COUPLING ENERGY USE, BODY SIZE, AND DIVERSITY DURING THE PLIO-PLEISTOCENE EXTINCTION EVENT

The incorporation of physiological variables into paleontological studies has led to a better understanding of the evolutionary history of organisms in the context of their metabolic demands. Here we evaluated changes in body size (BS), energy use (EU), and species turnover within two bivalve families that survived the Plio-Pleistocene Extinction Event [PPEE] and their response to environmental factors such as productivity and temperature. BS was determined using anterior-posterior measurements of 5,335 specimens belonging to 55 bivalve species from the families Pectinidae and Veneridae. The specimens were obtained from the Florida Museum of Natural History collections spanning the upper Pliocene Tamiami Fm., the lower Pleistocene Caloosahatchee Fm, and the middle Pleistocene Bermont Fm. EU was estimated from oxygen uptake that was measured in laboratory setting (20°C, 36.5 UPS) from closely related taxa (Mercenaria campechiensis and Argopecten irradians). EU values were scaled to each fossil specimen, assuming allometry. Distributions of EU and BS were plotted for each formation and family, and a Weibull function was fitted to the data. Proxies of temperature (d¹⁸O) and nutrient levels (P/Ca) were obtained from references, and logistic regressions were performed to test their effects on BS and EU. Finally, Whittaker's index was used to estimate species turnover. Pectinids show larger BS and higher EU before the extinction; however, they experienced a higher turnover rate and a drastic post-extinction reduction in BS and EU (mainly in the Caloosahatchee Fm.) relative to venerids. Logistic regression shows that only nutrient concentration had a significant effect on BS and EU, especially in pectinids. BS and EU show a unimodal distribution in all taxa and formations. However, after the PPEE, pectinids exhibited a right skewed distribution. Our work supports the idea the high late Pliocene productivity facilitated the expansion of larger-sized taxa with higher energetic demands. However, the decrease in the nutrient supply during Plio-Pleistocene may have triggered faunal changes with preferential turnover of small species with low metabolic rates. Our results support the hypothesis of “nutrient decline” and provide quantitative estimates of changes in energy use across an extinction event.