568 



Comparative Morphology of Chordates 



diversified. Most of its members remained terrestrial, but some ac- 

 quired burrowing habits (fossorial) and others became arboreal. Still 

 others reverted to water, becoming amphibious (seals) or wholly 

 aquatic (whales), and the aerial bats are the mammalian equivalent 

 of Mesozoic reptilian pterodactyls (Fig. 439). 



In adaptation to climates, the homothermous mammals did 

 what the reptiles could not: mammals range from the equator to the 

 polar regions. It is to be noted, however, that in the colder latitudes it 

 is the larger mammals that have the advantage, whereas the larger 

 reptiles inhabit the warmer regions. It is, in both, a matter of the rela- 

 tion of surface to volume. In a cold climate the necessity of conserving 

 internal heat gives the advantage to the larger mammal because of its 



Fig. 439. (Top) Skeleton of a pterosaur, Pteranodon. (Bottom) Skeleton of bat. 

 (Top, courtesy, American Museum of Natural History, New York. Bottom, 

 courtesy. Ward's Natural Science Establishment, Inc.) 



