Fig. 2

According to Christian et al. (1996), strong adductor muscles of the hip are common among all reptiles, while abductors are notoriously weak. Therefore, equilibrium against the moment of GRFs can best be maintained in the hip joint, if the pes is placed lateral to the hip. In all drawings, a-d, the lever arm of the vertical component of GRF at the hip joint GRF is the same. The position of the knee is unimportant (b, c, d). This configuration is secured in Plateosaurus and in several Sauropods by a very narrow pelvis with short distance between the acetabulae (=interacetabular distance; Christian et al., 1996). This keeps the hip reliably medial to the GRF. The pelvis in reptiles and mammals is exposed to contradicting stresses: If weight is supported by two limbs, as in standing, the symphysis is under tension (a, b horizontal arrows), and the sacrum compressed from one side to the other (a, b). If weight is supported by one limb alone, like in walking or trotting, the pubic region is under compressive stress, and the sacrum tensed (c, d). In the wide-hipped Savannasaurus, the width of the pelvis induces a markedly sprawling gait. This leads to particularly long lever arms and high forces in the pelvis which require high strength. (Inspired by Christian et al., 1996)