If you know where to look, you can find dumbbell-shaped fossils in rock outcrops all over the world: in Brazil, the United States, Canada, India and African and European countries. They are called Bifungites, and they are not fossilized animals but burrows left in an extinct creature’s wake. Most are found in rocks from the Paleozoic era more than 300 million years ago.
No one knows what made these Bifungites burrows, which are considered trace fossils, although scientists have hypothesized what they might have been. Daniel Sedorko, an invertebrate paleontologist at Brazil’s National Museum, has studied them for more than a decade, and during an expedition in June 2022, he noticed something unusual.
The burrows are typically empty because the creatures that constructed them were soft-bodied invertebrates that often don’t fossilize well. On exposed rocks in the bed of the Sambito River in northeastern Brazil, Dr. Sedorko saw an imprint of a small worm inside one Bifungites. Within hours, his team found seven other fossilized burrows with the same worm imprint, indicating that these organisms produced them.
Carlos Neto de Carvalho, an expert in the study of trace fossils, or ichnology, at the University of Lisbon who wasn’t involved in the work, called the find exciting.
“This is the best evidence you can get in the trace fossil record to figure out the producer,” he said. Emphasizing the rarity of the find, Dr. de Carvalho said that “it’s more common to find a new species of a dinosaur than to find a producer of a trace fossil.”
The imprints Dr. Sedorko and his team found suggest the marine worms that made Bifungites belonged to a group called Annulitubus. Species in the group lived in the shallow part of the ocean near the shores of prehistoric supercontinents and dug burrows into the seabed. The burrows typically have an inverted pi or u-shape, with the dumbbell-resembling horizontal chamber at the base and a vertical shaft at each end rising toward the ocean floor.
“It’s unusual to see the u-shape,” said Andrew Rindsberg, a paleontologist at the University of West Alabama and a co-author of the study. Over time, water currents erode the burrows, and the shafts are the first to go, but he added that the lower horizontal half could remain preserved.
The researchers suspect that the Annulitubus worms made these burrows to protect themselves against savage storms or probing predators. The worms potentially wedged themselves into the peculiar bulging or arrow-like ends of the chamber. “The animal was trying to be fixed,” Dr. Sedorko said. “But it’s just a hypothesis.”
Still, how did the fossil worms remain preserved in these burrows for millions of years? The region experienced frequent storms, Dr. Sedorko said, each depositing several feet of sediment that would quickly bury the worms. With time, their soft segmented bodies decayed, but their impressions remained preserved in the mud.
“It’s very beautiful,” Dr. Sedorko said, referring to the worm imprints. The team hopes that their find would encourage ichnologists across the globe to keep an eye out for makers of these fossil burrows. While they’re certain that ancient Annulitubus worms produced the Bifungites in the part of Brazil they studied, the researchers are not ruling out the involvement of arthropods in creating such burrows in other regions of the world.
Finding these animal preservations, though, “is always a moment of luck,” Dr. de Carvalho said. “It’s kind of like winning a lottery.”
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