Arthrodira
From Wikipedia, the free encyclopedia
| iArthrodira | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
 |
||||||||||
|
|
||||||||||
|
Conservation status: Fossil
|
||||||||||
| Scientific classification | ||||||||||
|
Arthrodira is an order of extinct armored jawed fishes of the Placodermi class who flourished in the Devonian period before their sudden extinction, surviving for about 50 million years and penetrating most marine ecological niches.
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 genus of this group, Dunkleosteus, was a true superpredator of the latest Devonian period, reaching 3 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 arthrodire body plan remained relatively conserved (that is, most arthrodires were bullet or torpedo-shaped)during the Devonian era, 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 clans of the Devonian, occupying a vast spectrum of roles from top 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.
The closest living relatives of the Arthrodires are the sharks and holocephalians.
[edit] External links
[edit] References
Long, John A. The Rise of Fishes: 500 Million Years of Evolution Baltimore: The John Hopkins University Press, 1996. ISBN 0-8018-5438-5
