It has spent some 175 years homeless, wandering many paths of taxonomy without a single branch to call its own. In the time since it was first described, this now-extinct, cone-shaped sea creature has known a number of presumed families — from mollusks to designations much more nebulous — but the tiny hyolith never quite fit in any of them.
You'd be forgiven for thinking the hyolith — with shells that resemble an ice cream cone and two spines that sprout like curved stilts — looks a bit odd. It is partly the creature's curious combination of parts that left scientists scratching their heads.
But now, a group of researchers believes it has found the hyolith a home in scientific classification, more than 500 million years after the now-extinct organism evolved onto the scene.
Drawing on more than 1,500 specimens, the group focused closely on fossils of a particular kind of hyolith, the Haplophrentis. And in the process, they discovered something crucial: The creature had short tentacles around a centrally located mouth, tucked between its two shells.
In other words, according to the study they published in the journal Nature, the ancient organism had a feeding structure called a lophophore. The researchers believe the Haplophrentis would elevate itself from the sea floor with those stilt-like spines (also known as "helens") and use its lophophore to filter and feed on material suspended in water.
"Only one group of living animals — the brachiopods — has a comparable feeding structure enclosed by a pair of valves," the lead author on the project, Joseph Moysiuk, told Phys.org. "This finding demonstrates that brachiopods, and not mollusks, are the closest surviving relatives of hyoliths."
The home that hyoliths can now call their own? The Lophophorata — or, the group of aquatic organisms, including brachiopods, that all share this signature organ.
The key to the discovery was the soft tissue preserved with the fossils Moysiuk and his team were studying, which were culled largely from the Burgess Shale in British Columbia. The New York Times explains:
"Typically when paleontologists find fossils they uncover the hard parts of an organism, like its teeth, bones or shells. Soft tissue is much harder to find because it does not fossilize easily. But some of the samples that Mr. Moysiuk came across had preserved soft tissue."
It was only in analyzing these "exceptionally preserved soft tissues" that Moysiuk, an undergraduate at the University of Toronto, found the tiny tentacles around the hyolith's mouth. Those soft tissues offered the crucial clue to solving a hard problem that long puzzled scientists.
"It's a real milestone," Martin Smith, a paleontologist on the research team, told The Toronto Sun. "It's enormously exciting to have solved such a major paleontological problem that has been such a mystery for so long, and I think it really does change the way we look at a large set of the fossil record."