They are often assumed to have appeared and radiated suddenly during the Cambrian Explosion about 541 to 510 million years ago, although it has always been suspected that their evolutionary ancestry was rooted in the Ediacaran Period.
The fossil reportedly consists of two rows of imprints that represent the earliest known record of an animal that has legs. However, without a fossil record, it's hard to make any solid assumptions about the creature.
The tracks are actually older than any fossil of a creature with legs, so scientists are puzzled by what created the footprints. The trackways also appear to be connected to burrows, suggesting the creatures periodically tunnelled down into the sediments, perhaps to mine oxygen and microbes as food. The rocks they come from are dated to between 551 million and 541 million years old.
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.
While the footprints were well-preserved, scientists don't know exactly what animal made the tracks since there were no body fossils found in the site.
They analysed the trackways in rock, usually created when an animal's weight causes a depression in the ground which is later filled in by a different sediment.
"This is considered the earliest animal fossil footprint record", the researchers wrote in the report.
Prior to this discovery, it was suspected that animals with legs first appeared during this period, but no evidence had ever been found.
Trackways and burrows excavated in situ from the Ediacaran Dengying Formation.
As modern arthropods and annelids served as appropriate analogs for the interpretation of this fossil, the researchers posit the animal in question could be the ancestor of either of the two groups. Maybe they were never preserved.