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The Osteostraci, or osteostracans, are a major clade (about 200 species) of fossil, armored jawless vertebrates which lived from the Early Silurian (about 430 million years) to the Late Devonian (about 370 million years). Most of them have a characteristic horseshoe-shaped head, which consists of a massive endoskeletal skull, covered with a shield of dermal bone. On the dorsal surface of the head are the closely-set eyes, a pineal foramen, and a median, keyhole-shaped nasohypophysial opening. In addition, there are peculiar "fields" (in fact, depressions of the braincase, covered with loose platelets of dermal bone), which have been regarded as either sense organs or electric organs. The mouth and gill openings are, like in the Galeaspida, situated on the ventral side of the head. Osteostracans also have large, pad-shaped paired fins. Most osteostracans are about 20 to 40 cm. in total length, but some species could be extremely small (about 4 cm in length). The largest species is about one metre in length.
Osteostracans, also known as "cephalaspids", played an important role in the history of vertebrate palaeontology, as they were the first fossil jawless fishes whose internal anatomy has been described in detail, thereby raising heated debates about the organization of the primitive vertebrate head.
Most osteostracans lived in marginal, marine environments, some of them being regarded as fresh water. Their horseshoe-shaped headshield suggests a benthic mode of life, but it is possible that some of them, such as the Boreaspididae or the Kiaeraspidida were more active swimmers. They are most diverse in the Early Devonian and become very rare in the Middle and Late Devonian. Osteostracans are widely distributed in North America, Europe, Siberia, and Central Asia, north of the Tian-Shan.