SCALE-DEPENDENT BODY SIZE EVOLUTION IN BRACHIOPODS: IS THERE EVIDENCE FOR COPE’S RULE?

Lineage increase in body size over time (Cope’s Rule) has been identified by both paleontologists and biologists in many different organisms. The attention body size evolution has received reflects its easy recovery from fossils and direct relevance to many life history characteristics and abiotic environmental parameters. It is accepted as a generalizable rule as often as it is criticized with counterexamples of body size decrease. Regardless of directionality, body size is a critically important trait to evaluate biotic response to changing environmental conditions. However, the relative lack of phylogenetic context for this trait limits our understanding of the mechanisms driving body size evolution. Cope’s Rule applies at a micro-evolutionary scale in populations, which complicates efforts to place body size data in a macroevolutionary framework. Can Cope’s Rule be extrapolated reliably to both taxonomic and phylogenetic patterns from the fossil record?

Heim et al. (2015) built the largest compendium of marine animal genus measurements per genus to date and demonstrated that at this broadest scale, body volume increases throughout the Phanerozoic. Studies focusing on brachiopods largely agree but differ on a key point: body size evolution is scale dependent. Whether they appear to follow Cope’s Rule depends on the taxonomic scale of analysis. This scale-dependency raises doubts: does Cope’s Rule truly apply to brachiopods, dominant in Paleozoic ecosystems? Answering this question requires examining body size trends at different scales and evaluating variation among clades, not simply taxa.

We tabulated biovolume data from Heim et al. (2015) to evaluate brachiopod body size evolution at the ordinal, subordinal, superfamilial, and familial level. We then analyzed phylogenetically a target group of superfamilies, focusing on representative genera of the four spire-bearing orders. Averaged volume over time at the largest taxonomic scale indicates broad agreement with previously identified patterns; however, few taxa at each rank are monophyletic, and some taxa in each rank increase in size while others decrease. We present these results with a preliminary examination of co-occurring morphological and ecological traits and argue for placing body size evolution within a phylogenetic framework.