Megarachne is a genus of eurypterid from the Carboniferous of Argentina. It was one of the few eurypterids, commonly know as sea scorpions, to be found in freshwater. It was famously confused for a species of spider when it was first discovered. If it was a spider, than it would have been the largest species of spider ever known. However, a reanalysis of the specimen confirmed that it was a eurypterid and not a spider.
In The Land Before Time
Megarachne in its spider form, referred to as Spiderdon, as well as being much larger than any individual in the fossil record, is seen as the fourth and final boss in The Land Before Time for GameBoy Advance. It is the boss of the fourth area in the game, Dark Cavern. It will jump at the player, attempting to land on them to deal damage, before rearing up and attempting to shoot poison at the player. However, the player can damage it by spinning into its soft underbelly when it rears up. It will be defeated after it has been hit nine times.