28 FROM FISH TO PHILOSOPHER 



heavy shield that covered the head. The Pteraspida 

 (pteron = wing; aspis = shield) ( = Heterostraci), il- 

 lustrated at the right by Anglaspis, had a heavy shield 

 over the whole front end of the body, and the trunk and 

 tail were covered with large rhomboid scales. 



Why all this armor of bony plates, scutes, and scales 

 covering the animal from snout to tail? Some years ago 

 A. S. Romer suggested that it was evolved as a defense 

 against the eurypterids, the large scorpionlike creatures 

 of the Silurian, shown at the bottom of Figure 4, be- 

 cause these presented the only visible enemies of the 

 early vertebrates in their fresh-water habitat. The argu- 

 ment, however, is not wholly convincing, or at least is 

 not the only argument that can be advanced. Though 

 the largest American eurypterid, Pterygotus buffaloensis 

 of western New York, grew to an estimated body length 

 of seven feet (the largest arthropod of all time) and 

 possessed pinching claws, the majority were but a few 

 inches to a foot in length and had no pinching claws 

 whatever. If not primarily mud crawlers Hke their de- 

 scendant the horseshoe crab, the eurypterids, which had 

 only heavy and clumsy paddles on the head, did not 

 have the appearance of powerful swimmers. The mouth 

 was a small opening on the underside of the head (as 

 in the horseshoe crab), and bore toothlike processes be- 

 ginning to function as chewing organs; it suggests the 

 scavenging stage of an advanced clumsy mud-strainer 

 rather than a 'predacious' and Voracious' animal capa- 

 ble of driving the vertebrates into armored safety. Even 

 though, as was probably the case, types like Eurypterus 

 remipes struck their prey with their pointed tails as do 

 the horseshoe crabs, and types like Eusarcus scorpionis 

 injected poison as do their other offspring, the scorpions, 

 their fearsomeness may be a matter more of psychologic 

 association than of fact. Moreover, the eurypterids did 

 not reach their climax until the Silurian; they are poorly 

 represented in the Ordovician, and it may be questioned 



