542 ORIGIN OF MAMMALS xix. 6 



mammalian formula of 2.3.3.3.3. The roof of the head was expanded 

 into a large dome, giving the name to the group. The later dicynodonts 

 were a still more specialized and successful offshoot, the first of the 

 many great tetrapod herbivores. They became the commonest of all 

 reptiles in the later Permian. They were large creatures (*Kanne- 

 meyeria and *Dicynodon, Fig. 318), some probably living in marshes. 

 The margins of the jaws were covered with a horny beak and the 

 teeth reduced to a single pair of upper 'canine' tusks. 



The most interesting of the therapsid reptiles are, however, those 

 placed in the suborder # Theriodontia, which, by the early Trias, had 

 produced a very mammal-like type of organization, apparently in 

 several independent lines. The temporal opening became progres- 

 sively wider, presumably for the accommodation of larger jaw muscles, 

 so that the parietal bone entered into the margin of the fossa and post- 

 orbital and squamosal bones no longer met above it. Eventually the 

 post-orbital bar itself became incomplete, leading to the typical mam- 

 malian condition, with the orbit and temporal fossa confluent. There 

 was a large columella, articulating broadly with the quadrate and 

 perhaps serving to brace the latter as well as to transmit vibrations 

 to the inner ear. A bone of this size could act as an efficient vibrator 

 (Hotton, 1959). The brain-case was high and large and the cerebellum 

 better developed than in modern reptiles. The cerebral hemispheres, 

 however, though large, probably remained mainly olfactory structures, 

 as indeed they were in many Eocene mammals and are in some that 

 survive today. 



The ribs were well developed and seem to have formed a cage, 

 which may have been used, with a diaphragm, for respiration. The 

 limbs came to support the body off the ground, and the dorsal parts 

 of the girdles were developed accordingly, the scapula becoming large 

 and bearing an acromial spine for muscle attachments, the coracoids 

 being reduced. The anterior portion of the ilium became large and 

 a hole appeared between the pubis and ischium. The head of the 

 femur lay at the side, and a special knob, the great trochanter, ap- 

 peared on it for the attachment of muscles running from the front 

 part of the ilium. In the hands and feet the toes show a reduction of 

 the formula to 2.3.3.3.3. The teeth became differentiated into incisors, 

 canines, and cheek teeth, the latter having several cusps in place of the 

 cone of a typical reptile tooth. In most forms the tooth replacement 

 was as in reptiles but in some it was more limited. 



These features were little developed in the earliest theriodonts, 

 such as *Scy?nnognathus and other Permian forms (Fig. 318). The 



