How and when did Old World ratsnakes disperse into the New World?

To examine Holarctic snake dispersal, we inferred a phylogenetic tree from four mtDNA genes and one scnDNA gene for most species of the Old World (OW) and New World (NW) colubrid group known as ratsnakes. Ancestral area distributions are estimated for various clades using divergence-vicariance analysis and maximum likelihood on trees produced using Bayesian inference. Dates of divergence for the same clades are estimated using penalized likelihood with statistically crosschecked calibration references obtained from the Miocene fossil record. With ancestral areas and associated dates estimated, various hypotheses concerning the age and environment associated with the origin of ratsnakes and the dispersal of NW taxa from OW ancestors were tested. Results suggest that the ratsnakes originated in tropical Asia in the late Eocene and subsequently dispersed to the Western and Eastern Palearctic by the early Oligocene. These analyses also suggest that the monophyletic NW ratsnakes (the Lampropeltini) diverged from OW ratsnakes and dispersed through Beringia in the late Oligocene/early Miocene when this land bridge was mostly composed of deciduous and coniferous forests.