Venom clade
| Toxicoferans Temporal range: Middle Jurassic to present |
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| Venomous snakes, such as the rattlesnake shown above, are the best-known venomous squamates | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Squamata |
| Clade: |
Toxicofera Vidal & Hedges, 2005 |
| Subgroups | |
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 4600 species, (nearly 60%) of extant squamata. 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 recent molecular analyses.
Toxicofera combines the following groups from traditional classification:
Detailed cladogram in Reeder et al., 2015; Fig. 1
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%). 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. In snakes, the venom gland is in the upper jaw, but in helodermatids, it is found in the lower jaw. 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.
In 2003 a study was published that described venom in snake subfamilies previously thought to lack it. 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. As a practical matter, Fry cautioned:
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