THE PROTOVERTEBRATE 2/ 



tors of the protovertebrate had radiated from the sea 

 into the brackish estuaries bordering the continents when 

 the Grand Canyon disturbance overtook them, and that 

 continental elevation, by accelerating the motion of the 

 rivers, fostered their evolution in the direction of the 

 dynamic vertebrate form. 



In wandering from the sea into the estuaries, the an- 

 cestors of the protovertebrate may have been driven by 

 predatory enemies, or they may have been radiating into 

 relatively unoccupied territory rich in algae and other 

 food; what is more likely, they may have been just ex- 

 ploring the periphery of their world, seeking peace in 

 the face of the never-ceasing drives of hunger and har- 

 assment. But peace they were destined never to find. 

 That they immediately encountered trouble is revealed 

 by the fact that when the fishes first make their appear- 

 ance in the fossil record it is as heavily armored, bottom- 

 hving forms far removed from the hypothetical proto- 

 vertebrate from which, by all evidences, they must have 

 been derived. 



Because of their heavy armor, these earliest fishes are 

 collectively called the Ostracodermi (ostrakon = shell; 

 derma = skin), and because they had no movable jaws 

 they are grouped with a few greatly modified descend- 

 ants ( the living cyclostome fishes which include the lam- 

 prey, Petromyzon, and the hagfish, Myxine) in the class 

 Agnatha, or jawless vertebrates. 



By the Devonian period, when ostracoderm remains 

 appear in the fossil record in fair abundance and a good 

 state of preservation, they are represented by three ma- 

 jor orders, of which type specimens are illustrated at the 

 top of Figure 4. The Anaspida (an = without; aspis = 

 shield), illustrated at the left by Birkenia, were very 

 small fishes lacking any carapace but with the body en- 

 tirely armored in heavy scales. The Cephalaspida (kep- 

 hale = head; aspis = shield) (= Osteostraci), illus- 

 trated by Cephalaspis, are so named because of the 



