Toxicofera

Toxicofera combines the following groups from traditional classification: }} 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 questioned 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. == Venom ==

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. 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 radiatus (formerly known as Elaphe radiata'') This taxonomic rational, known currently as the "Toxicofera hypothesis," estimated that the common ancestral species that first developed venom in the venom clade lived on the order of 200 million years ago. Criticism Other scientists such as Washington State University biologist Kenneth V. Kardong and toxicologists Scott A. Weinstein and Tamara L. Smith, have stated that the allegation of venom glands found in many of these animals "has had the effect of underestimating the variety of complex roles played by oral secretions in the biology of reptiles, produced a very narrow view of oral secretions and resulted in misinterpretation of reptilian evolution". According to these scientists "reptilian oral secretions contribute to many biological roles other than to quickly dispatch prey". These researchers concluded that, "Calling all in this clade venomous implies an overall potential danger that does not exist, misleads in the assessment of medical risks, and confuses the biological assessment of squamate biochemical systems". More recent research has found that the majority of genes used to support the establishment of the Toxicofera clade were not uniquely expressed in venom and venom structures, but rather in multiple body structures and are likely to more closely reflect maintenance genes. This evidence, pointing to misinterpretation of phylogenetic trees and incomplete tissue sampling in formation of the Toxicofera hypothesis, places doubt on the assumption that the common ancestor of the clade was indeed venomous, and suggests instead that venom has evolved multiple times in reptiles. == References ==