Cranial anatomy of the "round-headed" Amphisbaenian Zygaspis quadrifrons (Squamata, Amphisbaenia) based on high-resolution x-ray computed tomography.

Amphisbaenians are a poorly understood clade of fossorial lizards. Because of their derived anatomy and relative scarcity, the systematics of the clade and its placement within squamates has long been controversial. Traditional approaches grouped species into four assemblages according to burrowing behavior and cranial morphology, resulting in the recognition of "shovel-headed," "round-headed," "keel-headed," and "spade-headed" morphotypes.