More big arachnological fossil news came yesterday, November 20th. In the Royal Society journal Biology Letters, Simon J. Braddy, Markus Poschmann, and O. Erik Tetlie reported having found an enormous fossil claw of a sea scorpion (order Eurypterida). The claw was 1.5 feet long, suggesting that the animal must have had a body 8 feet long. It’s the largest arthropod ever discovered.

The creature was a Jaekelopterus rhenaniae that lived about 390 million years ago in freshwater swamps and rivers. Ancient sea scorpions are thought to be the ancestors of modern arachnids. (That is, a species related to Jaekelopterus rhenaniae was likely the ancestor of spiders, not necessarily this species.)

It is not known how sea scorpions got so big. Early theories assumed that high levels of atmospheric oxygen enabled such growth, but large sea scorpions appear to have existed prior to high oxygen levels. A theory more in vogue is that competition from and predation by vertebrates may have made such large sizes infeasible in later ages. Arthropods are limited in size on land because the force of gravity would crush them under their own weight.

For more information, see the Biology Letters abstract, Science Daily, or Nature News.