Toxicofera (Greek for "those who bear toxins") is a proposed clade of scaled reptiles (squamates) that includes the Serpentes (snakes), Anguimorpha (monitor lizards, gila monster, and alligator lizards) and Iguania (iguanas, agamas, and chameleons). Toxicofera contains about 4,600 species, (nearly 60%) of extant Squamata.[1] It encompasses all venomous reptile species, as well as numerous related non-venomous species. There is little morphological evidence to support this grouping; however, it has been recovered by all molecular analyses as of 2012.[2][3][4]
The oldest known toxicoferan is Cryptovaranoides, an anguimorph from the Late Triassic of England.[5]
The relationship between these extant groups and a couple of extinct taxa are shown in the following cladogram, which is based on Reeder et al. (2015; Fig. 1).[6]
Venom in squamates has historically been considered a rarity; while it has been known in Serpentes since ancient times, the actual percentage of snake species considered venomous was relatively small (around 25%).[7] Of the approximately 2,650 species of advanced snakes (Caenophidia), only the front-fanged species (~650) were considered venomous by the anthropocentric definition. Following the classification of Helodermatidae in the 19th century, their venom was thought to have developed independently.[1] In snakes, the venom gland is in the upper jaw, but in helodermatids, it is found in the lower jaw.[1] The origin of venom in squamates was thus considered relatively recent in evolutionary terms and the result of convergent evolution among the seemingly-polyphyletic venomous snake families.[citation needed]
In 2003 a study was published that described venom in snake subfamilies previously thought to lack it.[8] Further study claimed nearly all "non-venomous" snakes produce venom to a certain extent, suggesting a single, and thus far more ancient origin for venom in Serpentes than had been considered until then.[9][10] As a practical matter, Fry cautioned:[11]
Some non-venomous snakes have been previously thought to have only mild 'toxic saliva'. But these results suggest that they actually possess true venoms. We even isolated from a rat snake [Coelognathus radiatus (formerly known as Elaphe radiata)[9]], a snake common in pet stores, a typical cobra-style neurotoxin, one that is as potent as comparative toxins found in close relatives of the cobra. These snakes typically have smaller quantities of venom and lack fangs, but they can still deliver their venom via their numerous sharp teeth. But not all of these snakes are dangerous. It does mean, however, that we need to re-evaluate the relative danger of non-venomous snakes.