A new Assemblage of Mosasaurs from the Upper Cretaceous Savoy Pit, Austin Chalk, Northeast Texas

Chapman, Blake Russell
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The fossil assemblage at the Savoy Pit (Fannin County, Texas) was collected in 1940 by participants in the Works Progress Administration working alongside personnel from the Bureau of Economic Geology at The University of Texas at Austin. The locality is contained within the Ector Chalk (lower Coniacian), an argillaceous limestone, of the Austin Group. Previously published taxa from the site include the bird Ichthyornis and fish such as Belonostomus and Laminospondylus transversus; several types of shark teeth and invertebrates (e.g., crustaceans, asteroids, and bivalves) also have been found. Several mosasaur specimens are present at the site and include isolated elements and associated skeletons that have yet to be described. My collaborator Joshua Lively and I scored the material from the Savoy Pit using published and novel morphological characters and compared our results to about 250 mosasaur specimens from North America and Sweden. We then ran a phylogenetic analysis using maximum parsimony to hypothesize the taxonomic composition of the mosasaur assemblage. At least three taxa are identified: a russellosaurine represented by a frontal resembling Russellosaurus coheni, an undetermined basal mosasaurine represented by a partial skull and vertebrae, and Tylosaurus nepaeolicus based on a premaxilla and quadrate. The mosasaurine possesses a unique combination of characters including synapomorphies of both Mosasaurinae (e.g., striated quadrate tympanic rim, dorsally constricted suprastapedial process) and Russellosaurina (e.g., short premaxilla-maxilla suture, low surangular coronoid buttress). In our phylogenetic analysis, we recovered this specimen as a basal mosasaurine that does not align with any published genus, and we recognize it as a new taxon. The russellosaurine synapomorphies possessed by the new taxon may represent the plesiomorphic state of Mosasaurinae; some of those characters are shared with Kourisodon puntledgensis. The presence of a russellosaurine resembling Russellosaurus coheni may extend the range of this species into the Coniacian. The new taxon helps to fill a stratigraphic gap of approximately six million years between the earliest known mosasaurine, Dallasaurus turneri, and ‘Clidastes liodontus.’ Identification of Tylosaurus nepaeolicus at the Savoy Pit extends the lineage into the Early Coniacian and represents one of the earliest occurrences of that genus.