Amateur cave explorers have found a new family of spiders in the Siskiyou Mountains of Southern Oregon, and scientists have dubbed it Trogloraptor - Latin for cave robber - for their fearsome front claws.
The explorers sent specimens to the California Academy of Sciences in San Francisco, which has the West Coast's largest collection of spiders.
Entomologists there say the spider - reddish brown and the size of a half dollar - evolved so distinctly that it requires its own taxonomic family - the first new spider family found in North America since the 1870s.
"It took us a long time to figure out what it wasn't," said Charles Griswold, curator of arachnids at the academy.
"Even longer to figure out what it is. We used anatomy. We used DNA to understand its evolutionary place. Then we consulted other experts all over the world about what this was. They all concurred with our opinion that this was something completely new to science.
"It's a good example of how science works - professional and citizen scientists share information," he added.
The discovery is described in the Friday online edition of the journal ZooKeys.
Jonathan Coddington, curator of arachnids at the Smithsonian Institution and associate director for science at the National Museum of Natural History, agreed that the spiders represent a family never seen before.
"This is really a distinct event," he said. "To walk out in the woods and find an example of an ancient lineage that no one has ever seen before is special."
Norman Platnick, curator emeritus of spiders at the American Museum of Natural History, said the discovery was as exciting to spider scientists as the discovery of a new dinosaur to palaeontologists.
"Because it belongs to one of the more primitive groups of true spiders, it has the potential to change many of our current ideas about the early evolution of spiders," he said. "But it is better than a fossil, because we can study the entire organism, along with its behavior and physiology, not just those aspects that happen to have been fossilised."
Coddington said caves tend to keep primitive species from evolving, because they are sheltered from climatic and other changes.
"Once you figure out a lifestyle, you can just do it for millions of years," he said.
The Oregon spider's species name - marchingtoni - honours Deschutes County sheriff's Deputy Neil Marchington, who was on the first Western Cave Conservancy expedition in 2010 to find the spiders in a cave on private land outside Grants Pass. A year later he led academy scientists to the site to collect live specimens.
"A lot of times, caves are very unique ecosystems, and what we find in there can be very special," said Marchington from the jail in Bend where he works. "At other times, they can be completely normal."
He said about a dozen of the explorers collected bugs from the walls and ceiling of the cave and put them in vials of preservative, then sent them down to the academy.
"We don't know exactly how they work yet," Griswold said of the spiders. "We've seen these spiders alive. But we haven't seen them eat anything yet. They are very shy.
"They make a little web, but hang under this web. They hang some of their legs out in space. This is all in the dark in a cave. We think the legs are stretched out waiting for something to come by, like a fly, and when it hits the legs, the claws may just snap shut."
Though scientists built a small artificial cave for the spiders in their lab, the spiders would not eat any of the insects, and died.
Griswold said new families of spiders are described around the world every 20 years or so, but the last ones to be found in North America date to the 1870s, when two families of desert spiders were found in Southern California, Arizona and Mexico, and another was found in the Appalachian Mountains.
After the spiders were found in the Oregon cave, scientists from San Diego State University went looking in the redwoods of Northern California, and found a spider in the same family, but a new species, living in underground cavities beneath boulders and logs, Griswold said.