January 31. The Ediacara – 560-600 million years ago

The Ediacara Hills of South Australia contain some remarkable fossils discovered in 1946, but largely ignored. They were thought to be of Cambrian age, and they were vague enough that it was challenging to determine what they were. With improvements in age dating, these fossils were eventually determined to be about 560 to 600 million years old – the Precambrian, a time when no one thought life existed, at least not as large animals.

Dickinsonia costata was as much as a meter across, or more.

They appear to be soft-bodied organisms, preserved as imprints in fine sand. There are disk-like shapes, and ovoids, with radiating textures. Some frond-like fossils have a quilted appearance, and there are many other varieties. We don’t know what they are.

Early interpretations suggested they might be jellyfish, or worms, or sea anemones. They have a wide range in size, from a few millimeters to more than a meter across. Some researchers say they aren’t animals at all, but are some kind of fungus. Or they might belong to a phylum, or even a kingdom, that has no modern representatives. 

Examples have been found in Newfoundland, England, Namibia, and elsewhere, so they appear to have been pretty common in the oceans that appeared after the glaciations and possible snowball earth ended 650 million years ago, perhaps 50 million years before the Ediacarans appeared. And interestingly, the Ediacaran life seems to disappear just before the Cambrian Explosion of life, about 530 million years ago.

Whatever the Ediacarans are, they represent an important step in the evolution of multicellular life on earth. Figuring out what they were, or what they are related to, is one of the ongoing challenges of geology and biology.

With the Ediacaran fossils, we reach the end of the Precambrian. The Precambrian represents 88% of Earth history, but we’ve compressed it into the month of January alone. The rest of the year will cover the most recent 500 million years of Earth history, because we know a lot more about it. And because it contains cool things like fossils and mountain ranges and sedimentary basins and coal swamps and dinosaurs. Tomorrow the Cambrian begins.

—Richard I. Gibson

UPDATE: Mary Droser (UC Riverside) and colleagues have described a new critter from the Ediacara. This weird one gets as big as 30 cm across and apparently grew as fan-like branches, flat on the sea floor or within the upper part of the sediment on the sea floor.


The Wikipedia article is a good overview, and has extensive links to scientific literature.

Photo by Verisimilius from Wikipedia under GNU Free Documentation license.