Cretaceous roots of the amphisbaenian lizards

摘要

Amphisbaenians are highly specialized limbless burrowing lizards of controversial relationships. Among fossil lizards, the Eocene (47 Ma) Cryptolacerta is allegedly closest to the amphisbaenian ancestor, but this is put in doubt in this study. Similarities between Cryptolacerta and amphisbaenians, such as limb reduction and expansion of the skull roof, may be a result of parallel evolution. Instead, the Late Cretaceous lizard Slavoia with well-developed limbs and several plesiomorphic skull characters is proposed to be the oldest known stem amphisbaenian. This is supported by two different phylogenetic analyses and observations on numerous specimens together representing almost the whole skeleton. Among the unique features, that Slavoia shares with amphisbaenians, the most significant are vomers strongly underlapping palatines and pterygoid quadrate ramus tightly wrapping around posteromedial surface of quadrate. The anatomy of Slavoia suggests that the reinforcement of the snout in amphisbaenian evolution preceded the elongation of the postorbital part of the skull, and that of the body, as well as modification of the limbs. Reduction of its hindlimbs was more advanced than that of the forelimbs. The ancient geological age of the central Asiatic Slavoia suggests that diversification of the main North American amphisbaenian groups may have resulted from a faunal dispersals from Asia after the Late Cretaceous.