Devonian Placoderms

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 Subclass 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.

Ostracoderms are the earliest known vertebrates. They had bodies that were encased in a covering of dermal bone in the form of carapace or scales. Their internal skeletons were cartilaginous and rarely preserved. They had a notochord, which was the main axial support throughout adult life. They lacked the paired (pectoral and pelvic) fins of more advanced fish. In some cases, small spines were present at the points where paired fins developed in later fishes. Though they did lack jaws, they did have some moveable mouth plates. These mouth plates were arranged around a small circular mouth, which was located farther forward in the head than in jawed vertebrates. The lateral eyes were probably better than the sight organs of most insects but lacked the stereo or depth perception of other vertebrates with more frontal eyes.

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 groups, the more primitive heterostracans and the cephalapsids. The cephalapsids improved over the heterostracans because they had lateral stabilizers for more control of their swimming.

After true fish appeared about 400 million years ago, most ostracoderms rapidly became extinct.