Researchers first discovered the pancake-shaped creatures — a group known as Dickinsonia — in the late 1940s. The species were among the most common residents of the world’s oceans 558 million years ago, during the Ediacaran period. Whereas most living things during that time ranged in size from microscopic to a few millimeters long, some Dickinsonia grew up to 1.4 meters in length.
The creatures’ large size has puzzled scientists because Dickinsonia lived tens of millions of years before the Cambrian explosion, the period 541 million years ago when living things became bigger and most major animal groups emerged. Scientists have since debated whether Dickinsonia were primitive animals, giant single-celled organisms called protists, bacterial colonies or something else entirely.
The latest study attempts to end that debate by analyzing chemical biomarkers preserved in a unique set of Dickinsonia fossils from Russia, rather than by examining the ancient species’ body characteristics.
The surrounding rocks and algal mats contained only about 10% cholesterol and 75% of another sterol that is common in green algae, the Dickinsonia fossils contained 93% cholesterol — suggesting that they were ancient animals living 17 million years before the Cambrian explosion.
The findings from the chemical analysis reinforce other evidence that Dickinsonia were primitive animals, he says. This includes fossil ‘footprints’ that show the organisms moved from place to place for food, and growth patterns that match those of most animals today.
The latest findings also suggest that the transition between the Cambrian and the Precambrian, which includes the Ediacaran, was just another extinction event in which one animal community replaced another, Brocks says. “But now the jury’s out on all the other weirdos.”
Analyses that have compared the DNA of living creatures today to trace back their evolutionary trees suggest that animals originated more than 100 million years before the Cambrian — well before even Dickinsonia. But finding the fossils of these creatures, and then proving they are animals, remains challenging.