Evolution and systematics

Researchers have devoted considerable effort to reconstructing the phylogenetic history of the Viperidae, and, consequently, extensive revision of the classification of these snakes was done throughout the end of the twentieth century and has continued into the twenty-first century. Monophyly of the Viperidae is well supported by molecular and anatomical data. Available evidence suggests that the Viperidae is the most basal family of the Colubroidea, which also includes the Colubridae, Elapidae, and Atractaspididae. Four subfamilies are recognized.

The Causinae includes a single genus, Causus, with six species; the Viperinae includes 12 genera with 75 species; the Azemiopinae contains a single genus and species (Azemiops feae); and the pitvipers are classified in the Crotalinae, in which 22 genera and 174 species are recognized. With the possible exception of the Viperinae, the monophyly of each viperid subfamily is well supported by anatomical and molecular data. A potential synapomorphy of the Viperinae is the ventral course of the facial carotid artery, providing at least some support for the monophyly of this subfamily. The Causi-nae is thought to be sister to either all other viperids or only the Viperinae. The Azemiopinae is sister to the Crotalinae. As of 2002 almost all genera and subgenera recognized are monophyletic, although relationships among genera are not yet resolved fully.

Viperinae consists of Vipera, Macrovipera, Pseudocerastes, Eristicophis, Daboia, Echis, Cerastes, Atheris, Bitis, Proatheris, Adenorhinos, and Montatheris. The Eurasian genus Vipera is divided into the subgenera Vipera, Pelias, Montivipera, and

Acridophaga, each with an evolutionary history that can be traced back to the Miocene through the fossil record or "molecular clock" calibrations. It is the same with the African genus Bitis, which is divided into the subgenera Bitis, Macrocerastes, Calechidna, and Keniabitis, each with its own evolutionary history, as inferred from molecular data. Unfortunately, there are no viper fossils known from tropical Africa. The genera of Old World pitvipers are Calloselasma, Deinagkistrodon, Er-mia, Gloydius, Hypnale, Ovophis, Protobothrops, Triceratolepi-dophis, Trimeresurus, and Tropidolaemus, and the genera of New World pitvipers are Agkistrodon, Atropoides, Bothriechis, Both-riopsis, Bothrocophias, Bothrops, Cerrophidion, Crotalus, Lachesis, Ophryacus, Porthidium, and Sistrurus.

The region of origin for viperids remains undetermined. The earliest fossil specimens of this family are known from the Lower Miocene (ca. 20 million years ago) of Europe and West Asia. These findings are well documented, but there are no fossil records from the Oligocene, and there is a general opinion that the viperines arose somewhere else. The origin of African viperines has been dated to at least 50 million years before the present based on molecular evidence, and there is much support for a tropical African origin of the Viperidae.

The striking examples of convergent evolution between the Asian and American pitviper radiations had long obscured attempts to understand the phylogeny of these snakes; however, data from mitochondrial DNA sequences strongly suggest that pitvipers originated in Asia, and these findings are consistent with other lines of evidence. For example, the closest living relative of all pitvipers, Azemiops feae, is an Asian species. The earliest known fossil pitviper from the New World is from the Miocene. All New World pitvipers are descended

Pope's pitviper (Trimeresurus popeorum) in the montane rainforest of Malaysia. (Photo by Fletcher & Baylis/Photo Researchers, Inc. Reproduced by permission.)

from a single pitviper species that extended its range across the Bering Land Bridge into North America. Phylogenetic studies suggest that the original American pitviper resembled Gloydius blomhoffi in many respects.

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