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 conti nental records-the early Paleogene of the Bighorn Basin, Wyoming, and the Neogene Siwalik sequence of northern Pakistan. Both sequences cont ain a similar array of fluvial facies, and the abundance of these faci es differs among formations. We document environments of preservation of vertebrate localities over time to determine comparability of fossi l assemblages within and between sequences. Changes in sample size and species richness are noted to reveal potential sampling effects on pa tterns of faunal turnover. Preservational history determined the envir onment, 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 a re found in fossil productivity, species richness, and faunal composit ion. Both sequences contain some episodes of apparent faunal change in which appearances and disappearances of rare taxa can be attributed p rincipally to changes in sample size. The Paleogene record has high ta xonomic resolution (i.e., to genus or species) for most mammalian foss il remains. Temporal and spatial averaging of Paleogene fossil assembl ages 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 hi gh degrees of temporal averaging and low to high degrees of spatial av eraging. Different preservational circumstances impose different const raints on paleocological and evolutionary analyses. The best opportuni ties for paleocommunity reconstruction are provided by high taxonomic resolution, large samples, and varied environments of preservation. Th ese circumstances are found in limited portions of each record. The be st opportunities for documenting evolution within lineages and species -replacement patterns are provided by high taxonomic resolution, high temporal resolution, and consistent preservational context. These taph onomic attributes pertain to the more common Paleogene mammals, partic ularly from the rich paleosol localities of the Willwood Formation, an d to the more common Neogene small mammals from abandoned-channel fill s of the Siwalik record.