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Placodermi (from the Greek πλάξ = plate and δέρμα = skin, literally "plate-skinned") is an extinct 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 covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were among the first jawed fish; their jaws likely evolved from the first of their gill arches. Placoderms are paraphyletic, and comprise several distinct outgroups or sister taxa to all living jawed vertebrates, which originated among their ranks. This is illustrated by a 419-million-year-old fossil, ''Entelognathus'', from China, which is the only known placoderm with a type of bony jaw like that found in modern bony fishes. This includes a dentary bone, which is found in humans and other tetrapods,. The jaws in other placoderms were simplified and consisted of a single bone.〔(Entelognathus: The Fish with the First Modern Face )〕 Placoderms were also the first fish to develop pelvic fins, the precursor to hindlimbs in tetrapods, as well as true teeth. 380-million-year-old fossils of three other genera, ''Incistoscutum'', ''Materpiscis'' and ''Austroptyctodus'', represent the oldest known example of live birth. The first identifiable placoderms appear in the fossil record during the late Llandovery epoch of the early Silurian. The various groups of placoderms were diverse and abundant during the Devonian, but became extinct at the end-Devonian Hangenberg event 360 million years ago ==Characteristics== Many placoderms, particularly the Rhenanida, Petalichthyida, Phyllolepida, and Antiarchi, were bottom-dwellers. In particular, the antiarchs, with their highly modified, jointed bony pectoral fins, were highly successful inhabitants of Middle-Late Devonian freshwater and shallow marine habitats, with the Middle to Late Devonian genus, ''Bothriolepis'', known from over 100 valid species.〔Long 1983〕 The vast majority of placoderms were predators, many of which lived at or near the substrate. Many, primarily the Arthrodira, were active, nektonic predators that dwelled in the middle to upper portions of the water column. A study of the arthrodire ''Compagopiscis'' published in 2012 concluded that placoderms (at least this particular genus) likely possessed true teeth contrary to some early studies. The teeth had well defined pulp cavity and were made of both bone and dentine. However the tooth and jaw development were not as closely integrated as in modern gnathostomes. These teeth were likely homologous to the teeth of other gnathostomes.〔 One of the largest known arthrodires, ''Dunkleosteus terrelli '', was long, and is presumed to have had a large distribution, as its remains have been found in Europe, and North America and possibly Morocco. Some paleontologists regard it as the world's first vertebrate "superpredator", preying upon other predators. Other, smaller arthrodires, such as ''Fallacosteus'' and ''Rolfosteus'', both of the Gogo Formation of Western Australia, 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. Some placoderms were herbivorous, such as the Middle to Late Devonian arthrodire ''Holonema'', and some were planktivores, such as the gigantic, long arthrodire, ''Titanichthys''. Extraordinary evidence of internal fertilization in a placoderm was afforded by the discovery in the Gogo Formation, near Fitzroy Crossing, Kimberley, Western Australia,〔Long & Trinajstic 2010〕 of a small female placoderm, about in length, which died in the process of giving birth to a 6 cm ( in) live young one and was fossilized with the umbilical cord intact.〔Long et al. 2008〕 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. With this discovery, the placoderm became the oldest vertebrate known to have given birth to live young ("viviparous"),〔 pushing the date of first viviparity back some 200 million years earlier than had been previously known. Specimens of the arthrodire ''Incisoscutum ritchei'', also from the Gogo Formation, have been found with embryos inside them indicating this group also had live bearing ability.〔Long et al. 2009〕 The males reproduced by inserting a long clasper into the female. Long basipterygia are also found on the phyllolepid placoderms, such as ''Austrophyllolepis''〔Long 1984〕 and ''Cowralepis'', both from the Middle Devonian of Australia, suggesting that the basiptergia were used in copulation. The placoderm claspers' are not homologous with the claspers in cartilaginous fishes. The similarities between the structures has been revealed to be an example of convergent evolution. While the claspers in cartilaginous fishes are specialized parts of their paired pelvic fins that has been modified for copulation due to changes in the hox genes hoxd13, the origin of the mating organs in placoderms most likely relied on different sets of hox genes and were structures that developed further down the body as an extra and independent pair of appendages, but which during development turned into body parts used for reproduction only. Because they were not attached to the pelvic fins, as the claspers in fish like sharks, they were much more flexible and could probably be rotated forward.〔(The first vertebrate sexual organs evolved as an extra pair of legs )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Placodermi」の詳細全文を読む スポンサード リンク
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