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The Placodermi are armoured prehistoric fishes known from fossils dating from the late Silurian to the end of the Devonian Period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked. Placoderms were among the first of the jawed fish, their jaws likely evolving from the first of their gill arches. There are studies that attribute to the Placodermi the first development of teeth. The first identifiable Placoderms evolved in the late Silurian; they disappeared in the Late Devonian extinctions. The first appearance of late Silurian placoderm fossils, in China, show the fishes already differentiated into Antiarchs and Arthrodires; apparently Placoderm diversity originated long before the Devonian, somewhere in the middle Silurian, though earlier fossils of basal Placodermi, have yet to be discovered in these particular strata. The earliest studies of placoderms were published by Louis Agassiz, in his five volumes on fossil fishes, 1833 – 1843. The work of Dr. Erik Stensiö, at the Swedish Museum of Natural History, Stockholm, from the late 1920s established the details of placoderm anatomy, and identified them as true jawed fishes related to sharks.
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