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Scientists estimated that mobile and segmented animals with bilateral symmetry evolved during the Ediacaran period (between 635 and 542 million years ago), long before the age of dinosaurs or even the supercontinent Pangea. However, there was no convincing evidence to corroborate these estimates. Only fossil traces of trails, paths and burrows were known which, with few exceptions, were traces of unknown living beings.
A study published in the Nature magazine describes details of the trail produced by a worm-like animal shortly before its death, indicating that this species was mobile. This fossil being, which they have called Yilingia spiciformis, would have lived between 551 and 539 million years ago in what is now the Dengying training, in the area of the Yangtze Gorges (China). The discovery is vital because the animal and the signal it produced just before its death are preserved together.
“This discovery shows that segmented and mobile animals evolved 550 million years ago. Mobility made it possible for them to trace a unmistakable footprint on Earthboth literally and metaphorically, ”says Shuhai Xiao, a geoscientist at Virginia Tech (USA) and the study's principal investigator. "Those are the characteristics found in a group called bilateral, which includes humans and most animals," he adds.
The authors of this work collected 35 fossils of the species and they estimated that the animal was between 5 mm to 26 mm wide and up to 27 cm long, with approximately 50 segments. They also found 13 other fossil remains, including a trace that was directly connected to one of the fossils. The characteristics of the 25mm trail indicate that it was left by the Yilingia animal when it moved, making it the most compelling sign of ancient animal mobility.
A being similar to a millipede
Yilingia spiciformis, owes its name to a pointed insect Yiling which is, in turn, the name of a Chinese city near the site. The animal was a millipede-like creature that alternately dragged its body across the muddy ocean floor and rested along the way. Its shape was long and narrow, with a left and right side and a head and a tail.
The origin of bilaterally symmetric animals with segmented bodies and directional mobility is a fundamental event in early animal evolution.
The finding also marks what may be the first sign of decision-making among animals. "The trails suggest an effort to move towards something or to move away, perhaps under the guidance of a sophisticated central nervous system," says Xiao.
The mobility of animals caused environmental and ecological impacts on the Earth's surface system and ultimately, caused the Cambrian substrate or agronomic revolutions.
“We are the animal with the most impact on Earth. We make a big impression, not only because of locomotion, but also because of many other activities with greater impact related to our ability to move. When and how animal locomotion evolved defines an important geological and evolutionary context for anthropogenic consequences on the Earth's surface”, Emphasizes the scientist.
Rachel Wood, a professor at the Faculty of Geosciences at the University of Edinburgh in Scotland, who was not involved in the study, concludes: “This is a remarkable find of very significant fossils. We now have evidence that segmented animals were present and had the ability to move on the seafloor before the Cambrian. We can link the actual person in charge of the trail with their tracks. Such preservation is unusual and provides considerable insight into an important step in the evolution of animals. "
Zhe Chen, Chuanming Zhou, Xunlai Yuan, and Shuhai Xiao. "Death march of a segmented and trilobate bilaterian elucidates early animal evolution" Nature September 4, 2019. https://doi.org/10.1038/s41586-019-1522-7.