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A Dunkleosteus puking up the remains of a younger, cannibalized Dunkleosteus.
Arthrodira is an order of extinct armoured jawed fishes of the class Placodermi flourished in the Devonian period before their sudden extinction, surviving for about 50 million years and penetrating most marine ecological niches.
Facts
Greek for "jointed neck", the arthrodires had movable joint between 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 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, reaching 3 to as much as 9 metres in length. In contrast, the long-nosed Rolfosteus measured just 15 cm.
There is a common misconception that the arthrodires (along with all other placoderms) were sluggish bottom-dwellers that were out-competed by more advanced fish. Leading to this misconception is that the arthrodire body plan remained relatively conserved (that is, the majority of arthrodires were bullet or torpedo-shaped) during the Devonian period, save for increasing in size. However during their reign the arthrodires were one of the most diverse and numerically successful, if not the most successful, vertebrate orders of the Devonian, occupying a vast spectrum of roles from apex predator to detritus-nibbling bottom dweller. Despite their success, the arthrodires were one of many groups eliminated by the environmental catastrophes of the Late Devonian extinction, allowing other fish such as sharks to diversify into the vacated ecological niches during the Carboniferous period.
In Walking with... Series
Sea Monsters
Sea Monsters, Episode 1
This episode featured Dunkleosteus, the biggest of the armored fish and the only arthodira to be featured in Impossible Pictures up to date. It was an apex predator of the Devonian, overpowering and eating anything it came across, including its juveniles. It tried to eat Nigel too, but the round shark cage prevented that.
Sea Monsters, Episode 2
However, by the end of the Devonian, Dunkleosteus died out with the rest of the arthodires and placoderms, as Nigel Marven explained in the beginning of this episode.