Arthrodira
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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 covered by a bony ring, which supports the eye, 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 15cm. 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.[citation needed]
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, 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 Heintz, 1937 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)
Further reading
- Long, John A. (1996): The Rise of Fishes: 500 Million Years of Evolution. Johns Hopkins University Press, Baltimore. ISBN0-8018-5438-5