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.

The researchers believe it’s the first record revealing the invertebrates that made these Bifungites. They announced their discovery in the September issue of the journal Earth History and Biodiversity.

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.