Amphibians

Traditionally, the amphibians of today have been grouped into three classes:

1. Anura or Salientia - frogs and toads
2. Urodela or Caudata - newts, salamanders, mudpuppies, etc.
3. Gymnophiona - caecilians

These three groups have been classified together in the Lissamphibia.Frogs salamanders, and caecilians may have arisen separately from among different ancestral Paleozoic amphibian groups. One problem in establishing the main lines of amphibian evolution is the lack of fossil records - important groups like the caecilians have very few fossils and the early history of modern amphibian groups is not well known. Another problem is the fact that living anurans and caecilians are highly modified animals; frogs and toads are adapted for jumping, while caecilians have lost their limbs entirely and are modified for a burrowing lifestyle. Because these groups are so diverse and are specific to their habitats, it is not easy to find clues to their ancestry that are common to them all.

The Paleozoic amphibians ( amphibians of that period) are also the topic of controversy over their classification and relationships. In fact, if the likely ancestors of the Amniota are included in the Amphibia, then the Amphibia becomes a paraphyletic group.

Life of amphibians

The young amphibians, like the larval frog or tadpole, spend their early years in the water, breathing through gills in the side of their head in much the same way as fishes do. In many ways they resemble a fish more than they do their parents, for they have no legs, and swim by wriggling their tail, much in the manner of a fish. As they mature, amphibians will usually lose their gills and develop legs. A number of salamanders, such as the North American "mudpuppy" and the Mexican axolotl, develop legs but retain their larval gills and stay in the water throughout their lifetimes. This is a classic example of an evolutionary phenomenon known as neoteny - the retention of larval or juvenile features in mature adults.

Frogs and toads are distinguished by a segment in the hindleg, formed by two elongated tarsal bones, that is used for hopping and swimming. Teeth are usually found in the upper jaw.

In caecilians and salamanders, fertilization is usually internal. The male caecilian introduces sperm into the female by means of a protrusible portion of the cloaca - in the lower end of the digestive track. The male salamander deposits sperm on a gelatinous structure, which the female maneuvers into her cloaca where the ovum is present.

Most amphibians have soft skin which easily absorbs water, which puts them in very close contact withtheir surroundings. It also makes them particularly susceptible to certain toxins and pollutants released into the water by man. This may be why the number of amphibian species, and the size of many amphibian populations, has been declining at an increasing ratein recent years. Amphibians may be among the first of the organisms to suffer from the effects of global pollution and climate change - providing an early warning of environmental degradation.

There are three living groups of amphibians, all of which differ markedly in structure. The caecilians (order Apoda, or Gymnophiona of some authorities) are burrowing, wormlike animals that have no limbs and practically no tail, a simple intestine, and tiny eyes deep in a smooth skin. They seem to be rather primitive in this group. They are confined to moist habitats in the tropics.

The salamanders and newts (order Caudata) belong to a second group, prevalent in the southeastern United States and in Mexico. Salamanders have more distinct features, including forelegs, hindlegs, a tail and a neck. Teeth are found on the jaws and roof of the mouth. Some species remain in the water as permanent larvae with gills. The sirens, one of the two aquatic genera of salamanders, are elongated aquatic animals without hindlimbs but with anterior pectoral girdles to which the front legs are attached. They breathe through gills or gulp air at the surface. The eyes are buried in the smooth skin, and teeth are attached to the roof of the mouth. Tail fins assist locomotion.