Amphisbaenia - worm lizards

The Amphisbaenia are a poorly known group of limbless burrowing squamates, called amphisbaenians, or worm lizards. While the monophyly of Amphisbaenia is well supported, their affinities to other squamate groups remain unresolved. For more information regarding the position of the Amphisbaenia in squamate phylogeny, please refer to the Squamata main page.

Diversity and Lower Taxonomy:

There are approximately 169 species of amphisbaenian, contained within 24 well-established genera. The position of these genera at the family-level, however, has been the subject of scientific debate.

In 2003, Kearney performed a comprehensive morphological cladistic analysis of the Amphisbaenia, and proposed the following five families (including one new family, Blanidae, previously placed in Amphisbaenidae):

• Bipedidae - conaining three species in the single genus Bipes. These are the only amphisbaenians to have forelimbs.
• Blanidae - containing four species in the genus Blanus.
• Amphisbaenidae - containing 153 species divided between 13 genera. The most speciose genus is Amphisbaena (69 species).
• Trogonophidae - containing six species in four genera.
• Rhineuridae - containing the monospecific Rhineura floridana.

The family status of the four remaining genera - Aulura, Dalophia, Leposternon, and Monopeltis - was not resolved, but they were placed in the superfamily Rhineuroidea with the family Rhineuridae. These are united by the possession of a shovel-like skull shape, amongst other characteristics (see below).

See below for the synapomorphies of these clades.

Since Kearney's (2003) phylogeny, molecular studies have demonstrated a different evolutionary history for the Amphisbaenia. In 2004, Macey et al. analysed the full mitochodrial genomes of 12 amphisbaenian species in four families (Blanidae was not recognised in this study, and no Blanusspecies were used) - a total of 5797 parsimony informative sites. They found that the family Rhineuridae was the first to split from other extant amphisbaenians, followed by the family Bipedidae - a reversal of the relationships proposed by Kearney (2003). Both, however, agreed on the nested position and monophyly of the Amphisbaenoidea - the clade formed of the sister families Amphisbaenidae and Trogonophidae. The cladogram below illustrates Macey et al.'s results:

An even more recent study (Vidal et al. 2007) proposed, using molecular and biogeographic evidence, that the Cuban genus Cadea should be removed from the family Amphisbaenidae, and placed in its own new family, Cadeidae, as sister group to Blanidae. The Blanidae-Cadeidae clade was sister to the Bipedidae, together forming the sister group to the Amphisbaenoidea. Like Macey et al. (2004), Rhineuridae was the first family to diverge.

Description

Amphisbaenians are highly specialised for a fossorial (burrowing) mode of life. Almost all are completely limbless, except the three species of the Mexican genus Bipes, which have small, well-developed forelimbs. All are elongate, ranging in size from around 10 to 70 cm, with a long trunk region and short tail (the musculature required for burrowing is present in the trunk).

They have an extremely distinctive skin, consisting of conspicuous rings of scales, called annuli, that encircle the trunk. Only loosely connected to the main trunk, this specialised integument forms a tube in which the animal can move forwards and backwards. This is key to the ability of amphisbaenians to burrow. An amphisbaenian can use longitudinal muscular contractions between each annulus to bunch up the skin in order to anchor a part of the body in the soil. Trunk muscles can then be used to move the body forward within the integumentary tube. The front can then be anchored whilst the posterior integument is brought forward, after which the cycle can restart, allowing the amphisbaenian to advance.

Amphisbaenians are headfirst burrowers, and so the skull is highly modified for digging. While all have a rigid, compact skull, there are three main amphisbaenian skull types, which are functionally related to the way in which they burrow:

• "Shovel-headed" (e.g. Rhineura and Leposternon) - dorsoventrally flattened snout, with a sharp craniofacial angle. Amphisbaenians with this skull form dig by forcing the head forwards and slightly downwards, and then lifting the head dorsally to pack the soil onto the top of the tunnel. The sides of the tunnel are smoothed with the pectoral musculature.
• "Spade-headed" (trogonophids) - again a dorsoventrally flattened snout, with a strong craniofacial angle. The burrowing method, is however, quite distinct. Trogonophids use the sharp sides of their head (called lateralcanthi) to shave off soil from the front of the tunnel in an oscillatory motion. Soil is pushed and packed using the sides of the head and the body.
• "Keel-headed" (e.g. Anops and Mesobaenia) - laterally compressed head. These amphisbaenians dig by ramming the head forwards, and then push and pack soil rearwards by forcing the head alternately left and right.
• "Round-headed" or "bullet-headed" (the majority of amphisbaenians, e.g. Amphisbaenia, Blanus, Cadea,Zygaspis) - dig by using the head as a simple battering ram, followed by pushing the head in different directions to pack soil.

While the majority of amphisbaenians have a similar appearance, close to that described above, some are more distinct, and warrant further description. As already noted, members of the family Bipedidae (genus Bipes) have functional forelimbs. These are used for the initial surface excavation of a tunnel, and the first digit possesses extra phalanges to increase the efficiency of this digging process.

The trogonophids (family Trogonophidae) also have some features that make them distinct. First, they have a triangular cross-section, as opposed to cylindrical. Second, they have an acrodont dentition, in contrast to the pleurodont dentition of other amphisbaenians. Third, they do not exhibit caudal autotomy (the ability to self-amputate the tail). Finally, they are "spade-headed" and exhibit a unique burrowing behaviour (see above).


Distribution and Habitat:

While the majority of amphisbaenians are located in Africa and South America (members of Amphisbaenidae, Blanidae, and Trogonophidae), others are present in Florida (Rhineura floridana), Cuba (Cadea spp.), the Carribean (members of Amphisbaenidae), Mexico (Bipedidae), the Middle East (members of Trogonophidae), and the Mediterranean region (members of Blanidae).

All amphisbaenians burrow in soil or dry, loose sand.


Conservation Status (IUCN):

Only nine species of amphisbaenian are present on the IUCN Red List. All of these are listed as Least Concern (LC), and have been assessed since at least 2006.


Synapomorphies of the Amphisbaenia and its families (sensu Kearney 2003):

o Amphisbaenia:

o High degree of cranial consolidation.

o Brain completely surrounded by frontal bone (anteriorly) and enlarged, azygous orbitosphenoid plate (ventrally).

o Presence of annuli - ring-like arrangement of scales.

o Enlarged median tooth in the premaxillary bone of the upper jaw. This tooth fits into a groove between the two lower teeth, forming an efficient 'nipper' that can take a chunk of flesh from its prey.

o Modified extracolumellar system of the ear, sensitive to low frequency sounds - improving subterranean hearing.

o Absence of the epipterygoid.

o Right lung reduced in size or lost (in contrast to other limbless squamates, which show reduction of the left lung).

o Family Bipedia only:

o Presence of (functional) forelimbs, with anteriorly shifted pectoral girdle.

o Fused fronto-parietal complex.

o Digit I exhibits polyphalangy (the presence of extra phalanges).

o Family Blanidae only:

o Reduced clavicles.

o Anteriorly truncated nasals.

o Family Amphisbaenidae only:

o Absence of squamosal.

o Absence of the sternum.

o Family Trogonophidae only:

o Acrodont dentition.

o Absence of caudal autotomy - no fracture planes to self-amputate.

o Strong craniofacial angle.

o Enlarged sternal plate.

o Short tail.

o Family Rhineuridae only:

o Anterior edge of the snout is squared-off.

o External naris opens ventrally.

o Pterygoid vomer contact.

o Absence of posterodorsal rib processes.