Ancient Worm Died in Its Tracks, Shedding Light on Animal Movement

The 550 million-year-old fossil shows familiar animals evolved before the Cambrian Explosion

Worm Track
A 550-million-year-old worm track found in China. Virginia Tech

Until recently, it was believed that the ancestors of most animals on Earth today burst onto the scene during a brief period called the “Cambrian Explosion,” 570 to 530 million years ago. Fossils of the animals on the scene prior to that, in a period called the Ediacaran, seemed strange, mainly sedentary or passive like a jellyfish, and unrelated to anything recognizable today. But a fossilized "worm" and its final track, as well as several other newly described fossils, are starting to show that the Cambrian Explosion probably wasn't a big bang, but a slow evolution.

According to a press release, the millipede-like fossil is about 550 million years old and was found in rock formations near the city of Yiling in east central China. Paleontologists found 35 fossils of the new species, named Yilingia spiciformis, including 13 associated tracks and the final “death march” of the worm fossilized in its tracks.

That worm was about four inches long and a quarter to a full inch wide. It dragged itself along the ocean floor, resting along the way. The fossil indicates it was bilateral, had about 50 segments, a back, a belly, a head and a tail, all recognizable features in later Cambrian and modern animals.

“This discovery shows that segmented and mobile animals evolved by 550 million years ago,” Shuhai Xiao of Virginia Tech, co-author of the study in Nature, says. “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 motility.”

It’s incredibly rare to find a fossil animal at the end of a trail that it’s left, also known as a mortichnium. “Think about how many footprints a person would make in its lifetime," Xiao tells AFP. “What is the chance of this person being fossilized together with one of its footprints? Very slim.”

This new species, along with others identified in the past few years, show that Ediacaran animals were mobile and were recognizable as animals 12 million years before the Cambrian Explosion. “In the past, paleontologists emphasized the differences between the Ediacaran and Cambrian,” Xiao tells Colin Barras at Nature. “But when you think about it, life had to continue through the boundary. Some lineages had to survive.”

According to the press release, the find not only represents the earliest known independent movement of segmented animals, it may also represent the first signs of decision making among animals. The worms crawled toward or away from objects, perhaps under the direction of a central nervous system.

AFP reports that the finding also lends weight to a theory that the development of segmentation in animals led to the improved ability to move and maneuver. Xiao and his colleagues now want to understand why a creature like Y. spiciformis developed the ability to move. There are many possibilities including seeking out oxygen, looking for food, avoiding predators or looking for a mate.

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