Arthrodira

Arthrodira Temporal range: Late Silurian? to Late Devonian
Dunkleosteus terrelli
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Infraphylum:	Gnathostomata
Class:	Placodermi
Order:	Arthrodira Woodward, 1898
Suborders and infraorders
basal Actinolepina Phlyctaenioidei Phlyctaeniina Brachythoraci Coccosteina and see text

Arthrodira is an order of extinct armoured jawed fishes of the Placodermi class 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 metres in length. In contrast, the long-nosed Rolfosteus measured just 15 cm.

Heterosteus ingens from the Middle Devonian of Estonia

There is a common misconception that the arthrodires (along with all other placoderms) 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, 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

ORDER ARTHRODIRA

• Clade Aspinothoracidi
  • Superfamily Dinichthyloidea
    • Family Dinichthyidae
    • Family Trematosteidae
    • Family Rachiosteidae
    • Family Pachyosteidae
    • 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
  • Basal family Heterosteidae
  • Infraorder Coccosteina
    • Superfamily Buchanosteoidea
      • Family Buchanosteidae
    • Superfamily Gemuendenaspoidea
      • Family Gemuendenaspidae
    • Superfamily Homosteoidea
      • Family Homosteiidae
    • Superfamily Brachydeiroidea
      • Family Brachydeiridae
    • Superfamily Coccosteoidea
      • Family Pholidosteidae
      • Family Coccosteidae
      • Family Plourdostenidae
      • Family Torosteidae
      • Family Incisoscutidae
      • Family Camuropiscidae
    • Family Dunkleosteidae

Genera incertae sedis

Arthrodira of unresolved relationships

• Antarctolepis
• Aspidichthys - including Anomalichthys, Aspidophorus
• Atlantidosteus
• Callognathus
• Copanognathus
• Glyptaspis
• Golshanichthys
• Grazosteus
• Hollardosteus
• Laurentaspis
• Machaerognathus
• Murmur - including Euptychaspis, Ptychaspis
• Overtonaspis
• Phylactaenium
• Prescottaspis
• Qataraspis
• Taemasosteus
• 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
• Sepkoski, Jack (2002). "A compendium of fossil marine animal genera (Placodermi entry)". Bulletins of American Paleontology 364: p.560. http://strata.ummp.lsa.umich.edu/jack/showgenera.php?taxon=564&rank=class. Retrieved 2011-05-20. 

External links

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