Phlyctaenioidei

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.

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 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 as much as 6 m in length. In contrast, the long-nosed '' Rolfosteus'' measured just 15 cm.Dennis, Kim, and R. S. Miles. "Eubrachythoracid arthrodires with tubular rostral plates from Gogo, Western Australia." Zoological Journal of the Linnean Society 67.4 (1979): 297–328. 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.

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 and 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).

Classification

Order Arthrodira Woodward, 1891
* Phlyctaenioidei Miles, 1973
** Brachythoraci Gross, 1932 (includes the well-known '' Dunkleosteus'', '' Dinichthys'', etc.)
** Williamsaspididae White, 1952
** Groenlandaspididae Obruchev, 1964
** Arctolepididae Stensiö, 1959
** Phlyctaeniidae Fowler, 1947
* Actinolepidae Gross, 1940
* Phyllolepida Stensiö 1934
* Wuttagoonaspidae Ritchie 1973
* Genera ''incertae sedis''
** ''Aethaspis'' Denison, 1958 (may be within Actinolepidae)
** '' Aleosteus'' Johnson ''et al.'', 2000 (may be within Actinolepidae)
** ''Anarthraspis'' Bryant, 1934 (may be within Actinolepidae)
** ''Antarctaspis'' White, 1968 (previously within Antarctaspididae)
** '' Antarctolepis'' White, 1968
** ''Aspidichthys'' Newberry, 1873
** ''Baringaspis'' Miles, 1973 (may be within Actinolepidae)
** ''Bryantolepis'' Denison, 1958 (may be within Actinolepidae)
** '' Callognathus'' Newberry, 1890
** '' Copanognathus'' Hussakof & Bryant, 1920
** ''Carolowilhelmina'' Mark-Kurik & Carls, 2002
** ''Diplognathus'' Newberry, 1878
** '' Erikaspis'' Dupret, Goujet, & Mark-Kurik, 2007
** '' Eskimaspis'' Dineley & Yuhai, 1984
** '' Glyptaspis'' Newberry, 1890
** '' Grazosteus'' Gross, 1958
** '' Heightingtonaspis'' White, 1969 (may be within Actinolepidae)
** ''Hollardosteus'' Lehman, 1956
** '' Kujdanowiaspis'' Stensiö, 1942 (may be within Actinolepidae)
** '' Lataspis'' Strand, 1932 (may be within Actinolepidae)
** '' Laurentaspis'' Pageau, 1969
** '' Lehmanosteus'' Goujet, 1984 (may be within Actinolepidae)
** '' Machaerognathus'' Hussakof & Bryant, 1919
** ''Maideria'' Lelièvre, 1995
** '' Mediaspis'' Heintz, 1929 (may be within Actinolepidae)
** '' Murmur'' Whitley, 1951
** '' Overtonaspis'' White, 1961
** '' Phylactaenium'' Heintz, 1934
** ''Pinguosteus'' Long, 1990 (may be within Brachythoraci)
** '' Prescottaspis'' White, 1961
** ''Proaethaspis'' Denison, 1978 (may be within Actinolepidae)
** ''Qataraspis'' White, 1969
** ''Sigaspis'' Goujet, 1973 (may be within Actinolepidae)
** '' Simblaspis'' Denison, 1958 (may be within Actinolepidae)
** '' Taunaspis'' Schmidt, 1933
** '' Timanosteus'' Obrucheva, 1962
** '' Trachosteus'' Newberry, 18903 (may be within Coccosteidae)
** '' Wheathillaspis'' White, 1961
** ''Yujiangolepis'' Wang & Dupret, 2009 (previously within Antarctaspididae)

References

Further reading

* (1996): ''The Rise of Fishes: 500 Million Years of Evolution''. Johns Hopkins University Press, Baltimore.

{{Taxonbar, from=Q136606

Prehistoric fish orders
Articles which contain graphical timelines