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Brief Summary

    Eurypterid: Brief Summary
    provided by wikipedia

    Eurypterids, often informally called sea scorpions, are an extinct group of arthropods related to horseshoe crabs that include the largest known arthropods to have ever lived. They are members of the extinct order Eurypterida (Chelicerata); which is the most diverse Paleozoic chelicerate order in terms of species. The name Eurypterida comes from the Greek words eury- (meaning "broad" or "wide") and pteron (meaning "wing"). This name was chosen due to the pair of wide swimming appendages, reminiscent of wings, on the first fossil eurypterids discovered. The largest, such as Jaekelopterus, reached 2.5 metres (8 ft 2 in) in length, but most species were less than 20 centimetres (8 in). They were formidable predators that thrived in warm shallow water, in both seas and lakes, from the mid Ordovician to late Permian (467.3 to 252 million years ago).

    Although 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 had probably occurred by the Pennsylvanian subperiod. Some studies suggest that a dual respiratory system was present, which would allow short periods of time in terrestrial environments. Eurypterids are believed to have undergone ecdysis, making their significance in ecosystems difficult to assess, because it can be difficult to tell a fossil moult from a true fossil carcass. They became extinct during the Permian–Triassic extinction event or sometime before 251.902 million years ago. 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.

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Comprehensive Description

    Eurypterid
    provided by wikipedia

    Eurypterids, often informally called sea scorpions, are an extinct group of arthropods related to horseshoe crabs that include the largest known arthropods to have ever lived. They are members of the extinct order Eurypterida (Chelicerata); which is the most diverse Paleozoic chelicerate order in terms of species.[1] The name Eurypterida comes from the Greek words eury- (meaning "broad" or "wide") and pteron (meaning "wing").[2] This name was chosen due to the pair of wide swimming appendages, reminiscent of wings, on the first fossil eurypterids discovered. The largest, such as Jaekelopterus, reached 2.5 metres (8 ft 2 in) in length, but most species were less than 20 centimetres (8 in). They were formidable predators that thrived in warm shallow water, in both seas and lakes,[3] from the mid Ordovician to late Permian (467.3 to 252 million years ago).

    Although 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 had probably occurred by the Pennsylvanian subperiod. Some studies suggest that a dual respiratory system was present, which would allow short periods of time in terrestrial environments.[4] Eurypterids are believed to have undergone ecdysis, making their significance in ecosystems difficult to assess, because it can be difficult to tell a fossil moult from a true fossil carcass.[5] They became extinct during the Permian–Triassic extinction event or sometime before 251.902 million years ago. 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.

    Morphology

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    Eurypterus with body parts labelled.

    Like all other chelicerates, and other arthropods in general, eurypterids possessed segmented bodies and jointed appendages (limbs) covered in a cuticle composed of proteins and chitin. The chelicerate body is divided into two tagmata (sections); the frontal prosoma (head) and posterior opisthosoma (abdomen).[6] The prosoma was covered by a carapace (or "prosomal shield") on which the compound eyes and ocelli (simple eye-like sensory organs) were located.[7]

    The prosoma also bore six pairs of appendages which are usually referred to as appendage pairs I to VI by eurypterid researchers. The first pair of appendages, the only pair placed before the mouth, are referred to as the chelicerae (differently developed, but the same organs as spider fangs) and were equipped with small pincers used to manipulate food fragments and push them into the mouth.[7] In one lineage, the Pterygotidae, the chelicerae were large and long, with strong, well-developed teeth on specialised chelae (claws).[8] The subsequent pairs of appendages, numbers II to VI, possessed gnathobases (or "tooth-plates") on the coxae (limb segments) used for feeding. These appendages were generally walking legs that were cylindrical, spiny in some species, and tended to get larger the further back they are.[9] In the Eurypterina suborder, the larger of the two eurypterid suborders, the sixth pair of appendages is also modified into a swimming paddle to aid in traversing aquatic environments.[7]

    The opisthosoma compromises 12 segments and the telson, the very posteriormost segment which in most species takes the form of a blade-like shape.[7] In some lineages, notably the Pterygotioidea and the Hibbertopteridae, the telson was flattened and could possibly have been used as a rudder while swimming and some genera within the Carcinosomatoidea, such as Eusarcana, with a telson similar to that of scorpions, has been suggested to have been capable of using it to inject venom.[10][11] The coxae of the sixth pair of appendages are overlaid by a plate that is referred to as the metastoma. The opisthosoma itself can be divided either into a "mesosoma" (compromising segments 1 to 6) and "metasoma" (compromising segments 7 to 12) or into a "preabdomen" (compromising segments 1 to 7) and "postabdomen" (compromising segments 8 to 12).[7]

    The underside of the opisthosoma was covered in structures evolved from modified opisthosomal appendages. Throughout the opisthosoma, these structures form plate-like structures termed blatfüsse (German for "leaf-feet"). These create a branchial (relating to gills) chamber between preceding blatfüsse and the ventral surface of the opisthosoma itself, which contained the respiratory organs. The second to sixth opisthosomal segments also contained oval organs that have been interpreted as organs that aid in respiration. These organs, termed kiemenplatten, or "gill tracts", would potentially have aided eurypterids to breath air above water whilst blatfüssen, similar to organs in modern horseshoe crabs, would cover the parts that serve for underwater respiration.[7]

    The appendages of the opisthosomal segments 1 and 2 (the seventh and eighth segments overall) are fused into a structure termed the genital operculum, occupying most of the underside of the opisthosomal segment 2. Near the anterior margin of this structure, the genital appendage (also referred to as the zipfel or the median abdominal appendage) protruded. This appendage, often preserved very prominently, has consistently been interpreted as part of the reproductive system and occurs in two recognized types, assumed to correspond to male and female.[7]

    Biology

    Size

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    Size comparison of six of the largest eurypterid species, in the genera Jaekelopterus, Carcinosoma, Acutiramus (2 species), Pterygotus and Pentecopterus.

    Eurypterids were highly variable in size, depending on factors such as lifestyle, living environment and taxonomic affinity. Smaller eurypterids were likely formidable predators just like their larger relatives and sizes around 100 centimetres (3.3 ft) are common in most eurypterid groups.[12] The smallest eurypterids, Alkenopterus burglahrensis and Eocarcinosoma batrachophthalmus, measured just 3 centimetres (1.2 in) in length whilst the largest exceeded 2 metres (6.6 ft).[13]

    The largest eurypterid, and the largest known arthropod to have ever lived, is Jaekelopterus rhenaniae. A chelicera from the Emsian Klerf Formation of Willwerath, Germany measured 36.4 centimetres (14.3 in) in length, but is missing a quarter of its length, suggesting that the full chelicera would have been 45.5 centimetres (17.9 in) long. If the proportions between body length and chelicerae match those of its closest relatives, where the ratio between claw size and body length is relatively consistent, the specimen of Jaekelopterus that possessed the chelicera in question would have measured between 233 and 259 centimetres (7.64 and 8.50 ft), average 2.5 metres (8.2 ft), in length. With the chelicerae extended, another metre would be added to this length. This estimate exceeds the maximum body size of all other known giant arthropods by almost half a metre even if the extended chelicerae are not included.[14]

    The family of Jaekelopterus, the Pterygotidae, is noted for several unusually large species. Both Acutiramus, the largest species A. bohemicus measuring 2.1 metres (6.9 ft), and Pterygotus, the largest species P. grandidentatus measuring 1.75 metres (5.7 ft), were gigantic.[14] Several different contributing factors to the large size of the pterygotids have been suggested, including courtship behaviour, predation and competition over environmental resources.[15]

    Giant eurypterids were not limited to the Pterygotidae family. An isolated 12.7 centimetres (5.0 in) long fossil metastoma of the carcinosomatoid eurypterid Carcinosoma punctatum indicates that the animal would have reached a length of 2.2 metres (7.2 ft) in life, rivalling the pterygotids in size.[16] The early megalograptoid Pentecopterus decorahensis is estimated to have reached lengths of 1.7 metres (5.6 ft).[17] Typical of large eurypterids is a lightweight build. Factors such as locomotion, energy costs in moulting and respiration as well as the actual physical properties of the exoskeleton limits the size that arthropods can reach. A lightweight construction significantly decreases the influence of these factors. Pterygotids were particularly lightweight, with most large body segments preserving as thin and unmineralized.[14] Lightweight adaptations are present in other giant paleozoic arthropods as well, such as the giant insect Arthropleura, and is possibly vital for the evolution of giant size in arthropods.[18]

    In addition to the lightweight giant eurypterids, some deep-bodied walking forms of the family Hibbertopteridae were also very large. A carapace from the Carboniferous of Scotland referred to the species Hibbertoperus scouleri measures 65 cm wide. As Hibbertopterus was very wide compared to its length, the animal in question could possibly have measured just short of 2 metres (6.6 ft) in length. More robust than the pterygotids, this giant Hibbertopterus would possibly have rivalled the largest pterygotids in weight, if not surpassed them.[19]

    Locomotion

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    Illustration of subaqueous flight in Eurypterus in which the shape of the paddles and their motion through water is enough to generate lift.

    The locomotion of the eurypterids varied greatly from genus to genus. The legs of many genera were far too small to do much more than allow them to crawl across the sea bottom.[9] This is especially the case in the suborder Eurypterina. Eurypterines and Stylonurines (the other suborder) are separated by the morphology of the posteriormost prosomal appendage. In the Stylonurina, this appendage takes the form of a long and slender walking leg whilst in the Eurypterina, the leg is most usually modified and broadened into a swimming paddle.[20]

    Unlike the Eurypterines, a number of Stylonurines had elongated and powerful legs that would clearly have allowed them to walk even on land (not unlike modern crabs).[9] Studies of what are believed to be fossil trackways indicate that eurypterids used in-phase, hexapodous (six-legged) and octopodous (eight-legged) gaits.[21] 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.

    Eurypterines were more clearly aquatic, with their sixth appendage developed into a broad swimming paddle. The swimming paddles could only be moved in near-horizontal planes (not upwards or downwards).[22] As such, swimming Eurypterids are generally agreed to have utilized a rowing type of propulsion similar to that of crabs and water beetles.[23] Larger individuals may have been capable of underwater flying (or subaqueous flight) in which the motion and shape of the paddles are enough to generate lift, similar to the swimming of sea turtles and sea lions. It has a relatively slower acceleration rate than the rowing type, especially since adults have proportionally smaller paddles than juveniles. But since the larger sizes of adults mean a higher drag coefficient, using this type of propulsion is more energy-efficient.[22][24]

    Sexual dimorphism and reproduction

    Genital anatomy

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    Type A genital appendage of Adelophthalmus mansfieldi (top) and Type B genital appendage of Kokomopterus longicaudatus (bottom).

    As in many other entirely extinct groups, understanding and researching the reproduction and sexual dimorphism of eurypterids is difficult, as they are only known from fossilized shells and carapaces.[25] Sometimes two sexes of the same species have been interpreted as two different species, as was the case with two species of Drepanopterus (D. bembycoides and D. lobatus).[26] In other cases, there might not be enough apparent differences to differentiate different sexes based on morphology alone.[25]

    The eurypterid prosoma is made up of the first six exoskeleton segments fused together. The seventh segment is what is called the metastoma and the eighth plate-like segment (called the operculum) contains the genital aperature. In its center, as in modern Xiphosurans, is a genital appendage. This appendage is an elongated rod and exists in two distinct morphs, both with internal ducts. These morphs are generally referred to as "Type A" and "Type B".[25] Appendages are commonly preserved prominently in eurypterid fossils and have been the subject of various different interpretations of eurypterid reproduction and sexual dimorphism. A 1997 study by researchers Simon J. Braddy and Jason A. Dunlop examined fossils of the species Eurypterus tetragonophthalmus, one of the most completely known eurypterids and one of the few from which both types of genital appendages, and the prosomal appendages, are known. As E. tetragonophthalmus is so well known it is often used as a model eurypterid and its reproductive system should be indicative of that of the entire group.[27]

    Type A appendages are longer than type B appendages and are in E. tetragonophthalmus divided into three sections whilst the type B appendage is divided into two.[27] Such division of the appendage is common in eurypterids, but the number is not universal, for instance the appendages in the Pterygotidae family are completely undivided.[28] The type A appendage is also armed with two curved spines referred to as furca (Latin for "fork"). The presence of furca in the type B appendage is also possible and the structure may represent the unfused tips of the appendages. Located in between the dorsal and ventral surfaces of the blatfuss associated with the type A appendages were a set of organs traditionally described as either "tubular" or "horn organs" which have been interpreted as spermathecae, though this function can not be conclusively proven.[27] In arthropods, spermathecae are used to store the spermatophore received from males, which would imply that the type A appendage is the female morph and the type B appendage the male.[25] This interpretation has not been uncontroversial throughout the history of eurypterid research, with opinions shifting back and forth on which gender the longer (type A) appendage represents, with some researchers arguing that it represents a female ovipositor (used to deposit eggs) or that supposed organs used for clasping found associated with it would show that it was more likely to represent male organs. Comparison with the genital anatomy of Limulus, modern horseshoe crabs, have allowed researchers to lean towards the interpretation of type A as female and type B as male, especially since the clasping organs supposedly found associated with the type A appendages are not actually existant but similar structures are known from type B appendages. Additionally, type A appendages are clearly more complex than type B appendages, a general trend for female arthropod genitalia.[27]

    The different types of genital appendages are not necessarily the only feature that distinguishes between the sexes of eurypterids. Depending on the genus and species in question other features such as size, the amount of ornamentation and the width of the body can be the result of sexual dimorphism.[27]

    The primary function of the long, and assumed to be female, type A appendages was likely to take up spermatophore from the substrate into the reproductive tract rather than to serve as an ovipositor as arthropod ovipositors are generally much longer than eurypterid type A appendages. By rotating the sides of the operculum, it would have been possible to lower the appendage from the body. Due to the way different plates overlay at its location, the appendage would have been impossible to move without muscular contractions moving around the operculum and would thus have been kept in place whenever not used. The "horn organs", possibly spermathecae, are thought to have been connected directly to the appendage via tracts but these supposed tracts remain unpreserved in available fossil material.[27]

    Type B appendages, interpreted as belonging to male eurypterids, would have produced spermatophore and stored, and maybe shaped, it in a heart-shaped structure on the dorsal surface of the appendage. A broad genital opening would have allowed large amounts of spermatophore to be released at once. The long furca associated with type B appendages, perhaps lowered in a similar way to type A appendages, could have been used to detect whether or not a substrate would have been suitable for spermatophore deposition.[27]

    Mating

    Modern day horseshoe crabs mate in a manner where during a particular breeding time, the males mount females and deposit spermatophore onto their eggs directly. Stark differences in morphology means that this method would have been unlikely to have occurred in eurypterids but a possible mating method can be reconstructed from what is known of the form and function of the genital appendages. The male would likely have initiated some form of courtship of the female to draw her attention, possibly through some visual, chemical or tactile means. The male would then have used its claspers to lock on to the prosomal appendages of the female, allowing the male to manoeuvre the female to a suitable site for deposition of spermatophore and simultaneously avoiding possible offensive actions from the female.[27]

    Lowering his body and his furca, the male would deposit spermatophore directly onto the substrate below and then move the female to a position directly above the deposited spermatophore. Enveloping the spermatophore with her own furca, the female would penetrate it and free the sperm inside to traverse up the female genital appendage. Following the delivery, the eurypterid pair would separate and the female would store the sperm in the spermathecae for future usage.[27]

    Evolutionary history

    The earliest known eurypterids - Brachyopterus and Pentecopterus - are from the Middle Ordovician, and the last recorded members of the group - Campylocephalus - are from the Late Permian, giving a total range of over 200 million years. They were most diverse between the Middle Silurian and Early Devonian, peaking in diversity during the Late Silurian.[4]

    Origins

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    Sidneyia, a cambrian arthropod that has been described as a "missing link" between trilobites and true chelicerates (the group to which eurypterids belong).

    The origin of the eurypterids is obscure. Though "primitive eurypterids" have on occasion been described from deposits of Cambrian or even Pre-Cambrian age,[29] these fossils are not recognized as eurypterids (and sometimes not even as related forms) today. Some animals previously seen as "primitive eurypterids", such as the genus Strabops from the Cambrian of Missouri,[29] are now classified as aglaspidids or strabopids. The aglaspidids, once seen as primitive chelicerates are now seen as a group more closely related to trilobites.[30]

    A predatory arthropod known as Protichnites,[31] whose traces are 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.[32]

    The appearance of several morphologically different clades of eurypterids during the Darriwilian stage of the Ordovician indicate that the eurypterids experienced an explosive radiation during the Early Ordovician or that they first appeared during the Cambrian.[33]

    Ordovician

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    Megalograptus, a member of the Megalograptoidea, the first truly successful eurypterid group.

    Until 1882 no eurypterids were known from before the Silurian, though discoveries throughout the twentieth century and modern times have expanded the knowledge of early eurypterids from the Ordovician period.[29] The earliest eurypterids known today date from the Middle Ordovician.[4] There are reports of fossil eurypterids in deposits of Tremadocian (Early Ordovician) age in Morocco, but these have yet to be thoroughly studied.[34]

    Both major eurypterid groups, the Stylonurina and Eurypterina, had already been established 460 million years ago. The presence of members of both groups, notably Stylonurine Brachyopterus[4] and Eurypterine Pentecopterus,[33] indicate that primitive stem-eurypterids would have preceded them, though these are so far unknown in the fossil record. Furthermore, the phylogenetic positions of the few Ordovician eurypterids known indicate that they must have been very diverse during this early period of their evolution, despite being rare in the fossil record.[35]

    Indeed, the fossil record of Ordovician eurypterids is very poor. The majority of eurypterids once reportedly known from the Ordovician have since proven to be misidentifications or pseudofossils. Today only 11[33][36] species can confidently be identified as representing Ordovician eurypterids. These taxa fall into two distinct ecological categories; large and active predators of Laurentia and demersal and basal animals from Avalonia and Gondwana.[33] The Megalograptoidea of Laurentia likely represent the first truly successful eurypterid group, experiencing a radiation during the Late Ordovician.[4]

    Some genera of eurypterids appearing during the Ordovician period include:[33]

    Silurian

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    Pterygotus, a member of the Pterygotioidea, a successful group of giant predatory eurypterids that achieved a cosmopolitan distribution during the Silurian and Devonian.

    Of the 150-160 species of eurypterids known in 1916 more than half were from the Silurian, with a third from the Late Silurian alone. As such, it has been concluded that the group peaked in number and diversity during the Silurian period.[29] Approximately 100 additional eurypterid species have been described since 1916,[36] though there is still a distinct absolute peak in eurypterid diversity seen during the Late Silurian.[4]

    A vast majority of eurypterid groups are first recorded from the Silurian, among them the Stylonuroidea, Kokomopteroidea, Mycteropoidea, Pterygotioidea, Eurypteroidea and Waeringopteroidea,[36] though at least some were likely already present during the Late Ordovician.[33] The Stylonurina remained rare, as they had been during the Ordovician, but the Eurypterina saw a rapid rise in diversity and number.[4]

    The Silurian Eurypterus was the most successful eurypterid by far, dominating many marine eurypterid faunas and accounting for more than 90% of all known eurypterid specimens.[4] The Pterygotioidea, large predatory forms, achieved a cosmopolitan distribution, but would not survive for long, going extinct during the Early Devonian.[4]

    Some genera of eurypterids appearing during the Silurian period include:[36]

    Devonian

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    Fossil of Adelophthalmus, the only swimming (eurypterine) eurypterid to survive beyond the Devonian.

    The eurypterids were one of many groups affected by the Late Devonian extinction. A major decline in diversity had begun during the Early Devonian and they were rare in marine environments by the Late Devonian. Elevated extinction rates have been observed during the Frasnian (extinction of four families) and Famennian (extinction of five families) stages. The families surviving into the Carboniferous were all freshwater groups.[37] Though the diversity and number of the Eurypterina waned, a slow extinction possibly tied to the emergence of jawed vertebrates, the Stylonurina were relatively unaffected, adapting new strategies (such as sweep-feeding) to avoid competition.[38]

    The radiation of the Stylonurine Hibbertopteridae in the Late Devonian and Carboniferous was the last major radiation of eurypterids before their extinction in the Permian.[39]

    Some genera of eurypterids appearing during the Devonian period include:[36]

    Carboniferous and Permian

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    Campylocephalus, the last known surviving eurypterid.

    By the Carboniferous, only three families of eurypterids remained. The Stylonurines survived in the Hibbertopteridae and Mycteroptidae,[39] whilst the Eurypterina was represented by a single genus, Adelophthalmus.[4] Both Carboniferous Stylonurine families and Adelophthalmus would last into the Permian.[36]

    Adelophthalmus, an able swimmer, was already widespread during the Devonian, with fossils having been found in both Siberia and Australia. With the amalgamation of Pangaea during the Carboniferous and Permian, the genus gained an almost cosmopolitan distribution.[4]

    No eurypterids are known from fossil beds higher than the Permian, indicating that the last eurypterids died either during the period or in the catastrophic extinction event at its end. The Permian-Triassic extinction event was the most devastating mass extinction event recorded and also rendered many other Paleozoic groups, such as the trilobites, extinct.[40] The last known surviving eurypterid was the Late Permian Campylocephalus permianus, recovered from deposits in Russia about 250 million years old.[4]

    Some genera of eurypterids appearing during the Carboniferous and Permian periods include:[36]

    History of discovery

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    Figure of Eurypterus remipes by James E. De Kay (1825).

    The earliest discovered eurypterid fossil was unearthed in the fossil-rich Siluric rocks of New York. Dr S. L. Mitchill erroneously identified the fossil as an example of the fish Silurus in 1818, likely due to the strange, catfish-like appearance of the carapace. Seven years later, in 1825, James E. DeKay recognized the fossil as clearly belonging to an arthropod. He identified it as a crustacean of the order Branchiopoda and concluded that the genus Triops was likely closely related to it. DeKay further suggested that the fossil, which he named Eurypterus remipes, might be a missing link between the trilobites and the branchiopods.[41]

    Early descriptions of other eurypterids from New York preceded discoveries in Europe by a considerable amount of time and many European species later recognized as distinct, such as Eurypterus tetragonophthalmus from the Baltic, were first seen as examples of E. remipes.[41]

    Nieszkowski's De Euryptero Remipede (1858) featured an extensive description of E. fischeri (now seen as synonymous with E. tetragonophthalmus), which along with the monograph On the Genus Pterygotus by Huxley and Salter and an exhaustive description of the various eurypterids of New York in volume 3 of the Palaeontology of New York (1859) by James Hall contributed massively to the understanding of eurypterid diversity and biology. These publications were the first to fully describe the whole anatomy of eurypterids, recognizing the full number of prosomal appendages and the number of pre-abdominal and post-abdominal segments. Both Nieszkowski and Hall recognized that the eurypterids were closely related to modern horseshoe crabs.[41]

    In the work Anatomy and Relations of the Eurypterida (1893), Laurie added considerably to the knowledge and discussion of eurypterid anatomy and relations, focusing on how the eurypterids related to each other and to trilobites, crustaceans, scorpions, other arachnids and horseshoe crabs. The description of Eurypterus fischeri by Holm (1896) was so elaborate that the species became one of the most completely known of all extinct animals, so much so that the knowledge of E. fischeri was comparable with the knowledge of its modern relatives (such as Limulus). The description also helped solidify the close relationship between the eurypterids and the horseshoe crabs by showcasing numerous homologies between the two groups.[41]

    Classification

    Eurypterids have traditionally been regarded as close relatives of horseshoe crabs (Xiphosura), together forming a group called Merostomata.[42] Subsequent studies placed eurypterids closer to the arachnids in a group called Metastomata.[43]

    There has also been a prevailing idea that eurypterids are closely related to scorpions, which they resemble.[44] 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,[45] and the two groups were sometimes confused with one another.

    A recent summary of the relationships between arachnids and their relatives recognised Eurypterida, Xiphosura and Arachnida as three major groups, but was not able to resolve the phylogenetic relationship of any shared details between them.[46] Another suggested the eurypterids were sister group to the chasmataspids, with these two groups in turn sister group to the horseshoe crabs.[42]

    Phylogeny

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    Genera found in New York, illustrated by Charles R. Knight

    The cladogram presented here is simplified from a study by Tetlie (2007)[47] with the Stylonurina following Lamsdell et al. (2010).[39] 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.

    Seventy-five percent 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.[47] 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[48] (specifically the pterygotids) although this has yet to be verified by detailed analysis.[49] An increase in fish diversity is tied to a decline in eurypterid diversity in the Lower Devonian,[50] although it is not thought that this represents competitive replacement; in fact, this is rare in the fossil record.[51]

    .mw-parser-output table.clade{border-spacing:0;margin:0;font-size:100%;line-height:100%;border-collapse:separate;width:auto}.mw-parser-output table.clade table.clade{width:100%}.mw-parser-output table.clade td{border:0;padding:0;vertical-align:middle;text-align:center}.mw-parser-output table.clade td.clade-label{width:0.8em;border:0;padding:0 0.2em;vertical-align:bottom;text-align:center}.mw-parser-output table.clade td.clade-slabel{border:0;padding:0 0.2em;vertical-align:top;text-align:center}.mw-parser-output table.clade td.clade-bar{vertical-align:middle;text-align:left;padding:0 0.5em}.mw-parser-output table.clade td.clade-leaf{border:0;padding:0;text-align:left;vertical-align:middle}.mw-parser-output table.clade td.clade-leafR{border:0;padding:0;text-align:right} Eurypterida Stylonurina[Note 2]

    Rhenopteroidea

         

    Stylonuroidea

         

    Kokomopteroidea

       

    Mycteropoidea

            Eurypterina

    Megalograptoidea[Note 1]

         

    Eurypteroidea

         

    Carcinosomatoidea

         

    Waeringopteroidea

         

    Adelophthalmoidea

    Pterygotioidea

    Hughmilleria

         

    Herefordopterus

         

    Slimonia

         

    Pterygotidae

                         
    1. ^ The position of the megalograptids is uncertain.
    2. ^ The position of the stylonurines is highly uncertain, as noted by Tetlie.[47] [49]

    See also

    References

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