The Oststracoderms are a class of extinct fish. They were bony-armoured jawless fish, the Osteostraci ("bony shields"). They lived in what is now North America, Europe and Russia from the Middle Silurian to Upper Devonian. They were the first fish to use gills just for respiration.
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Cardipeltis bryanti, a lower Devonian ostracoderm from the Bighorn Mountains of Wyoming. Ventral (underside) exposed.
Ostracoderm Facts for Kids. Kiddle Encyclopedia.