ROM researchers discover spiders’ 500 million-year-old ancestor

Mollisonia Plenovenatrix Illustration by Joanna Liang

If you think spiders at the cottage are gross just wait until you see what researchers from Toronto’s Royal Ontario Museum discovered in British Columbia’s Canadian Rockies. Digging through the iconic Burgess Shale, a fossil-bearing deposit in southeastern B.C., paleontologists uncovered the fossil of a Mollisonia plenovenatrix. Dating back to over 500 million years ago, the creature is the oldest known chelicerate, a group of 115,000 species that includes spiders, scorpions, and horseshoe crab.

It is not a direct ancestor, clarifies Jean-Bernard Caron, the ROM’s curator of invertebrate paleontology, “but from an animal like Mollisonia, you will have a group of organisms that evolved like the chelicerate.”

The Mollisonia existed during the Cambrian period, prior to the Cambrian explosion, an event that occurred approximately 500 million years ago when most major animal groups began to appear in the fossil record. “Basically, you can trace most of the major animal groups that you know today, like anthropoids and invertebrates, through the Cambrian period,” Caron says.

This isn’t the first time, however, that paleontologists have come face-to-face with a Mollisonia fossil. In fact, the creature’s discovery can be traced back to over a century ago when it was described by the man who first found the Burgess Shale, Charles Doolittle Walcott. Until recently, however, paleontologists had only found fossilized versions of the creature’s shell.

“Think about a suit of armour with no knight inside. You have the armour, but you don’t actually know who owned the armour,” Caron says. “It was the same idea. Who was the owner of this carapace? We didn’t know.” The most recent Mollisonia fossil, however, had soft tissue, giving researchers a full sketch of what the creature looked like and how it functioned.

Mollisonia Plenovenatrix
Photo by Jean-Bernard Caron

“Based on its eyes and the series of legs it has around its head, we think it was a predator—a tiny one,” Caron says. The Mollisonia was about the size of a thumb and lived underwater near the sea bottom. “At the time, Canada was not Canada, of course. But this area of the Canadian Rockies was actually under a tropical sea.”

The Burgess Shale has preserved much of the marine life from the Cambrian period, acting like a “time capsule.” But unlike previous Mollisonian fossils, this most recent one was discovered at Marble Canyon quarry in Kootenay National Park. The site is approximately 40 kilometres southeast of the original site in Yoho National Park where Charles Doolittle Walcott first discovered the Mollisonia.

Since 2012, researchers have been unearthing a treasure trove of fossils at Marble Canyon, finding over 50 Mollisonia specimens, many including tissue. Caron is hopeful that the discoveries will help drive research in the field. “When you find a fossil which has a popular feature like the chelicerate group, then that probably means there’s a fossil that’s older than this guy somewhere on this planet. I don’t know where, but potentially this will help push more studies and collecting in different parts of the world.”

The Mollisonia fossil will be displayed in the ROM’s new Willner Madge Gallery, Dawn of Life, set to open in 2021. “It’s actually a big deal for us and all the people of Ontario because [the gallery] focuses on the first four billion years of life on Earth with a big Canada angle in terms of which fossils will be on display,” Caron says. And it will include “many fossils from cottage country,” he adds.

If people are interested in learning more about the Burgess Shale, the ROM will be premiering a Nature of Things CBC documentary about the fossil site on October 18. “Viewers will see where the fossils come from,” Caron says. “It’s a beautiful place in the Canadian Rockies.”

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