PRE-CAMBRIAN AND PALEOZOIC ERAS 



157 



the Ordovician. Much more complete remains of them are found in De- 

 vonian deposits (Fig. 8.14). Like modern cyclostomes (lampreys and hag- 

 fishes) they had mouths without jaws, and they had no paired fins. Some 

 of them had a pair of movable flippers attached just behind the head but 

 these did not correspond to the pectoral fins of true fishes. The name 

 "armored fishes'' is based on the armor plate of fused scales covering the 

 head and part of the body. This armor may have served as protection from 

 predatory eurypterids (Romer, 1959) or it may have served to reduce the 

 amount of body surface exposed to unfavorable osmotic action. 



FIG. 8.12. Scorpions of Silurian age. A, dorsal view of 

 one species; B, ventral view of another. (After Pocock; re- 

 printed by permission from Textbook of Geology, Part II, 

 Historical Geology, by Louis V. Pirsson and Charles Schu- 

 chert, published by John Wiley & Sons, Inc., 1915, p. 670.) 



Ostracoderms were the forerunners and probably the ancestors of higher 

 types of fishes which appeared in the Devonian. Among these were the 

 placoderms (Fig. 8.15), the first fishes to possess jaws. The term "placo- 

 derm" as used here includes the acanthodians, which are sometimes re- 

 garded as a separate group. Placoderms form a varied assemblage but the 

 anatomy of some of them suggests that they were the ancestors of two 

 great groups of fishes appearing about this time: ( 1 ) Chondrichthyes, fishes 

 with cartilaginous skeletons, such as dogfishes and sharks; and (2) Oste- 

 ichthyes, fishes with skeletons composed mainly of bone, such as sturgeon, 

 gar pike, trout, salmon, perch, bass, tuna. 



