RELATION TO MAMMALS 



303 



Many of the Theromorpha ^ reached a considerable size, massive 

 skulls of one foot in length being not uncommon. The tail 

 was comparatively short. 



The many resemblances of these strange creatures to 

 Mammals have naturally suggested that the Mammalia have 

 sprung from some such Theromorpha or " beast-shaped " animals. 

 The resemblances are chiefly the dentition, the zygomatic arch, 

 the pelvis, the cruro-tarsal joint, the scapula which is sometimes 

 possessed of a spine, and the occasionally double occipital con- 

 dyle. The general shape of the skull of Cynogjiuthus is indeed 

 strikingly like that of a Carnivorous Mammal, and the shape of the 

 whole body suggests rather a Mammal than a reptile ; and when 

 we have to deal with the fragmentary skulls of Tritylodon (cf p. 

 309) it is, indeed, difficult to decide to which of the two classes 

 such a creature belongs. But the Theromorpha possess a 

 number of important characters by which they reveal themselves 

 at once as reptiles: (1) the large and fixed quadrate bone, which 

 is still the sole support of the lower jaw; (2) the compound 

 mandible, which is composed of at least an articular, dentary, 

 angular, supra-angular, and splenial element ; (3) the inter- 

 parietal foramen ; (4) the possession of prefrontal and post- 

 frontal bones, sometimes also postorbital, supratemporal, and 

 quadrato -jugal bones. Of course, any of these ancestral 

 bones may be lost, and the interparietal hole may be closed as in 

 tortoises and crocodiles. We can also imagine that the quadrate 

 may be relieved of its jaw-bearing function and become loosened, 

 but this is not easy, considering the strong development of 

 the squamoso-quadrate pedicle. Those Theromorpha in which the 

 quadrate itself is small, whilst the squamosal reaches down, or 

 at least approaches the mandible, as in Bicynodon and Gordonia, 

 are so hopelessly pledged, or specialised in other directions, that 

 it is impossible to connect them ancestrally with Mammals. 



However, it is beyond reasonable question that the Mammals 

 have sprung from some reptilian stock (the attempts to derive 



^ Cope, the inventor of this most appropriate name, soon changed it, un- 

 necessarily, into Theromora (juupos = sluggish), perhaps in order not to emphasise 

 too much their possible Mammalian affinities ; while others rashly called them 

 Sauro-Mammalia. For detailed illustrations of Theromorpha reference should be 

 made to Owen, British Fossil lieptiles, 4to, London, 1849-55, and to numerous 

 papers by Seeley, Phil. Trans. 178 (1887), 186 (1895), and by E. T. Newton in 

 Fhil. Trans. 184 (1893), 185 (1894). 



