylid “Crocodylus” acer is considerably rarer than the two previous forms. It is represented by very fragmentary remains restricted to lake and lake margin deposits. The enigmatic “Crocodylus” sulciferus is represented by its very distinctive teeth that bear strongly developed enamel folds and sulci. These teeth are rare and are thus far restricted to Basin-Center deposits of Br1b and younger age. When described, the teeth of “C.” sulciferus were permanently separated from its associated skull, but a skull of a large broad snouted crocodylian (lacking a premaxillary notch) can now be associated with these distinctive teeth. Further preparation, description, and analysis of this skull are required before its systematic position can be assessed. Fragmentary remains, mostly isolated blade-like and serrated teeth, probably referable to Boverisuchus (Pristichampsus) vorax are also present. These presumably terrestrial crocodylians were somewhat smaller than the above forms and are quite rare except in Basin-Margin deposits of the earliest Bridgerian (Br1a).