Most of the synapomorphies of the leg that appear in Ornithosuchus, advance in Lagosuchus, and come to full development in the dinosaurs are concerned with the acquisition of an erect gait – or the fully upright posture. It is important to note that erect or upright gait does not necessarily mean bipedal. Cows and horses have the erect gait and posture, just as much as humans do.
The first archosaurs were sprawlers, like modern lizards and salamanders. The limbs stuck out sideways from the body, and the elbows and knees form right angles at all times as the animal walks. Even at speed, a lizard generally swings its limbs far out to the side of its body, and it is assumed that the Early Triassic archosaurs moved in a similar way. During the Middle Triassic, most archosaurs adopted a semi-erect posture in which the body could be lifted clear of the ground, with the arms and legs tucked partly underneath for rapid locomotion. Finally, in the Middle and Late Triassic, the two archosaur lineages noted above – the crocodilian and dinosaur lines – adopted an erect posture in which the limbs were tucked underneath the body at all times. This seems to have happened independently in each line.
The aetosaurs, rauisuchians and early crocodilians evolved an erect posture in which the acetabula shifted beneath the hip bones and the heads of the femurs fitted straight up into them, like straight columns beneath a building. The members of the dinosaur line used the approach seen in mammals, in which the acetabula remain on the side of the hip bones but the femurs develop right-angled heads that fit in from the sides. In this design the relationship of hip girdle and leg is more like a buttress on the side of a church building, rather than a column beneath its roof, but the result is the same. The legs of dinosaurs, and of mammals, come together in a slightly knock-kneed fashion beneath the body, and this is a crucial feature.
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