428 



THE CHANGING GENERATIONS 



The bony fishes suddenly appeared in abundance in the fresh waters of 

 Middle Devonian time, and since then have steadily increased in impor- 

 tance. By the end of the Paleozoic they dominated the lakes and streams 

 and had invaded the seas, and today they comprise all the fresh-water 



and most of the marine fishes. The 

 group probably arose from acantho- 

 dians or similar placoderms during 

 the late Silurian. In the early forms 

 the head was enclosed in a nearly 

 solid case of dermal bones, the gill 

 slits were covered by a bony plate 

 (the operculum) as in all later bony 

 fishes, and the body was armored 

 with close-fitting, enameled scales 

 of bone. There were paired pectoral 

 and pelvic fins, and the tail was 

 usually bilobed, with the spinal 

 axis bent up into the larger dorsal 

 lobe. 



At their earliest appearance the 

 bony fishes were already separated 

 into two divisions. One of these, the 

 ray-finned fishes, made a slow be- 

 ginning but later expanded rapidly 

 and today includes the great major- 

 ity of all fishes. Their fins are sup- 

 ported by parallel rays of bone and 

 cartilage, and they lack internal 

 nostrils; primitive members of the 

 group had a single dorsal fin and 

 "ganoid" scales heavily covered 

 with an enamel-like material called 

 ganoine. The ray fins have been 

 most successful as fishes but have 

 given rise to no higher types. The 

 other group comprised the choanate 

 fishes, which possessed internal nostrils (choanae), two dorsal fins, 

 and bony "cosmoid" scales covered with a thin outer layer of enamel. 

 Some of the choanates were lung fishes (dipnoians) and others were lobe 

 fins (crossopterygians) ; both groups were abundant in the Devonian and 

 later Paleozoic but have since declined nearly to extinction. Though 

 relatively unsuccessful as fishes, the lobe fins are of great interest as the 

 stock from which the land vertebrates evolved. 



Fig. 27.11. The evolution of fish jaws and 

 the spiracle. The gill slits (black) are sep- 

 arated by gill arches, each of the latter with 

 a hinged supporting bar of cartilage. A, the 

 primitive chordate condition without jaws. 

 B, jaws have developed, but only the brain 

 case is involved in their support. The 

 hyomandibular arch (h) and the gill slit in 

 front of this arch are unaffected. C, the 

 jaws extend farther back, the hyomandibu- 

 lar arch has become a jaw support, and the 

 gill slit in front of this arch has been re- 

 stricted to a small opening, the spiracle. 

 (Redraivn from Romer, by permission The 

 Williams & Wilkins Company.) 



