New information on the skull of Keichousaurus hui (Reptilia: Sauropterygia) with comments on sauropterygian interrelationships

Abstract

ABSTRACT Newly prepared material of Keichousaurus hui demonstrates that the nasals are very reduced and do not contribute to the margins of external nares, the quadrate lacks a posterior condylar process, the premaxilla and anterior dentary teeth are enlarged and procumbent, the maxilla has a pair of ‘canines,’ the vomers are co-ossified, the suborbital fenestrae are very reduced or lost, a distinct coronoid process is present, and both the mandibular symphysis and retroarticular process are elongated. The presence of an ectopterygoid could not be confirmed. The shape of the prefrontal, length of the quadrate ramus of the squamosal, and length of the quadrate and quadratojugal all indicate that the skull profile of Keichousaurus is much lower than previously thought. A new reconstruction attributes a distinctly eusauropterygian gestalt to the skull. A phylogenetic analysis, although equivocal, suggests that Keichousaurus is more appropriately considered a basal nothosauroid rather than a pachypleurosauroid, and that the Eusauropterygia is paraphyletic.