VERTEBRATE MICROFOSSIL BONEBEDS AS BENCHMARKS FOR TERRESTRIAL VERTEBRATE DIVERSITY AND PALEOECOLOGY THROUGH TIME

The terrestrial vertebrate fossil record is beset by a complicated set of biases (not all of which are known) that frustrate attempts to understand long-term trends in diversity and paleoecology. Most recent efforts have focused on analytical access to the existing record, but systematic improvements can also be made in the record itself. Specifically, sets of isotaphonomic samples can be used to standardize many collecting and preservational biases. This allows more robust, detailed comparisons to be made over broad spatiotemporal scales.

We propose that vertebrate microfossil bonebeds (VMBs) represent one such sampling regime. These specimen-rich, locally restricted concentrations of small vertebrate fossils are especially valuable sources for small and rare taxa. More importantly, they occur throughout the terrestrial record, only form under a limited set of taphonomic settings, and provide high sample sizes that afford opportunities to assess abundances and diversities.

We made and analyzed VMB collections from the Cretaceous Cloverly and Judith River formations to assess patterns of diversity, abundance, and paleoecology. In addition to finding several undiscovered taxa and refining spatiotemporal ranges for others, we could make much finer-scale comparisons with penecontemporaneous VMBs than is typically possible for Mesozoic terrestrial faunas.

We also attempted to address the potential biases and difficulties associated with VMB abundance data. Differentiating between taphocoenotic and biocoenotic effects remains a challenge, along with the differing contributions to the biocoenotic sample from organisms with unequal tooth replacement rates. Once solved, VMBs will be a powerful tool for examining geographic, environmental, and temporal variations in paleoecosystems across a wide swath of geologic time.