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Toxicofera
Venomous snakes, such as the rattlesnake shown above, are the most well-known venomous squamates.
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: Sauropsida
Subclass: Diapsida
Infraclass: Lepidosauromorpha
Superorder: Lepidosauria
Order: Squamata
(unranked): Toxicofera
Beaded Lizard


Toxicofera (Greek for "those who bear toxins"), is a hypothetical clade which represents about 4600 species (nearly 60%) of extant squamates (scaled lizards.)[1] It encompasses all venomous reptile species, as well as numerous related non-venomous species.

Contents

Cladistics

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

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 the result of convergent evolution among the seemingly-polyphyletic venomous snake families.

In 2003 however, a study was published that described venom in snake subfamilies previously thought to lack it.[2] 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.[3][4]

This prompted still 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).[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 a previously suggested clade name: Toxicofera.[5]

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.[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]

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 pseudogenes) or actually do produce venom in small quantities (e.g. 'toxic saliva'), likely sufficient to assist in small prey capture, but not normally cause 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.[1] Previously known venomous squamates have already provided the basis for medications such as Ancrod, Captopril, Eptifibatide, Exenatide and Tirofiban.

References

  1. ^ a b c d e f Fry, B. et al. (February 2006). "Early evolution of the venom system in lizards and snakes" (PDF). Nature 439: 584–588. doi:10.1038/nature04328. http://www.nature.com/nature/journal/v439/n7076/abs/nature04328.html. 
  2. ^ a b Fry, B. et al. (July 2003). "Molecular Evolution and Phylogeny of Elapid Snake Venom Three-Finger Toxins" (PDF). Journal of Molecular Evolution 57 (1): 110–129. doi:10.1007/s00239-003-2461-2. 
  3. ^ Fry, B. et al. (October 2003). "Isolation of a Neurotoxin (α-colubritoxin) from a Nonvenomous Colubrid: Evidence for Early Origin of Venom in Snakes" (PDF). Journal of Molecular Evolution 57 (4): 446–452. doi:10.1007/s00239-003-2497-3. 
  4. ^ Fry, B. and Wüster, W. (May 2004). "Assembling an Arsenal: Origin and Evolution of the Snake Venom Proteome Inferred from Phylogenetic Analysis of Toxin Sequences" (PDF). Molecular Biology and Evolution 21 (5): 870–883. doi:10.1093/molbev/msh091. 
  5. ^ Vidal, N. and Hedges, S. (October-November 2005). "The phylogeny of squamate reptiles (lizards, snakes, and amphisbaenians) inferred from nine nuclear protein-coding genes" (PDF). Comptes Rendus Biologies 328 (10-11): 1000–1008. doi:10.1016/j.crvi.2005.10.001. 

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