Xenosaurus is an enigmatic clade of Mexican and Central American lizards distinguished by knob-like scalation and flattening of the head and body associated with living in cracks within cliff faces. The position of Xenosaurus within the larger clade Anguimorpha is difficult to determine owing to a combination of primitive features and a unique, highly modified anatomy that obscures useful characters. Evidently, the phylogenetic stem of Xenosaurus represents a long independent history of evolution. Fortunately, several fossil taxa exist that can elucidate this history. These taxa include the extinct Exostinus lancensis (Cretaceous), Exostinus serratus (Oligocene), and Restes rugosus (Paleocene), the latter two known from substantial, cranial material (Bhullar, 2007; 2010). Using osteological and alcohol-preserved specimens, fossils, and high-resolution x-ray CT scans thereof, I attempted to reconstruct the relationships of the three fossil taxa and the six extant species of Xenosaurus that are available in U.S. collections.
Despite the considerable phylogenetic importance of Xenosaurus and its stem, this is the first phylogenetic analysis of the group. An exhaustive search of the skeleton, including osteoderms embedded in the skin visualized using CT scanning, yielded 274 new characters, substantially more than have been used previously in gross anatomy–based analyses of such a restricted group of reptiles. The great number of characters is largely the result of the availability of disarticulated skeletal material, CT scans showing internal bone structure and bones embedded in the skin, and attention to subtle anatomical differences whose validity could be assessed in terms of intraspecies variation because of the availability of large sample sizes for certain taxa.
My results suggested that R. rugosus is sister to the other xenosaurs, resolving a polytomy with other Anguimorpha recovered by previous work. Exostinus lancensis is problematic in that it may represent several distinct taxa, but it was recovered as sister to E. serratus Xenosaurus, making Exostinus paraphyletic. Exostinus serratus emerged as sister to Xenosaurus. Xenosaurus comprises a northern clade consisting of Xenosaurus newmanorum and Xenosaurus platyceps; the remaining taxa are united as a southern clade. Within the southern clade, Xenosaurus agrenon and Xenosaurus rectocollaris are sister to Xenosaurus grandis and Xenosaurus rackhami. North–south splits within Xenosauridae mirror those of several other lizard clades and may be the legacy of the equatorial contraction of early Tertiary tropical forests. The fully resolved nature of the phylogeny and the congruence of the extant portion with molecular results indicates the continued relevance and efficacy of morphological systematics when an exhaustive anatomical analysis is performed to search for new characters.