TAXONOMIC HOMOGENIZATION AFTER THE END-PERMIAN MASS EXTINCTION

In the wake of the end-Permian mass extinction, Early Triassic fossil assemblages are often dominated by a few abundant and widely occurring species, often referred to as “disaster taxa.” This pattern suggests a more general taxonomic homogenization of marine communities during the early recovery phase of the Triassic, but global, quantitative assessment of this hypothesis remains scarce. Here, we test this hypothesized increase in cosmopolitanism using three measures of taxonomic similarity: Biogeographic Connectedness, Jaccard Similarity and the semimetric Czekanowski coefficient, each of which measures the degree of taxonomic similarity on a scale from 0 (totally endemic) to 1 (globally widespread). To do this, we downloaded fossil occurrence data for classes Bivalvia and Gastropoda spanning the Changhsingian (~254.4 - 251.9 Ma) and Induan (~251.9 -251.2 Ma) ages from the Paleobiology Database (2,140 occurrences of 240 genera), and grouped the occurrences into a global grid of 812 equal-area (~630,000 km²) hexagonal cells. All occurrences were used for the calculation of Biogeographic Connectedness. Cells containing at least 30 occurrences for a given age were included in calculations of the Jaccard and Czekanowski coefficients. All three measures indicate that taxonomic similarity increased after the end-Permian mass extinction. These changes in similarity occur independently in bivalves and gastropods and are not explained by the biogeography of sampling because they occur even after controlling for pairwise distance between compared cells. We further divided our dataset by survival status (victims, survivors, and originators). Our results reveal a greater degree of provinciality in victims than in survivors during the Changhsingian, which may have been a control in survivorship for this mass extinction event. Genera that originated in the Induan had similarity indices that were similar to survivors during that age. A key remaining question is whether this taxonomic homogenization of the oceans was controlled by ecology, environment, or both.