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

Abstract

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.

Ecology, conservation status, and phylogenetic placement of endemic Pristimantis frogs (Anura: Craugastoridae) in Trinidad and Tobago and genetic affinities to northern Venezuela

“…The time tree (Figure 5) suggests that the presence of P. turpinorum in Tobago is likely the result of a vicariant event roughly at the time of the detachment of Trinidad from northern Venezuela circa 4–5 Ma, followed by a stepping‐stone colonization event to Tobago with extinctions of intermediate populations in Trinidad thereafter. A different scenario implies a direct colonization event from the mainland to Tobago, as is the case for other species such as the glass frog Hyalinobatrachium orientalis , which is only present in Tobago and northern Venezuela (Jowers et al, 2015) or the Hallowell's ground snake A. fuliginosus , which is present in Tobago and central‐western Venezuela (Jowers et al, 2021). Under this last scenario, Tobago would have been positioned several kilometers to the west of its current position and it could had been easier to colonize directly from the mainland, explaining the absence of the species in Trinidad, as has already been proposed for another anuran (Jowers et al, 2015).…”

“…Until the work reported here, no other study had found that some mainland Pristimantis species are closely related to some insular species. Four scenarios support the presence of closely related species between northern Venezuela and Tobago: (1) when Tobago's position was much further west of its current position (Jowers et al, 2015), shallow marine depths (<50 m) in the region and sea‐level falls during the Pliocene (Miller et al, 2005) would have facilitated land‐bridge connections between northern Venezuela and Tobago (Jowers et al, 2015); (2) through stepping‐stone connection of several Pliocene islands that once existed between Tobago and the mainland and Trinidad, and all of the islands were connected by very shallow and likely emergent coastal systems (Jowers et al, 2021; Samlal, 2016); (3) over‐water dispersal with frogs rafting on vegetation, possibly from river mouths or an estuary such as the Orinoco Delta (Camargo et al, 2006; Yanek et al, 2006); and (4) a stepping‐stone colonization from Trinidad with subsequent extinctions locally, which remains more unlikely due to the much larger area of suitable habitat present in Trinidad's Northern Range. Further sampling and sequencing of other loci are required to run accurate phylogenetic interpretations and models.…”

“…(2) through stepping-stone connection of several Pliocene islands that once existed between Tobago and the mainland and Trinidad, and all of the islands were connected by very shallow and likely emergent coastal systems (Jowers et al, 2021;Samlal, 2016); (3) over-water dispersal with frogs rafting on vegetation, possibly from river mouths or an estuary such as the Orinoco Delta (Camargo et al, 2006;Yanek et al, 2006); and (4) a stepping-stone colonization from Trinidad with subsequent extinctions locally, which remains more unlikely due to the much larger area of suitable habitat present in Trinidad's Northern Range. Further sampling and sequencing of other loci are required to run accurate phylogenetic interpretations and models.…”

“…Interestingly, some of the Tobago endemics seem to be more closely related to species of the Venezuelan coastal montane complex than to Trinidad. Some examples include Tobago's dendrobatid Mannophryne olmonae (Manzanilla et al, 2009), the Tobago glass frog, Hyalinobatrachium orientale (Jowers et al, 2015), the Tobago stream snake, Erythrolamprus pseudoreginae (Murphy et al, 2019) and the fossorial Hallowell's ground snake Atractus fuliginosus (Jowers et al, 2021). Despite some of these species having members of the same genus in Trinidad, they are not closely related to them, suggesting that colonization of Tobago did not happen through a stepping-stone process from Trinidad.…”