214 VERTEBRATE ZOOLOGY 



call it provisionally, Protopoda. It was ancestral to both and both 

 classes have advanced since their divergence, the Amphibia some, the 

 Reptilia much. Could we find, as some time we hope that we may, 

 in mid-Mississippian or late Devonian times, a skeleton of those 

 ancestral creatures, we should perhaps not call it by the name of any 

 known order; it would be the old question over again of the difference 

 between animals and plants. At present we know the Protopoda 

 only by their footprints." 



This statement of Williston is somewhat radical in that it places 

 the Amphibia and the Reptilia on the same level, neither being ances- 

 tral to the other, but both derived from a common ancestor about 

 which we know nothing except the characters revealed by its foot- 

 prints. Some authors have assumed that this creature was an ances- 

 tral amphibian, a view that has gained wide acceptance. For our pur- 

 poses it seems advisable to think of the creature that made the 

 footprints (Fig. 101) as the earliest known land vertebrate, and to 

 call it the ancestral amphibian. 



Palaeontologists generally agree that the reptiles go back nearly 

 as far as do Amphibia, and that their evolution has been to a large 

 extent parallel. Both experimented with adaptations for land life 

 and the reptiles were much more successful in these ventures than 

 were their rivals. During the Permian, however, they were neck-and- 

 neck in the race; for every reptilian type of that period there was a 

 parallel amphibian type. The reptiles finally outstripped the Am- 

 phibia, largely through their adoption of the amnion-allantois com- 

 plex and their consequent emancipation from the water. 



Permian Reptiles 



The reptiles of the Permian were partly archetypal forms and partly 

 precociously specialized and already senescent types. Williston finds 

 four assemblages of Palaeozoic reptiles (belonging to the American 

 Permo-Carboniferous). Each of these assemblages has evidently the 

 systematic value of a sub-class and has one or more modern repre- 

 sentatives. 



a. Diapsida; represented to-day by Sphenodon, crocodiles, birds 



and the great extinct dinosaurs, pterosaurs, parasuchians, etc. 



b. Synapsida; represented to-day by the mammals, and by the 



extinct plesiosaurs, anomodonts, therapsidans and ther- 

 omorphs, etc. 



