Arthrodira (Greek for "jointed neck") is an

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

armored, jawed fishes of the class

Placodermi Placodermi (from Greek πλάξ 'plate' and δέρμα 'skin', literally ' plate-skinned') is a class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devonian period. Their head and thorax were co ...

that flourished in the

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...

period before their sudden extinction, surviving for about 50 million years and penetrating most marine

ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of resources and competitors (for ...

s. Arthrodires were the largest and most diverse of all groups of Placoderms.

Description

Arthrodire placoderms are notable for the movable joint between

armor Armour (British English) or armor (American English; see spelling differences) is a covering used to protect an object, individual, or vehicle from physical injury or damage, especially direct contact weapons or projectiles during combat, or f ...

surrounding their heads and bodies. Like all placoderms, they lacked distinct

teeth A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, ...

; 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 Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1842, alth ...

. Early arthrodires, such as the genus ''

Arctolepis ''Arctolepis'' is an extinct genus of placoderm fish, which lived during the Early Devonian period (408 – 307 million years ago). Fossils of ''Arctolepis'' have been found in what is now Norway and Michigan Michigan () is a state in t ...

'', were well-armoured fishes with flattened bodies. The largest member of this group, ''

Dunkleosteus ''Dunkleosteus'' is an extinct genus of large armored, jawed fishes that existed during the Late Devonian period, about 382–358 million years ago. It consists of ten species, some of which are among the largest placoderms to have ever lived: ...

'', was a true superpredator of the latest Devonian period, reaching as much as 6 m in length. In contrast, the long-nosed ''

Rolfosteus ''Rolfosteus'' is an extinct monospecific genus of arthrodire placoderm from the Early Frasnian stage of the Late Devonian period, found at the Gogo Formation of Western Australia.DENNIS, KIM, and R. S. Miles. "Eubrachythoracid arthrodires with t ...

'' 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 ''Incisoscutum'' is an extinct genus of arthrodire placoderm from the Early Frasnian Gogo Reef, from Late Devonian Australia. The genus contains two species ''I. ritchiei'', named after Alex Ritchie, a palaeoichthyologist and senior fellow of ...

'' 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 A body plan, ( ), or ground plan is a set of morphological features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many. This term, usually applied to animals, envisages a "blueprin ...

remained relatively conserved (that is, the majority of arthrodires were

bullet A bullet is a kinetic projectile, a component of firearm ammunition that is shot from a gun barrel. Bullets are made of a variety of materials, such as copper, lead, steel, polymer, rubber and even wax. Bullets are made in various shapes and co ...

- or

torpedo A modern torpedo is an underwater ranged weapon launched above or below the water surface, self-propelled towards a target, and with an explosive warhead designed to detonate either on contact with or in proximity to the target. Historically, ...

-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 An apex predator, also known as a top predator, is a predator at the top of a food chain, without natural predators of its own. Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the highest trophic lev ...

detritus In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts comm ...

-nibbling

bottom dweller The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning ...

. Despite their success, the arthrodires were one of many groups eliminated by the environmental catastrophes of the

Late Devonian extinction The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch, which collectively represent one of the five largest mass extinction events in the history of life on Earth. The term primarily refers to a major ex ...

, allowing other fish such as

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...

s to diversify into the vacated ecological niches during the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...

period.

Phylogeny

The

Arthrodira belongs to the

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently ...

, the large group of extinct prehistoric armored fish that is thought to have diverged over 400 million years ago from all

sharks Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorp ...

and

bony fishes Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilage ...

(and thus also all subsequent

tetrapods Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct therapsi ...

, including mammals, birds, reptiles and amphibians). However, recent

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...

studies have found Placodermi to be

paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...

, and rather an evolutionary grade towards

Eugnathostomata Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living ...

, the

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

grouping that contains sharks and bony fish and all tetrapods. Arthrodira was traditionally divided into the

Actinolepida, the Phlyctaenii (now also paraphyletic), and the

Brachythoraci Brachythoraci is an extinct suborder of arthrodire placoderms, armored fish most diverse during the Devonian. Phylogeny Arthrodira is divided into three main groups: the paraphyletic Actinolepida and Phlyctaenii, and then the monophyletic B ...

Phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...

studies have since found two of those groups as paraphyletic, as shown in the

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...

below, from Dupret ''et al.'' (2009).

Classification

Order Arthrodira Woodward, 1891 * Phlyctaenioidei Miles, 1973 **

Gross, 1932 (includes the well-known ''

'', ''

Dinichthys ''Dinichthys'' (from el, δεινός , 'terrible' and el, ἰχθύς 'fish') is an extinct monospecific genus of giant, marine arthrodire placoderm from the Late Devonian (Famennian stage), comparable in size, shape, and ecological role ...

'', 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 ' () or ''problematica'' is a term used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertain ...

'' ** '' Aethaspis'' Denison, 1958 (may be within Actinolepidae) ** ''

Aleosteus ''Aleosteus eganensis'' is an extinct arthrodire placoderm fish. Its fossils have been found in the Late Emsian The Emsian is one of three faunal stages in the Early Devonian Epoch. It lasted from 407.6 ± 2.6 million years ago to 393.3 ± 1.2 ...

'' 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

) ** '' 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 Coccosteidae is a family of arthrodire placoderms from the Early to Late Devonian. Fossils appear in various strata in Europe, North America and China. Phylogeny Coccosteidae belongs to the larger clade Coccosteomorphi, which together with it ...

) ** '' Wheathillaspis'' White, 1961 ** '' Yujiangolepis'' Wang & Dupret, 2009 (previously within Antarctaspididae)

Description

Phylogeny

Classification

References

Further reading