POLES APART: CENOZOIC SIZE DIFFERENTIATION BETWEEN HIGH-LATITUDE BIVALVE FAUNAS

Although regions at the same latitudes in different hemispheres are assumed to be (and, relative to different latitudes, generally are) quite similar in taxonomic, functional, and morphological diversity, the contrasting histories and configurations of the high latitudes might promote greater divergence in their trait diversity. Using global modern occurrences of bivalves, fossil polar bivalves from the Paleocene and Eocene, and presence/absence data for Miocene bivalves of Patagonia, analyses comparing size, functional groups, and taxonomic affiliation were performed. Modern Antarctic bivalves were found to be consistently smaller than Arctic bivalves, whether considering the whole fauna, species only from families present at both poles, species only from genera present at both poles, or species in the same pool of functional groups, though the differing family compositions of the two regions contributes somewhat to the size contrast. More broadly, bivalves from high temperate latitudes in the Northern Hemisphere did not differ significantly in size from tropical bivalves, whereas bivalves from the same latitudes of the Southern Hemisphere were far smaller than tropical bivalves and their northern counterparts. However, polar Paleocene and Eocene bivalves did not show any size differentiation, and were not significantly different from modern Arctic bivalves. Meanwhile, a slight majority of globally extant genera now locally extinct in Patagonia - but present in the Miocene - were larger than their remaining Patagonian family members, potentially displaying an early point in the size-reduction of the southern high latitude bivalves. Although the precise drivers of size differentiation of high latitude bivalve faunas are still unclear, the consistent contrast in size between the hemispheres despite the cooling of both poles implies that there are pressures in the Southern Hemisphere favoring small bivalves that began in the early Neogene.