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The timeline of a significant group in the animal kingdom, consisting of millions of insect, arachnid, and nematode species, has been unclear until now. A research team has successfully pinpointed the oldest recognized ecdysozoan in the fossil timeline and the sole representative from the Precambrian era.
Every living creature has a history, including the vast array of insects, arachnids, and nematodes that form a substantial category known as Ecdysozoa.
Details surrounding the ancient origins of this group have remained a mystery until recently. However, a team led by researchers at UC Riverside has discovered the oldest known ecdysozoan within the fossil record, specifically one from the Precambrian. Their findings pertain to Uncus dzaugisi, a worm-like organism rarely exceeding a few centimeters in length, as detailed in an article released today in Current Biology.
“For years, scientists have speculated that this group must predate the Cambrian period, but its beginnings have been shrouded in uncertainty until now. This finding bridges a significant gap between expectations derived from genetic data and the absence of defined ecdysozoans before the abundant Cambrian fossil records, enhancing our grasp of animal evolution,” stated Mary Droser, a geology professor at UCR who spearheaded the research.
Ecdysozoans represent the largest and most diverse animal group globally, comprising over half of all animal species. They are identified by their cuticle— a tough outer skeleton that they shed periodically— and include three main subcategories: nematodes (microscopic worms), arthropods (which encompass insects, spiders, and crustaceans), and scalidophora (a varied group of small, scaly marine organisms).
“Similar to many contemporary animal categories, ecdysozoans were widespread in Cambrian fossil records, with evidence of all three subgroups evident from the onset of this period, around 540 million years ago,” explained Ian Hughes, a Harvard University graduate student specializing in marine biology and the paper’s lead author. “We understand they didn’t just emerge suddenly, so the ancestors of all ecdysozoans must have existed during the earlier Ediacaran period.”
Analysis based on DNA comparisons, which estimates the ages of animal categories by looking at their closest living relatives, supports this theory. Nevertheless, the fossil remains of ecdysozoan creatures have been difficult to find among the many animal fossils excavated from the Ediacaran Period.
Living between 635-538 million years ago, Ediacaran organisms inhabited marine environments, leaving behind imprints in the seabed that solidified into rock. Hughes noted that excavating these fossils is a painstaking and delicate endeavor, involving peeling back layers of rock, turning them over, cleaning them up, and reconstructing them to reveal a clear picture of the ancient seafloor.
This careful excavation has only been conducted at Nilpena Ediacara National Park in South Australia, where Droser and her group have been researching for 25 years, a location recognized for its exceptionally preserved Ediacaran fossils.
“Nilpena stands out as perhaps the leading fossil site globally for understanding the early phases of animal evolution. It features fossils from a time of significant diversity, and we can excavate comprehensive layers of rock that maintain these vital snapshots,” remarked Scott Evans, an assistant professor focusing on Earth-Life interactions at Florida State University and co-author of the research. “The layer where Uncus was discovered is particularly intriguing because the sediment is so fine that we can clearly see all the fossil details.”
While the team did not initially aim to uncover an ancient ecdysozoan during their 2018 dig, they were intrigued by an unusual, worm-like impression they referred to as “fishhook.”
“Sometimes our excavations yield surprising finds, and other times we carefully sift through a site and think, ‘Hmm, I wonder about that?'” Hughes recounted. “In this instance, we all had noticed this ‘fishhook’ squiggle in the rock. It stood out because it was quite pronounced and very deep.”
After observing more worm-like impressions, the team focused on identifying the features of the fishhook.
“Since it was deeply embedded, we knew it hadn’t been easily compressed, indicating it likely possessed a relatively rigid body,” Hughes explained. Other notable features included its unique curvature and its capability for movement, evidenced by trace fossils in the vicinity. Paul De Ley, an associate professor in nematology at UCR, validated its classification as an early nematode, excluding other worm varieties.
“At that moment, we realized this was a novel fossil creature belonging to the Ecdysozoa group,” Hughes stated.
The team named the new species Uncus, derived from the Latin word for “hook,” highlighting its similarities to contemporary nematodes. Hughes expressed the excitement of discovering evidence that supports long-held scientific predictions regarding the presence of ecdysozoans during the Ediacaran Period.
“This finding is also crucial for our understanding of the appearance and lifestyle of these early animal groups, especially since ecdysozoans would later become dominant in marine ecosystems during the Cambrian,” he concluded.
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