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Eurypterids Fossil range: 460–248 Ma PreЄ Є O S D C P T J K Pg N Ordovician-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

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) and predate the earliest fishes. The largest, such as Jaekelopterus, reached 2 metres (6 ft 7 in) or more in length, but most species were less than 20 centimetres (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.

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 large and long enough 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.¹

Relationships

Mixopterus, a Eurypterid of the Silurian

Eurypterids have traditionally been regarded as close relatives of Horseshoe Crabs; together forming a group called Merostomata. Subsequent studies² placed eurypterids closer to the arachnids in a group called Metastomata. There has also been a prevailing idea³ 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 eurypterids⁴ and the two groups were sometimes confused with one another. The most recent summary⁵ 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.

Eurypterid fossils

Stylonurus, a Eurypterid from the Late Devonian

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 million years ago. A predatory arthropod whose traces are known as Protichnites,⁶ found in Cambrian strata dating from 510 million years ago, is a possible stem group eurypterid, and is among the first evidence of animals on land.⁷

In 2007, a group paleontologists led by Simon Braddy at the University of Bristol discovered the claw of a 390 million year old sea scorpion which would have been larger than a human - making it the largest arthropod ever found.⁸

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

Wikimedia Commons has media related to: Eurypterida

Wikispecies has information related to: Eurypterida

• Life-like reconstruction of a eurypterid
• Eurypterida from the Palaeos website
• Associated Press article on recent giant eurypterid discovery
• Jaekelopterus rhenaniae at the Palaeoblog

Wikipedia content modification information:

• This page was last modified on 2 January 2009, at 17:26.

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