Imma Perfetto
Cosmos science journalist
Palaeontologists have uncovered a fossilised worm-like creature at Nilpena Ediacara National Park in South Australia.
The discovery is the first to date ecdysozoans – a group of animals which today includes arthropods and nematode worms – back to the Ediacaran period, when complex life first evolved.
“Scientists have hypothesized for decades that this group must be older than the Cambrian,” says Mary Droser, professor of geology at the University of California, Riverside, in the US, who led the study published in Current Biology.
The Cambrian, which spanned from about 538-485 million years ago (mya), is when life began to explode in diversity.
“But until now its origins have remained enigmatic,” sats Droser.
“This discovery reconciles a major gap between predictions based on molecular data and the lack of described ecdysozoans prior to the rich Cambrian fossils record and adds to our understanding of the evolution of animal life.”
The ecdysozoans are the largest and most species-rich animal group on Earth. They include: nematodes, which are microscopic worms; arthropods, which include insects, spiders, and crustaceans; and scalidophora, a group of small, scaly marine creatures.
“Like many modern-day animal groups, ecdysozoans were prevalent in the Cambrian fossil record,” says Ian Hughes, a graduate student in marine biology at Harvard University and first author of the paper.
“We can see evidence of all three subgroups right at the beginning of this period, about 540 million years ago.
“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.”
The Ediacaran period, named after the Ediacara Hills in Nilpena, 600km north of the state capital Adelaide, spanned from 635-538 mya and marks the first widespread appearance of complex multicellular animals.
“Nilpena is perhaps the best fossil site for understanding early animal evolution in the world,” says Scott Evans, an assistant professor of Earth-Life interactions at Florida State University in the US and co-author of the study.
“[This is] because the fossils occur during a period of heightened diversity and we are able to excavate extensive layers of rock that preserve these snapshots.”
The layer the team discovered the new species in is particularly exciting, according to Evans, because: “the sediment grains are so small that we really see all the details of the fossils preserved there.”
The species is named Uncus dzaugisi, uncus meaning “hook” in Latin, after the fossils’ fishhook-like appearance.
“Sometimes we make dramatic discoveries and sometimes we excavate an entire bed and say ‘hmmm, I’ve been looking at that thing, what do you think?’” Hughes says.
“That’s what happened here. We had all sort of noticed this fishhook squiggle on the rock. It was pretty prominent because it was really, really deep.
“Because it was deep, we knew it wasn’t smooshed easily so it must have had a pretty rigid body.”
Ecdysozoan animals are characterised by their rigid outer exoskeleton, which they periodically moult as they grow. They also realised Uncus was motile (capable of moving on its own) due to trace fossils behind the specimens.
Droser and her team have been excavating the site at Nilpena for 25 years. Their prior discoveries also include the earliest evidence of an animal with left-right asymmetry.
