HOW TO MAKE MONSTERS: CRANIOFACIAL ONTOGENY IN TYLOSAURINAE

Mosasaurs were large, globally distributed aquatic lizards that lived during the Late Cretaceous. Despite numerous specimens of varying maturity, a detailed growth series has not been proposed for any mosasaur taxon. Two taxa–Tylosaurus proriger and T. kansasensis/nepaeolicus––have robust fossil records with specimens spanning a wide range of sizes and are thus ideal for studying mosasaur ontogeny. Tylosaurus is a genus of large mosasaurs with edentulous rostra that lived in Europe and North America during the Late Cretaceous. An analysis of growth in Tylosaurus provides an opportunity to test hypotheses of the synonymy of T. kansasensis with T. nepaeolicus, sexual dimorphism, anagenesis, and heterochrony.

The goals of this project were to use quantitative cladistic analysis to (1) recover a growth series of T. proriger and T. kansasensis/nepaeolicus; (2) test whether two measures of size (total skull length and quadrate height) are appropriate proxies for relative maturity in these species; (3) test for sexual dimorphism; (4) test the hypothesis that T. kansasensis are juveniles of T. nepaeolicus; (5) test the hypothesis that two character states, the presence of a frontal midline crest and convex lateral borders of the parietal table, in T. proriger are paedomorphic relative to T. nepaeolicus; (6) test for anagenesis in these species using growth data; (7) propose revised cranial diagnoses of T. proriger and T. nepaeolicus/kansasensis within an ontogenetic context; and (8) identify conserved patterns of growth in Tylosaurus.

Fifty-nine hypothetical growth characters were identified, including size-dependent, size-independent, and phylogenetic characters. Growth series were recovered for both taxa: T. proriger, 17 growth stages, Consistency Index (CI) = 0.7; T. nepaeolicus/kansasensis, 12 stages, CI = 0.6. The results supported the hypothesis that T. kansasensis are juvenile T. nepaeolicus. A Spearman rank-order correlation test resulted in a significant (p < 0.05) correlation between size and maturity. Neither of the ontogram topologies showed evidence of skeletal sexual dimorphism, 11 growth changes were shared across both taxa, and paedomorphy in T. proriger was supported. A novel hypothesis of anagenesis in Western Interior Seaway Tylosaurus species, driven by peramorphy, is proposed here.