Anyone familiar with the harrowing Bobbit worm (Eunice aphroditois) sequence from Blue Planet II may remember the impressive but sinister hunting skills of these alien-like carnivorous worms. Coated in a beautiful iridescence, the Bobbit conceals most of its body in long, narrow burrows within the seafloor, with just its mouthparts showing while it keeps tabs on passersby. Should an unlucky fish swim within Bobbit range, they’re hastily snapped up and dragged under. That’s the kind of nightmare-fuel creature described in an eye-opening new study published in the journal Scientific Reports, which details a potential ancestor to the Bobbit that may have once colonized the Eurasian continent’s seabed around 20 million years ago.

The study looked at a large L-shaped burrow recreated from 319 specimens preserved within what was the seafloor across northeast Taiwan back in the Miocene. Using this, a 2-meter (6.6 foot) trace fossil, roughly 3 centimeters (1 inch) wide, was imagined by researcher Ludvig Löwemark, from National Taiwan University, Taipei, and colleagues. Researchers can use such trace fossils as a means of investigating ancient organisms’ behavior by looking at physical structures such as burrows from evidence in the fossil record.

Using this trace fossil, the researchers were able to theorize the organism that once occupied it, which they have named Pennichnus formosae. They suggest it was likely some kind of giant, predatory marine worm, similar to the Bobbit worm found in our oceans today. The study authors believe the specific way in which P. formosae’s burrows showed signs of collapse could be evidence that these enormous worms also hunted by dragging prey down into their lair, just like the Bobbit.

They decided to take a closer look at the burrow specimens and analyses revealed that there was a high concentration of iron near the top of the burrow. This could well have come from the worm using mucus as a means of repairing damage to the burrow. This mucus in extant animals is known to be a veritable feast for certain bacteria, which are known to contribute towards iron-rich marine environments.

Big or small, marine worms are tricky customers for scientists who specialize in ancient species and specimens. Their bodies are mostly made up of soft tissues that rarely make it into the fossil record, and so instead they must be studied from trace fossils such as P. formosae which are crafted from clues in the ancient environment. While in the eyes of some this is not quite as sexy as wielding an ancient, giant Tremors-esque death-worm specimen, these trace fossils provide a valuable insight into life beneath the sand for a sub-surface ambush predator.