The Tetrapoda: Mammals 627 



successful and diversified reptiles of that period. A few of the therapsids 

 must have survived, but they were so overshadowed by the towering 

 reptiles that they could not have been of any significance. Until the 

 dinosaurs and their innumerable relatives were destroyed in the cata- 

 clysmic changes which marked the end of the Mesozoic era, the mammal- 

 like reptiles could not go about their business of development. 



The Cretaceous marks the beginning of our modern period. The 

 continents gradually assumed their present form, the modern plants 

 began their rise, and the mammals started their diversification. Dur- 

 ing the Cretaceous, the three modern forms of the mammals were pres- 

 ent. These were the Prototheria as represented by the duck-billed 

 platypus; the Metatheria, the marsupials; and the Eutheria or true 

 mammals. These three forms differentiated during the Jurassic, but 

 the fossil remains are so sparse that the details are obscure. The egg- 

 laying prototherians appeared during the Jurassic, and are left today 



Fig. 208. — Lycaenops. the mammal-like reptile from the late Permian of South 

 Africa. (After Colbert, from Romer: The Vertebrate Body, W. B. Saunders 

 Company.) 



as only a few species living in the continent of Australia and neigh- 

 boring islands. These highly specialized animals, however, cannot 

 be regarded as ancestors of the higher mammals. The other two 

 groups of mammals must have separated very early for the marsupials 

 were very abundant during the latter Cretaceous. At present, the 

 marsupials are abundant only in Australia where they have evolved in 

 numerous different forms adapted to many different types of habitats. 

 During much of the Tertiary, the continent of South America had many 

 diverse marsupials, but these failed to survive after the North Ameri- 

 can continent was again joined to South America. This permitted the 

 more assertive placental carnivores to flood down to South America 

 and usurp the places occupied by the marsupials. At present, only 



