An artistic representation of Megasiphon thylakos, a non-moving relative of vertebrates that lived in the ocean half a billion years ago and is related to modern tunicates (sea squirts).Franz Anthony
When Karma Nanglu was growing up in Toronto, the fossil collection at the Royal Ontario Museum was a source of endless fascination. But while the dinosaur and mastodon skeletons were fun to look at, it was always the weirder and more ancient specimens that caught his eye.
“The more alien an animal looks, I think the more interesting it is and the more I want to know about it,” said Dr. Nanglu, now a postdoctoral researcher at Harvard University’s Museum of Comparatively Zoology.
Now, new research by Dr. Nanglu and colleagues reaches back half-a-billion years to reveal the history of one of the most alien-looking creatures on Earth. In the process, the team has shed light on a branch of the evolutionary tree that includes our closest non-vertebrate cousins.
At the centre of the work is an early and remarkably well-preserved fossil of a tunicate, also known as a sea squirt, which appears to be surprisingly similar to those that live in the ocean today.
A tunicate is a marine animal that resembles a pitcher with two openings. As adults, they plant themselves in one spot and exist to filter the water for particles of food, which they do while siphoning water in through one opening and expelling it out the other.
Megasiphon thylakos, a previously unrecognized fossil species of tunicate (sea squirt) was discovered in a museum collection in Utah in 2019. The fossil shows preservation of soft tissue, including a barrel-shaped body with two siphons. Credit: Rudy Lerosey-AubriRudy Lerosey-Aubri
It is only in their larval stage, when they look more like tadpoles, that tunicates reveal their connection to our own branch of the animal family tree. As unlikely as it may seem based on appearance, they are far more closely related to humans than are lobsters or squid, for example.
Exactly how they relate is a harder question to answer because tunicates lack bones, teeth, shells or exoskeletons that would allow them to show up in the fossil record.
But when colleagues who were visiting the Utah Museum of Natural History in 2019 came across an unidentified fossil in its collection with a trunk and branch shape, Dr. Nanglu was given the task of figuring out what it might be.
The find, dubbed Megasiphon thylakos, not only proved to be the best example yet of a tunicate from the distant past, it shows what kind of body design the ancestors of modern tunicates had, including traces of the muscles that they used to keep water flowing in and out, which appear in the fossil as dark bands.
It also shows that the tunicate body design, and the survival strategy that goes with it, was already in place during the Cambrian period, when all of the major animal groups such as vertebrates, arthropods, molluscs and others, appeared.
The find is documented in a study published Thursday in the journal Nature Communications.
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Tetsuto Miyashita, a paleontologist with the Canadian Museum of Nature in Ottawa who was not involved in the study, said that the find was a valuable addition to science because it is the first “good” fossil tunicate from this period.
“Now that we have one, we have something to compare other dubious fossils with,” he said. “More than half a billion years ago, our fish ancestors went in one direction and we are their descendants, but tunicate ancestors kind of went ricocheting [into] the outfield. It’s really curious how their evolutionary trajectory went off that way.”
Dr. Nanglu said his excitement over working with the specimen also underscores the fact that there are likely other unknown gems that have been collected by fossil hunters over the years but have yet to be recognized as major finds.
“It’s definitely a call to go back to museum collections and start looking through those drawers of unnamed fossils,” he said.
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