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. Scientific classification or biological classification is how biologists group and categorize extinct and living species of organisms. ... Phyla Porifera (sponges) Ctenophora (comb jellies) Cnidaria (coral, jellyfish, anemones) Placozoa (trichoplax) Subregnum Bilateria (bilateral symmetry) Acoelomorpha (basal) Orthonectida (parasitic to flatworms, echinoderms, etc. ... Typical Classes Subphylum Urochordata - Tunicates Ascidiacea Thaliacea Larvacea Subphylum Cephalochordata - Lancelets Subphylum Myxini - Hagfishes Subphylum Vertebrata - Vertebrates Petromyzontida - Lampreys Placodermi (extinct) Chondrichthyes - Cartilaginous fishes Acanthodii (extinct) Actinopterygii - Ray-finned fishes Actinistia - Coelacanths Dipnoi - Lungfishes Amphibia - Amphibians Reptilia - Reptiles Aves - Birds Mammalia - Mammals Chordates (phylum Chordata) include the vertebrates, together with... Typical classes Petromyzontidae (lampreys) Placodermi - extinct Chondrichthyes (cartilaginous fish) Acanthodii - extinct Actinopterygii (ray-finned fish) Actinistia (coelacanths) Dipnoi (lungfish) Amphibia (amphibians) Reptilia (reptiles) Aves (birds) Mammalia (mammals) Vertebrata is a subphylum of chordates, specifically, those with backbones or spinal columns. ... Groups Myxini (hagfish) Hyperoartia Petromyzontidae (lampreys) Pteraspidomorphi Thelodonti Anaspida Cephalaspidomorphi Galeaspida Pituriaspida Osteostraci Agnatha (Greek, no jaws) is a paraphyletic superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. ... In biology and ecology, extinction is the ceasing of existence of a species or group of taxa. ... Atlantic herring, Clupea harengus: the most abundant species of fish in the world. ... Grays illustration of a human femur, a typically recognized bone. ... A fossil Ammonite Fossils (from Latin fossus, literally having been dug up) are the mineralized or otherwise preserved remains or traces (such as footprints) of animals, plants, and other organisms. ... The Ordovician period is the second of the six (seven in North America) periods of the Paleozoic era. ... The Devonian is a major division of the geologic timescale that extends from the end of the Silurian period (360 million years ago (mya)) to the beginning of the Mississippian subperiod of the Carboniferous (408. ... Stratigraphy, a branch of geology, is basically the study of rock layers and layering (stratification). ... World map showing North America A satellite composite image of North America. ... World map showing Europe Europe is conventionally considered one of the seven continents of Earth which, in this case, is more a cultural and political distinction than a physiogeographic one. ...

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. Subgroups †Conodonta Hyperoartia   Petromyzontidae (lampreys) †Pteraspidomorphi †Thelodonti †Anaspida †Cephalaspidomorphi   †Galeaspida   †Pituriaspida   †Osteostraci Gnathostomata (jawed vertebrates)   †Placodermi   Chondrichthyes (cartilaginous fish)   †Acanthodii   Osteichthyes (bony fish)     Actinopterygii (ray-finned fish)     Sarcopterygii (lobe-finned fish)       Actinistia (coelacanths)       Dipnoi (lungfish)       Tetrapoda (four-limbed vertebrates)         Amphibia (amphibians)         Amniota (amniotic embryo)           Sauropsida (reptiles)             Aves (birds)           Synapsida (mammal... Cartilage is a type of dense connective tissue. ... The notochord is a flexible rod-shaped body found in embryos of all chordates. ... A fin is a surface used to produce lift and thrust or to steer while traveling in water, air, or other fluid media. ... Classes & Orders Subclass:Apterygota Orders Archaeognatha (Bristletails) Thysanura (Silverfish) Monura - extinct Subclass:Pterygota Infraclass: Paleoptera (paraphyletic) Orders Ephemeroptera (mayflies) Palaeodictyoptera - extinct Megasecoptera - extinct Archodonata - extinct Diaphanopterodea - extinct Protodonata - extinct Odonata (dragonflies and damselflies) Infraclass: Neoptera Superorder: Exopterygota Orders Caloneroptera - extinct Titanoptera - extinct Protorthoptera - extinct Grylloblattodea (ice-crawlers) Mantophasmatodea (gladiators) Plecoptera...

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. It has been suggested that Gas exchange be merged into this article or section. ... gills of a Smooth Newt Gills inside of a tuna head In aquatic organisms, gills are a respiratory organ for the extraction of oxygen from water and for the excretion of carbon dioxide. ... Invertebrate is a term coined by Jean-Baptiste Lamarck to describe any animal without a spinal column. ... This article is about an organelle. ... Prey can refer to: Look up Prey in Wiktionary, the free dictionary A prey animal eaten by a predator in an act called predation. ...

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. Groups Myxini (hagfish) Hyperoartia Petromyzontidae (lampreys) Pteraspidomorphi Thelodonti Anaspida Cephalaspidomorphi Galeaspida Pituriaspida Osteostraci Agnatha (Greek, no jaws) is a paraphyletic superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. ... Cyclostomata is a subclass of Agnatha. ... Subfamilies Geotriinae Mordaciinae Petromyzontinae A lamprey is a jawless fish with a toothed, funnel-like sucking mouth, with which most species bore into the flesh of other fishes to suck their blood. ... Genera Eptatretus Myxine Nemamyxine Neomyxine Notomyxine A hagfish is any of several marine chordates of the class Myxini, also known as Hyperotreti. ...