EARLIEST KNOWN FISHES 



i6i 



Mountains of Wyoming and the Black Hills of South Dakota. 

 Small spines referred to acanthodian sharks are also abundant 

 in the Ordovician of Canon City, Col. Since they were slow- 

 moving types protected with the beginnings of a dorsal arma- 

 ture composed of small calcareous tubercles, to which the 



BIRDS MAMMALS 



ORDER OF APPEARANCE AND EXPANSION OF THE CLASSES OF VERTEBRATE ANIMALS 



Fig. 42. Chronologic Chart of Vertebrate Succession. 



Successive geologic appearance and epochs of maximum adaptive radiation (expansion) 

 and diminution (contraction) of the five classes of vertebrates, namely, fishes, amphi- 

 bians, reptiles, birds, and mammals. 



group name Ostracoderm refers, probably these earliest known 

 pro-fishes were not primitive in external form but followed 

 upon a long antecedent stage of vertebrate evolution. In the 

 form evolution of the vertebrates relatively swift-moving, de- 

 fenseless types are invariably antecedent and ancestral to slow- 

 moving, armored types. Ancestral to these Ordovician chor- 

 dates there doubtless existed free-swimming, quickly darting 



