Arachnid Fossils Change Ideas of Spider Ancestry

Ancient arachnid fossils from New York's Gilboa Fossil Forest show traces of a jointed tail

Arachnid fossils unearthed in New York's Gilboa Fossil Forest have led paleontologists Paul Selden, of Kansas University , and William Shear at Hampden-Sydney College to reevaluate how spiders first evolved the ability to spin webs. Their research focuses on the remains of the ancient arachnid Attercopus fimbriunguis, which date back to 386 million years. It was formerly considered the most ancient spider.

Now these researchers suggest that this bug was in fact more primitive than a true spider. Modern spiders make silk threads with mobile appendages called spinnerets, but Attercopus exuded broad sheets of silk from "spigots" on plates on their undersides.

Selden and Shear first discovered remains of Attercopus almost 20 years ago at Gilboa, the world's oldest known petrified forest. Additional specimens found near the original site caused the paleontologists to reinterpret their initial findings. They were able to see a long, jointed tail not found in any spider previously known, but common among certain putative spider ancestors.

"We think these 'tailed spiders' represent an entirely new kind of animal, not known before from living or fossil examples." Shear said. "They were more primitive than spiders in many ways, and may be spider ancestors." Besides having tails and spinning silk from broad plates, the animals also seem to lack poison glands.

Selden added, "This new information also allows us to reinterpret other fossils once thought to be spiders, and this evidence suggests these Uraraneida, or pre-spiders, existed for more than 100 million years, living alongside real spiders, which evolved later."

Selden and Shear suggest silk spigots were the precursors of modern spiders' spinnerets, which are organs that move freely enough to produce a web. They think Attercopus probably used its spigots to line burrows, make homing trails and perhaps to bind prey. But, due to the limited mobility of the spigots, they think these proto-spiders weren't capable of making the intricate traps woven by their modern descendants.

Or, if they did — Oh, what a tangled web they wove!