Aquatic, worm-like animals capable of crawling through mud appeared at least 550 million years ago, according to new fossil evidence. The discovery is helping to resolve a longstanding question as to when segmented, mobile animals first appeared on the planet.
It’s the paleontological equivalent of finally being able to put a face to a name.
In this case, a team of scientists from Virginia Tech and the Chinese Academy of Sciences were able to connect an ancient species to the trail marks it left behind. Prior to this study, the same group of scientists detected animal tracks in rocks dated to between 551 million and 539 million years old. Trouble is, these tracks could not be connected to a specific organism, leaving the features ambiguous in nature; it’s exceptionally rare to find a fossilized creature resting next to its fossilized trail marks.
New research published today in Nature showcases one such example. The fossilized remnants of a newly described creature, dubbed Yilingia spiciformis, were found in rocks pulled from China’s Dengying Formation in the Yangtze Gorges area. These rocks date back to the Ediacaran period, long before the appearance of dinosaurs and the Pangea supercontinent. The track marks found in these rocks are among the oldest known on Earth—and we finally know who made them.
“This discovery shows that segmented and mobile animals evolved by 550 million years ago,” said Shuhai Xiao, a paleontologist from Virginia Tech and the lead author of the new study, in a press release. “Mobility made it possible for animals to make an unmistakable footprint on Earth, both literally and metaphorically. Those are the kind of features you find in a group of animals called bilaterans. This group includes us humans and most animals. Animals and particularly humans are movers and shakers on Earth. Their ability to shape the face of the planet is ultimately tied to the origin of animal [mobility].”
Yilingia spiciformis featured a back, stomach, head, and tail. It looked similar to a millipede, featuring a long, narrow body composed of around 50 body segments. Yilingia spiciformis was around 26 millimeters (1 inch) wide and about 27 centimeters (10.6 inches) long. The creature would’ve dragged its body across the muddy ocean floor, creating trails as long as 58 centimeters (23 inches). In total, the scientists managed to identify 35 fossils of this species, along with 13 trace fossils of the trails. The “smoking gun” fossil, however, shows the creature with its associated trail mark.
The new research, in addition to offering a date for the emergence of bilateral symmetry in animals, also establishes an origin of directional mobility. Scientists had previously estimated an emergence date between 635 million and 539 million years ago based on other evidence, and this latest discovery further constrains these approximations.
“I think that this is a thorough piece of work based on years of detailed study of the Shibantan fossils,” Graham Shields, a professor of Geology at University College London, told Gizmodo. “Bilaterian animals are rare finds in the Ediacaran Period, and these are undoubtedly amazing examples with both trace and trace maker being preserved.”
Shields, who wasn’t involved with the new study, said the fossils “also have implications for the wider Earth system, as such energetic metabolisms were a first for the world and suggest that for the first time large organisms could actively mine the shallow seafloor, redistributing nutrients.” Prior to the emergence of these sea creatures, the only animals around were rooted to the seafloor, or they floated passively through the water, he said.
The discovery wasn’t a huge surprise for Shields, who said the newly discovered species is “in line” with modern animal groups such as annelid worms and arthropods—groups that would go on to dominate and diversify during the ensuing Cambrian period.
The discovery also made sense to Shields in that it’s consistent with his own work. New research published earlier this week shows that a weird carbon isotope anomaly dating back to the same period can be linked to the oxygenation of the world’s oceans, facilitating the rise of complex new organisms. A story of “co-evolution between life and the environment during these key early stages of animal evolution and diversification is all coming together,” Shields said.