xv. i 3 ARCHOSAURS 417 



that have remained at the reptilian level. They have, of course, aban- 

 doned the bipedal habit and survive as a specialized amphibious 

 remnant. The birds, which are also undoubtedly descendants of this 

 archosaurian group, give us in some ways a better idea of the charac- 

 teristic structure than do the crocodiles. 



All the lines of archosaurs are characterized by certain common 

 tendencies, mostly associated with bipedalism, which is possible also 

 in some lizards (Fig. 240); features barely indicated in the earlier 

 forms become developed in the later. In all archosaurs the hind legs 

 were much longer than the front and the acetabulum formed a cup, 

 open below, so that the legs were held vertically below the body. At 

 the same time the ischium and pubis became elongated, presumably 

 to allow for the attachment of muscles producing a fore-and-aft move- 

 ment (see p. 375). In later forms the ilium became fused with several 

 sacral vertebrae. The femur has a lateral head and the tibia becomes 

 long and strong and sometimes fused with the proximal tarsals; the 

 distal tarsals may fuse with the metatarsals as in birds, and the digits 

 are reduced, usually to three long ones turned forward while the first 

 is turned back. The skull is typically diapsid, but tends to have cer- 

 tain modifications, such as the development of antorbital vacuities 

 behind the nostrils and other spaces in the palate, presumably serving 

 to give lightness without loss of strength. 



12. Order *Pseudosuchia 



The earliest archosaurs were the Triassic pseudosuchians, creatures 

 evidently not far removed from the Permian eosuchians. These 

 animals (*Saltoposuchus) can be visualized as lizards that ran on their 

 hind legs (Fig. 241). They were small and carnivorous, having sharp 

 teeth set in sockets along the edges of the jaws (hence 'thecodont'). 

 The skeleton showed all the archosaur characters in a most interesting 

 incipient form. Thus the bones of the pelvis were still plate-like, but 

 arranged in the characteristic triradiate manner. The front legs were 

 already much shorter than the hind. Antorbital vacuities were present 

 and there was no pineal foramen. 



13. Order *Phytosauria 



Even in the Triassic at least one line, the phytosaurs, abandoned 

 the bipedal habit, becoming amphibious. These creatures were not 

 actually ancestral to the crocodiles, but show remarkable parallelism 

 to them in the elongated jaws and general build (Fig. 241). How- 

 ever the nostrils were set far back. There can be no doubt that the 



