"The footprints are organised in two parallel rows, as expected if they were made by animals with paired appendages", Xiao told the Independent.
The Chinese and American team led by Dr Shuhai Xiao, from Virginia Tech in the U.S., wrote in the journal Science Advances: "The irregular arrangement of tracks in the trackways may be taken as evidence that the movement of their trace maker's appendages was poorly coordinated and is distinct from the highly coordinated metachronal (wave-like) rhythm typical of modern arthropods". The newfound trace fossils are some of the earliest known evidence for animal appendages on record.
An global team of scientists, including researchers from the Nanjing Institute of Geology and Palaeontology, and Virginia Tech in the United States, conducted the study.
The trackways were found in the Yangtze Gorges area of South China, and date to the Ediacaran Period 541 million to 635 million years ago. The new findings suggest animals evolved primitive "arms" and "legs" earlier than previously thought.
The trackways are irregular, the scientists found, with two rows of imprints that suggest they were created by a bilaterian animal whose appendages raised it above the ground.
Before about 580 million years ago, most organisms were simple, composed of individual cells occasionally organised into colonies. They are one of the most diverse animal groups in existence today.
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Animals with bilaterally paired appengages are assumed to have appeared during the Cambrian Explosion, but now their ancestry may be traceable to even further back in history. "Also, they are organised in repeated groups, as expected if the animal had multiple paired appendages".
"Animals use their appendages to move around, to build their homes, to fight, to feed, and sometimes to help mate", Xiao told The Guardian.
As Xiao explains, knowing when the first legged animal appeared on Earth is a crucial detail, considering that the movement of sediments triggered by that first walking creature as it trotted over our planet's surface could have had a major impact on the Earth's geochemical cycles and climate.
Unfortunately for scientists, the creature that made the footprints did not die nearby and leave an equally well preserved fossil to be studied. This means that the mystery animal might have periodically dug into the ocean floor's sediments and microbial matts, possibly to mine for oxygen and food, the researchers said.
Now, the discovery of the trackways and burrows shows that animals with appendages lived during the Ediacaran period, the researchers said.