The enigmatic biogeography of Tobago's marooned relics: The case study of a fossorial snake (Squamata, Dipsadidae)

The islands of Trinidad and Tobago form a southern extension of the Lesser Antilles. Unlike the continental island of Trinidad, the more northerly Tobago formed as an older oceanic island volcanic arc. Their reptile biodiversity reflects colonization events from the South American mainland through land bridge connections at times of glacial maxima. Most of Tobago's herpetofauna has colonized through stepping-stone events from Trinidad. However, the enigmatic presence of a rare and poorly known fossorial snake in Tobago, Western Venezuela and Colombia, but absent in Trinidad and Eastern Venezuela, raises interesting questions regarding its biogeography, mode and timing of colonization of the island. Here, we sequence for the first time gene fragments from three individuals from Western Venezuela and one from Tobago and include them in the largest phylogeny of Atractus to date. We validate the monophyly of the species based on morphology and molecular data, with an unexpected low genetic divergence between island and mainland specimens. Despite more than 1000 km separating them, our time tree indicates a mean 550,000 year divergence. We examine alternative scenarios to explain the biogeography and conclude on an ancient corridor of coastal land bridges at times of very low (>100 m) sea-level falls that connected Venezuela to Tobago.