A DWARF SPHENODONTIAN FROM THE JURASSIC LA BOCA FORMATION OF TAMAULIPAS, MEXICO

A new genus and species of sphenodontian lepidosaur, Zapatadon ejidoen sis, is described, based upon a nearly complete skull of a posthatchli ng specimen from the late Early Jurassic La Boca Formation, Tamaulipas , Mexico. It is diagnosed by its extremely small size, the presence of a depression around the hatchling tooth series on the dentary, an exp anded prefrontal that surrounds most of the dorsal process of the maxi lla, and a broad jugal that nearly excludes the maxilla from the orbit . A phylogenetic analysis of its relationships supports the hypothesis that it is among the closest relatives of the only extant sphenodonti an, Sphenodon. However, the relationships of Z. ejidoensis could not b e resolved with respect to Eilenodontinae, Opisthias, and a Cynospheno don-Sphenodon clade. The most significant character diagnosing the cla de composed of these five taxa is wear facets indicative of propalinal mandibular movement.