 <unk> have been known since the early 19th century , and were initially thought to be reptiles . They were described at various times as <unk> , <unk> , and <unk> , although these names are now rarely used . Animals now grouped in <unk> were spread out among several amphibian groups until the early 20th century , when they were found to belong to a distinct taxon based on the structure of their vertebrae . <unk> means " cut vertebrae " , as each <unk> is divided into several parts . 
