Creatures without hard shells or bones, such as jellyfish, almost never survive in the fossil record. Preservation is even more difficult in sandstone, a rock made of coarse grains that allow water to pass easily and typically formed in turbulent environments shaped by waves and storms. These conditions usually obliterate the fine biological remains long before they can fossilize.
Yet something remarkable happened about 570 million years ago, during the chapter of Earth’s history known as the Ediacaran period. Soft-bodied organisms living on the sea floor have been buried by sand and preserved with extraordinary precision, leaving behind detailed fossil impressions that defy expectations.
The global mystery of the Ediacaran biota
Today, fossils of these organisms, collectively called the Ediacara Biota, have been discovered at sites around the world. Their unusual preservation has sparked the intense interest of scientists who want to understand how such fragile life forms were so clearly recorded in sandstone. Solving this puzzle could help shed light on a major missing chapter in the history of large, visible life on Earth.
“The Ediacara Biota look absolutely bizarre. Some of them have triradial symmetry, some have spiral arms, some have a fractal pattern,” says Dr. Lidya Tarhanová, a paleontologist from Yale University. “It’s really hard when you first look at them to figure out where to place them on the tree of life.”
Life before the Cambrian explosion
These organisms lived only tens of millions of years before the Cambrian explosion, a key period that began about 540 million years ago and marked the rapid rise of complex and diverse animal life. For a long time, this event was thought to represent a sudden biological breakthrough. Increasingly, however, scientists see it as the culmination of a much longer build-up.
Tarhan describes this process as a “long association,” with the Ediacara Biota representing an important early stage in the gradual expansion of size, complexity, and ecological roles among animals.
New clues from an unusual fossilization process
Understanding how these organisms were preserved is essential to interpreting their place in evolution and to understanding how complex life emerged early. A recent study by Tarhan and her colleagues, published last month in the journal Geology under the title “Authogenous clays shaped exceptional Ediacaran-style fossilization,” offers new insight into this process.
“If we want to understand the origin of complex life on Earth, the Ediacaran Biota really occupies a critical point in that trajectory,” says Tarhan. “It is incredibly important, not only for the Ediacaran Biota, but for all exceptionally preserved fossil assemblages, that we try to find out what the mechanisms are behind this exceptional fossilization, so that we can better estimate the extent to which these fossil assemblages provide a faithful reflection of life on the ancient seafloor.”
Fossil tracking through lithium isotopes
To find out what happened during burial and fossilization, Tarhan’s team used an innovative chemical approach. They analyzed lithium isotopes in Ediacaran fossils collected from Newfoundland and northwestern Canada and studied samples preserved in both sandy and muddy sediments.
These isotopes helped determine whether clay minerals played a role in fossilization and whether these clays came from land, known as detrital clays, or formed directly on the sea floor, known as authigenic clays.
How ancient seafloor chemistry preserved soft life
The results showed that detrital clay particles were already present in the sediment that covered the organisms. These particles then provided surfaces where new clays could form directly on the sea floor. Fueled by silica- and iron-rich seawater and the unusual chemistry of the Ediacaran oceans, these authentic clays grew around the buried organisms.
In effect, the clays acted as a natural cement, binding the sand grains together and preserving the detailed outlines and imprints of the soft tissues in the sandstone.
Rethinking why these fossils survive
This finding challenges the long-held idea that the Ediacara Biota were preserved because their bodies were unusually tough or chemically resistant. Instead, their survival in the fossil record appears to depend on environmental conditions rather than biological durability.
According to Tarhan and his colleagues, it was the chemistry of the ancient seawater and sediments that allowed for this exceptional preservation.
What this means for the history of life
Tarhan plans to apply the same lithium isotope method to fossils from other areas and time periods to see if similar processes are at work elsewhere. Even now, the results provide a clearer view of Earth at a pivotal moment in the evolution of animal life.
“It’s hard to overemphasize how dramatic a change this is from the small and microbial life forms that dominated much of the Precambrian to a large increase in size and complexity,” says Tarhan. “A clearer understanding of the processes responsible for fossilization in this interval will allow us to more thoroughly evaluate long-term hypotheses for the drivers not only of the appearance of the Ediacaran Biota, but also of their subsequent disappearance at the end of the Ediacaran period.”
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