Body Size and Climate Change in Deep-Sea Ostracodes: Testing Models of within-Lineage Evolution

Documenting evolutionary trends in body size--and understanding the mechanisms underpinning such trends--has been an important focus of paleobiological research. There are many proposed causes of trends in body size such as Cope's Rule, but there are increasing indications that, at least among microfossils, persistent body size trends can be related to long-term secular changes in environmental and climatic conditions. Here we present an analysis of body size evolution for the entire ostracode fauna from a site in the deep Indian Ocean (DSDP 253, 1962m water depth). For the interval spanning the last 40 million years, we tracked body size (measured as the projected area of valves) within specified instars in over 20 species-level evolutionary lineages and compared it to deep-sea temperature changes as recorded by published oxygen isotope and Mg/Ca proxies. For evolutionary transitions across all lineages considered jointly, we fit using maximum-likelihood four different evolutionary models: (i) an unbiased random walk, (ii) directional evolution (as in Cope's Rule), (iii) evolutionary stasis, and (iv) a optimum-tracking model in which body size evolution follows changes in deep-sea temperature.

Results obtained thus far reveal that many but not all lineages show net increases in body size over time. In addition, there is a significant negative correlation between temperature and body size changes: these ostracode lineages have generally become larger-bodied with cooling of the deep ocean waters. Of the four evolutionary models considered, temperature tracking is the most highly supported. These results support views suggesting that long-term body size trends arise not from generic advantages of larger bodies, but instead from long-term directional changes in environmental circumstances.