This leggy, spiny fella is Douglassarachne acanthopoda, whom we’ll call Doug for short (only kidding). You might look at this fossilized specimen and confidently declare, “That there’s a spider.” But wait! While D. acanthopoda certainly shares many of the characteristics of modern arachnids, it has enough unique features to give palaentologists a headache when it comes to classifying it.

Spiders and their ancestors have been around on this planet for about the last 400 million years. If you were thrust back in time to the Carboniferous and landed in the coal forests of North America or Europe, you’d probably see some critters you’d recognize as spiders, harvestmen, or scorpions. But even among that pack, D. acanthopoda would have stood out.

“Douglassarachne acanthopoda comes from the famous Mazon Creek locality in Illinois and is about 308 million years old,” said Paul Selden, lead author of a new study on this enigmatic fossil, in a statement. “This compact arachnid had a body length of about 1.5 centimeters [0.6 inches] and is characterized by its remarkably robust and spiny legs – such that it is quite unlike any other arachnid known, living or extinct.”

So unlike, in fact, that Selden and co-author Jason Dunlop were forced to conclude that it simply doesn’t fit into any of our known orders of arachnids. 

“The fossil’s very spiny legs are reminiscent of some modern harvestmen, but its body plan is quite different from a harvestman or any other known arachnid group,” Dunlop explained.

Being over 300 million years old, it’s understandable that the fossil is missing a few details, like clear mouthparts, which could help narrow down an appropriate classification. This period of our planet’s history is thought to have been the first when most living groups of arachnids roamed around together, and the Mazon Creek location – where D. acanthopoda was found in the 1980s – is one of our most important portals to this time. 

“Whatever its evolutionary affinities, these spiny arachnids appear to come from a time when arachnids were experimenting with a range of different body plans,” Selden said. “Some of these later became extinct, perhaps during the so-called ‘Carboniferous Rainforest Collapse,’ a time shortly after the age of Mazon Creek when the coal forests began to fragment and die off. Or perhaps these strange arachnids clung on until the end Permian mass extinction?”

The fossil spent some years on display in the Prehistoric Life Museum as part of the David and Sandra Douglass Collection. And although this particular critter is proving tricky to classify, it did still need a scientific name. 

“The genus name Douglassarachne acknowledges the Douglass family, who kindly donated the specimen to the Field Museum of Natural History in Chicago for scientific study once it became apparent that it represented an undescribed species. Then, acanthopoda refers to the unique and characteristic spiny legs of the animal,” said Dunlop.

In some places on our planet today, you almost feel that you can’t escape from spiders, no matter how hard you try (paging Australia). New species, and some we thought we’d lost, are popping up all over the shop. 

But back in the day, Dunlop explains, “Spiders were a rather rare group.” Their ancestors would have shared the world stage with many impressive beasties that have long since been lost to us, with fossils our only chance now to discover their many-legged diversity.

And for Dunlop, “Douglassarachne acanthopoda is a particularly impressive example of one of these extinct forms.”

The study is published in the Journal of Paleontology.