Placoderms
Dunkleosteous was an enormous placoderm from the Devonian period, 380–360 million years ago. It measured up to 10 meters in length and weighed up to 3.6 tons. As gnathostomes, they were more mobile and could exploit more food resources than the agnathostomes.
Source
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"OpenStax College, Fishes. October 17, 2013."
http://cnx.org/content/m44687/latest/Figure_29_02_03.jpg
OpenStax CNX
CC BY 3.0.
