placoderm definition by American Heritage Dictionary
The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Any of various extinct fishes of the Silurian and Devonian periods, characterized by bony plates of armor covering the head and flanks, hinged jaws, and paired fins.
Origin: New Latin Placodermī, class name : Greek plax, plak-, flat stone, plate; see plāk-1 in Indo-European roots + -derm.
placoderm - Science Definition
Any of various extinct fishes of the class Placodermi of the Silurian and Devonian Periods, characterized by bony plates of armor covering the head and flanks. The bodies of placoderms were spindle-shaped or flattened, and their skeletons were usually partially bony and included a cranium. Placoderms were the first group of fish to evolve jaws, but are not closely related to the jawed fish of today.