Fossils of a 305-million-year-old "almost spider" fill in a major evolutionary gap between modern spiders and ancient arachnids.
The specimen, aptly named Idmonarachne brasieri after the Greek mythological figure Idmon - the father of the skilled weaver Arachne, was found preserved in iron carbonate from central France.
While the legs, mouthparts and body of this one-centimeter-long arachnid were all spiderlike, researchers discovered the ancient arthropod lacks a key feature of all true spiders: spinnerets used to turn silk into webs.
"When I first saw it, I was unsure what kind of arachnid it was," said Russell Garwood, a paleontologist from the University of Manchester.
At first only the specimen's abdomen was visible in the fossil sample, but high-resolution CT scans later revealed more attributes preserved in the Carboniferous-age stone.
"The legs and entire front half of the body was buried in the rock," Garwood added.
The fossils of Idmonarachne were actually found decades ago, but it was not until recently that advanced technology was available to examine them in great detail without destroying them.
Following high-resolution CT scans, researchers created a 3-D model and restoration of Idmonarachne that illustrated various arachnid traits, including very spider-like fangs. However, it also identified some key differences.
In addition to missing its silk-spinning organs, Idmonarachne also lacked a tail-like appendage found on known arachnid ancestors that lived at the same time during the late Carboniferous period, suggesting it was in fact a unique species.
Furthermore, unlike modern spiders, with a single bulbous structure on the entire back half of their bodies, Idomarachne retained segments along its abdomen, similar to archaic forms of arachnids such as the pseudoscorpions.
Nonetheless, the critter's age and unique body structure is helping scientists learn more about the manner in which spiders evolved. Based on fossil analyses conducted so far, Idmonarachne is believed to be one of the closet spider relatives that likely went extinct as its cousins continued to evolve into modern spiders.
"It's not quite a spider, but it's very close to being one," Garwood said. "Arachnids as a whole are an incredibly successful group. They're the most diverse group of living organisms after insects. They're really, really successful - but we have a very limited understanding of how they are related to each other."
Their findings were recently published in the journal Proceedings of the Royal Society B: Biological Sciences.
