Ediacaran Nematode-Like Worm Fossils Unearthed in Australia

Paleontologists have hypothesized for decades that a major animal group called Ecdysozoa must be older than the Cambrian period, but until now its origins have remained enigmatic. The discovery of Uncus dzaugisi reconciles a major gap between predictions based on molecular data and the lack of described ecdysozoans prior to the rich Cambrian fossil record and adds to our understanding of the evolution of animal life, according to a research team headed by University of California – Riverside’s Professor Mary Droser.

Ecdysozoa is the largest and most species-rich animal group on Earth, encompassing more than half of all animals.

Characterized by their cuticle — a tough external skeleton that is periodically shed — the group comprises three subgroups: nematodes, which are microscopic worms; arthropods, which include insects, spiders, and crustaceans; and scalidophora, an eclectic group of small, scaly marine creatures.

“Like many modern-day animal groups, ecdysozoans were prevalent in the Cambrian fossil record and we can see evidence of all three subgroups right at the beginning of this period, about 540 million years ago,” said Ian Hughes, a graduate student in marine biology at Harvard University.

“We know they didn’t just appear out of nowhere, and so the ancestors of all ecdysozoans must have been present during the preceding Ediacaran period (635-539 million years ago).”

“DNA-based analyses, used to predict the age of animal groups by comparing them with their closest living relatives, have corroborated this hypothesis.”

“Yet ecdysozoan fossil animals have remained hidden among scores of animal fossils paleontologists have discovered from the Ediacaran period.”

The newly-described Ediacaran ecdysozoan, Uncus dzaugisi, was just a few centimeters in length.

An assemblage of 82 Uncus dzaugisi specimens was found in the Nilpena Ediacara National Park in South Australia.

“Nilpena is perhaps the best fossil site for understanding early animal evolution in the world because the fossils occur during a period of heightened diversity and we are able to excavate extensive layers of rock that preserve these snapshots,” said Florida State University’s Dr. Scott Evans.

“The layer where we found Uncus dzaugisi is particularly exciting because the sediment grains are so small that we really see all the details of the fossils preserved there.”

“We were excited to find evidence of what scientists had long predicted — that ecdysozoans existed in the Ediacaran period,” Hughes said.

“It’s also really important for our understanding of what these early animal groups would have looked like and their lifestyle, especially as the ecdysozoans would really come to dominate the marine ecosystem in the Cambrian.”

The discovery is reported in a paper in the journal Current Biology.

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Ian V. Hughes et al. An Ediacaran bilaterian with an ecdysozoan affinity from South Australia. Current Biology, published online November 18, 2024; doi: 10.1016/j.cub.2024.10.030