Digital reduction in Sitana (Reptilia: Agamidae) and the dual roles of the fifth metatarsal in lizards

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Abstract
The agamid lizard S. ponticeriana shows complete loss of the 5th digit of the pes. This is unusual in a normal-limbed lizard and especially so as the region of the 5th digit serves as the focal point of the crural muscles operating to extend the foot. The pattern of digit reduction does not resemble that seen in lizards showing serpentiform tendencies. Sitana is bipedal at high speeds and has well developed, greatly elongated hind limbs. It thus provides an ideal natural experiment for testing recently advanced hypotheses concerning the functional role of the hooked 5th metatarsal and 5th digit of the pes in normal-limbed lizards. While the phalanges of the 5th digit and the shaft of the 5th metatarsal are absent, the proximal end of the 5th metatarsal remains, with a similar shape and topographic relationship to that seen in other lizards. The functional morphology of the remainder of the pes is only very slightly affected by the loss of the 5th digit, indicating that it is functionally independent of the rest of the pes. The role of the proximal part of the 5th metatarsal as a heel bone is strongly supported by the available evidence.