QUANTIFYING THE IMPACTS OF PRESERVATION ON PALEOBIOLOGICAL PATTERNS: A LITERATURE-BASED SURVEY OF BODY SIZE IN CRETACEOUS AND PALEOGENE GASTROPODS

One major challenge in understanding the history of life lies in detangling the effects of preservational biases from true ecological and evolutionary patterns in deep time. For example, from the late Mesozoic onward, unlithified sedimentary deposits become more common, and the average preservational quality of aragonitic fossils like gastropods increases. Within the same time interval, the diversity and size range of fossil gastropods also appears to increase. Here, Cretaceous and Paleogene gastropods are used as a model system to explore relationships between preservational condition and shell size. Data were collected from illustrations accompanying taxonomic descriptions from over 200 published monographs, journal articles, and books. This literature-based approach allows a broad geographic sampling and will complement more detailed analyses of specific faunal assemblages or geographic regions. For the most complete specimen of each species in each formation, we recorded the length and width of the shell and the aperture, as well as several variables describing quality of preservation: 1) overall completeness of the fossil, 2) pristineness of the shell surface, and 3) a very general measure of the degree of lithification based on characteristics observable from photographed specimens. As suggested by previous studies, our preliminary analyses indicate that average preservational quality does increase through the Cretaceous and Paleogene and that small fossils are undersampled when preservation is poor. We develop new quantitative estimates of the intensity of both of these patterns that should provide a foundation for further work assessing the effects of preservational biases on patterns like diversity trends. Interestingly, small shells were particularly common in our data set in well-preserved assemblages from the Paleocene; this may be a consequence of the end-Cretaceous extinction, although further study is needed.