Arthrodira

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.

Arthrodira
Temporal range: Devonian, 419.2–358.9 Ma
Arthrodira
Artist's reconstruction of the arthrodire placoderm Dunkleosteus terrelli
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Infraphylum: Gnathostomata
Class: Placodermi
Order: Arthrodira
Woodward, 1891
Subgroups

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. Fossils of Incisoscutum have been found containing unborn fetuses, indicating that arthrodires gave birth to live young.

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]

Arthrodira 
Evolution and extinction of placoderms. The diagram is based on Michael Benton, 2005.

Phylogeny

The order Arthrodira belongs to the class Placodermi, 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.

Arthrodira was traditionally divided into the paraphyletic Actinolepida, the Phlyctaenii (now also paraphyletic), and the Brachythoraci. Phylogenetic studies have since found two of those groups as paraphyletic, as shown in the cladogram below, from Dupret et al. (2009).

Lunaspis broilii

Eurycaraspis incilis

Arthrodira

Yujiangolepis liujingensis

Antarctaspis mcmurdoensis

Wuttagoonaspidae

Yiminaspis shenme

Wuttagoonaspis fletcheri

Aethaspis major

Aethaspis utahensis

Lehmanosteus hyperboreus

Aleosteus eganensis

Simblaspis cachensis

Kujdanowiaspis buczacziensis

Kujdanowiaspis podolica

Erikaspis zychi

Sigaspis lepidophora

Eskimaspis heintzi

Baringaspis dineleyi

Proaethaspis ohioensis

Anarthraspis chamberlini

Heightingtonaspis anglica

Phyllolepida

Gavinaspis convergens

Phyllolepis orvini

Austrophyllolepis sp.

Cowralepis mclachlani

Placolepis budawangensis

Actinolepidae

Bollandaspis woschmidti

Actinolepis spinosa

Actinolepis magna

Actinolepis tuberculata

Bryantolepis brachycephalus

Phlyctaenioidei
Phlyctaeniidae

Phlyctaenius acadicus

Pageauaspis russelli

Groenlandaspidae

Groenlandaspis antarctica

Tiaraspis subilis

Arctaspidae

Dicksonosteus arcticus

Arctolepidae

Arctolepis decipiens

Heintzosteus brevis

Brachythoraci

Antineosteus lehmani

Buchanosteus confertituberculatus

Coccosteus cuspidatus

Actinolepidoidei
Phlyctaeniina

Classification

Arthrodira 
The pelagic selenosteid Amazichthys trinajsticae
Arthrodira 
Artist's reconstruction of the arthrodire placoderm Coccosteus cuspidatus
Arthrodira 
Lower jaw plate of Eastmanosteus pustulosus from the Middle Devonian of Wisconsin
Arthrodira 
Heterosteus ingens from the Middle Devonian of Estonia

Order Arthrodira Woodward, 1891

References

Further reading

  • Long, John A. (1996): The Rise of Fishes: 500 Million Years of Evolution. Johns Hopkins University Press, Baltimore. ISBN 0-8018-5438-5

Tags:

Arthrodira DescriptionArthrodira PhylogenyArthrodira ClassificationArthrodira Further readingArthrodira

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