A new fossil discovered in France promises to shed light on the murky evolutionary history of spiders. The rare three-dimensional fossil of the new species shows that it is nearly, but not quite, a spider, lacking the key silk-spinning adaptation that defines spiders. This 300-million-year-old arachnid is our closest view yet of the ancestor to all spiders.

The new species, discovered in Montceau-les-Mines, France and named Idmonarachne brasieri, dates to between 309 and 299 million years ago, around the time that spiders first evolved. Researchers led by Russell Garwood at the University of Manchester, used a high-resolution x-ray-based technique called microtomography to visualise the extinct arachnid’s 3D shape in incredible detail. They found that although I. brasieri shows many spider-characteristics, it lacked the key trait that defines true spiders – the presence of silk-spinning organs known as spinnerets, on the underside of the body. The authors say that Idmonarchne instead represents one of the closest relatives of all living spiders. Spinnerets, which give spiders fine control over silk production, may have been key to their success, and this new discovery suggests they were also one of the last evolutionary innovations that made spiders, spiders.

Want to know more?

• Garwood et al (2016) Almost a spider: a 305-million-year-old fossil arachnid and spider origins. Proceedings B