Arthrodira

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 Coccosteina and see text

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.

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 member of this group, Dunkleosteus, was a true superpredator of the latest Devonian period, reaching 3 to as much as 9 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

Below is a cladogram 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 Phlyctaenii

ORDER ARTHRODIRA

• Clade Aspinothoracidi
  • Superfamily Dinichthyloidea
    • Family Dinichthyidae
    • Family Trematosteidae
    • Family Rhachiosteidae
    • Family Hadrosteidae
    • Family Titanichthyidae
    • Family Bungartiidae
    • Family Selenosteidae
    • Family Mylostomatidae
• Basal infraorder Actinolepina
  • Family Actinolepidae
• Suborder Phlyctaenioidei
  • Basal family Holonematidae
  • Infraorder Phlyctaeniina
    • Family Groenlandaspididae
    • Family Phlyctaeniidae
• Suborder Brachythoraci
  • Infraorder Coccosteina
    • Superfamily Buchanosteoidea
      • Family Buchanosteidae
    • Superfamily Gemuendenaspoidea
      • Family Gemuendenaspidae
    • Superfamily Homosteoidea
      • Family Homostiidae
    • Superfamily Brachydeiroidea
      • Family Brachydeiridae
      • Family Leptosteidae
    • Superfamily Coccosteoidea
      • Family Pholidosteidae
      • Family Coccosteidae
      • Family Incisoscutidae
      • Family Camuropiscidae
    • Superfamily Dunkleosteoidea
      • • (basal genus) Westralichthys
      • Family Panxiosteidae
      • Family Dunkleosteidae

Genera incertae sedis

Arthrodira of unresolved relationships

• Antarctolepis
• Aspidichthys - including Anomalichthys, Aspidophorus
• Callognathus
• Copanognathus
• Carolowilhelmina
• Diplognathus
• Glyptaspis
• Grazosteus
• Hollardosteus
• Laurentaspis
• Machaerognathus
• Maideria
• Murmur - including Euptychaspis, Ptychaspis
• Overtonaspis
• Phylactaenium
• Pinguosteus
• Prescottaspis
• Qataraspis
• Taunaspis
• Timanosteus
• Trachosteus
• Wheathillaspis

Timeline of genera

See also

List of placoderms

References

• Haaramo, Mikko (2005): Mikko's Phylogeny Archive: †Arthrodira. Version of 2005-NOV-15. Retrieved 2007-SEP-13.
• Long, John A. (1996): The Rise of Fishes: 500 Million Years of Evolution. Johns Hopkins University Press, Baltimore. ISBN 0-8018-5438-5

External links

• http://www.palaeos.com/Vertebrates/Units/Unit060/060.100.html

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