Temporal range: Cretaceous – Recent 99–0Ma
|Blanus cinereus, Spain|
|black: range of Amphisbaenia|
Amphisbaenia (called amphisbaenians or worm lizards) is a group of usually legless squamates, comprising over 180 extant species. Amphisbaenians are characterized by long bodies, reduction or loss of the limbs, and rudimentary eyes. As many species possess a pink body coloration and scales arranged in rings, they have a superficial resemblance to earthworms. All are limbless except for Bipes, which retains forelimbs. Although superficially similar to the snakes and Dibamidae, recent phylogenetic studies suggest that they are most closely related to the Lacertidae. Amphisbaenians are widely distributed, occurring in North America, Europe, Africa, South America, and the Caribbean. Most species are less than 6 inches (150 mm) long. Little is known of them outside of their anatomy, and even that is difficult to study due to the mechanics of dissecting such small animals.
Despite a superficial resemblance to some primitive snakes, amphisbaenians have many unique features that distinguish them from other reptiles. Internally, their right lung is reduced in size to fit their narrow bodies, whereas in snakes, it is always the left lung. Their skeletal structure and skin are also different from those of other squamates. Both genetic and recent fossil evidence indicate that amphisbaenians lost their legs independently from snakes.
The head is stout, not set off from the neck, and either rounded, sloped, or sloped with a ridge down the middle. Most of the skull is solid bone, with a distinctive single median tooth in the upper jaw. It has no outer ears, and the eyes are deeply recessed and covered with skin and scales. These rudimentary eyes have a cornea, lens, and complex ciliary body which allows them to detect light but they are reduced in size and do not have an anterior chamber. The body is elongated, and the tail truncates in a manner that vaguely resembles the head. Their name is derived from Amphisbaena, a mythical serpent with a head at each end. The four species of Bipes are unusual in having a pair of forelimbs, but all limbless species have some remnants of the pelvic and pectoral girdles embedded within the body musculature.
The skin of amphisbaenians is only loosely attached to the body, and they move using an accordion-like motion, in which the skin moves and the body seemingly just drags along behind it. Uniquely, they are also able to perform this motion in reverse just as effectively.
Amphisbaenians are carnivorous, able to tear chunks out of larger prey with their powerful, interlocking teeth. Like lizards, some species are able to shed their tails (autotomy). Most species lay eggs, although at least some are known to be viviparous.
The white worm lizard (Amphisbaena alba) is often found in association with leafcutter ants. This reptile is thought to forage in the ants' deep galleries where the insects deposit their waste. The larvae of certain large beetles live in these galleries and it is these on which the reptile preys.
Taxonomic classification of amphisbaenians was traditionally based on morphological characters such as the number of preanal pores, body annuli, and tail annuli. Such characters are vulnerable to convergent evolution; in particular, loss of the forelimbs and the evolution of specialized shovel-headed and keel-headed morphs appear to have occurred multiple times in the history of the group. Classifications based on mitochondrial DNA sequences and nuclear DNA sequences better reflect their true evolutionary history, and are now being used to distinguish genera of amphisbaenians.
The most ancient branch of the tree is the Rhineuridae. The remaining five families form a group to the exclusion of rhineurids. Bipedidae, Blanidae and Cadeidae represent the most ancient divergences within this grouping, with the Trogonophidae and Amphisbaenidae diverging more recently.
The Amphisbaenia has usually been considered a suborder of squamates. However, more recent studies indicate that it is part of the Lacertoidea clade of lizards, ranked only as a superfamily, so it is now commonly described as an unranked clade.
Five families of amphisbaenians are currently recognised:
- Amphisbaenidae Gray, 1865 – Amphisbaenids, tropical worm lizards (17 genera)
- Bipedidae Taylor, 1951 – Ajolotes It should not be confused with the axolotl (1 genus)
- Rhineuridae Vanzolini, 1951 – North American worm lizards (1 genus)
- Trogonophidae Gray, 1865 – Palearctic worm lizards (4 genera)
- Blanidae Kearney & Stuart, 2004 - Anatolian, Iberian, and Moroccan worm lizards (1 genus)
- Cadeidae Cuban keel-headed worm lizards. Represented by only two species. Traditionally assigned to Amphisbaenia, but studies of DNA suggest that they represent a more primitive lineage, whose affinities are poorly known.
- Wu X.-C., D. B. Brinkman, A. P. Russell, Z.-M. Dong, P. J. Currie, L.-H. Hou, & G.-H. Cui (1993). "Oldest known amphisbaenian from the Upper Cretaceous of Chinese Inner Mongolia." Nature 366: 57–59.
- Gans, Carl (1998). Cogger, H.G. & Zweifel, R.G., ed. Encyclopedia of Reptiles and Amphibians. San Diego: Academic Press. pp. 212–215. ISBN 0-12-178560-2.
- Muller, J et al. (2011). "Eocene lizard from Germany reveals amphisbaenian origins." Nature 473: 364–367.
- Foureaux, G., Egami, M.I., Jared, C., Antoniazzi, M.M., Gutierre, R.C., Smith, R.L., 2010. Rudimentary eyes of squamate fossorial reptiles (amphisbaenia and serpentes). John Wiley & Sons, Hoboken, NJ. 293(2): 351-7.
- Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
- Kearney, Maureen, and Bryan L. Stuart. "Repeated evolution of limblessness and digging heads in worm lizards revealed by DNA from old bones." PROCEEDINGS-ROYAL SOCIETY OF LONDON B 271 (2004): 1677-1684.
- Mott, T., Vieites, D.R., 2009. Molecular phylogenetics reveals extreme morphological homoplasy in Brazilian worm lizards challenging current taxonomy. Molecular Phylogenetics & Evolution. 51(2): 190-200.
- Vanzolini, P.E., 2002. An aid to the identification of the South American species of Amphisbaena (Squamata, Amphisbaenidae). Pap. Avulsos Zool, São Paulo, 42(15): 351-362.
- Vidal, Nicolas, et al. "Origin of tropical American burrowing reptiles by transatlantic rafting." Biology Letters 4.1 (2008): 115-118.
- "Amphisbaenidae". Integrated Taxonomic Information System. Retrieved 19 August 2007.
- "Bipedidae". Integrated Taxonomic Information System. Retrieved 19 August 2007.
- "Rhineuridae". Integrated Taxonomic Information System. Retrieved 19 August 2007.
- "Trogonophidae". Integrated Taxonomic Information System. Retrieved 19 August 2007.
- "Blanidae". Dahms Tierleben. www.dahmstierleben.de.
- Branch, Bill. 2004. Field Guide to Snakes and Other Reptiles of Southern Africa. Third Revised edition, Second Impression. Sanibel Island, Florida: Ralph Curtis Books. 399 pp. ISBN 0-88359-042-5. (Suborder Amphisbaenia, pp. 201–202.)
- Gans, C. 2005. Checklist and Bibliography of the Amphisbaenia of the World. Bull. American Mus. Nat. Hist. (289): 1-130.
- Goin, C.J.; O.B. Goin; G.R. Zug. 1978. Introduction to Herpetology, Third Edition. San Francisco: W.H. Freeman and Company. xi + 378 pp. ISBN 0-7167-0020-4. (Suborder Amphisbaenia, pp. 276–278.)
- Gray, J.E. 1844. Catalogue of the Tortoises, Crocodiles, and Amphisbænians, in the Collection of the British Museum. London: Trustees of the British Museum. (E. Newman, printer.) viii + 80 pp. (Order "Amphisbænia", p. 68.)