Now, the discovery of the trackways and burrows shows that animals with appendages lived during the Ediacaran period, the researchers said. They are often assumed to have appeared and radiated suddenly during the "Cambrian Explosion" about 541-510 million years ago, although it has always been suspected that their evolutionary ancestry was rooted in the Ediacaran Period.
The findings are reported in Science Advances.
On closer analysis, the team found the marks comprise two rows of imprints that they believe were left by a creature scurrying along a riverbed, at a time when life had not yet colonised dry land.
"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 Chinese and American team led by Dr Shuhai Xiao, from Virginia Tech in the U.S., wrote in the journal Science Advances.
The trackways' characteristics indicate that a bilaterian animal - that is, a creature with bilateral symmetry that has a head at one end, a back end at the other, and a symmetrical right and left side - made the tracks. "It is important to know when the first appendages appeared, and in what animals, because this can tell us when and how animals began to change to the Earth in a particular way".
These trackways, preserved near burrows, were discovered in Dengying Formation - a rich fossil preserve in China's south - and constitute the first evidence confirming that an ancient group of animals called bilateria actually pre-dates the Cambrian explosion.
The trackways are the earliest discovered indication of when animals evolved appendages. 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.
While the researchers are unable to identify the animal behind the footprints, there are three types of living animals with paired appendages: arthropods such as bumble bees, annelids such as bristle worms, and tetrapods which include humans. The newfound trace fossils are some of the earliest known evidence for animal appendages on record.
An worldwide team of scientists, including researchers from the Nanjing Institute of Geology and Palaeontology, and Virginia Tech in the United States, conducted the study. The body fossils of the animals that made these traces, however, have not yet been found.