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Zhe Chen/Nanjing Institute of Geology and Paleontology

Ancient wormlike animal caught in its tracks sheds light on early locomotion

Some fossil beds are full of scratches, grooves, and repeating imprinted patterns likely left behind by some moseying creature. But rarely are the sources of these marks confirmed because the animal who made them is nowhere to be found.

Now, scientists have discovered an exception. In rock about 560 million years old—as ancient as animals themselves—they unearthed a fossil preserved in the act of moving: a flat, segmented animal with lots of tiny “legs,” not too unlike a modern millipede, but with a softer body. In the rock behind one fossil was a long, continuous trail with grooved edges (as seen in this artist’s rendition).

The researchers named their find Yilingia spiciformis: Yilingia after the area in the Yangtze River’s gorges region where the fossil was found, and spiciformis because the body is a little spikey. Each segment has three lobes: a large central lobe flanked by smaller, backward-pointed lobes. The longest of the 35 Yilingia specimens collected stretches 27 centimeters from head to tail—about the length of a large adult human foot.

Based on the fossil’s anatomy and the nature of its trail, other tracks made at this site were likewise made by animals with paired “legs” of some sort, the team reports today in Nature, a rather advanced characteristic for such an early evolving animal. (Most animals with paired legs didn’t evolve until 15 million years later.)

The evolution of segments, they suggest, helped make these organisms mobile and thereby helped spur the great diversification of animals that occurred soon after. Indeed, this fossil could be a missing link between animals with no segments, like simple worms, and arthropods, like insects and lobsters.