Systematics and evolution of the Amphisbaenia (Lepidosauria: Squamata): A phylogenetic hypothesis based on morphological evidence from fossil and recent forms

Amphisbaenians are a group of squamate reptiles that live almost exclusively under loose or sandy soil in tropical to temperate areas around the world, and their anatomy is modified in many ways related to this lifestyle. Only one genus—Bipes—retains external limbs, and these are uniquely adapted forelimbs that appear to be used mainly to dig entrances to burrows. Most recent studies recognize four families of living amphisbaenians—Bipedidae, Amphisbaenidae, Trogonophidae, and Rhineuridae. Only the Rhineuridae are well represented in the fossil record.; A systematic study of the Amphisbaenia is presented based on morphological characters surveyed from fossil and living forms. This study tests previous ideas regarding amphisbaenian evolution with the first cladistic analysis of the group and provides new data evaluating the homology of amphisbaenian features with those of other squamates. Amphisbaenian monophyly is supported but the recent identification of the late Cretaceous Sineoamphisbaena hextabularis as a primitive amphisbaenian is not supported. The monophyly of two out of the four families of extant amphisbaenians is upheld. The Family Amphisbaenidae is substantially revised: several taxa previously included in the group are more closely related to rhineurids, and the genus Blanus is a basal amphisbaenian. The Family Rhineuridae is also revised to include the previously recognized fossil family Hyporhinidae. Trogonophidae is the sister-group to the remaining amphisbaenids, and together they form the sister-group to the expanded rhineurid clade. Bipes is the most basal amphisbaenian, and Blanus is the sister-group to all other amphisbaenians. The fossil amphisbaenian family Crythiosauridae is removed from the Amphisbaenia because no support was found for a relationship with amphisbaenians.; Morphological investigations of squamates conducted in this study include many characters associated with reduced or absent limbs and girdles, and other specializations presumed to be related to a burrowing lifestyle. These characters are often hypothesized to have been convergently acquired among limbless squamate clades and have been excluded or downweighted in some previous analyses based on presumed correlation. This is difficult to justify here because variation was found with further anatomical investigations of these structures useful for resolving relationships within amphisbaenians.