430 



THE CHANGING GENERATIONS 



During dry periods the pools become stagnant, low in oxygen, and foul with the 

 decaying bodies of other fishes; but this does not seem to affect the lungfishes, 

 though at such times they depend chiefly upon their lungs for respiration. The 



African and South American lungfishes 

 live in marshes bordering rivers, where 

 they feed on frogs, insects, and other 

 animals. When the marshes are flooded 

 they live much as does the Australian 

 form, though they are more dependent 

 upon their lungs and will drown if pre- 

 vented from coming to the surface. 

 During the dry seasons, when the 

 marshes may be without water for 

 months at a time, these fishes burrow 

 into the mud and coil up at the bottom 

 of their holes. There they secrete around 

 themselves a moisture-holding "cocoon" 

 of slime, tightly closed everywhere 

 except over the mouth; and there they 

 stay, breathing air and keeping moist in 

 their slimy covering, until the return of 

 water to the marshes. In the light of such 

 facts our guesses concerning the habits 

 of the Devonian air-breathing fishes 

 seem entirely reasonable and probably 

 correct. 



Fig. 27.14. Primitive fish scales in cross 

 section. The cosmoid scale of the early 

 choanate fishes (above) has a thin outer 

 layer of enamel. The ganoid scale of the 

 primitive ray-finned fishes (below) has a 

 thick outer layer of enamel-like ganoine. 

 (Redrawn from Romer, Vertebrate Paleon- 

 tology, by permission University of Chicago 

 Press.) 



The origin of amphibians. The 



oldest known four-footed, land-dwell- 

 ing vertebrates are primitive amphibians found in Upper Devonian rocks. 

 Except that they had legs instead of fins, they are almost indistinguishable 

 from lobe-finned fishes, certain of which were without question their 

 ancestors. The early amphibians and the lobe fins are parallel in the 



Fig. 27.15. The Devonian lobe-finned fish, Osteolepis. (Redrawn after Jarvik, 1948.) 



arrangement of the bones of the skull, in the microscopic structure of the 

 teeth, in the presence of internal nostrils, and particularly in the arrange- 

 ment of the bones of the fish fin base and the amphibian leg. In the lobe 

 fins there was a single proximal bone corresponding to the humerus or 

 femur, followed by two bones corresponding to those of the forearm or 



