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Ostracoderms ("shell-skinned") are any of several groups of extinct, primitive, jawless fishes that were covered in an armor of bony plates. They belong to the taxon Ostracodermi, and their fossils are found in the Ordovician and Devonian Period strata of North America and Europe. They were often less than 30 cm (1 ft) long and were probably slow, bottom-dwelling animals.

Another innovation of ostracoderms was the use of gills not for feeding, but exclusively for respiration. In all previous life that had them, gills were used for both respiration and feeding. They had separate pharyngeal gill pouches along the side of the head, which were permanently open with no protective operculum. Unlike invertebrates that use cilliated motion to move food, ostracoderms used their muscular gill pouch to create a suction that pulled in small and slow moving prey.

Ostracoderms existed in two major groups, the more primitive heterostracans and the cephalaspids. The cephalaspids improved over the heterostracans because they had lateral stabilizers for more control of their swimming.

After the appearance of jawed fish (placoderms, acanthodians, sharks, etc.) about 400 million years ago, most ostracoderm species underwent a decline, and the last ostracoderms became extinct at the end of the Devonian period.

The Subclass Ostracodermi has been placed in the Superclass Agnatha along with the extant Subclass Cyclostomata, which includes lampreys and hagfishes. The term does not often appear in classifications today because it is paraphyletic or polyphyletic. However, "ostracoderm" is still used as an informal term for the armored jawless fishes of the Paleozoic.

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