A LATE WASATCHIAN (LATE EARLY EOCENE) VERTEBRATE ASSEMBLAGE PRESERVED IN MEANDERING STREAM CHANNEL DEPOSITS, NORTHERN RED DESERT, WYOMING

The northern Red Desert of the northeastern Green River Basin contains limited exposures of the uppermost Main Body of the Wasatch Formation. A small outcrop belt know as The Pinnacles preserves approximately 50 meters of the fluvial Wasatch Formation directly under the stromatoliticlimestone benches of the lacustrine Tipton Shale of the Green River Formation. The sequence consists primarily of thin crevasse splay sandstones and flood basin mudstones overprinted by mature paleosols. Isolated ribbon sand bodies punctuate the section. The thickest of these units preserves a rich and diverse assemblage of latest Wasatchian (Wa7, Lostcabinian, Lambdotherium Zone) terrestrial vertebrates.

The Pinnacles area has yielded 400 catalogued specimens making it one of the richest latest Wasatchian sites. The assemblage includes at least 40 species of mammals (including Lambdotherium) and 14 species of reptiles, as well as subordinate numbers of other vertebrates. The mammalian assemblage is strikingly equable both in terms of diversity and occurrence (larger mammals such as Hyracotherium are as abundant as smaller taxa such as Hyopsodus and small mammals such as rodents and insectivores). The reptilian portion of the assemblage is dominated by lizards, which account for eight species, five of which are glyptosaurines.

Although fossil vertebrates have been recovered from the over-bank mudstones, the vast majority are preserved as isolated teeth and small cranial and mandibular fragments within the thickest of channel sandstone bodies. The fossils occur in the lower portion of the sand body as a basal lag, in the coarse toes of trough cross-bed sets, and at the tops of individual cross-beds within sets. Although their position with the sand body clearly indicates transportation and deposition on the lower point bar, the relatively un-abraded nature of the remains argues against significant transportation. Given the aggrading nature of the stream system, the lack of significant incision into underlying flood basin deposits, and the poorly fossiliferous nature of the flood basin mudstones, it is postulated that the vertebrate remains originally accumulated (and degraded) in the stream channel during an abandonment episode and were locally remobilized and re-deposited in the early stages of stream reactivation.