Pity the poor hyolith; for the past 175 years, this tiny cone-shaped marine creature, which lived 530 million years ago, has been lumped together with molluscs, the same group of creatures that includes squid, octopi, clams and scallops.
But with the publication of a recent study in the scientific journal Nature, based on evidence provided by the Burgess Shale fossil sites at Marble Canyon and the Stanley Glacier in Kootenay National Park, the hyolith has been set in its rightful place as a relative of brachiopods.
Now, before you yawn, shrug your shoulders and say “meh,” consider this: saying hyolith is related to mollusks when it is in fact related to brachiopods is much like saying a Canadian is no different than an American.
Outwardly, many molluscs and brachiopods – at least the shelly ones – do look very similar. Both have two shells, but, like Canadians and Americans, once you get beyond the similar exterior, they are two very different things.
Mollusks are bivalves, which means their two shells, also known as valves, open along the centre in a left-right arrangement. Brachiopod shells, meanwhile, open top-bottom across the centre line and, unlike mollusk shells, the shells of brachiopods are not identical. Brachiopods also happen to be an extremely common fossil, even here in the Bow Valley.
Like its brachiopod relatives, hyolith’s shell has a top and bottom and, in its case, the bottom is a long tapering cone and the top looks like a small lid.
“Hyolith shells are very well known in the fossil record,” said Joseph Moysiuk, lead author of the study. “They’re not only found at the Burgess Shale, they’re found all around the world, and they occur in the sediments of 530 million years old as well as 250 million years, so they were around for about 280 million years; however, we really didn’t know much about the soft tissues of these organisms.”
And like all of the creatures of the Burgess Shale, soft tissue has been key to identifying their features and their connections to other animals, both living and extinct.
The soft-tissue feature that allowed Moysiuk and his collaborators Jean-Bernard Caron of the Royal Ontario Museum and Martin Smith of Durham University identify hyolith, which measures between 1-5 centimetres long, is a band of 12 to 16 ciliated tentacles that sits between the upper and lower shells and fans away from the creature’s mouth. Known as a lophophore, this structure is unique to brachiopods.
Moysiuk, an undergraduate student at the University of Toronto in the Depts. of Earth Sciences and Ecology & Evolutionary Biology, studied 1,500 fossilized hyolith as part of this study. Only 254 specimens showed any preserved soft tissue. He found the lophophore with its tentacles in only a few dozen hyolith specimens.
“This feature is really characteristic of a group of animals that we call lophophorates,” he said.
“In our paper, we suggest hyolith is showing characteristics of both brachiopods and another group of lophophorates, which is still alive today, called the horseshoe worm and, although the horseshoe worm doesn’t have a shell like brachiopods and hyolith, they have this elongate body which hyoliths have in this elongate shell.”
However, as the elongated body and shell is not typical of brachiopods, Moysiuk said he and his collaborators are suggesting that while hyolith is sitting on a branch close to brachiopods, it retained the body of the common ancestor it shares with the horseshoe worm.
Hyoliths found at Burgess Shale sites in the Canadian Rockies also have another interesting structure, a curving pair of spines known as helens. These spines, which look a lot like the tusks of a mammoth, acted like stilts, allowing hyolith to feed above the sea floor.
Charles Doolittle Walcott, who discovered the original Burgess Shale fossil site located in Yoho in 1909, originally described the helens as a separate creature, which he named after his wife, Helena, and daughter, Helen.
“Those helens were originally found isolated in the fossil record and people didn’t know what these things were. (But) based on Burgess Shale fossils, somebody else figured out that, ‘hey these are actually a part of hyolith,’ ” said Moysiuk, adding that while the understanding of the nature of those spines changed, the name did not.
Moysiuk completed this study first as part of the University of Toronto Research Opportunity Program, an undergraduate research program, and then as a volunteer. The life-long fossil hunter, who once found a foot long fossilized trilobite near his home in Toronto, has also taken part in three field seasons at the Marble Canyon fossil site, discovered in 2008.
Without the discovery of the Burgess Shale fossil sites at Marble Canyon and Stanley Glacier, it may not have been possible to resolve hyolith’s identity.
“Hyoliths have also been found at the original Burgess Shale site at the Walcott quarry, which is 30 kilometres to the north. But those ones don’t show very well preserved soft tissue. It was these new sites, and the new fossils discovered in them, that revealed a lot of these details in our study that weren’t previously known about hyoliths,” said Moysiuk.
Even though hyolith has been identified as a lophophorate, Moysiuk said there are still questions about this diminutive marine creature that have yet to be resolved. Just as it can be challenging to quantify exactly how Canadians and Americans differ, how exactly this tiny marine creature fits within the lophophorate group has yet to be determined; however, that answer may yet come. But for now, researchers like Moysiuk are much farther than they were 175 years ago when hyolith was identified as a mollusk.
“The Burgess Shale is a unique window into the early time of evolution of these animals, which we call the Cambrian Explosion. And if we want to figure out what the animals’ early life looked like, we can learn a lot looking at sites like the Burgess Shale and that is exactly what we have done in this study,” said Moysiuk. “We’ve added a new branch to the tree of life that could not be placed confidently before.”
