DESCRIPTION AND PHYLOGENETIC ANALYSIS OF CRYPTOCLEIDOID PLESIOSAURS BASED ON NEW MATERIAL OF TATENECTES LARAMIENSIS FROM THE SUNDANCE FORMATION (WYOMING, USA)

The majority of cryptocleidoid plesiosaurs have been recovered from Jurassic units in the United Kingdom, but Tatenectes laramiensis is one of two species documented from the Sundance Formation of Wyoming. The purpose of this research was to describe new fossil material of Tatenectes and to reevaluate the phylogenetic position of this genus within the cryptocleidoid lineage based on this material. The Sundance Formation was deposited in the Sundance Seaway, a shallow isolated epicontinental seaway, which inundated the northern Rocky Mountain states in several pulses during the Jurassic. The fossils were collected during fieldwork in the Bighorn Basin in North-central Wyoming, from Redwater Shale Member sediments near Shell, Wyoming. This new material includes a partial squamosal, a humerus missing part of the shaft, and a partial pectoral girdle, among other elements. Nine new phylogenetic characters based on morphological features of the humerus and pectoral girdle were integrated into previously existing character matrices for cryptocleidoids to generate new phylogenetic trees using parsimony criteria. The pectoral girdle is very short anteriorly and most closely resembles that of Tricleidus seeleyi. Two equally parsimonious trees were obtained, and the resulting strict consensus tree solidifies the phylogenetic position of Tatenectes and strengthens the relationship between the highly derived pliosauromorph polycotylidae and the cimoliasauridae plesiosaurs.