The long Permarachne spinneret now known to be a tail (centre bottom)
The species once described as the world's oldest spider is a more primitive version of the web-spinning modern spider, scientists have found.
The parts of the Attercopus spider's described as spinnerets - the appendages that allow web-spinning - were not spinnerets after all.
That means that the oldest "true" spider may have arrived 80m years later than previously thought.
The results appear in Proceedings of the National Academy of Sciences.
Paul Selden of the University of Kansas and his colleagues first described Attercopus fossil remains in 1989.
Most of what the group initially found among fossil remains belonged to a group of extinct arachnids called trigonotarbids, but one bit seemed to have the modified hairs called spigots from which spider silk emerges, as well as the external, flexible appendages known as spinnerets that facilitate web-spinning.
That led the group to believe they were looking at the world's oldest known spider.
However, also present in the mix of material from the rock was a tail-like structure, which the researchers could not identify.
The analysis stopped there until more material from the site was recovered and analysed. This time the researchers found a piece of Attercopus abdomen, with a tail attached.
The existence of such a tail was also relevant to the more recently discovered Permarachne genus, whose tail was originally described as an "elongated spinneret".
The finding prompted the team to return to their original samples, which on further study showed that the spinnerets they had earlier identified were in fact rolled-up pieces of cuticle, the animals' external skeletal material.
It seems that Attercopus is a missing link, capable of producing silk but not of weaving it.
"The thing that had been called the oldest known spider we have now shown is in fact more primitive than a true spider," Professor Selden told BBC News.
The oldest "true spider", like the ones seen today, dates from the late Carboniferous period that ended about 300m years ago, though Professor Selden says that true spiders may have existed earlier but have not yet been discovered.
The 'smoking gun': a tail attached to Attercopus abdomen
The process of identifying the fossilised spider parts started with solid rock that was dissolved in acid, leaving behind organic matter that was sifted through to determine which belong to animals or plants.
"They're all microscopic fragments. What you've got is a jigsaw puzzle, with half the pieces and no picture on the box lid," Professor Selden said.
"You don't know what it's going to be if you haven't got all the pieces, so having these additional pieces means it changed the idea of what it was."
The finding is important for evolutionary biologists trying to unravel the origin of spider silk.
"The puzzle about silk was this: we knew that it wasn't used for making webs initially, for catching insects, because there were no flying insects when the earliest spiders were around," Professor Selden said.
"Here we clearly have a spider-like animal that could produce silk but didn't yet have these flexible spinnerets for weaving it into webs; we think that this sort of spider would leave a trail of silk as it moved along, using it to find its way back to its burrow."
To clean up the incomplete record of different species, the team has suggested a new order be instituted, containing the Attercopus and Permarachne species.