A new burrowing diapsid from the Jurassic La Boca Formation of Tamaulipas, Mexico

Abstract

Two articulated skulls and associated material from the Early or Middle Jurassic La Boca Formation of Mexico represent a new genus and species of small diapsid, Tamaulipasaurus morenoi. The skull is superficially similar to that of burrowing squamates, especially amphisbaenians and dibamids, but the presence of plesiomorphic characters, such as a complete lower temporal bar, contradict a phylogenetic relationship within Squamata. Among its autapomorphies are extensive fusion of braincase bones and of postdentary bones, a broad thin squamosal, absence of three circumorbital bones, an extensive side wall to the braincase, an anteriorly directed fenestra ovalis, an unusual course for the stylar process of the stapes in a groove between the quadrate and braincase, and a large longitudinal foramen through the base of the transverse process of cervical vertebrae. The phylogenetic relationships of Tamaulipasaurus morenoi are difficult to assess because its burrowing adaptations obscure its structure. It shares several features with diapsids, such as an emarginated quadrate with a tympanic recess and a lower temporal bar, and several characters with squamates, such as absence of a descending process of the squamosal and procoelous vertebrae with fused neural arches, suggesting that it may be closely related to Lepidosauria.