London (CNN) The oldest known animal in the geological record has been identified, in a discovery that scientists are calling "the Holy Grail of palaeontology."

Fat molecules discovered on the fossil of a mysterious creature called Dickinsonia have confirmed that that it lived 558 million years ago, making it the earliest known member of the animal kingdom.

The findings place its existence 20 million years before the Cambrian Explosion event, when major animals began appearing on the fossil record.

The fossil was first discovered by Australian scientists on a remote Russian cliff face by the White Sea in 1947 and the study, published Friday, brings to an end a decades-long debate to identify what it was.

The oval-shaped fossil was so well preserved that scientists could extract molecules of fat.

"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," said Jochen Brocks, part of the team from the Australian National University (ANU) that led the research.

The symmetrical, oval-shaped organism grew up to around 1.4 meters in length and had rib-like features across its body.

Its fossil was so well preserved that scientists found cholesterol, which they have called "the hallmark" of animal life.

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

"The fossil fat now confirms Dickinsonia as the oldest known animal fossil."

New approach leads to breakthrough

Scientists had been puzzled by the fossil for so long because the rocks containing the fossils had been weathered by heat and pressure, according to ANU.

But the team made the groundbreaking discovery after extracting molecules from inside the fossil, rather than studying its structure as scientists usually do.

"I took a helicopter to reach this very remote part of the world -- home to bears and mosquitoes -- where I could find Dickinsonia fossils with organic matter still intact," said Ilya Bobrovskiy, the paper's lead author.

"I had to hang over the edge of a cliff on ropes and dig out huge blocks of sandstone, throw them down, wash the sandstone and repeat this process until I found the fossils I was after," he added.

Dickinsonia was found to contain a "striking abundance" of cholesterol -- 93%, compared to levels of just 10.6% and 11.9% found in the deposits immediately above and below the fossil.

The team worked alongside researchers from the Russian Academy of Science, as well as the Max Planck Institute for Biogeochemistry and the University of Bremen, both in Germany.