Amphisbaenia

Amphisbaenians Temporal range: Ma PreꞒ Ꞓ O S D C P T J K Pg N
Blanus cinereus , Spain
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Squamata
Superfamily:	Lacertoidea
Clade:	Amphisbaenia Gray, 1844
Families
Cadeidae Rhineuridae Trogonophidae
black: range of Amphisbaenia

Amphisbaenia

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.^[2] Both genetic and recent fossil evidence indicate that amphisbaenians lost their legs independently from snakes.^[3]

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

Amphisbaenians have a distinctive skin made up of rings of scales (annuli) that form a tube in which the loosely attached trunk of the body moves. Burrowing is achieved with an accordion-like motion, with longitudinal muscles in the skin bunching up the annuli, anchoring it to the surrounding soil, and trunk muscles moving the body forward or backwards within the integumentary tube.^[6]

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 (

The red 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 presence of these reptiles is easily explained by the fact that they prey on the larvae of large beetles that also inhabit the leafcutter ants' galleries.^[7]

Amphisbaenians have often been categorized by their skull shape. The specialized skull shape is hypothesized to be driven by environmental and ecological conditions, such as soil type, and is an instance of convergent evolution.^[8]

Distribution

Amphisbaenians are found in North America, Europe, Africa, South America, the Middle East, and the Caribbean, a surprisingly large distribution despite being small subterranean animals that rarely ever leave their burrows. Initially, this large distribution was thought to be due to

The fully limbed Slavoia darevskii from the Late Cretaceous (Campanian) of Mongolia may represent an early relative of amphisbaenians.^[11] The oldest known modern amphisbaenians are members of Rhineuridae and the extinct family Oligodontosauridae from the Paleocene of North America. Modern amphisbaenians likely originated in North America, before dispersing to South America, Africa and Europe via rafting during the Paleogene.^[12]

Taxonomy

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 Trogonophidae and Amphisbaenidae diverging more recently.[16] South American amphisbaenids apparently are derived from African amphisbaenids that rafted across the Atlantic in the Eocene, about 40 million years ago.^[16] Cuban cadeids may be similarly derived from blanids that rafted across from northwestern Africa or southwestern Europe in a similar time frame.^[16]

Historically considered to be lizards, some studies have suggested that they should be considered separate from lizards,^[17] though many modern studies consider them to be true lizards, as they are closely related to other lizards of the clade Lacertoidea.^[1]

Families

Six families of amphisbaenians are currently recognised:^[18]

• Amphisbaenidae Gray, 1865 – Amphisbaenids, tropical worm lizards of South America, some Caribbean islands, and Sub-Saharan Africa.^[19] (12 genera, 182 species)^[20]
• Bipedidae Taylor, 1951 – Only in Mexico and commonly called ajolotes,^[21] but not to be confused with axolotls (1 genus, 3 species)^[22]
• Blanidae Kearney & Stuart, 2004^[23] – Anatolian, Iberian, and Moroccan worm lizards (1 genus, 7 species)^[24]
• Cadeidae Vidal and Hedges, 2008 – Cuban keel-headed worm lizards (1 genus, 2 species).^[25] Traditionally amphisbaenids, but shown by DNA to be closest to Blanidae.^[16]
• Vanzolini, 1951 – North American worm lizards^[26] (1 genus)^[27]
• Trogonophidae Gray, 1865 – Palearctic worm lizards^[28] (4 genera, 6 species)^[29]

Phylogeny

The following cladogram shows the relationships between the six amphisbaenian families determined in the phylogenetic analysis of mitochondrial and nuclear genes by Vidal et al. (2008).^[16]

References