Temnospondyli ( from Greek τέμνειν ( temnein , " to cut " ) and σπόνδυλος ( spondylos , " vertebra " ) ) is a diverse subclass of extinct small to giant tetrapods — often considered primitive amphibians — that flourished worldwide during the Carboniferous , Permian , and Triassic periods . A few species continued into the Cretaceous . Fossils have been found on every continent . During about 210 million years of evolutionary history , they adapted to a wide range of habitats , including fresh water , terrestrial , and even coastal marine environments . Their life history is well understood , with fossils known from the larval stage , metamorphosis , and maturity . Most temnospondyls were semiaquatic , although some were almost fully terrestrial , returning to the water only to breed . These temnospondyls were some of the first vertebrates fully adapted to life on land . Although temnospondyls are considered amphibians , many had characteristics , such as scales , claws , and armour @-@ like bony plates , that distinguish them from modern amphibians .