56 FROM FISH TO PHILOSOPHER 



tached to the rear edge a web of sldn which made them 

 more effective as stabilizers. All the higher fishes are 

 presumed to be derived from a primitive acanthodian 

 root. 



The Arthrodira (arthron = joint; deire = neck), com- 

 monest of Devonian vertebrates, were large fishes derived 

 from some imidentified acanthodian. They possessed a 

 heavily armored head flexibly joined to the armored 

 body by a ball-and-socket joint, so that for the first time 

 the animal could raise and lower its head as well as move 

 it from side to side, and to implement this motion they 

 had elaborated on the shoulder girdle, to which the mus- 

 cles that moved the head were anchored. Some (as 

 Coccosteus) possessed paired pectoral and pelvic fins, the 

 former still carrying the old ostracoderm spines. The 

 posterior part of the body generally lacked armor, indi- 

 cating that improvements in the structure of the skin had 

 enabled them to begin the abandonment of heavy water- 

 proofing. 



The Antiarchi {anti — opposite; archos = anus) were 

 grotesque Httle fishes related to the arthrodires. They 

 possessed jointed flippers or creepers that were neither 

 spines nor fins and resembled nothing before or since, 

 but which are accepted as having been derived from 

 spines. It has been suggested that, peculiar in all other 

 ways, they also possessed lungs— certainly, if true, a case 

 of 'convergent' evolution, since they are in no way re- 

 lated to the later air-breathing fishes. 



The Stegoselachii (stegos = roof; selachos = the 

 Greek name for fishes having cartilage instead of bones ) 

 lacked the ball-and-socket joint in the neck but possessed 

 pectoral fins attached directly to the shoulder girdle, and 

 in the brain and gill arches foreshadowed the later 

 elasmobranch fishes. 



Whereas spinous processes were once thought to be a 

 sign of extreme evolutionary specialization, the spines of 

 the placoderms are now seen to be derived from the 

 ostracoderm armor and to have served to give the animal 



