Ostracoderm definitions

ŏs'trə-kŏ-dûrm'
Any of various extinct primitive jawless fishes of the Ordovician through the Devonian periods, having bodies encased in an armor of bony plates.
noun
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Any of various extinct primitive jawless fishes of the Ordovician through the Devonian Period. Most ostracoderms had their heads or entire bodies encased in fused bony plates. Some or all ostracoderms are sometimes classified together with the modern cyclostomes (lampreys and hagfish).
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Origin of ostracoderm

Greek ostrakon shell ost- in Indo-European roots –derm