THE DIVERGENT EVOLUTIONARY HISTORIES OF MODERN POLAR FAUNAS: A ROLE FOR PRIMARY PRODUCTIVITY?

High-latitude faunas changed significantly through the Cenozoic as these regions shifted from temperate to polar climate zones. However, despite many similarities in the Paleocene/Eocene, the composition of marine faunas at the two poles differ strongly today, with the Antarctic generally richer in species but exhibiting archaic community structures more reminiscent of Paleozoic than modern communities. Combining fossil data from the Arctic and Antarctic Paleogene with a family-level molecular phylogeny of living marine bivalves, we show that differences between the polar faunas arose via contrasting phylogenetic patterns of extinction through the Cenozoic. Two measures of the phylogenetic relatedness of taxa, mean pairwise distance and mean nearest taxon distance, verify that while Arctic extinctions are distributed randomly across the phylogenetic tree, Antarctic losses are significantly clumped in the more derived portion of the tree, resulting in the loss of up to 35% of recorded evolutionary history from the region and leaving a fauna dominated by basal lineages. These results provide the first phylogenetic support for the “retrograde” hypothesis of Antarctic faunal evolution, though the underlying mechanisms for the phylogenetically structured Antarctic extinctions remain unclear. Preliminary data suggest a strong role for primary productivity in producing the divergent patterns. Antarctica today has the lowest rates of primary productivity in the world, despite a lack of nutrient limitations there. In the Antarctic, body sizes, which can correlate with extreme variations in productivity, were indistinguishable from the global distribution in the Eocene but are significantly smaller today, and families with large mean and median body sizes were preferentially removed from Antarctica. We suggest that productivity in Antarctica dropped as temperatures cooled and the continent became isolated, potentially disfavoring the active, high energy modes of life typical of the derived bivalve families.