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In the interval between 541 million years ago and 530 million years ago, almost all the major phyla of animals appeared, which, in evolutionary terms, is a very short period of time. This event is called the Cambrian explosion, which occurred at the start of the Cambrian era of paleontology. We can understand almost every modern animal, be it fish, insect, or parasite, in terms of some kind of ancestor found in the Cambrian era.
However, the fauna of the Cambrian era were not the first fauna. the history of evolution extends into periods many millions of years before the Cambrian explosion. However, fossils of creatures from the Precambrian era, called the Ediacaran era, are rare and mysterious. They are rare because they were soft-bodied, and it’s always the hard parts of creatures that are easiest to fossilize. They are mysterious because they are so different from modern organisms. It’s hard to tell how they ate, moved, or reproduced. Further, it is unknown if any of these ancient creatures are related to modern organisms.
A new report analyzes how extremely well-preserved fossils from the Ediacaran era formed, and what that tells us about the organisms observed.
"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."
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"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."
The fossils in question, found in Newfoundland, were preserved in sandstone. That means that the preserved creatures were in the environment of a sandy seabed. By itself, a sandy seabed is not an ideal spot to preserve fossils because they are porous to water currents and vulnerable to turbulence. So by what process were these remarkably detailed fossils preserved? It turns out that the grains of sand served as a substrate for the formation of clays whose origin were from the iron and silicate rich seawater. In essence, the combination of the sand and the precipitating clay formed a natural cement, which preserved the detailed impressions of the remarkable organisms of the Ediacaran era. Importantly, this process of preservation does not depend on the toughness of the organism being preserved, as some had previously believed, but entirely by environmental conditions. Even the most delicate of soft-bodied organisms can be preserved and fossilized by this process.
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