VERTEBRATE PALAEONTOLOGY 



135 



and amphibian fossils, the next have fish, amphibian and reptile 

 fossils, whilst the most recent rocks will have fish, amphibian, 

 reptile and mammal fossils (see Fig. 40)." 



"The most important point is that one never finds a mammal 

 fossil in rocks that are pre-reptilian ; in fact the finding of a single 

 mammal fossil in such an early stratum would seriously question 

 the correctness of evolutionary concepts. Such a fossil has never 

 been found and the evidence now accumulating strongly supports 

 the view that the fish gave rise to the amphibia, the amphibia to 

 the reptiles, and the reptiles to the mammals." 



Coenozoic 



Cretaceous 



Jurassic 



Triassic 



Carboniferous 



Devonian 



Cambrian 



T 



Mammals 



Reptiles 



Amphibia 



Fish 



No vertebrate fossils 



Fig. 40. Diagram to illustrate a simple view of the level of origin of 

 the various vertebrate fossils. Note that the sequence runs 



Fish-Amphibia-Reptilia-Mammals. 



This account, though a simple one, contains one serious fault. 

 The figure shows not the time of origin of the different classes of 

 the vertebrates but instead the time of dominance of that class. If 

 we consider the time of origin we get a more complex picture (Fig. 

 41). Thus instead of having the reptiles, amphibia, bony fish and 

 elasmobranch fishes all separated from each other by hundreds of 

 millions of years, they all arose during the course of less than 100 

 million years. It is of course difficult to decide just when any of 

 the groups did arise, but some estimate can be made. 



The earliest fossil vertebrates, the Agnatha, are found in the 

 Silurian (fragments are found in the Ordovician). The next group, 

 the Placoderms, are found in the Upper Silurian. The bony fish 

 arose in the Devonian as did also the elasmobranches and the 

 Amphibia. (It is of interest to note here that there is one school 

 of thought, examplified by Save Soderberg (1934) and Jarvik 



