Placoderms are prehistoric fishes, which had a heavy bony armor about the head and neck. The first scant fossil evidence of these fishes is found in the Late Silurian, but by the beginning of the Devonian Period there was a wide diversity of species occupying virtually every niche of their aquatic ecosystem. Placoderm fossils are found in salt, brackish and fresh water. They were some of the first vertebrates to colonize fresh water systems. This diverse group of fishes varied in size from less than a foot to the gigantic Titanichthys, which is over thirty feet in length. Although these fishes dominated their environment, they went completely extinct at a time when many families of marine animals also perished in what is referred to as the Late Devonian Extinction.
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