Life on Earth would have resumed quickly after the 4th mass extinction
Complex ecosystems developed earlier than previously thought thought so after the greatest extinction in the history of the planet.
Illustration of what the marine fauna of the Guiyang biota might have looked like, with its multiple species .
The discovery in China of a thousand fossils of exceptionally well-preserved marine species testifies to the existence of a complex ecosystem barely a million years later the Permian-Triassic mass extinction that occurred more than 250 million years ago.
Until recently, scientists believed that life had been dominated by single species for the 10 million years following that dark period in which about 95% of marine and 75% of terrestrial species have disappeared from the surface of the globe.
The fossil of a fish collected near Guiyang in southern China.
Even today, the exact causes of this extinction remain unknown, but they could include the the eruption of one or more supervolcanoes, the fall of meteorites or an unfavorable evolution of the environment following the formation of Pangea, the last supercontinent which brought together almost all of the emerged lands.
The fossil of a Watsonulus fish.
The numerous fossils were found in the province of Guizhou in southern China, in what paleontologists call deposits of exceptional preservation. There is an abundant diversity of organisms which are also very well preserved, which makes it possible to study their anatomical characteristics in a relatively detailed way and to specify their evolution, explained the paleontologist Arnaud Brayard of the University of Burgundy. , one of the authors of this work, during an interview on the show Les Années lumière.
“These exceptionally well-preserved deposits are real time windows on the ecosystems that existed in a given place at a given time. »
— Arnaud Brayard, University of Burgundy
The bottom of the Guizhou Basin, an ancient ocean during the Permian-Triassic transition which contains many fossils.
An international group of scientists including researchers from the University of Quebec in Montreal analyzed the fossils discovered in these deposits.
The fossils dated to 250.8 million The years reveal a marine ecosystem rich in diverse species that formed a complex food chain including plants, bony fish, finfish, crabs, lobsters, shrimps and even molluscs.
“We sampled around 40 species from 19 different major groups with primary species at the base of the food chain all the way up to top predators. »
— Arnaud Brayard, University of Burgundy
The existence of these deposits therefore suggests that a great diversity of species populated the Earth relatively soon after the great extinction. Species would have diversified rapidly, only a million years after extinction. This reality calls into question the theory that life on Earth would have been dominated by simple species in the 5 to 10 million years after extinction and then became more complex.
The fossil of a coelacanth fish.
Dating such old fossils is not an easy task, but it has been made possible by a relatively new technique: high-precision geochronology. This method uses uranium and lead isotopes to establish the age of fossils.
Two specialists in this technique are associated with the University of Quebec in Montreal.
One of them, the researcher Morgann Perrot, explains under what conditions the fossils found themselves imprisoned in space-time.
“You have to imagine a basin filled with water like a lake, a sea or an ocean. There are sediments that settle in this basin. And during a volcanic eruption, the ashes will settle in this basin. »
— Morgann Perrot, University of Quebec in Montreal
In these ashes, there are minerals, in particular zircon, which we are able to date. In fact, these minerals are formed during the eruption and will be deposited and allow us to precisely date the deposition of this ash and the age of the basin at this time, adds the researcher.
“If there are animals dying and settling on the bottom of the pond at the same time, that allows us to estimate when they died. »
— Morgann Perrot, University of Quebec in Montreal
Earth science professor Josh Davies of UQAM explains how the composition of zircon isotopes allows establish the age of zircon in the laboratory.
“Zircon grains are dissolved one at a time and placed under a microscope to separate the uranium from the lead and analyze its isotopic composition which is proportional to its age.
—Professor Josh Davies, UQAM
You should know that over time, uranium decays into lead. Since the rate of disintegration of uranium into lead is known, by analyzing the ratios of uranium and lead isotopes, it is possible to go back in time to know the age of the zircon and therefore of the fossils which are found in the same environment.
This dating has, according to Morgann Perrot, implications for our understanding of how quickly life can respond to extreme crises. It also requires a reassessment of early Triassic ocean conditions, she notes.
The details of this work are the subject of an article published in the journal Science (in English).
1. Ordovician – 445 millions of years
Species extinction rate: 60-70%.
Cause: short but intense ice age.
2. Devonian – 360 millions of years
Disappearance rate: up to 75%.
Cause: ocean oxygen depletion.
3. Permian – 252 millions of years
Disappearance rate: 95%.
Causes : asteroid impacts, volcanic activity
4. Triassic – 200 millions of years
Disappearance rate: 70 to 80%.
Probable causes: multiple, still under discussion.
5. Cretaceous – 66 millions of years
Disappearance rate: about 75%.
Cause: Asteroid impact in present-day Yucatan Peninsula.
6. Currently
Appeared in the scientific literature in the last three decades, the idea of a sixth extinction is increasingly widespread. It would be associated with the collapse of biodiversity linked to human activities, which destroy and fragment natural habitats.
