placoderm
noun
plac·o·derm ˈpla-kə-ˌdərm 
: any of a class (Placodermi) of extinct chiefly Devonian fishes with an armor of bony plates and primitive jaw structures
Example Sentences
Recent Examples on the Web From one layer to the next one above, no more trilobites or nightmarish placoderms or dimetrodons—entire animal reigns and ecosystems have vanished. 
Manu Saadia, Ars Technica, 4 Sep. 2017
Manu Saadia, Ars Technica, 4 Sep. 2017 Word History
Etymology
New Latin Placodermi, ultimately from Greek plak-, plax + derma skin — more at derm-
First Known Use
circa 1859, in the meaning defined above
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