In 2019, two Harvard researchers visiting the Natural History Museum of Utah pulled open a drawer of confiscated Cambrian fossils and spotted something that had been overlooked for years: a barrel-shaped impression, roughly the size of a thumb, with two openings and faint muscle bands pressed into the rock. That specimen, now formally named Megasiphon thylakos, has since been identified as the oldest definitive tunicate in the fossil record, placing a roughly 500-million-year-old sea creature at the exact evolutionary branch point between invertebrates and every animal that has a backbone.
The discovery, published in Nature Communications, matters because tunicates, commonly known as sea squirts, are widely regarded as the closest living invertebrate relatives of all vertebrates, though some molecular analyses have debated whether cephalochordates (lancelets) might hold that position instead. In their larval stage, tunicates possess a notochord, the flexible rod that in vertebrates develops into a spinal column. Finding a tunicate body plan fully formed at 500 million years old means the split between the tunicate lineage and the vertebrate line happened even earlier, pushing the origin of the chordate blueprint deeper into the Cambrian period. As of June 2026, no older fossil with comparably clear tunicate anatomy has been reported.
What the fossil preserves
Megasiphon thylakos comes from the Marjum Formation in Utah’s House Range, a site recognized as one of the state’s premier Cambrian Lagerstatten, meaning a deposit where conditions allowed exceptional preservation of soft tissues. The fossil dates to the Drumian stage of the mid-Cambrian, within a biozone (Ptychagnostus punctuosus) that places it at approximately 500 million years old.
What makes the specimen remarkable is what survived. The rock preserves two siphons, one for drawing in water and one for expelling it, along with transverse muscle bands arranged around the body wall. Those are the signature features of ascidians, the largest group of modern tunicates. Living sea squirts use the same basic plumbing: water enters one siphon, passes over a mucus-coated filter that traps food particles, and exits through the other. The muscle bands control the pumping. Finding both structures together in a single Cambrian fossil allowed the research team to argue this is not merely a tunicate but specifically an ascidian-type tunicate, pushing that body plan back hundreds of millions of years earlier than previously confirmed.
Karma Nanglu and Javier Ortega-Hernandez, both affiliated with Harvard’s Department of Organismic and Evolutionary Biology at the time of the study, led the analysis. Their methods included detailed imaging of the preserved tissues and side-by-side muscle comparisons between the fossil and Ciona, a modern tunicate widely used in laboratory research. The convergence between fossil morphology and living anatomy is why the authors describe the identification as “definitive,” even though it rests on a single specimen.
A fossil with an unusual backstory
The specimen belongs to a collection of 2,260 Cambrian fossils that reached the Natural History Museum of Utah through a Bureau of Land Management enforcement case. The museum serves as Utah’s state repository for paleontological material recovered from federal land, and the collection had been sitting in storage before researchers recognized the tunicate during their 2019 visit. That chain of custody, from BLM seizure to state repository to peer-reviewed description, adds institutional credibility but also underscores how significant specimens can sit unnoticed for years when collections outpace the staff available to study them.
Why earlier candidates fell short
Megasiphon thylakos is not the first fossil proposed as the oldest tunicate. A specimen called Shankouclava shankouense, described in the Proceedings of the National Academy of Sciences, was recovered from Early Cambrian deposits in South China and could technically be tens of millions of years older. But Shankouclava‘s classification has been contested because its preserved features are less clearly diagnostic. The impressions can be interpreted as belonging to more than one soft-bodied group, leaving its tunicate identity uncertain.
Other candidate fossils have relied on overall shape or fragmentary impressions rather than the specific combination of characters, paired siphons plus muscle bands, that pins Megasiphon to the ascidian body plan. The distinction the Nature Communications team draws is not about absolute age but about anatomical clarity: this is the oldest tunicate whose identity can be confirmed without ambiguity.
Open questions
Some uncertainty remains around precise dating. The Marjum Formation spans a range of depositional time; the University of Utah’s reference page lists an approximate age of 520 million years for the formation as a whole, while the specific layer yielding the tunicate falls within the younger portion of that window. The 500-million-year figure relies on biostratigraphic markers rather than absolute radiometric dates, leaving room for refinement as geochemical work in the House Range continues.
The preservation mechanism itself is also not fully explained. Lagerstatte conditions, typically low oxygen and rapid sediment burial, favored the survival of soft tissues, but no published study has confirmed the specific process for this fossil. Answering that question could reveal whether more soft-bodied organisms remain to be found in the same rock units, potentially filling other gaps in the Cambrian record.
What a sea squirt tells us about our own origins
For readers outside paleontology, the practical significance comes down to a single fact: tunicates are the bridge organisms between the invertebrate world and every vertebrate alive today. If you have a spine, or if your ancestors ever had one, your lineage passes through something that looked and functioned much like a sea squirt. Adult tunicates are sessile blobs anchored to rocks and seafloors, filtering water for food. They do not look like the ancestors of fish, let alone mammals. But their larvae carry a notochord and a rudimentary nervous system, the same developmental toolkit that vertebrates later elaborated into spinal columns and complex brains.
If that toolkit was already in place 500 million years ago, the genetic and developmental machinery used to build fish, amphibians, reptiles, birds, and mammals was being assembled in small, soft-bodied animals on the Cambrian seafloor far earlier than the vertebrate fossil record alone would suggest. Megasiphon thylakos does not rewrite the tree of life, but it anchors one of its most important branch points to a specific time and place: a shallow sea in what is now western Utah, half a billion years before anything with a jaw or a limb existed.
The discovery also sharpens the picture of the Cambrian explosion itself. Rather than a sudden appearance of fully formed modern groups, the record emerging from sites like the Marjum Formation points to a more gradual assembly of familiar body plans, with lineages such as tunicates experimenting with the chordate toolkit long before vertebrates left the hard shells and bones that fossilize easily. Some of the most consequential steps in our own evolutionary story, it turns out, were taken by soft-bodied animals that almost never leave a trace. This one did.
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*This article was researched with the help of AI, with human editors creating the final content.
