"If an animal makes footprints, the footprints are depressions on the sediment surface, and the depressions are filled with sediments from the overlying layer".
Animals left the first footprints on Earth up to 551 million years ago, according to ancient tracks found in China.
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 USA and the Nanjing Institute of Geology and Palaeontology (NIGP) of the Chinese Academy of Sciences.
Both the footprints and the borrows are known as "trace fossils", a term that refers to fossilized remnants that animals leave behind, such as fossilized poop, rather than fossils of the animals themselves.
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.
Experts previously believed this type of creature appeared during the "Cambrian Explosion" around 541 to 510million years ago.
The odd-looking prehistoric trackways show two rows of imprints that resemble a series of repeated footprints, the researchers said.
Well, the team has it narrowed down to a bilaterian animal - a creature with bilateral symmetry, that has a head at one end and the back end at the other, as well as a symmetrical right and left side, Live Science explains. That's hundreds of millions of years before dinosaurs started roaming Earth, about 245 million years ago.
Bilaterian animals such as arthropods and annelids have paired appendages and are among the most diverse animals today and in the geological past.
The identity of the creature that made the 546-million-year-old tracks is still unknown, but they come from the period when the earliest animals are thought to have evolved.
"Also, they are organised in repeated groups, as expected if the animal had multiple paired appendages". "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".
"Previously identified footprints are between 540 and 530 million years old".
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.
These trace fossils represent some of the earliest known evidence for animal appendages and extend the earliest trace fossil record of animals with appendages from the early Cambrian to the late Ediacaran Period.