Amphibians are studied by scientists because of their role in the evolution of life. Fossil remains of the earliest known amphibians indicate they probably developed from fishes called crossopterygians. These fishes possessed an early form of lungs. Although they also had gills, their lungs allowed them to absorb the oxygen necessary to live when they were on land. These fishes also developed moveable, paired fins to help them move on dry land. About 350 million years ago, during a time of alternating dry and wet periods called the Devonian period, the early amphibians could cross land to find other bodies of water if the ones they inhabited dried up. They became the first four-footed animals. (See also Animals, Prehistoric.)
The earliest known amphibians are believed to be the ichthyostegalians. These creatures had short stubby legs with five toes and a tail fin. They had scales on their bellies and tails.
Most early amphibians were much larger than the species that exist today. Some of them were as long as 15 feet (4.6 meters). They fed on large insects. There were no other land animals. They were the dominant land animals for at least 75 million years.
After the Devonian period came the Coal, or Carboniferous, period during which many land areas were under water. This environment was hospitable to the amphibians. At the end of this period the land became dryer. Those species that best adapted to the new conditions evolved into the early reptiles.
Main Page - Kinds of Amphibians - Physical Characteristics - Behavior
The Evolutionary Record - Anatomy of the Frog