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Osteostracans are known from the Silurian and Devonian of Europe, Siberia and North America. They are characterized by peculiar "cephalic fields" of unknown function, on the dorsal surface of the head-shield (red). Although jawless, they share with jawed vertebrates well-developed paired fins, an epicercal tail, cellular bone, and a sclerotic ring in eyes. Their mouth and gill opening are ventrally placed, as in galeaspids and pituriaspids. Their median, dorsal, nasohypophysial aperture, anterior to the eyes, is strikingly similar to that of lampreys but is now regarded as a convergence. All the osteostracans reconstructed here belong to the major clade Cornuata, whose generalized 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.