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We divide snakes into two main groups: the scolecophidians (small, burrowing snakes with a limited gape size that mainly feed on small insects and termites) and the alethinophidians (large gape size, thus capable of feeding on larger prey, including some large-bodied vertebrates).
The alethinophidians are the largest group in terms of number of species, and they are far more ecologically diverse then the scolecophidians. They include all the well-known species such as pythons, boas, cobras and vipers. The alethinophidians include the familiar Boidae (pythons and boas), and the more basal, and lesser known tropidophiids and aniliids.
All the venomous snake species are found within the most derived group of alethinophidians, the advanced snakes (Caenophidia).
The relationships between the different families of advanced snakes have long been controversial. Some scientists used dentitonal characters for establishing relationships, and came to the conclusion that the cobras and vipers (that both posses front-fangs of rather different morphology), must have evolved from rear-fanged snakes (previosuly all dumped into the 'Colubridae'). But new molecular data suggests that vipers and pitvipers (Viperidae) are actually basal advanced snakes, while cobras and seasnakes (Elapidae) are more derived . The Colubridae was a paraphyletic group, but now several new families have been split off from the old group of the 'Colubridae'. Most of this work was done by Nicolas Vidal and Blair Hedges.
Establishing the relationships of snakes to each other is an essential prerequisite for studying the evolution of snakes, and their special adaptations (for example venom and fangs). Without a robust family tree, we cannot say what the polarity of evolutionary change is (i.e. which features are primitive and which are derived) and we cannot identify the condition in common ancestors. Furthermore we cannot make statements about the potential independent evolution of characters in different lineages if we do not know their evolutionary relationships. The origin of snakes in general is another controversial subject, not yet completely understood. It is generally agreed that snakes evolved from lizards.However, there are two major interpretations of currently available evidence: (i) snakes evolved on land, from burrowing or semi-burrowing lizards (ii) snakes evolved in the ocean, from marine lizards, than colonized land.
Both these hypotheses find support from both molecular and morphological data of extant reptiles and fossils. Some snake fossils have been described and shed some light on this controversial subject, although the results are still inconclusive. The answer may still be hidden somewhere in Mesozoic rocks, formed around 100 million years ago.