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placoderm

[plak-uh-durm] /ˈplæk əˌdɜrm/
noun
1.
any of various extinct jawed fishes of the class Placodermi, dominant in seas and rivers during the Devonian Period and characterized by bony armored plates on the head and upper trunk.
Origin
1855-1860
1855-60; < Neo-Latin Placodermi name of the class, plural of placodermus, equivalent to placo- (< Greek; see placoid, -o-) + -dermus -derm
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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British Dictionary definitions for placoderm

placoderm

/ˈplækəˌdɜːm/
noun
1.
any extinct bony-plated fishlike vertebrate of the class Placodermi, of Silurian to Permian times: thought to have been the earliest vertebrates with jaws
Word Origin
C19: from Greek plac-, plax a flat plate + -derm
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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placoderm in Science
placoderm
  (plāk'ə-dûrm')   
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.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Article for placoderm

any member of an extinct group (Placodermi) of primitive jawed fishes known only from fossil remains. Placoderms existed throughout the Devonian period (416,000,000-359,000,000 years ago), but only two species persisted into the succeeding Carboniferous period. During the Devonian they were a dominant group, occurring in all continents except South America in a variety of marine and freshwater sediments.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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