722 phylum chordata : class reptilia 



Relationships 



Reptiles, in their widest sense, form a central assemblage 

 among Vertebrates. Some of the extinct forms exhibit 

 affinities with Amphibians, others with Birds, others again 

 with Mammals. Though we cannot with certainty point to 

 any of the extinct types as directly ancestral to Birds or 

 Mammals, it seems likely that the ancestors of both were 

 derived from the plastic Reptilian stock. 



Adaptive Radiation 



One of the many interesting features of the Mesozoic 

 Reptiles is what H. F. Osborn has called adaptive radia- 

 tion. Just like the Mammals who succeeded them, the 

 ancient Reptiles tried all sorts of haunts and ways of living. 

 Some were aerial, like the Pterodactyls ; some aquatic, like 

 the Ichthyosaurs ; some amphibious, like certain Dino- 

 saurs ; some cursorial, like many Dinosaurs ; others, 

 perhaps, fossorial and arboreal. There is a striking par- 

 alellism between Reptilian and Mammalian ways of living. 



Grouping of Vertebrates 



If we leave the primitive Chordates out of account, and 

 the Cylostomes as well, we may recognise, as Huxley did, 

 three great groups of Vertebrates — contrasted on the next 

 page : 



I. IcHTHYOPSiDA : Fishcs and Amphibians. 

 II. Sauropsida : Reptiles and Birds. 

 . III. Mammalia. 



In spite of appearances, Amphibians must be ranked 

 along with Fishes, and Birds along with Reptiles. 



