GENERAL ZOOLOGY 



determining the types that survived. These reptile-mammals became extinct 

 with the rise of the reptiles in the Mesozoic, but they seem to have left 

 descendants that were truly mammals, and contemporaneous with the 

 dinosaurs. Small mammals of this type, no larger than a rat, are known 

 from fragmentary remains scattered throughout the Mesozoic. The fossils are 

 principalK' jaws and teeth; not a single complete skeleton has been discovered, 

 and not even a complete skull, until a time near the end of the Age of 

 Reptiles. Judging from their teeth, some of these small mammals seem to 

 have been insect eaters, and their prey probably included other small 

 animals in addition to insects. Others had teeth indicating a herbivorous 

 diet. It appears from a variety of evidence that they were arboreal; they 

 may have been nocturnal, like many small mammals of today. With the 

 reptiles so diversified and occupying the land, the water, and the air, there 

 remained perhaps a greater degree of safety in the arboreal habitat. In any 

 event, small mammals appear early in the Mesozoic, and they seem to have 

 remained much as they were until the diversification of the Eutheria began 

 near the end of this period. 



Evolutionary changes have occurred not only in the adult structure of 

 mammals, as compared with reptiles, but also in their embryonic stages. 

 The reptilian ancestors probably laid eggs, as do modern reptiles. The 

 Prototheria, as represented in the modern fauna by the platypus and the 

 spiny anteater (Fig. 18.30), lay eggs somewhat resembling those of reptiles. 



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