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Conservation biologists are working to understand the causes of these problems and to resolve them.
The use of the common names "frog" and "toad" has no taxonomic justification.
From a classification perspective, all members of the order Anura are frogs, but only members of the family Bufonidae are considered "true toads".
The use of the term "frog" in common names usually refers to species that are aquatic or semi-aquatic and have smooth, moist skins; the term "toad" generally refers to species that are terrestrial with dry, warty skins.
The Neobatrachia suborder is further divided into the two superfamilies Hyloidea and Ranoidea.
This classification is based on such morphological features as the number of vertebrae, the structure of the pectoral girdle, and the morphology of tadpoles.
The study postulated that Lissamphibia originated no earlier than the late Carboniferous, some 290 to 305 million years ago.
Besides living in fresh water and on dry land, the adults of some species are adapted for living underground or in trees.
The skins of frogs are glandular, with secretions ranging from distasteful to toxic.
Other researchers, while agreeing with the main thrust of this study, questioned the choice of calibration points used to synchronise the data.
They proposed that the date of lissamphibian diversification should be placed in the Permian, rather less than 300 million years ago, a date in better agreement with the palaeontological data.
Frogs are valued as food by humans and also have many cultural roles in literature, symbolism and religion.