Megarachne servinei

In 1980 a scientist discovered a fossil of what he thought was the biggest spider to ever live. As large as a small dog, Megarachne had a heavily armored body, giant eyes and a leg span of up to 20 inches! For many years, the life of this huge swamp-dwelling creature was a mystery. Only now do we know the true story-Megarachne was actually a giant prehistoric sea scorpion."What Big Eyes Do You Have! Megarachne's large compound eyes measured up to half an inch in diameter. When the creature was underwater, its eyes probably poked up from the water like a crocodile's.""In Good Company: Like modern spiders and scorpions, Megarachne had a pair of feeding appendages at the front of its mouth called Chelicerae, which it used to pass food into its mouth."

Spider or Scorpion?
The first Megarachne fossil was locked away soon after its discovery. Since no one could examine the specimen, they couldn't prove that it really was a prehistoric spider. Finally, in 2004, a British spider expert was allowed to study the fossil, and he quickly found that Megarachne actually belonged to the family of prehistoric sea scorpions known as eurypterids. Unlike spiders, which have eight small eyes, Megarachne had two big ones. And it had a hard shell with ridges across its back-unlike any spider known to science."Clean Sweep: Though most eurypterids were predators, scientists believe Megarachne was a 'sweep feeder.' Its feeding appendages had long combs on them, like baleen plates in a whale's mouth, which Megarachne used to sift through mud and water and filter out food.""Time to Change: Megarachne most likely lived on land and in the water, like a crab. It probably molted, or shed, its outer shell in water-its bare legs would have been too soft to support its weight until its new exoskeleton hardened." Megarachne servinei is a extinct species of land eurypterid (sea scorpions) that lived in Argentina during the Carboniferous period.