Physical characteristics

Certain features of trogonophids are common to most or all amphisbaenians. These include: a unique modification of the middle ear in which an elongated structure, the extracol-umella, attaches to the stapedial bone of the middle ear ex tending forward to attach to tissue along the sides of the face and allowing the reception and transmission of vibrations to the inner ear; reduction or absence of the right lung; an enlarged, medial, premaxillary tooth; the periodic shedding of the skin in a single piece; a heavily ossified and robust skull; absence of eyelids and external ear openings; and a forked tongue.

Trogonophids are all limbless but retain both pectoral and pelvic girdle vestiges. The head is spade-shaped, with sharp lateral edges on the spade. The body shape of trogonophids is unique among amphisbaenians, being higher than wide, and resembling an upside down "U" in cross-section rather than the more common circular shape. The ventral surface of the body appears excavated, or concave. This shape is partially due to the elongation of the ribs, the ends of which dig into the ground, helping to balance the animal against the forces created during the oscillating motion that trogonophids use in burrowing. The dentition of trogonophids is acrodont, another unique feature among amphisbaenians. The tail is extremely short, pointed, and downward curving, and may be keeled in some species. Caudal autotomy is always absent in

A spade-headed (or shorthead) wormlizard (Trogonophis wiegmanni). (Photo by Axel Kwet. Reproduced by permission.)

these species. Striking pigmentation with spotting and checkerboard patterns occurs in some species. Beyond those easily recognizable features, trogonophids are also characterized by a number of unique internal conditions, including an enlarged, pectoral sternal plate and a greatly enlarged premax-illa in the facial portion of the skull.

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