 Originally , <unk> were classified according to the structure of their vertebrae . Early forms , with complex vertebrae consisting of a number of separate elements , were placed in the suborder <unk> , and large <unk> aquatic forms with simpler vertebrae were placed in the suborder <unk> . With the recent growth of <unk> , this classification is no longer viable . The basic <unk> condition is found in many primitive <unk> , and is not unique to one group of <unk> . Moreover , the distinction between <unk> and <unk> vertebrae is not entirely clear . Some <unk> have <unk> , <unk> , and <unk> vertebrae at different points in the same <unk> column . Other taxa have intermediate morphologies that do not fit into any category . <unk> is no longer recognized as a group , but <unk> is still considered valid . Below is a simplified taxonomy of <unk> showing currently recognized groups : 
