TESTING COPE'S RULE AND THE EXISTENCE OF AN UPPER BOUND FOR BODY SIZE IN NON-AVIAN DINOSAURS

Although their body masses span at least five orders of magnitude, non-avian dinosaurs have only recently attracted the attention of scientists interested in the dynamics of body size evolution. Using femoral length as a proxy for overall body size in dinosaurs, we apply several statistical models to explore its evolution in a phylogenetic context. Targeted comparisons among models can be used to shed light on aspects of body-size evolution, and here we focus on two issues: (1) the pervasiveness of directional trends, and (2) the presence of detectable upper limits for body size. Directionality was assessed by comparing the fit of the non-directional model of Brownian motion (BM) to that of BM with an underlying trend. Results indicate support for a trend of increasing body size (Cope’s rule) in some, but not all dinosaur clades. There is little indication that body-size trends vary systematically across time intervals, but they do vary substantially across dinosaur subclades.

To assess the macroevolutionary evidence for an upper limit to dinosaur body size we compared the fit of the BM model to that of BM in the presence of an upper reflecting boundary, a scenario similar to Stanley’s classic explanation of Cope’s rule as a passive trend. This model was fit using a novel likelihood function, and its improvement in fit relative to BM was judged using a likelihood ratio test with the null distribution generated by simulation. Results support the existence of a macroevolutionary upper limit to body sizes in theropods, but not in sauropods or ornithopods.