page_title: Megarachne
section_title: Description
hierarchical_section_title: Megarachne / Description
caption_attribution_description: Megarachne servinei, an eurypterid from the Upper Carboniferous of Argentina, after description by Selden et al., 2005, pencil drawing, digital coloring
context_page_description: 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 meaning "great" and Ancient Greek ἀράχνη meaning "spider", translates to "great spider", because the fossil was misidentified as a large prehistoric spider.
  With a body length of 54 cm, 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.
context_section_description: 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). 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.
  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.
  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, and heads with small compound eyes that were roughly trapezoidal in shape.
caption: Reconstruction of Megarachne. Parts missing from its fossils, such as the telson, are based on the related Woodwardopterus.
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