Understanding How Lithification Acts as a Taphonomic Filter on Diversity, Abundance Distributions, and Body Size: a Case Study from the Early Paleocene of the Gulf Coastal Plain, USA

While taphonomic processes have long been known to alter the diversity and composition of fossil assemblages, the magnitude of these biases is contentious. The degree to which sediments are lithified is one such bias that has received quantitative treatment only recently and appears to significantly impact estimates of past biodiversity. The secular trend of increasingly unlithified sediments through the Mesozoic and Cenozoic has been advanced as a possible cause of the large rise in global diversity during this time. On a regional level, an analogus pattern of decreasing lithification and increasing diversity is seen in early Paleogene molluscan dominated assemblages in the Gulf Coastal Plain, USA (GCP). Specifically, the GCP contains a mixture of preservational types following the Cretaceous-Paleogene (K-Pg) mass extinction and through the early Paleocene, but predominately unlithified sediments from the late Paleocene onwards. Using a combination of abundance data derived from the literature and body size measurements made on museum specimens, we find that lithified sediments are relatively undersampled and contain significantly fewer small individuals and taxa than unlithified units. Lithification also causes a loss of diagnostic features, resulting in decreased taxonomic resolution. Importantly, existing standardization techniques, such as rarefaction, cannot correct for these effects, and, likely, neither will an increase in collection effort. This bias is predicted to be most severe during periods when organisms have especially small body sizes and when multiple preservational styles are available for study. Within the GCP, lithification causes the recovery from the K-Pg mass extinction to appear protracted, because lithified assemblages are less diverse and less even than those in unlithified sediments. Since lithification causes the size distribution of organisms to be skewed towards larger size classes, a pattern counter to modern and fossil distributions, this bias can be recognized and accounted for by focusing on specific size classes.