Arthrodire
| Arthrodire | |
|---|---|
 | |
| Dunkleosteus terrelli | |
 | |
| "Coccosteus" cuspidatus | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | †Placodermi |
| Order: | †Arthrodira Woodward, 1891 |
| Suborders and infraorders | |
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 a hinged joint between the dermal skull and armoured pectoral fin support structure covering the front chest. As the lower jaw moved down, the dermal skull could lift because of the hinge, allowing a large mouth opening, depending on the species. 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 many fish and reptiles.
Early arthrodires, such as the genus Arctolepis, were well-armoured fishes with large spines and flattened bodies. The largest member of this group, Dunkleosteus, was a true superpredator of the latest Devonian period, currently considered between 3.5 m (11 ft 5 in) to 4 m (13 ft 1 in) in length. In contrast, the long-nosed Rolfosteus measured just 15 cm (6 in).
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 open-water apex predators to detritus-nibbling bottom dweller. The arthrodires were one of many groups eliminated by the mass extinctions of the Late Devonian. This let cartilaginous fish diversify into the vacant ecological niches during the Carboniferous period.