The mass extinction of biodiversity on Earth could be the seventh – not the sixth

Scientists call it the “Ediacaran fauna”, named after the hills in southern Australia where it was discovered in 1946. It consists of animal fossils that are visible to the eye and are 550 million years old, forming a unique biodiversity . You have to imagine the scene “printed” in the sediments: strange “petalamide” with bodies in the shape of feathers caressed the shoulders of Kimberellasimilar to modern slugs, when the ancestors of jellyfish (Cnidaria) were already floating.

But when we compare this rich Ediacara fauna with the “fossil record” (all known fossils) dated only 10 million years later, the picture changes completely: 80% of the species have not been found. What happened in the meantime?

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Several hypotheses are considered. For example, trilobites – marine arthropods with bodies covered in “armor” – can compete with other species to the point of driving them to extinction. Or, the conditions conducive to the formation of fossils – temperature and pressure that allow shells and other remains of dead animals to be preserved until the present – temporarily cease to exist, which can give the illusion that the fauna disappeared.

The first mass extinction in Earth’s history?

Another possibility: a mass extinction will occur across the planet. A hypothesis that just received a serious “boost”, so to speak, in a new study published in the journal Proceedings of the National Academy of Sciences (11/2022). According to its authors, the first mass extinction crisis on Earth occurred 550 million years ago (Ma). Currently, the five main biodiversity crises that are generally recognized are:

  • The Ordovician-Silurian crisis (-440 Ma)
  • The Devonian crisis (-365 Ma)
  • The Permian-Triassic crisis or “great extinction” (-250 Ma)
  • The Triassic-Jurassic crisis (-210 Ma);
  • The Cretaceous crisis (-66 Ma) which dramatically saw the extinction of all dinosaurs (except the ancestors of birds).

The team, led by the American Scott Evans, a post-doctoral researcher at the Polytechnic Institute and State University of Virginia (Virginia Tech), compiled a database of fossils dated 550 million years ago and described in scientific literature, before classifying each specimen. according to various factors (location, size, feeding method, etc.). In total, 70 species have been identified, of which only 14 are still found after about ten million years.

In their analysis, the authors did not observe any significant changes in the conditions needed to preserve fossils, or even differences in feeding patterns between “extinct” and “surviving” genera – thus undermining the two other hypotheses mentioned above.

On the other hand, the researchers highlighted a remarkable trend. “We looked at the selection model – which (organisms) went extinct, which survived and then evolved.“, explained Professor Shuhai Xiao, geobiologist at Virginia Tech and co-author of the study, quoted by LiveScience. “It appears that organisms that cannot tolerate low oxygen levels are selectively eliminated.

Survives low oxygen levels

Thus, all living things that have survived all have one thing in common: their bodies are characterized by a high surface-to-volume ratio – a characteristic known to help animals cope with low oxygen conditions. . This observation, along with geochemical evidence of a decrease in oxygen 550 million years ago, indicates that the “Ediacaran” (geological period associated with the Ediacara fauna) may have ended with a mass extinction caused by the low availability of oxygen in the ocean.

Greenhouse gases raise fears of mass extinction in the oceans

However, the study does not conclude on the cause of the decrease in oxygen levels. The main author discusses the LiveScience the track of volcanic eruptions, the movements at the level of tectonic plates, or even the possible impacts of asteroids. Without including a less “fantastic” explanation, such as changing the amount of nutrients in the oceans – which would approach, all things considered, a phenomenon observed today.

In fact, agriculture that uses synthetic fertilizers and the disposal of sewage releases large amounts of nutrients – particularly phosphorus and nitrogen – into marine and river ecosystems. These discharges cause an increase in algae, which destroys the environment by consuming all the oxygen available, creating what scientists call “dead zones”. “This study helps us understand (what could be) the long-term ecological and geological effects of periods of oxygen deficiency.“, commented Professor Xiao.

Destruction of natural habitats and global warming

But pollution is not the only cause of the current crisis, called the “sixth mass extinction” by some researchers – and therefore it may now be necessary to call the “seventh mass extinction”, if the results of the American team are confirmed only. . Today, the main reason for the decline of biodiversity is none other than the destruction of natural habitats (for the benefit of cities, animal pastures and agricultural fields), followed by the strict exploitation of natural resources. wealth and the poaching.

Nearly 70% of wild animals have disappeared since 1970, according to the WWF

Global warming – caused by greenhouse gases emitted mainly by human use of fossil fuels – is set to become the main cause of the extinction of animal and plant species, according to the WWF, whose Planet Index Vivante (IPV) showed a 69% loss of populations (in number and not in number of species) of wild vertebrates – fish, birds, mammals, amphibians and reptiles – between 1970 and 2018 (WWF, 2022).

Also read:

Sixth extinction: is the scale of the crisis underestimated?

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