Some of the world's oldest animal trails have been found in Hubei province, China – and they are roughly half-a-billion years old. This is one of the earliest examples of an animal capable of movement, say geoscientists writing in the journal Nature.
The 550-million-year-old fossils include the bodies and trails left by an ancient millipede-like creature called a Yilingia spiciformis (or Yiling bug), so named because of the city (Yiling) near to where it was found. It would have been just 1-2.5 centimeters (0.25 to 1 inch) wide and up to 27 centimeters long (11 inches), though researchers say the lack of complete fossils makes estimating total length difficult. Its body would have been made of some 50 body segments and it would have inched forward by hauling itself across the ocean floor.
Researchers co-led by Virginia Tech's Shuhai Xiao dated the age of this mini-beast to between 551-539 million years ago, before the Cambrian explosion, an evolutionary burst of life that occurred 540 million years ago. The Ediacaran Period occurred between 635 million to 542 million years ago – before the dinosaurs and before the formation of Pangea, when most life (as per the fossil record) appears to have been algae and bacteria, but more complex lifeforms were starting to emerge.
The same team found another fossil trail from this period in the Yangtze Gorges area of southern China during digs between 2013 and 2018, however, it did not contain the fossilized remains of the animal that made them. These aren't the earliest known fossils of tracks showing animal locomotion ever discovered either, those were found in Canada in 2010 dating back 565 million years, and also did not include the animal that made the trails. Shuhai Xiao, a professor of geosciences with the Virginia Tech College of Science and co-author of the study, however, says these fossils are the most convincing signs of ancient animal mobility and show segmented and mobile animals had evolved by this time.
“This discovery shows that segmented and mobile animals evolved by 550 million years ago,” Xiao said. "Mobility made it possible for animals to make an unmistakable footprint on Earth, both literally and metaphorically."
The existence of this fossil confirms hypotheses that propose bilaterally symmetric animals (bilaterians) built with segmented bodies and with the capacity for directional mobility evolved during the Ediacaran Period. However, this is the first time we have physical proof in the form of fossil evidence.
"This is a remarkable finding of highly significant fossils," explained Rachel Wood, a professor in the School of GeoSciences at the University of Edinburgh, Scotland, who was not involved in the research.
"We now have evidence that segmented animals were present and had gained an ability to move across the seafloor before the Cambrian, and more notably we can tie the actual trace-maker to the trace. Such preservation is unusual and provides considerable insight into a major step in the evolution of animals."
But that is not all, says Xiao. The trail may also be the first indication of decision-making. The shape of the trail appears to show the animal attempting to move to – or away – from something.
"[Humans] make a huge footprint, not only from locomotion, but in many other and more impactful activities related to our ability to move," said Xiao.
"When and how animal locomotion evolved defines an important geological and evolutionary context of anthropogenic impact on the surface of the Earth."