| Eurypterid Temporal range:Ordovician–Permian | |
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| Eurypterid fromErnst Haeckel's Kunstformen der Natur (1904) | |
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| Order: | † Eurypterida |
Theeurypterids, related toarachnids, were the largest knownarthropods. They are members of the extinctorderEurypterida. It is a most diverseChelicerate order.[1] They are also calledsea scorpions.
The largest, such asJaekelopterus, reached 2½ metres in length, but most species were less than 20 cm (8 inches). They were the largestarthropods of all time. They werepredators which thrived in the warm, shallow seas and lakes of theOrdovician to thePermian periods, around 460 to 248 million years ago. Recent research suggsts their eyesight was not very good.[2]
The move from the sea to fresh water probably occurred by thePennsylvanian period. Eurypterids went extinct during thePermian–Triassic extinction event 251 million years ago, and their fossils have a near global distribution.
The typical eurypterid had a large, flat, semicircularcarapace, 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 of the eurypterids were twelve body segments. These segments are formed by a dorsal plate, called a tergite, and a ventral plate, called a sternite. The tail, known as thetelson, is spiked in most eurypterids.[3]
A recent discovery of a new fossil species of eurypterid,Pentecopterus, has been made. It is two meters long, and lived 467 million years ago, in the MiddleOrdovician period.[4][5]
