Evolution and systematics

The two major families of venomous snakes are Elapidae, or the elapid snakes, and Viperidae, the vipers and pitvipers. The snakes in these families are similar in that they have fangs in the front of the mouth. The two groups arose independently from nonvenomous snake ancestors, however, so there are important differences between them. The overall appearance of elapids is much more like that of the primarily non-venomous colubrids than it is of the generally plump, short-tailed vipers. The main differences between elapids and vipers are in the structure of the venom delivery apparatus and the nature of the venom. Elapids have proteroglyphic dentition, which means "fixed front fangs." Vipers have solenoglyphic dentition, which means "movable front fangs." In elapids, the fangs are fixed in one position and are relatively short to avoid puncturing the snake's bottom lip. Vipers have long fangs that are hinged and fold back into the mouth. The venom of elapids is quite different from that of vipers. Elapids have neurotoxic venom (nerve poison), which acts mainly on the central nervous system. The venom affects heart function and breathing but causes little or no damage at the bite site. Vipers have primarily hemotoxic and myotoxic venom, which produces severe damage at the bite site, including complete necrosis of the surrounding tissue.

The venomous elapid snakes include 60 genera and more than 300 species. Because elapids represent approximately 10% of living snake species and more than 50% of species of venomous snakes, they are of considerable medical importance. The elapids are fantastically diverse in size, shape, color, ecology, and behavior, but they can be classified as follows according to size and distribution: cobras and mambas;

coral snakes; terrestrial kraits; Australo-Papuan elapids, which include brown snakes, taipans, and death adders; sea kraits; and seasnakes.

Little is known about the origin of elapid snakes except that they are related to some African forms that seem to have "protoelapid" fangs. For example, the African and Middle East members of the genus Atractaspis are venomous and have front fangs, but they also have a number of characteristics that differentiate them from elapids and unite them with primarily nonvenomous species. The southern African genus Ho-moroselaps is confusing in that it has elapid fangs and venom but seems to have characteristics of Atractaspis.

Despite the confusion, elapids seem to form a mono-phyletic group, as does each of the major elapid lineages. Together elapids are primarily defined by the presence of a venom delivery system comprising two small permanently erect front fangs. Diverse data sets have been used to elucidate relationships among and within elapid lineages, including various aspects of morphology, protein albumins, karyotypes, allozymes, venom protein sequences, and DNA sequences. There is still some disagreement about the relationships between the major groups of elapids.

The number and content of elapid families and subfamilies have varied widely. Depending on perceived levels of differentiation, various authors have recognized either a single family, Elapidae, with two to six subfamilies or two families: the Elapidae, terrestrial elapids, and the Hydrophiidae, seasnakes. Evidence from studies of morphology and DNA sequences shows that seasnakes are most closely related to Australo-Papuan elapids and thus are part of elapid radiation.

An Egyptian banded cobra (Naja naja annulifera). (Photo by Animals Animals ©Ralph Reinhold. Reproduced by permission.)

The fully marine seasnakes evolved from terrestrial live-bearing Australian ancestors, and the partially marine sea kraits seem to be most closely related to terrestrial elapids in Asia and Melanesia. Most authorities recognize a single family, Elapidae, that has two subfamilies: the Elapinae, including coral snakes, cobras, mambas, and terrestrial kraits, and the Hydrophiinae, including all the Australo-Papuan elapids, sea kraits, and seasnakes.

Definite elapid snake fossils are rare but have been found in Miocene deposits in Europe, North America, Africa, and Australia. Because there are so few, these fossils have contributed little to the understanding of elapid evolution.

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