Giant predatory worm's ancient fossil burrows discovered

Millions of years ago, giant predatory worms as long as an adult human terrorized the ocean. The fearsome creatures hid under the sea floor, waiting to seize unwitting prey with their slicing jaws and drag them underground to be consumed — like they do today, recently discovered fossils suggest.

The fossils are "very, very distinctive," said Shahin Dashtgard, a professor of earth sciences at Simon Fraser University in Burnaby, B.C., who co-authored a new study describing them.

"They're like nothing we've ever seen before in the rock record."

Unlike traditional fossils that are usually formed from the hard parts of an animal's body, such as its bones or shell, the worm fossils are "trace fossils" consisting of non-biological traces such as footprints or, in this case, a burrow. The fossils are described in a study published this week in the journal Scientific Reports.

Dashtgard noted that because worms have soft bodies, they're rarely fossilized.

"So, the burrows they make is really the only record we have of what the ecosystem would look like and how diverse the ecosystem was."

Evoke the monsters of science fiction

The researchers propose that the ancient worm was similar to the modern-day Bobbit worm or sand striker, a marine predator that lives in tropical and subtropical seas in the Indo-Pacific Region and grows up to three metres long. It hides in underground burrows with just its head exposed, striking and grabbing prey, such as fish or shellfish with sharp, scissor-like jaws and dragging them into its burrow.

Bobbit worms are named for the slicing ability of their jaws, which was likened to the slicing that abused wife Lorena Bobbit did to remove her husband's penis in 1989. They have also been compared to sand crawling monsters in science fiction worlds such as Star Wars, Dune and Tremors.

Bobbit worms and their relatives are thought to have existed for a very long time. Fossil jaws of what is thought to be the oldest Bobbit worm have been found in a 400 million year old rock formation in Ontario.

But because they're soft, worms are rarely found in the fossil record.

That's why researchers have begun looking for trace fossils of soft-bodied marine animals. Ludvig Löwemark, a professor of geosciences at National Taiwan University and Masakazu Nara, a professor of biological sciences at Kochi University in Japan, two co-authors of the study, were looking for trace fossils of another ancient animal when they came across something unusual in a 20 million-year-old sandstone formation in Taiwan.

Figuring out what it was became the project of Yu Yen Pan, a master's student working with Löwemark who is now a PhD student at Simon Fraser University.

Key piece of the puzzle

The rock where the fossils were originally found, Badouzi promontory, was an ancient continental shelf about 30 or 40 metres below the surface of the ocean, said Pan.  It was likely similar to the environment found off the coast of Taiwan today. Other fossil evidence shows that it was likely a coral reef populated by animals such as stingrays and other fish, sea urchins and crustaceans such as shrimp and lobsters.

The first fossils were mostly fragments left behind by erosion, so the researchers decided to look for similar fossils in another part of the same rock layer some distance away in an area called Yehliu Geopark.

It wasn't long before Löwemark called Pan over. He had found a complete fossil,  starting with a funnel at the top that narrows to a cylindrical tube about three centimetres in diameter, descending straight into the ground for 70 or 80 centimetres, before bending horizontally into an L-shape, reaching a total length of about two metres

"We were super excited," Pan recalled. "This really could help us to connect the puzzle together and make the story more complete."

In total, the researchers found 319 fossil specimens at the two sites. A chemical analysis of the fossils found they were high in iron, which is typical of burrows made by soft-bodied animals. That's because they tend to stabilize their burrows with mucus that attracts microbes that enrich the sediment with iron.

The fact that the tunnel was L-shaped also suggested that it was made by a soft-bodied animal, as such animals can't dig too deep before the ground gets too hard and compacted for them to continue, and they need to start digging horizontally.

The burrows were different in size and shape from burrows made from other animals, such as eels or razor clams. 

But when the researchers compared the fossil burrows to the burrows of modern Bobbit worms, which inhabit modern ecosystems not much different from those that the fossil was found in, they appeared very similar.

Dashtgard suggests that means the worms have been living in a similar environment for quite a long time — about 20 million years.

'Feathery footprint' from Taiwan

The researchers named their new fossil Pennichnus formosae. The first part of the name refers to the feathery ("penna" in Latin) "footprint" ("ichnus" in Latin) left in the top "funnel" of the burrow by the way the sediments were disturbed when the animal pulled its prey inside. "Formosae" after Formosa, a former name for Taiwan, honours the place it was found, 

Pan said the fossil is notable because it provides clues about hunting behaviour of an ancient invertebrate, something that is quite rare.

David Rudkin was one of the researchers who studied the Ontario Bobbit worm jaw fossils but was not involved in the trace fossil study. Rudkin, a retired assistant curator at the Royal Ontario Museum and a retired lecturer at the University of Toronto, said while he isn't an expert in trace fossils, he found the interpretation in the new study "pretty convincing."

"The kicker, of course, would be finding a direct association in the form of either 'jaw' elements or soft-body bits within the burrows, left after the animal died in place," he said in an email.

Unfortunately, the conditions that preserve burrows and those that preserve bodies tend to be quite different, so they're rarely found together, he said. 

"Under the circumstances," he said, "I think the authors have done a nice job of making the case for these being Bobbit burrows!"

More burrows likely to be found

Murray Gingras is professor at the University of Alberta who studies traces made by modern animals and compares them to the fossil record. He wasn't involved in the new study but has gone to Australia to study the burrows of modern Bobbit worms as part of his own research.

One challenge with trace fossils, he said, is that many animals can make very similar traces and figuring out which one any given trace came from requires some interpretation. But in this case, he thinks the researchers' interpretation is reasonable and well argued.

"I think it's a fun discovery," he said. 

He said he's surprised such fossil burrows haven't been found before given how widespread Bobbit worms are and how conspicuous their burrows are.

He suspects that many more will be found now that other researchers know what to look for, and that will help uncover the animals' movements and distribution over the past 20 million years.