SPECIES TURNOVER DURING THE PLIO-PLEISTOCENE EXTINCTION EVENT: COUPLING METABOLIC RATES AND BODY SIZE

Recently, paleontological studies have begun to incorporate explicit physiological variables to better evaluate the evolutionary history of organisms in the context of their metabolic demands. Here we examine the influence of body size [BS], metabolic rate [MR], and productivity on species turnover within two guilds of bivalve mollusks that persisted across the Plio-Pleistocene Extinction Event [PPEE].

Antero-posterior measurements for 2,497 specimens that represent 75 bivalve species from two families (Pectinidae and Veneridae) were compiled from FLMNH collections spanning the late Pliocene-early Pleistocene Tamiami Fm. (Pinecrest beds), the lower Pleistocene Caloosahatchee Fm., and the middle Pleistocene Bermont Fm. For each formation and family, we used kernel density estimation to quantify the modality of the BS distributions. Mean MR per species was estimated following the model of metabolic theory of ecology, which scales BS with the ambient temperature at which metabolism occurs (warm temperate, range: 17-23°C, × ̅=20 °C). Proxies of paleotemperature and nutrient data were obtained from the literature. We performed logistic regressions to evaluate extinction selectivity in terms of BS, MR, and nutrient variables. In addition, beta turnover rates in faunal composition were estimated using the Jaccard similarity measure.

On average, pectinids recorded larger BS and higher MRs through the extinction event (mainly, Caloosahatchee Fm.) and experienced higher turnover rates and more drastic post-extinction reduction in size relative to veneroids. Logistic regression indicated that nutrient concentration had a significant effect on species extinction, especially in pectinids. Finally, after the PPEE, both veneroid and pectinids were characterized by smaller BS and lower MR than their pre-extinction counterparts.

During PPEE, taxa with higher MRs and larger sizes were prone to extinction relative to taxa of smaller size, while after extinction, colonizing taxa were generally characterized by smaller body size and lower metabolic rates than their pre-extinction counterparts. These results strongly support the hypothesis that changes in nutrient supply played an important role in the turnover of taxonomic composition during the PPEE.