EOCENE TURTLES FROM THE DISTAL DEPOSITS OF THE CATHEDRAL BLUFFS TONGUE (WASATCH FORMATION), RED DESERT, WYOMING

This study describes an unusual assemblage of fossil turtles from Eocene deposits of the Cathedral Bluffs Tongue of the Wasatch Formation in the Green River Basin of Wyoming. Cathedral Bluffs deposits range from conglomerates, coarse sandstones, and conglomeratic to sandy mudstones deposited by alluvial fans close to the Wind River mountain source (basin margin environment), through braided and meandering stream sandstones and mudstones, to lake-margin mudflat and meandering stream fine sandstones and mudstones farthest from the mountains (basin center environment). The fossils are recovered from the most distal deposits of the Cathedral Bluffs Tongue along Bush Rim where it thins, interfingers with, and pinches-out into the lacustrine deposits of the Laney Shale Member of the Green River Formation.

Typical basin center assemblages contain a diverse array of trionychid (soft-shelled), emydid (slider, pond, box, and painted), baenid (extinct snapper-like), and dermatemydid (river) turtles. The upland deposits of the basin margin contain only an undescribed emydid (informally referred to as “Southpassemys”) that was adapted to the faster flowing alluvial fan streams. At Bush Rim, however, “Southpassemys” occurs alongside the common basin center (lowland) turtles. Several other unusual co-occurrences of mixed upland and lowland reptile and mammal groups have also been noted in the Bush Rim assemblage.

It appears that during the earliest Bridgerian (Br1a), rapid movement on the Wind River Thrust may have caused the alluvial fan (upland) environments to rapidly expand into the basin center lake-margin environment, bringing together what had been geographically distinct upland and lowland vertebrate faunas (“a forced fauna”).

In other vertebrate groups, where upland and lowland forms have been forced together, it has been noted that the most similar animals tend to evolve away from one another morphologically. A morphometric analysis of the large sample of Bush Rim “Southpassemys” specimens indicates little morphological difference from basin margin populations other than a reduction in size. The smaller size of this turtle in the Bush Rim sample, however, may be an evolutionary response to competition with the larger bodied basin center emydid species Echmatemys wyomingensis and E. septaria.