Taphonomy and preservation of a monospecific titanothere assemblage from the Washakie formation (Late Eocene), southern Wyoming. An ecological accident in the fossil record
Abstract More than twenty partial skeletons, all cf. Mesatirhinus sp., were discovered in overbank deposits associated with sandstone channels within the Adobe Town Member of the Washakie Formation (Washakie Basin, southwest Wyoming). Taphonomic and sedimentologic data indicate a mass mortality event with reduced post-mortem transport of the carcasses. Such a mass-death assemblage of a large mammal taxon is a rare occurrence for the Paleogene record. Accordingly, position and orientation of all the bones were recorded to make possible an accurate detailed map of the deposit. The remains principally include disarticulated skeletons of both sexes, from juveniles to old adult. Some axial portions and limbs remained articulated; others suggest partial articulation. The collected remains were scattered in loose associations within an area of about 7.5 × 9 m. Recent erosion (sheetwash) has destroyed an unknown extent of the original mass burial to the east and south of the excavation, but the 68 oriented bonebearing blocks collected represent about 50% of the exposed site. The skeletal remains occurred within a single unit of poorly sorted but generally upward-fining quartz grit with a basal mudflake conglomerate. The bones are not abraded, but have suffered from severe compaction damage and recent freeze-thaw action. The sediment represents a mass flow accumulation deposited during the waning stages of a flash flood. The large number of individuals of a single taxon suggests that part of a herd of titanotheres was killed in a single mass mortality event. The cause of death is not known, but it is likely that the accumulation represents a small fraction of a large herd that was crossing a flooded river. The carcasses drifted downstream to be left as a carcass jam on the flood plain as the floodwaters withdrew. It follows that the assemblage represents a close approach to a single taxon slice from a terrestrial biocoenosis, and allows interpretations to be made about the autecology (social behavior, age composition and community structure) of titanotheres. Decomposition of the integument and other tissues of partially buried carcasses may have been a factor contributing to the “pockety” nature of the sediment.