566 Comparative Morphology of Chordates 



there is some evidence that the animal had two successive sets of teeth, 

 early "milk-teeth" later replaced by a permanent dentition, as in most 

 modern mammals. In vertebrates other than mammals a tooth is rela- 

 tively temporary. Loss and replacement of a tooth occurs repeatedly. 



The legs of theromorphs resembled those of mammals. In typical 

 quadruped amphibians and reptiles, the legs are relatively short, 

 project laterally outward from the body, and do not raise the body off 

 the ground. Cynognathus, found in the early Triassic of South Africa, 

 was an animal 4 feet or more in length. Its legs were relatively longer 

 and more slender and must have raised the body well above the ground. 

 Instead of projecting laterally, the foreleg is rotated backward at the 

 shoulder-joint so that the bend at the elbow-joint points backward, 

 while the hindleg is rotated forward at the hip-joint so that the bend 

 at the knee points forward as in modern mammals (Figs. 437, 438). 

 The result is to bring the support more nearly directly underneath the 

 body and the several joints are brought into positions which greatly 

 facilitate speed and agility. 



It would be of utmost interest if we could know whether these 

 theromorphs were scaly or hairy. Or were they covered by something 

 intermediate between horny scales and hair? Were they warm-blooded? 

 How did they reproduce? Did they have mammary glands? Probably 

 answers to these questions can never be obtained. As for reproduction, 

 however, it is very likely that it was typically reptilian, because even 

 now there are mammals which "lay eggs" and in all respects reproduce 

 in the reptilian manner. 



It can hardly be said that fossils show us an uninterrupted series 

 transitional between typical reptiles and typical mammals. It is an 

 undeniable fact, however, that the skeletons of certain ancient verte- 

 brates, while for the most part distinctly reptilian, yet possess some of 

 the most characteristic features of mammalian skeletons. It is true 

 also that in regard to some of these mammalian features — the second- 

 ary palate, the jaw-joint, and the teeth — the theromorphs show a 

 graded series which progresses from the reptilian toward the mam- 

 malian condition. 



The chronology of the groups is significant. Theromorphs, so far 

 as is known, first appear in the Permian. In the Triassic they reached 

 their height in diversity of species and were widely distributed. Their 

 remains occur most abundantly in North America and South Africa, 

 but have been found also in South America, Europe, and Asia. But 

 the group seems to have died out before the end of the Triassic. The 

 earliest fossils which are certainly mammalian occur in the late Triassic 

 of South Africa and Europe. They represent small animals which, 

 judging from skulls and teeth, must have resembled modern rodents. 



