Scientists Found an Exceptionally Preserved Fossil in Utah. Then Noticed Something No One Expected

A 500-million-year-old fossil found in Utah is giving scientists a rare look at one of the strangest branches of the animal family tree. Called Megasiphon thylakos, it shows how early tunicates lived and helps clarify their link to vertebrates. For a group with almost no fossil record, this discovery stands out.

Tunicates are easy to overlook today. They often sit motionless on the seafloor and look nothing like animals we usually think of as relatives. Yet they are the closest living relatives of vertebrates, which makes them key to understanding where we come from.

The problem is that fossils of tunicates are extremely rare. Most ideas about their evolution have come from studying living species, leaving major gaps around their origins, anatomy, and lifestyle. This new specimen begins to fill those gaps with direct evidence.

A Fossil With Details Scientists Almost Never See

The Megasiphon thylakos specimen comes from the Marjum Formation in Utah and stands out immediately. The study published in Nature Communications describes preserved soft tissues, something almost never seen in tunicates.

Dark bands run along the fossil’s body. Researchers compared them to the muscles of a modern tunicate called Ciona, and the resemblance is clear. These muscles control the siphons, which the animal uses to draw in water and filter food. A press release featured on the official Harvard department website.

“It’s not easy to do as tunicates are almost completely absent from the entire fossil record, with only a handful of fossils appearing convincingly as members of the group. which makes this specimen unique.

Traces of a Creature Adapted to the Seabed

A long-standing question has been whether early tunicates swam freely or lived attached to the seafloor. The shape of Megasiphon offers a clear direction. Karma Nanglu, a researcher at Harvard University, explained that:

“Megasiphon’s morphology suggests to us that the ancestral lifestyle of tunicates involved a non-moving adult that filter fed with its large siphons,“ he said. “It’s so rare to find not just a tunicate fossil, but one that provides a unique and unparalleled view into the early evolutionary origins of this enigmatic group.”

Its barrel-shaped body and two siphons back that idea up. The fossil also suggests tunicates started out as free-swimming larvae before settling down and becoming stationary adults, which is still how they develop today.

A Timeline Being Revised

At around 500 million years old, Megasiphon thylakos is older than previous estimates for this group. Earlier molecular data had placed the origin of similar tunicates closer to 450 million years ago.

The findings suggest that the basic tunicate body plan was already established not long after the Cambrian Explosion, which began about 538 million years ago. That period marked a rapid diversification of animal life, even if tunicates themselves remained largely missing from the fossil record.

Javier Ortega-Hernández, associate professor of organismic and evolutionary biology, recalls that the fossil’s resemblance to modern tunicates was immediately apparent, prompting closer study.

“The fossil immediately caught our attention,” noted Ortega-Hernández, “although we mostly work on Cambrian arthropods, such as trilobites and their soft-bodied relatives, the close morphological similarity of Megasiphon with modern tunicates was simply too striking to overlook, and we immediately knew that the fossil would have an interesting story to tell.”

Enjoyed this article? Subscribe to our free newsletter for engaging stories, exclusive content, and the latest news.