The Placodermi are fish known from fossils dating to the Devonian period. The front of its body and head were covered by armoured plates and the rest of its body was scaled or naked.
The placoderms are among the most ancient of jawed fish, and, along with the Acanthodii, the only class of (gnathostomes) to become completely extinct.
Most placoderms were quite small, about 10 or 15 cm in length, but one specialized group of carnivorous types, Dinichthyidae included a few genera reached 4 to 9 meters in length, making them the largest carnivore of the Devonian.
The placoderms were seemingly little affected by the extinction at the end of the Frasnian Age, yet the group dwindled during the Famennian and died out at the Devonian/Carboniferous boundary, without leaving any descendents.
Placoderms bore heavy bony armor on the head and neck, often with an unusual joint in the dorsal armor between the head and neck regions; this joint apparently allowed the head to move upwards as the jaw dropped downwards, creating a larger gape.
In a sense, placoderms represent an "early experiment" in the evolution of jawed fish; they radiated into a number of body shapes and ecological niches, which were occupied by other fish lineages after the extinction of the placoderms.
Placoderms survived until the very end of the Devonian, and their extinction appears to have been quite sudden, but its causes are still unknown.
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