Fossil Reptile Assemblages and Depositional Environments of Selected Early Tertiary Vertebrate Bone Concentrations, Bighorn Basin, Wyoming.

Abstract

Fossil vertebrate remains occasionally occur in extraordinary concentrations ("quarry beds") in otherwise unfossiliferous strata. These isolated occurrences are often the only local record of terrestrial vertebrate faunas for the stratigraphic intervals they represent, leading to their use in documenting "faunal" evolution. This study attempts to explain the formation of some of these concentrations and to assess the validity of their use in faunal, paleoecologic, and paleoclimatic reconstructions. Four highly fossiliferous Early Tertiary localities in the Northern Bighorn Basin were selected for this investigation: Rock Bench Quarry (middle Paleocene); Cedar Point; and Princeton quarries (late Paleocene); and SC-188 quarry (earliest Eocene). The sedimentology and microstratigraphy of each quarry deposit was examined in order to determine the depositional environments responsible for their formation. Data on the orientation, physical condition, and hydraulic class of bone elements were then combined with compilations of the ecological types, sizes, and frequencies of vertebrate species in order to evaluate the relationships between the depositional environments of the quarry beds and the origin of the bone concentrations. The reptilian assemblages from each of the quarries and adjacent localities are described and analyzed. Four new taxa are named: Proxestops gastrodon; Procaimanoidea tanyrhynchus; Leidyosuchus oligodon; and Allognathosuchus leptodon. Information from the quarry stratigraphy and the fossil reptiles and their associated biota, indicate that individual quarry beds are the result of unique combinations of taphonomic processes, paleoecologic conditions, and depositional environments that do not uniformly sample the vertebrate faunas extant locally at the time of their formation. There is little evidence for significant climatic deterioration during the Paleocene in the Bighorn Basin.