 Modern amphibians ( frogs , <unk> , and <unk> ) are classified in <unk> . <unk> appear to have arisen in the <unk> . Molecular clock estimates place the first <unk> in the Late <unk> , but the first member of <unk> ( frogs and <unk> , but not <unk> ) is estimated to have appeared in the Middle <unk> using the same technique . Using fossil evidence , there are three main theories for the origin of modern amphibians . 
