THE PHYLUM CHORDATA 



Figure 61. A The- 

 RAPSiD, Lycaenops. 

 (After Colbert, from 

 Romer, "The Verte- 

 brate Body," 2nd 

 Ed., W. B. Saunders 

 Co., 1955. ) 



The therapsids (Figure 61) expanded rapidly and were among the most 

 common Permian vertebrates. But apparently they were unable to com- 

 pete with the Ruling Reptiles which rose to prominence in the Triassic, 

 for all but the smallest therapsids became extinct. In other words, they 

 were able to hold only those ecological niches for which the Ruling Rep- 

 tiles did not compete. 



The major mammal-like trends include the rotation of the limbs so that 

 the elbows and knees were brought in under the body. This made it pos- 

 sible to lift the weight of the body without working against a leverage, 

 a very important advance which made possible rapid and long sustained 

 running. The rotation of the limbs also requires modification of the girdles 

 and strengthening of the vertebral column. The skull was simplified some- 

 what, but the major changes in the skull involve the jaws and the articula- 

 tion of the lower jaw to the skull. In typical reptiles, the lower jaw is 

 composed of many bones. In the therapsids one of these, the dentary, 

 which bears the teeth, tends to replace the others. In typical reptiles, the 

 articular bone of the lower jaw articulates with the movable quadrate 

 bone, which in turn articulates with the temporal region of the skull. In 

 the therapsids, the dentary forms a second joint with the temporal, and the 

 articular-quadrate-temporal joint becomes reduced in size and in func- 

 tional importance. The articular and the quadrate become loosely attached 

 and tend to become associated with the columella, an ossicle of the ad- 

 jacent ear. Finally, a certain amount of regional differentiation of the teeth 

 occurred in the therapsids. 



A few mammalian remains of Jurassic age are available. It is probable 

 that the remote ancestors of the order Monotremata were separated from 

 the general mammalian stock as early as this. Of all the orders of mammals 

 ( thirty-two, including fourteen extinct orders ) , this is the only one which 

 lays eggs. The young are fed by milk which is secreted into shallow de- 

 pressions on the abdomen of the mother. As there are no nipples, the 

 young must lap up the milk. The order is represented by only two Hving 

 species, the duckbilled platypus and the spiny anteater, both of which 

 are confined to Australia. Like all mammals, they are covered by a coat 

 of hair, an insulating material which assists in the maintenance of a rela- 

 tively constant body temperature. 



A few fossils from the late Cretaceous show that the order Marsupialia 

 had appeared by that time, but it is only in the Tertiary that they became 

 numerous. These are mammals which no longer lay eggs, but rather the 



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