PELVIC OR HIP GIRDLE
The pelvic girdle or pelvis in reptiles and higher animals consists of three bones on each side, often closely fused in adult reptiles and together known as the innominate bone. The upper or dorsal one of these three bones—that to which the sacrum is attached—is the ilium; the one on the lower or ventral side in front is the pubis; and that on the ventral side behind is the ischium. On the outer side, where these three bones meet, is a cup-like depression, sometimes a hole, called the acetabulum, for the articulation of the head of the thigh bone, homologous with the glenoid articulation of the pectoral girdle, which, as we have seen, was originally formed by three bones, the scapula, coracoid, and metacoracoid, the two latter bones, like the pubis and ischium, meeting in the middle line below. In all the primitive and early reptiles the pubis and ischium form a continuous plate of bone without holes in it, except a small one just below the acetabulum in the pubis, called the obturator foramen, and corresponding to the supracoracoid foramen of the coracoid. One may almost always recognize these two bones by the presence of the foramen. This “plate-like” condition of the pelvis has been lost in all late and modern reptiles by the appearance of a larger or smaller vacuity between the pubis and ischium, either paired, when it corresponds quite with the so-called obturator opening of mammals, or singly in the middle. This old-fashioned character, like the old-fashioned type of pectoral girdle, disappeared entirely about the close of the Mesozoic period, the Choristodera, described in the following pages, being the last of the kind.
Fig. 22.—Pelvis of Ophiacodon: A from side; B from above; pu, pubis; il, ilium; is, ischium.
The ilium in reptiles usually has a more or less prolonged process or projection turned backward by the side of the anterior caudal vertebrae, but in those animals which walked erect on the hind legs, the dinosaurs and pterodactyls, as also some of the more erect-walking reptiles ancestral to the mammals, this process was directed forward, as in birds and mammals. The crocodilia, unlike all other known reptiles, have the pubes excluded from the acetabulum, and they do not meet in a median symphysis. This character alone will distinguish any crocodilian from all other reptiles. But there is some doubt as to the homology of the bones usually called pubes in the crocodiles. Some of the bipedal dinosaurs have the pubis forked, the anterior part directed downward and forward, and not meeting its mate in a symphysis, the posterior process long and slender, lying below the long ischium, as in birds. Indeed, when this peculiarity of the dinosaurian pubis was first discovered, it was thought to be an evidence of the immediate relationship of birds; its structure is now interpreted differently.