Placoderms were early jawed fish that lived in the Silurian and Devonain (439-362 Mya). The name placoderm means 'plated skin' and refers to the dermal armour of large plates that covered the head and trunk region of the majority of the taxa in the Class Placodermi.
Although placoderms have been recorded from the Silurian of China, they do not become widespread until the Devonian, when they briefly became one of the most successful groups of fishes before their extincion in the end Devonian mass extinctinon event.
The mass extinction at the end of the Devonian was a drawn out affair compared to the other "Big Five" mass extinctions and was made up of a sucession of 8-10 events. The cause remains unknown, but it may have been due to global cooling caused by a fall in atmospheric CO2 levels or a series of bolide impact events.
The main victims of this event were marine organisms that lived in shallow tropical seas such as the goniatites (early ammonoids), benthic foraminifera, brachiopods, conodonts, rugose and tablulate corals, jawless fishes, placoderms, stromatoporoid sponges and trilobites.
In total out of 70 families of fishes present in the Late Devonian, 51 died out , an extinction rate of 73%. Even the families that survived into the Carboniferous were much depleted in species diversity.
Author: Simon Neil Atkinson
e-mail address: firstname.lastname@example.org
Last updated: 14/11/2004
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