CHAPTER 23 



A History of Vertebrates: 

 Amphibians and Reptiles 



202. The Transition from Water to Land 



The transition tiom fresh water to land was a momentous step in 

 vertebrate evolution that opened up vast new areas for exploitation. It 

 was an extremely difficult step because the physical conditions on land 

 are so very different from those in water. Air neither affords as much 

 support, nor offers as much resistance as water. The terrestrial environ- 

 ment provides little of the essential body water and salts. Oxygen is more 

 abundant in the air than in water, but it must be extracted from a 

 different medium. The ambient temperature fluctuates much more on 

 the land than in the water. Air and water have different refractive 

 indices. 



Successful adaptation to the terrestrial environment necessitated 

 changes throughout the body. Stronger skeletal support and different 

 methods of locomotion evolved. Changes occurred in the equipment for 

 sensory perception and changes in the nervous system were a natural 

 corollary of the more complex muscular system and altered sense organs. 

 An efficient method of obtaining oxygen from the air evolved, as did 

 adaptations to prevent desiccation. The delicate, free-swimming, aquatic 

 larval stage was suppressed, and reproduction upon land became pos- 

 sible. Finally, the ability to maintain a fairly constant and high body 

 temperature was achieved, and terrestrial vertebrates could then be 

 active under a wide range of external temperatures. 



In view of the magnitude of these changes, it is not surprising that 

 the transition from water to land was not abrupt, but took millions of 

 years, and involved the participation of many groups. Indeed the main 

 theme in the evolution of the terrestrial vertebrates, or tetrapods, has 

 been a continual improvement in their adjustment to terrestrial condi- 

 tions. 



The crossopterygians unwittingly made the first steps in this transi- 

 tion. Their lungs, as we have seen, were probably an adaptation to 

 survive conditions of stagnant water or temporary drought. Their rela- 

 tively strong, lobate, paired fins enabled them to squirm from one drying 

 and overcrowded swamp to another more favorable one. Crossopterygians 

 were not trying to get onto the land, but, in adapting to their own 

 environment, they evolved features that made them viable in a new and 



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