Arthrodira
Encyclopedia
Arthrodira is an order of extinct armoured jawed fishes of the Placodermi
class that flourished in the Devonian
period before their sudden extinction, surviving for about 50 million years and penetrating most marine ecological niche
s.
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 3 to as much as 9 metres in length. In contrast, the long-nosed Rolfosteus measured just 15 cm.
There is a common misconception that the arthrodires (along with all other placoderms) were sluggish bottom-dwellers that were out-competed 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 shark
s to diversify into the vacated ecological niches during the Carboniferous
period.
Placodermi
Placodermi is a class of armoured prehistoric fish, known from fossils, which lived from the late Silurian to the end of the Devonian Period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were...
class that flourished in the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
period before their sudden extinction, surviving for about 50 million years and penetrating most marine ecological niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
s.
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
Arctolepis
Arctolepis is an extinct genus of placoderm fish, which lived during the Early Devonian period .Fossils of Arctolepis have been found in Norway and Michigan....
, were well-armoured fishes with flattened bodies. The largest member of this group, Dunkleosteus
Dunkleosteus
Dunkleosteus is a genus of prehistoric fish, one of the largest arthrodire placoderms ever to have lived, existing during the Late Devonian period, about 380-360 million years ago.This hunter, measuring up to and weighing , was a hypercarnivorous apex predator...
, was a true superpredator of the latest Devonian period, reaching 3 to as much as 9 metres in length. In contrast, the long-nosed Rolfosteus measured just 15 cm.
There is a common misconception that the arthrodires (along with all other placoderms) were sluggish bottom-dwellers that were out-competed by more advanced fish. Leading to this misconception is that the arthrodire body plan
Body plan
A body plan is the blueprint for the way the body of an organism is laid out. An organism's symmetry, its number of body segments and number of limbs are all aspects of its body plan...
remained relatively conserved (that is, the majority of arthrodires were bullet
Bullet
A bullet is a projectile propelled by a firearm, sling, or air gun. Bullets do not normally contain explosives, but damage the intended target by impact and penetration...
or torpedo
Torpedo
The modern torpedo is a self-propelled missile weapon with an explosive warhead, launched above or below the water surface, propelled underwater towards a target, and designed to detonate either on contact with it or in proximity to it.The term torpedo was originally employed for...
-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
Apex predator
Apex predators are predators that have no predators of their own, residing at the top of their food chain. Zoologists define predation as the killing and consumption of another organism...
to detritus
Detritus
Detritus is a biological term used to describe dead or waste organic material.Detritus may also refer to:* Detritus , a geological term used to describe the particles of rock produced by weathering...
-nibbling bottom dweller. Despite their success, the arthrodires were one of many groups eliminated by the environmental catastrophes of the Late Devonian extinction
Late Devonian extinction
The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, , about 374 million years ago...
, allowing other fish such as shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
s to diversify into the vacated ecological niches during the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
period.
Systematics
ORDER ARTHRODIRA- Clade AspinothoracidiAspinothoracidiThe Aspinothoracidi were a clade of placoderms, armored fish most diverse during the Devonian. The gigantic apex predator Dinichthys, is the most well-known member of this group...
- Superfamily DinichthyloideaDinichthyloideaThe Dinichthyloidea were a superfamily of placoderms, armored fish most diverse during the Devonian. They contain numerous extremely large species, such as Titanichthys agassizi which fed on small animals and the apex predator Dunkleosteus terrelli....
- Family Dinichthyidae
- Family Trematosteidae
- Family Rachiosteidae
- Family Pachyosteidae
- Family Titanichthyidae
- Family Bungartiidae
- Family Selenosteidae
- Family Mylostomatidae
- Superfamily Dinichthyloidea
- Basal infraorder Actinolepina
- Family ActinolepidaeActinolepidaeActinolepidae is an extinct family of placoderm fishes which lived during the Early Devonian period....
- Family Actinolepidae
- Suborder Phlyctaenioidei
- Basal family Holonematidae
- Infraorder Phlyctaeniina
- Family Groenlandaspididae
- Family PhlyctaeniidaePhlyctaeniidaePhlyctaeniidae is an extinct family of placoderm fishes that lived during the Devonian period, mainly in Norway and North America.-Phylogeny:ORDER ARTHRODIRA* Infraorder Actinolepina** Family Actinolepidae* Suborder Phlyctaenioidei...
- Suborder BrachythoraciBrachythoraciThe Brachythoraci were a suborder of arthrodire placoderms, armored fish most diverse during the Devonian. They used to contain only the Coccosteina, but the more basal family, Heterosteidae is now placed there too....
- Basal family Heterosteidae
- Infraorder CoccosteinaCoccosteinaThe Coccosteina were an infraorder of placoderms, armored fish most diverse during the Devonian.-Systematics:* Basal genus Maideria* Superfamily Buchanosteoidea** Family Buchanosteidae* Superfamily Gemuendenaspoidea** Family Gemuendenaspidae...
- Superfamily Buchanosteoidea
- Family Buchanosteidae
- Superfamily Gemuendenaspoidea
- Family Gemuendenaspidae
- Superfamily Homosteoidea
- Family Homosteiidae
- Superfamily Brachydeiroidea
- Family Brachydeiridae
- Superfamily Coccosteoidea
- Family Pholidosteidae
- Family Coccosteidae
- Family Plourdostenidae
- Family Torosteidae
- Family Incisoscutidae
- Family Camuropiscidae
- Family DunkleosteidaeDunkleosteidaeThe Dunkleosteidae were a family of placoderms, armored fish most diverse during the Devonian. The gigantic apex predator Dunkleosteus terrelli is the most well-known member of this group...
- Superfamily Buchanosteoidea
Genera incertae sedis
Arthrodira of unresolved relationships- Antarctolepis
- Aspidichthys - including Anomalichthys, Aspidophorus
- Atlantidosteus
- Callognathus
- Copanognathus
- Glyptaspis
- Golshanichthys
- Grazosteus
- Hollardosteus
- Laurentaspis
- Machaerognathus
- Murmur - including Euptychaspis, Ptychaspis
- Overtonaspis
- Phylactaenium
- Prescottaspis
- Qataraspis
- Taemasosteus
- Taunaspis
- Timanosteus
- Trachosteus
- Wheathillaspis