More than half a billion years ago, a worm-like creature wriggled its last, creating a groove preserved as a fossil that offers new insights into some of the earliest animal movement.
The origins of movement in animals remains fairly murky, although there is evidence of “directional movement” — as opposed to the meandering drift of a jellyfish, for example — as early as 560 million years ago.
However, records of such early movement are very rare, which makes a series of fossils that provide evidence of the life, and death, of Yilingia spiciformis a key find.
Photo: AFP / Virginia Polytechnic Institute and State University
Collected from 2013 to last year in southern China, the fossils show a segmented creature similar to a millipede that lived 550 million years ago, long before dinosaurs roamed the Earth.
They include something even rarer: a fossilized “death march” or “mortichnium” — the trail produced by a Y spiciformis just before it died.
The fossils provide the first “direct supporting evidence” of early movement by a segmented animal, said Xiao Shuhai, a professor at Virginia Polytechnic Institute and State University.
Experts had long theorized that segmented animals were capable of movement in this time period, but there was no fossil evidence to support the idea.
“Yilingia spiciformis is thus far the oldest known segmented animal that was demonstrably capable of directional movement,” Xiao said. “We show that animal motility evolved nearly 550 million years ago, although in a rather modest way.”
Experts generally believe that animals began to move during a period known as the Ediacaran era, about 635 million to 540 million years ago, but those animals left individual footprints or tooth marks from scraping as they passed over a surface.
“Yilingia is different, because it produced long and continuous trails,” Xiao said.
The animal that made those trails, which are seen regularly in the fossil record from the period, was a mystery until the discovery of the mortichnium — which neatly displayed a deceased Y spiciformis at the end of the trail.
The conclusions Xiao and his colleagues draw, published yesterday in the journal Nature, are thanks to the mortichnium.
“Think about how many footprints a person would make in its lifetime,” Xiao said. “What is the chance of this person being fossilized together with one of its footprints?”
The team examined other fossil samples of Y spiciformis that provided more insights into the creature, which appears to have been up to 50cm long and lived in the sea.
There are still plenty of questions, including how and why Y spiciformis came to be on the move.
The fossils could also lend weight to the theory that body segmentation was linked to a greater ability to move and maneuver.
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