These Tracks Were Made by a Foot-long Worm on a Death March 550 Million Years Ago

Scientists have discovered the tracks of a worm-like animal that lived and died around 550 million years ago—a key point in the evolution of life on Earth. The creature, which has been named Yilingia spiciformis, represents one of the few animals from this period that were capable of movement, so finding both its fossilized tracks and remains together provides huge clues into early animal evolution.

Virginia Tech's Shuhai Xiao and colleagues found 35 fossils of the species—which existed between 551 to 539 million years ago—in the Dengying Formation in south China. "These carbonate rocks were deposited in balmy shallow waters between 550 and 540 million years ago, more than twice older than the oldest dinosaurs," Xiao said in a statement.

In a study published in Nature, the team has now described the species, saying it would have been about 11 inches in length and between 0.1 and 1 inch wide. It was made up of about 50 segments and could not only move around independently, were capable of making decisions—the trail appears to suggest the creature was trying to move away from something, researchers say.

The team also said that Y. spiciformis was on a "death march"—the fossil appears to show the creature shedding its guts shortly before dying, leaving a trail known as a mortichnium.

Independent movement is a trait thought to have evolved at some point around 580 million years ago. Before this, animals were immobile, like sponges and corals today. "The earliest convincing evidence for animal mobility is probably around 560 million years, maybe up to 570 million years," Xiao told Newsweek. "So our fossils are not the oldest evidence for animal mobility, but they are thus far the oldest known evidence for the mobility of segmented animals."

Yilingia spiciformis
Artist impression of Yilingia spiciformis. Researchers believe the creature could move and make decisions. Zhe Chen Nanjing Institute of Geology and Paleontology) and Shuhai Xiao (Virginia Tech

He said plenty of ancient trails and tracks can be found, but working out what animal made them is challenging. "It was still a surprise when we discovered the trace and its trace maker were preserved together and physically connected," he said. "This helped us to unambiguously determine the kind of animals that made this kind of trails."

Y. spiciformis belonged to a group of animals known as bilaterians—which today includes most animals, including humans. "The evolution of animal mobility, and particularly the evolution of mobile segmented animals, is a key evolutionary innovation," Xiao said. "Animals are movers and shakers in [the] modern world. The rise of mobile animals more than half a billion years ago changed the Earth in a significant way because they modify the geochemical cycles."

He said the team now plans to place Y. spiciformis into the family tree to constrain its place in history. They also hope to find out more about the marine ecosystem of this period.

Rachel Wood, from Scotland's University of Edinburgh, who was not involved in the research, commented on the discovery. "This is a remarkable finding of highly significant fossils," she said in a statement. "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."