300-million-year-old fossil reveals evolution of spiders

A new fossil found in France is almost a spider, but not quite.

The arachnid, locked in iron carbonate for 305 million years, reveals the stepwise evolution of arachnids into spiders. Dubbed Idmonarachne brasieri after the Greek mythological figure Idmon, father of Arachne, a weaver turned into a spider by a jealous goddess, the “almost spider” lacks only the spinnerets that spiders use to turn silk into webs.

“It’s not quite a spider, but it’s very close to being one,” said study researcher Russell Garwood, a paleontologist at the University of Manchester in the United Kingdom.

Locked in rock

Arachnids are an ancient group with murky origins, Garwood told Live Science. The creatures were among the first land-dwellers, adopting a terrestrial life at least 420 million years ago. There are very few rocks laid down on land from that time, so little of arachnids’ early history is preserved, Garwood said. And figuring out arachnid evolutionary relationships from DNA is likewise difficult because arachnids diversified so early, leaving few traceable evolutionary changes in their genes.

The oldest known spider fossil comes from the Montceau-les-Mines, a coal seam in eastern France. That spider was 305 million years old. The newfound fossil from the same time period reveals that these ancient spiders lived alongside not-quite-spider cousins.

The 0.4-inch-long (10 millimeters) arachnid was discovered decades ago, but no one could make much of it, because the front half of the fossil is buried in rock. Computed tomography unlocked the mystery by allowing Garwood and his colleagues to peer inside the rock at the arachnid’s walking legs and mouthparts, which are important for identifying the genus and species of this kind of creature.