MESOZOIC ERA 189 



times. Seemingly we have here another example of that parallel evolution 

 which we have discussed in other connections. 



ORIGIN OF MAMMALS 



It is fitting that our discussion of the Mesozoic should 

 conclude with the origin of the animals which were about "to inherit the 

 earth." 



We noted (p. 171) the occurrence in the Permian period of therapsid 

 or mammal-like reptiles. It will be recalled that these reptiles approached 

 mammaUan structure in many ways (Fig. 8.25, p. 171 ). including the fol- 

 lowing: (1) teeth differentiated into incisors, canines, premolars, and mo- 

 lars; (2) two occipital condyles; (3) reduction in number of skull bones; 

 (4) single temporal opening (fossa) having boundaries similar to those of 

 the mammalian temporal fossa; (5) lower jaw in which the dentary bone 

 was predominant; (6) presence of a secondary or "hard" palate; (7) 

 limbs arranged for more efficient locomotion than that characterizing most 

 reptiles. 



These therapsid reptiles are regarded as the ancestors of mammals (Fig. 

 9.3). The therapsids themselves continued into the Triassic but only a few 

 remnants of the group persisted until the Jurassic. Animals which were 

 clearly mammals occurred in the Jurassic. Evidently, then, mammals arose 

 from their therapsid ancestors late in the Triassic or early in the Jurassic. 



Unfortunately the fossil record is most fragmentary at this point. Also, 

 it is difficult to decide whether the remains which have been found are 

 those of advanced therapsid reptiles or those of early mammals. This un- 

 certainty is hardly surprising in view of the fact that the therapsids had 

 already approached closely to mammalian skeletal structure. The problem 

 arises as to where to draw the line between therapsid reptiles and mam- 

 mals. The decision would be much easier if we knew more of these Triassic 

 animals than is revealed by their skeletons. Did they have hair like a mam- 

 mal or scales like a reptile? Did they lay eggs like a reptile or were the 

 young born as in a mammal? (The diagnostic value of this point is some- 

 what lessened by the fact that a small group of living mammals, the mono- 

 tremes — duckbilled platypus and spiny anteater — lay eggs much as do 

 reptiles.) Were the young nourished with milk secreted by mammary 

 glands of the mother? Were they warm-blooded? These and other mam- 

 maUan characteristics we should wish to know about. Since such knowl- 

 edge is denied us, however, we must draw what conclusions we can from 

 the skeletons. 



