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

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 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
Phlyctaenii

ORDER ARTHRODIRA

Genera incertae sedis

Arthrodira of unresolved relationships

Timeline of genera

Devonian Famennian Frasnian Givetian Eifelian Emsian Pragian Lochkovian Stenosteus Paramylostoma Holdenius Gymnotrachelus Gorgonichthys Titanichthys Selenosteus Diplognathus Aspidichthys Synauchenia Rhinosteus Pachyosteus Microsteus Erromenosteus Enseosteus Braunosteus Oxyosteus Brachydeirus Hadrosteus Dinichthys Heintzichthys Groenlandaspis Rolfosteus Latocamurus Tubonasus Plourdosteus Melanosteus Mcnamaraspis Incisoscutum Fallacosteus Camuropiscis Bruntonichthys Dunkleosteus Tropidosteus Rhenonema Hollardosteus Eastmanosteus Holonema Belemnacanthus Taemasosteus Parabuchanosteus Goodradigbeeon Errolosteus Buchanosteus Arenipiscis Tiaraspis Murmur Devonian Famennian Frasnian Givetian Eifelian Emsian Pragian Lochkovian

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.

See also

List of placoderms

References

External links

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