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Rear fanged snakes are, simply, snakes that have their fangs in the the back of their mouth. This is rather awkward, and requires the snake to open their mouth very wide (~170 degrees) in order to inject the venom.

The rear fanged snakes are a polytypic group, and are all classified in the wastebasket taxon of Colubridae, a family in which all unclassified snake taxons are dumped. It appears, in fact, that originally all snakes were rear fanged snakes, and some genera found it advantageous to push the fangs forward along the jaw until they eventually came to replace the frontmost teeth. This range of variation is possible largely because the fangs are formed from a separate bit of embryonic tissue than the rest of the jaw, allowing more flexibility in mutational variation.

In general, rear fanged snakes tend to prey on smaller and less aggressive animals, generally catching their prey and biting repeatedly to inject venom, to the point where they appear to be chewing on their prey, rather than striking. In contrast, snakes with fangs at the front of their mouth tend to use a single, sudden strike and then withdraw while the venom takes effect.

Rear fanged snakes are not likely to strike at humans, and if they do are less likely to inject an effective dose of venom. This has led to some surprises for us humans; the deadly boomslang and twig snake were both thought to be non-venomous by scientists, until incautious herpetologists died unexpectedly. Because this sort of bite is rare, it can be all the more deadly; we still have not developed an antivenom for the twig snake. However, the majority of these snakes have venom that is targeted at killing small reptiles and amphibians, rather than mammals.

Further reading:
The Reptile Blog: Rear Fanged Snakes
Leiden University: Snakes were originally rear-fanged

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