page_title: Eurypterid
section_title: Origins
hierarchical_section_title: Eurypterid / Evolutionary history / Origins
caption_attribution_description: English: Pentecopterus sea scorpion. Pentecopterus lived 467 million years ago and could grow to nearly six feet, with a long head shield, a narrow body, and large, grasping limbs for trapping prey. It is the oldest described eurypterid — a group of aquatic arthropods that are related to modern spiders, lobsters, and ticks.
context_page_description: Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is likely to have appeared first either during the Early Ordovician or Late Cambrian period. With approximately 250 species, the Eurypterida is the most diverse Paleozoic chelicerate order. Following their appearance during the Ordovician, eurypterids became major components of marine faunas during the Silurian, from which the majority of eurypterid species have been described. The Silurian genus Eurypterus accounts for more than 90% of all known eurypterid specimens. Though the group continued to diversify during the subsequent Devonian period, the eurypterids were heavily affected by the Late Devonian extinction event. They declined in numbers and diversity until becoming extinct during the Permian–Triassic extinction event 251.9 million years ago.
  Although popularly called "sea scorpions", only the earliest eurypterids were marine; many later forms lived in brackish or fresh water, and they were not true scorpions.
context_section_description: Until 1882 no eurypterids were known from before the Silurian. Contemporary discoveries since the 1880s have expanded the knowledge of early eurypterids from the Ordovician period. The earliest eurypterids known today, the megalograptid Pentecopterus, date from the Darriwilian stage of the Middle Ordovician, 467.3 million years ago. There are also reports of even earlier fossil eurypterids in deposits of Late Tremadocian (Early Ordovician) age in Morocco, but these have yet to be thoroughly studied.
  Pentecopterus was a relatively derived eurypterid, part of the megalograptid family within the carcinosomatoid superfamily. Its derived position suggests that most eurypterid clades, at least within the eurypterine suborder, had already been established at this point during the Middle Ordovician. The earliest known stylonurine eurypterid, Brachyopterus, is also Middle Ordovician in age. The presence of members of both suborders indicates that primitive stem-eurypterids would have preceded them, though these are so far unknown in the fossil record. The presence of several eurypterid clades during the Middle Ordovician suggests that eurypterids either originated during the Early Ordovician and experienced a rapid and explosive radiation and diversification soon after the first forms evolved, or that the group originated much earlier, perhaps during the Cambrian period.
  As such, the exact eurypterid time of origin remains unknown. Though fossils referred to as "primitive eurypterids" have occasionally been described from deposits of Cambrian or even Precambrian age, they are not recognized as eurypterids, and sometimes not even as related forms, today. Some animals previously seen as primitive eurypterids, such as the genus Strabops from the Cambrian of Missouri, are now classified as aglaspidids or strabopids. The aglaspidids, once seen as primitive chelicerates, are now seen as a group more closely related to trilobites.
  The fossil record of Ordovician eurypterids is quite poor.  The majority of eurypterids once reportedly known from the Ordovician have since proven to be misidentifications or pseudofossils. Today only 11 species can be confidently identified as representing Ordovician eurypterids. These taxa fall into two distinct ecological categories; large and active predators from the ancient continent of Laurentia, and demersal (living on the seafloor) and basal animals from the continents Avalonia and Gondwana. The Laurentian predators, classified in the family Megalograptidae (compromising the genera Echinognathus, Megalograptus and Pentecopterus), are likely to represent the first truly successful eurypterid group, experiencing a small radiation during the Late Ordovician.
caption: A reconstruction of Pentecopterus, the earliest known eurypterid. The family to which Pentecopterus belongs, the Megalograptidae, was the first truly successful eurypterid group.
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