Scientists have discovered remnants of fat in an ancient fossil that belongs to the earliest confirmed animal.

The fat, which was discovered by researchers from the Australian National University, dates back 558 million years, and belonged to a strange creature called Dickinsonia.

Dickinsonia grew up to 1.4 metres in length and was oval shaped with rib-like segments running along its body.

Its fossilised remains were discovered in a remote area near the White Sea in Russia.

Dr Jochen Brocks, who led the study, said: “The fossil fat molecules that we’ve found prove that animals were large and abundant 558 million years ago, millions of years earlier than previously thought.

“Scientists have been fighting for more than 75 years over what Dickinsonia and other bizarre fossils of the Edicaran Biota were: giant single-celled amoeba, lichen, failed experiments of evolution or the earliest animals on Earth.

“The fossil fat now confirms Dickinsonia as the oldest known animal fossil, solving a decades-old mystery that has been the Holy Grail of palaeontology.”

Until now, researchers have struggled to find Dickinsonia fossils that retain some organic matter.

Mr Ilya Bobrovskiy, an author of the study, said: “Most rocks containing these fossils such as those from the Ediacara Hills in Australia have endured a lot of heat, a lot of pressure, and then they were weathered after that – these are the rocks that palaeontologists studied for many decades, which explained why they were stuck on the question of Dickinsonia’s true identity.”

To understand how old the fossil was, the researchers analysed molecules inside it.

Dr Brocks said: “When Ilya showed me the results, I just couldn’t believe it. But I also immediately saw the significance.”