Giant worms may have burrowed into the ancient seafloor to ambush prey

Around 20 million years ago, giant ocean worms may have burrowed into the seafloor and burst forth like the space slug from Star Wars to ambush unsuspecting fish.

Ancient underground lairs left behind by these animals appear in rocks from coastal Taiwan, researchers report January 21 in Scientific Reports. The diggers may have been analogs of modern bobbit worms (Eunice aphroditois), known for burying themselves in sand to surprise and strike their prey.

The burrows are trace fossils — evidence of animal activity preserved in the geologic record (SN: 6/15/14) such as footprints (SN: 4/27/20) or even fossilized poop (SN: 9/21/17). These newly reported fossils were first spotted in 2013 at Taiwan’s Badouzi promontory by paleontologist Masakazu Nara of Kochi University in Japan. More turned up later amid the otherworldly rock structures of Yehliu geopark, a popular tourist attraction that was once a shallow ocean ecosystem 20 million to 22 million years ago.

From 319 fossil specimens, the team was able to reconstruct the burrows.The animals drilled L-shaped paths into the seafloor. The paths had a funnel structure at the top that looks like a feather in vertical cross sections. The burrows were about 2 meters long and 2 to 3 centimeters wide.

“Compared to other trace fossils, which are usually only a few tens of centimeters long, this trace fossil was huge,” says Yu-Yen Pan, a geologist at Simon Fraser University in Burnaby, Canada. She dubbed the trace fossil Pennichnus formosae, combining the Latin words for feather, footprint and beautiful.

The burrows were most likely dug by some kind of giant worm, the researchers conclude, because they lacked the hallmark pellets lining crustacean tunnels and had smoother lining than bivalve tunnels. Iron deposits along the inside suggest the digger must have been long and slender and used mucus to reinforce the walls. Funneling at the top of the burrow also points to the ancient worm emerging from its hideout, retreating and then rebuilding the top sections over and over again.

“These [funnels] suggest that the worm repeatedly dragged its prey down into the sediment,” says study coauthor Ludvig Löwemark, a geoscientist at National Taiwan University in Taipei.

These hunting tactics are consistent with those of modern bobbit worms, which conceal their 3-meter-long bodies in sand and surge forth to grab unsuspecting prey with scissorlike teeth. While the oldest evidence of bobbit worms comes from the early Paleozoic Era, around 400 million years ago, how or if the ancient worms relate to bobbit worms is unknown.

Because the worms that lived in these ancient tunnels were invertebrates, they didn’t have skeletons to leave behind in the fossil record. If soft tissue or teeth from bobbit worms were found preserved inside a burrow, that would confirm that these animals were living in the area 20 million years ago. But teeth break easily, and soft tissue degrades. Both are unlikely to turn up in the fossil record, and that’s normal for trace fossils.

“It is almost always a challenge to link fossil traces to specific trace makers,” says David Rudkin, an invertebrate paleontologist at the Royal Ontario Museum in Toronto, who was not involved with this study. Still, Rudkin thinks that the case for ancient bobbit worms hiding in these burrows is convincing.

If ancient bobbit worms did terrorize the seafloor back then, their burrows are a rare example of invertebrates hunting vertebrates — usually it’s the other way around. Their presence also makes the local ecosystem more complex than previously thought, says Löwemark. “There was obviously a whole lot more going on at the seafloor 20 million years ago than one would imagine when seeing these sandstones,” he says.