How did these strange, ancient organisms turn into such remarkable fossils?

How Did These Strange, Ancient Organisms Turn into Such Remarkable Fossils?

New Research in Geology reveals why the 570-million-year-old Ediacaran Biota were so exceptionally preserved.

Boulder, Colo., USA: In Earth’s fossil record, soft-bodied organisms like jellyfish rarely stand the test of time. What’s more, it’s hard for any animal to get preserved with exceptional detail in sandstones, which are made of large grains, are porous, and commonly form in environments swept by rough storms and waves. But around 570 million years ago, in a geologic time interval called the Ediacaran period, strange-looking, soft-bodied organisms died on the seafloor, were buried in sand, and fossilized in incredible detail. 

Now, these fantastical fossils are found in deposits around the world. Scientists want to discover why the Ediacara Biota were so well preserved—in particular, the reasons for their very unusual fossilization as impressions in sandstone—in part to fill in a critical gap in the evolution of macroscopic life on Earth. 
 

“The Ediacara Biota look totally bizarre in their appearance. Some of them have triradial symmetry, some have spiraling arms, some have fractal patterning,” says Dr. Lidya Tarhan, a paleontologist at Yale University. “It's really hard when you first look at them to figure out where to place them in the tree of life.”
 

The Ediacara Biota lived at a liminal moment just a few tens of millions of years before what geologists call the Cambrian Explosion, an episode that began about 540 million years ago, marked by flourishing diversity and complexity across nearly all evolutionary lineages of almost all animals living today. But more and more research suggests what Tarhan calls a “long fuse” to the explosion, and the rise of the Ediacara Biota formed a key stage of that slow burn.
 

Understanding how and why these Ediacara organisms are so exceptionally preserved is central to placing them in their evolutionary context and illuminating the origins of the complex life forms from which many animals, including humans, ultimately descend. A study from Tarhan and her colleagues published last month in a Geology paper titled “Authigenic clays shaped Ediacara-style exceptional fossilization” helps fill in that gap.
 

“If we want to understand the origins of complex life on Earth, the Ediacara Biota really occupies a critical point in that trajectory,” says Tarhan. “It's incredibly important, not just for the Ediacara Biota but for all exceptionally preserved fossil assemblages, that we try to figure out what are the mechanisms behind that exceptional fossilization so we can better gauge to what extent these fossil assemblages provide a faithful reflection of life on the ancient sea floor.”
 

To learn more, Tarhan and her team used a novel approach to determining what processes and minerals were at work when the Ediacara Biota organisms experienced death, burial, and fossilization. They measured isotopes of the element lithium in Ediacara Biota fossils found in Newfoundland and northwest Canada in deposits that are both sandy and muddy. The lithium isotopes helped the scientists determine whether clay minerals were involved in fossilization and, in particular, whether these were detrital, meaning they washed off the continents, or authigenic, meaning the clays precipitated in the sea floor.
 

The researchers found that detrital clay particles were present in the sediments that buried these organisms on the sea floor. These minerals then served as nucleation sites for authigenic clays to form from silica- and iron-rich seawater in the upper sea floor, driven by the unusual chemistry of the Ediacaran seawater. These clays acted like cement, holding together sand particles in the sandstone and preserving outlines and replicas of the soft-bodied forms of the Ediacara Biota.
 

This counters a longstanding idea that the exceptional preservation of the Ediacara Biota might have occurred because their bodies were made of a uniquely hardy substance. Instead, it was the chemistry of the environment that lent itself to fossilization, according to Tarhan and colleagues.
 

Going forward, Tarhan wants to apply this lithium isotope technique to more fossils from different locations and geologic ages to see if the same mechanism applies. In the meantime, Tarhan says their findings help fill in the picture of what the world was like at a critical time in the evolution of complex animal life on Earth.
 

“It's hard to overemphasize how dramatic of a change it is from the small and microbial life forms that dominate much of the Precambrian to the big step up in size and complexity” seen in the Ediacara Biota and Cambrian Explosion, says Tarhan. "A clearer understanding of the processes responsible for fossilization across this interval will allow us to more robustly evaluate longstanding hypotheses for drivers of not only the appearance of the Ediacara Biota but also for their subsequent disappearance at the close of the Ediacaran period."
 

About the Geological Society of America

The Geological Society of America (GSA) is a global professional society with more than 17,000 members across over 100 countries. As a leading voice for the geosciences, GSA advances the understanding of Earth's dynamic processes and fosters collaboration among scientists, educators, and policymakers. GSA publishes Geology, the top-ranked geoscience journal, along with a diverse portfolio of scholarly journals, books, and conference proceedings—several of which rank among Amazon's top 100 best-selling geology titles.

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