Is this a fossilized lair of the dreaded bobbit worm?

Not to use my own horn, but I know a thing or two about bizarre animals. And I can tell you without a doubt that the Bobbit worm is by far the most bizarre. The worm grows to 10 feet long and digs a burrow in the ocean floor, leaving only the bear trap of a protruding mouth. When a fish approaches, the Bobbit worm shoots out of its burrow at astonishing speed and snaps its jaws around its prey. With violent tugs, the worm then pulls the victim into its hiding place, where it eats the fish alive. (Oh, there's a video.)

Now scientists say they have found evidence that an ancestor of the bobbit worm may have threatened fish 20 million years ago. The researchers write in Scientific Reports today that hundreds of fossilized worm caves found in what is now Taiwan show tell-tale signs of combat. They didn't find the worms themselves, mind you, as boneless creatures like worms (known as invertebrates because they lack spinal columns) very rarely petrify. Instead, they discovered trace fossils, geological features that indicate the behavior of ancient animals, in sandstone that was once a seabed.

"We believe this is the first time we've actually found a trace fossil that shows how invertebrates like worms fed on vertebrates," said National Taiwan University sedimentologist Ludvig Löwemark, co-author of the new paper. "Because in the sediment record we usually find animals moving through the sediment." For example, invertebrates could dig tunnels through the ocean floor and pump water through their burrows to filter out particles. "But this is a record of much more active behavior," he continues. "The worms actually hid in the sediment, jumped out, caught their prey, and then dragged that prey down into the sediment."

Enlarge /. The sandstone formation in Taiwan where giant worms once hunted.

Ludvig Löwemark

The petrified caves are approximately 6.5 feet long. From their openings on the surface of the ocean floor, they would have run more or less directly into the dirt. Halfway up, they bent about 45 degrees and formed the shape of an L or a boomerang. Near the entrances to the tunnels, Löwemark and his colleagues noticed "collapse funnels" or heaps of sediment that had accumulated during construction. The researchers argue that this is a sign of the struggle that has persisted in the fossil record for millions of years: if a worm pulled a squirming fish down into its hiding place, sediment would invade to fill the void.

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A cross-section of a structure is shaped a bit like a spring, with the main channel being the shaft and the collapse funnels branching off in the sediment on either side. The researchers argue that this is indicative of the worms' eating habits. "When the worm has digested its prey, it reappears on the surface," says Löwemark. "It re-creates a tunnel system in the middle of these collapse structures, and so these spring-like structures are created around the tube."

Enlarge /. If you look closely, you can see the faint, fluffy lines of the feathery structures.

Ludvig Löwemark

Now, bobbit worms and their ancestors aren't the only animals digging burrows in the ocean floor. Some types of shrimp and clams (shelled mollusks like clams) do the same thing. So how could the researchers be sure that they were actually digging up a worm's house?

Shrimp are shrimp, but they can still dig long tunnels. But because they are working with grains of sand that don't normally stick together, they have to reinforce the walls of their caves with mud so that they don't collapse. Shrimp tunnels also tend to be Mazelian, and the animals build chambers that they can use as roundabouts so they can reverse directions. In clams, their cavities are usually oval because their body plan is made up of two shells glued together.

These 20 million year old caves, on the other hand, are neatly circular. This tells the scientists that its inhabitants were likely round, like modern bobbit worms are. The researchers can also determine that these caves were not reinforced, suggesting that whatever lived inside them instead supported the structures with their bodies to keep them from collapsing. Given the caves are over 6 feet long, it means we might be dealing with a massive worm.

Or more precisely, the unfortunate fish of the distant past could have been dealing with a massive worm. "The fact that we have a very nice, round shape, but without strong food, suggests that most of the time the animal actually sat in its burrow and then rushed out," says Löwemark. "And the collapse funnels – those feathers on top – then represent the struggle of the prey as it is drawn into the sediment."

Enlarge /. A look into the trace fossil of a building.

Ludvig Löwemark

Such trace fossils thus retain evidence of an ancient battle between predator and prey, the researchers argue. This gives scientists a better idea of ​​how long the bobbit worm and its ancestors ruined the lives of fish, and provides clues that a fossilized body alone could never reveal. Even if a worm's soft tissues were fossilized rather than rotten quickly, a preserved bobbit worm would provide information about morphology, but likely not behavior. "If we understand how the interaction between prey and predators worked in the past, we can better understand the paleo-ecosystem," says Löwemark.

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Enlarge /. This fieldwork was not easy.

Ludvig Löwemark

But the lack of morphology is actually a problem, says Terrence Gosliner, senior curator of invertebrate zoology at the California Academy of Sciences, who was not involved in the work. Bobbit worms belong to a class of worms known as polychaetes, some of which are vegetarian and some of which can grow as large as the predator in question. So it is possible that the leakage of a burrow is not a sign that the resident has hunted fish, but just sticks their head out to feed on other things. "I think every time a worm retreats, it will leave what I see as similar types of feathers and the collapse structures they're talking about," says Gosliner. "You might well be correct, but there are many other explanations as well."

Really, even the modern bobbit worm remains a largely mysterious creature. "Nobody has ever really investigated what their structure is and whether it is L-shaped," says Gosliner. "And so I think it's a really interesting paleontological find. But in my eyes it raises as many questions as it does answers."

Löwemark notes, however, that the feather-like structures of the den extend quite a long way vertically, which is more indicative of a ferocious struggle than peaceful grazing. The environment at that time also provides clues. "It is possible that other worms could form similar burrows," says Löwemark, "but we believe that the fact that the burrows in our paper were found in a shallow marine paleo-environment where any plant material would have arrived as small fragments contradicts a speaks herbivorous worm. "

It is of course food for thought.

This story originally appeared on wired.com.