Fig. 3

Schematic cross section through the shoulder region of a Titanosaur is sketched for two-limb support (a) and one-limb support (b). In the former, the sternal region is under tension. In the latter, the sternal area is compressed from both sides. The ways of loading are the same as those in the pelvis, shown in Fig. 2. The pectoral muscle must be active in one-limb support as well as in two-limb support. This in particular, if load arms (i.e. horizontal distance between manus and shoulder joint) are long. Contraction of the (strong) pectoral muscle evokes compressive stresses in the coracoid (which is traversed, Fig. 3 c) as well as in the sternal plates (from where the muscle takes its origin). In addition to the activity of the M. pectoralis, the M. serratus, which carries trunk weight, exerts in two-limb standing a dorso-medial torque at the scapula, because of which the (weak) abducting muscles must be activated. The dorso-medially directed torque becomes greater if only one limb supports trunk weight, since M. rhomboideus and M. trapezius are acting to sustain the tensile forces by which the free limb is suspended. Both muscles rotate the scapula medially. Their rotating moments are much smaller than that of body weight in the gleno-humeral joint, but anyway must be compensated by abducting muscles. (Inspired by Fig. 11.9 in Hohn, 2011). c Schematic view from ventral side on the major elements of the shoulder girdle. The size and exact outline of the cartilaginous sternal plate (stippled) are difficult to predict. It probably contained joints, like in modern lizards and crocodiles. In its centre, bony plates (white, heavy outlines) reinforce the cartilage. The humeri are abducted (as in sprawling positions) and three fibres of the M. pectoralis are illustrated as double lines. The system sustains compressive as well as tensile forces such as shown in Fig. 4. (Inspired by fig. 11.10 B in Hohn, 2011)