AMPHIBIAN

any member of a class (Amphibia) of cold-blooded vertebrate animals that are midway in evolutionary development between fishes and reptiles. Included among the more than 4,400 living species are the frogs and toads, salamanders and newts, and caecilians. Amphibians (from Greek amphi, both; bios, life) were the first vertebrates to move from an aquatic environment to land, and they are the ancestors of all reptiles, birds, and mammals. Although a few species spend their lives in water, most live at least part of the time on land. They are distributed worldwide, but the majority are found in the tropics. Most amphibians have an aquatic larval, or tadpole, stage that metamorphoses into a terrestrial adult. 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, the lower end of the digestive track. The male salamander deposits sperm on a gelatinous structure, which the female maneuvers into her cloaca. Fertilization is external in most frogs and toads, with the male clasping the female and releasing sperm over the eggs as the female extrudes them (amplexus). The eggs of amphibians do not have shells and must be deposited in water or in a moist place, such as damp soil or on a female's back. 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, practically no tail, a simple intestine, and minute eyes buried in a smooth skin. 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 forelegs, hindlegs, a tail, smooth skin, 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. Frogs and toads (order Anura) constitute the largest group of amphibians. These animals 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. The glandular skin, usually smooth and soft, is occasionally dry and rough in terrestrial species. Amphibians are believed to have evolved from either the lobe-fin fishes (Crossopterygii) or the lungfishes (Dipnoi) of the Early Devonian Period (408 to 387 million years ago). These fishes had lungs, muscular fins supported by bones, and, in some species, passages from nasal sacs to internal openings in the roof of the mouth. In time of drought, these creatures probably propelled themselves out of the drying pools on their muscular fins to search for other water. They then perhaps added insects and other small arthropods to their diet and became less dependent on water. A single caecilian fossil from the Paleocene Epoch (66.4 to 57.8 million years ago) has been found in Brazil. Fossils of sirens have been found in rocks of the Cretaceous Period (144 to 66.4 million years ago) and salamanders in rocks of the Jurassic Period (208 to 144 million years ago). Both sirens and salamanders apparently evolved in the Northern Hemisphere, but the earliest frogs, dating from the Jurassic Period, evolved south of the Equator. any member of the class Amphibia, vertebrates distinguished by their ability to exploit both aquatic and terrestrial habitats. They include the frogs and toads, salamanders and newts, and caecilians. The name, derived from the Greek amphibios meaning living a double life, reflects this dual life strategy. Despite this distinction, however, some species are permanent land dwellers, while other species have a completely aquatic mode of existence. Being the earliest tetrapods to adapt to a terrestrial existence, primitive amphibians are regarded as intermediary life-forms between fishes and reptiles. Modern amphibians are not, however, strictly transitional in their morphology; during their successful radiation throughout the world, they have achieved a variety of modifications that do not exemplify this intermediate status but are specific adaptations to their environment. One such example is the skin, which is kept moist by mucus-secreting glands and is involved in respiration and maintenance of water balance. Members of the three extant orders of amphibians, Anura (frogs and toads), Caudata (salamanders and newts), and Apoda (or Gymnophiona of some authors; caecilians), differ markedly in their structural appearance. Frogs and toads (anurans) are tailless, somewhat squat amphibians with long, powerful hindlimbs modified for leaping. Salamanders and newts have tails and two pairs of limbs of roughly the same size and have less-specialized structures than the other two orders. Caecilians are limbless, wormlike, and highly adapted for a burrowing existence. Traditionally, these orders have been united in one class by the feature unique to them among all tetrapods, the anamniotic egg. Other general defining characteristics include glandular skin that lacks epidermal structures such as hair or feathers, two lungs, a three-chambered heart, and a biphasic lifestyle common to most groups in which aquatic larvae metamorphose into adult forms. Additional reading Overviews of amphibians and their natural history may be found in the following works: Fernand Angel, Vie et moeurs des amphibiens (1947), a popular account; Doris M. Cochran, Living Amphibians of the World (1961, reissued 1972), a lively and beautifully illustrated popular account; Hans Gadow, Amphibia and Reptiles (1901, reprinted 1968), a classic work, old but still worth reading; Robert Mertens, The World of Amphibians and Reptiles (1960; originally published in French, 1939), a beautifully written and illustrated popular book; Robert L. Carroll, Vertebrate Paleontology and Evolution (1988), which contains a comprehensive treatment of fossil amphibians; David M. Green and Stanley K. Sessions (eds.), Amphibian Cytogenetics and Evolution (1991), on the chromosomal and molecular evolution in amphibians; William E. Duellman and Linda Trueb, Biology of Amphibians (1986, reissued 1994), a comprehensive treatment of the group; Martin E. Feder and Warren W. Burggren, Environmental Physiology of the Amphibians (1992), a thorough compendium of amphibian physiology; George R. Zug, Herpetology: An Introductory Biology of Amphibians and Reptiles (1993), a college-level textbook; G. Kingsley Noble, The Biology of the Amphibia (1931, reprinted 1954), the classic English work on amphibians; Darrel R. Frost (ed.), Amphibian Species of the World: A Taxonomic and Geographic Reference (1985), which includes a complete list of the species recognized taxonomically through 1985; and Linda Trueb and R. Cloutier, A Phylogenetic Investigation of the Inter- and Intrarelationships of the Lissamphibia (Amphibia: Temnospondyli), in Hans-Peter Schultze and Linda Trueb (eds.), Origins of the Higher Groups of Tetrapods (1991), a thorough analysis of the relationships of the living orders of amphibians. James A. Peters, Dictionary of Herpetology (1964), is a useful guide, especially for nonspecialists, to understanding the technical terms used by herpetologists. William E. Duellman