The first fossil evidence of a terrestrial vertebrate, an amphibian Ichtyostega, is found in the Devonian Period, between 390 and 350 million years ago. The Devonian Period was a time of great ecological changes, which resulted from massive disruptions of surface features of the Earth.
As water levels rose then receded, the sea left behind organic materials in which plants could thrive. This tended to encourage the development of areas of lush vegetation in swamplike coastal regions. These conditions favored creatures that could obtain their oxygen from the air as well as from water, and it is probable that it was in such environment that the first amphibian evolved.
How Fins Became Limbs
In the late Devonian period, around 365 million years ago, fish-like creatures started venturing from shallow waters onto land with the help of eight-fingered limbs. The limbs had evolved from fins. The loss of genes that guide the development of fins may help to explain how fish evolved into four-limbed vertebrates, according to a study.
Groups of Amphibians
There are three orders of amphibian (class Amphibia):
Although the majority of amphibians inhabits the tropics, representatives of the class are found throughout the world, except in places where there is no water at all, or there is permanent frost. In cold climates, they hibernate during winter. They are cold-blooded, and their body temperature changes with fluctuations in the temperature of the environment.
Amphibians vary in length from an inch or less to more than 5 feet, the largest amphibian being the Japanese salamander (Megalobatracus japonica). The smallest frog measures only 7.7 millimeters – the same size as a fly – Paedophryne amauensis. This tiny amphibian lives in Papua New Guinea and is the smallest animal with a backbone ever discovered.
The skin of all amphibians is toughened on the animal’s upper surface and smooth on the lower surface. All shed their skin regularly, and this molting is under hormonal control. The skin produces slimy or poisonous secretion, which makes these animals unpalatable to most predators. In many species the skin color can change, usually for camouflage or mating purpose.
Respiration takes place in the gills, lungs, lining of the mouth or the external skin. The role of the skin in respiration varies from species for which it is particularly important, to others in which the skin is the sole means of absorbing oxygen from the aerated water of the mountain streams in which they live.
In many species, the skin color can change. In almost all species the sense of touch is well developed, but the development of organs for sensing things at a distance (for sight, hearing and smell) varies greatly. Cave dwellers such as the Olm (Proteus anguinus) have little need of sight, and their eyes are vestigial.
Burrowing Caecilians from Sri Lanka also have only rudimentary eyes. However, they have another organ like a small feeler which is associated with eye muscle and seems to have a sensory function. The existence a hearing sense in amphibians seems to depend on the presence of a voice in the animal.
Apart from anuran larvae (tadpoles), amphibians are carnivores, their primary sources of food being insects and small invertebrates. Caecilians have a more varied diet, which includes other amphibians, fishes, and even some reptiles, small mammals and birds.
Amphibians’ teeth have no roots and grow all the time as they are worn down. Some urodeles, such as cave salamanders have tongues that can protrude to catch prey.
Most amphibians lay eggs, and fertilization usually takes place outside the body. The young pass through a larval stage, although in tailed amphibians and caecilians the distinction between this and the adult form is less obvious than in anurans. Some species (most caecilians) bear live young, other lay eggs that contain partially developed young, and others again produce spawn in which the whole process of development occurs from the first cell division onward.