REPTILES, BIRDS, AND MAMMALS 



459 



specialized in peculiar ways. The most typical were active carnivores, 

 about 5 feet long, with limbs already shifted toward the typical mam- 

 malian position, and mammal-like changes in the skull. In them many 

 of the skull bones that are lost in mammals were becoming small or had 

 already disappeared; the dentary was enlarged and the other jaw bones 

 reduced in size; a double occipital condyle had developed, and also a 

 palate separating the nasal and 

 mouth cavities; the teeth had 

 differentiated into incisors, canines, 

 and cheek teeth. There was still 

 only a single auditory bone. In spite 

 of their mammalian characteristics 

 the therapsids were still reptiles in 

 most respects. Whether they were 

 warm-blooded, or had hair, or 

 nursed their young is a matter for 

 conjecture. 



Egg-laying mammals. Two very 

 specialized survivors of an egg- 



Fig. 28.25. The skull of Cynognathus, a 

 Permian dog-toothed reptile. Note the 

 mammal-like differentiation of the teeth, 

 increased size of the dentary bone of the 

 lower jaw, and reduction of the bones of the 

 jaw hinge region (Redrawn from Romer, 

 Vertebrate Paleontology, by Permission Uni- 

 versity of Chicago Press.) 



Fig. 28.26. The duckbill or platypus. 

 Ornithorhynchus, a primitive egg-laying 

 Australian mammal. (From Burrell, The 

 Platypus.) 



laying stock of mammals occur today in Australia, where they have been 

 able to persist through the protection afforded by their geographic isola- 

 tion and their retiring habits. These are the duckbill (Platypus) and the 

 spiny anteater (Echidna). They have fur and nurse their young but lay 

 shelled eggs. Neither has teeth. The duckbill has webbed feet and is 

 mainly a stream dweller, nesting in burrows in the banks; the anteater 

 has a covering of stout spines, strong digging feet, and a slender snout. 

 It is probable that viviparity appeared early in mammalian evolution, 

 and that the viviparous stocks prevailed over the egg-laying types even 

 during the Mesozoic era. 



