Subterranean Habitats 



37 



these qualities it is not difficult for animals to change from marshy 

 ocean to swampy fresh water or from aquatic habitats to land. 

 Paludal environments apparently have been important highways 

 from sea to land. 



Case (1919) believes that tetrapod vertebrates came into existence 

 when paludal conditions obtained over wide areas of the earth. 

 Many paleontologists agree that bony fishes had their origin in 

 fresh water and that land vertebrates came largely from swamps. 



Fig. 6. Animals on a marine mud flat There is little or no oxygen in 

 the mud, so animals breathe by creating respiratory currents. (From 

 Miner, 1912.) Courtesy of the American Museum of Natural History. 



"Indeed, it may now be said to be highly probable that although 

 no known genus of lobed-finned fish was the immediate ancestor of 

 the amphibians, yet the group as a whole has the characters to be 

 expected in the descendants of an earlier common stock that gave 

 rise, on the left to the lung-fishes, near the center to the known 

 lobe-fins, and on the right to the earliest tetrapods or amphibians" 

 (Gregory, 1933) . 



