Meet Megarachne, the giant prehistoric spider, with a whopping 50cm leg span, that turned out to be something else entirely – BBC Discover Wildlife

Will NewtonIn 1980, a palaeontologist announced the discovery of the largest ‘spider’ ever; except this eight-legged giant wasn’t a spider, but a sea scorpion that would have to wait 25 years for its true identity to be revealed…The first remains of Megarachne were discovered by palaeontologist Mario Hünicken in 300 million-year-old rocks belonging to the Bajo de Véliz rock formation, located in central Argentina. In 1980, Hünicken described his discovery and, based on its spider-like features and estimated half-a-metre leg-span, named it Megarachne, or “Great Spider”. This discovery captured a lot of attention and soon after plaster casts of Hünicken’s giant fossil ‘spider’ were sent to museums around the world.It’s clear, looking at photos of these plaster casts, why Megarachne was initially identified as a spider – it looks just like an enormous tarantula, with a large abdomen, three long, spindly legs (the other five are missing), and two eye-like structures on top of the frontmost part of its carapace. In fact, this is exactly what Hünicken thought when he first discovered the fossil, initially identifying it as a giant mygalomorph spider – a group that includes tarantulas.For nearly 25 years, Megarachne was described as the ‘largest spider ever’; with an estimated body length of 34cm and a leg-span of 50cm, it was thought to have been larger than even the goliath birdeater (Theraphosa blondi), which has a leg-span of ~30cm and is the biggest spider in the world. However, a re-examination of Hünicken’s holotype and the discovery of a second, more complete specimen in 2005 quickly turned this interpretation of Megarachne on its head.After a thorough re-examination of Megarachne was undertaken in 2005, it was announced by palaeontologists Paul Selden, José Corronca, and Hünicken himself, that the giant arthropod discovered some 25 years earlier was not actually spider, but a eurypterid – an extinct group of aquatic arthropods more commonly known as ‘sea scorpions’.In a paper published in Biology Letters, Selden noted that Megarachne’s carapace bore more similarities with a eurypterid’s than it did with a spider’s. The shape, structure, and ornamentation of the carapace also helped Selden and his team assign Megarachne to a rare family of eurypterids known as mycteroptids. These eurypterids lived in freshwater environments and are known only from South Africa and Scotland.Although Hünicken’s initial description of Megarachne was wrong, he wasn’t far off with his initial identification – both spiders (arachnids) and eurypterids belong to the same overarching group, the chelicerates. This group of arthropods also includes scorpions, horseshoe crabs, ticks, and mites.While big for a spider, Megarachne was relatively small for a eurypterid. The largest eurypterid, Jaekelopterus, is estimated to have reached lengths of ~2.5m, which makes it the largest arthropod currently known to science.Jaekelopterus and other large eurypterids were highly predatory and used fierce-looking pincers, known as chelicerae, to grab their prey. These pincers looked a lot like a modern lobster’s, with well-developed ‘teeth’ that were capable of subduing even the slipperiest of prey.Megarachne, on the other hand, is thought to have been a ‘sweep feeder’ that used its front-facing limbs to rake through soft sediment and hoover up crustaceans and other small invertebrates.It’s often thought that the largest representative of any group of animals is long extinct. That may be the case for some groups, but it’s not true for spiders. The largest spider ever discovered, the goliath birdeater, is alive today and lives deep in the rainforests of northern South America. This giant has a body length of ~13cm, a leg-span of ~30cm, and a weight of 175g; which isn’t huge per se, but you’d definitely struggle to hold one in the palms of your hands.Despite its name, the goliath birdeater very rarely eats birds. Instead, its diet consists of other large arthropods, worms, amphibians, and even small vertebrates. After catching its prey, a goliath birdeater will drag it back to its burrow where it’ll liquify its insides before proceeding to suck it dry – like a child trying to get the last few drops out of a juice box.Like other tarantulas, Goliath birdeaters have fangs that are sharp and long enough (~4cm) to pierce human skin. They also carry venom in their fangs, though it’s relatively harmless and its effects are comparable to those of a wasp’s sting. If anything, it’s humans that pose the most threat to goliath birdeaters. In parts of northern South America they’re often caught, roasted in banana leaves, and eaten; apparently they taste just like shrimp…It’s entirely possible that, during some parts of Earth’s history, spiders larger than a Goliath birdeater existed. The problem with finding these giants is that spiders are fragile animals that don’t fossilise very well, so they very rarely appear in the fossil record. Those that do appear also tend to be quite small.If a giant spider did exist, then it likely did so during the Carboniferous – a period of Earth’s history renowned for its massive invertebrates, such as the double-duvet-sized millipede Arthropleura and the sparrowhawk-sized griffinfly Meganeura.Just how big this mystery giant would have been is anyone’s guess, but taking some anatomical and physiological constraints into account, it’s unlikely that it would have been a lot bigger than a goliath birdeater (think more Incy Wincy from the popular nursery rhyme, than Shelob from J.R.R Tolkien’s The Lord Of The Rings).Main image: cast of the holotype specimen of Megarachne exhibited at the Naturalis Biodiversity Center in Leiden, Netherlands by Ghedoghedo, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia CommonsWill Newton

Source: https://www.discoverwildlife.com/prehistoric-life/megarachne-spider-sea-scorpion