About 20 million years ago, giant oceanic worms may have entered the seabed and exploded like the Star Wars space slug to ambush unsuspecting fish.
Ancient underground hollows left by these animals appear in rocks off the coast of Taiwan, according to researchers on Jan. 21 in Scientific Reports. The excavators may have been analogous to the modern bobbit worms (Eunice aphroditois), known for burying themselves in the sand to surprise and strike their prey.
The Madrilenians are fossil traces: evidence of animal activity preserved in the geological record (SN: 15/06/14) as footprints (SN: 27/04/20) or even fossilized poop (SN: 21/09/17). These recently reported fossils were first detected in 2013 on the Badouzi promontory in Taiwan by paleontologist Masakazu Nara of Kochi University in Japan. They later appeared among the otherworldly rocky structures of the Yehliu Geopark, a popular tourist attraction that was a shallow ocean ecosystem between 20 and 22 million years ago.
From 319 fossils, the team was able to rebuild the burrows. The animals drilled L-shaped paths at the bottom of the sea, leaving a funnel structure at the top that looks like a feather in vertical cross sections. The burrows were about 2 feet long and 2 to 3 inches wide.
“Compared to other fossil traces, which are usually only a few tens of centimeters long, this fossil trail was huge,” says Yu-Yen Pan, a geologist at Simon Fraser University in Burnaby, Canada. He nicknamed the fossil trace Pennichnus formosae, combining the Latin words for feather, footprint and beautiful.
The researchers conclude that the tunnels were likely to have been dug by some kind of giant worm because they lacked the pellet badges that covered the crustacean tunnels and had a softer coating than bivalve tunnels. Iron deposits along the interior suggest that the digger should be long and thin and use mucus to reinforce the walls. The pipe at the top of the burrow also points to the old worm that emerges from its hiding place, removing and rebuilding the upper sections over and over again.
“These (funnels) suggest that the worm has repeatedly dragged its prey into the sediment,” says study co-author Ludvig Löwemark, a geoscientist at Taiwan National University in Taipei.
These hunting tactics are consistent with those of modern bobbit worms, which hide their 3-meter-long bodies in the sand and emerge to catch unsuspecting prey with scissor teeth. Although the earliest evidence of bobbit worms comes from the early Paleozoic era, about 400 million years ago, it is unknown how or whether ancient worms are related to bobbit worms.
Because the worms that lived in these ancient tunnels were invertebrates, they had no skeletons to leave behind in the fossil record. If soft tissues or teeth of bobbit worms preserved inside a burrow were found, this would confirm that these animals lived in the area 20 million years ago. But teeth break easily and soft tissues degrade. Both are unlikely to appear in the fossil record, and this is normal for fossil traces.
“It’s 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 did not participate in this study. Still, Rudkin thinks the case of the old bobbit worms hiding in these burrows is compelling.
If the ancient bobbit worms terrorized the seabed of that time, their burrows are a rare example of invertebrates hunting vertebrates; it is usually the other way around. Its presence also makes the local ecosystem more complex than previously thought, says Löwemark. “Obviously there was a lot more on the seabed 20 million years ago than you could have imagined when you saw these sandstones,” he says.
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