A HISTORY Of VERTEBRATES: BIRDS AND MAMMALS 491 



24.14 A), the milk is discharged onto the hairs and the young lap it up, 

 but in other mammals, nipples or teats are associated with the glands 

 and the young are suckled. When the young finally leave their mother, 

 they are at a relatively advanced stage of development, and are equipped 

 to care for themselves. 



212. Primitive Mammals 



Monofremes. The most primitive mammals are the platypus 

 (Ornithorhynchus), and its close relative, the spiny anteater (Tachyglos- 

 sus) (Fig. 24.14 A and B). In addition to the egg-laying habit, these 

 mammals retain many other reptilian characteristics, including a cloaca. 

 The ordinal name for the group, Monotremata, refers to the presence of 

 a single opening for the discharge of feces, excretory and genital prod- 

 ucts. In other mammals, the cloaca has become divided, and the opening 

 of the intestine, the anus, is separate from that of the urogenital ducts. 



Monotremes are curious animals that have survived to the present 

 only because they have been isolated from serious competition in the 

 Australian region. The platypus is a semiaquatic species with webbed 

 feet, short hairs and a bill like a duck's used in grubbing in the mud for 

 food. Spiny anteaters have large claws and a long beak adapted for 

 feeding upon ants and termites. The animal can burrow very effectively 

 with these claws, completely burying itself in fairly hard ground in a 

 few minutes. Many of its hairs are modified as quills. 



A\4ien the first skins of the platypus were shipped to Europe in the 

 late 18th century, many zoologists viewed them as skillful fakes such 

 as the then current Chinese mermaids (the forepart of a monkey sewn 

 onto the tail of a fish). After the authenticity of the platypus was estab- 

 lished, a long controversy ensued as to whether to consider it a reptile 

 or a mammal. Monotremes were finally regarded as mammals, but as 

 such primitive and unusual ones that they are placed in a separate sub- 

 class—the Prototheria. Many investigators now believe that monotremes 

 evolved from mammal-like reptiles earlier than, and independently of, 

 the other mammals. If this is true, mammals have had a polyphyletic 

 rather than a common evolutionary origin (Fig. 22.2). A corollary of such 

 a view is that hair and mammary glands either evolved independently 

 in monotremes and other mammals, or were attributes of the mammal- 

 like reptiles. 



hAarsup'ials. All other mammals are believed to have had a com- 

 mon origin, and are placed in the subclass Theria. Therian mammals 

 were present in the last half of the Mesozoic era, but they did not 

 become abundant until the extinction of the ruling reptiles. During the 

 Cenozoic, they increased rapidly, radiated widely, and became the domi- 

 nant terrestrial vertebrates. 



Contemporary therians fall into two infraclasses— (1) the Meta- 

 theria, which includes the opossum, kangaroo and other pouched mam- 

 mals of the order Marsupialia (Fig. 24.14); and (2) the Eutheria, or true 

 placental mammals. Both groups are viviparous, though the placental 

 arrangement of marsupials is less effective than that of eutherians. In 



