THE PALEOBIOGEOGRAPHICAL EFFECTS OF THE PARIETAL FORAMEN ON MOSASAURS

Modern vertebrates living in high-latitude environments require a larger pineal gland or parietal foramen (PF) than those living in low-latitude environments. This correlation, however, may not apply to ancient PF-bearing vertebrates because the PF may have served a different purpose or had no relationship to latitudinal distribution. The PF has rarely been studied in modern organisms; it has not been studied in ancient organisms. Mosasaurs (Squamata, Mosasauridae) are a group of extinct marine lizards that have a PF and lived worldwide during the Late Cretaceous. Global distribution of mosasaurs makes them ideal for testing the biogeographical effects of the size of PF. To test if the size of the PF follows a latitudinal pattern—the higher the paleolatitude, the greater the diameter of the PF––the ratio of the length of the PF to the length of the parietal bone (PB) is used as a proxy for the diameter of the PF. This ratio can then be used to compare mosasaurs from different paleolatitude, regardless of preservation quality. The hypothesis to be tested is that the PF/PB increases as paleolatitude increases for among and within genera. The first test is to compare this ratio in specimens––averaged within the same genus––among different genera. The second test is to compare this ratio in specimens within the same genus of varying latitudinal distribution. The first test did not support the prediction that the PF/PB increased with increased paleolatitude. Mosasaurs bearing a large-diameter PF, such as Platecarpus and Plioplatecarpus, had a similar southern latitudinal distribution as mosasaurs, such as Tylosaurus, bearing a small-diameter PF. The second test did not support the prediction as Clidastes, Platecarpus, and Plioplatecarpus had similar PF sizes throughout their latitude gradient. Results of the second test may indicate that individuals of these genera lived in a diverse latitudinal habitat (i.e., cosmopolitan) and not in localized areas (i.e., endemic). There may, however, be a relationship between the PF size and deep diving behavior. A large PF could be used in deep diving to orient the animal at depth. This could explain why the alleged shallow-dwelling Clidastes had a smaller PF compared to the alleged deep-diving Platecarpus. More specimens need to be studied before a robust evaluation can be made for all tests.