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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 Peiod 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 theor oxygen from air as well as from water and it is probable that it was in such environment that the first amphibian evolved.
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.
There are three orders of amphibian (class Amphibia): newts and salamanders (urodela); frogs and toads (Anura); and caecilians (Apoda). 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 devlopment of organs of sensing things at 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, although they have another organ like a small feeler which is associated with eye muscle and seems to have a sensory function. The existence of a sense of hearing in amphibians seems to depend on wether or not the animal has a voice.
Apart from anuran larvae (tadpoles), amphibians are carnivores, their main 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.
The anurans, loosely called frogs and toads, form the largest of the three orders of Amphibia, with some 2,600 species that have adapted to a wider range of habitats than the others and live on every continent except Antarctica. Although most are terrestrial rather than aquatic, some, such as the tree frogs (Hylidae), are primarily arboreal.
The terms "frogs" and "toads" are based on appearance and do not relate to actual genetic distinctions. Anural classification is based on skeletal features, such as the presence of ribs, which distinguishes lower from higher anurans. Anurans show enormous variety.
Most anurans are nocturnal. They mainly depend on their eyes to catch prey; their vision system responds to small, irregular moving objects, and they will not react to even a preferred food source unless it moves.
An important feature distinguishing anurans from other amphibians is their voice. In the breeding season the male anuran's call serves to attract females and enables a female to recognize a male from the same species.
In most frogs and toads eggs and seminal fluids are emitted at the same time; fertilized eggs are then deposited singly or in clumps or strings.
The tailed amphibians (Urodela) make up the second of the 3 orders of Amphibia. There are about 450 species of salamanders and newts, most of which inhabit the temperate regions of the Northern Hemisphere. Many are terrestrial, retreating to crevices in hot, dry weather. Some live in trees, some never leave the water, and some live in the total darkness of deep caves.
Mating behavior differs from that of amphibians. Salamanders and newts have no voice, so mating display is all important. Male alpine newts, for example, develop crests and striking coloration. Many species indulge in complex courtship rituals. During mating the male deposits a spermatophore (sperm pocket), which the female takes up in her cloaca. The sperms are then released as the female releases her eggs. When these hatch, the larvae resembles adults, although they have gills that disappear during metamorphosis.
The third amphibian order, and the one about which least is known, is the Apoda. Apods, or caecilians, have no limbs and look like large worms or smooth snakes. Length vary from just over 2 inches to nearly 5 feet. Nearly all species live in warm-temperate and tropical regions. They usually burrow beneath the ground, seldom being seen in daylight above the ground. Eyes are of little use in such a habitat, but caecilians have developed a sensitive "feeler" or tentacle which probably helps them search for worms and insects that are the main constituents of their diet. Reproduction is by internal fertilization, and it is believed that caecilians either lay eggs or retain the eggs until the young hatch.