Eurypterid

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Eurypterids
Fossil range: Cambrian-Permian
Eurypterid from Ernst Haeckel's Kunstformen der Natur, 1904.
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 scorpions) include the largest known arthropod that ever lived (with the possible exception of Arthropleuridae). 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 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.

Contents

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. 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 small claws at the front, chelicerae. 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.

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 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. They may have preyed upon Brontoscorpio and Cephalaspis. 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, indicating that J. rhenaniae was 2.5 meters in length.[1]

They had traditionally been considered close relatives to the common Horseshoe Crab, but most recent evidence places them closer to the arachnids.[2]

Eurypterid fossils have a near global distribution. 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. About two dozen families of eurypterids are known. They went extinct during the Permian-Triassic extinction event 251.4 million years ago. A predatory arthropod known as Protichnites, found in Cambrian strata dating from 510 million years ago, is a possible stem group eurypterid, and was one of the first animals to walk on land.

Very recently, British paleontologist at the University of Bristol discovered a sea scorpion larger than a human being, claimed to be the largest arthropod ever lived. This discovery was made by a finding of a 390 million year old rock containing the fossil of a huge claw, that of a sea scorpion.

Main article: List of eurypterids
Image:Splitsection.svg
 It has been suggested that this section be split into a new article. (Discuss)

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:

This list is incomplete; you can help by expanding it.

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 (phylogenetically troublesome)

  1. ^ http://www.newscientist.com/article/dn12941-giant-claw-points-to-monster-sea-scorpion.html
  2. ^ Boudreaux H. B., 1979. Arthropod phylogeny with special reference to insects. John Willey & sons, New York, Chichester, Brisbane, Toronto. 320 pp.
  • Braddy, S. J. 2001. "Eurypterid Palaeoecology: palaeobiological, ichnological and comparative evidence for a ‘mass-moult-mate’ hypothesis". Palaeogeography, Palaeoclimatology, Palaeoecology 172, 115-132.
  • 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.
  • Gupta, N. S., Tetlie, O. E., Briggs, D. E. G. and Pancost, R. D. 2007. "The fossilization of eurypterids: a result of molecular transformation". Palaios 22, 439-447.
  • Manning, P. L. and Dunlop, J. A. 1995. "The respiratory organs of Eurypterids". Palaeontology 38, 287-297.
  • Tetlie, O. E. 2007. "Distribution and dispersal history of Eurypterida (Chelicerata)". Palaeogeography, Palaeoclimatology, Palaeoecology 252, 557-574.
  • Tetlie, O. E. and Cuggy, M. B. 2007. "Phylogeny of the basal swimming eurypterids (Chelicerata; Eurypterida; Eurypterina)". Journal of Systematic Palaeontology 5, 345-356.
  • Whyte, Martin A. "Palaeoecology: A gigantic fossil arthropod trackway". Nature 438, 576-576 (01 December 2005).

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