A strange, ancient creature that stood on stilts has finally found its place in the tree of life.
Behold the hyolith — a bizarre Cambrian-period creature that dwelt on the ocean floor alongside other armored invertebrates like trilobites more than 500 million years ago. Its body was encased in a pair of shells that resembled an ice cream cone with a lid like a trap door. Two tusklike spines protruded from the soft tissue near the hinge, and on top of its mouth was a row of fluttering tentacles.
Since its discovery in the 19th century, the hyolith has puzzled paleontologists. Some thought it was a mollusk, like a snail or clam. Others said it belonged to its own group of animals.
Joseph Moysiuk, an undergraduate student at the University of Toronto, thinks he has solved the mystery. After analyzing more than 1,500 hyolith fossils, Mr. Moysiuk has concluded that the hyolith belonged to a known group of organisms called lophophorates, which would make them most closely related to present-day brachiopods.
Brachiopods are shell-bearing marine animals that resemble bivalve mollusks like clams, but have different shell structures.
“This is the first time that this group of animals can be solidly placed in the evolutionary tree of life,” Mr. Moysiuk said. He and his colleagues published their finding on Wednesday in the journal Nature.
The key to deciphering the hyolith mystery was the creature’s soft tissue. 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. Many of these specimens came from deposits near the Burgess Shale in British Columbia, which is a well-known fossil hunting site that contains prehistoric remains that date back more than 500 million years.
Using a scanning electron microscope and other tools that tested for traces of specific elements like sulfur, Mr. Moysiuk found several samples had tentacles near their mouths. After comparing the hyolith specimens, which are only a few centimeters long, with known mollusks and brachiopods, Mr. Moysiuk discovered that the tentacles were actually feeding structures called lophophores, the signature structures of the Lophophorata group, which includes brachiopods.
“When we first observed this in some of the fossils, that was really a ‘wow’ moment,” said Mr. Moysiuk. “It’s nice to be able to conclude this mystery that has been around for 175 years.”
In addition to discerning the hyoliths’ place in the tree of life, Mr. Moysiuk and his colleagues also discussed a purpose for the tusklike structures, known as helens. Mr. Moysiuk and his colleagues suggest the structures were used to prop the creature up from the sea floor to help it filter-feed.
“It’s a pretty surprising discovery,” said Uwe Balthasar, a paleontologist from Plymouth University in Britain, who was not involved in the study but reviewed the paper. “This group, which has confidently been placed in the group of mollusks, now has to be interpreted as being closer to brachiopods.”
Mark Sutton, a paleontologist from Imperial College London who also reviewed the paper, said the finding solved a longstanding mystery about the hyolith’s place in the tree of life.
“Finding them with enough soft tissue to actually place them is a major coup,” Dr. Sutton said. “This is the sort of thing that will rewrite textbooks — at least the ones that talk about hyoliths.”