Ostracoderm

thumb|Various ostracoderms of the class Osteostraci ('bony-shields') thumb|Cardipeltis bryanti, a lower Devonian ostracoderm from the [[Bighorn Mountains of Wyoming. Ventral (underside) exposed.|upright]] Ostracodermi () or ostracoderms is an informal group of vertebrate animals that include all armored jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic (excluding jawed fishes and possibly the cyclostomes if anaspids are closer to them) 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.

An innovation of ostracoderms was the use of gills not for feeding, but exclusively for respiration. Earlier chordates with gill precursors used them for both respiration and feeding. Ostracoderms had separate pharyngeal gill pouches along the side of the head, which were permanently open with no protective operculum. Unlike invertebrates that use ciliated motion to move food, ostracoderms used their muscular pharynx to create a suction that pulled small and slow-moving prey into their mouths.