BIOSTRATIGRAPHY AND PALEOBIOLOGY OF THE LATE PALEOCENE TITANOIDES LOCALITY, FORT UNION FORMATION, BISON BASIN, WYOMING

The Titanoides locality is a poorly documented site of late Paleocene age in the Bison Basin, Fremont County, south-central Wyoming. Since its discovery in 1953, a moderately diverse mammal fauna has been reported from the Titanoides locality, including the holotypes of Litocherus lacunatus and Haplaletes serior. Previous research identified Plesiadapis fodinatus at the Titanoides locality, and the site has long been considered to be late Tiffanian (Ti-5) in age on the basis of its plesiadapids, which have been interpreted as part of a gradually evolving lineage in western North America. However, key index fossils for Ti-5 including the invasive clades Dinocerata (Probathyopsis) and Arctostylopidae (Arctostylops) have never been reported from the Titanoides locality. Recent fieldwork has significantly expanded our knowledge of the mammalian fauna from the Titanoides locality. Our revised mammalian faunal list includes: Litocherus lacunatus, Plesiadapis churchilli, Ignacius frugivorus, Aphanocyon sp., Chriacus sp., Thryptacodon cf. australis, Mimotricentes fremontensis, Haplaletes serior, Phenacodus grangeri, Ectocion cedrus, Titanoides primaevus, and Dissacus sp. Based on much larger samples than were previously available, we reidentify the plesiadapid occurring at the Titanoides locality as P. churchilli rather than P. fodinatus. We also note the presence of other taxa characteristic of Ti-4 in our updated faunal list, including Aphanocyon sp. and Ignacius frugivorus. The Ti-4 biochron is subdivided into Ti-4a and Ti-4b on the basis of the first occurrence of Phenacolemur, which remains unknown from the Titanoides locality. Because Phenacolemur archus was common during Ti-4b, its absence at the Titanoides locality may indicate the locality is Ti-4a in age. Our results highlight the advantage of using the first appearances of invasive clades over purported members of gradually evolving lineages in biostratigraphic correlation. Our refined age for the Titanoides locality will inform future investigations of paleoecology, paleobiogeography, and paleoclimate.