Mammalian paleontology of freshwater limestones from the Paleocene -Eocene of the Clarks Fork Basin, Wyoming.

Abstract

The Clarks Fork Basin of northwestern Wyoming includes more than 2200 meters of richly fossiliferous fluvial strata representing the upper Paleocene Fort Union and lower Eocene Willwood formations. Fossiliferous freshwater limestones are known from 30 localities spanning faunal zones Cf-1 of the Clarkforkian land-mammal age through Wa-4 of the Wasatchian land mammal age. Sedimentological and petrographical features of the limestones are most similar to those of ponded-water carbonate crusts. Limestones are associated with wet soils and possibly swampy microhabitats. Mammal communities differing from other more commonly sampled communities may have occupied these habitats. Mammals probably entered limestone assemblages as articulated skeletons that were subsequently bioturbated. It is likely that predation and scavenging, pit-trapping, and normal attritional processes all contributed to concentration of bone. The limestone faunal assemblages are mostly composed of mammals and lizards with virtually no aquatic animals, have mammalian faunas mostly composed of insectivores, and preserve a proportionally larger sample of small mammals than do the surrounding mudstone deposits. Limestones from the Clarks Fork Basin contain a diversity of exceptionally well preserved small mammals. The Clarkforkian limestone faunal assemblage has at least 42 species of mammals, nine of which are new and described here. The Wasatchian limestone faunal assemblage has at least 47 species of mammals, 13 of which are new and described here. Sampling from this unique depositional environment is documenting range extensions and, in at least one case, the co-occurrence of species pairs previously regarded as ancestor and descendant (anachronistic taxa). Diversity and size distributions of small mammals in the limestones remain constant across the Paleocene-Eocene transition (not including Wa-0). Small mammal diversity increases significantly by Wa-4. This change in diversity, which is coincident with a significant floral turnover, may reflect a mammalian response to a rapid increase in mean annual temperature at this time. Using careful preparation and documentation techniques, it is possible to recover partial or even complete skeletons of Paleogene micromammals from the limestones. New skeletons of small Paleocene mammals from limestones provide evidence bearing on current paleobiogeographic, phylogenetic, and functional hypotheses.