COMPARISON OF EOCENE BASIN-MARGIN AND BASIN-CENTER CROCODILES FROM THE BRIDGER FORMATION, GREEN RIVER BASIN, WYOMING

This study describes new specimens of Eocene crocodylids from basin-margin deposits and compares them morphologically to age-equivalent specimens from basin-center environments.

Fossil crocodylians are abundant and diverse elements in Rocky Mountain Paleogene faunas. The majority of described specimens and taxa are known only from basin-center deposits representing distal fluvial, lake-margin, and lacustrine depositional environments well removed from upland source areas. Several different crocodile species have been described from the Green River Basin, the most common being the large and relatively broad snouted “Crocodylus” affinis that is found in meandering stream deposits of the basin-center. Other species include the narrow snouted “Crocodylus” acer, which was the dominant species in the lacustrine environment, and “Crocodylus” sulciferus, and Borealosuchus wilsoni which may also have been environmentally restricted.

The South Pass area of the northern Green River Basin preserves several hundred meters of lacustrine deposits of the Green River Formation alternating with predominantly fluvial deposits of the Wasatch and Bridger Formations. The crocodiles described in this study were recovered from meandering stream channel sandstones of the lower part of the Bridger Formation exposed on the flank of Continental Peak. Based on mammalian biostratigraphy and magnetostratigraphy, these deposits are early Blacksforkian (Br1b, early Bridgerian, "Bridgerian A") in age.

The basin-margin crocodile specimens described in this study have been prepared and compared to the descriptions of the known basin-center crocodile species. The cranial morphology of these basin-margin specimens are most similar to “Crocodylus” affinis and Borealosuchus wilsoni but differ in several minor ways from the basin-center specimens.

Vertebrate faunas from basin-margin settings are characterized by anachronistic taxa (appearing earlier or later than in basin-centers), rare taxa (common in basin-margins but rare in basin-centers), unique taxa (found only in basin-margins), and morphologically distinct forms assignable to basin-center taxa. The crocodiles described here appear to be an example of this last pattern.