EVOLUTIONARY PERSPECTIVE (amphibia, living a double life)
Class Amphibia includes frogs. roads, salamanders and caecilians. Class name shows that amphibians move between water and land. They spend one stage or their life in water and another on land. Amphibians are tetrapods. Term tetrapod is a non-taxonomic It is used for vertebrates other than fishes. Most tetrapods are adapted on land.
Evolution of land vertebrates: Many adaptive radiations produced in vertebrates during first 250 million years of vertebrate history. Thus the vertebrates filled most aquatic habitats Prehistoric waters contained many active, powerful predators however vertebrates sere absent on land. Only some arthropods lived there as predators. Therefore, there was no vertebrate predator on land to capture animals that moved near the edge. Some vertebrates developed lungs for breathing and muscular fins for moving. These animals found ample food on land in the form of arthropods. The arthropods are the major component of the diet of most modern amphibians. Thus evolution of amphibians took place. Then adaptive radiations were produced in amphibians. It produced greater variety of amphibians than exists today. Later convergent and parallel evolution took place in amphibians. Then widespread extinction took place.
First amphibian: No one knows what the first amphibian was. The structure of limbs, skulls and teeth of lchthyostega are similar to the earliest amphibians. Two lineages of amphibians are formed in the late Devonian and early Carboniferous periods. These lineage can be differentiated by the structure of roof and the attachment of posterior portion of the skull to each other.
1. Amniotic lineage: One lineage of amphibians became extinct in the late Carboniferous period. An amniotic egg evolved in this group. It resisted dryness. This lineage is called the amniotic lineage. This lineage formed reptiles, birds; and mammals.
2. Nonamniote lineage: A second lineage flourished in the Jurassic period. Most of animals of lineage have become extinct. But some of them gave rise to the three orders of living amphibians. This lineage is called the nonamniote lineage.