Arthrodira

Arthrodira is an order of extinct armoured, 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.

Arthrodira
Temporal range: Devonian
Dunkleosteus terrelli
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: †Placodermi
Order: †Arthrodira
Woodward, 1891
Suborders and infraorders

basal

  • Actinolepina

Phlyctaenioidei

  • Phlyctaeniina

Brachythoraci

and see text

Lower jaw plate of Eastmanosteus pustulosus from the Middle Devonian of Wisconsin

Greek for "jointed neck", the arthrodires had movable joint between armor surrounding the head and body. 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 1 to as much as 6 m in length. In contrast, the long-nosed Rolfosteus measured just 15 cm.

Heterosteus ingens from the Middle Devonian of Estonia

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.

Systematics[1]

Below is a cladogram from Dupret et al. (2009).[2]

 
 

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

ORDER ARTHRODIRA

Genera incertae sedis

Arthrodira of unresolved relationships

Timeline of genera
DevonianFamennianFrasnianGivetianEifelianEmsianPragianLochkovianStenosteusParamylostomaHoldeniusGymnotrachelusGorgonichthysTitanichthysSelenosteusDiplognathusAfricanaspisAspidichthysSynaucheniaRhinosteusPachyosteusMicrosteusErromenosteusEnseosteusBraunosteusOxyosteusBrachydeirusHadrosteusDinichthysHeintzichthysGroenlandaspisCosmacanthusRolfosteusLatocamurusTubonasusPlourdosteusMelanosteusMcnamaraspisIncisoscutumHarrytoombsiaFallacosteusCamuropiscisBruntonichthysDunkleosteusYinosteusTropidosteusRhenonemaKiangyousteusHollardosteusHeterosteusEastmanosteusHolonemaBelemnacanthusTaemasosteusParabuchanosteusGoodradigbeeonErrolosteusBuchanosteusArenipiscisTiaraspisMurmurDevonianFamennianFrasnianGivetianEifelianEmsianPragianLochkovian

Footnotes
  1. For sources, see Haaramo (2005)
  2. Dupret, V.; Zhu, M. I. N.; Wang, J. N. Q. (2009). "The morphology of Yujiangolepis liujingensis (Placodermi, Arthrodira) from the Pragian of Guangxi (south China) and its phylogenetic significance". Zoological Journal of the Linnean Society. 157: 70. doi:10.1111/j.1096-3642.2009.00519.x.
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See also

List of placoderms

References
  • Long, John A. (1996): The Rise of Fishes: 500 Million Years of Evolution. Johns Hopkins University Press, Baltimore. ISBN 0-8018-5438-5
  • Sepkoski, Jack (2002). "A compendium of fossil marine animal genera (Placodermi entry)". Bulletins of American Paleontology. 364: 560. Archived from the original on 2004-10-14. Retrieved 2011-05-20.
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