The typical frog has long hind legs, a large head, a short body, and no tail; an amphibian, it may live entirely in water or may spend much of its life on land. Frogs are found on every continent except Antarctica. They are thought to have evolved from a tailed, four-limbed, amphibious ancestor; their means of locomotion has changed since the time of that ancestor as a result of the loss of the tail, reduction in the number of trunk vertebrae, and lengthening of the hind legs and feet. Their feeding system has also altered, either because the use of the tongue was virtually abandoned or, conversely, because its prey-catching use was stressed. Frogs have adapted to a great variety of ecological situations.
The classification of frogs at the family level is currently debated. It is usually determined by characteristics of skeletal and muscular structures in larvae and adults, features of the frogs' life histories, and certain genetic and biochemical components. One current theory states that the Leiopelmatidae of New Zealand and the Pacific Northwest are the most primitive living frogs. The Discoglossidae of Europe, Asia Minor, China, northwestern Africa, and the Philippines are also considered primitive. The family Pipidae of South America and Africa south of the Sahara is a highly specialized group; the African clawed toad, Xenopus laevis, is a member of this family. The southern Mexican Rhinophrynidae, with its single species, is related to pipids. The Pelobatidae, spadefoot toads of Europe, North America, and Southeast Asia, and the Pelodytidae, with only two species, are a distinct group. The family Ranidae�the true frogs, such as the leopard frog and bullfrog�is large and widely distributed in North and South America, Europe, Asia, and northern Australia. Ranid relatives include the following: the Hyperoliidae of Africa, Madagascar, and the Seychelles; the Mantellidae, also of Madagascar; the Hemisotidae and Arthroleptilidae of Africa; and the Rhacophoridae of Africa and Southeast Asia. The Bufonidae, or true toads, inhabit all landmasses except Australia, New Zealand, New Guinea, and Greenland. Several Central and South American groups are closely related to bufonids, as are the Sooglossidae, two genera on the Seychelles Islands and Madagascar. The Heleophrynidae of South America and the Myobatrachidae of Australia and New Guinea have affinities with the Leptodactylidae of the New World. Also related are the Hylidae, widespread only in the Americas but with one genus, Hyla, occurring nearly worldwide; the arrow poison frogs, Dendrobatidae, and the green-boned frogs, Centrolenidae, of Central and South America; the Pseudidae of South America; and the Rhinodermatidae of Chile. The Microhylidae of the Americas, Southeast Asia, and Africa constitute a separate group.
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