UNDERSTANDING THE DIVERSIFICATION OF MOSASAURINAE BASED ON TWO NEW GENERA FROM THE WESTERN INTERIOR SEAWAY

I discuss two new genera of mosasaurines from the Campanian of North America and their importance for understanding mosasaur diversification. Mosasaurs are marine lizards that traditionally are divided into four sub-familial clades. The most diverse of those clades was the Mosasaurinae. By the mid-Campanian through Maastrichtian, mosasaurines diversified into numerous lineages and included the genera Mosasaurus, Plotosaurus, Globidens, Prognathodon, and Plesiotylosaurus. Throughout its earlier history, the clade was represented only by Dallasaurus (Turonian) and Clidastes (Coniacian – Campanian). I describe a new, large mosasaurine from the Campanian of Texas and Alabama, which possesses a suite of characters similar to early-diverging mosasaurines like Clidastes. However, the new taxon also exhibits derived characters in the quadrate, dentition, and lower jaw that are shared with more highly-nested taxa like Globidens and Prognathodon. In addition to the new taxon, undescribed material of the Campanian taxon Prognathodon stadtmanialso possesses a combination of plesiomorphic and derived characters.

To place two these taxa into an evolutionary context, I scored a combination of published and novel discrete characters for North American mosasaurine specimens and performed a phylogenetic analysis to model the evolutionary relationships within Mosasaurinae. The new taxon was found to be nested within a clade of mosasaurs with derived character states, in spite of its overall similarity to Clidastes. I found that stadtmani is not monophyletic with other species of the genus Prognathodon, and should be assigned to a new genus. My analysis calls into question the monophyly of the clade Globidensini (Globidens + Prognathodon), potentially rendering characters traditionally used to diagnose that clade as homoplastic.

These taxa demonstrate that Mosasaurinae underwent a major diversification during the early Campanian, with lineages leading to Prognathodon, Globidens, and Mosasaurus originating by the mid Campanian. These taxa call into question the monophyly of traditionally recognized clades within Mosasaurinae. Many of the characters previously optimized as synapomorphies for those clades are instead homoplastic and likely represent similar paleoecology rather than shared ancestry.