Abstract
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 particularly large mosasaurs with long, edentulous anterior extensions of the premaxilla and dentary 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. Fifty-nine hypothetical growth characters were identified, including size-dependent, size-independent, and phylogenetic characters, and quantitative cladistic analysis was used to recover growth series for the two taxa. The results supported the synonymy of T. kansasensis with T. nepaeolicus and that T. kansasensis represent juveniles of T. nepaeolicus. A Spearman rank-order correlation test resulted in a significant correlation between two measures of size (total skull length and quadrate height) and maturity. Eleven growth changes were shared across both species, neither of the ontogram topologies showed evidence of skeletal sexual dimorphism, and a previous hypothesis of paedomorphy in T. proriger was not rejected. Finally, a novel hypothesis of anagenesis in Western Interior Seaway Tylosaurus species, driven by peramorphy, is proposed here.
Highlights
Mosasaur ontogeny Mosasaurs (Squamata: Mosasauridae) were a group of large, predatory marine lizards with a global distribution that lived during the Late Cretaceous
Quantitative cladistic analysis Size-independent assessment of maturity In fossil taxa, it is difficult to discern whether morphologically similar, but differently sized, individuals are different species or different growth stages of a single species; adults of a small species may be mistaken for juveniles of a large species, or different growth stages of a single species may be mistaken for separate species altogether (Rozhdestvensky, 1965; Brinkman, 1988; Carr, 1999)
Growth series of T. proriger One ontogram was recovered with a length of 82 steps, consistency index (CI) of 0.65, homoplasy index (HI) of 0.35, retention index (RI) of 0.76, and rescaled consistency index (RC) of 0.49 (Fig. 5)
Summary
Mosasaur ontogeny Mosasaurs (Squamata: Mosasauridae) were a group of large, predatory marine lizards with a global distribution that lived during the Late Cretaceous. The fossils of several taxa span a wide range of sizes and are presumably of varying maturity. The first published study of growth in mosasaurs was done by Caldwell (1996), which sought to determine the patterns of ossification in the autopodial skeleton across mosasauroids and to test the congruence between these growth processes and mosasaur phylogeny. Caldwell (1996) found that many ossified carpals and tarsals is the ancestral condition, whereas more derived species have less ossified carpals and tarsals; a low number of ossified carpals and tarsals is characteristic of juveniles.
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