Eurypterid

Taxobox
name = Eurypterids
fossil_range = Fossil range|510|248 Cambrian-Permian

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image_caption = Eurypterid from Ernst Haeckel's "Kunstformen der Natur", 1904.
regnum = Animalia
phylum = Arthropoda
subphylum = Chelicerata
classis = Eurypterida
subdivision_ranks = Orders
subdivision =†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 "Jaekelopterus", reached convert|2|m|ftin|0 or more in length, but most species were less than convert|20|cm|ftin|0. 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 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.

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. 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 large 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.

Many eurypterids had legs big and long to do more than allow them to crawl over the sea bottom; a number of species 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.

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, indicating that "J. rhenaniae" was 2.5 meters in length. [http://www.newscientist.com/article/dn12941-giant-claw-points-to-monster-sea-scorpion.html Giant claw points to monster sea scorpion - life - 21 November 2007 - New Scientist ] ]

They had traditionally been considered close relatives to the common Horseshoe Crab, but most recent evidence places them closer to the arachnids. [Boudreaux H. B., 1979. Arthropod phylogeny with special reference to insects. John Willey & sons, New York, Chichester, Brisbane, Toronto. 320 pp.]

Eurypterid fossils

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 Ma|251.4. A predatory arthropod whose traces are known as "Protichnites",cite journal
author = Owen, R.
year = 1852
title = Description of the impressions and footprints of the Protichnites from the Potsdam Sandstone of Canada
journal = Quarterly Journal of the Geological Society of London
volume = 8
pages = 214–225] found in Cambrian strata dating from Ma|510, is a possible stem group eurypterid, and is among the first evidence of animals on land.cite journal
author = Burling, L.D.
year = 1917
title = Protichnites and Climactichnites. A Critical Study of Some Cambrian Trails
journal = American Journal of Science
volume = 44
pages = 387–398]

In 2007, a group paleontologists led by Simon Braddy at the University of Bristol discovered a sea scorpion larger than a human being, which was claimed to have been the largest arthropod which ever lived. This discovery was made in a 390 million year old rock containing the fossil of a huge claw or chelicera.

See also

* List of eurypterids
* Nepidae - an unrelated insect, commonly known as a "water scorpion"
* Hibbertopterus

References

* 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 (1 December 2005).

External links

* [http://paranormal.about.com/od/ghostphotos/ig/2006-Photo-Hoax-Contest-Slides/Weird-Crustacean.htm Life-like reconstruction of a eurypterid]
* [http://www.palaeos.com/Invertebrates/Arthropods/Chelicerata/Eurypterida/Eurypterida.html Eurypterida from the Palaeos website]
* [http://ap.google.com/article/ALeqM5gQ9X4-OozXU0E52gi-xaqJR9WsEAD8T1QCIO0 Associated Press article on recent giant eurypterid discovery]
* [http://palaeoblog.blogspot.com/2007/11/largest-sea-scorpion-discovered.html Jaekelopterus rhenaniae at the Palaeoblog]