Placoderms
| The Placodermi are armoured
prehistoric fishes known from fossils dating to 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 the first of the jawed fishes,
their jaws likely evolving from the first of their gill arches. 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, though earlier fossils, of basal Placodermi, have yet to be discovered. The earliest studies of placoderms were published by Louis Agassiz, in his five volumes on fossil fishes, 1833 – 1843. The work of Dr. Erik Stensio, 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. |
