A new assemblage of mosasaurs from the Upper Cretaceous Savoy Pit, Austin Chalk, North Texas

• In Event: Systematics and Evolutionary Biology Oral Session & Section Meeting

Abstract

We describe an assemblage of mosasaurs from the Savoy Pit of Fannin County, in northeastern Texas. The site was excavated in 1940 by the Works Progress Administration (WPA) in collaboration with The University of Texas Bureau of Economic Geology. Based on field notes from the WPA, specimens were recovered from the Upper Cretaceous (Coniacian) Austin Chalk, approximately 7 m above the contact with the Eagle Ford Shale. We scored material from the Savoy Pit using a combination of published and novel morphological characters and compared our results to over 200 other mosasaur specimens. We then ran a phylogenetic analysis to hypothesize the taxonomic composition of the assemblage. We identified at least three mosasaur taxa from the Savoy Pit assemblage, including Tylosaurus nepaeolicus, a russellosaurian identified from a frontal similar to that of Russellosaurus coheni, and a new basal mosasaurine. The latter specimen exhibits a unique combination of characters that include synapomorphies of Mosasaurinae and those observed in russellosaurians. Phylogenetic analyses place the specimen as an early-diverging member of Mosasaurinae and the characters of Russellosaurina could represent plesiomorphic conditions. The presence of Tylosaurus nepaeolicus and the russellosaurian represent potential range extensions of those lineages into the Coniacian. The basal mosasaurine fills a stratigraphic gap in the record of that clade between early members. Our study not only presents new data that are critical to understanding the distribution and evolution of mosasaurs, but also highlights the continued importance of specimens collected by the WPA to our understanding of the fossil record.

Authors

• Blake Russell Chapman, The University of Texas at Austin

• Joshua Ryan Lively, The University of Texas at Austin