LATITUDINAL LIFE-HISTORY GRADIENTS IN FOSSIL BIVALVES

Modern bivalves display a latitudinal gradient in life history parameters (LLHG): tropical bivalves tend to grow fast and die young, whereas mid- and high-latitude bivalves grow more slowly and they can live much longer. Environmental factors such as temperature and seasonal food availability, which affect metabolic rates, are thought to be at least partially responsible for this pattern. Consequently, we expect that its intensity should vary over the Phanerozoic. However, little work has been done on life histories of fossil bivalves. Here, we determine lifespans and growth rates of two Pliocene bivalves, Glycymeris americana and Glycymeris subovata, widely distributed from Florida to Virginia on the US Atlantic Coastal Plain. Their life histories are reconstructed from annual growth increments counted and measured in the hinge region of sectioned and polished individuals. We also discuss the need for further exploration of the LLHG through time and its implications. The Pliocene is Earth’s most recent analog for ongoing climate change, offering a window through which to examine the LLHG when temperatures were at least 2-3°C warmer than today. Our results indicate that G. americana follows the expected pattern. Southern populations were faster growing, as approximated by von Bertalanffy k values, and shorter lived than their northern counterparts. In G. subovata, the predicted pattern is not recognized. Higher latitude populations appear to have grown more slowly, as expected, but individuals did not live as long as those in low latitude populations. Reasons for these differences in pattern are uncertain but some facet of the ecologies and/or biogeographic histories of these organisms may be playing a role. Our findings suggest that the LLHG may be recognized in some circumstances through geologic time, but its nuances remain to be worked out.