QUANTIFYING THE MORPHOLOGICAL DIVERSITY OF MOSASAURS: IMPLICATIONS FOR MACROEVOLUTION AND PALEOECOLOGY IN THE WESTERN INTERIOR SEAWAY

During the Late Cretaceous, mosasaurs became the dominant predators of marine environments until the eventual extinction of many marine reptile groups at the K-Pg boundary. Based on tooth morphotypes and preserved stomach contents, they filled a number of predatory niches within the ecosystem. Because of their large sample sizes and high taxonomic diversity, mosasaurs are an exceptional study system for examining macroevolution during the Late Cretaceous.

I examined the morphological diversity of all members of the Mosasauridae from the Western Interior Seaway (WIS) by quantifying disparity using discrete anatomical features. To accomplish this, I built a novel matrix of morphological features, focused on taxa collected from deposits of the WIS, Gulf, and Atlantic coastal plains. I augmented taxon-character matrices used for phylogenetic analyses with autapomorphic and variable features to more accurately quantify changes in disparity. I ran a principal coordinates analysis and calculated mean pairwise dissimilarity using the R package Claddis. I calculated dissimilarity and 95% confidence intervals in equal-length time bins through the Late Cretaceous for all Mosasauridae and for individual sub-familial clades of mosasaurs.

My results show no significant change in disparity across all Mosasauridae from the WIS. However, individual clades of mosasaurs showed significant changes throughout the Late Cretaceous. Plioplatecarpines exhibited an early peak in disparity during the Santonian to earliest Campanian, followed by a significant decrease in anatomical diversity throughout the rest of the Cretaceous. Within the Mosasaurinae, low levels of disparity through their first 8 Ma of evolution were followed by an expansion in morphospace occupation during the early to middle Campanian. That morphological diversification of mosasaurines was preceded by an increase in the number of tooth morphotypes exhibited by the clade, including novel morphotypes not present in other clades of mosasaurs. This occupation of novel feeding ecologies by the Mosasaurinae was likely a major factor in the taxonomic diversification exhibited by the clade during the later Campanian and Maastrichtian and ultimately led to a latest Cretaceous peak in mosasaur diversity just prior to their extinction.