Paleozoic jawless fish

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

Cephalaspidomorphi (alternatively called Monirhina, or simply cephalaspids) is a class of jawless fishes that is presently regarded as uniting the osteostracans, galeaspids and pituriaspids. Most biologists regard this taxon as extinct, but the name is still sometimes used in the classification of lampreys because they were once thought to be descended. If lampreys are included they would extend the known range of the group from the Silurian and Devonian periods, when they are traditionally assumed to have lived, to the present day. Modern works typically assume the cephalaspidimorphs to be th

thumb|254px|right|Anaspids are characterized by a large, tri-radiate spine (red) posteriorly to the series of branchial openings. It is assumed that the most primitive anaspids, such as Pharyngolepis (top), possessed a long, ribbon-shaped, [[ventrolateral fin-fold (green). More advanced forms, such as Rhyncholepis (bottom), possessed a shorter paired fin-fold (green) and enlarged, spine-shaped, median dorsal scutes. – Philippe Janvier]] Anaspida ("shieldless ones") is an extinct group of jawless fish that existed from the early Silurian period to the late Devonian period. They were classi

thumb|right|The osteostracans reconstructed here belong to the major clade Cornuata, whose generalised morphology is exemplified by the [[zenaspidid Zenaspis (bottom left). Some highly derived head-shield morphologies are exemplified by the benneviaspidids Hoelaspis (top right) and Tauraspis (top left), or the thyestiid Tremataspis (bottom right). The latter has lost the paired fins, possibly as a consequence of an adaptation to burrowing habits.]] The class Osteostraci (meaning "bony shells") is an extinct taxon of bony-armored jawless fish, termed "ostracoderms", that lived in what is now No

Thelodonti (from Greek: "nipple teeth") is a class of extinct Palaeozoic jawless fishes with distinctive scales instead of large plates of armor. These fish lived in both freshwater and marine environments, first appearing during the Late Ordovician, and perishing shortly after the Frasnian–Famennian extinction event in the Late Devonian.

Loganellia is a genus of jawless fish which lived between 430 and 370 million years ago, during the Silurian and Devonian periods of the Paleozoic. Loganellia belonged to the Thelodonti class and like other thelodonts possessed scales instead of plate armor.

Ateleaspis is an extinct genus of primitive ostracoderm fish that lived in the Silurian period (Wenlock Epoch) to the Lower Devonian. Like other ostracoderms, Ateleaspis had a head shield similar to that of Cephalaspis. Species from Silurian period were found in Norway and Scotland, but now has been found also in Siberia from Early Devonian period.