544 



ORIGIN OF MAMMALS 



xix. 6- 



temporal fossa still resembled that of pelycosaurs, there was a single 

 condyle, no secondary palate, and a phalangeal formula of 2.3.4.5.3. 

 *Cynognathus and other Triassic forms were typical theriodonts, 

 showing the above 'mammalian' features. They were carnivores, of 

 distinctly dog-like appearance, although they still retained many signs 

 of a heavy reptilian build. *Bauria was of another type, still more 

 advanced than *Cynognathus in that the orbit and temporal fossa were 



Symphysis 



Outer Aspect 



Articular"* 

 Angular Angular' 



Cynognabhus 



Inner Aspect Symphysis 



Fig. 319. Reduction of the articular and other bones and increase in the dentary in 



therapsid reptiles. (From Neal and Rand, Comparative Anatomy, The Blakiston 



Company, after Watson.) 



confluent (Fig. 318). The foramina of the maxilla of some of these 

 animals suggest the passage of nerves and blood-vessels for a facial 

 musculature, which is characteristic of mammals but absent in other 

 vertebrates. Other types of theriodont were more specialized, with 

 rodent-like features (*Tritylodon, *Oligokyphus). 



These # Theriodontia were the dominant carnivores of the early 

 Trias, but by the end of that period they had almost disappeared. 

 The latest synapsids include in some classifications the *Ictidosauria, 

 such as *Diarthrognathus of the Upper Trias. They are rare fossils, 

 showing almost completely mammal-like structure. There was 

 no post-orbital bar and no prefrontal, post-frontal, or post-orbital 

 bones. A well-developed secondary palate was present. The bones 

 at the hind end of the lower jaw had become very small (Fig. 319). In 

 at least one form the dentary articulates with the squamosal, though 



