“It’s the oldest evidence of animals with mandibles in the fossil record,” Jean-Bernard Caron, a University of Toronto paleontologist and a Royal Ontario Museum curator, told The Washington Post. Caron and Cedric Aria, then a paleontology graduate student at the University of Toronto, published their findings in the journal Nature on Wednesday.
Until the discovery of Tokummia, mystery clouded the origin of animals called mandibulates. The word comes from mandible; mandibles are to insects and crustaceans as jaws are to humans. Our jaws, though, suffer by comparison. Specialized mandibles allow leaf cutter ants to saw leaves, aphids to siphon sap, mosquitoes to suck blood and stag beetles to wrestle.
The 508-million-year-old mouthparts were remarkable by dint of existence, not sophistication. “These mandibles are fairly simple, structures with little tiny teeth on the side,” Caron said. “Nothing really too striking or specialized.”
Tokummia lived during the Cambrian explosion, a time named for its evolutionary burst of biodiversity. The arthropod had a host of bizarre neighbors. One fossilized Cambrian critter, Hallucigenia, was a nightmare of spines, legs and a toothed gullet. Opabinia had five eyes. Anomalocaris looked like a cross between a flounder and a pineapple.
(If Tokummia was big, Anomalocaris was a three-foot-long behemoth. In 1985, paleontologists Derek Briggs and Harry Whittington determined that Anomalocaris was a single creature and not, as scientists had previously suspected, several animal fossils clumped together.)
Hallucigenia turned out to be an evolutionary dead end. But the mandibulates endured and now include insects, crustaceans, millipedes and centipedes; they are the most successful type of animals on the planet, representing eight in 10 named species.
And Tokummia was an ancient one. “This new discovery of a 508-million-year-old fossil from British Columbia preserves clear evidence of mandibles and other features typical of crustaceans,” said Briggs, of Anomalocaris fame and who was not involved in the new research. Briggs, curator of invertebrate paleontology at the Yale Peabody Museum of Natural History, said the discovery demonstrated that the origin of this major group went all the way back to the Cambrian period.
In 2012, Caron and Aria were examining the Marble Canyon in the Canadian Rockies, a previously unexplored part of the rich Burgess Shale deposits. “At that time, we had found only two specimens: a pair of molted claws, and a carapace with the frontal pincers but not much else,” Aria wrote in an email to The Post. They returned to the site two years later.
“It is really in 2014 that the bulk of the material was unearthed, thanks to a larger quarrying team, heavy power tools, skilled helicopter pilots and a lot of sweat,” Aria said. “It was quite something to open those very large rectangular slabs of shale and see those complete, hand-sized, pristine Tokummia specimens posing for us after half a billion years of concealment.”
Paleontologist Stephen Jay Gould once remarked that the Burgess Shale Formation was the location of “the world’s most important animal fossils.” The area is unusually rich in the fossils of soft-bodied animals; though Tokummia had a shell, it was closer to a shrimp's, not as hard as a lobster's. Why, exactly, this spot preserved squishy and ancient animals is not fully understood, Caron said. Tokummia fossils retained tiny details: guts, eyes and possibly nerve tissues, too.
Its preserved anatomy included tiny projections at the tip of its limbs. Aria called these “endites,” another feature that placed Tokummia among the early mandibulates. Scientists hypothesized that these limb tips gave way to the tremendous diversity of later insect and crustacean legs, including mandibles.
Thanks to Tokummia's relatively large size, the scientists could perform what Caron described as a “dissection” of the fossils. (“A tiny fossil that's one centimeter long? Good luck,” Caron said.) In lieu of a knife, they used a device like a microengraving tool for carving glass. They peeled back the fossilized carapace, revealing a body that had been compressed in the shale from three centimeters to about three millimeters thick. And on its head below the carapace were the mandibles.
The predator probably ignored trilobites and other armored animals, as its slender pincers would have struggled with shells. “They seem to be really adapted to pinch and perforate soft prey,” Caron said.
Aria likened the hooked claws to “can openers,” he said, when he first started working on the animal. “They strike me as particularly delicate in comparison to claws of some modern crustaceans.” He pictured Tokummia fishing out worms that lived in collagen tubes — which would have been plentiful at the time — and then slicing the soft animals into small pieces with its mouth bits.
Caron and Aria said that this discovery should encourage researchers to a new look at old Cambrian specimens. “We have to look at them again,” Aria said, “and try to find mandibles and other features we might have also missed.”
As for the rest of us, Caron said, perhaps we'll have a new appreciation of the long lineage behind the shrimp in our cocktails.