Toxicofera

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Toxicofera (Latin for "those who bear toxins"), is a hypothetical clade which represents about 4600 species (nearly 60%) of Squamates; it encompasses all venomous reptile species, as well as numerous related non-venomous species.

Cladistics

Toxicofera would combine the following groups from traditional classification:^[1]

• suborder Serpentes (snakes)
• suborder Iguania (iguanas, agamid lizards, chameleons, etc.)
• infraorder Anguimorpha, consisting of:
  • family Varanidae (monitor lizards)
  • family Anguidae (alligator lizards, glass lizards, etc.)
  • family Helodermatidae (Gila monster and Mexican beaded lizard)

Research

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%). Following the classification of Helodermatidae in the 19th century, their venom was thought to have developed independently. The origin of venom in Squamates was thus considered relatively recent in evolutionary terms and convergent among the various venomous snake families.

In 2003 however, a study was published that described venom in subfamilies previously thought to lack it.^[2] Further study claimed nearly all "non-venomous" snakes produce venom to a certain extent, suggesting a far more ancient origin for venom in Serpentes than had been considered until then.^[3][4] As a practical matter, Dr. Bryan Fry cautioned:^[5]

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)^[3]], 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.

Still further research 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).^[1] 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.^[1] The venom clade included Anguidae for phylogenetic reasons and adopted the previously suggested clade name Toxicofera.^[6]

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, approximately 100 million years before snakes evolved.^[1] The venoms are thought to have resulted after genes normally active in various parts of the body duplicated and the copies found new use in the salivary glands.^[2]

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.^[1]

References