Arthrodire

The Arthrodires (Arthrodira) are an order of extinct armoured, jawed fishes of the class Placodermi. They flourished in the Devonian period before their sudden extinction, surviving for about 50 million years and living in most marine ecological niches.
Greek for "jointed neck", the arthrodires had movable joints between armour plates surrounding the head and body. The mouth is interesting. As the lower jaw moved down, the head shield moved, allowing a large opening. Lacking distinct teeth, like all placoderms, they used the sharpened edges of a bony plate as a biting surface. The eye sockets are protected by a bony ring, a feature shared by birds and some ichthyosaurs. 
Early arthrodires, such as the genus "Arctolepis", were well-armoured fishes with flattened bodies. The largest member of this group, "Dunkleosteus", was a true superpredator of the latest Devonian period, from 3 to 9 m in length. In contrast, the long-nosed "Rolfosteus" measured just 15 cm.
A common misconception is the arthrodires were sluggish bottom-dwellers that were outcompeted by more advanced fish. But during their reign, the arthrodires were one of the most diverse and numerically successful vertebrate orders of the Devonian. They occupied a vast spectrum of roles from apex predator to detritus-nibbling bottom dweller. The arthrodires were one of many groups eliminated by the mass extinctions of the late Devonian. This let other fish such as sharks diversify into the vacant ecological niches during the Carboniferous period.