To date, the Ordovician-Silurian extinction, which occurred about 440 million years ago, is considered the first. Mass destruction of our planet. This may have occurred following a major glaciation, and almost 85% of the species would have died out because they did not succeed in adapting to these new conditions. But evidence now suggests that another extinction event preceded that: a decrease in global oxygen availability that wiped out most animals at the end of the Ediacaran, about 550 million years ago.
The first extinction in Earth’s history
The sudden decline in fossil diversity around 550 million years ago has long been known, but scientists have not been able to pinpoint it. Extant species may have competed for survival, eliminating each other, or environmental conditions at the time were not conducive to preserving Ediacaran fossils. A new study Published in Proceedings of the National Academy of Sciences This makes it possible today to confirm that this decline was indeed the result of a mass extinction.
According to Shuhai Xiao, professor of geology at Virginia Tech, our planet has had five known mass extinctions, the “Big Five”: Ordovician-Silurian extinction (440 million years ago), the Late Devonian Extinction (370 million years ago), the Permian-Triassic Extinction (250 million years ago), the Triassic-Jurassic (200 million years ago) and finally, the Cretaceous-Paleogene Extinction (65 million years ago). ), which wiped out about 75% of plants and animals. Including non-avian dinosaurs.
All are linked to larger and larger scale environmental changes. A Climate change Or the oxidation phenomenon can lead to mass extinction of animals, as well as deep disruption and restoration of ecosystems. This first extinction event, which occurred during the Ediacaran period, was no exception to the rule: it too was triggered by a significant change in the environment.
During this first mass extinction, 80% of the animals living on Earth would have disappeared. ” This includes the loss of a wide variety of animals, but their body plans and behaviors indicate a significant oxygen dependency, especially in those affected. », Scott Evans explainsis a postdoctoral researcher and first faculty member in the Department of Geosciences at Virginia Tech A study describing the phenomenon.
An “incentive” for evolution?
The soft-bodied fossils of the Ediacara biota — named after the mountains in South Australia where the fossils were discovered in 1946 — are among the oldest known complex multicellular organisms. Fossil footprints from the Ediacaran period – about -635 to -539 million years ago – show that the animals that died out during this mass extinction had very strange appearances in the form of leaves, feathers or tubes.
According to Evans, organisms of the time seemed to be experimenting with different ways to build their large, multicellular bodies. Therefore, fossils found before the extinction do not always correspond to the current classification of animals. ” This extinction may have paved the way for the evolution of animals as we know them. The researcher concludes. That is, most of the animal system plans that exist today originated in the Cambrian period (i.e., the period after the Ediacaran).
Fossil footprints from the Ediacaran period – Dickinsonia (left) and the related but rare form Antiva (right) – found in the Ediacaran sandstone of South Australia’s Nilpena Ediacara National Park. These animals may have died out during the extinction. Thanks to: Scott Evans.
Evans and his colleagues carefully examined and cataloged all Ediacaran period fossils described in the literature. Thus they identified 70 species of animals, of which only 14 were still around 10 million years later. However, they found no signs that these animals competed with early Cambrian animals or anything that could explain the lack of fossil preservation.
In contrast, animals that survived showed an organizational program that favored survival in the event of anoxia: greater body surface area relative to their size. Geochemical evidence confirms low oxygen availability in oceans 550 million years ago.
An anoxia whose cause remains to be elucidated
What causes this drop in overall oxygen availability? ” We don’t really know the short answer to how that happened. Evans said. In fact, the scientist explains that many events, individual or combined, could be at the origin of the phenomenon: volcanic eruptions, movements of tectonic plates, asteroid impact, etc. Changes in marine nutrient levels may be another possible cause.
However, this extinction greatly affected the evolution of life on Earth, and this study provides insight into the long-term impact of oxygen deprivation on aquatic life. In fact In another studyVirginia Tech scientists recently discovered that the world’s freshwater lakes are now being rapidly depleted of oxygen.
This phenomenon is linked not only to the warming of water induced by climate change, but also to the excessive flow of pollutants (phosphorus, nitrogen) associated with agricultural practices: ” Warming water reduces the capacity of freshwater to retain oxygen, while nutrient degradation by freshwater microorganisms reduces oxygen. », Researchers explain.
In other words, the discovery of this new extinction provides insight into the dangers of the current climate crisis for animal life.