COMPARATIVE TAPHONOMY OF SURFACE COLLECTED AND BULK SAMPLED FOSSIL COLLECTIONS FROM THE UPPER CRETACEOUS JUDITH RIVER FORMATION, NORTH-CENTRAL MONTANA

The Judith River Formation preserves numerous vertebrate microfossil bonebeds that represent skeletal accumulation in both lacustrine and fluvial depositional settings. Previous studies have suggested that microvertebrate fossils preserved in fluvial facies were reworked from preexisting concentrations in lacustrine facies. Testing this hypothesis requires careful comparison of collections from both channel-hosted and lake-hosted sites. Here we describe the taphonomic characteristics of a large sample of vertebrate bioclasts (n=7655) derived from two richly fossiliferous lacustrine bonebeds, focusing on the comparison of surface-collected material and bulk-sampled material sieved and picked down to 500 microns. Surface collections, which represent over a decade of recovery effort by field crews, were characterized with regard to taxonomy, size, shape, breakage, and weathering. Fossil shape was classified as compact, platy, elongate, and conical. Weathering and breakage patterns were scored using existing schemes specifically designed for fossil assemblages. Maximum dimensions of surface collected fossils were measured with digital calipers. The taphonomy of sieved samples was characterized in similar fashion, although the size of bioclasts in this extremely fine-grained fraction was documented using an image analysis system coupled with a stereomicroscope. With regard to taxonomic representation, the surface and sieve assemblages are quite distinct, with surface assemblages dominated by dinosaur, fish (mostly ganoid scales and skull fragments), and turtle and sieved samples dominated by fish (mostly teleost vertebrae and scales). Shape characteristics are somewhat more comparable between site types, but still distinct, with surface assemblages dominated by platy elements and sieved samples dominated by compact elements. Elongate and conical elements are relatively rare in both collection types. Surface collected samples range in size from 2.1 – 82.9 mm, averaging 12.3 mm long axis. In contrast, sieved samples range in size from 0.4 – 15.7 mm, averaging 1.7 mm long axis. Distinctions between collections highlight the potential biases in microvertebrate collections, and provide a means of assessing the quality of faunal reconstructions derived from similar localities.