When reptiles first evolved from amphibians (Sect. 1.3), they inherited the short, stumpy legs of their ancestors. Modern newts and salamanders (Uro-dela) as well as modern reptiles such as lizards with elongated bodies and small laterally projecting legs show the most primitive form of tetrapod locomotion. If they are frightened and need to move exceptionally fast, urodeles and lizards usually wriggle along with the belly resting on the substrate, as though they were swimming, with their legs scarcely touching the ground. This eel-like movement is similar to the serpentine locomotion of a snake (Sect. 4.2, Fig. 16). When moving deliberately, however, both amphibians and quadrupedal reptiles carry their weight clear of the ground. The movement of one forelimb is followed by that of the hindlimb on the opposite side of the body. The forelimb of that side is then moved forward, followed by the rear leg on the opposite side. When these movements are accelerated, however, and the animals begin to trot, the limbs opposite each other tend to be moved simultaneously and, during most of each strike, only two feet are on the ground at the same time (Gray 1968, summarised in Cloudsley-Thompson 1999).
Correlated with the basic similarity of movement between small, elongated reptiles and urodeles, there is a general similarity in the plans of their skeletons. Nevertheless, certain advanced features are characteristic of all reptiles. The head is usually carried above the level of the ground, and the neck is well developed. The first two cervical vertebrae are modified to form the atlas and axis bones, and all the vertebrae articulate with one another by a system of interlocking processes much more elaborate than anything found in the Amphibia.
The vertebral column provides a stiff but flexible support for the body, and the legs are connected to it by the pectoral and pelvic limb girdles. In the case of amphibians and most reptiles, excluding dinosaurs (Sect. 7.3), the limbs are held out from the sides of the body and the animal adopts a sprawling posture. To maintain this position involves the expenditure of a considerable amount of energy, especially in the case of the heavier reptiles such as large turtles and crocodilians - which therefore spend most of the time with their bodies resting on the ground. Legs do more than merely support the body, however: they also provide locomotion. These two functions do not operate synergistically. Weight bearing is provided for most efficiently by adopting an erect posture with the legs articulated below the limb girdles. Speedy movement, on the other hand, is better achieved by having the limbs inclined outwards, as in lizards and other small reptiles. In practice, a compromise between these two objectives is reached.
Locomotor performance in an erect posture is improved when the leg segments are inclined towards one another. Thus, the humerus slopes forwards, the radius and ulna backwards; while the femur slopes forwards and the tibia and fibula slope backwards. The inclination of the limb segments is maintained by tension in the muscles and ligaments. This provides for a natural springing action that allows for the storage and release of strain energy. Speed is increased by having the limb segments move relatively to each other, because the speed of the entire leg represents the sum of speeds of its component parts (McGowan 1991). Quadrupedality has been independently derived from bipedal ancestors in the sauropods (Saurischia) and ankylosaurs, stegosaurs, ceratopsians, hadrosaurs, and iguanodonts (Ornithischia).
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