Slimonidae

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Slimonidae
Temporal range: Silurian, 436–419  Ma
Slimonia acuminata 1.jpg
Fossil of Slimonia acuminata housed at the Senckenberg Museum of Frankfurt
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Order: Eurypterida
Infraorder: Diploperculata
Superfamily: Pterygotioidea
Family: Slimonidae
Novojilov, 1962
Type species
Slimonia acuminata
Salter, 1856
Genera

Slimonidae (the name deriving from the type genus Slimonia , which is named in honor of Welsh fossil collector and surgeon Robert Slimon) is a family of eurypterids, an extinct group of aquatic arthropods. Slimonids were members of the superfamily Pterygotioidea and the family most closely related to the derived pterygotid eurypterids, which are famous for their cheliceral claws and great size. Many characteristics of the Slimonidae, such as their flattened and expanded telsons (the posteriormost division of their bodies), support a close relationship between the two groups.

Contents

Slimonids are defined as pterygotioid eurypterids with swimming legs similar to those of the type genus, Slimonia, and the second to fifth pair of appendages being non-spiniferous. The family contains only two genera, the almost completely known Slimonia and Salteropterus , which is known only from the telson and the metastoma (a large plate part of the abdomen).

Both slimonid genera preserve flattened and expanded telsons that end in elongated telson spikes. The discovery of several articulated specimens of Slimonia with the tail segments preserved in tight curves, suggesting that the tail segments were considerably more flexible than previously thought and would have been capable of considerable side-to-side movement. Unlike the related pterygotids, the slimonids did not possess robust and powerful cheliceral claws and as such, these telson spikes may have been the primary weaponry used by Slimonia, although this theory is considered unlikely by contemporary researchers. The telson spike of Salteropterus was likely not used as a weapon and was highly distinct and different from that of any other eurypterid.

Description

Reconstruction of Slimonia Slimonia acuminata reconstruction.jpg
Reconstruction of Slimonia

Slimonid eurypterids ranged in size from 12 centimetres (5 inches) to 100 centimetres (39 inches) in length. The largest species was Slimonia acuminata , which was also the first slimonid to be described and is known from the Early to Middle Silurian of Lesmahagow, Scotland. [1]

Like all other chelicerates, and other arthropods in general, slimonid 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). The appendages were attached to the prosoma, and were characterized in slimonids as being non-spiniferous (lacking spines). [2] The telson (the posteriormost division of the body) was expanded and flattened (similarly to the telsons of the more derived pterygotid eurypterids) and ended in a thin and elongated telson spike. [3] In Salteropterus , this telson spike was even more elongated than in Slimonia and ended in a tri-lobed structure unique to the genus. [4]

Though Slimonia itself is very well known, the other genus of the family, Salteropterus, is less well known with its fossils only preserving the telson and the metastoma (a large plate part of the abdomen). [2] As such it is difficult to establish exactly which traits distinguish the family as a whole from the other pterygotioid eurypterid families (Pterygotidae and Hughmilleriidae), even though several defining traits are known of Slimonia. Many of the unique traits of Slimonia are found in the carapace (the "head"), which is not known in Salteropterus. Among these is the quadrate (square) shape of the carapace itself and the placement of the compound eyes on the frontal corners. [5] Though they were closely related, the Slimonidae had small and non-developed chelicerae (frontal appendages) in comparison to the Pterygotidae, which possessed well-developed and powerful cheliceral claws. [5]

History of research

The type species of Slimonia, S. acuminata, was first described as a species of Pterygotus , "Pterygotus acuminata", by John William Salter in 1856 based on fossils that had been discovered in Lesmahagow, Scotland. That same year David Page erected a new genus to contain the species, as several distinctive characteristics made the species considerably different from other known species of Pterygotus, among them the shape of the carapace and P. acuminata lacking the large cheliceral claws otherwise characteristic of Pterygotus. [6] The genus was named "Slimonia" after Robert Slimon, honoring the Welsh fossil collector and surgeon who was the first to discover eurypterid fossils in Lesmahagow. [7]

In the late 1800s and early 1900s new specimens were discovered of a previously fragmentary species of Eurypterus , E. abbreviatus, in Herefordshire, England. [4] One of these specimens, BGS GSM Zf-2864 (discovered in 1939), revealed a very distinct telson and features similar to Slimonia (such as the last three opisthosomal segments tapering in a way similar to in Slimonia), which suggested a close relation between this species and Slimonia. [4]

The family Slimonidae was erected as a taxon by Nestor Ivanovich Novojilov in 1962 to contain the genus Slimonia, which was considered sufficiently distinct from the genera housed in its previous family, the Hughmilleriidae. After several features had been noted that suggested a close relationship between Salteropterus and Slimonia (particularly similarities in the abdominal segments), Salteropterus was finally classified as a slimonid in 1989 by Victor P. Tollerton. [8]

Paleobiology

The large and flattened telson of Slimonia (it is also flattened in Salteropterus, but not to the full extent of that of Slimonia) is distinctive and shared only with the pterygotid eurypterids and with the derived hibbertopterid Hibbertopterus and mycteroptid Hastimima , where a flattened telson had convergently evolved. [9]

The function of these specialized telsons has historically been controversial and disputed, and whilst study has mainly been focused on telsons within the Pterygotidae, the similarity between the telson of Slimonia and its close relatives should mean that the function would likely have been similar. The pterygotids were hypothesized to have moved by undulating the entire opisthosoma (the large posterior section of the body) by moving the abdominal plates, as such undulations of the opisthosoma and telson would have acted as the propulsive method of the animal, rendering the swimming legs used by other eurypterid groups useless. [10] Fossil evidence contradicts such an hypothesis however, as eurypterid bodies were stiff dorsally (up and down) and preserve no evidence for any sort of tapering or other mechanism that would have increased flexibility. Any flexing of the body would require muscular contractions, but no major apodemes (internal ridges of the exoskeleton that supports muscular attachments) or any muscle scars indicative of large opisthosomal muscles have been found. [9] Instead, propulsion was likely generated by the sixth pair of appendages, the swimming legs used by other eurypterine eurypterids. [9]

Whilst stiff dorsally, fossil evidence suggests that Slimonia was very flexible laterally (side to side). A specimen of Slimonia acuminata from the Patrick Burn Formation of Scotland preserves a complete and articulated series of telsonal, postabdominal and preabdominal segments. In the specimen, the "tail" is bent to a considerable degree previously unseen in any eurypterid. Capable of bending its tail from side to side, it has been theorised that the tail may have been used as a weapon. The telson spine, serrated along the sides and exceeding the flattened telson in length, ends in a sharp tip and would likely have been capable of piercing prey. [3] In pterygotids, it is likely that the cheliceral claws came to replace telson spikes as weaponry as the telson spikes of that family are relatively shorter than those of the Slimonidae. [3] However, this theory has been proven to be erroneous as the fossil specimen in question was a molt, rather than an actual carcass, and did show apparent signs of disarticulation. [10]

The telson of Salteropterus is very distinctive and though its function remains unknown (possibly used for additional balancing), it was likely not used as a weapon in the same way the telson of Slimonia was. The flattened portion is trigonal and smaller than that of Slimonia but the telson spike is far longer, forming something akin to a stem with knobs running alongside it and ending in a tri-lobed organ unseen in any other eurypterid. [4]

Classification

Size comparison of the two genera in Slimonidae. Slimonidae Size.svg
Size comparison of the two genera in Slimonidae.

Slimonid eurypterids are classified as part of the Pterygotioidea superfamily, within the Diploperculata infraorder and Eurypterina suborder. [11] The Slimonidae is often interpreted as a sister-taxon (the most closely related group) to the Pterygotidae. The other Pterygotioid family, the Hughmilleriidae, has also been interpreted as the most closely related sister-taxon to the pterygotids. The discovery of Ciurcopterus , currently the most primitive known pterygotid, allowed researchers to study its features which showed that the primitive pterygotid combined characteristics of more derived members of its own family and Slimonia. In particular, the similar appendages shared between the two genera suggested that the Slimonidae was the most closely related group to the Pterygotidae. Hughmilleriid eurypterids are thus seen as a group more basal than both the slimonids and the pterygotids. [12]

The cladogram below is based on the conclusions drawn by O. Erik Tetlie (2004) on the phylogenetic positions of Herefordopterus , Salteropterus and the Pterygotioidea at large following his redescriptions of various eurypterids from Herefordshire, England, including Salteropterus. The partial lack of spines on the appendages of both slimonid genera unites them as a group and showcases that they are more derived than the hughmilleriids, on which spines appear on four to five of their podomeres (leg segments). [8]

Pterygotioidea

Hughmilleria

Herefordopterus

Slimonidae

Slimonia

Salteropterus

Pterygotidae

See also

Related Research Articles

<span class="mw-page-title-main">Eurypterid</span> Order of arthropods (fossil)

Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is likely to have appeared first either during the Early Ordovician or Late Cambrian period. With approximately 250 species, the Eurypterida is the most diverse Paleozoic chelicerate order. Following their appearance during the Ordovician, eurypterids became major components of marine faunas during the Silurian, from which the majority of eurypterid species have been described. The Silurian genus Eurypterus accounts for more than 90% of all known eurypterid specimens. Though the group continued to diversify during the subsequent Devonian period, the eurypterids were heavily affected by the Late Devonian extinction event. They declined in numbers and diversity until becoming extinct during the Permian–Triassic extinction event 251.9 million years ago.

<i>Pterygotus</i> Extinct genus of eurypterid

Pterygotus is a genus of giant predatory eurypterid, a group of extinct aquatic arthropods. Fossils of Pterygotus have been discovered in deposits ranging in age from Middle Silurian to Late Devonian, and have been referred to several different species. Fossils have been recovered from four continents; Australia, Europe, North America and South America, which indicates that Pterygotus might have had a nearly cosmopolitan (worldwide) distribution. The type species, P. anglicus, was described by Swiss naturalist Louis Agassiz in 1839, who gave it the name Pterygotus, meaning "winged one". Agassiz mistakenly believed the remains were of a giant fish; he would only realize the mistake five years later in 1844.

<i>Slimonia</i> Extinct genus of arthropods

Slimonia is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Slimonia have been discovered in deposits of Silurian age in South America and Europe. Classified as part of the family Slimonidae alongside the related Salteropterus, the genus contains three valid species, S. acuminata from Lesmahagow, Scotland, S. boliviana from Cochabamba, Bolivia and S. dubia from the Pentland Hills of Scotland and one dubious species, S. stylops, from Herefordshire, England. The generic name is derived from and honors Robert Slimon, a fossil collector and surgeon from Lesmahagow.

<i>Carcinosoma</i> Extinct genus of arthropods

Carcinosoma is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Carcinosoma are restricted to deposits of late Silurian age. Classified as part of the family Carcinosomatidae, which the genus lends its name to, Carcinosoma contains seven species from North America and Great Britain.

<i>Hughmilleria</i> Genus of extinct arthropods

Hughmilleria is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Hughmilleria have been discovered in deposits of the Silurian age in China and the United States. Classified as part of the basal family Hughmilleriidae, the genus contains three species, H. shawangunk from the eastern United States, H. socialis from Pittsford, New York, and H. wangi from Hunan, China. The genus is named in honor of the Scottish geologist Hugh Miller.

<i>Acutiramus</i> Extinct genus of arthropods

Acutiramus is a genus of giant predatory eurypterid, an extinct group of aquatic arthropods. Fossils of Acutiramus have been discovered in deposits of Late Silurian to Early Devonian age. Seven species have been described, five from North America and two from the Czech Republic. The generic name derives from Latin acuto and Latin ramus ("branch"), referring to the acute angle of the final tooth of the claws relative to the rest of the claw.

<i>Jaekelopterus</i> Extinct Devonian genus of the Eurypterida (sea scorpions)

Jaekelopterus is a genus of predatory eurypterid, a group of extinct aquatic arthropods. Fossils of Jaekelopterus have been discovered in deposits of Early Devonian age, from the Pragian and Emsian stages. There are two known species: the type species J. rhenaniae from brackish to fresh water strata in the Rhineland, and J. howelli from estuarine strata in Wyoming. The generic name combines the name of German paleontologist Otto Jaekel, who described the type species, and the Greek word πτερόν (pteron) meaning "wing".

<i>Alkenopterus</i> Extinct genus of arthropods

Alkenopterus is a genus of prehistoric eurypterid classified as part of the family Onychopterellidae. The genus contains two species, A. brevitelson and A. burglahrensis, both from the Devonian of Germany.

Salteropterus is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Salteropterus have been discovered in deposits of Late Silurian age in Britain. Classified as part of the family Slimonidae, the genus contains one known valid species, S. abbreviatus, which is known from fossils discovered in Herefordshire, England, and a dubious species, S. longilabium, with fossils discovered in Leintwardine, also in Herefordshire. The generic name honours John William Salter, who originally described S. abbreviatus as a species of Eurypterus in 1859.

<i>Erettopterus</i> Extinct genus of arthropods

Erettopterus is a genus of large predatory eurypterid, an extinct group of aquatic arthropods. Fossils of Erettopterus have been discovered in deposits ranging from Early Silurian to the Early Devonian, and have been referred to several different species. Fossils have been recovered from two continents; Europe and North America. The genus name is composed by the Ancient Greek words ἐρέττω (eréttō), which means "rower", and πτερόν (pterón), which means "wing", and therefore, "rower wing".

<i>Unionopterus</i> Genus of eurypterid

Unionopterus is a genus of eurypterid, an extinct group of aquatic arthropods commonly known as "sea scorpions". Fossils have been registered from the Early Carboniferous period. The genus contains only one species, U. anastasiae, recovered from deposits of Tournaisian to Viséan stages in Kazakhstan. Known from one single specimen which was described in a publication of Russian language with poor illustrations, Unionopterus' affinities are extremely poorly known.

<span class="mw-page-title-main">Pterygotidae</span> Extinct family of eurypterids

Pterygotidae is a family of eurypterids, an extinct group of aquatic arthropods. They were members of the superfamily Pterygotioidea. Pterygotids were the largest known arthropods to have ever lived with some members of the family, such as Jaekelopterus and Acutiramus, exceeding 2 metres (6.6 ft) in length. Their fossilized remains have been recovered in deposits ranging in age from 428 to 372 million years old.

<span class="mw-page-title-main">Carcinosomatidae</span> Extinct family of arthropods

Carcinosomatidae is a family of eurypterids, an extinct group of aquatic arthropods. They were members of the superfamily Carcinosomatoidea, also named after Carcinosoma. Fossils of carcinosomatids have been found in North America, Europe and Asia, the family possibly having achieved a worldwide distribution, and range in age from the Late Ordovician to the Early Devonian. They were among the most marine eurypterids, known almost entirely from marine environments.

<span class="mw-page-title-main">Pterygotioidea</span> Extinct superfamily of eurypterids

Pterygotioidea is a superfamily of eurypterids, an extinct group of aquatic arthropods. Pterygotioids were the most derived members of the infraorder Diploperculata and the sister group of the adelophthalmoid eurypterids. The group includes the basal and small hughmilleriids, the larger and specialized slimonids and the famous pterygotids which were equipped with robust and powerful cheliceral claws.

<span class="mw-page-title-main">Adelophthalmidae</span> Family of eurypterids, an extinct group of aquatic arthropods

Adelophthalmidae is a family of eurypterids, an extinct group of aquatic arthropods. Adelophthalmidae is the only family classified as part of the superfamily Adelophthalmoidea, which in turn is classified within the infraorder Diploperculata in the suborder Eurypterina.

<span class="mw-page-title-main">Hughmilleriidae</span> Extinct family of eurypterids

Hughmilleriidae is a family of eurypterids, an extinct group of aquatic arthropods. The hughmilleriids were the most basal members of the superfamily Pterygotioidea, in contrast with the more derived families Pterygotidae and Slimonidae. Despite their classification as pterygotioids, the hughmilleriids possessed several characteristics shared with other eurypterid groups, such as the lanceolate telson.

<i>Herefordopterus</i> Extinct genus of arthropods

Herefordopterus is a genus of eurypterid, an extinct group of aquatic arthropods. Herefordopterus is classified as part of the family Hughmilleriidae, a basal family in the highly derived Pterygotioidea superfamily of eurypterids. Fossils of the single and type species, H. banksii, have been discovered in deposits of Silurian age in Herefordshire and Shropshire, England. The genus is named after Herefordshire, where most of the Herefordopterus fossils have been found. The specific epithet honors Richard Banks, who found several well-preserved specimens, including the first Herefordopterus fossils.

<i>Necrogammarus</i> Extinct genus of arthropods

Necrogammarus salweyi is the binomial name applied to an arthropod fossil discovered in Herefordshire, England. The fossil represents a fragmentary section of the underside and an appendage of a pterygotid eurypterid, a group of large and predatory aquatic arthropods that lived from the late Silurian to the late Devonian. The Necrogammarus fossil is Late Silurian in age and its generic name means "dead lobster", deriving from Ancient Greek νεκρός and Latin gammarus ("lobster").

<i>Ciurcopterus</i> Extinct genus of arthropods

Ciurcopterus is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Ciurcopterus have been discovered in deposits of Late Silurian age in North America. Classified as part of the family Pterygotidae, the genus contains two species, C. sarlei from Pittsford, New York and C. ventricosus from Kokomo, Indiana. The genus is named in honor of Samuel J. Ciurca, Jr., who has contributed significantly to eurypterid research by discovering a large amount of eurypterid specimens, including the four specimens used to describe Ciurcopterus itself.

<span class="mw-page-title-main">Timeline of eurypterid research</span>

This timeline of eurypterid research is a chronologically ordered list of important fossil discoveries, controversies of interpretation, and taxonomic revisions of eurypterids, a group of extinct aquatic arthropods closely related to modern arachnids and horseshoe crabs that lived during the Paleozoic Era.

References

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