Fundamental Adaptations in the Lacertilian Hind Limb: A Partial Analysis of the Sprawling Limb Posture and Gait

Lizards lacking extreme locomotor adaptations exhibit a type of limb posture and locomotion termed 'sprawling.' Two major mechanical problems inherent in this type of posture can be identified, one concerning the support of the body between laterally projecting limbs, the other focussing on the efficiency of the limb in producing effective locomotor thrust. Analysis of distinct morphological features of the hind limb, including knee and mesotarsal joints, and the geometry of the pes, reveals an integrated set of adaptations which maximize the effective participation of all limb segments in locomotion. It is suggested that the lacertilian pes represents a key innovation potentiating the exploitation of new locomotor niches. Functional demands stimulating the early evolutionary development of the reptilian pes may have preadapted that of lizards for a new role. It is suggested that the prime potential of the sprawling limb posture is in scansorial or arboreal locomotion.