Bipedal Locomotion

The earliest dinosaurs did not exceed 2 m in length and were lightly built. They were bipedal, as already stated, and their hind legs were considerably longer than their forelimbs - as were those of ancestral crocodilians (Sect. 4.6.3). In dinosaurs, a bipedal stance and parasagittal gait were both primitive and obligatory. (Parasagittal means parallel to an animal's midline.) Dinosaurs would certainly not have been able to sprawl (Padian 1997; Parrish 1997). Although the earlier dinosaurs (Fig. 65) - like most of their archosaurian predecessors and contemporaries - were bipedal and had erect stances, the tibia and

Bipedal Reptiles
■ Fig. 65. Plateosaurus (Sauropodomorpha; Upper Triassic; length ca. 7 m)

fibula were usually longer in dinosaurs than was the femur. In contrast, the femur was longer than the tibia and fibula in most of the non-dinosaurian archosaurs. Dinosaurs also held the metatarsus off the ground in a digitigrade stance, whereas the other archosaurs retained the primitive plantigrade condition in which the entire foot was placed on the ground during locomotion (Parrish 1997).

Plateosaurus, the genus illustrated in Fig. 65, is the best known of the Sauropodomorpha or prosauropods (Sect. 10.3) because numerous well-preserved fossils have been unearthed in Triassic sandstones throughout Europe. In some places, groups of complete individuals have been found together, suggesting that the animals may well have been gregarious and travelled together in herds throughout the desert landscape, searching for new feeding grounds. Alternatively, of course, the bodies of animals that had died previously might have been washed together in periodic flash floods or have been piled up at the ends of wadis. Plateosaurus was a large reptile. Its tail, towards which the intestines had extended (see below) was nearly as long as its body.

The tail would have had a major effect on locomotion in bipedal dinosaurs, especially when they were running. The trunk was erect and, when one foot was set down in front of the centre of gravity, it would not have been lifted until it was behind the centre of gravity, as in the case of human beings. A stiff tail, useful also as a weapon (Sect. 9.2.2) would have been better for running than would a springy tail. Had bipedal dinosaurs jumped, as kangaroos do, springy tails would doubtless have evolved, as they have in kangaroos, to reduce the rocking movements of the trunk (Alexander 1989).

A bipedal stance is more favourable for carnivores than for herbivorous animals. This is due to the fact that the digestion of vegetable matter necessitates relatively larger intestines which have to be accommodated in larger, heavier bodies in front of the hips. These, in turn, cause problems of balance. Selection was consequently pulling in two opposite directions. Shorter torsos provided better balance for running, while longer bodies were needed for digesting leaves (Bakker 1987). The way in which the dilemma was solved evolutionarily was by bending the pubic bones backward from their attachment to the other hip hones so that the intestines could lengthen, while the hip joint remained as it had been. The pubic bones now slanted downward and backward instead of directly down, and the intestinal canal could be prolonged from the belly to the base of the tail (Galton 1970a). Bipedal balance was thereby actually improved by the increased weight of the body.

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