Climate-driven body-size trends in the ostracod fauna of the deep Indian Ocean

Abstract

Body size is a common focus of macroevolutionary, macroecological and palaeontological investigations. Here, we document body-size evolution in 19 species-level ostracod lineages from the deep Indian Ocean (Deep Sea Drilling Program Site 253) over the past 40 myr. Body-size trajectories vary across taxa and time intervals, but most lineages (16 ⁄ 19) show net gains in body size. Because many modern ostracod taxa are larger in colder parts of their geographical range, we compared the timing and magnitude of these size changes to established Cenozoic deep-water cooling patterns confirmed through d18 O measurements of benthic foraminifera in the samples studied. These data show a significant negative correlation between size changes and temperature changes (ostracods get larger as temperatures get colder), and that systematic size increases only occur during intervals of sustained cooling. In addition, statistical support for an explicit temperature-tracking model exceeds that of purely directional evolution. We argue that this Cope's Rule pattern is driven by secular changes in the environment, rather than any universal or intrinsic advantages to larger body sizes, and we note some difficulties in the attempts to link Cope's Rule to observations made within a single generation.

Adaptive evolution toward larger size in mammals

“…Taken together, our results demonstrate that mammals have consistently evolved toward larger size, almost certainly reflecting an adaptive response to new selective circumstances, such as competition (48), climate changes (7,49), or dietary specialization (11). These results are not compatible with purely passive explanations for trends through time (24,50).…”

“…In support of these conjectures, analyses from a range of different taxonomic groups demonstrate that larger individuals within populations have significantly enhanced survival, fecundity, and mating success (4,5). If these advantages are general and have played out over long time scales, they could explain the existence of Cope's rule (6): a broad trend toward increasing size through time (4,5,7).…”

Precocious sexual dimorphism and the Lilliput effect in Neo‐Tethyan Ostracoda (Crustacea) through the Permian–Triassic boundary

“…Referring to the observation of ostracods getting larger in colder climate (Hunt et al . ), we interpret the trends of Palaeocopida ( Paraparchites chenshii and Cyathus caperata ) as temperature dependent, reflecting the global warming during the overall Permian. It is also interesting to note that the size change of Cyathus caperata is much stronger from the Middle to Late Permian than from the Early to Middle Permian.…”

“…In the fossil record, the body size of deep‐sea ostracods has been linked to climate change, ostracods getting larger as climate got colder (Hunt et al . ). Similarly, dwarfing has been documented during the Paleocene–Eocene Thermal Maximum (PETM) in response to high temperatures: ostracod food consumption rates and lifetime might have been reduced compared to their pre‐PETM counterparts, increasing their growth rates (Yamaguchi et al .…”

A Shift in the Long-Term Mode of Foraminiferan Size Evolution Caused by the End-Permian Mass Extinction

“…Size evolution in terrestrial mammals may be driven largely by ecological interactions (Alroy 1998; Clauset and Erwin 2008; Sookias et al 2012) with climate change being relatively unimportant (Alroy et al 2000). In contrast, climate change has significantly impacted the size evolution of diatoms, dinoflagellates, and deep‐sea ostracods through the Cenozoic (Schmidt et al 2004; Finkel et al 2005, 2007; Hunt and Roy 2006; Hunt et al 2010). Taken together, these studies suggest a limited applicability for more general theories because controls on size evolution have varied substantially across taxa and habitats.…”