Exploring the Relationship between Paleobiogeography, Deep-Diving Behavior, and Size Variation of the Parietal Eye in Mosasaurs

Abstract

The parietal eye (PE) in modern squamates (Reptilia) plays a major role in regulating body temperature, maintaining circadian rhythms, and orientation via the solar axis. This study is the first to determine the role, if any, of the PE in an extinct group of lizards. We analyzed variation in relative size of the parietal foramen (PF) of five mosasaur genera to explore the relationship between PF size and paleolatitudinal distribution. We also surveyed the same specimens for the presence of avascular necrosis—a result of deep- diving behavior—in the vertebrae. Plioplatecarpus had the largest PF followed by Platecarpus, Tylosaurus, Mosasaurus, and Clidastes. A weak relationship exists between paleolatitudinal distribution and PF size among genera, as Plioplatecarpus had the highest paleolatitudinal distribution (~78°N) and the largest PF among genera. Clidastes, Mosasaurus, Platecarpus, and Tylosaurus, however, shared a similar northern paleolatitude (~55°N) extent despite Platecarpus having a statistically larger PF than the other three genera (p<0.001 in Fisher’s LSD test). Mosasaurus, Plioplatecarpus, and Tylosaurus also shared a similar southern paleolatitude (~64°S) despite Plioplatecarpus having a larger PF. There is no correlation between PF size and paleolatitudinal distribution for specimens within genera. We found no relationship between PF size and presence of avascular necrosis. Tylosaurus and Mosasaurus, which exhibited avascular necrosis, had a similar PF size to Clidastes, which did not avascular necrosis. The PE of mosasaurs may have functioned primarily for navigation and orientation related to migration; however, this possibility requires further study of modern PE-bearing organisms and its function.