GLOBAL GEOGRAPHIC DISPARITY OF MARINE BIOTAS THROUGH THE PHANEROZOIC

In the assessment of Phanerozoic marine diversification, it has long been suspected that the major Cenozoic radiation observed in raw compilations was fueled in part by growing provinciality. Following the breakup of Pangea, coastal areas associated with several paleocontinents are thought to have become increasingly isolated from one another, enhancing the taxonomic regionalization of biotas worldwide and helping to fuel the diversity increase. While this suggestion is not without merit, there has not been an assessment of provinciality trends that maintains consistent standards of province delineation on the broad temporal scale of the Phanerozoic.

Here, instead of attempting to assess provinciality per se, we use a new analytical protocol through which we quantify directly the compositional disparities of marine biotas on a geographic basis during several key Phanerozoic intervals by: a) determining the paleogeographic locations of marine genera in collections obtained from the Paleobiology Database (http://paleodb.org); b) assigning these occurrences to five-by-five degree latitude-by-longitude cells; and c) quantifying compositional similarities between all pairs of cells that contain a sufficient quota of occurrences, in relation to the distances that separated them. Similarities among cells are determined using the Raup/Crick and percent similarity indices, the former applied to presence/absence data and the latter to sampling-standardized, quantitative data based on the number of occurrences of each genus within a cell.

Results suggest that, at multiple geographic scales, there is no Cenozoic increase in the geographic disparities of marine biotas. If anything, it appears that, at distances greater than ~4000 km, Paleozoic biotas tend to be more disparate than their Mesozoic and Cenozoic counterparts. Furthermore, by mapping similarity assessments directly onto paleogeographic maps, we observe that the greatest levels of similarity are exhibited among Paleozoic cells that are in close proximity to one another, within epicontental seas. By contrast, high levels of similarity among Cenozoic biotas are often exhibited among cells on opposite sides of ocean basins, perhaps reflecting fundamental differences in the circulation regimes of open oceans relative to epicontinental seas.