A combination of two of humans biggest fears alligators and scorpions Eurypterid was like a scorpion which, from one found fossil, could reach the size of an alligator. Primarily an oceandweller, this creature was not a true scorpion. Found all over the world, Eurypterid finally went extinct during the PermianTriassic extinction event 252.17 million years ago. Most only grew up to 8 inches (20 cm) long, but the infamous subspecies Jaekelopterus was bigger, making it the largest arthropod yet discovered.