Macroevolution in the greenhouse : morphological disparity of Mosasauridae and the evolution of Late Cretaceous marine ecosystems
Creators: name: Lively, Joshua Ryan
name: 0000-0001-8524-9515
Publisher: name: The University of Texas at Austin
Year: 2019
Subjects: Mosasauridae
Mosasaurinae
Phylogeny
Disparity
Late Cretaceous

description: Climate researchers assert that our planet is not only entering an interglacial period, but potentially a time of prolonged warming, or even a greenhouse climate. The fossil record provides the opportunity to better understand the effects of prolonged warming on the evolution of the Earth’s biota because we can examine species and lineages across their entire stratigraphic range. I examined the macroevolutionary dynamics of mosasaurs with a focus on new mosasaurines from the Western Interior Seaway (WIS). Mosasaurs represent the dominant marine tetrapod predators of the latest Cretaceous. Because they both originated and reached peak diversity during the Late Cretaceous, mosasaurs are an excellent study system for understanding macroevolutionary dynamics during a greenhouse interval. My work combined phylogenetic comparative methods, analysis of anatomical diversity (disparity) using discrete morphological features, analysis of intraspecific variation, and insights from new mosasaur genera to better understand the diversification of mosasaurs. The mosasaur Prognathodon stadtmani was originally named in 1999 based on fragmentary material from the Mancos Shale of Colorado. New cranial elements that have been prepared since its original description helped to elucidate its phylogenetic position. My analyses support the placement of this taxon outside of the genus Prognathodon, in spite of many gestalt features that suggest otherwise. I also described a new mosasaur from the lowermost Campanian of Texas and Alabama. This new animal, which in many respects resembles the early-diverging mosasaurine Clidastes propython, exhibits derived character states observed in later mosasaurines, including Prognathodon overtoni. Including this animal in a global disparity analysis of Mosasaurinae demonstrates that the lineage exhibited low morphological diversity throughout the first 8 million years of its evolution, followed by a significant increase in disparity during the early Campanian. Based on the phylogenetic position and chronostratigraphic distribution of this new taxon, that diversification was preceded by an expansion in the feeding ecology of this clade. I also modeled disparity across all mosasaur clades within the WIS. Within the seaway, mosasaurs exhibit two significant diversification events; the first of those occurred during the Santonian with the diversification of Plioplatecarpinae. The second period of diversification occurred during the Campanian, with a significant increase in disparity of mosasaurines. To contextualize the role of intraspecific variation in the macroevolution of mosasaurs, I collected discrete character data from a large number of Clidastes propython and plioplatecarpines from the Coniacian-early Campanian of the WIS and quantified variation within these two lineages. Early members of the Mosasaurinae exhibited significantly higher levels of variation in tooth-bearing elements than their contemporaries from the Plioplatecarpinae. This is notable because while the plioplatecarpines retain relatively low diversity throughout their evolutionary history, mosasaurines diversified into at least five separate lineages during the Campanian, filling a number of predatory niches within the marine ecosystem. Early mosasaurines like Clidastes propython potentially represent a zone of variability in the clade’s evolution, with variation exhibited by that taxon becoming fixed interspecifically within later members of the clade