Oldest known spider ancestor fossil found in B.C.'s Burgess Shale
Credit to Author: The Canadian Press| Date: Wed, 11 Sep 2019 19:26:49 +0000
Tiny eyes blinking at him from the rockface of the Burgess Shale drew Jean-Bernard Caron to the fossil of the oldest known ancestor of today’s spiders and scorpions.
“I was sitting there along the quarry and I turned my head to the right and I see this glowing light coming from this rock,” he said. “Two eyes, almost staring at me.”
The eyes turned out to belong to a 500-million-year-old specimen of Mollisonia plenovenatrix — so well preserved that Caron and his colleague Cedric Aria were able for the first time to definitively place the long-gone beastie at the root of a family tree that now boasts thousands of branches.
It was only thumb-sized, a scurrier of ancient sea bottoms. Still, the two paleontologists from the Royal Ontario Museum say it would have a been a fierce predator.
Large eyes spotted prey. Long limbs propelled it across the sediments. Its head was like a modern multi-tool with limbs that could sense, grasp, crush and chew.
The tiny pair of structures in front of its mouth really got Caron and Aria excited.
Those same pincers can be seen on all members of the family Chelicerata. That’s 115,000 different species, and here was their progenitor.
The underside of this modern-day horseshoe crab, Limulus polyphemus, also known as the Atlantic horseshoe crab, shows its pincer type chelicerae at the top of the photos. In the lower half of the photo, a section of book gills can be seen by lifting its ridged protective flaps called opercula. Opercula are absent in Mollisonia and therefore represent a further step in the evolution of chelicerates. B.C.’s famed Burgess Shales have yielded another ground-breaking fossil find — this time the oldest known ancestor of today’s spiders and scorpions.
“I was really excited about this,” said Caron, who published his findings Wednesday in the journal Nature.
“Those fossils tell us about the origin of key innovations in animal evolution. It’s important to understand how they happen. Because when they happen, there is often an explosion of life that is the consequence.”
Intriguingly, the species was clearly some distance along its evolutionary path, well-adapted to its environment and breathing through thin gills layered like the pages of a book.
“This discovery tells us that at the time of the Cambrian they were already there,” Caron said. “They probably evolved earlier than that.”
That means there’s probably another, even older ancestor out there — maybe waiting in the same Burgess Shale of southeastern British Columbia.
Those rocks are renowned the world over for their wealth of fossils from the middle Cambrian period, a time when the Earth’s biodiversity exploded. What sets Burgess specimens apart is the clarity with which the soft parts of the animals are preserved.
“I feel like a boy in a candy store,” Caron said. “There are a lot of candies to choose from and the question is which one I’m going to pick and describe first.
“We could work for decades there and still feel we haven’t scratched the surface. There’s still a lot ground we haven’t covered.”
Look for the eyes, said Caron.
“The eyes are very reflective. It’s very striking. Many fossils that we discover in the rocks, the first thing you see are the eyes, like shiny spots in the rock.”
— Follow Bob Weber on Twitter at @row1960
British Columbia’s famed Burgess Shale has yielded another ground-breaking fossil find — this time the oldest known ancestor of today’s spiders and scorpions. Chutney National Park’s Burgess Shale site is near Marble Canyon shown here.