Meet Cambroraster falcatus, the sediment-sifting ‘Roomba’ of the Cambrian

Nowadays, the sight of a footlong horseshoe crab probably wouldn’t stop you in your tracks. But half a billion years ago, the threat of such a creature would have made the tiny denizens of the Cambrian-era seafloor quake in their boots.

Such was the reign of terror wrought by Cambroraster falcatus, an early arthropod relative recently unearthed from the Burgess Shale deposit in British Columbia, Canada. Equipped with rake-like claws, a vacuum-like mouth rimmed with tiny teeth, and an enormous, armored head, this predator was probably a strange sight to behold.

But for all its unnerving accoutrements, Cambroraster wasn’t exactly your typical assassin, new research suggests. Unlike some of its fast-swimming, prey-snatching cousins, this crustacean-like critter seems to have spent most of its time sifting through the sands at the bottom of the prehistoric sea, hoping to suck up a meal.

And at a time when the tree of life was rapidly sprouting new branches—including many of the animal groups that still exist today—Cambroraster’s kooky combination of traits may have been the hard-shelled cherry on the paleontological sundae that is Cambrian arthropod diversity.

“This is basically the Roomba of the Cambrian,” says Rhiannon LaVine, a paleontologist and arthropod expert at the University of Chicago who was not involved in the study. “It shows that, in terms of their feeding strategies, arthropods at the time were a lot more diverse than people may have thought.”

The millennia around the Cambrian period, which began roughly 540 million years ago, ushered in a series of evolutionary booms that not-so-discreetly upended the state of life on Earth. Animals that resembled little more than sluggish, amorphous blobs were rapidly replaced by swift, complex critters sporting funhouse variants of traits still seen today: keen sets of eyes that occurred in odd numbers, segmented limbs so robust they seemed to take the place of heads.

With a whole suite of novel anatomical features to stretch and flex, organisms wasted no time scurrying into new habitats—driving newly minted predators with a hankering for flesh to follow suit.

That’s the world Cambroraster was born into, says study author Jean-Bernard Caron, an invertebrate paleontologist at the Royal Ontario Museum in Canada. But when Caron and his graduate student Joseph Moysiuk first stumbled upon a cache of 506-million-year-old Cambroraster fossils in Kootenay National Park in 2014, the remains were so peculiar that the pair wasn’t totally sure what to make of them.

One of the first features Moysiuk took notice of was Cambroraster’s oddball of a head, which appeared to sport a stocky, horseshoe-shaped helmet. The shield’s silhouette bore an uncanny resemblance to the Millennium Falcon, the iconic Star Wars spaceship, Moysiuk says, and the fossil quickly became known as the “spaceship.” (The latter half of Cambroraster’s scientific name, falcatus, is actually a nerdy nod to the moniker.)

But without more of the fossil to go on, the spaceship’s origins remained mostly mysterious—until subsequent excavations unveiled a series of rock slabs that had enshrined Cambroraster fossils by the dozen. More and more spaceships appeared until, in the summer of 2018, Moysiuk and Caron found themselves with a collection of fossil fragments from more than 150 individuals.

The sheer number of specimens is “one of the things that’s most exciting about this discovery,” especially given that fossil records from this time period are often sparse, says Alycia Stigall, a paleoecologist at Ohio University who was not involved in the study. “This allows the authors to provide a comprehensive description of this species…and makes this animal really come alive.”

By piecing together fossil snapshots from their Cambroraster cache, Moysiuk and Caron deduced that it was a member of a group of creatures called radiodonts, a now-extinct offshoot of the lineage that eventually gave rise to modern arthropods like insects, spiders, and crustaceans. One of the most iconic radiodonts was Anomalocaris canadensis: a three-foot-long shrimplike carnivore that used a pair of barbed, dexterous feeding appendages protruding from its face to snare a smorgasbord of seafaring prey.

But Cambroraster was no Anomalocaris.

The new fossil certainly shared some commonalities with its kin, including the radiodonts’ namesake mouth: a gaping “O” encircled by a ring of serrated plates, which tends to fossilize into the shape of a pineapple ring with an unsettlingly toothy interior. Many of its other features, however, suggested that Cambroraster was something of a black sheep, even in a family of bizarros.

Below its nonexistent neck, Cambroraster’s body was pretty much a soft, stubby sack of flesh adorned with gill-like flaps. But all this paled in comparison to its humongous noggin—a structure so big it took up more than half the length of Cambroraster’s footlong form, possibly even pushing the placement of its eyes onto the back half of its body, Moysiuk says.

Cambroraster was, in effect, a “floating head,” says Javier Ortega-Hernández, an invertebrate paleontologist at Harvard University who was not involved in the study. This extreme physique—reminiscent of a modern horseshoe crab—makes this radiodont very different from its relatives at the time, he says, and strongly suggests it lived close to the sea floor.

And a life on the sea floor seems to with what the researchers found beneath the early arthropod’s hefty helmet: a pair of bristled appendages that shrouded the creature’s gyre of a mouth. These claws, which look like an assemblage of forward-facing combs, probably functioned like rakes, allowing Cambroraster to sift through ocean sediment for its meals, Moysiuk says.

The claws’ colander of hooked spines would have trapped prey rustled from their sandy homes and drawn them close to Cambroraster’s waiting maw, which slurped them up like a toothy vacuum. Even the head shield may have played a role in this process, helping the shell-encrusted creature plow through the grainy sludge for prey.

This meticulous feeding strategy suggests that Cambroraster fed on a lot of tiny creatures, perhaps down to half a millimeter in length, Moysiuk says. But it’s possible this early arthropod was also gorging itself on slightly bigger catches, like soft-bodied worms.

All in all, this paints a picture of a creature that was starkly different from many of its closest relatives. Rather than taking after more aggressive hunters like Anomalocaris, Cambroraster carved out its own niche—something that might have allowed the two species to more easily coexist.

As such, the team’s discovery adds to a growing list of unusual arthropods that speckled the Cambrian landscape. Cambroraster probably wasn’t the only shell-encrusted sediment sifter around at the time. Still, its unique mash-up of traits showcases just how specialized predators could get—even during the early days of animal diversification on Earth, Ortega-Hernández says. “Early arthropods were diverse and abundant, just as they are today,” he says. “This is a group that keeps surprising us.”

Of course, that the evolutionary tactics of Cambroraster and its kin only got them so far: From the lethal Anomalocaris to the benign, bottom-feeding Cambroraster, all the world’s radiodonts eventually disappeared, leaving their distant relatives—the direct ancestors of today’s arthropods—to inherit the Earth. It’s still unclear why one group triumphed over the other. But more research on these lineages may someday tell us why certain species persisted, while others perished for good.

“All this allows us to understand life as it might have happened at that time,” Caron says. “Anything that helps us to understand the ecology of species like these can help us to understand the origin of modern animals.”