CENOZOIC RELATIONSHIPS BETWEEN GEOGRAPHIC RANGE AND ASSEMBLAGE-LEVEL ABUNDANCE: THE ROLE OF RARE TAXA

Most paleontological samples include only a few abundant taxa; taxonomic richness is derived in large part from less common or rare taxa. In paleoecological analyses that use abundance data, common taxa tend to contribute overwhelmingly to the recognition and interpretation of spatial patterns, while rare taxa play minor roles and are often omitted entirely. Nevertheless, rare taxa may reveal paleoecological and paleogeographic features when not overwhelmed in analyses by common taxa. Rare taxa are often considered to be habitat specialists and may be more sensitive to environmental processes and perturbations, such as variation in oceanographic conditions and fluctuations in climate.

Here, we investigate whether the proportional abundance of molluscan taxa within samples correlate with variations in their geographic ranges, whether geographic distribution relates to the rarity or commonness of taxa among samples, and whether these relationships change through time. The data used here are derived from two sources 1) bulk samples of fossil assemblages collected from Neogene localities of the western Atlantic and 2) global Cenozoic occurrence and abundance data obtained from the Paleobiology Database (http://paleodb.org). Previous research using these data have illustrated patterns of increased ecospace utilization and taxonomic richness at tropical latitudes through the Cenozoic, corresponding to a strengthening latitudinal biodiversity gradient. The present research investigates whether rare taxa contribute disproportionately to this strengthening, and whether this is reflected both at the community level, as measured by relative abundance, and at the regional level, as measured by geographic range.