DIVERSIFICATION OF MOSASAURINAE: INSIGHTS FROM ANATOMICAL DISPARITY, PHYLOGENETICS, AND A NEW MOSASAUR FROM THE WESTERN INTERIOR SEAWAY

I discuss the diversification of mosasaurine mosasaurs from the perspective of morphological diversity, novel phylogenetic hypotheses, and a new taxon from the Western Interior Seaway. During the latest Cretaceous, the Mosasaurinae were the most diverse of the four sub-familial clades of mosasaurs. From the Coniacian through the Santonian, they exhibited low taxonomic diversity. However, the lineage diversified during the Campanian, with over eight genera known from the last 15 Ma of the Cretaceous. The stratigraphic occurrence, morphology, and phylogenetic relationships of a new mosasaurine from the Campanian of Texas and Alabama helps to contextualize this diversification.

I scored a combination of published and novel characters for mosasaurines and performed a phylogenetic analysis to form a hypothesis of relationships within the clade. To model occupancy of morphospace through time, I ran a principal coordinates analyses for temporally equivalent time bins throughout the Late Cretaceous using my matrix of discrete characters. I added variable and autapomorphic characters to the dataset to more completely model morphospace and morphological evolution.

Through the Coniacian and Santonian, mosasaurines occupied a narrow region of morphospace. Anatomical diversity increased dramatically in the early and middle Campanian, coinciding with an increase in taxonomic diversity and the co-occurrence of early-diverging mosasaurines, the new mosasaurine, and other highly-nested taxa including Globidens and Mosasaurus. In the late Campanian and Maastrichtian, continued high disparity coincided with high global taxonomic diversity, though the morphospace previously occupied by early-diverging mosasaurines was vacated by the clade.

Many of the characters previously optimized as synapomorphies for mosasaurine clades, including those related to the dentition and quadrate, are instead homoplastic and may reflect paleoecology rather than shared ancestry. The new taxon exhibits an increased level of heterodonty compared to Clidastes. It also possesses a large fossa on the lateral surface of the surangular, an indication of large jaw muscles and an increased bite force. This animal provides evidence for the exploration of new feeding strategies early in the diversification of Mosasaurinae.