It boasts a pair of bulgy egg-shaped eyes, a dorsal skeleton that resembles a shield of sharp armour, and numerous pairs of limbs that could grasp, crush and chew.

And its name is a mouthful: Mollisonia plenovenatrix.

Fossils uncovered by an expedition team from the Royal Ontario Museum in Toronto have led to the discovery of this thumb-sized sea-dwelling predator that predates arachnids, the family that includes spiders and scorpions.

It was a pair of fang-like appendages at the front of the creature's mouth that most excited ROM paleontologist Jean-Bernard Caron, whose findings were published Wednesday in the science journal Nature.

The appendages, called chelicerae, meant the species was a member of the family Chelicerate. Not only that, it was that family's oldest known ancestor, at about 506 million years old.

Mollisonia plenovenatrix shown from side view. The creature is defined by its fang-like pincers, called chelicerae, seen at the front of its head. It used them to kill, hold and sometimes cut its prey. (Submitted by ROM)

"Chelicerate is a group that is familiar with everybody because it includes spiders, scorpions and horseshoe crabs. Those represent 115,000 species of animals today. And they are part of an even bigger group called arthropods which are defined by a jointed body and limbs," said Caron.

But while they are one of the largest groups of species, Caron said "their origin has always been puzzling to scientists."

That's why the discovery is so special, he told CBC Toronto.

"It's important to try to understand the origin of this group. In many ways, these fossils help provide the answer of where they come from. They tell us about the origin of innovations in animal evolution and also about ourselves."

Top photo shows the chelicerae on a horseshoe crab; bottom shows the fang-like chelicerae of a spider, which can use them to puncture the skin of a victim and inject venom. (Submitted by ROM)

Discovery made at new Burgess Shale site

The fossils were discovered in the world-renowned Burgess Shale located in the Canadian Rockies of southeastern British Columbia.

It is famous for being one of the earliest fossil beds containing fossil imprints from the Cambrian period, a time when the Earth's biodiversity exploded.

"What's exceptional about the Burgess Shale is the preservation of soft tissue. It is extraordinary, the amount of detail we see. We've discovered some really early fish," said Caron.

Since 2012, a ROM expedition team has been uncovering fossils in a new site called Marble Canyon.

But it took detailed observations and research, conducted by Caron and his colleague Cédric Aria, to be certain of their discovery of the Mollisonia.

Even though it is only as big as a thumb, Mollisonia plenovenatrix would have been a fierce predator in its day. Caron and Aria believe it preferred to hunt close to the sea floor due to its well-developed walking legs. (Submitted by ROM)

"The animals lived at the bottom of the sea. They were buried very quickly in the mud that became rock, and they were buried at different angles."

Bit-by-bit the two paleontologists were able to make out the characteristics of the prehistoric mini-beast from numerous slabs of rock.

"Before this discovery, we couldn't pinpoint the chelicerae in other Cambrian fossils, although some of them clearly have chelicerate-like characteristics," said Aria.

Paleontologist Cedric Aria, seen here at the Burgess Shale, says 'evidence is converging towards picturing the Cambrian explosion as even swifter than what we thought.' (Submitted by ROM)

An even older ancestor waiting to be discovered?

The fossils also showed the creature having back limbs like gills, suggesting it had already adapted to its watery environment.

"This discovery tells us that at the time of the Cambrian they were already there. They probably evolved earlier than that," Caron said.

"We know this is the oldest fossil showing chelicerae. But it's likely there are fossils before that we haven't found yet."

Caron intends to head back to Burgess Shale next summer to continue exploring.

In the meantime, as curator of invertebrate paleontology at the ROM, he is busy planning for a new gallery entitled Dawn of Life.

The Mollisonia will be just one of many fossils from the Burgess Shale that will be on display in the gallery, scheduled to open in 2021.