Ostracoderms (lit. 'shell-skins') are the armored jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic or polyphyletic, and thus does not correspond to one evolutionary lineage. However, the term is still used as an informal way of loosely grouping together the armored jawless fishes. Ostracoderms existed in two major groups, the more primitive heterostracans and the cephalaspids. The cephalaspids were more advanced than the heterostracans in that they had lateral stabilizers for more control of their swimming.