 <unk> ( from Greek <unk> ( <unk> , " to cut " ) and <unk> ( <unk> , " <unk> " ) ) is a diverse <unk> of extinct small to giant <unk> — often considered primitive amphibians — that flourished worldwide during the <unk> , <unk> , and <unk> periods . A few species continued into the Cretaceous . <unk> 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 <unk> stage , <unk> , and maturity . Most <unk> were <unk> , although some were almost fully terrestrial , returning to the water only to breed . These <unk> were some of the first vertebrates fully adapted to life on land . Although <unk> are considered amphibians , many had characteristics , such as scales , claws , and armour @-@ like bony plates , that distinguish them from modern amphibians . 
