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THE RISE OF ANIMAL LIFE 



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Fig. 13-55. Relative sizes of a few mammals. 



origin of mammals took place approxi- 

 mately 150 million years ago (Fig. 13-41). 

 These early forms were neitlier reptiles nor 

 mammals as we know them, but probably 

 intermediate in character, with resem- 

 blances to both. It is generally agreed that 

 mammals had their beginnings in some 

 reptilian type and the first ones were prob- 

 ably more lizard-like than mammal-like 

 (Fig. 13-41). One of the earliest animals 

 that showed mammalian features was 

 Cynognathtis ("dog-jawed"), in which sev- 

 eral skeletal improvements over the typical 

 reptile are observed. The legs had assumed 

 a more ventral position so that they not 

 only lifted the body farther off the ground 

 but in addition were capable of forward 

 and backward motion, resulting in more 

 forward speed. Thus, instead of crawling 

 as its reptilian ancestors did, it was able to 

 carry itself a considerable distance above 

 the ground and undoubtedly travel much 

 faster. This characteristic has culminated 

 in such modern animals as the horse and 

 giraffe. 



The skull also had certain modifications, 

 such as the more posterior position of the 

 entrance of the internal nostrils into the 

 mouth, for example. Being warm-blooded 

 now, it was necessary for the animal to 

 breathe continuously and rapidly so that 

 sufficient oxygen could be supplied to the 

 tissues to maintain the high temperature. In 



order that breathing might not be inter- 

 rupted during feeding, the internal nostrils 

 opened into the mouth farther back and 

 were walled off by a plate, the palate. 

 These characters are present in this primi- 

 tive mammal. Furthermore, the teeth show 

 the beginnings of a typical mammalian 

 pattern, consisting of the front nippers ( in- 

 cisors) and the large fang-like canines, 

 followed by the shearing molars. Reptiles 

 have no such dental pattern. 



These primitive mammals were present 

 on the earth long before the coming of the 

 great dinosaurs but they were small and in- 

 conspicuous, and probably kept in hiding 

 throughout the reign of these great beasts. 

 During this long period of perhaps 15 mil- 

 lion years they were a source of food and, 

 consequently, were forced to live "by their 

 wits," which gave them an opportvmity to 

 try out many devious plans for a better 

 body as an aid to survival. They remained 

 small and insignificant until the dinosaurs 

 met their end. The world was then left free 

 for any animal that was ready to take over. 

 The mammals apparently "saw their chance 

 and took it." Even then, while the mamma- 

 lian characteristics in general persisted, 

 many "ideas" were evolved that produced 

 animals not able to survive very long — the 

 mammoth, saber-tooth tiger and the giant 

 sloth, for example. The more intelligent and 

 versatile did survive, however, and gave 



