TAPHONOMY OF VERTEBRATE ASSEMBLAGES FROM THE PALEOGENE OF NORTHWESTERN WYOMING AND THE NEOGENE OF NORTHERN PAKISTAN

We compare the taphonomy of vertebrate assemblages from two long continental records-the early Paleogene of the Bighorn Basin, Wyoming, and the Neogene Siwalik sequence of northern Pakistan. Both sequences contain a similar array of fluvial facies, and the abundance of these facies differs among formations. We document environments of preservation of vertebrate localities over time to determine comparability of fossil assemblages within and between sequences. Changes in sample size and species richness are noted to reveal potential sampling effects on patterns of faunal turnover. Preservational history determined the environment, sample size, quality of specimens, taxonomic composition, and spatial and temporal resolution of fossil assemblages and thereby the quality of the fossil record and its suitability for further analyses. In both sequences, changes in prevailing taphonomic processes reflect changes in lithofacies and habitat distribution. Correlated changes are found in fossil productivity, species richness, and faunal composition. Both sequences contain some episodes of apparent faunal change in which appearances and disappearances of rare taxa can be attributed principally to changes in sample size. The Paleogene record has high taxonomic resolution (i.e., to genus or species) for most mammalian fossil remains. Temporal and spatial averaging of Paleogene fossil assemblages changes with lithofacies. The Neogene record has higher taxonomic resolution for remains of small mammals (<2 kg) than of large mammals. Ah formations have several fossiliferous facies, with moderate to high degrees of temporal averaging and low to high degrees of spatial averaging. Different preservational circumstances impose different constraints on paleocological and evolutionary analyses. The best opportunities for paleocommunity reconstruction are provided by high taxonomic resolution, large samples, and varied environments of preservation. These circumstances are found in limited portions of each record. The best opportunities for documenting evolution within lineages and species-replacement patterns are provided by high taxonomic resolution, high temporal resolution, and consistent preservational context. These taphonomic attributes pertain to the more common Paleogene mammals, particularly from the rich paleosol localities of the Willwood Formation, and to the more common Neogene small mammals from abandoned-channel fills of the Siwalik record.