Cold-blooded animals that spend part of their time on land and part in the water, amphibians are able to breathe through their skin. ( This permeable skin makes them particularly vulnerable to environmental disturbances, from chemical pollution to the thinning ozone layer and global climate change.) The first major groups developed about 400 million years ago, from fishes similar to modern coelocanths that were about 15 feet long. Later amphibians moved up the food chain, eating insects on land and fish in the water. By about 250 million years ago, amphibians were competing with proto-crocodiles, pushing their distribution north and south from the equator and causing a reduction in amphibian size in temperate zones. 

Globally, 1,808 species of amphibians, or 29 percent of the total number of 5,918 described amphibian species, were deemed at risk of extinction by IUCN's 2007 Red List. Fifty-three out of about 287 amphibians, or about 18 percent, are at risk in the United States.