A groundbreaking fossil discovery in remote South Australia sheds light on one of evolution's greatest mysteries: the origins of Ecdysozoa, a superphylum of molting animals including insects, crustaceans, and nematodes.
Ecdysozoans are the largest, most species-rich animal group on Earth. Although more than half of all animals are ecdysozoans, scientists have long grappled with the lack of evidence of ecdysozoan ancestors in the fossil record, despite molecular evidence suggesting they should exist in the Precambrian.
Now, in a new study published in Current Biology , researchers describe Uncus dzaugisi, a 555-million-year-old worm-like organism preserved in the Precambrian rocks of Nilpena Ediacara National Park (NENP). This tiny fossil, barely over a few centimeters long, represents the oldest confirmed member of the Ecdysozoa and the only one known from the Precambrian period.
"It's an incredibly exciting discovery," said lead author Ian Hughes, a Ph.D. candidate in the Department of Organismic and Evolutionary Biology at Harvard. "Ecdysozoans were prevalent in the Cambrian fossil record and we know they didn't just appear out of nowhere. But, until now, we had no concrete fossil evidence to confirm it."
Hughes discovered Uncus in the Precambrian sandstone rocks at NENP, a site renowned for its exceptionally well-preserved Ediacaran lifeforms. Once an ancient seafloor, the park is dedicated to the research and preservation of Ediacaran fossils found in the sandstone sediments in the area. The site's fine-grained sediments actually allowed for the extraordinary preservation and detail of Uncus.
"A lot of the beds that we have are relatively coarse-grained," said Hughes, "but the sandstone in several new beds at Nilpena is so fine-grained that it allows for finer details and preservation of smaller animals." The researchers worked carefully, using hand tools to dig into the rocky hillside. The Ediacaran seafloor reveals a time before animals began burrowing up and down between sediment. The top few ancient millimeters were covered in thick microbial mat materials where these first animals lived. Eventually, sediment covered and compressed these communities, creating a mold of their ancient ecosystem.
"Imagine pouring concrete over the ocean, waiting a half-billion years, and then flipping it over to find an entire ecosystem," Hughes said. "That's what's really unique about this locality. Because they were all smothered at the same time, we can actually also conduct ecological work on the first animal ecosystems on Earth."
There are over one hundred genera of Ediacaran fossils, yet none had ever been confidently assigned to the Ecdysozoa. The discovery of Uncus helps address the dilemma of the origins and rapid diversification of the Ecdysozoa lineages in the Cambrian. For the first time, Uncus offers direct evidence of early ecdysozoan life forms from the Precambrian.
The fossil's unique features—including its cylindrical body, a consistent length-to-width ratio, remarkably rigid cuticle, and distinct curved traces indicating motility—point to similarities with modern nematodes. The team was especially excited to find trace fossils, as unlike many of its squishy contemporaries, which were not especially mobile, Uncus was quite active. It wasn't until the researchers moved to a different section of the site in 2019 and found a shift in the composition of the microbial mat that they were able to see the ecdysozoan fossils and traces that revealed this ancient species and its motility.
"It's a remarkable find, as it places Ecdysozoans in the Precambrian Eon, supporting the theory that this lineage predates the Cambrian explosion," said Hughes. "Ecdysozoans are so diverse and occupy so many niches, that to actually identify an early one and see what Ecdysozoans were doing is just amazing."
Nilpena Ediacara National Park (NENP) is more than a field site for Hughes and the team – it's been a part of Hughes and his family's lives for 25 years. After so many years of working there, Hughes and his family have built a deep relationship with the indigenous community and the cattle ranch owners who sold part of their land to develop NENP and preserve the Ediacaran fossils. Hughes and his team are very careful to not disturb the structure and layout of the park, as no rock is allowed to be removed from the site.
With a grant from NASA, they were able to secure a 3D laser scanner that provides high-resolution images of all of their findings in the field. "We do everything we can to make sure every rock stays on site," said Hughes, "The only exception is if we discover and describe a new species. Then, we deposit a holotype in the Museum in South Australia. This is really important to our work ethic, to not disrupt the land that belongs to the indigenous people."
Moving forward, the researchers plan to use the laser scan data to study the curvature and movement of this species. "Uncus body fossils and associated trace fossils give us a really unique opportunity to look in detail into the sinuosity and curvature to model the potential musculature of this ancient species," Hughes said. "Moreover, because these fossils are preserved on fossil bed surfaces, we can investigate the ecology of this taxon and try to understand whether its distribution on the seafloor might correlate with another species or a particular type of microbial mat structure."
Uncus dzaugis is named in honor of Mary, Matthew, and Peter Dzaugis for their past and continuing contributions to fieldwork at NENP
