Creepy New Fossil Shows the Dawn of Spiders

The 305-million-year-old arachnid acts as a bridge between known proto-spiders and today's elegant weavers.

A beautifully preserved fossil from France has given paleontologists a look at an animal that is almost, but not quite, the earliest known spider.

The eight-legged creature lived 305 million years ago and has been named Idmonarachne brasieri, a reference to Idmon, the father of the skilled weaver Arachne in Greek mythology. Amateur fossil hunter Daniel Sotty discovered it in the ancient rock of Montceau-les-Mines in eastern France, and it was later placed in the collections of the Muséum National d’Histoire Naturelle in Paris.

“When I first saw it, I was unsure what kind of arachnid it was,” says University of Manchester paleontologist Russell Garwood. Only the abdomen was visible in the sample, but already Garwood could see hints of a relationship with arachnids. Later, when his team created high-resolution CT scans, they found far more preserved in the Carboniferous-age stone.

A 3D model of the ancient arachnid reveals its bulbous body, fangs, and distinct lack of silk-spinning appendages. [Illustration By Garwood Et Al 2016, Museum National D’histoire Naturelle, Paris]

“The legs and entire front half of the body was buried in the rock,” says Garwood, whose team describes the fossil this week in Proceedings of the Royal Society B. His team created a 3D model and restoration of Idmonarachne that revealed various arachnid traits, including very spider-like fangs. But the detailed models also highlighted some significant differences.

If you look closely at a modern spider (sorry, arachnophobes), the entire back half of the body is a single bulbous structure. “Not so in this creature!”, Garwood says of Idmonarachne. The ancient invertebrate retains segments along its abdomen, more like archaic forms of arachnids such as the pseudoscorpions.

Something's Missing
Idmonarachne also lacks something special that defines so much of the spider lifestyle: The fossil arachnid doesn’t have any spinnerets.  While delicately constructed webs seem synonymous with spiders, we know from the fossil record that the ability to secrete silk came before the ability to carefully control it.

Spider relatives called uraraneids, which lived from 385 million years ago through the time of Idmonarachne, could produce silk but could not build webs.

“We know from uraraneid fossils that they had spigots but not spinnerets,” Garwood says. Rather than making exquisite, sticky traps, the uraraneids and Idmonarachne might have used their silk to line their burrows or wrap up egg sacs. But the lack of extra appendages to better manage that silk separates them from true spiders, Garwood says.

Still, the results show that Idmonarachne “falls along the line of evolution towards true spiders,” says arachnologist Jonathan Coddington of the Smithsonian's National Museum of Natural History. The fossil’s combination of features “implies a fairly neat set of transitions to true silk production,” he adds.

In that sense, Idmonarachne acts as a bridge between early spider-like creatures brewing up blobs of silk and the skilled weavers that we see today—which have been so unabashedly successful that there are probably several of them within a few feet of you right now.

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