Hidden under the seabed around coral reefs, giant worms wait until an unlucky fish swims close enough for them to catch their jagged and agile jaws and retreat to their sandy burrows. These rapid and deadly attacks have given worms the name of sand attackers.
Now, recently discovered fossilized burrows detailed in the diary Scientific reports indicates that versions of these sea worms probably tasted unsuspected fish about 20 million years ago in what is now northern Taiwan.
Discovered in Yehliu Geopark and on the promontory of Badouzi Island, the burrows are what are known as trace fossils – preserved footprints left behind by the activities of ancient animals. The following fossils are valuable because they can keep clues about a creature’s behavior. In this case, the prehistoric tubes, each over 6 meters long and about an inch wide, are traces of fossils probably left behind by creatures that lived in the Cenozoic era, when this part of the world was under the ocean.
Although modern sand attackers have been known to scientists since the late 18th century, researchers have only recently studied them in detail. Newly discovered fossils indicate that these vicious sea worms have probably been part of ocean ecosystems since time immemorial, consolidating the evolutionary advantages of their hunting technique.
A 3D model shows the feeding behavior of sand-weighted worms and the proposed formation of fossil traces, now known as Pennichus bun. Sand strikers wait inside their L-shaped burrows and use their strong jaws to catch fish passing through the burrow openings.
The kindness of Ludvig Löwemark
The burrows remained in the rock
Modern sand liquids are polychete worms, belonging to the same group of animals as sand worms that make small bubbles as the wave falls on the beach. But sand attackers can get much bigger than anything you’ll probably see on shore.
These perfect ambush predators range from just a few inches to almost 10 feet in length and are extremely hidden. In 2009, workers at the Blue Reef Aquarium in England were puzzled when their fish disappeared, to find a large sand attacker – nicknamed Barry – who went undetected in the corners and habitats of the reef.
In 2013, Kochi University biologist Masakazu Nara searched 20-million-year-old rocks in Taiwan for fossil impressions of stingray feeding behavior when he observed a series of strange burrows. At first, it seemed that these L-shaped burrows were made by ancient shrimp, says paleontologist and study co-author Ludvig Löwemark. Many creatures sank to the bottom of the sea, so that traces of fossils did not seem particularly out of the ordinary.
However, a definitive identification of the fossil has prevented scientists. Only in 2017, when an international conference of fossil experts gathered in Taipei, Taiwan, Löwemark and colleagues were able to compare the notes. Burrows did not match any of the fossils previously recorded.
“The fact that no one has seen anything like it has convinced us that this is a new trace of fossil species,” says Löwemark.
Determining exactly what the burrows did, however, required more detective work. “It was not a feature that convinced us that this den was made by a worm,” says Löwemark, “but the combination of features.” The tops of the burrows seemed to have collapsed and left impression like a feather in the rock, suggesting that they were used everywhere by an animal moving in and out. “Funnels indicate a violent event,” adds Löwemark, like a worm exploding from a burrow, rather than a shell coming out of the sand.
A key element of geochemical evidence has solidified the case. The tops of the burrows are very rich in iron, which suggests that whatever made them mucus infiltrated along the upper walls to help maintain the shape of the structure. The bacteria then fed on this goo, producing iron sulfide. This fortification of mucus matches the dens of today’s sand attackers. Also, the ancient sand at the tops of the burrows seems to have been disturbed on a regular basis, indicating that it was probably occupied by an ambush predator. Sand weights are suitable for fossils.
“Such huge burrows with those feather-like interruption features are dead marks for these worms,” said Jakob Vinther, a paleontologist at the University of Bristol who was not involved in the new study. The size of the burrows and how the sand was altered by the behavior of invertebrates also fits, he says.
Sand strikers throughout history
Most often, fossil traces are described and named without identifying the specific creature that created them, says University of Alberta paleontologist Murray Gingras. This is because fossil traces and fossils of the body are rarely found together. The new work, which gives the name Pennichnus beautiful! for fossil burrows, it is a good case for an interpretation of the sand attacker, he observes, but the body’s fossils would help confirm what the traces involve.
“Because the worm is made entirely of soft tissue,” says Löwemark, “the chances of preservation are extremely low.” However, the characteristic, multi-directional jaws of sand weights are made of hardened proteins and can be tilted with zinc, so they are more likely to appear in the fossil record. “I think these types of jaws go back to the Ordovician period,” says Vinther, referring to a period of over 443 million years.
There are several older fossils attributed to these types of worms. The approximately 400 million-year-old rocks in Ontario, Canada, contain signs of worms that behave similarly to today’s sand attackers. However, it is somewhat strange that no more fossils like these have yet been found. Based on how large and distinct these burrows can be, such traces should be relatively common in rocks of the last 20 million years, says Gingras.
Maybe scientists are just learning to recognize them, and with a little luck, paleontologists could trace these worms back to the oldest burrows – if they dare.