Megarachne

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Megarachne
Temporal range: Gzhelian
303.7–298.9  Ma
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Megarachne servinei.jpg
Cast of the holotype specimen of Megarachne exhibited at Royal Ontario Museum
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Order: Eurypterida
Superfamily: Mycteropoidea
Family: Mycteroptidae
Genus: Megarachne
Hünicken, 1980
Species:
M. servinei
Binomial name
Megarachne servinei
Hünicken, 1980

Megarachne is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Megarachne have been discovered in deposits of Late Carboniferous age, from the Gzhelian stage, in San Luis, Argentina. The fossils of the single and type species M. servinei have been recovered from deposits that had once been a freshwater environment. The generic name, composed of the Ancient Greek μέγας (megas) meaning "great" and Ancient Greek ἀράχνη (arachne) meaning "spider", translates to "great spider", because the fossil was misidentified as a large prehistoric spider.

Contents

With a body length of 54 cm (21 in), Megarachne was a medium-sized eurypterid. If the original identification as a spider had been correct, Megarachne would have been the largest known spider to have ever lived. Eurypterids such as Megarachne are often called "sea scorpions", but the strata in which Megarachne has been found indicates that it dwelled in freshwater and not in marine environments.

Megarachne was similar to other eurypterids within the Mycteropoidea, a rare group known primarily from South Africa and Scotland. The mycteropoids had evolved a specialized method of feeding referred to as sweep-feeding. This involved raking through the substrate of riverbeds in order to capture and eat smaller invertebrates. Despite only two specimens having been recovered, Megarachne represents the most complete eurypterid discovered in Carboniferous deposits in South America so far. [1] Due to their fragmentary fossil record and similarities between the genera, some researchers have hypothesized that Megarachne and two other members of its family, Mycterops and Woodwardopterus , represent different developmental stages of a single genus.

Description

Reconstruction of Megarachne. Parts missing from its fossils, such as the telson, are based on the related Woodwardopterus. Megarachne BW.jpg
Reconstruction of Megarachne. Parts missing from its fossils, such as the telson, are based on the related Woodwardopterus .

Known fossils of Megarachne indicate a body length of 54 cm (21 in). Whilst large for an arthropod, Megarachne was dwarfed by other eurypterids, even relatively close relatives such as Hibbertopterus which could reach lengths exceeding 1.5 m (59 in). [2] Though originally described as a giant spider, a multitude of features support the classification of Megarachne as a eurypterid. Among them, the raised lunules (the vaguely moon-shaped ornamentation, similar to scales) and the cuticular sculpture of the mucrones (a dividing ridge continuing uninterrupted throughout the carapace, the part of the exoskeleton which covers the head) are especially important since these features are characteristic of eurypterids. [3]

Megarachne possessed blade-like structures on its appendages (limbs) which would have allowed it to engage in a feeding method known as sweep-feeding, raking through the soft sediment of aquatic environments in swamps and rivers with its frontal appendage blades to capture and feed on small invertebrates. Megarachne also possessed a large and circular second opisthosomal tergite (the second dorsal segment of the abdomen), the function of which remains unknown. [3]

Megarachne was very similar to other mycteroptid eurypterids in appearance, a group distinguished from other mycteropoids by the parabolic shape of their prosoma (the head plate), hastate telsons (the hindmost part of the body being shaped like a gladius , a Roman sword) with paired keel-shaped projections on the underside, [4] and heads with small compound eyes that were roughly trapezoidal in shape. [5]

History of research

Outdated and incorrect reconstruction of Megarachne as a giant spider exhibited at the Museum d'histoire naturelle in Geneva, Switzerland. Outdated Megarachne.jpg
Outdated and incorrect reconstruction of Megarachne as a giant spider exhibited at the Muséum d'histoire naturelle in Geneva, Switzerland.

Megarachne servinei was originally described in 1980 by the Argentinean paleontologist Mario Hünicken. The generic name, composed of the Ancient Greek μέγας (megas) meaning "great" and Latin arachne meaning "spider", translates to "great spider". The holotype (now stored at the Museum of Paleontology at the National University of Córdoba) was recovered from the Pallero Member of the Bajo de Véliz Formation of Argentina, which has been dated to the Gzhelian age, 303.7 to 298.9 million years ago. [6] [7] The specimen preserves the carapace, the first two tergites, three partial appendages and what is possibly a coxa (the proximalmost limb segment). [1] [6]

Hünicken wrongly identified the specimen as a mygalomorph spider (the group that includes tarantulae) based on the shape of the carapace, the 15-millimetre (0.59 in) wide circular eye tubercle (round outgrowth) located in the center of the head between the two eyes and a circular structure behind the first body segment which he identified as the "moderately hairy" abdomen. Hünicken's identification relied heavily on X-ray microtomography of the holotype. Additional hidden structures – such as a sternum and labium, coxae and cheliceral fangs – were also extrapolated from the X-radiographs. [6]

With an estimated length of 33.9 cm (13.3 in) based on the assumption that the fossil was of a spider, and a legspan estimated to be 50 centimetres (20 in), Megarachne servinei would have been the largest spider to have ever existed, exceeding the goliath birdeater (Theraphosa blondi) which has a maximum legspan of around 30 cm (12 in). Because of its status as the "largest spider to have ever lived", Megarachne quickly became popular. Based on Hünicken's detailed description of the fossil specimen and various other illustrations and reconstructions made by him, reconstructions of Megarachne as a giant spider were set up in museums around the world. [8] [7]

The identification of the specimen as a spider was doubted by some arachnologists, such as Shear and colleagues (1989), who stated that whilst Megarachne had been assigned to the Araneae, it "may represent an unnamed order or a ricinuleid". [9] Even Hünicken himself acknowledged discrepancies in the morphology of the fossil that could not be accommodated with an arachnid identity. These discrepancies included an unusual cuticular ornamentation, the carapace being divided into frontal and rear parts by a suture and spatulate (having a broad, rounded end) chelicerae (already noted by Hünicken as a strange feature as no known spider possesses spatulate chelicerae), all features unknown in other spiders. However, the holotype was by then deposited in a bank vault so other paleontologists only had access to plaster casts. [8]

In 2005, a second, more complete specimen consisting of a part and counterpart (the matching halves of a compression fossil) was recovered, preserving parts of the front section of the body, as well as coxae possibly from the fourth pair of appendages, was recovered from the same locality and horizon. [6] A research team led by the British paleontologist and arachnologist Paul A. Selden and also consisting of Hünicken and Argentinean arachnologist José A. Corronca reexamined the holotype in light of the new discovery. They concluded that Megarachne servinei was a large eurypterid (a group also known as "sea scorpions"), not a spider. [3] [7] Although Hünicken had misidentified Megarachne, his identification as an arachnid was not entirely absurd as the two groups are closely related. [10] A morphological comparison with other eurypterids indicated that Megarachne most closely resembled another large Permo-Carboniferous eurypterid, the mycteroptid Woodwardopterus scabrosus which is known only from a single specimen. [3] Selden and colleagues (2005) concluded that despite only being represented by two known specimens, Megarachne is the most complete eurypterid discovered in Carboniferous deposits in South America so far. [1]

Classification

Cast of the holotype specimen of Megarachne exhibited at the Naturalis Biodiversity Center in Leiden, Netherlands Megarachne servinei Naturalis.JPG
Cast of the holotype specimen of Megarachne exhibited at the Naturalis Biodiversity Center in Leiden, Netherlands
Figure by Edward Drinker Cope (1886) of the carapace and part of the abdomen of the related and possibly congeneric Mycterops. Mycterops.png
Figure by Edward Drinker Cope (1886) of the carapace and part of the abdomen of the related and possibly congeneric Mycterops .

Megarachne was part of a relatively rare clade of eurypterids, the stylonurine suborder of extinct "sea scorpions". Within the stylonurines, Megarachne is a member of the superfamily Mycteropoidea (synonymous to Hibbertopteroidea) and its constituent family Mycteropidae, which includes the close relatives Woodwardopterus and Mycterops . [4]

Fossilized remains of the second tergite of the mycteroptid Woodwardopterus were compared to the fossil remains of Megarachne by Selden and colleagues (2005), which revealed that they were virtually identical, including features previously not noted in Woodwardopterus, such as radiating lines covering the tergite. It was concluded that Megarachne and Woodwardopterus were part of the same family by Selden and colleagues (2005), with two primary differences; the tergites and the mucrones on the carapace are more sparsely packed in Megarachne and the protrusion of the anteroedian (i.e. before the middle) carapace, seen prominently in Megarachne, does not occur in Woodwardopterus. [3]

It has been suggested that three of the four genera that constitute the Mycteroptidae, Mycterops, Woodwardopterus and Megarachne, might represent different ontogenetic stages (different developmental stages of the animal during its life) of each other based on their morphology and the size of the specimens. [4] Should this interpretation be correct, the sparse mucrones of Megarachne might be because of its age, as Megarachne is significantly larger than Woodwardopterus. The smallest genus, Mycterops, has even more densely packed ornaments on its carapace and tergite and might thus be the youngest ontogenetic stage of the animal. [3] Should Mycterops, Megarachne and Woodwardopterus represent the same animal, the name taking priority would be Mycterops as it was named first, in 1886. [11]

The cladogram below is adapted from Lamsdell and colleagues (2010) [4] and shows the relationship of Megarachne within the suborder Stylonurina.

Stylonurina
Rhenopteridae

Brachyopterus

Kiaeropterus

Brachyopterella

Rhenopterinae

Alkenopterus

Rhenopterus

Stylonuroidea

Parastylonurus

Stylonurella

Stylonuridae

Pagea

Stylonurus

Laurieipterinae

Laurieipterus

Ctenopterus

Kokomopteroidea
Kokomopteridae

Lamontopterus

Kokomopterus

Hardieopteridae

Hardieopterus

Tarsopterella

Hallipterus

Mycteropoidea

Drepanopterus

Hibbertopteridae

Cyrtoctenus

Hibbertopterus

Mycteroptidae

Woodwardopterus

Megarachne

Mycterops

Paleoecology

The size of Megarachne compared to a human Megarachne Size.svg
The size of Megarachne compared to a human

Both known specimens of Megarachne have been recovered from the Bajo de Véliz Formation of Argentina, dated to the Gzhelian stage of the Late Carboniferous. [6] [7] The environment of the Bajo de Véliz formation was, unlike the typical living environments of eurypterids (especially the swimming eurypterids of the suborder Eurypterina), a freshwater environment [6] in a floodplain. [12] Similar Late Carboniferous floodplains with fossilized remnants discovered in modern-day Australia suggest a flora dominated by different types of pteridosperms with pockets of isoetoid lycopsids. [13]

During Megarachne's time, Argentina and the rest of South America was part of the ancient supercontinent Gondwana which was beginning to fuse with the northern continents of Euramerica, North China, Siberia and Kazakhstania to form Pangaea. [13] In addition to Megarachne, the Bajo de Véliz Formation preserves a wide array of fossilized flying insects, such as Rigattoptera (classified in the order Protorthoptera), [14] but as a freshwater predator, Megarachne would probably not have fed on them. Instead, the blades on the frontal appendages of Megarachne would have allowed it to sweep-feed, raking through the soft sediment of the rivers it inhabited in order to capture and feed on small invertebrates. [3] This feeding strategy was common to other mycteropoids. [4]

In comparison to the comparatively warm climate of the earlier parts of the Carboniferous, the Late Carboniferous was relatively cold globally. This climate change likely occurred during the Middle Carboniferous due to falling CO
2
levels in the atmosphere and high oxygen levels. The Southern Hemisphere, where Argentina was and still is located, may even have experienced glaciation with large continental ice sheets similar to the modern glacial ice sheets of the Arctic and Antarctica, or smaller glaciers in dispersed centers. The spread of the ice sheets also affected sea levels, which would rise and fall throughout the period. Late Carboniferous flora was low in diversity but also developed uniformly throughout Gondwana. The plant life consisted of pteridosperm trees such as Nothorhacopteris , Triphyllopteris and Botrychiopsis , and lycopsid trees Malanzania , Lepidodendropsis and Bumbudendron . The plant fossils present also suggest that it was subject to monsoons during certain time intervals. [13]

During the production of the 2005 British documentary Walking with Monsters , Megarachne was slated to appear as a giant tarantula-like spider hunting the cat-sized reptile Petrolacosaurus in the segment detailing the Carboniferous, closely by following what was thought to be known of the genus at the time the series began production. The actual identity of the genus, as a eurypterid, was only discovered well into production and by then it was far too late to update the reconstructions. The scenes were left in, but the giant spider was renamed as an unspecified species belonging to the primitive spider suborder Mesothelae, a suborder that actually exists but with genera much smaller than, and looking considerably different from, the spider featured in the program. [7]

Related Research Articles

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Eurypterid 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.

Stylonurina one of two suborders of eurypterids

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<i>Hibbertopterus</i> genus of arthropods (fossil)

Hibbertopterus is a genus of eurypterid, a group of extinct aquatic arthropods. Fossils of Hibbertopterus have been discovered in deposits ranging from the Devonian period in Belgium, Scotland and the United States to the Carboniferous period in Scotland, Ireland, the Czech Republic and South Africa. The type species, H. scouleri, was first named as a species of the significantly different Eurypterus by Samuel Hibbert in 1836. The generic name Hibbertopterus, coined more than a century later, combines his name and the Greek word πτερόν (pteron) meaning "wing".

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<i>Mycterops</i> Extinct genus of sea scorpions

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Mycteroptidae group of extinct chelicerate arthropods

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<i>Pentecopterus</i> genus of Eurypterid

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Eurypterina suborder of eurypterid

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Mycteropoidea taxon

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Strobilopteridae

Strobilopteridae is an extinct family of eurypterids that lived in the Silurian and Devonian periods. The family is one of three families contained in the superfamily Eurypteroidea, which in turn is one of the superfamilies classified as part of the suborder Eurypterina. The family contains two genera, Buffalopterus and Strobilopterus.

<i>Ciurcopterus</i>

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.

Timeline of eurypterid research

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.

Bajo de Véliz Formation Carboniferous and Permian stratigraphic formation in Argentina

The Bajo de Véliz Formation is stratigraphic formation of the Paleozoic located in Paganzo Basin, Argentina. This formation lies in the northwestern sector of San Luis, about 25 km west of Santa Rosa de Conlara. Containing a slice of the Carboniferous-Permian transition, it is one of the key upper Paleozoic successions characterizing that era in westernmost Gondwana, which South America was a part of in the Paleozoic.

References

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