The undersea lair of a giant worm that ambushed passing marine creatures 20m years ago has been uncovered by fossil hunters in Taiwan.

Researchers believe the 2-metre-long burrow found in ancient marine sediment once housed a prehistoric predator that burst out of the seabed and dragged unsuspecting animals down into its lair.

The creature may have been similar to the ferocious “Bobbit worms” of today that lie in wait in sandy seafloor burrows with antennae protruding to sense passersby. Though soft-bodied, the worms possess sharp and powerful jaws that can slice a fish in two.

“After 20m years, it’s not possible to say whether this was made by an ancestor of the Bobbit worm or another predatory worm that worked in more or less the same way,” said Prof Ludvig Löwemark, a sedimentologist at National Taiwan University. “There’s huge variation in Bobbit worm behaviour, but this seems very similar to the shallow water worms that reach out, grab fish and pull them down.”

Bobbit worms, or Eunice aphroditois, take their names from the John and Lorena Bobbitt case, in which the latter – after years of physical and sexual abuse – cut off the former’s penis with a kitchen knife.

Löwemark and his colleagues discovered the fossilised lair and others like it while studying 20m-year-old sedimentary rock on the north-eastern coast of Taiwan. The burrows are strengthened with mucus and are more resilient to weathering, meaning they sometimes protrude from the fine sandstone rock faces.

The scientists were initially mystified by the trace fossils, but gradually converged on a likely suspect. At the top of the 3cm-wide burrows they noticed a distinctive pattern that looked like several inverted funnels stacked on top of each other. This gave the opening of the lair a feathered appearance in cross-section.

Having ruled out other burrowing creatures such as shrimp, and marks left by stingrays that blast the seabed with water jets to expose cowering prey, the researchers concluded the feathered entrance to the lair was caused by a hunting strategy similar to the Bobbit worm’s.

When the worms pull their prey down into their lair, the top of the burrow collapses and the worms have to rebuild it before ambushing their next meal. “This results in the stack of cone-in-cone structures that form the ‘feathers’ around the uppermost part of the tube,” said Löwemark.

Writing in the journal Scientific Reports, the researchers describe 319 such shallow water burrows preserved in 20m-year-old sandstone in Yehliu Geopark and on the nearby Badouzi promontory, suggesting the local seafloor was colonised with the beasts. The trace fossil burrows, named Pennichnus formosae, are vertical for the top metre, then run horizontal for another metre or so, perhaps because deeper sediment is harder to burrow into, and the water there is less oxygenated. Bobbit worms breathe by absorbing oxygen through their skin.

The researchers hoped the burrows might contain fossilised remains of prey or the worms themselves, but have found none so far. One reason, Löwemark said, is that burrowing worms often inject their faeces into the water and let it drift away, spreading bone fragments from past meals far and wide.

Löwemark harbours a dream to study Bobbit worms in the wild one day. “They are impressive animals,” he said. “You don’t necessarily want to snorkel too close if you find one.”