Evolution and systematics

There are few snake fossils that can be incontrovertibly identified as woodsnakes. The fossil genus Boavus dating from the Eocene may belong to the Tropidophiidae. More recent fossils identified as Tropidophis are known from Pleistocene deposits in caves.

Woodsnakes are believed to have diverged from basal macrostomatan stock sometime after the divergence of the macrostomatans from alethinophidians at the end of the Cretaceous. Macrostomatan snakes are distinguished by characters of the skull and musculature that allow them increased jaw flexibility, a greater gape, and the ability to consume larger prey.

The woodsnakes are believed to have originated in northern South America and from there spread into Central America and the West Indies. The greatest diversity of woodsnakes exists in Cuba, where in the relative absence of other snakes the tropidophiid lineage became the dominant family of snakes on the large island. The origin and relationships of the two Asian species of spinejaw snakes in the genus Xenophid-ion are unclear at this time.

The phylogenetic relationship of the Tropidophiidae to other snake families is unclear. Traditionally, the sister taxon has been identified as the Boidae, but in recent years many authors have come to identify the Bolyeriidae as the most likely sister taxon.

As is typical of most macrostomatan snakes, most tropi-dophiid species have the vestiges of a pelvic girdle and cloacal spurs. However, in the Tropidophiidae, only male snakes have the vestigial pelvic girdle and cloacal spurs; Tropidophis semi-cinctus and both species of Xenophidion do not have the pelvic girdle and spurs.

Tropidophiid snakes do not have a functional left lung, as is characteristic of all caenophidian snakes. They do have a well-developed tracheal lung, a characteristic of many colu-bridoid snakes.

The Tropidophiidae has been well investigated and is relatively well-known. However, it is a widely distributed lineage that may be more speciose than is currently recognized. It seems likely that future phylogenetic analyses based on genetic characters will recognize some subspecies and some disjunct populations as new species. Two species, Tropidophis celiae and Tropidophis spiritus, were described as recently as 1999. The Asian genus Xenophidion was recognized in 1996, and tentatively placed in the Tropidophiidae in 2001.

Some authors recognize three distinct lineages within the Tropidophiidae, namely the Tropidophiinae comprised of the genera Tropidophis and Trachyboa; the Ungaliophiinae comprised of Ungaliophis and Exiliboa; and the Xenophidiinae with Xenophidion. It has been proposed that each of the three lineages should be recognized as a family. Future investigations will undoubtedly address and resolve the relationships of these genera.

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