Fossil discovery uncovers early history of Earth’s largest animal group

A fossil discovery in a remote part of South Australia has shed light on one of evolution’s great mysteries: the origins of Ecdysozoa, Earth’s largest, most species-rich animal group.

In a new study, published last month in the journal Current Biology, researchers describe a 555-million-year-old worm-like organism beautifully preserved in the rocks of Nilpena Ediacara National Park (NENP). This tiny fossil, just a few centimeters long, is the oldest confirmed member of the Ecdysozoa, representing more than half of all animals including insects and crustaceans. It’s the only known Ecdysozoa dating to the Precambrian, an early stretch of history pre-dating the “explosion” of biodiversity that began during the Cambrian period more than 500 million years ago.

“It’s an incredibly exciting discovery,” said lead author Ian Hughes, a Ph.D. candidate in the Department of Organismic and Evolutionary Biology at the Griffin Graduate School of Arts and Sciences. “Ecdysozoans were prevalent in the Cambrian fossil record, and we knew they didn’t just appear out of nowhere. But, until now, we had no concrete fossil evidence to confirm it.”

Hughes discovered the organism in the sandstone rocks at NENP, a site renowned for well-preserved lifeforms that date to the late Precambrian period known as the Ediacaran (spanning 635 to 542 million years ago). Once an ancient seafloor, the park is now dedicated to the research and preservation of Ediacaran fossils. The site’s fine-grained sandstone sediments allowed for the extraordinary preservation and detail of the organism, named Uncus dzaugisi in honor of a family who has provided financial support for fieldwork at the site.

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 organism’s unique features — with a cylindrical body, a consistent length-to-width ratio, and remarkably rigid cuticle — point to similarities with modern nematodes, a variety of microscopic, transparent worm.

“It’s a remarkable find, as it places Ecdysozoans in the Precambrian Eon, supporting the theory that this lineage predates the Cambrian explosion,” Hughes offered. “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.”

This research was supported by the National Aeronautics and Space Administration (NASA).