Arthrodira (Greek for "jointed neck") is an

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

armored, jawed fishes of the class

Placodermi Placoderms (from Ancient Greek πλάξ 'plax'', ''plakos plate' and δέρμα 'dermaskin') are vertebrate animals of the class Placodermi, an extinct group of prehistoric fish known from Paleozoic fossils during the Silurian and the De ...

that flourished in the

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

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 Resource (biology), resources an ...

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 (Commonwealth English) or armor (American English; see American and British English spelling differences#-our, -or, spelling differences) is a covering used to protect an object, individual, or vehicle from physical injury or damage, e ...

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, tear ...

; 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 An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

, a feature shared by

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s and some

ichthyosaurs Ichthyosauria is an taxonomy (biology), order of large extinction, extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides. Ichthyosaurians thrived during much of ...

. Early arthrodires, such as the genus '' Arctolepis'', were well-armoured fishes with flattened bodies. The largest member of this group, ''

Dunkleosteus ''Dunkleosteus'' is an extinct genus of large arthrodira, arthrodire ("jointed-neck") fish that existed during the Late Devonian period, about 382–358 million years ago. It was a pelagic fish inhabiting open waters, and one of the first verteb ...

'', 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 A body plan, (), or ground plan is a set of morphology (biology), morphological phenotypic trait, features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many. This term, usually app ...

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. They are made of a variety of materials, such as copper, lead, steel, polymer, rubber and even wax; and are made in various shapes and constru ...

- or

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

-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 or superpredator, 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 hig ...

detritus In biology, detritus ( or ) is organic matter made up of the decomposition, decomposing remains of organisms and plants, and also of feces. Detritus usually hosts communities of microorganisms that colonize and decomposition, decompose (Reminera ...

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

Late Devonian extinction The Late Devonian mass extinction, also known as the Kellwasser event, was a mass extinction event which occurred around 372 million years ago, at the boundary between the Frasnian and Famennian ages of the Late Devonian period.Racki, 2005McGh ...

, allowing other fish such as

shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...

s to diversify into the vacated ecological niches during the

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

period.

Phylogeny

The

Arthrodira belongs to the

class Class, Classes, 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 d ...

, 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 cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...

and bony fishes (and thus also all subsequent

tetrapods A tetrapod (; from Ancient Greek τετρα- ''(tetra-)'' 'four' and πούς ''(poús)'' 'foot') is any four- limbed vertebrate animal of the clade Tetrapoda (). Tetrapods include all extant and extinct amphibians and amniotes, with the lat ...

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

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

studies have found Placodermi to be

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

, and rather an

evolutionary grade A grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit. Phylogenetics The concept of evolutionary grades ...

towards Eugnathostomata, the

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

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

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

Phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

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

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

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

Classification

Order Arthrodira Woodward, 1891 * Phlyctaenioidei Miles, 1973 ** Brachythoraci Gross, 1932 (includes the well-known ''

'', '' 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 or is a term used for a taxonomy (biology), 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, uncertainty ...

'' ** '' 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 ''Kujdanowiaspis'' is an extinct genus of arthrodire placoderm fish from the Early Devonian of Podolia (Ukraine), Poland and Spain. ''Kujdanowiaspis'' is known from many fragmentary head shields and body armours. Classification ''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)

Description

Phylogeny

Classification

References

Further reading