MESOZOIC ERA 179 



the body, with knees turned forward, a position better calculated for effi- 

 cient support of weight. 



The body form resulting from this adaptation of the thecodonts for a 

 bipedal gait provides the key to an understanding of dinosaur structure. 

 The inheritance from thecodont ancestry was never completely obliter- 

 ated, even in those dinosaurs which became huge in size and returned to 

 a four-footed or quadripedal locomotion. The thecodont body plan is, as 

 Colbert (1951) has said, "the blueprint to dinosaurian body form." 



Orders of Dinosaurs 



The dinosaurs arose from thecodonts in the Triassic and continued as 

 the dominant land animals throughout the remainder of the Mesozoic. In 

 reality the dinosaurs did not constitute a single group; from the first they 

 were divided into two great orders, the Saurischia and the Ornithischia. 

 These names refer to the most clear-cut distinction between the two: the 



ORNITHISCHIAN PELVIS 



FIG. 9.5. Pelvic girdles of saurischian and ornithischian dinosaurs. (By permission from 

 The Dinosaur Book, by Colbert, p. 65. Copyright, 1951. McGraw-Hill Book Company, Inc.) 



structure of the pelvic girdle. The Saurischia retained a triradiate arrange- 

 ment of the pelvic bones similar to that of their thecodont ancestors (Fig. 

 9.5). The ilium was the bone attaching the girdle to the vertebral column. 

 To the ilium attached two bones, the pubis, extending ventrally and an- 

 teriorly, and the ischium, extending ventrally and posteriorly (Fig. 9.5). 

 The socket (acetabulum) for the head of the femur was located at the 

 junction of these three bones. Since the ischium and pubis on one side of 

 the body were usually attached to the corresponding bones of the other 



