Traces of cholesterol suggest that these mysterious fossils were animals, not mushrooms

Cholesterol has won out: a group of strange Precambrian fossils are among the oldest known animals in the rock.

Organic molecules preserved with fossils of the genus Dickinsonia confirms that the creatures were animals rather than fungi or lichens, a 21 September study Science said. Researchers led by paleontologist Ilya Bobrovski of the Australian National University of Canberra have analyzed levels of steroids in fossils dating back to 571 million and 541 million years ago. The team found an abundance of cholesterol that points firmly to the animal kingdom.

The discovery "gets rid of the most extravagant assumptions about the nature of these objects," says MIT geoscientist Roger Summons, who wrote a related commentary in the same issue of Science. "You can not argue with chemistry."

Dickinsonia are part of the enigmatic Ediacara biocide, the collective name of an explosion of strange and extraterrestrial life forms that flourished during the Precambrian Eon. The ediacarans, originally named for the Hills of Ediacara, Australia, where they were first discovered, are now found in Precambrian rocks from around the world.

The new study was conducted on fossils of Ediacaran extracted from a remote coastline of northwestern Russia, along the White Sea. The site is difficult to access – Bobrovskiy had to helicopter and abseil down a cliff to collect fossils – but paleontologists claim that the rewards are worth it: the tectonic forces have not cooked nor twisted rocks containing fossils. The rocks are so intact that they still contain traces of soft tissues containing organic molecules, which researchers can use as biomarkers to help identify fossils.

This is especially useful for the Ediacaran fossils, which have proved difficult to place on the tree of life, as they hardly resemble any known creature. (SN 18/05/13, p. 20). Ediacarans were macrofossils, which means that several centimeters in diameter, they are large enough to be seen with the naked eye. But their strange shapes – for example, Dickinsonia look like ribbed ovals that are symmetrical around a central axis – leftist scientists are baffled. Most paleontologists suspected that Dickinsonia were animals. But some scientists have argued that they can be fungi, lichens or even giant unicellular creatures called protists (SN: 1/26/13, p. 15).

Tiny matter

Scientists have carefully extracted thin layers of organic matter (a layer shown here) preserved with Dickinsonia fossils. The team found a very high percentage of cholesterol compared to other steroids in organic matter, suggesting that creatures belonged to the animal kingdom. This makes them among the oldest animal macrofossils.

Biomarkers in soft tissues, however, provide a new source of data. To collect organic molecules, scientists removed thin layers of organic matter from the rocks and then extracted the molecules from the laboratory using solvents. The high-precision work must be conducted "under extremely clean conditions to avoid contaminants," says co-author Jochen Brocks, also of the National University of Australia.

As a control, the team also extracted organic materials from the rocks Dickinsonia fossils. Isolated steroids in surrounding rocks have been shown to be 70% stigosteroids, an organic molecule found in algae. This is consistent with the likely marine context of Ediacara biota: it was thought that creatures lived in shallow waters at the top of microbial mats.

But the fossils themselves were remarkably high in cholesterol compared to other steroids, accounting for between 85% and 93% of all the steroids in the different fossils of the genus. "The proportion of almost 100 percent cholesterol in the Dickinsonia The fossils tell us it must be an animal, "says Brocks. Cholesterol is produced by bacteria in the bowels of animals.

Then the team plans to look for biomarkers of other ediacarans in the rocks of the White Sea – and there are many intriguing options, says Brocks. "The coolest target is the rangeomorphs. These strange creatures are built like a fractal. No modern animal is built like that.