Abstract

Extant alligator lizards of the genus Abronia are found in montane cloud forests and pine-oak forests of Mesoamerica and are iconic among the public and scientific communities. Here, we describe a fossilized partial skull from the Miocene of southern California (~12.5–11.0 Mya) that is the first definitive fossil and only recognized extinct species of Abronia. The locality of the fossil is substantially removed from the range of extant species of Abronia. This remarkable biogeographical discovery corroborates previous speculation that Abronia was distributed north of Mexico during the Neogene, a scenario that could not be inferred from the geographical ranges and phylogeny of the extant species alone. Additionally, the fossil preserves a distinctive morphology, osteoderm sails, that appears unique to the new taxon among alligator lizards. The finding emphasizes the importance of the fossil record for historical biogeography and could motivate new avenues of biogeographical research in Mesoamerica and the USA