Frogs and toads, the Anura, are amphibians, a group of vertebrates that first evolved in the Devonian era, about 400 million years ago. Two other groups of amphibians are the Caudata, salamanders and newts, and the Gymnophiona, caecilians. More than four thousand species of frogs and toads currently inhabit the planet. In contrast, about 400 species of salamanders and newts and 165 species of caecilians are extant. Frogs and toads thus account for approximately 88 percent of all living amphibians. Amphibians have many unique characteristics that separate them from other vertebrates. In terms of skeletal features, the skull articulates with the vertebral column by a specialized vertebra called the atlas, and ribs are lacking in most frogs and toads. Many amphibians have a twophase life history, an aquatic larval stage and a terrestrial adult stage, although some species may be entirely terrestrial or entirely aquatic. Amphibian skin is unlike that of other vertebrates in that it is glandular and lacks scales, feathers, or hair. Respiration is by means of the skin, which is porous, freely permitting the exchange of oxygen and carbon dioxide, with gills in aquatic larvae and adults, and lungs in terrestrial species. Scientists classify frogs and toads into twentyfive families. Although the terms "frog" and "toad" are commonly used, toads are simply a specialized group of frogs. The true toads are in the family Bufonidae. They are distinguished from other families by a number of osteological (bony) features, and they also typically have thick, glandular skin with wartlike pustules. Many species have large parotoid glands that produce toxic secretions located on the back of the head. In contrast, most frogs have smooth skin and no parotoid glands. Toads in the family Bufonidae can generally withstand drier conditions than frogs. Other kinds of frogs have common names that include the word "toad", such as spadefoot toads, which are in the family Pelobatidae, or narrowmouthtoads, which are inthe family Microhylidae.
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