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.
Placoderms were armored fish that lived between 430 and 360 million years ago
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