Cannibalism is common among the millions of modern arthropod species. A praying mantis devours its mate after mating, termites suck blood from injured congeners and mosquitoes eat larvae. But how far does this horrific way of dining go back in the history of life feeding on life?
Previous studies place the earliest cannibalism about 450 million years ago in the Late Ordovician period. But a study published last month in the journal Palaeogeography, Palaeoclimatology, Palaeoecology says even older evidence of cannibalism can be found in a 514-million-year-old treasure trove of trilobites on an island off the South Australian coast, at a site called Emu Bay. There, there are many ancient wounds on trilobite shells, and fossil excrement, probably produced by trilobites, contains even more trilobite shells. These indicate that cannibalism dates back to the early Cambrian era — more than 50 million years earlier than previously thought.
Paleontologists consider a preserved meal in fossilized guts the best evidence that one animal has eaten another. But such fossils are rare.
However, the site at Emu Bay had optimal conditions to preserve a different kind of evidence of who ate who: fossilized injuries and fossilized feces.
Trilobites have hard exoskeletons, like modern arthropods, such as horseshoe crabs or lobsters. When trilobites escaped attack, their shells registered those close calls with bite marks, crushed sections and missing chunks.
In the new study, Russell Bicknell, a paleontologist at the University of New England in Australia, focused on healed injuries in two Emu Bay trilobite species: Redlichia takooensis and Redlichia rex. dr. Bicknell collected 38 fossils of the two species from Emu Bay, some from fieldwork and others in the South Australian Museum collection. While examining the fossils, Dr. Bicknell for patterns that could tell him about attack styles, and thus who the attacker was.
The specimens with healed injuries — trilobites that survived attacks, including R. rex — were large, so something even bigger must have attacked them. Small scarred trilobites were nowhere to be seen and Dr. Bicknell had a hunch where they ended up: in fossilized feces, known as coprolites.
The coprolites in Emu Bay were large, at least 10 percent as long as the body of an adult trilobite. It’s often impossible to tell which species a shell fragment comes from, said Dr. Bicknell, but the researchers are confident that the coprolite fragments reflect the two species in the study.
“Anything smaller is consumed and turns into these beautiful coprolites,” he said. “Anything bigger gets something out of it, but it was able to get away from the attack.”
The attacker, he suspected, was most likely R. rex, which grew to nearly four inches (10 centimeters) in length, making it the “king trilobite” of its time. dr. Bicknell describes R. rex as “a horseshoe crab, but on steroids.” He sees it prowling the Cambrian seabed, chasing easy, small targets, including smaller members of its own kind.
So if R. rex produced the feces, as Dr. Bicknell suspects, the Emu Bay massacre is the oldest example of cannibalism in the fossil record.
Loren Babcock, a paleontologist at Ohio State University who has studied trilobite predation for decades, said he hoped similar studies would be conducted elsewhere to look for similar patterns or predation traits and perhaps even gut contents, which he noted could be revealed using X-rays and micro-CT scans. Whether trilobites made the coprolites, he said, “is an open question, but trilobites are a good guess for now.”
dr. Babcock added that he would be surprised if trilobites hadn’t been cannibalistic. But he also thought it was possible that another Cambrian predator, Anomalocaris, produced some of the coprolites in Emu Bay identified in Dr. Bicknell were used.
dr. Bicknell doubts whether Anomalocaris, despite its size, had the ability to crush trilobites with its spindly appendages.
Allison Daley, a paleontologist from the University of Lausanne who was not involved in the study, said the size of the coprolites in Emu Bay helped convince her that large trilobites such as R. rex could be responsible for the site’s predation.
“There just aren’t many things big enough to make those coprolites,” she said.
But she added that trilobites were unlikely to be purely cannibalistic.
“Let’s face it, if you could find something to eat that isn’t mineralized,” she said, referring to trilobite shells, “you would probably prefer to eat that.”
A completely cannibalistic species wouldn’t last long.