Craniofacial ontogeny in Tylosaurinae

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.

A transitional species of Daspletosaurus Russell, 1970 from the Judith River Formation of eastern Montana

“…Namely, D. wilsoni is stratigraphically, phylogenetically, and morphologically intermediate between these taxa (see Geologic Context, Results, and Diagnosis, respectively), and occurs within the same general geographic range (all three species of Daspletosaurus are found within Montana or Alberta; Carr et al, 2017 ). These points correspond to the criteria proposed by Carr et al (2017) (and later Zietlow, 2020 ) for defensible hypotheses of anagenesis: (1) lack of stratigraphic overlap (but see above), (2) close phylogenetic relationships, (3) intermediate morphologies, and (4) similar geographic ranges. It should be noted that while the fulfillment of these criteria establishes anagenesis as a defensible hypothesis, it does not preclude cladogenesis in Daspletosaurus as the driving factor of the evolution of this genus (with successively more derived clades, e.g ., D. wilsoni and more derived tyrannosaurines, representing cladogenetic events rather than portions of an anagenetic sequence).…”

Jormungandr walhallaensis: A New Mosasaurine (Squamata: Mosasauroidea) from the Pierre Shale Formation (Pembina Member: Middle Campanian) of North Dakota

“…18, 28). The left bone preserves at least 16 tooth positions, and the right preserves at least 15 tooth positions, which is most like Clidastes , Plotosaurus , and basal mosasauroids (e.g., Tethysaurus ; Bardet et al, 2003); such a high tooth count for a large (>5 m body length) mosasaur individual is unusual, given that other large-bodied taxa (e.g., Mosasaurus , Tylosaurus , Prognathodon ) typically only have between eight and 10 tooth positions in each pterygoid (Russell, 1967; Lingham-Soliar, 1995; Konishi et al, 2011; Zietlow, 2020; but see Konishi and Caldwell [2007: 71] for the high pterygoid tooth count variation in Plesioplatecarpus planifrons ). The pterygoid teeth are distinctly smaller than the marginal teeth; they are largest in the middle of the toothrow and become progressively smaller posteriorly.…”