There have never been enough amphisbaenians on Tet Zoo. In fact, the only time I’ve written about them at any sort of length is in the 2008 (and 2012) April Fool’s article wherein they were convincingly (cough) shown to be the true ancestors of mammals. In reality, amphisbaenians – popularly called worm lizards – are highly specialized, subterranean squamates; that is, part of the same group of diapsid reptiles as lizards and snakes. The 140 or so living amphisbaenian species inhabit the tropical Americas, western Asia, Spain, Portugal and parts of Africa. Most (but not all) are limbless; they have reduced eyes and ears, a distinctly segmented body, and often a keeled or spade-like snout. The word amphisbaenian is pronounced something like ‘am-fis-bay-nee-an’.

I think I recall saying in one of my pygopodid articles (see links below) that among the earliest of reptile images to inspire a far younger Darren Naish were the remarkable and sometimes exceptional paintings produced by Alan Male for Philip Whitfield’s 1983 book Reptiles and Amphibians, one of my favourite childhood books (Whitfield 1983). Amphisbaenians get just a single page of illustrations therein, but among the six species illustrated we see... this (below): Male’s depiction of a Somali edge snout or Angled worm lizard Agamodon anguliceps. What the hell? If you’re interested in weird, obscure animals, this is an arresting image.

Agamodon is a trogonophid, or short-headed amphisbaenian, and it really isn’t easy to find good images of it, or indeed information on it. Indeed, there aren’t that many good places to go on amphisbaenians in general: they tend to get brief coverage in herpetology books, and you need to go to more technical sources if you want to know more. I recommend in particular the several reviews produced by the great Carl Gans (Gans 1969, 1974, 2000, 2005).

Six ‘family-level’ amphisbaenian groups are presently recognized, and a very quick run-through of these groups will serve as a crash course in amphisbaenian diversity. Rhineurids, represented by Rhineura floridana from Florida, possess depressed, spade-shaped snouts and are the sister-group to remaining amphisbaenians in most studies (Conrad 2008, Vidal et al. 2008, Gauthier et al. 2012, Pyron et al. 2013). Several fossil rhineurids are known, the oldest of which (Plesiorhineura) is from the Paleocene. Bipedids – the one group that possesses limbs (and are exclusive to Mexico) – might be very closely related to blanids (Conrad 2008, Vidal et al. 2008), a group currently unique to the fringes of the Mediterranean but with fossil representatives known from the Eocene of England, France and Belgium. Cadea blanoides, a morphologically unexciting amphisbaenian from Cuba, has recently been identified as the sister-taxon to Blanus, raising interesting questions about amphisbaenian biogeography (Vidal et al. 2008). Cadea has been given its own 'family': Cadeidae.

Amphisbaenids (note: the terms ‘amphisbaenid’ and ‘amphisbaenian’ are not interchangeable) are present in both Africa and South America and include the majority of tropical amphisbaenians while trogonophids are the African and Middle Eastern short-headed amphisbaenians, all of which have formidable teeth fused to the edges of their jaws. Gauthier et al. (2012) named the amphisbaenid + trogonophid clade Afrobaenia. For a substantial review of amphisbaenian phylogeny and diversity see Kearney (2003).

The biology, behavior and functional morphology of amphisbaenians are areas of special interest and much has been written about them. All species are predatory, preying on arthropods, worms and small vertebrates. They catch animals in tunnels, often immobilising them by holding them against the side of the tunnel with a curve of the body. Some amphisbaenians, such as the large South American Amphisbaena species, take surprisingly large prey and regularly kill other reptiles as well as rodents. When hunting surface-dwelling prey, an amphisbaenian will lunge forward with its mouth open, grasping the prey with a powerful bite before dragging it underground. Alternatively, it might tear a chunk from the prey animal's body, sometimes by powerfully twisting after initially grabbing hold. Amphisbaenians return to disabled or deceased prey and it has even been suggested that they may scavenge from dead bodies lying on the surface.

Exactly how amphisbaenians relate to other squamates has long been controversial. Several radically different hypotheses have been published, and so much could be said about this subject that I’m deliberately going to avoid it for now.

If you were hoping for a big and comprehensive review of amphisbaenian evolution, diversity and biology – sorry, this isn’t it. But I’ll come back to them in time, and at least Tet Zoo now has something more than parody about them.

Tet Zoo now features some fairly reasonable coverage of squamate diversity... but there is still so much to do...

Dibamids and amphisbaenians

Gekkotans

Lacertoids

Iguanians

Scincomorphs and cordyliforms

Anguimorphs

Snakes

Refs - -

Conrad, J. L. 2008. Phylogeny and systematics of Squamata (Reptilia) based on morphology. Bulletin of the American Museum of Natural History 310, 1-182.

Gans, C. 1969. Amphisbaenians – reptiles specialized for a burrowing existence. Endeavour, 28, 146-151.

- . 1974. Biomechanics: An Approach to Vertebrate Biology. Lippincott (Philadelphia).

- . 2000. Amphisbaenians. In Cogger, H. G., Gould, E., Forshaw, J., McKay, G. & Zweifel, R. G. (consultant eds) Encyclopedia of Animals: Mammals, Birds, Reptiles, Amphibians. Fog City Press (San Francisco), pp. 650-655.

- . 2005. Checklist and bibliography of the Amphisbaenia of the world. Bulletin of the American Museum of Natural History 289, 1-130.

Gauthier, J. A., Kearney, M., Maisano, J. A., Rieppel, O. & Behlke, A. D. B. 2012. Assembling the squamate tree of life: perspectives from the phenotype and the fossil record. Bulletin of the Peabody Museum of Natural History 53, 3-308.

Kearney, M. 2003. Systematics of the Amphisbaenia (Lepidosauria: Squamata) based on morphological evidence from Recent and fossil forms. Herpetological Monographs 17, 1-74.

Pyron, R. A., Burbrink, F. T. & Wiens, J. J. 2013. A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes. BMC Evolutionary Biology 2013, 13:93 doi:10.1186/1471-2148-13-93

Vidal, N., Azvolinsky, A., Cruaud, C. & Hedges, S. B. 2008. Origin of tropical American burrowing reptiles by transatlantic rafting. Biology Letters 4, 115-118.

Whitfield, P. 1983. Reptiles and Amphibians: an Authoritative and Illustrated Guide. Longman, Harlow (UK).