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Toxicofera (Latin for "toxin-bearers") is a proposed clade of squamata (squamates) that includes the snake (snakes), Anguimorpha (, Helodermatidae, and ) and Iguania (, agamas, and ). Toxicofera contains about 4,600 species (nearly 60%) of extant taxon 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 molecular analyses as of 2012.
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).
Alongside these groups, Mosasauria, an extinct group including large marine reptiles primarily known from the Late Cretaceous, has been placed as part of the group. It has often been supposed that mosasaurs are most closely related to snakes, with the group containing the two dubbed Pythonomorpha, however, other studies have quesitioned this, finding that the closest relatives of mosasaurs are members of Varanoidea. Polyglyphanodontia, a group of extinct herbivorous lizards known from the Cretaceous, have also been placed as part of this group in some studies as the sister group to Iguania, though other studies have instead suggested that they are most closely related to Teiioidea and thus placed outside Toxicofera.
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:
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, 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.''
This prompted further research, which led to the discovery of venom (and venom genes) in species from groups which were not previously known to produce it, e.g. in Iguania (specifically Pogona barbata from the family Agamidae) and Varanidae (from Varanus varius). It is thought that this was the result of descent from a common venom-producing squamate ancestor; the hypothesis was described simply as the "venom clade" when first proposed to the scientific community. The venom clade included Anguidae for phylogenetic reasons and adopted a previously suggested clade name: Toxicofera.
It was estimated that the common ancestral species that first developed venom in the venom clade lived on the order of 200 million years ago. The venoms are thought to have evolved after genes normally active in various parts of the body Gene duplication and the copies found new use in the salivary glands.
Among snake families traditionally classified as venomous, the capacity seems to have evolved to extremes more than once by parallel evolution; 'non-venomous' snake lineages have either lost the ability to produce venom (but may still have lingering venom ) or actually do produce venom in small quantities (e.g. 'toxic saliva'), likely sufficient to assist in small prey capture, but not normally causing harm to humans if bitten.
The newly discovered diversity of squamate species producing venoms is a treasure trove for those seeking to develop new pharmaceutical drugs; many of these venoms lower blood pressure, for example. Previously known venomous squamates have already provided the basis for such as Ancrod, Captopril, Eptifibatide, Exenatide and Tirofiban.
The world's largest venomous lizard and the largest species of venomous land animal is the Komodo dragon.
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