Placodermi Fossil range: Early/Mid Silurian – Late Devonian
Dunkleosteus attacking Gorgonichthys
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Infraphylum: Gnathostomata Class: Placodermi McCoy, 1848
Orders
Antiarchi † Arthrodira † Brindabellaspida † Petalichthyida † Phyllolepida † Ptyctodontida † Rhenanida † Acanthothoraci † ?Pseudopetalichthyida † ?Stensioellida †

The Placodermi were a class of armoured prehistoric fishes, known from fossils, which lived from the late Silurian to the end of the Devonian Period. The Silurian is a geologic period and system that extends from the end of the Ordovician period about 443 The Devonian is a geologic period and system of the Paleozoic era spanning from to  million years ago. Dunkleosteus (From "(David Dunkle" + Greek: osteus / οστεος = bone; meaning "Dunkle's Bone" is a Chordates ( Phylum Chordata) are a group of Animals that includes the Vertebrates together with several closely related Invertebrates Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes Gnathostomata is the group of Vertebrates with Jaws The group is traditionally a superclass, including the familiar classes of Fish, This article is about the taxonomic rank for the sequence of species in a taxonomic list see Taxonomic order In scientific classification used The Antiarchi ("Opposite anus" were the 2nd most successful order of Placoderms known after the Arthrodira. In Biology and Ecology, extinction is the cessation of existence of a Species or group of taxa. Arthrodira is an order of extinct armored jawed fishes of the Placodermi class who flourished in the Devonian period before their sudden extinction surviving Brindabellaspis stensioi (" Erik Stensiö 's Brindabella Ranges Shield" was a long-snouted placoderm from the Early The Petalichthyida was an order of small flattened Placoderm Fish. The order Phyllolepida ("leaf scales" was an order of flattened Placoderms found throughout the world with fossils being found in Devonian strata The ptyctodontids ("beak-teeth" are the Monotypic order Ptyctodontida of unarmored Placoderms containing only the family Rhenanida (" Rhine (fish" was an order of primitive lightly armored (relatively speaking Placoderms Unlike most other placoderms the Description The Acanthothoraci ("Spine Chests" were a group of Chimaera -like Placoderms who were closely related to the Rhenanid The Pseudopetalichthyida was a group of extinct skate-like fishes known only from rare fossils in Lower Devonian strata in Hunsrück, Germany Stensioella heintzi ("Heintz's Little Stensio " was an enigmatic placoderm of arcane affinity Prehistoric fish are various groups of Fishes that lived before recorded History. FOSSIL is a standard protocol for allowing serial communication for Telecommunications programs under the DOS Operating system. The Silurian is a geologic period and system that extends from the end of the Ordovician period about 443 The Devonian is a geologic period and system of the Paleozoic era spanning from to  million years ago. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. In Anatomy, the head of an Animal is the Rostral part (from Anatomical position that usually comprises the Brain, Eyes The thorax is a division of an Animal 's body that lies between the head and the Abdomen. In most biological nomenclature a scale ( Greek lepid, Latin squama) is a small rigid plate that grows out of an Animal Placoderms were among the first jawed fishes; their jaws likely evolved from the first of their gill arches. The jaw is either of the two opposable structures forming or near the entrance to the Mouth. Fish are aquatic Vertebrate animals that are typically ectothermic (previously Cold-blooded) covered with scales, and equipped with two A gill is an anatomical structure found in many aquatic organisms A 380 millions year old fossil of one of the species represents the oldest-known example of live birth. ^[1]

The first identifiable Placoderms evolved in the late Silurian; they began a dramatic decline during the Late Devonian extinctions, and the class was entirely extinct by the end of the Devonian. The Late Devonian extinction was one of five major Extinction events in the history of the Earth's Biota. The oldest known fossils were found in China. China ( Wade-Giles ( Mandarin) Chung¹kuo² is a cultural region, an ancient Civilization, and depending on perspective a National

Contents 1 Fossil record 2 Ecology and lifestyles 3 History of study 4 See also 5 Notes 6 External links 7 References

Fossil record

The earliest identifiable placoderm fossils are from China and date to the mid to late Silurian. Homostius is an extinct Genus of Arthrodire Placoderm The Silurian is a geologic period and system that extends from the end of the Ordovician period about 443 They are already differentiated into Antiarchs and Arthrodires, along with the other, more primitive groups. The Antiarchi ("Opposite anus" were the 2nd most successful order of Placoderms known after the Arthrodira. Arthrodira is an order of extinct armored jawed fishes of the Placodermi class who flourished in the Devonian period before their sudden extinction surviving Apparently Placoderms diversified long before the Devonian, somewhere in early or mid Silurian, though earlier fossils of basal Placodermi have not been discovered in these particular strata. In Phylogenetics, a basal Clade is the earliest clade to branch in a larger clade it appears at the base of a cladogram

The Silurian fossil record of the placoderms is quite (literally) fragmented. All known Silurian placoderms exist today only as fragments, either scraps of armor, or isolated scales, of which some have been tentatively identified as either antiarch or arthrodire due to histological similarities. Although they have been identified, many of the Silurian arthrodire and antiarch species have not yet been formally described, or even named. Paradoxically, the best known, or rather, most commonly cited example of a Silurian placoderm, Wangolepis of Silurian China, is known only from a few fragments that currently defy attempts to place them in any of the recognized placoderm orders.

Paleontologists and placoderm specialists suspect that the scarcity of the Silurian fossil record of placoderms is due to placoderms living in environments unconducive of fossil preservation, rather than a genuine scarcity. This hypothesis helps to explain the placoderms' seemingly miraculous appearance and diversity at the very beginning of the Devonian.

During the Devonian, the placoderms went on to inhabit and dominate almost all known aquatic ecosystems, both freshwater and saltwater, in stark contrast to the Silurian. Freshwater is a word that refers to bodies of water such as Ponds lakes rivers and streams containing low concentrations of dissolved Salts and other Total dissolved But this diversity ultimately suffered many casualties during the extinction event at the Frasnian–Famennian boundary, the Late Devonian extinctions. The Late Devonian extinction was one of five major Extinction events in the history of the Earth's Biota. The remaining species then died out during the Devonian/Carboniferous extinction event; not a single species survived into the Carboniferous. The Carboniferous is a geologic period and system that extends from the end of the Devonian period about 359

Ecology and lifestyles

Many placoderms, particularly the Rhenanida, Petalichthyida, Phyllolepida, and Antiarchi, were bottom-dwellers. Phyllolepis ("Leaf scale" is the Type genus of Phyllolepida, an Extinct order of Placoderm Fish Rhenanida (" Rhine (fish" was an order of primitive lightly armored (relatively speaking Placoderms Unlike most other placoderms the The Petalichthyida was an order of small flattened Placoderm Fish. The order Phyllolepida ("leaf scales" was an order of flattened Placoderms found throughout the world with fossils being found in Devonian strata The Antiarchi ("Opposite anus" were the 2nd most successful order of Placoderms known after the Arthrodira. As such (paraphrasing Palaeos), Placodermi has been popularly misunderstood as a tribe of bottom-feeding snails and "garbage trucks", even though they were actually the dominant vertebrate group during the Devonian. Palaeoscom is a web site on Biology, Paleontology, Cladistics and Geology and which covers the history of Earth The word snail is a Common name that can be used for almost all members of the Molluscan class Gastropoda which have coiled shells in the The vast majority of placoderms were predators, many of which lived at or near the bottom. Many, primarily the Arthrodira, were mid- to upper-water dwellers, and were active predators. Arthrodira is an order of extinct armored jawed fishes of the Placodermi class who flourished in the Devonian period before their sudden extinction surviving The largest known arthrodire, Dunkleosteus telleri, was 8 to 11 meters long, and is presumed to have had a nearly worldwide distribution, as its remains have been found in Europe, North America and Morocco. Dunkleosteus (From "(David Dunkle" + Greek: osteus / οστεος = bone; meaning "Dunkle's Bone" is a In fact, it is regarded as the world's first vertebrate "super-predator". Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes Other, smaller arthrodires, such as Fallacosteus and Rolfosteus of Gogo, had streamlined, bullet-shaped head armor, strongly supporting the idea that many, if not most, arthrodires were active swimmers, rather than passive ambush-hunters whose armor practically anchored them to the sea floor. Rolfosteus canningensis is an extinct species of Arthrodire Placoderm from the late Devonian of the Gogo Reef formation in Australia

Extraordinary evidence of internal fertilisation in a placoderm was afforded by the discovery in the Gogo Formation, near Fitzroy Crossing Kimberley, Western Australia, of a small female placoderm, about 25 cm in length, which died in the process of giving birth to a 6 cm live young one and was fossilised with the umbilical cord intact. The Gogo Formation in the Kimberley region of Western Australia is a world famous Lagerstätte that exhibits exceptional preservation of a Devonian Fitzroy Crossing is a small town in the Kimberley region of Western Australia, 400 km east of Broome and 300 km west of Halls Creek. Kimberley may refer to Kimberley Northern Cape in South Africa Kimberley in Western Australia Kimberley Nottinghamshire The fossil, named Materpiscis attenboroughi (after scientist David Attenborough), had eggs which were fertilised internally, the mother providing nourishment to the embryo and giving birth to live young. Materpiscis ( Latin: mother fish) is a Genus of Ptyctodontid placoderm (a class of Extinct, superficially shark-like Sir David Frederick Attenborough OM, CH, CVO, CBE, FRS (born 8 May 1926 in London, England With this discovery, the placoderm became the oldest vertebrate known to have given birth to live young ("viviparous"),^[1] pushing the date of first viviparity back some 200 million years earlier than had been previously known. A viviparous Animal is an animal employing vivipary: the embryo develops inside the body of the mother as opposed to outside in an egg ( Ovipary

It was thought that placoderms went extinct due to competition from the first bony fish, and the early sharks, given a combination of the supposed inherent superiority of bony fish, and the presumed sluggishness of placoderms. Sharks ( Superorder Selachimorpha) are a type of Fish with a full cartilaginous Skeleton and a highly streamlined body But after more accurate summaries of prehistoric organisms, it is now thought that the last placoderms died out one by one as each of their ecological communities suffered the environmental catastrophes of the Devonian/Carboniferous extinction event.

History of study

The earliest studies of placoderms were published by Louis Agassiz, in his five volumes on fossil fishes, 1833–1843. Dunkleosteus (From "(David Dunkle" + Greek: osteus / οστεος = bone; meaning "Dunkle's Bone" is a Jean Louis Rodolphe Agassiz ( May 28 1807 — December 14 1873) was a Swiss - American Zoologist, Glaciologist In those days, the placoderms were thought to be shelled jawless fish akin to ostracoderms. Ostracoderms ("shell-skinned" are any of several groups of extinct, primitive jawless Fishes that were covered in an armor of bony plates Some naturalists even suggested that they were shelled invertebrates, or even turtle-like vertebrates. Turtles are Reptiles of the Order Testudines (all living turtles belong to the Crown group Chelonia) most of The work of Dr. Erik Stensiö, at the Swedish Museum of Natural History, Stockholm, from the late 1920s established the details of placoderm anatomy, and identified them as true jawed fishes related to sharks. Erik Helge Osvald Stensiö ( 2 October 1891 &ndash 11 January 1984) was a Swedish paleozoologist. The Swedish Museum of Natural History (in Swedish Naturhistoriska riksmuseet, literally the National Museum of Natural History in Stockholm, is Sharks ( Superorder Selachimorpha) are a type of Fish with a full cartilaginous Skeleton and a highly streamlined body He took fossil specimens with well-preserved skulls, and ground them away, one tenth of a millimeter at a time. Between each grinding, he made an imprint in wax. Wax has traditionally referred to a substance that is secreted by Bees ( Beeswax) and used by them in constructing their Once the specimens had been completely ground away (and so completely destroyed), he made enlarged, three dimensional models of the skulls in order to examine the anatomical details more thoroughly. Many other placoderm specialists suspected that Stensiö was trying to shoehorn placoderms into a relationship with sharks, but when more fossil specimens were found, especially the exceptionally well-preserved fossils from the Gogo Reef formation in Australia, Stensiö's theory of sharks and placoderms as sister groups became accepted as fact. Sharks ( Superorder Selachimorpha) are a type of Fish with a full cartilaginous Skeleton and a highly streamlined body FOSSIL is a standard protocol for allowing serial communication for Telecommunications programs under the DOS Operating system.

See also

• List of placoderms
• Ostracoderm
• Acanthodii

Notes

External links

• Annetta Markussen-Brown, "Devonian Armoured Fish" 2000
• Introduction to the Placodermi Extinct armored fishes with jaws
• PALAEOZOIC FOSSILS UK
• Placoderms
• Placodermi: Overview
• BBC—report on Dunkleosteus terrelli

References

• Janvier, Philippe. Early Vertebrates Oxford, New York: Oxford University Press, 1998. ISBN 0-19-854047-7
• Long, John A. The Rise of Fishes: 500 Million Years of Evolution Baltimore: The Johns Hopkins University Press, 1996. ISBN 0-8018-5438-5

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