Comparative taphonomy and paleoecological reconstruction of two microvertebrate accumulations from the late Cretaceous (Maastrichtian) Hell Creek Formation, eastern Montana

Abstract

Although microvertebrate accumulations are commonly used for paleoecological reconstructions, the taphonomic processes affecting the final taxonomic composition of an accumulation are often ignored. To explore the correlation between taphonomic processes and taxonomic structure, one floodplain and one channel-lag deposit from the Maastrichtian Hell Creek Formation were compared. Specimens were quarried to collect high-resolution vertical and horizontal distribution data. Distribution of skeletal elements with specific physical attributes and relative abundance of taxa are shown to correlate with the hydraulic indicators (i.e., grain size, sedimentary structures) of the depositional facies. Conor's Microsite (CM) is located within a laminated sandstone, siltstone, and mudstone facies interpreted as a fluvial overbank deposit. Just Past Celeste Microsite (JPC) is located in a massive sandstone facies and is interpreted as a channel lag deposit. Relative abundance data in conjunction with statistical test results show size, shape, abrasion, and taxonomic compositions to vary significantly between assemblages. Chisquared tests and Correspondence Analyses of CM and JPC data sets indicate significant differences between taxonomic compositions.

The CM assemblage is dominated by tabular, low density elements such as cycloid scales and fish vertebrae. Dense, equidimensional elements such as teeth and ganoid fish scales dominate the JPC assemblage. Transport distances, hydraulic equivalencies of dominant skeletal elements, amount of hydraulic sorting and reworking, and degree of time-averaging varies between deposits and significantly affect taxonomic distributions. Chi-squared tests and rank order analysis results demonstrate that relative abundance of taxa can be determined for portions of the assemblages despite different taphonomic processes. Most notably, relative abundance of hydraulically equivalent skeletal elements from morphologically similar organisms can be compared regardless of accumulation in non-isotaphonomic deposits. Statistical comparisons were made among osteichthians using ganoid scales, caudates using vertebrae, ornithischians using teeth, and testudinates using shell fragments. Results show that portions of the assemblage analyzed using hydrodynamically equivalent elements are not significantly different.