INVESTIGATING THE FORMATION OF VERTEBRATE MICROFOSSIL BONEBEDS FROM THE LATE CRETACEOUS HELL CREEK FORMATION IN GARFIELD COUNTY, MONTANA

The accumulation of vertebrate microfossils has long been debated, with either carnivore behavior or hydraulic processes typically favored as accumulation models. However, recent studies have hypothesized that vertebrate microfossil bonebeds (VMBs) accumulate in small lacustrine environments and are only later reworked by higher-energy fluvial systems. If this is the case, one would expect to see specific taphonomic and paleoecological differences: fluvially deposited VMBs should exhibit poorer preservation and more size winnowing than those deposited in lacustrine environments. Smaller taxa may be present in lacustrine VMBs, but absent in fluvial ones due to this size-winnowing. This study investigates the relationship between the taphonomy of microvertebrate fossils and their depositional environment using assemblages preserved at multiple VMBs from the Hell Creek Formation in central Montana.

We collected 20 kg bulk samples from five fluvial and lacustrine VMBs identified in the Late Cretaceous Hell Creek Formation in Garfield County, MT. After screening, identifying, and sorting the samples, we measured taphonomic characteristics of the fossils, such as wear, size, and shape. The taphonomic data were statistically analyzed to see whether preservation differed significantly according to paleoenvironment. Additionally, we quantified and compared species abundance, richness, and evenness among VMBs to assess paleoecological differences between sites deposited in lacustrine versus fluvial environments.

Preliminary results from all sites indicate a high abundance of osteichthyans (particularly representatives of Lepisosteidae) with successively rarer occurrences of reptiles, amphibians, and mammals. The majority of the fossils possess aquatic or semiaquatic affinities. Lastly, the samples derived from lacustrine deposits appear to have greater species richness than those samples from fluvial deposits. Together these data provide support for a common depositional source across multiple sites.