An accidental discovery
In 1946, geologist Reginald Sprigg was surveying abandoned mines in the remote Ediacara Hills of the Flinders Ranges. During a lunch break, he noticed disc-like impressions on the surface of sandstone slabs. He suspected they were fossils of soft-bodied creatures like jellyfish, far older than any complex life then known. His findings, suggesting a Precambrian age, were initially met with skepticism. The scientific establishment of the time held that no complex life existed before the Cambrian Period. His paper submitted to the journal Nature was rejected, and his presentation at the 1948 International Geological Congress failed to persuade his peers.
Decades later, Sprigg's assessment was proven correct. The fossils he found, preserved as impressions in ancient sandy seafloors, were the remains of a bizarre community of organisms that lived between 635 and 541 million years ago. The rock formations containing these fossils, part of what is now called the Rawnsley Quartzite, captured entire communities of these soft-bodied creatures, buried by storm-driven sand. This discovery was so deep that in 2004, the International Union of Geological Sciences ratified the Ediacaran Period, the first new geological timescale division declared in over 120 years.
Life on a microbial mat
The Ediacaran biota is a strange period of life's history. These organisms inhabited shallow seas when the seafloor was covered in dense microbial mats. They were soft-bodied, and their construction was unlike most life today. Many had a "quilted" or segmented body plan, while others were frond-like, tubular, or disc-shaped. Organisms like Dickinsonia were flat and oval, reaching up to 1.4 meters in length, and appear to have absorbed nutrients directly through their bodies as they moved slowly across the microbial mats. Another common fossil, Spriggina, was a segmented organism a few centimeters long with a distinctive horseshoe-shaped "head."
The classification of these creatures is a subject of scientific debate. Some scientists argue that forms like Dickinsonia were early animals. This hypothesis gained support when chemical analysis of preserved soft tissues revealed molecules of cholestane, a fossilized form of cholesterol, which is a sign of animal life. Others propose the Ediacaran organisms were experiments in multicellular life that belong to an extinct kingdom, the Vendobionta, which had no modern descendants. Some have even suggested they were giant single-celled protozoans, fungi, or lichens. This diverse biota this diverse biota largely disappeared from the fossil record just before the Cambrian Explosion, a period when the ancestors of most modern animal phyla suddenly appeared.