Megarachne is a mycteropid eurypterid merostomate arthropod from the Pennsylvanian of Argentina. It was discovered in 1980 by Mario Hünicken. It was thought to be a mygalomorph spider, before finally being discovered to be a stylonurine eurypterid.
Like its semi-close cousin, Hibbertopterus, Megarachne more resembled a horseshoe crab than the generic eurypterid. It had a large, rounded cephalothorax, mid-sized eyes, a pair of small claws, 4 pairs of walking legs, and a mid-sized tail ending in a shaft-like telson. Its body would have been covered in a hard, chitinous exoskeleton.
Megarachne was a predator, feeding on small invertebrates by capturing them with its claws. As well as this, it would sometimes settle for detritus suspended in the water.
So far, all we know about Megarachne is that it used to be known as a large, tarantula-esque spider that would hide in a burrow and wait for prey to get close before pouncing at them and eating them, but now, it is known as a eurypterid that behaved similarly to Hibbertopterus; as well as this, the title of "biggest spider" now goes to the opisthothele Mongolarachne.
In popular culture
Megarachne was going to be featured in the 2nd episode of the 2005 documentary Walking with Monsters, where it would be shown as the same giant spider, living in burrows and hunting small reptiles such as Petrolacosaurus. However, it was discovered to be a eurypterid when the Carboniferous segment was already made, and so, the terrestrial arachnid was left in and recast as an unidentified giant mesothele spider.