Arthrodira
From Paleos
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Orders:
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Arthrodira ("Jointed Neck") was an order of extinct armored jawed fishes of the Placodermi class which arose during the Silurian and flourished in the Devonian period before their sudden extinction, surviving for about 50 million years and penetrating most marine ecological niches.
The arthrodires had movable, "ball-in-socket" joints between the plates of their armor surrounding the head and body. The mouth is interesting because as the lower jaw moved down the head shield moved allowing for a large opening. Lacking teeth, like all placoderms, they used the sharpened edges of two pairs of bony plates as a biting surface. The arthrodire's eyeballs, like with most other placoderms, were ossified, thus protecting the eyes in a manner similar to the way the eyes of birds, mosasaurs, and some ichthyosaurs were protected by a bony ring. Early arthrodires, such as the genus Arctolepis, were well-armoured fishes with flattened bodies. The largest genus of this group, Dunkleosteus, was a true superpredator of the latest Devonian period, reaching up to as much as 9 meters in length. In contrast, the long-nosed Rolfosteus measured just 15 cm.
There is a common misconception that the arthrodires were sluggish bottom-dwellers that were out-competed by more advanced fish. Leading to this misconception is that the order changed little during the Devonian era, save for increasing in size. Contrary to this perception, the arthrodires were among the most diverse and numerically successful of any vertebrate clans of the Devonian, occupying a spectrum of roles ranging from giant predator to dirt-nibbling bottom dweller. By the Late Devonian, about one out of every two placoderm species was an arthrodire. The arthrodires' extermination during the Late Devonian extinction allowed other fish such as sharks to diversify into the vacated ecological niches during the Carboniferous period.
