A NEW FOSSIL VERTEBRATE ASSEMBLAGE FROM THE FRONTIER FORMATION (CRETACEOUS: CENOMANIAN) OF PARK COUNTY, WYOMING

The Frontier Formation of Wyoming has received considerable study due to its economic importance, but its vertebrate paleontology remains virtually unknown. The “Final Frontier” locality in Park County, Wyoming, collected by the Raymond M. Alf Museum of Paleontology at The Webb Schools, preserves a relatively diverse faunal assemblage that adds important new data on the Frontier Formation. Fossils were collected in tailings from a bentonite mine, within a highly indurated conglomeratic siltstone, here interpreted as a lag deposit. The fossiliferous horizon can be constrained to Cenomanian (between 94.1 and 97 Ma) using local stratigraphy, biostratigraphy, and published radiometric dates.

All fossil vertebrates collected between 2021 and 2023 (n=167) were identified to the finest taxonomic resolution possible. Osteicthyes dominate (75% of the assemblage, n=126), with Enchodus (n=23) most common, followed by Pycnodontiformes (n=8). Enchodus specimens show morphologies potentially consistent with several taxa, including E. shumardi, E. petrosus, and E. dirus; additional study is required to refine the identifications for Enchodus at this locality. Chondricthyes (10%, n=16) include Lamniformes (Cretoxyrhina agassizensis, Archaeolamna kopingensis, and Odontaspididae indet.), Hybodontiformes (Meristodonoides sp.), and Rajiformes (Pseudohypolophus mcnultyi). Tetrapoda (15%, n=25) include Testudines, Crocodyliformes, and Plesiosauria. At least two plesiosaur taxa occur in the sample, based on tooth morphology. A fragment of maxilla represents the first known occurrence of the pholidosaurid Terminonaris sp. in Wyoming.

The assemblage at the “Final Frontier” locality is similar to that at penecontemporaneous localities throughout the western margin of the Western Interior Seaway, consistent with previous hypotheses of faunal homogeneity in the WIS during the Cenomanian and also consistent with a nearshore marine environment. Additional sampling is required to refine the understanding of the Frontier Formation in Wyoming.