The revolutionary animal lived and died in the muck. In its final hours, it inched across the sea floor, leaving a track like a tyre print, and finally went still. Then geology set to work. Over the next half a billion years, sediment turned to stone, preserving the deathbed scene. The fossilized creature looks like a piece of frayed rope measuring just a few centimetres wide. But it was a trailblazer among living things.
This was the earliest-known animal to show unequivocal evidence of two momentous innovations packaged together: the ability to roam the ocean floor, and a body built from segments. It was also among the oldest known to have clear front and back ends, and a left side that mirrored its right. Those same features are found today in animals from flies to flying foxes, from lobsters to lions.
Palaeontologist Shuhai Xiao marvels at the tracks left by this creature, Yilingia spiciformis, and how they captured evidence of its movement. In his cluttered office at Virginia Tech in Blacksburg, he shows off a slab of beige resin — a reproduction of the fossil, which was found in China’s Yangtze Gorges region and is now kept in a Chinese research institute. The replica captures a snapshot of a moment from 550 million years ago. Xiao, whose team formally described Yilingia last year1, traces the bumpy tracks it made immediately before its death. “It was just moving around, and it died suddenly,” he says.
But that’s not the end of this creature’s story. Although nobody knows which category of life it belonged to — the group that includes earthworms is one possibility — Yilingia is helping to fill in key details about the evolution of animals. Most importantly, Yilingia shows that some quintessential animal traits had appeared half a billion years ago, earlier than previous definitive evidence, Xiao says.
Yilingia is not the only creature from that region to provide some of the earliest fossil evidence for an important animal feature. In 2018, Xiao and his team reported2 on tracks found in the Yangtze Gorges consisting of two parallel rows of dimples. The researchers propose that the trails were made by an animal from 550 million years ago that might have been able to burrow and had multiple pairs of appendages — which would make it one of the earliest-known animals with legs.
These Chinese fossils hail from a time right before the Cambrian explosion, the evolutionary transformation when most of the animal groups that populate the planet today first made their appearance in the fossil record. Scientists long regarded the boundary between the Cambrian period and the Precambrian as a dividing point in evolution — a transition from a world in which simple, strange organisms flourished, to a time when the seas teemed with complex creatures that are the forebears of nearly everything that followed.