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INTRODUCTION TO EVOLUTION 



dorsal surface of the skull of labyrinthodonts was composed of a mosaic 

 of small bones fitted together edge to edge. The first diagram in Fig. 3.6 

 (p. 31) represents this mosaic pattern. These bones correspond in detail 

 to the bony plates covering the heads of the Crossopterygii (Fig. 8.17). 

 These and other similarities cannot be mere coincidence; they leave no 

 doubt that amphibians arose from Crossopterygii. 



Although these first amphibians possessed many preadaptations for life 

 in the air, it is likely that most of them spent the greater portion of their 

 lives in the water, as many amphibians do to this day (Romer, 1959). They 

 had one great advantage over most fishes, however: they could leave the 



FIG. 8.19. A labyrinthodont amphibian, Diplovertebron; about 2 feet long. (Reprinted 

 by permission of the publishers from Percy Edward Raymond, Prehistoric Life, Cambridge, 

 Mass.: Harvard University Press, 1939, p. 111.) 



water when necessity arose. Under what conditions would it have been 

 desirable to do so? Apparently they did not leave to escape predatory ani- 

 mals. In their fresh-water environment the ancestors of amphibians were 

 the largest animals present. Abundant food supply on land can hardly have 

 been the explanation since these animals were carnivorous, and prospec- 

 tive prey in the form of animals living on land was, as we have seen, 

 much less abundant than was prey living in the water. The most generally 

 accepted answer to the question is based on the idea that the ancestors 

 of amphibians lived in pools that dried up periodically, as do the pools 

 in which some lungfishes live today. Under conditions of overcrowding in 

 stagnant water, followed perhaps by complete evaporation of that water, 

 a premium would be placed on being able to breathe air directly and to 

 move about on land, perhaps at first in search of a neighboring pool 

 having better living conditions. Animals able to survive such stringent 

 conditions were on their way to becoming true land dwellers. Thus, as so 

 often happens, progress occurred under the lash of adversity. 



