Abstract

Mosasaurs were large aquatic lizards that lived during the Late Cretaceous. Their fossils are found across the globe, but despite a multitude of specimens of varying maturity, detailed growth series have not been proposed for any mosasaur taxon. Four taxa – Tylosaurus proriger, T. kansasensis/nepaeolicus, Tethysaurus nopcsai, and Mosasaurus hoffmannii – have robust fossil records with specimens spanning a wide range of sizes and are thus ideal for studying mosasaur ontogeny. Furthermore, an analysis of growth provides an opportunity to test the synonymy of T. kansasensis with T. nepaeolicus, and by sampling several mosasaur taxa, ancestral patterns of mosasaur growth can be identified. 59 hypothetical growth characters were identified, including size-dependent, size-independent, and phylogenetic characters, and quantitative cladistic analysis was used to recover growth series for these four mosasaur taxa. The analysis supported the synonymy of T. kansasensis with T. nepaeolicus but did not support a previous hypothesis that T. kansasensis represent juveniles of T. nepaeolicus. A Spearman rank-order correlation revealed a significant correlation between two measures of size (total skull length and quadrate height) and maturity for all taxa except in M. hoffmannii, which is likely due to its relatively small sample size and limited data availability. Finally, 11 growth changes – eight of which involving the quadrate – were shared across two or more taxa and none of the ontogram topologies showed evidence of sexual dimorphism.