These animals have a primary shoulder girdle consisting of a scapula and precoracoid, the glenoid cavity has lost all trace of the screw shape of primitive reptiles and permits considerable freedom of motion. In the larger and more advanced lizards the anterior part of the scapula and coracoid is much enlarged, and these bones are perforated by fenestrae.
The clavicles have an expanded, and sometimes fenestrated, lower end, and the interclavicle is usually cross-shaped. There is a large sternum, which is usually calcified although not ossified, with whose antero-lateral borders the precoracoids articulate.
The pelvis of these reptiles is of very characteristic pattern, the ilia are narrow rods with an expanded lower end which con tributes to the acetabulum. It slopes downward and forward and is firmly held by its articulation with the two sacral ribs. The pubis and ischium are separated by a large obturator foramen which, in many cases in the bony skeleton, is confluent with that of the opposite side. The hind limb presents few features of in terest, but it may be noted that a patella is sometimes present, and that most of the motion at the ankle-joint takes place between the two rows of tarsals and not, as in mammal-like reptiles, between the tarsus and lower leg.
One universal and unexplained feature of the hind foot of these reptiles is that the fifth distal tarsal is absent, and that the upper end of the fifth metatarsal has moved up into contact with the calcaneum, and has become much widened so that the whole bone is hook-shaped. As a result, the fifth toe tends to be widely separated from the other four.
This feature occurs in Rhynchocephalia, Thecodontia, Croco dilia, Dinosauria, Squamata, and Chelonia, and has been held to imply a close relationship between these orders.
fingers. The pelvis had an ilium which was antero-posteriorly extended, but so low that the acetabulum lay on the level of the vertebrae. The pubis and ischium were plate-like, but much elon gated and directed largely downward. The hind legs were much longer and more massive than the fore, a condition made possible to a quadripedal animal by the presence of a long tail, which acted as a counterpoise to the body. Although it is certain that these animals had a straddling gait, it is probable that the feet were placed unusually near to the middle line and the feet were not so asymmetrical as those of most lizards.
From such reptiles the Saurischia, the carnivorous dinosaurs arose by an increase in the length of the hind legs, and concurrent reduction of the arms. They became predominantly bipedal, a habit which necessitates the raising of the body so far above the ground that the whole animal balances about the pelvis. This pose can only be attained if the thighs are turned in until they lie parallel to the body of the animal, and the feet are placed on the line which marks the middle of the track. Such an arrange ment ensures that the body need no longer be thrown from side to side, as it is in all more primitive reptiles.
A further result is that, as the powerful muscles which are used for propelling the animal forward no longer press the heads of the femora into the acetabula, this depression no longer needs a floor and becomes perforated. At the same time, in order to lengthen the muscles attached to them and thus enable the leg to swing through a larger arc, the pubis and ischium, both elongated, stretch downwards and away from one another, meeting only at the acetabulum. The reduction in size of the fore limb, which occurs because it is no longer required to carry the weight of the body, results in a reduction and final loss of the clavicular arch, and in a reduction in size of the precoracoid. Sub sequently, certain carnivorous dinosaurs increased greatly in size and became quadripedal again, retaining in many parts of their skeleton features which arose during the bipedal stage in their ancestry.