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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 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 notochordThe notochord consists of a rod of cells situated on the ventral aspect of the neural tube; it constitutes the foundation of the axial skeleton, since around it the segments of the vertebral column are formed. Its appearance synchronizes with that of the, which was the main axial support throughout adult life. They lacked the paired (pectoral and pelvic) finA fin is a surface used to produce thrust while traveling in water, air, or other fluid media. The first use of the word was for the limbs of fish, but has been extended to include other animal limbs and man made devices. The foremost use of fins are to es 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 insectSubclass Apterygota Symphypleona globular springtails Subclass Archaeognatha (jumping bristletails) Subclass Dicondylia Monura extinct Thysanura (common bristletails) Subclass Pterygota Palaeodictyoptera extinct Ephemeroptera (mayflies) Odonata ( dragonfls but lacked the stereo or depth perception of other vertebrates with more frontal eyes.

In all previous life that had them, gillGill In aquatic organisms In aquatic organisms, gills are a respiratory organ for the extraction of oxygen from water and for the excretion of carbon dioxide. Many small aquatic animals absorb oxygen through the surface of their bodies in general, but mors were used for both respiration and feeding. Another innovation of ostracoderms was the use of gills not for feeding, but exclusively for respirationRespiration can refer to: Cellular respiration, which is the use of oxygen in the metabolism of organic molecules. Physiological respiration, which exchanges oxygen and carbon dioxide between an organism and the external environment.. They had separate pharyngeal gill pouches along the side of the head, which were permanently open with no protective operculum. Unlike invertebrateInvertebrate is a term coined by Chevalier de Lamarck to describe any animal without a backbone or vertebra, like insects, squids and worms. He divided them into two groups, the Insecta and the Vermes. However, the invertebrates are not a coherent group os that use cilliaA cilium (plural cilia is a fine projection from a eukaryotic cell that constantly beats in one direction. They are structurally identical to eukaryotic flagella, and the terms are often used interchangeably. In general, though, the term cilium is used whted motion to move food, ostracoderms used their muscular gill pouch to create a suction that pulled in small and slow moving preyIn biology, the term prey refers to an animal that is sought, captured, and eaten by a predator in an act called predation. A prey species is the food of a predator species, and therefore its source of energy and material for activity and growth. The Volt.

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.

The ostracoderms are placed in the Class Agnatha along with the extant Subclass Cyclostomata, which includes lampreys and hagfishes.