Class Placodermi
Phylum
Chordata
Subphylum Vertebrata
Infraphylum Gnathostomata
Class Placodermi |
Placodermi
is a class of armoured fish that known from
fossils dating from the late Silurian
to the end of the
Devonian. As fierce as some Placoderms were, they persisted
only 50 million years, which pales in comparison with
the
400 million year history of sharks.
Their head
and 
thorax were covered by articulated
armoured plates, while the rest of the body had no scales
or small scales. The Placoderms were some of the earliest
fish having jaws that
probably evolved from
the first of their gill arches. Placoderms had bony plates
in their jaws that served the function of
teeth they lacked, and did not descend from toothed
ancestors. These plates were razor-like and self-sharpening.
The oldest placoderms in the fossil record are known
from China, from the Early Silurian. However, placoderms
enjoyed their greatest diversity in the Devonian,
during the so-called "Age of Fishes". The
Devonian saw the greatest diversity of a large number
of fish
taxa, including not only placoderms, but armored jawless
fishes, early Chondrichthyes,
and the first Actinopterygian
ray-finned fishes and lobe-finned
fishes. Many of these taxa died out around the end
of the Devonian Period for reasons that
are still not well understood. Placoderms survived
until the very end of the Devonian, and their extinction
appears to have been quite sudden, but its causes are
still unknown. Bothriolepis was
the most successful of all the placoderms, with some
100 species known from every continent, including Antarctica. Dunkleosteus was
one of the largest of the placoderms that reached some
20 feet, and was a violent predator in the Devonian
seas.
Placoderm
Armoured Fish Fossils
