Worms, arachnids, insects, etc. together form the largest and most diverse group of animals in the world, the so-called molting animals. Although researchers had long suspected that this group of animals existed before the beginning of the Cambrian period, 540 million years ago, corresponding fossils were still missing. In a fossil deposit in South Australia, a research team has now discovered, for the first time, prints of nematodes that probably lived around 550 million years ago. The newly discovered species Uncus dzaugisi is the oldest known representative of all molting animals.
Even at the beginning of the Cambrian period, 540 million years ago, there were numerous different ancestors of today’s nematodes, spiders and insects on earth. What these animals have in common is that they shed their skin over the course of their lives, as their stable outer shell can only grow to a limited extent. Scientifically, they are therefore grouped together as the so-called Ecdysozoa, the molting animals. This group includes three subgroups: the nematodes, which are tiny worms, the arthropods, which include insects, spiders and crustaceans, and the Scalidophora, a group of small marine invertebrates.
Hook-shaped patterns in the rock
“The Ecdysozoa were widespread in the Cambrian and we can find evidence for all three subgroups right at the beginning of this period, around 540 million years ago,” says Ian Hughes of Harvard University. “We know they didn’t just appear out of nowhere. Therefore, the ancestors of all molting animals must have been present in the previous Ediacaran period.” Comparative DNA analyzes of various molting animals also suggest that their last common ancestor lived before the beginning of the Cambrian. So far, however, there has been a lack of corresponding fossil evidence.
But now Hughes and his team have found such evidence. During excavations in the Nilpena Ediacara National Park in South Australia, they discovered numerous worm-like imprints in the rock layers formed from the sediment of the Precambrian seafloor. The impressions were about half a millimeter deep – which is significant for fossils of this type. “We all noticed this squiggly hook pattern on the rocks,” Hughes says. “It was quite noticeable because it was very deep. “So the creatures that left these prints must have had a fairly sturdy body that couldn’t be easily crushed.”
The researchers identified a total of 82 of these impressions in the rock and subjected them to a more detailed analysis. Accordingly, the prints come from a newly discovered species of ancient roundworm, which the research team named Uncus dzaugisi. “Uncus” is the Latin word for hook and refers to the body shape of the worms, which are approximately six to 31 millimeters long. Trace fossils near the body prints provided evidence that the animals could move.
Oldest known moulting animal in the world
It is not yet completely clear when exactly Uncus dzaugisi lived. What is clear, however, is that it is the only known representative of molting animals from the Precambrian. “Although exact dates are not yet available, the rock layers date from before the beginning of the Cambrian period. The associated fossils can be narrowed down to a period between 560 and 550 million years ago,” explains the team.
Since the newly discovered species can best be assigned to the nematodes, they must have split off from their sister group, the Scalidophora, before this time. “This discovery resolves a major discrepancy between predictions based on molecular clocks and the fossil record,” write Hughes and his team. “It thus contributes to our understanding of the origins and diversification of the most species-rich group of animals on earth.”
Source: Ian Hughes (Harvard University, Cambridge, USA) et al., Current Biology, doi: 10.1016/j.cub.2024.10.030