When did animals leave their first footprint on Earth?

Trackways and burrows excavated in situ from the Ediacaran Dengying Formation. Credit NIGP

Trackways and burrows excavated in situ from the Ediacaran Dengying Formation. Credit NIGP

Researchers may have uncovered the oldest fossil footprints on record, dating back to the Ediacaran Period 600 million years ago, in China.

It was a commonly held belief in scientific circles that animals with limbs and jointed appendages did not appear and diversify till the Cambrian Explosion that took place around 541 to 510 million years ago.

The study was published online today (June 6) in the journal Science Advances.

Scientists announced Wednesday that they have discovered the oldest known footprints of an animal.

This means that the symmetrical creature appeared before the Cambrian Period, Chen noted.

"Ediacaran trace fossils provide key paleontological evidence for the evolution of early animals and their behaviors", researchers write in their study. However, Xiao said they are uncertain if the creature belonged to the arthropod family or whether it has many or two legs.

Near the ancient footprints, the team found fossilized burrows, which suggests that the animal might have been periodically tunneling into sediments and microbial mats, either in search of food or perhaps to mine for oxygen.

Researchers from the Nanjing Institute of Geology and Palaeontology under the Chinese Academy of Sciences and Virginia Tech in the United States studied trackways and burrows discovered in the Ediacaran Shibantan Member of the Dengying Formation (551 to 541 million years ago) in the Yangtze Gorges area of southern China.

"If an animal makes footprints, the footprints are depressions on the sediment surface, and the depressions are filled with sediments from the overlying layer", Shuhai Xiao told the Independent while speaking of the finding.

"We do not know exactly what animals made these footprints, other than that the animals must have been bilaterally symmetric because they had paired appendages", said Chen.

Because the footprints are trace fossils, not fossils of the animal itself, it's going to be hard to credit a particular branch of the Tree of Life with the title of "First to Develop Limbs". Those footprints were dated to be between 11,000 and 14,000 years old, making them twice as old as the earliest human civilization. The footprints are present in parallel tracks in the mud.

"Arthropods and annelids, or their ancestors, are possibilities".

The fossil trackways-preserved between two ancient layers of rock-are very narrow, measuring about half an inch in width.

The trackways are the earliest discovered indication of when animals evolved appendages.

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