Amphibians are important in an evolutionary sense because early amphibians were the ancestors of all reptiles, birds and mammals. They left the water in the Devonian period about 270 million years ago to become the first vertebrates to colonise land. Their dominance was brief, because one of the early amphibians encased its eggs in a hard shell and so was able to leave the water behind entirely. Those early reptiles went on to dominate the earth for millions of years as the dinosaurs, but some amphibians hung on and are still here today, long after the dinosaurs disappeared.
Amphibians are not as numerous as they once were, but they can occur in large numbers and so are important in the food chains of swamps and fresh water communities. They are characterised by soft glandular skin, four pentadactyl limbs (having five fingers/toes) and a life cycle that begins with an unshelled egg followed by an aquatic larval stage or tadpole, that metamorphoses into an air-breathing adult. Because of their soft skin and unshelled eggs, most amphibians must remain in moist environments near sources of fresh water, although a few have adapted to seemingly dry habitats by burrowing.
There are three orders of modern amphibians. The most numerous and well-known are the frogs and toads, order Salientia. The four thousand known species of frogs are characterised by large heads, short, fat, tailless bodies and long hind legs suitable for hopping. They are common around swamps and marshes and can often be identified by the distinctive calls of the amorous males. Unfortunately frog numbers have plummeted in many places because of swamp drainage and pollution. Frogs need healthy environments to survive.
The second order, Caudata, is made up of about five hundred species of lizard-like newts and salamanders. Salamanders are built like skinks, with thin bodies, long tails and four legs that poke out to the sides, rather than being positioned below the body. They differ from reptiles because they have soft, porous skins and shell-less eggs that must be laid in water. Like frogs, the eggs hatch into an aquatic larval stage characterised by gills. Most salamanders eventually metamorphose into air-breathing adults. The exception is the axolotl or Mexican walking 'fish', which keeps its gills and lives in the water for its entire life.
The third order of Amphibia is the Gymnophiona, known as caecilians. There are about 100 known species of these little, worm-like burrowing animals. Unlike other living amphibians, gymnophionids have small scales on their skin and well-developed copulatory organs. Most are blind and have sensory tentacles between their tiny eyes and their nostrils.
Amphibians have been described as canaries in the coal mine because they are sensitive creatures who are often the first to die because of pollution. They may not seem that important but they are part of the food chain. Storks in Europe are dependent on frogs for survival and may disappear because frog populations there are plummeting, between pollution and draining of swamps. It would be a sad endictment of our stewardship of this planet if these animals, that have survived numerous catastrophes in geological history, were to become extinct under our watch.
For more information:
http://www.gymnophiona.org/ has information on caecilians while
http://animals.nationalgeographic.com/animals/amphibians.html has good information on a variety of amphibians