TYLOSAURUS THROUGH TIME: CLADISTIC ANALYSIS OF ONTOGENY RECOVERED EVIDENCE OF PERAMORPHY-DRIVEN ANAGENESIS IN NORTH AMERICAN TYLOSAURINES

Mosasaurs were large, globally distributed aquatic lizards that lived during the Late Cretaceous. Tylosaurus is a genus of particularly large mosasaurs with long, edentulous anterior extensions of the premaxilla and dentary that lived in Europe and North America during the Late Cretaceous (89 to 66 million years ago). Three species—Tylosaurus proriger, T. kansasensis, and T. nepaeolicus—have robust fossil records with specimens spanning a wide range of sizes, and previous work has proposed that T. kansasensis are juvenile T. nepaeolicus, rather than a separate taxon. Therefore, these species are ideal models for studying mosasaur intraspecific variation (e.g., ontogeny, sexual dimorphism, individual variation) and testing hypotheses of synonymy, anagenesis (i.e., evolution within a single lineage), and heterochrony.

One hundred fifteen specimens (65 T. proriger, 21 T. kansasensis, 24 T. nepaeolicus, five Tylosaurus sp.) were scored for 194 characters, including 168 phylogenetic (i.e., purportedly diagnostic) and 29 ontogenetic (9 size-dependent, 20 size-independent) characters. Quantitative cladistic analysis was then used to: (1) recover a growth series for each taxon individually; (2) test an existing hypothesis of synonymy of T. kansasensis and T. nepaeolicus by analyzing them together; and (3) test a hypothesis of anagenesis in Western Interior Seaway Tylosaurus by analyzing specimens of all 3 species together.

A Spearman rank-order test resulted in a significant (p < 0.05) correlation between size (skull length, quadrate height) and maturity for each species. Evidence for skeletal sexual dimorphism was not found, which is consistent with what is seen in most extant squamates; however, size-based dimorphism cannot yet be ruled out. The separate analyses of each taxon recovered eleven growth characters, including the development of a knob-like rostrum and increase in quadrate height relative to skull length, that were shared by all 3 taxa. The second analysis, including specimens of T. kansasensis and T. nepaeolicus, supported their synonymy and that T. kansasensis are immature individuals. Finally, the third analysis: (1) failed to falsify the hypothesis that T. kansasensis and T. nepaeolicus are different species; (2) recovered evidence of intermediate morphologies shared by mature T. nepaeolicus and immature T. proriger; (3) recovered evidence of peramorphy of skull size and development in T. proriger, supporting the hypothesis of peramorphy-driven anagenesis in Western Interior Seaway Tylosaurines.