Megarachne

Megarachne is a genus of eurypterid, a 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 typespecies 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 Latin arachne meaning "spider", translates to "great spider", because the fossil had been misidentified as a large prehistoric spider.

With a body length of 54 centimetres (1.8 ft), Megarachne was a medium-sized eurypterid. If the original identification as a spider would have 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 suggest that it dwelled in fresh water and not in marine environments.

Megarachne was similar to other eurypterids within the Mycteropoidea, a rare group known primarily from South Africa and Scotland that had evolved a specialized method of feeding known as sweep-feeding in which they raked through the substrate of bodies of water in order to capture and eat smaller invertebrates. Despite only two known 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 the other members of its family, Mycterops and Woodwardopterus, represent ontogenetic stages (different developmental stages of the animal throughout its life) of a single genus.