EXPLORING THE IMPACTS OF PRESERVATIONAL BIASES ON GASTROPOD DIVERSITY TRENDS FROM THE JURASSIC TO THE PALEOGENE

Preservation biases cause fossil assemblages to be imperfect reflections of living communities, confounding the ability of paleobiologists to discern ancient evolutionary and ecological trends. Such biases include the lithification of sedimentary deposits and the preferential dissolution of aragonitic material, including gastropod shells. The abundance of unlithified sediments in the geologic record increases from the late Mesozoic into the Cenozoic. This trend coincides with an apparent taxonomic diversification within Gastropoda. To better understand the extent to which this radiation is real versus an artifact of preservation, we created a dataset of over 8000 gastropod specimens described and illustrated in the published literature from the Jurassic to the Oligocene. For each specimen we recorded attributes including shell size, age, collection locality, and taxonomic affinities. We also graded the quality of preservation on a semi-quantitative scale for three variables: shell completeness, surface pristineness, and apparent sediment lithification. We observed an overall increase in preservation quality with decreasing age as well as a bias against smaller taxa among poorly preserved remains. Together, these findings support the contention that changes in preservation affect the apparent magnitude of the Mesozoic-Cenozoic gastropod radiation, although we have not yet quantified the strength of this effect. Preliminary data indicate that trends through time in quality of preservation differ among lithologies (sandstone, mudstone, carbonates), suggesting that environmental and geological context will be important to consider when documenting the effects of lithification and aragonite dissolution on diversity trends.