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 (nā′thəl, năth′əl)

[Greek gnathos, jaw; see genu- in Indo-European roots + -al.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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The blades, called gnathal plates, looked so peculiar that most scientists thought that the three-part jaw originated in an early bony fish and that placoderms were just a side branch in the vertebrate family tree.
It is a member of an extinct group of fish called placoderms whose heads and bodies were mostly covered in armor, with jaws that had bony plates, called gnathal plates, which acted like teeth.
In other, more derived crustacean lineages, the most anterior thoracic segments are transformed into maxillipeds - supplementary feeding appendages that resemble in part the gnathal segments.