Hidden beneath the seabed around coral reefs, giant worms wait for an unfortunate fish to swim close enough to grab their jagged, nimble jaws and retreat to their sandy burrows. These fast, deadly attacks have earned the worms the name sand strikers.
Now, newly discovered fossilized burrows detailed in the diary Scientific reports indicate that versions of these ravenous sea worms likely snacked unsuspecting fish in what is now northern Taiwan about 20 million years ago.
Discovered in the Yehliu Geopark and the island’s Badouzi promontory, the caves are so-called trace fossils – preserved prints left by the activities of ancient animals. Trace fossils are valuable because they can hold clues about a creature’s behavior. In this case, the prehistoric tubes, each more than six feet long and about an inch wide, are trace fossils likely left behind by creatures that lived in the Cenozoic Era, when this part of the world was under the ocean.
Although modern sandstakers have been known to scientists since the late 18th century, researchers have only recently studied them in detail. The newly discovered fossils indicate that these cruel sea worms likely have been part of ocean ecosystems over the centuries, reinforcing the evolutionary benefits of their devious hunting technique.
A 3D model shows the feeding behavior of sand dachshunds and the proposed formation of the trace fossils, now known as Pennichus good. Sand strikers wait in their L-shaped burrows and use their strong jaws to catch fish that pass through the hole openings.
Thanks to Ludvig Löwemark
Caves left in the rock
Modern sandbars are polychaete worms, belonging to the same group of animals as the sandworms that make tiny bubbles when the tide is ebbing on the beach. But sand traps can get much bigger than anything you’re likely to see on the shore.
These consummate ambush predators range from just a few inches to nearly 10 feet in length, and they are extremely sneaky. In 2009, workers at the Blue Reef Aquarium in England were amazed when their fish continued to disappear, but they found a large sand striker – nicknamed Barry – that had gone unnoticed in the nooks and crannies of the reef habitat.
In 2013, Kochi University biologist Masakazu Nara was looking for 20-million-year-old rocks in Taiwan for fossil impressions of the feeding behavior of stingrays when he saw a series of strange burrows. At first, it looked like these L-shaped burrows were made by ancient shrimp, says National Taiwan University paleontologist and co-author Ludvig Löwemark. Many creatures burrow into the sandy sea floor, so the trace fossils didn’t seem particularly uncommon.
Still, the scientists missed a definitive identification of the fossil. It wasn’t until 2017, when an international conference of trace fossil experts met in Taipei, Taiwan, that Löwemark and colleagues were able to compare notes. The burrows did not match anything previously seen in the fossil record.
“The fact that no one had seen anything like it convinced us that this is a new fossil spore species,” says Löwemark.
However, more detective work was required to determine exactly what made the burrows. “It is not a feature that convinced us that this hole was made by a worm,” says Löwemark, “but the combination of features.” The tops of the burrows appeared to have collapsed, leaving feather-like impressions in the rock, suggesting they were used over and over by an animal moving in and out. “The funnels indicate a violent event,” adds Löwemark, like a worm exploding from its hole rather than a shell crawling its way out of the sand.
An important piece of geochemical evidence confirmed the case. The tops of the burrows are very rich in iron, which suggests that whatever made them was slime along the top walls to help maintain the shape of the structure. Bacteria then fed on this goo and produced iron sulfide. This slime build-up matches today’s sandstriker burrows. Also, the ancient sand on the tops of the burrows appears to have been regularly disturbed, indicating that it was likely occupied by an ambush predator. Sand traps go well with fossils.
“Such huge burrows with those feather-like disruptive features are dead spores for these worms,” said University of Bristol paleontologist Jakob Vinther, who was not involved in the new research. The size of the burrows and the way the sand was changed by the behavior of the invertebrates are also similar, he says.
Sandstakers throughout history
Mostly, trace fossils are described and given names without identifying the specific creature that created them, says paleontologist Murray Gingras of the University of Alberta. That’s because trace fossils and body fossils are rarely found together. The new paper that gives the name Pennichnus beautiful! to the fossil burrows, argues well for an interpretation of sand traps, he notes, but body fossils would help confirm what the tracks imply.
“Because the worm consists almost entirely of soft tissue,” says Löwemark, “the chances of preservation are extremely small.” Still, the characteristic multi-pointed jaws of sand snares are made of hardened proteins and can be supplied with zinc, giving them a greater chance of appearing in the fossil record. “These kinds of jaws, I believe, go back to the Ordovician period,” says Vinther, referring to more than 443 million years ago.
There are some older fossils attributed to these types of worms. Rocks about 400 million years old in Ontario, Canada, show signs of worms acting like sandsticks today. However, it is somewhat strange that no more such fossils have been found yet. Based on how distinctive and large these burrows can be, such traces would be relatively common in rocks from the past 20 million years, Gingras says.
Scientists may just be learning to recognize them, and with luck, paleontologists may be able to spot these worms in their oldest burrows – if they dare.