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Trilobites (, ; meaning "three lobes") are a fossil group of extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (), and they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, all trilobite orders except Proetida died out. Trilobites finally disappeared in the mass extinction at the end of the Permian about . The trilobites were among the most successful of all early animals, roaming the oceans for over 270 million years.〔http://firstlifeseries.com/learn-more/〕 By the time trilobites first appeared in the fossil record they were already highly diversified and geographically dispersed. Because trilobites had wide diversity and an easily fossilized exoskeleton an extensive fossil record was left behind, with some 17,000 known species spanning Paleozoic time. The study of these fossils has facilitated important contributions to biostratigraphy, paleontology, evolutionary biology and plate tectonics. Trilobites are often placed within the arthropod subphylum Schizoramia within the superclass Arachnomorpha (equivalent to the Arachnata), although several alternative taxonomies are found in the literature. Trilobites had many life styles; some moved over the sea-bed as predators, scavengers or filter feeders and some swam, feeding on plankton. Most life styles expected of modern marine arthropods are seen in trilobites, with the possible exception of parasitism (where there are still scientific debates). Some trilobites (particularly the family Olenidae) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food. ==Fossil record== The earliest trilobites known from the fossil record are redlichiids and ptychopariid bigotinids dated to some .〔 Contenders for the earliest trilobites include ''Profallotaspis jakutensis'' (Siberia), ''Fritzaspis'' spp. (western USA), ''Hupetina antiqua'' (Morocco) and ''Serrania gordaensis'' (Spain). All trilobites are thought to have originated in present-day Siberia, with subsequent distribution and radiation from this location.〔 All Olenellina lack facial sutures (see below), and this is thought to represent the original state. The earliest sutured trilobite found so far (''Lemdadella''), occurs almost at the same time as the earliest Olenellina however, implying that trilobite origin lies before the start of the Atdabanian, but without leaving fossils.〔 Other groups show secondary lost facial sutures, such as all Agnostina and some Phacopina. Another common feature of the Olenellina also suggests this suborder to be the ancestral trilobite stock: early protaspid stages have not been found, supposedly because these were not calcified, and this also is supposed to represent the original state. Earlier trilobites may be found and could shed more light on the origin of trilobites.〔〔(【引用サイトリンク】title=First Trilobites )〕 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「trilobite」の詳細全文を読む スポンサード リンク
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