The dinosaurs were on the decline before the fall of the asteroid…, not so sure!

Were dinosaurs already declining when the Chicxulub asteroid crashed into Earth 66 million years ago? The debate rages in the scientific community. If many studies lean for the yes, a new one contradicts this idea, based on the modeling of the trophic networks of the end of the Cretaceous.

66 million years ago, a asteroidasteroidabout 10 kilometers in diameter crashed into the current peninsulapeninsula of the Yucatán, causing the fall of non-avian dinosaurs and many other speciesspeciesthe so-called mass extinction of the CretaceousCretaceous. Today, scientists are trying to reconstruct little by little the events that took place before and after the meteoritemeteorite. In particular concerning the dinosaurs: were they already in decline when the meteorite fell?

According to many studies, yes. They are based in particular on the rate of appearance of new species, which began to decrease millions of years before the great extinction. A decline that could be explained by various causes, such as a change in the weatherweather, continental fragmentation, or sea level rise. But today, a new study published inScience Advancescalls this long-held assumption into question.

Major changes in food webs

For their study, the researchers from the University of Edinburgh reconstructed part of the food webs from several periods, before and after the impact of the asteroid. They make it possible to study the interactions between species, and therefore the ecological diversity of certain places. In this case, the new study targets the North American continent. The team thus reviewed more than 1,600 specimens over 22 million years: 18 million years before the racing carracing carand 4 Ma later, making it possible to encompass the campanien, leMaastrichtianet leof Denmark.

The fossilsfossils belonged to all species types of the food chainfood chain of the time, ranging from frogs to large dinosaurs, including many reptiles and mammalsmammals ! And if the researchers have noted the same decrease in the diversity of species in the dinosaurs, for them, this decrease is not interpreted as a decline.“We are seeing a change in recent faunesfaunes of Cretaceous dinosaurs, as mid-sized species offset a loss of megaherbivores, but dinosaur niches were otherwise stable and static, potentially contributing to their demise,”they write.

Only one ecological guild (containing several different species), that of the« dinosaures ornithischiensornithischiens herbivores de grande taille »,appears to have declined in North America prior to the asteroid’s arrival. According to the researchers, if the impact of some dinosaurs has indeed diminished in the food web, the fact that their position does not change means a stable position, further supported by “ their anatomicallyanatomically and their very diverse functional capacities until the end of the Cretaceous”describes the study.

“It’s a kind of compromise”says Jorge García-Girón, first author of the study and ecologist at the Finnish University of Oulu and the Spanish University of León. “Dinosaurs were much more stable in their ecological environments. They were, of course, masters of their ecosystemecosystem. Mammals, on the other hand, were diversifying and starting to colonize different habitats and different environments. »

A boon for mammals

Because the extinction of the Cretaceous-Paleogene, if it caused the disappearance of 75% of the species, allowed the developmentmammals! Indeed, only species capable of going under terreterre, or to adapt their diet, survived. Which was not the case with dinosaurs, according to the study:“Niche stability could have disadvantaged dinosaur-dominated guilds in the event of an abrupt food web shutdown, such as that encountered at the Cretaceous-Paleogene extinction event. »But it would then be this flexibility that would have been the salvation of the mammals!

Indeed, the researchers counted more and more species in the three targeted periods, clearly showing an increase in their ecological impact.“The little ones vertebratesvertebratesincluding mammals, followed a consistent trajectory of increasing trophic impact and loosening niche boundaries beginning in the late Cretaceous and continuing after the mass extinction,”describes the study.

The researchers conclude on the usefulness of this research for forecasting our future.« The species ofpure waterpure waterfor example, are already rapidly disappearingalarmed Jorge García-Girón.If we are able to discern the type of factors that have determined the survival of freshwater fauna in the past, we may be able to use this information to predict the consequences of the loss of biodiversitybiodiversity of fresh water that occurs in our time. »

The dinosaurs were already in decline before the fall of the asteroid

If the biological crisis marking the end of the Cretaceous is often represented as a brutal event, associated with the fall of the asteroid of ChicxulubChicxulubthe extinction of the dinosaurs would in reality only be the result of a long decline, which began several million years ago.

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Article of Morgane GillardMorgane Gillardpublished on September 24, 2022

About 66 million years ago, the dinosaurs died out, leaving behind only meager descendants. Only the birds are indeed today the heirs of these giants who reigned over the continents and oceans of the globe for more than 180 million years.

This mass extinctionmass extinction is known as the biological crisis of the end of the Cretaceous, or Cretaceous-Tertiary crisis. Its exact causes are still poorly understood, even if it is generally presented as the result of the fall of a large asteroid, or of massive volcanic eruptions at the level of what are now called the Trapps du DeccanTrapps du Deccan.

Of them natural disastersnatural disasters which would therefore have combined to bring down all the species of dinosaurs. But did this extinction occur suddenly, as the images of the devastation of the asteroid often suggest, or did it stage much longer in time?

Dinosaurs already in bad shape long before the fall of the asteroid

For researchers at the Chinese Academy of Sciences, there is now no longer any doubt. The second option would be the most realistic. The dinosaurs would indeed have undergone a long decline before finally dying out.

If the paleontological data show that, over the last million years marking the end of the Cretaceous, the dinosaurs were still present all over the globe, we cannot however say that they were in very good shape! It would indeed seem that their population was no longer very diversified, and that long before the fall of the Chicxulub asteroid.

This is what Chinese researchers report in an article published in the journal PNAS. To reach this conclusion, they studied in detail more than 1,000 dinosaur egg fossils found in central China, dating from the last two million years before the total extinction of the dinosaurs. While the region is well known for its abundance of fossils and the large number of species represented, the researchers show that this period (-68.2 to -66.4 million years ago) was marked by a sharp decline in the diversity of dinosaur populations.

Abnormal species poverty long before extinction

The eggs found are in fact only from three different species: Macroolithus yaotunensis, Elongatoolithus elongatuset Stromatoolithus pinglingensis. Two of these species are oviraptors, small toothless dinosaurs equipped with a beak certainly used to break shellsshells eggs to eat. The third species is a herbivore of the Hadrosaur family, decked out with a duck’s beak. Next to that, a few rare bones of tyrannosaurestyrannosaures and of sauropodessauropodes were found in the area. This anomalous poverty of dinosaur biodiversity during the last million years before total extinction was also observed in North America and suggests that dinosaurs were already on the verge of global extinction during the late Late Cretaceous.

The causes of this long decline are still poorly understood. Among them, a modification of the climate, perhaps in connection with the intense continental fragmentation which took place at that time, a modification of the food chain with the beginning of the domination of flowering plants over conifers, or simply the consequences of too great a diversification of species. We must not forget the certainly major role of volcanic eruptionsvolcanic eruptions of the Deccan Traps during this period. This volcanic episode of phenomenal magnitude, active from -68 million years ago, would have led to climatic changesclimatic changes and environmental factors, destabilizing the dinosaur populations most adapted to their environment and most sensitive to ecosystem changes.

The Chicxulub asteroid, whose fall is dated at 66 million years, would have been only the cerisecerise on the cake, ending up ravaging a population already in agony for several million years.

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The dinosaurs were on the decline before the fall of the asteroid

Dinosaurs. In the collective imagination, they were the undisputed masters of our Earth. Until a meteorite comes to put an abrupt end to their reign. But researchers are now offering a new reading of the fossil record. They feature dinosaurs already in decline before the disaster. Implicating, in particular, a sudden climate change. Fabien Condamine, CNRS researcher at the Institute of Evolutionary Sciences in Montpellier, explains to us how his team arrived at these conclusions which revive the debate.

Article of Nathalie MayerNathalie Mayer published on July 4, 2021

66 million years ago, a meteorite more than ten kilometers in diameter lit up the sky of our Earth. She ended her race on the side of the Yucatan peninsula, in Mexico. Ending the reign of the dinosaurs. “Bad timing”, comments for us Fabien Condamine, CNRS researcher at the Institute of Evolutionary Sciences in Montpellier. “For the collision occurs in a world under pressionpression. A world that is experiencing global changes in its vegetation, climate and sea level. A world in which the dinosaurs were already not doing well. »

Extinction of dinosaurs: the identity of the culprit has been formally established by scientists

In any case, this is the conclusion of the work he has just carried out with a Franco-Anglo-Canadian team. A conclusion based on facts. Direct evidence from the fossil record. Evidence of the presence of certain species at a given time, in a given place. From which the researchers then made estimates of the rate of formation or the rate of extinction of the species. Thanks to a sophisticated method of modelizationmodelization statistics which made it possible to limit the biases linked to the shortcomings of the register. However, “With the knowledge that continues to accumulate, we are not immune to different conclusions falling in a few years. That’s also science.”points out Fabien Condamine.

But, in the meantime, let us return to this famous conclusion. Because it still has something to surprise. Because until then, it was quite commonly accepted that, towards the end of the Cretaceous, the groups of dinosaurs were rather prosperous. That they were diversifying and finding solutions to fill different niches. However, it is quite the opposite that the researchers suggest today.

Some 10 million years before the fall of the meteorite that caused the extinction of the dinosaurs, the rate of appearance of new species (blue curve) decreased and that of extinctions (red curve) increased sharply. What lead to a rapid decrease in the number of dinosaur species (black curve) during this period. © Fabien Condamine, ISEM, CNRS

“We have identified a peak of diversity in dinosaurs — or at least, among the six large families targeted by the study — at least 10 million years before their extinction. Between this peak and the fall of the meteorite in the Yucatan Peninsula, we observed a marked decline in species diversity — Understand, the number of species, explains Fabien Condamine. We sought to shed light on the dynamics of dinosaur diversification before understanding what factors could have influenced it in the long term. We weren’t expecting a decline. It just appeared to us in the data. »

A cascading extinction phenomenon

Once this decline was taken for granted — if science can take anything for granted at such a stage… — the researchers chose to adopt an integrative approach including a dozen possible causes. Among them, so-called causes abioticabioticrelated to the environment physiquephysique in which the dinosaurs evolved such as temperature, sea level or continental fragmentation. “Because at that time, our Earth was going through a lot of tectonic changes. » Researchers have also explored so-called biotic causes. Thus, the changes observed at the level of the vegetation.

“This is the time when flowering plants took over conifers”, situates Fabien Condamine for us. Another possible biotic cause, perhaps more surprising, is the intrinsic diversity of dinosaurs. “The diversity specific to a group influences the diversification of species. We talk about diversity dependence. As the group diversifies, there are fewer opportunities to form a species because the main niches are filled. The group finds fewer solutions to fill new niches. A kind of competition for resources sets in until, why not, leading to a decline. »

When flowering plants took over conifers, dinosaurs failed to adapt to them. The researchers find negative correlations that show that the change harmed the dinosaurs without being significantly so. © Elenarts, fotolia

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Finally, the researchers emerge from their work, two causes that strongly influence the process ofspecies extinctionspecies extinction. First, there is the global temperature, which decreases sharply over the period. About 7°C. One would have thought that the dinosaurs, being mesothermal animals that they were, would be only slightly affected. But, “we observe that the dinosaurs ultimately seem to have depended substantially on the temperature of their environment”notes Fabien Condamine.

Did you know ?

Dinosaurs were so-called mesothermal animals. Understand, halfway between lizards, turtles and crocodiles who depend on environmental temperatures to carry out their biological activities, and us mammals and birds who create our own body temperature.

Another identified cause of the decline of dinosaurs some 10 million years before their extinction: a decline in the diversity of herbivorous dinosaurs. “It makes ecological sense. Large herbivore species are structuring species on which the balance of an ecosystem depends. Ankylausaurs or Triceratops probably played a huge role in the Cretaceous ecosystem. When these species began to decline, perhaps due to greater sensitivity to temperature changes, other herbivores followed, and then carnivorescarnivores. It’s a cascading extinction phenomenon that seems to show up in the data. »

Extinction: large herbivorous animals are more threatened than carnivores

The researcher nevertheless remains cautious in these conclusions. Because when herbivores and carnivores are separated, “the data lose their statistical power”. To confirm all this, it will therefore be necessary to extend the studies to more dinosaurs. “We only worked on six families. The most known. It would be necessary to integrate groups from the southern hemisphere, but the registers are less extensive. We should also be interested in the Titanosaurs — which are among the heaviest animals to ever walk our Earth. They may well tell us a slightly different story. » Anyway, the debate seems to be revived…

Were the dinosaurs destined to disappear?

A more accurate estimate of the appearance and disappearance rates of dinosaur species tens of millions of years before the Cretaceous-Tertiary crisis reveals that these fascinating animals were in decline. One can speculate on the fact that, sooner or later, they would inevitably give way to mammals.

Article of Laurent SaccoLaurent Sacco published on 20/04/2016

A T-Rex skeleton on display at the Field Museum of Natural History, Chicago, USA. The number of carnivorous dinosaur species declined less rapidly than that of sauropods during the Cretaceous. © Terence Faircloth, Flickr, CC by-nc-nd 2.0

For decades, assumptions were rife about the disappearance of one of the most successful groups of animals in the evolution of life: the dinosaurs. From plants that have become toxic to mammals devouring their eggs, to climate changes caused by marine regressions, almost everything has happened. These are finally the hypotheses of the eruptions at the origin of the step you Deccan and the impact of a celestial body causing theastroblèmeastroblème from Chicxulub who imposed themselves on the front of the stage at the beginning of the 1990s.

However, an article published inPnas by a group of paleontologistspaleontologists just complicated the picture. They took advantage of the accumulation of data discovered in the geological archives of the Earth and, thanks to sophisticated techniques of statistical analysis, they were able to estimate the rate of appearance of new species and that of their disappearance during the 50 million years preceding the famous Cretaceous-Tertiary crisis, which occurred about 66 million years ago.

Surprisingly, signs of senescencesenescence appeared because the total balance shows a constant decrease in the diversity of dinosaur speciesmore marked in the case of herbivores long-necked. Something was therefore already in the process of weakening these animals and dethroning them from the biospherebiosphere, although we do not yet know what. Eruptions basaltbasalt and the impact of a celestial body may have been fatal to them only because they had already been in decline for tens of millions of years.



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