 Later in the 19th century , <unk> were classified as various members of <unk> , a name coined by American paleontologist Edward <unk> Cope in 1868 . Cope placed <unk> in the class <unk> , the name then used for <unk> . <unk> means " roof @-@ headed " in Greek , a reference to the wide , flat heads of <unk> and other early <unk> . During this time , <unk> considered <unk> to be amphibians because they possessed three main features : gill arches in juvenile skeletons , indicating they were amphibious for at least the first part of their lives ; ribs that do not connect at the underside of the rib cage ; and deep pits in the skull that were interpreted as space for <unk> glands . 
