The homology of the pelvic elements of Zygaspis quadrifrons (Squamata: Amphisbaenia).

Limb attenuation with element loss has occurred multiple times among the Squamata (lizards, snakes, and amphisbaenians). Although most of the attention has been focused on the appendicular bones, we found that the pelvic rudiments have been studied less thoroughly and recurring disagreement is common among different authors studying the same species. We studied the osteology of the pelvic region of female and male Zygaspis quadrifrons with high-resolution X-ray computed tomography data. We report an osteological landmark (acetabulum) not previously detected in this taxon, the presence of which has repercussions that call for a reconsideration of the primary homology hypothesis for the identity of these bones in amphisbaenians and other squamates. Finally, we observed that the acetabulum and limb rudiments in amphisbaenians (i.e., the stylopodium when present) are situated medial to the pelvic girdle, contrasting with the large majority of tetrapods where these structures are laterally oriented.