Eurypterida

Eurypterids (sea scorpions) are an extinct group of arthropods related to arachnids, which include the largest known arthropods that ever lived. They are members of the extinct class Eurypterida (Chelicerata). The word Eurypterid comes from the Greek word eury meaning "broad" or "wide" and the Greek word pteron meaning "wing". They predate the earliest fishes. The largest, such as Jaekelopterus, reached 2 meters (6 ft 7 in) or more in length, but most species were less than 20 centimeters (8 in). They were formidable predators that thrived in warm shallow water in the Ordovician to Permian from 460 to 248 million years ago. 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 probably occurred by the Pennsylvanian period. They went extinct during the Permian-Triassic extinction event 251 Ma, and their fossils have a near global distribution.

About two dozen families of eurypterids are known. Eurypterus is perhaps the most well-known genus of eurypterid, of which 18 fossil species are known. The genus Eurypterus was created in 1825 by James Ellsworth DeKay, a zoologist. He recognized the arthropod nature of the first ever described eurypterid specimen found by Dr. S. L. Mitchell. In 1984, Eurypterus remipes was named the State Fossil of New York.

Body structure
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 of the eurypterids were twelve body segments. These segments are formed by a dorsal plate called tergite, and a ventral plate called sternite. The tail, known as the telson, is spiked in most eurypterids like in modern scorpions and in some species it may have been used to inject venom, but so far there is no certain evidence 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. It is possible that it was used for both. Underneath, in addition to the pair of swimming appendages the creature had 4 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, in between the other larger 2 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 whose traces are known as Protichnites, found in Cambrian strata dating from, is a possible stem group eurypterid, and is among the first evidence of animals on land.



Among the largest eurypterids are the hibbertopteroids, named after the British palaeontolgist S. Hibbert, who described Hibbertopterus scouleri at a limestone quarry in East Kirkton, Scotland, in 1836. Fossil tracks (a form of trace fossil) were identified recently in East Lothian, Scotland, as made by a 1.6 meter long Hibbertopterus (Whyte, 2005). The largest well-described genus of sea-scorpion was Pterygotus, an arthropod the size of a crocodile. Fossils of Pterygotus are relatively common although complete fossils are rare. At 2.1 meters long, they were until recently the largest known arthropod ever to have lived. Their fossils have been found worldwide, except in Antarctica. Arthropleura came close in size, growing to slightly over 2 meters long. In 2007, a 46 cm claw belonging to Jaekelopterus rhenaniae (a species originally described in 1914) was discovered by a group of paleontologists led by Simon Braddy at the University of Bristol, indicating that J. rhenaniae was 2.5 meters in length, making it the largest arthropod ever found.

Relationships
Eurypterids have traditionally been regarded as close relatives of Horseshoe Crabs; together forming a group called Merostomata. Subsequent studies placed eurypterids closer to the arachnids in a group called Metastomata. There has also been a prevailing idea that eurypterids are closely related to scorpions, which they obviously resemble. This hypothesis is reflected in the common name 'sea scorpion'. More recently it has been recognised that a little-known, extinct group called chasmataspids also share features with eurypterids and the two groups were sometimes confused with one another. The most recent summary of relationships between arachnids and their relatives recognised Eurypterida, Xiphosura and Arachnida as three major groups, but was not able to resolve details between them.