334 VERTEBRATE ZOOLOGY 



cestors of the first mammals it is impossible to say, but there is noth- 

 ing inherently improbable about such a theory. 



The cynodonts (Fig. 173) were mammal-like in a number of ways: 

 a, they had a well-defined heterodont dentition, with incisors, canines 

 and molars; h, they had two condyles; c, the lower jaw was composed 

 primarily of the dentaries, but there were sometimes small vestigial 



angulare, articulare, and other rep- 

 tiUan bones; d, the quadrate was 

 often greatly reduced and must 

 have been functionless as a con- 

 nection between the mandible and 

 the skull. These and many minor 

 features of the skull and limb skel- 

 FiG. 173.— Skull of cynodont reptile, etons were modified in a mam- 

 ''£r7J''Z^:iJ^l.Tll -alian direction, but no single spe- 

 ferentiation, but complex reptilian cies of cynodont approached very 

 lower jaw. Ang, angulare; Art, articu- closely a true mammaUan condi- 

 lare; Dent, dentary; Ju, jugal; h, lach- Posqihlvthpfiitiirehasinst^rp 



rymal; Mx, maxillary; iVa, nasal; Fa, ™n. rossiDlytneluturenasm store 

 parietal; Pmx, premaxillary; PoO, post- for the palaeontologists the discov- 

 orbital; Pr. F, prefrontal; S. Ang, gjy ^f i^ie real ancestral mammal. 



™aSrol.r'"°"' ^^"" Now, the cynodonts belong to 



the sub-class Synapsida and the 

 order Therapsida, which Williston places very low in the series of 

 reptilian orders, far below the Ichthyosauria, Squamata, Rhyncho- 

 cephalia, Crocodilia, Dinosauria, and Pterosauria. The only orders 

 of lower rank are Cotylosauria, Chelonia, and Theromorpha. It seems 

 highly probable that the Therapsida were derived from an early very 

 generalized group of Permo-Carboniferous theromorphs, probably the 

 pelecosaurs, of which Varanosaurus appears to be the most general- 

 ized representative. This type was a long lizard-like reptile with 

 very generalized proportions and with the maxillary teeth somewhat 

 more prominent than the others. 



In addition to the mammal-like skull characters referred to above, 

 these South African cynodonts had modifications of the limbs (Fig. 

 123, E) that appear to have had to do with rapid locomotion, char- 

 acters that might well have served to introduce the habit of migration 

 and thus to have given these reptiles an advantage over their more 

 sluggish relatives. Migrations would have a tendency to increase 

 the powers of observation and in turn to have served to accelerate 



