The fossil record is chock-full of strange extinct forms, but by far the most bizarre prehistoric critters come from some of the world's most ancient rock deposits, like the famous Burgess Shale in the Canadian Rockies. The newest weirdo to come out of these sediments wields two long, pincer-tipped arms, a dome-like shell and one of Earth's earliest pairs of mandibles.

Say hello to Tokummia. It prowled tropical waters in search of prey some 508 million years ago, during the Cambrian Period. Its beefy legs were great for stalking the seafloor, but like a lobster, it was also capable of swimming. This was a time long before whales or sharks appeared, and at an impressive ten centimetres (four inches) long, Tokummia was one of the largest predators of its time.

"The pincers of Tokummia are large, yet also delicate and complex, reminding us of the shape of a can opener," says Cédric Aria, who led the study of the new creature. "But we think they might have been too fragile to be handling shelly animals, and might have been better adapted to the capture of sizeable soft prey items."

This reconstruction shows what Tokummia may have looked like back when it was a living, swimming sea predator. Image: Lars Field

For the scientists, the most intriguing features of the animal are its mouthparts. It has sharp-edged mandibles, similar to those of insects and crustaceans today. This may not seem like anything special now, but back in the Cambrian, it was a formidable evolutionary innovation.

"Once [prey was] torn apart by the spiny limb bases under the trunk, the mandibles would have served as a revolutionary tool to cut the flesh into small, easily digestible pieces," Aria explains.

Mandibles are a hallmark of a group of animals aptly called the Mandibulata, which includes all crustaceans, insects and myriapods (millipedes and centipedes). In the modern world, the scientists point out, this is "the most diverse and abundant group of animals".

Tokummia provides hints at how these gnashing arthropods got their start – it might be one of the earliest members of this successful lineage, and the family resemblance goes beyond its jaws. Underneath the two-part shell is a centipede-like body composed of more than 50 segments, and the specialised claws in front of its mouth (called maxillipeds) are similar to those of many shrimp and crabs.

"In spite of their colossal diversity today, the origin of mandibulates had largely remained a mystery," says Aria. "Before now we've only had sparse hints at what the first arthropods with mandibles could have looked like."

The excellently preserved fossils of Tokummia provide detailed clues to the lifestyle of the species. Image: Jean-Bernard Caron

Thanks to some phenomenal fossilisation, that image is becoming clearer. The Burgess Shale is famous for its remarkably preserved fossils, representing some of the earliest experiments in animal life. The site is so important for understanding ancient history that it has been designated a UNESCO World Heritage Site.

Tokummia can now join the growing list of unusual Burgess Shale residents like the claw-faced Anomalocaris and the nearly incomprehensible Hallucigenia, both quite unlike anything that exists today.

And it looks like Tokummia might have lived among cousins. Since the researchers were able to get such a clear understanding of this animal's features, they were able to identify a handful of close relatives from the same fossil formation. Strange crustacean-like animals such as Branchiocaris, Canadaspis and Odaraia, formerly of mysterious affinities, seem to form a family alongside Tokummia – the world's earliest mandibled animals, and the predecessors of all the diversity around today.


Top header image: Royal Ontario Museum/Lars Fields via YouTube