THE PHYLA HEMICHORDATA AND CHORDATA 



they were the most common of all reptiles. Some of these animals, such as 

 Cynognathus (Fig. 18.29), might as well be called mammals as reptiles; if it 

 were known that they had given up the egg-laying habit, they would cer- 

 tainly be classified as mammals. No single genus among these fossils can be 

 fixed as the specific ancestor of mammals, but it appears that in the African 

 region a type of reptile gradually became modified so that it walked with 

 its limbs more beneath the body, and hence with the body off the ground. 

 From such members of the reptilian stock the mammals arose. One outcome 

 of this change that may have been important was the ultimate possibility of 

 more rapid and efficient locomotion. 



How and when these animals became warm-blooded, hairy, and viviparous 

 are matters of speculation. It has been suggested that increasing dryness 

 of the climate, and so a drier land surface (for which there is evidence in 

 the geologic records of the period), may have been factors of importance in 



Fig. 18.30. Representative Prototheria. A, the platypus, 

 Ormlhorhynchus analinus. B, Echidna aculeala, the spiny ant- 

 eater. (Photographs courtesy New Yori< Zoolo8;ical Society.) 



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