A ‘Woollgari-Zone Mosasaur’ (Squamata; Mosasauridae) from the Carlile Shale (Lower Middle Turonian) of Central Kansas and the Stratigraphic Overlap of Early Mosasaurs and Pliosaurid Plesiosaurs

Abstract

Anew specimen of russellosaurine mosasaur (FHSM VP-17564) from the middle Fairport Chalk Member (Collignoniceras woollgari zone, early Middle Turonian), Carlile Shale of central Kansas is among the earliest records of a derived mosasaur known from the North American Western Interior Seaway. Unequivocally the oldest recorded occurrence of a russellosaurine mosasaur from Kansas, and one of only four Kansas occurrences consisting of more than a single element, the specimen includes seven successive proximal caudal vertebrae bearing articulating hemal facets. No demonstrable characters ally the specimen to a more refined taxonomic subgroup, although the relative dimensions of the vertebral centra are significantly longer than equivalent caudal series in Platecarpus and Tylosaurus. This suggests that the tails of early russellosaurines were composed of a lesser number of longer caudal vertebrae than their successors, with less hydrodynamic adaptation from the terrestrial condition. Size of the vertebrae in relation to Platecarpus and Tylosaurus indicates a relatively large animal roughly 3.2 m in total body length, although this may be an over-estimate given the difference in vertebral dimensions as compared to more derived russellosaurines of the Niobrara Formation. The middle of the Fairport Chalk interval continues to produce a large number of vertebrate specimens despite the rarity of exposures. A pliosaurid plesiosaur skull (FHSM VP-321) is known from similar geographic and stratigraphic position as FHSM VP-17564, and the record of pliosaurs (FHSM VP-17469) continues into younger Blue Hill Shale (Prionocyclus hyatti zone, middle Middle Turonian) demonstrating with certainty that pliosaurs and early mosasaurs inhabited the same stretch of seaway for roughly one million years.