BODY SIZE TRENDS IN MESOZOIC MARINE REPTILES

During the Mesozoic, the oceans were populated by several clades of highly successful marine reptiles including the ichthyosaurs, plesiosaurs, and mosasaurs. These marine reptiles spanned a long range of geologic time, occupied a wide spectrum of ecological niches, and grew to a variety of sizes. An organism’s size has a significant influence on its physiology, life history, and ecology, but the body size trends of marine reptiles are not fully understood. We document size trends in ichthyosaurs, plesiosaurs, and mosasaurs to determine if size is affected by extinction events, if size increases through time (Cope’s rule), and if so, whether that size increase is due to neutral drift or active selection. Ichthyosaur size did not show a directional trend but fluctuated over time, with a notable decrease in size at the end-Triassic extinction. Although previous studies suggest size increase in plesiosaurs and mosasaurs, we find that the pattern is better explained by a random walk than an overarching selective advantage to being big.