The authors can't tell exactly what kind of animal made the tracks, but they can narrow it down to something with pairs of matching legs.
Scientists in China believe tracks left by tiny animals that crawled in sea-shore mud around 550 million years ago are the oldest footprints on Earth.
This places them perhaps even 10 million years before the "Cambrian Explosion" (roughly 541 million years ago), the moment in time which sparked the incredible evolution of life that led to the awesome diversity of species that we see today.
"Although the exact identity of the trace maker of the Shibantan trackways is hard to determine in the absence of body remains at the end of the trackways, we suggest that the trace maker was probably a bilaterian animal with paired appendages", the authors reported.
The scientists found the trackways in the Dengying Formation, a site in the Yangtze Gorges area of southern China.
"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 United States, wrote in the journal Science Advances. This is considered the earliest animal fossil footprint record.
This remarkable discovery is hailed in a study, published yesterday in the journal Science Advances by a research team from Virginia Tech University in the US and the Nanjing Institute of Geology and Palaeontology (NIGP) of the Chinese Academy of Sciences. "They consist of two rows of imprints arranged in poorly organized series or repeated groups", the researchers wrote in their paper.
Until now, there had been no evidence of limbed creatures prior to the Cambrian Explosion, say the researchers from Virginia Tech in the US. They are one of the most diverse animal groups in existence today.
The ancient trackways and burrows are pictured.
Until the current discovery, however, no fossil record of animal appendages had been found in that period. Take that, rest of the pre-Cambrian life forms!
These legs raised the animal's body above the sediment it was moving across.
Still, due to the proximity of the track marks to fossilised burrows discovered nearby, the researchers hypothesise the creature may have exhibited "complex behaviour", such as periodically digging into sediments to mine oxygen and food among its riverbed habitat.
He added: "At least three living groups of animals have paired appendages (represented by arthropods, such as bumblebees; annelids, such as bristle worms; and tetrapods, such as humans)".
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