Certain features of bipedids are common to most or all amphisbaenians. These include: reduction or absence of the right lung; an enlarged, medial, premaxillary tooth; the peri odic shedding of the skin in a single piece; a heavily ossified and robust skull; absence of eyelids; a forked tongue; and the absence of external ear openings.
The family Bipedidae includes three species that attain a body length of 4.5-9.4 in (115-240 mm) and a body width of 0.27-0.39 in (7-10 mm). The most striking feature of these species is the presence of short, robust forelimbs positioned close to the back of the head, a unique condition among amphisbaenians. Bipedids retain all typical elements of the forelimb, including the humerus, radius, ulna, carpals, meta-carpals, and phalanges. In addition, Bipes exhibits a unique condition termed hyperphalangy, which refers to an extra element occurring in the first digit of the hand, one more than is typically found in reptiles. This condition is presumably related to digging functions. The tail of bipedids is very short, as in most amphisbaenians, representing between 10-20% of the entire length of the animal. The eyes are reduced and sometimes covered by a head scale. The head is rounded and blunt. The teeth are conical, slightly recurved, and attached to the jaw in pleurodont fashion. There is no pigment or pattern to the skin, making the external appearance pale pink or flesh-colored. Caudal autotomy occurs at a single autotomy
constriction site, but bipedids do not regenerate tails. Bipedids are also recognized by a number of unique internal anatomical characteristics, which are evident through x rays or dissection. For example, despite the absence of external rear-limbs, bipedids retain elements of the pelvic girdle and a small vestige of the leg bone (the femur) internally. Also, the skull of bipedids is characterized by fusion of the frontal and parietal bones.
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