178 



INTRODUCTION TO EVOLUTION 



f'emporoii^ fossci 



postorbi+al 



orbit 



squoimosod 



SYNAPSID 



temporal fossae 



postorbi+al 



eye orbit 



nostril 



squoimosoi 



DIAPSID 



FIG. 9.4. Basic structural plans of the skulls of Synopsida and Diapsida. Note the 

 single temporal fossa in the synapsid skull, the two temporal fossae in the diapsid 

 skull. (After Colbert, The Dinosaur Book, McGraw-Hill Book Company, Inc., 1951.) 



temporal fossae on each side of the skull — one near the midline, the other 

 more lateral in position. The two are separated by a bridge of bone (Fig. 

 9.4). To the diapsid hne belong the dinosaurs. 



Thecodonts 



Diapsid ancestors were found in the Permian. They resembled lizards in 

 having long bodies and slender hmbs. Their descendants in the Triassic 

 were the thecodonts, direct ancestors of the dinosaurs (Fig. 9.3). Most of 

 the thecodonts were small reptiles. They had narrow skulls which lacked 

 a pineal opening but preserved the diapsid characteristic of two temporal 

 openings on each side. The most distinctive characteristic of these reptiles, 

 however, lay in their method of locomotion. Instead of walking or running 

 on all four legs, as a dog does, they adopted a bipedal (two-footed) type 

 of locomotion, running on the two hind legs as do many birds, e.g., robin 

 and ostrich. The hind legs were elongated, forming a support upon which 

 the body was balanced as on a fulcrum (Fig. 9.3). The body projected for- 

 ward from this fulcrum, its weight counterbalanced by a long tail project- 

 ing backward. The forelimbs, freed from locomotor duties, were available 

 for use in grasping and handling. Since all the weight of the body was 

 concentrated on the hind legs, the attachment of the latter to the body was 

 of necessity greatly strengthened. This involved strengthening of the pelvic 

 girdle and of its attachment to the vertebral column, as well as the devel- 

 opment of a more perfect ball-and-socket joint for attachment of the leg 

 to the pelvic girdle. The legs no longer sprawled broadly at the sides of the 

 animal, as they had in many earlier reptiles, but were placed well under 



