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Eurypterids

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Eurypterids

Eurypterida
Temporal range: OrdovicianPermian
Eurypterid from Ernst Haeckel's Kunstformen der Natur (1904)
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Class: Merostomata
Order:  Eurypterida
Burmeister, 1843
Suborders

† Stylonurina Diener, 1924
† Eurypterina Burmeister, 1843

Eurypterids (sea scorpions) are an extinct group of million years ago, and their fossils have a near global distribution.

About two dozen families of eurypterids are known. Perhaps the best-known genus of eurypterid is Eurypterus, of which around 16 fossil species are known. The genus Eurypterus was described in 1825 by James Ellsworth De Kay, a zoologist. He recognized the arthropod nature of the first ever described eurypterid specimen, found by Dr. S. L. Mitchill. In 1984, that species, Eurypterus remipes was named the state fossil of New York.

Body structure


Eurypterids have been formally described as follows:[5]

"Small to very large merostomes with elongate lanceolate, rarely trilobed body; prosoma [head] of moderate size; opisthosoma [body] with 12 moveable segments and styliform to spatulate telson [tail], with division commonly into 7-segmented preabdomen and 5-segmented postabdomen; prosomal [head] appendages 6, comprising 3-jointed chelicerae, walking legs, the last pair commonly transformed into swimming legs. Mouth central, bordered posteriorly by endostoma and metastoma. Operculum with median genital appendage, abdominal appendages plate-shaped with nonlaminate gills. Ordovician-Permian."

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, between the other larger pair of 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

The largest sea-scorpion and the largest known arthropod ever to have lived is Jaekelopterus rhenaniae, supported by the finding of a 46 cm (18 in) long claw in 2007, indicating a body length of 2.5 m (8 ft 2 in).[9]

Relationships with other groups


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.[10] There has also been a prevailing idea that eurypterids are closely related to scorpions, which they resemble.[11] This hypothesis is reflected in the common name "sea scorpion". More recently it has been recognised that a little-known, extinct group called Chasmataspida also shares features with Eurypterida,[12] 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 any shared details among them.[13]

List of families and genera



There are 246 valid species of eurypterids as of 2011. All of them are extinct. They are grouped into the following:[14]

Suborder Stylonurina Diener, 1924
  • Superfamily Rhenopteroidea Størmer, 1951
  • Superfamily Stylonuroidea Kjellesvig-Waering, 1959
  • Parastylonuridae Waterston, 1979
  • Superfamily Kokomopteroidea Kjellesvig-Waering, 1966
  • Kokomopteridae Kjellesvig-Waering, 1966
  • Hardieopteridae Tollerton, 1989
  • Superfamily Hibbertopteroidea Kjellesvig-Waering, 1959
  • Drepanopteridae Kjellesvig-Waering, 1966
  • Hibbertopteridae Kjellesvig-Waering, 1959
Suborder Eurypterina Burmeister, 1843
  • Superfamily Moselopteroidea Lamsdell, Braddy, & Tetlie, 2010
  • Moselopteridae Lamsdell, Braddy, & Tetlie, 2010
  • Moselopterus Størmer, 1974
  • Vinetopterus Poschmann & Tetlie, 2004
  • Superfamily Megalograptoidea Caster & Kjellesvig-Waering, 1955
  • Superfamily Eurypteroidea Burmeister, 1843
  • Erieopteridae Tollerton, 1989
  • Superfamily Mixopteroidea Caster & Kjellesvig-Waering, 1955
  • Micopteridae Caster & Kjellesvig-Waering, 1955
  • Superfamily Waeringopteroidea
  • Waeringopteridae (not formally published)
  • Superfamily Adelophthalmoidea Tollerton, 1989
  • Adelophthalmidae Tollerton, 1989
  • Superfamily Pterygotioidea Clarke & Ruedemann, 1912
  • Hughmilleriidae Kjellesvig-Waering, 1951
  • Slimonidae Novojilov, 1968

The trace fossil trackways produced by eurypterids are placed in the ichnogenus Palmichnium.[15]

Phylogeny


The cladogram presented here is simplified from a study by Tetlie.[16] The most important phylogenetic breakdown is based on the two major innovations that characterise the evolution of the eurypterids. The most important was the transformation of the posteriormost prosomal appendage into a swimming paddle (as found in the clade Eurypterina). The second innovation was the enlargement of the chelicerae, (as found in the family Pterygotidae), allowing these appendages to be used for active prey capture.

75% of eurypterid species are eurypterines; this represents 99% of specimens.[1] The superfamily Pterygotioidea is the most species-rich clade with 56 species, followed by the Adelophthalmoidea with 43 species; as sister taxa, they comprise the most derived eurypterids. Pterygotioidea includes the pterygotids, which are the only eurypterids to have a cosmopolitan distribution.[16] This clade is one of the best supported within the eurypterids.

It has been suggested that the development of dermal armour in certain groups of jawless vertebrates (such as the Heterostraci and the Osteostraci) is in response to predation pressure by increasingly sophisticated eurypterid predators[17] (specifically the pterygotids) although this has yet to be verified by detailed analysis.[18] An increase in fish diversity is tied to a decline in eurypterid diversity in the Lower Devonian,[19] although it is not thought that this represents competitive replacement; in fact, this is rare in the fossil record.[20]

Eurypterida


Stylonurina[Note 1]



Eurypterina

Megalograptoidea[Note 2]




Eurypteroidea




Mixopteroidea




Waeringopteroidea




Adelopthalmoidea


Pterygotioidea

Hughmilleria




Herefordopterus




Slimonia




Pterygotidae












See also

Arthropods portal
Palaeontology portal

References

Further reading

External links

  • Eurypterids.co.uk – eurypterid information website
  • Life-like reconstruction of a eurypterid
  • Eurypterida from the Palaeos website
  • at the Palaeoblog
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