Placoderms are a group of extinct armored fish. They are noted for
their heavily armored head and thorax, which are covered with armor
plates (whereas the rest of the body either has scales or is unarmored
depending on the particular species). Additionally, Placoderms
are noteworthy for using sharp bony plates as teeth
(recent research suggests that Placoderms may have evolved
their jaws independently from other jawed vertebrates).
The earliest known fossils
date from the mid-Silurian period of the Paleozoic Era
(about 430 million years ago) and are found in
China.
During this time, Placoderms are not believed to have been uncommon,
although fossils
from rocks of this age seem to be relatively rare - paleontologists
attribute this to Placoderms living in environments where
fossilization
was unlikely.
During the following Devonian period (beginning about 408 million years
ago), a diverse array of Placoderms are known to have lived, in both freshwater
and saltwater environments.
Placoderms however under went a rapid decline
during and after the late Devonian mass extinction (about 364 million years ago),
and were completely extinct by the end of the Devonian period (about 360 million
years ago).
The first scientific descriptions and studies of Placoderms were published by
Louis Agassiz in the first half of the 19th century. Since that time,
a variety of theories about Placoderms classification have been proposed
(in the past, they have been at times described as related to jawless fish, related to
cartilaginous fish such as sharks, and even as shelled invertebrates),
however today, they are usually classified to fall within the
Gnathostomata
(the group of animals that includes all jawed fish and other vertebrates),
but to be a sister group of all other known Gnathostomata.
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![Isolation of Sexual Strains of Placoderm Desmids [extracted from the Bulletin of the Torrey Botanical Club]](https://m.media-amazon.com/images/I/01RmK+J4pJL._SL160_.gif){kind=small}
