Divergence Dating and Biogeography of Xenosauridae Including Fossils as Terminal Taxa

Abstract.— Middle-American knob scaled lizards of the genus Xenosaurus are viviparous lizards characterized by flattened bodies, peculiar knob-like scales, and extreme specialization to a crevice-dwelling ecology. Species within this rarely seen and enigmatic genus are distributed throughout much of Mexico and Guatemala. Xenosaurus is the sole surviving genus of the ancient family Xenosauridae and little is known about the origins and biogeographic history of members of the group. To address this deficit, we estimated divergence times across Xenosaurus under the fossilized birth–death model, including published ddRADseq data, morphological data for six extant and three fossil relatives to genus Xenosaurus, and fossil occurrence data for the three fossil relatives. Crown Xenosaurus was estimated to be 35 million years old, considerably younger than the stem age of the family, implying high rates of extinction. The distribution of Xenosaurus populations across Mexico and the location of the three fossil relatives in western North America suggest a Nearctic origin with a north to southeast direction of range evolution in Xenosaurus. To test this hypothesis, we performed a Bayesian biogeographic analysis under the Dispersal Extinction Cladogenesis (DEC) model, in order to estimate the historical ranges and patterns of dispersal across the genus. The results of the biogeographic analysis supported this historical north to southeast movement. In conjunction with our diversification estimates, our DEC results suggest that climatic events such as cooling during the Miocene and Pliocene may have shaped the divergence and current distributions in Xenosaurus.