xix. 6 THERAPSIDS 541 



rhachitomous amphibia. They did not survive long, however, being 

 apparently replaced by their descendants. From some form similar 

 to *Dimetrodon arose a whole series of lines classed together as Therap- 

 sida and leading on to the mammals. 



6. Order *Therapsida 



In these animals there was never any trace of the bipedalism that 

 developed in the Archosauria; the fore-limbs were always at least as 

 long as the hind, and tended to be turned under the body and to carry 

 it off the ground. The skull became deeper and its brain-case enlarged ; 

 a bony secondary palate developed from flanges of the premaxillae, 

 maxillae, palatines, and pterygoids. The teeth became differentiated 

 for various functions and the bones of the lower jaw, except the 

 dentarv, were gradually reduced. 



These features seem to have developed in several different lines 

 descended from pelycosaur ancestors; the sorting out of the various 

 genealogies is not yet complete. It is therefore still difficult to decide 

 for certain the interesting question whether parallel evolution occurred, 

 and especially whether similar mammalian features appeared inde- 

 pendently in animals of different habits. The fossils are nearly all 

 found in South Africa and have been studied in great detail by R. 

 Broom. 



Much of the surface of the southern part of Africa is covered by 

 rocks known as the Karroo system. These consist partly of shales and 

 mud-stones formed by the matter brought down by a large Mesozoic 

 river, and the remainder are sandstones composed of blown sand. 

 Both sorts of rock were particularly favourable for the preservation 

 of the remains of terrestrial animals. Unfortunately the absence of 

 marine fossils makes it difficult to give dates for these rocks. Altogether 

 the "strata present a thickness of some 15,000 ft, laid down over a 

 period corresponding probably to that from the Middle Permian to 

 the Upper Trias in Europe, that is to say, from 250 to 180 million 

 years ago. 



The therapsids fall into two groups, the mainly herbivorous di- 

 cynodonts and the carnivorous theriodonts, probably preying upon 

 the former. *Gafepus (Fig. 318) perhaps shows a stage of evolution of 

 dicynodonts from pelycosaurs. The temporal opening was small, the 

 teeth all alike and the bones at the hind end of the jaw large. The 

 early dicynodonts include the herbivorous 'dinocephalia', such as 

 *Moschops, which retained many primitive features, but the legs were 

 turned under the body and the phalanges became reduced to the 



