ANCIENT ANIMALS 



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but are close to the stock from which the bony fishes arose. The arthrodires or 

 joint-necked fishes included the largest and most powerful animals of the time. A 

 complete bony armor covered the anterior parts of their bodies, the head armor 

 hinged to that of the shoulder region by a joint at each side of the neck. The very 

 large mouth was armed with powerful cutting and piercing "teeth" which were 

 not teeth at all but projections of the bony jaws; in biting it appears that the 

 lower jaw remained fixed while the upper part of the head moved up and down 

 on the neck hinges. The earlier arthro- 

 dires were small inhabitants of fresh 

 water, but some of the later marine 

 forms reached a length of more than 

 30 feet and had a tremendous gape of 

 jaw. The whole group suddenly dis- 

 appeared at the end of the Devonian. 

 Another equally strange group of 

 placoderms were the little fresh-water 

 antiarchs. Like their arthrodire rela- 

 tives they had a complete bony armor 

 on the front part of the body, but 

 they lacked the neck hinge and had a pair of armored "flippers" with which 

 they crawled and swam. They did not survive past the end of the period. 



All later fishes, including those living today, belong to one or the other 

 of two great groups which arose during the Devonian period. One of these 

 includes the cartilaginous fishes — the sharks, rays, skates and their allies — 

 the other, the bony fishes. Both groups possess modern- type jaws, the 

 upper jaw being attached at the rear to the bones of the first gill arch 



Fig. 27.9. The Devonian joint-necked fish 

 or arthrodire, Dinichthys. (Redrawn from 

 Romer, by permission The Williams & 

 Wilkins Company.) 



Fig. 27.10. The Devonian flippered fish or antiarch, Pterichthyodes (Pterichthys). (.Redrawn 

 from Romer, by permission The Williams & Wilkins Company.) 



instead of solely to the brain case (Fig. 27.11) ; both have fins and body of 

 more familiar and standardized type than those of the placoderms from 

 which they were derived. However, the two groups evolved along quite 

 different lines. In the sharklike fishes bony structure was lost, the scales 

 were reduced to denticles imbedded in a tough skin, smell became the 

 dominant food-finding sense, and the group became almost wholly marine. 

 None of the cartilaginous fishes ever developed lungs, and the group 

 played no part in the development of terrestrial vertebrates. 



