The erect posture of dinosaurs is often said to be the key to their success. Why is this? An important reason is that an erect posture is mechanically more satisfactory than a sprawling one. The weight of the body is supported entirely from below. In a sprawler, the weight of the body is supported from the sides. While gravity effectively pulls straight down from the center of the body mass, in a sprawler this force has to be converted into a sideways component along the femur or humerus (upper arm bone), and then a vertical component down the tibia and fibula, and the radius and ulna (forearm bones), which causes great stresses are avoided if the gravitational force of the animal’s mass is transferred down through a straight, erect limb.
This mechanical advantage is important. Firstly, erect animals can run in a more sustained way: not necessarily faster, but with more stamina, because the effort of supporting the body weight is much less than in a sprawler. This would have been an immediate advantage to an archosaur chasing sprawling prey animals or escaping from a sprawling carnivore. Interestingly, the main plant-eating groups of the Middle Triassic, the pig-like rhynchosaurs and dicynodonts, were evolving semi-erect gaits at the same time. Indeed, the ancestors of the mammals, the cynodonts – which were moderate-sized carnivores at that stage – also showed similar advances.
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