LATITUDINAL GRADIENTS IN LIFESPAN AND GROWTH RATE FOR TWO SPECIES OF GLYCYMERIS (BIVALVIA) FROM THE MID-PLIOCENE OF THE ATLANTIC COASTAL PLAIN

One of the most fundamental biological patterns on the planet today is the latitudinal gradient (LDG) of species diversity, which decreases with increasing latitude. A newly proposed phenomenon is the latitudinal life history gradient (LLHG), recently documented in marine bivalves. Across the Bivalvia as a whole and within various taxonomic subgroups, there is a tendency for lifespan to increase and growth rate to decrease with latitude. These two patterns of change in diversity and life history may not be unrelated. The faster pace of life and shorter generation times in the tropics may account for higher rates of mutation and faster turnover that lead to higher rates of speciation. Conversely, high-latitude bivalves have lower metabolic rates (leading to lower mutation rates) and longer generation times, and thus might be less likely to spin off new species. While much work has been done on the LDG through time, the fossil record of bivalve lifespans and growth rates, especially in the context of the LLHG, remains unstudied. Here, we examine lifespans and growth rates of Glycymeris americana and G. subovata from the Pliocene U.S. Atlantic Coastal Plain to discern whether a LLHG is apparent in this well-studied interval. Specimens range from 28 to 38 °N latitude. Their life histories are reconstructed from annual growth increments counted and measured in sectioned and polished individuals from multiple sites. Initial indications are that Pliocene Glycymeris does show a gradient in lifespan and growth rate with latitude – individuals from more northern sites lived longer and grew more slowly than those from more southern sites. G. americana is a much longer-lived species (at least 88 years) than G. subovata (at least 34 years), but both show growth rates, as approximated by von Bertalanffy k values, in the range of modern bivalves living at similar latitudes. These data are preliminary, but our findings support the hypothesis that the LLHG could have played the same role in the maintenance of the LDG in the Pliocene as it does today.