The fossils were found embedded in limestone columns called stromatalites that were formed by mats of ancient bacteria. ((Adam Maloof/Princeton University)) Fossils of sponge-like animals that lived around 650 million years ago — 70 million years earlier than any other known animal fossils — have been found in South Australia.

The fossils were found under a 635-million-year-old glacial deposit, reported a study published Tuesday in Nature Geoscience.

The rock was photographed after each of 500 thin layers were removed, producing images like this one. The fossil of interest is outlined in blue. ((Maloof Lab/Situ Studio)) Geoscientists Adam Maloof and Catherine Rose, researchers at Princeton University in New Jersey who led the study, discovered centimetre-long wishbone, ring and anvil-shaped forms embedded in limestone columns called stromatalites that were formed by mats of bacteria, which were common in ancient seas.

The researchers suspected they might be parts of the bodies of animals, and 3-D models created from the fossils support that theory.

The finding was surprising because there was a very severe ice age 635 million years ago.

"No one was expecting that we would find animals that lived before the ice age," said Maloof in a statement, "and since animals probably did not evolve twice, we are suddenly confronted with the question of how a relative of these reef-dwelling animals survived the 'snowball Earth.'"

According to the National Science Foundation, which funded the study, the find is at least 70 million years older than any other evidence of animal body forms in the fossil record.

Because the animals were made of the same material as the rock itself, they couldn't be removed or imaged with X-rays. Instead, the researchers collaborated with Situ Studio, a design company based in Brooklyn, N.Y., with an expertise in digital modelling techniques.

Stacking the images resulted in a 3-D model showing an irregularly shaped body with a network of channels through it. ((Situ Studio)) By shaving off thin layers of the rock, each about half the width of a human hair, and photographing it after each layer was removed, the team managed to create a 3-D model of the animals. The model revealed marble-sized bodies, irregularly shaped with an internal network of interconnected millimetre-wide channels.

At least three of the specimens had a short, tube-shaped appendage at its base that may have been used to attach it to a solid surface. The researchers believe the fossils likely belonged to a sponge-like filter feeding animal.

The oldest fossil sponges previously found lived around 520 million years ago. However, chemical traces from materials of cell membranes of sponges were discovered in 2009 in sedimentary rocks more than 635 million years old.