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Eurypterids (sea scorpions) are an extinct group of arthropods that are related to arachnids and include the largest known arthropods to have ever lived. They are members of the extinct order Eurypterida (Chelicerata); which is the most diverse Paleozoic chelicerate order in terms of species. The name Eurypterida comes from the Greek words ' (meaning "broad" or "wide") and ' (meaning "wing").〔''Webster's New Universal Unabridged Dictionary''. 2nd ed. 1979.〕 This name was chosen due to the pair of wide swimming appendages on the first fossil eurypterids discovered. The largest, such as ''Jaekelopterus'', reached in length, but most species were less than . They were formidable predators that thrived in warm shallow water, in both seas and lakes, from the mid Ordovician to late Permian (). Although informally called "sea scorpions", only the earliest ones were marine (later ones lived in brackish or freshwater), and they were not true scorpions. According to theory, the move from the sea to fresh water had probably occurred by the Pennsylvanian subperiod. Eurypterids are believed to have undergone ecdysis, making their significance in ecosystems difficult to assess, because it can be difficult to tell a fossil moult from a true fossil carcass. They became extinct during the Permian–Triassic extinction event . Their fossils have a near global distribution. About two dozen families of eurypterids are known. Perhaps the best-known genus of eurypterid is ''Eurypterus'', of which around 16 fossil species are known. The genus ''Eurypterus'' was described in 1825 by James Ellsworth De Kay, a zoologist. He recognized the arthropod nature of the first-ever described eurypterid specimen, found by Dr. S. L. Mitchill. In 1984, that species, ''Eurypterus remipes'' was named the state fossil of New York. ''Pentecopterus decorahensis'', which lived as early as 467.3 million years ago, is the oldest described eurypterid; with an estimated length of , it has been described as "the first real big predator". == Body structure == Eurypterids have been formally described as follows: "Small to very large merostomes with elongate lanceolate, rarely trilobed body; prosoma () of moderate size; opisthosoma () with 12 moveable segments and styliform to spatulate telson (), with division commonly into 7-segmented preabdomen and 5-segmented postabdomen; prosomal () appendages 6, comprising 3-jointed chelicerae, walking legs, the last pair commonly transformed into swimming legs. Mouth central, bordered posteriorly by endostoma and metastoma. Operculum with median genital appendage, abdominal appendages plate-shaped with nonlaminate gills. Ordovician-Permian." The typical eurypterid had a large, flat, semicircular carapace, followed by a jointed section, and finally a tapering, flexible tail, most ending with a long spine at the end (''Pterygotus'', though, had a large flat tail, possibly with a smaller spine). Behind the head, eurypterids had twelve body segments, each of which was formed from a dorsal plate (tergite) and a ventral plate (sternite). The tail, known as the telson, is spiked in most eurypterids (just like in modern scorpions) and in some species it may have been used to inject venom, but so far there is no certain evidence that any eurypterids were venomous. Most eurypterids have paddles toward the end of the carapace and beyond, which were used to propel themselves through water. The suborder Stylonurina have walking legs instead of paddles. Some argue that the paddles were also used for digging. Underneath, in addition to the pair of swimming appendages, the creature had four pairs of jointed legs for walking, and two claws at the front, chelicerae, which were enlarged in pterygotids. The walking legs had odd hairs, similar to modern day crabs. Other features, common to ancient and modern arthropods of this type, include one pair of compound eyes and a pair of smaller eyes, called ocelli, located between the other larger pair of eyes. Many eurypterids had legs large and long enough to do more than allow them to crawl over the sea bottom; a number of species (particularly hibbertopterids) had large stout legs, and were probably capable of terrestrial locomotion (like land crabs today). Studies of what are believed to be their trackways indicate that eurypterids used in-phase, hexapodous (six-legged) and octopodous (eight-legged) gaits. Some species may have been amphibious, emerging onto land for at least part of their life cycle; they may have been capable of breathing both in water and in air. A predatory arthropod known as ''Protichnites'', whose traces are found in Cambrian strata dating from , is a possible stem group eurypterid, and is among the first evidence of animals on land. The largest sea-scorpion and the largest known arthropod ever to have lived is ''Jaekelopterus rhenaniae'', supported by the finding of a long claw in 2007, indicating a body length of . 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「eurypterid」の詳細全文を読む スポンサード リンク
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