Arthrodira (Greek for "jointed neck") is an
order of
extinct armored, jawed fishes of the class
Placodermi that flourished in the
Devonian period before their sudden extinction, surviving for about 50 million years and penetrating most marine
ecological niches. Arthrodires were the largest and most diverse of all groups of placoderms.[1]
Description
Arthrodire placoderms are notable for the movable joint between
armor surrounding their heads and bodies. Like all placoderms, they lacked distinct
teeth; instead, they used the sharpened edges of a bony plate on their jawbone as a biting surface. The eye sockets are covered by a
bony ring, which supports the
eye, 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 as much as 6 m in length. In contrast, the long-nosed Rolfosteus measured just 15 cm.[2] Fossils of Incisoscutum have been found containing unborn fetuses, indicating that arthrodires gave birth to live young.[3][4]
A common misconception is the arthrodires (along with all other placoderms) were sluggish bottom-dwellers that were outcompeted 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.[citation needed]
Phylogeny
The
order Arthrodira belongs to the
classPlacodermi, the large group of extinct prehistoric armored fish that is thought to have diverged over 400 million years ago from all
sharks and
bony fishes (and thus also all subsequent
tetrapods, including mammals, birds, reptiles and amphibians). However, recent
phylogenetic studies have found Placodermi to be
paraphyletic, and rather an
evolutionary grade towards
Eugnathostomata, the
clade grouping that contains sharks, bony fish, and all tetrapods.[6][7]
^Dennis, Kim, and R. S. Miles. "Eubrachythoracid arthrodires with tubular rostral plates from Gogo, Western Australia." Zoological Journal of the Linnean Society 67.4 (1979): 297–328.
^You-An Zhu; Min Zhu (2013). "A redescription of Kiangyousteus yohii (Arthrodira: Eubrachythoraci) from the Middle Devonian of China, with remarks on the systematics of the Eubrachythoraci". Zoological Journal of the Linnean Society. 169 (4): 798–819.
doi:
10.1111/zoj12089.