What is Eurypterid?
{{Taxobox | name = ''Eurypterid'' | fossil_range = [[Ordovician]]–[[Permian]] | image = Haeckel Eurypterid.jpg | image_width = 230px | image_caption = Eurypterid from [[Ernst Haeckel]]'s <br/>''Kunstformen der Natur'' (1904) | regnum = [[Animal]]ia | phylum = [[Arthropod]]a | classis = [[Merostomata]] | ordo = [[extinction|†]] Eurypterida }} [[File:Eurypterus Smithsonian.jpg|thumb|right|''Eurypterus'' model, exhibited in Smithsonian National Museum of Natural History: Hall of Fossils]] The '''eurypterids''', related to [[arachnid]]s, were the largest known [[arthropod]]s. They are members of the extinct [[Order (biology)|order]] '''Eurypterida'''. It is a most diverse [[Chelicerata|Chelicerate]] order.<ref name='dunlop2008'>{{Cite journal| issn = 0161-8202| volume = 36| issue = 2| pages = 267–272| last = Dunlop| first = J.A. |display-authors = etal | title = How many species of fossil arachnids are there?| journal = Journal of Arachnology| year = 2008| doi = 10.1636/CH07-89.1| s2cid = 42371883| url = https://www.biodiversitylibrary.org/part/229074}}</ref> They are also called '''sea scorpions'''. The largest, such as ''[[Jaekelopterus]]'', reached 2½ metres in length, but most species were less than 20 cm (8 inches). They were the largest [[arthropod]]s of all time. They were [[predator]]s which thrived in the warm, shallow seas and lakes of the [[Ordovician]] to the [[Permian]] periods, around 460 to 248 million years ago. Recent research suggsts their eyesight was not very good.<ref>Anderson R.P. ''et al'' 2014. What big eyes you have: the ecological role of giant pterygotid eurypterids. ''Biology Letters'' '''10''' (7): 20140412. [http://www.sciencedaily.com/releases/2014/07/140710183616.htm]</ref> The move from the sea to fresh water probably occurred by the [[Pennsylvanian]] period. Eurypterids went extinct during the [[Permian–Triassic extinction event]] 251 million years ago, and their fossils have a near global distribution. == Description == The typical eurypterid had a large, flat, semicircular [[ca