When researchers first described hyoliths roughly 175 years ago, flummoxed paleontologists bestowed them with the Latin moniker Incertae sedis—the binomial equivalent of “We don’t know,” writes Stepanie Pappas at Live Science. Where did this strange little creature fit in the tree of life? Now, almost two centuries later scientists may have finally have an answer, positioning the hyolith on the proper twig in the tree of life.
Nicholas St. Fleur at The New York Times reports that researchers have debated from the beginning where the hyolith belongs. Less than half an inch long, the tiny extinct creature lived during the Cambrian period 540 million years ago. It resembles “a tentacled ice cream cone with a lid,” he writes, and has characteristics similar to snails or clams but is also very different. It sports a pair of “helens,” or curved stilts, that likely allowed the creature to prop itself up on the ocean floor to feed.
To crack the mystery, Joseph Moysiuk, and undergraduate at the University of Toronto, and his colleagues took a fresh look at 1,500 hyolith fossils from the Burgess Shale in the Canadian Rockies and the Spence Shale, a formation in Idaho and Utah. Pappas reports that 254 of those fossils included impressions of the hyolith's soft tissue, giving the researchers new insights into the structure of the animals.
The researchers used a scanning electron microscope and other instruments to test for trace elements on the fossils. Those tests revealed that the hyoliths had tentacled structures coming out of their mouths. A band of tissue protruded from the hyolith “lid,” and between 12 and 16 tentacles, depending on the exact species, extended off that tissue, Fleur reports.
That type of feeding structure is called a lophophore, which is one of the primary organs of a group of creatures called Lophophorata, which includes modern day bryozoa—aquatic “moss animals”—and brachiopods, a group of marine animals with hard shells.
Based on these analyses the hyolith, appears to combine characteristics from several modern lophophorata groups, reports Pappas. The creature has a shell like a brachiopod and has a tubular body similar to a group called phoronids or horseshoe worms. The hyolith, says Moysiuk, was likely a stationary filter feeder, and is more closely related to brachiopods than molluscs, like clams. “We suggest that hyoliths may be distant cousins of brachiopods who have retained a tubular-shaped body from an ancestor they share with phoronids,” he tells Pappas. “It’s adding this new branch to the tree of life.”
Paleontologist Mark Sutton at Imperial College London tells St. Fleur that the finding finally solves the 175-year-old mystery. “Finding them with enough soft tissue to actually place them is a major coup,” he says. “This is the sort of thing that will rewrite textbooks—at least the ones that talk about hyoliths.”
The real star of the show, however, is the Burgess Shale deposit. The area, first discovered in 1909, is still answering questions more than 100 years later about the Cambrian explosion, a period when huge numbers of new life forms, like the hyolith, evolved in the Earth’s oceans. Who knows what secrets remain buried in this deposit—just waiting to be found.