The discovery of an ancient reef north of Adelaide by Australian scientists may push back the evolution of the earliest animals by 80 million years.

The unpublished research, by geoscientists Associate Professor Malcolm Wallace, Estee Woon and Jonathan Giddings from the University of Melbourne, will be presented at the Selwyn Symposium on Thursday.

The researchers say they have uncovered complex organisms that in some ways resemble multicellular life in a large reef located in the Northern Flinders Ranges, 700 kilometres north of Adelaide in South Australia.

If the fossils - which are around 650 million years old - are of multicellular organisms, they would be the earliest examples of primitive animal life discovered so far, the researchers say.

The fossils are yet to be described scientifically, but look like cauliflowers and were probably sponge-like organisms up to two centimetres in diameter, Associate Professor Wallace says.

'Nothing else like them'

He says the reef-building organisms were "certainly more complex than any fossil of their age anywhere on Earth".

"They've never been described from anywhere else in the world," Associate Professor Wallace said.

"There's nothing else like them."

The ancient reef, which is now exposed on the surface, was 10 times higher than the Great Barrier Reef, and consisted partly of stromatolites, layered structures built by microbes, and partly of the sponge-like organisms.

The find is especially significant because it may be the missing piece of the puzzle in the evolution of early animal life.

Before the Ediacaran geological period, 635 million years ago, the only life forms were simple, single-celled organisms.

Then suddenly, 570 million years ago, very complex animals appear in the fossil record.

Scientists have long debated just what caused this evolutionary explosion in life.

"When you see the Ediacara they resemble jellyfish and modern arthropods [the group that contains insects and spiders]," Associate Professor Wallace said.

"There is no doubt they are animals. The real puzzle is why they appeared 570 to 540 million years ago.

"Maybe this reef system will tell us something about that."

'Extremely important'

Dr Jim Gehling, a palaeontologist from the South Australian Museum in Adelaide, says if verified the find will be "extremely important" and "very exciting".

"We know these things [the Ediacara] must have had ancestors,[but] when they occurred is the great debate," Dr Gehling said.

He says the find would confirm predictions based on the molecular record that pinpoint evolutionary steps based on the rates of DNA mutation.

Most branching of the molecular tree occurred in the Ediacaran when there was an evolutionary explosion in life.

But according to the molecular record, sponges should have branched off 650 to 680 million years ago, which is about when these reefs occur, Dr Gehling said.

"If he's found evidence of sponges, and that would need to be verified, that's exactly what the molecular records predict and that would be very exciting," he said.

The Selwyn Symposium will have international and Australian scientists debating theories on what led to the "evolutionary explosion" of life at the time of the first animals.