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Eurypterid

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Eurypterids
Temporal range: Cambrian-Permian
Eurypterid from Ernst Haeckel's Kunstformen der Natur, 1904.
Scientific classification
Kingdom:
Animalia
Phylum:
Arthropoda
Subphylum:
Chelicerata
Class:
Eurypterida
Orders

†Stylonuroidea Diener, 1924
†Eurypteroidea Burmeister, 1843

The eurypterids (sea scorpion) were the largest known arthropods that ever lived (with the possible exception of the Arthropleurids). They are members of the extinct class Eurypterida (Arachnomorpha, Chelicerata) and predate the earliest fishes. The largest, such as Pterygotus, reached 2 m or more in length, but most species were less than 20 cm. They were formidable predators that thrived in warm shallow water in the Cambrian to Permian from 510 to 248 million years ago. Although called "sea scorpions", only the earliest ones were marine (most lived in brackish or freshwater), and they were not true scorpions. The move from the sea to fresh water probably occurred by the Pennsylvanian period.

Eurypterus is perhaps the most well-known genus of eurypterid, of which 200 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, with a long spine at the end. 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, which is spiked, and in some species may have been used to inject venom, like in modern scorpions, is known as the telson. Some eurypterids have paddles, which were used to propel themselves through water. Some argue that the paddles were also used for digging. Underneath, in addition to the pair of swimming appendages the creature had 4 pairs of jointed legs for walking, and two small claws at the front, chelicerae. 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.

Although many eurypterids had legs too tiny to do more than allow them to crawl over the sea bottom, a number of forms had large stout legs, and were clearly capable of terrestrial locomotion (like land crabs today). While functional studies suggest that eurypterids used out-of-phase walking techniques, their trackways indicate that they 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.

The largest species of sea-scorpion was Pterygotus, an arthropod the size of a crocodile.Fossils of Pterygotus are relativeely common although complete skeletons are rare. They may had preyed on Brontoscorpio and Cephalaspis. At 2.8 meters long, they are the largest Arthropod ever to have lived. Their fossils have been found worldwide, except in Antartica.

They are close relatives to the common Horshoe crab.

Eurypterid fossils

Eurypterid fossils have been found on nearly every continent. Eurypterid fossils are routinely found at Ridgemount Quarry, in Fort Erie, Ontario Canada. A commercial eurypterid quarry has been open for several years in Mohawk, New York.

Among the largest eurypterids are the Hibbertopterina, 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).

Eurypterids are related to the modern marine horseshoe crabs and land scorpions. About two dozen families of eurypterids are known. They went extinct in the Permian-Triassic extinction event 248 million years ago (m.y.a.).The possible ancestor of Eurypterids was a predatory Cambrian Arthropod known as Protichnites which has been found in Cambrian strata dating from 510 Ma and was one of the first animals to walk on land.

Classification by Tollerton, 1989

There are more than 300 identified species of the extinct sea scorpions. They have been classified by Tollerton (1989) and others in more than 60 genera and in about 20 families:

Glyptoscorpioidae

Slimonioidea

Hughmillerioidea

Mixopteroidea

  • Mixopteridae
  • Lanarkopteridae

Megalograptoidea

Eurypteroidea

  • Eurypteridae

Stylonuroidea

Dolichocephala

  • Claypolidae (?)
  • Drepanopteridae
  • Parastylonuridae
  • Laurieipteridae

Kokomopteroidea

  • Kokomopteridae
  • Hardieopteridae

Brachyopterelloidea

  • Brachyopterellidae

Rhenopteroidea

Mycopteropoidea

Pterygotoidea


Incertae sedis (phylogeneticly troublesome)

See also

References

  • Ciurca, Samuel J. (1998). The Silurian Eurypterid Fauna (http://www.eurypterid.net/ ). Retrieved July 25, 2004.
  • Clarke, John M. & Rudolf R. The Eurypterida of New York. Albany: New York State Education Department, 1912.
  • Whyte, Martin A. "Palaeoecology: A gigantic fossil arthropod trackway". Nature 438, 576-576 (01 December 2005).

Life-like reconstruction of a eurypterid, photo[1]

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