A spider that isn't really a spider? Paleontologists may have stumbled on one of the earliest forms of spiders, which appeared even before the dinosaurs.

University of Manchester paleontologist Russell Garwood and his colleagues discovered that the fossil embedded on a rock that dates back to about 305 million years ago is actually an arachnid that shares similar, as well as different, features with modern-day spiders.

The arachnid, which they now call Idmonarachne brasieri, has the larger part of its body, including its legs, embedded deep into the rock, but only the abdomen can be studied properly from the sample.

Garwood, who specializes in X-ray techniques to understand the evolution of early ecosystems and animals, and his team used CT scans and created 3D models. Based on the reconstruction of the arachnid, they noted some of the major differences.

For example, the body of the Idmonarachne is not entirely "bulbous" like the back of the current spiders and has features similar to that of pseudoscorpions with defined segments that run along the arachnid's abdomen.

Perhaps the biggest difference that could explain the gap in the evolution of spiders is the missing spinneret and flagellum.

The flagellum is a tail-like projection that was present in the Attercopus family, which arrived 80 million years before the Idmonarachne but has since been extinct. With the appearance of this arachnid, it could be that they had evolved, losing the flagellum and gaining limbs and fangs in the process.

However, like the Attercopus family, the Idmonarachne also doesn't have the spinneret, which means it could make silk but not web because of the missing appendage.

"Our creature probably split off the spider line after [Attercopus], but before true spiders appeared," said Garwood.

But even if it cannot be classified as a "true" spider, it still "falls along the line of evolution towards true spiders," said Jonathan Coddington, an arachnologist of the National Museum of Natural History of the Smithsonian.

The ancient rock, which was discovered by Daniel Sotty, a fossil hunter, many years ago in the fossil-rich Montceau-les-Mines in eastern France, is just one of the many collections that have been kept in a box after they had been borrowed from the Museum National d'Histoire Naturelle in the 1980s.

A study about the discovery is published in Proceedings of the Royal Society B.

Photo: Ryan Steele | Flickr