Frogs

During the 200 million years of their existence, frogs have been evolving in response to varying environmental conditions. Common elements of their adaptations have given rise to clusters of species that share certain morphological, physiological, and behavioral traits. A system of classification has been established, mostly based on morphological features of adult frogs and their larvae. In the one presented here, two suborders, five superfamilies, and 21 families are recognized. It should be emphasized, however, that several systems of classification are recognized by scientists.

The families are often distinguished by such characters as the kind and number of vertebrae, the shape of the pectoral girdle, the presence of ribs, the kind and number of limb bones, and other elements. The structure of the pectoral girdle is an especially distinctive feature that separates large groups of otherwise similar-looking frogs.

The two genera of the family Leiopelmatidae are thought to be relics of an ancient group of frogs. They differ from all other frogs, and are also quite different from each other in habits and distribution, reflecting a long separation. Members of the genus Leiopelma are small terrestrial frogs of New Zealand, whereas Ascaphus, the "tailed frog" of western North America, inhabits streams. (The "tail" is actually an extension of the cloaca of males, and is used to place sperm into the cloaca of the female.) The Discoglossidae are another primitive group, made up of Asian and European pond frogs.

Other primitive frogs include the burrowing frog of Mexico (Rhinophrynus dorsalis) and a number of highly aquatic frogs, the Pipidae of Africa and South America. (One of this group, the African clawed frog [Xenopus laevis], has escaped from captivity and established wild populations in coastal California.) The spadefoot and parsley frogs (Ascaphidae) of North America and Europe are adapted to arid regions. They fall between the "primitive" and "advanced" frogs in structure, and show no close relationship to either. Their larvae are adapted to the rigors of desert life, and have very short periods of aquatic life.

Most of the world's frogs are included in the modern suborder Neobratrachia, with the superfamily Bufonoidea including several large families such as the Australian Myobatrachidae, the South American Leptodactylidae, and the widespread Hylidae. A number of smaller, specialized families are associated with them because of the common possession of a similarly structured pectoral girdle (known as arciferal).

The superfamily Ranoidea includes the large and widespread family Ranidae (the true or water frogs), the arboreal Rhacophoridae of Asia and Madagascar (flying frogs and allies), and the sedge frogs of Africa. Both of the latter appear to be derived from the ranids. The Ranoidea also includes the narrow-mouth toads, or Microhylidae. This widespread family of distinctively shaped ant-eating frogs has a so-called firmasternal pectoral girdle, and does not appear to be closely related to any other frog family, differing especially in their larval morphology. It has been placed only tentatively with the ranoid group.

In general, the species and genera of frogs in any region are relatively easy to recognize on the basis of their external features. These include the skin texture and color, the shape of the pupil of the eye (horizontally elliptic, vertically elliptic, or round), the amount of toe webbing, and the general body proportions, together with the geographic location and habitat. For example, a toad (family Bufonidae) is easily recognized throughout the world because of its warty skin. Water-dwelling frogs (Ranidae) are usually distinguished by their webbed hind feet. Tree-frogs usually have expanded toe-tips, although this can be misleading because some hylids (such as the cricket frogs of North America; genus Acris) have taken up a terrestrial existence and lost their climbing pads. Also, there are three quite different families of treefrogs: the Hylidae, which is primarily South American but with some members in North America and northern Eurasia; Centrolenidae, found only in the American tropics; and the Rhacophoridae of Asia, with a few species in Madagascar and Africa. The classification of the tree-frogs of Australia is still under consideration.

Nevertheless, the skin texture and color, the shape of the pupil of the eye (horizontally elliptic, vertically elliptic, or round), the amount of toe webbing, and the general body proportions together with the geographic location, are useful as local means of identification.