Rhineura is a member of Amphisbaenia, a lineage of mostly limbless burrowing lizards known from the Paleocene onward. Amphisbaenians share a highly modified skull architecture, a unique modification of the ear called the extracolumella, and skin with annuli (rings, like a worm). There are four major amphisbaenian groups: Bipedidae (the only lineage to have forelimbs); Amphisbaenidae (the most diverse and widespread lineage); Trogonophidae (a lineage that uses an oscillating excavation pattern); and Rhineuridae (to which Rhineura belongs). Rhineuridae includes a number of fossil forms (restricted to western North America), but only one living species - Rhineura floridana - which occurs in central Florida and Georgia.

Amphisbaenians, like other lizards (including snakes), have paired evertible hemipenes, a transverse cloacal slit, and shed their skin in its entirety. It is difficult to discern exactly where amphisbaenians fit in the lizard family tree. Even the earliest-known fossil representatives already exhibit the highly derived cranial morphology of living amphisbaenians. Their skull is characterized by remarkable adaptations for a burrowing lifestyle, in that it is solidly built, with complex interdigitation of the sutures between bones. These features, readily discernible in these CT-generated renderings of a fossil Rhineura from the Oligocene White River Formation, are not found in other lizards. Final resolution of the exact position of amphisbaenians among lizards will require the discovery of older fossils which are less derived.

Little is known of the lifestyle of living amphisbaenians because of their burrowing habit. It is thought that most lay eggs and are insectivorous. Males and females cannot be differentiated from external appearance. Their modern distribution includes northern and sub-saharan Africa, southwest Asia, the eastern and western Mediterranean, South America east of the Andes, the West Indies, western Mexico, Baja California, the southeasternmost United States, and Cuba.

References:

Estes, R., de Queiroz, K., and J. Gauthier. 1988. Phylogenetic relationships within Squamata, pp.119-281. In R. Estes and G. Pregill (eds.), Phylogenetic Relationships of the Lizard Families. Stanford University Press, Stanford.

Gans, C. 1978. The characteristics and affinities of the Amphisbaenia. Transactions of the Zoological Society of London 34:347-416.

Gans, C. 1990. Patterns in amphisbaenian biogeography: a preliminary analysis, pp. 133-143. In G. Peters and R. Hutterer (eds.), Vertebrates in the Tropics. Museum Alexander Koenig, Bonn.

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