About 66 million years ago, a celestial body with a diameter of at least 10 kilometers passed through the ancient atmosphere and hit the present-day Gulf of Mexico, setting off a tsunami over a hundred meters high; the impacted stratum was shattered, and the fragments were together with the fragments of the meteorite. Sprayed into the atmosphere, and the fall of this second wave of bolide caused widespread forest fires and rapidly acidified the ocean.
In this cold world where dust shades the sun, the survivors who have not been extinct by earthquakes and fires have also fallen amid the fragmented food chain.
This sudden catastrophe is one of the five major extinctions in the current history of animal evolution: the most dramatic and well-known "Cretaceous-Paleogene extinction event", commonly known as the "Dinosaur Extinction". This incident destroyed 75% of the creatures at that time. Except for birds, dinosaurs, pterosaurs , various aquatic reptiles, ammonites, and other countless animals and plants could not escape this fate.
(2021) A new study published at the end of June, after investigating and integrating 1,600 records of dinosaur species in the six major groups of the Cretaceous, it is believed that during the last 10 million years of the Cretaceous, climate change and dinosaurs may have occurred first. The diversity of the country is generally declining. The meteorite was the last straw that was laid on this pressure and caused the extinction of the dinosaurs.
Such a sudden change cannot completely disappear from the earth. The 180-kilometer diameter "Chicxulubcrater" is still lying on the land-sea junction at the northern end of Yucatan Peninsula; Cretaceous At the junction with the Paleogene strata, those dark strata with iridium elements, and burned plant fossils, have also witnessed this global catastrophe.
Chicxulub Crater
Cretaceous-Paleogene boundary
The Cretaceous-Paleogene boundary is located near Interstate 25 in Colorado, USA .The red arrow is the Cretaceous-Paleogene boundary (famous for being rich in iridium).
Discover the hidden information of dinosaur fossils with "evolutionary dynamics"
Meteorite impact has been recognized as the main reason for this extinction event, but are there other factors involved?
A recent study published in "NatureCommunication", based on the six main groups in the Cretaceous dinosaur -hadrosaurs, horns, ankylosaurs, Tyrannosaurus, Chiosaurus, was used as the object to analyze the global fossil records of these groups to understand other possible influencing factors; at the end of the study, a total of 247 species and 1,636 dinosaur fossil records were accumulated.
This study uses a database integrated with these fossil records to conduct a simulation analysis of the "evolution dynamics" of various groups, mainly involving the formation of new species and the rate of extinction of existing species. Of course, because the objects are fossils, these so-called "formation" and "extinction" are actually judged based on the age of the first and last fossil records.
In the process of analysis, in addition to the evolutionary branch as the basis, the analysis of the diversity trend of the species within each taxa, also based on the herbivorous (hadrosaurus, horned dragon, ankylosaurus) and carnivorous (tyrannosaurus) Group, and then integrate all the groups to know whether there is a difference in the changes of species diversity among different diets, and the trend of changes in the overall dinosaur group.
Overall, research and analysis show that approximately 10 million years before the end of the Cretaceous extinction event, which is about 76 million years ago, the diversity of dinosaurs reached the peak of the entire Cretaceous, but afterwards it became global General decline.
(left) analyzes the fossil records of 6 dinosaur groups. The trend of species generation (blue curve) has declined since 76 million years ago and the trend of extinction (red curve) has increased.(Picture right) The dinosaur diversity of the Cretaceous may decline in the last 10 million years of the Mesozoic.
Environmental change and competition among congeners, the complex effects of internal and external interactions
If you look at it by eating habits, the diversity of "phytophagous dinosaurs" began to decline 76 million years ago; "carnivorous dinosaurs" "Sex dinosaurs" came later, beginning to decline about 72 million years ago. This may be because the two types of dinosaurs have different positions in the food chain, and the decline in ecological carrying capacity caused by the decline in the diversity of herbivorous dinosaurs, after about 4 million years, is delayed reflected in the meat of secondary or higher consumers. Sex dinosaur.
In addition, competition within the same group will also affect the diversity of each other. For example, in the herbivorous taxa, the diversity of hadrosaurs declined the slowest, indicating that the diversity of hadrosaurs may also indirectly exclude ceratosaurus and ankylosaurus. The competition among carnivorous dinosaurs is far more serious than that of herbivorous dinosaurs. For example, the emergence of every new tyrannosaurus will cause the subsequent new species formation rate to drop by 23.4%; but the herbivorous ceratopsaurus Said that the emergence of a new horned dragon caused a decline in the formation rate of other new horned dragons, which was only 13.1%.
The body type comparison of 18 primitive ceratops
Compared with the time point when the species diversity of dinosaurs declined, " geological age" was 76 million years ago, which was the penultimate Cretaceous The two stratigraphic periods are the "Campanian", and from 72 million years ago to the end of the Cretaceous 66 million years ago, it was the "Maastrichtian".
Past studies have pointed out that the sea temperature of the North Atlantic Ocean at 35º north latitude during the Campa period was 35 degrees Celsius, but in the Maastricht period, it dropped to 28 degrees Celsius.
The change of sea temperature is linked to the change of land temperature, and it also involves the rise and fall of sea level. The newly published research believes that this trend of "long-term temperature reduction",It is probably the main reason for the decline in dinosaur diversity during the last 10 million years of the Cretaceous. Changes in the geographical environment and the gradual succession of plant groups from gymnosperms and ferns to angiosperms may also have an impact, but not So remarkable.
The drop in temperature caused by the change of sea temperature may be the main reason for the decline in dinosaur diversity
"Dinosaur extinction hypothesis" long-term ongoing debate topic
research history of modern paleontology, In fact, there have been numerous hypotheses about the reasons for the extinction of dinosaurs. Although the "celestial body impact theory" proposed in 1980 is now widely accepted by the scientific community, it was only after long-term follow-up research and accumulation of relevant geological evidence that everyone was truly convinced.
Putting aside whimsical ideas such as the extinction of aliens, others such as dinosaur eggs were eaten by dinosaurs/mammals, abnormal eggshells of dinosaurs made the fertility rate decline, unable to regulate the ecosystem where angiosperms gradually increased , Evolution is too extreme and eliminated... Although it seems absurd from the present point of view, it may also be an unreasonable inference under the background of the research knowledge at that time.
There are various hypotheses about the extinction of dinosaurs, including dinosaurs eating their own eggs
In modern times, various hypotheses about the extinction of dinosaurs, some people have proposed "whether dinosaurs were gradually fragile at the end of the Cretaceous" This proposition, even though it has been largely replaced by the "celestial body impact theory", has been changed to "before the meteorite impact, whether the dinosaurs have been fragile and unable to recover from the catastrophe", and continue to be among the supporters of different factions. debate.
For example, another article in 2019 that was also published in "NatureCommunication" held a completely opposite view, thinking that between the last two geological stages of the Cretaceous, taking North America as an example, the diversity of habitats And the habitability has not declined; in other words, the overall environment has not become less suitable for dinosaurs to survive.
This paper also proposes the difference in dinosaur fossil records between the Campanian and Maastrichian stages,It may be attributed to the fact that the fossil collection sites are only concentrated in a few areas, and these places happen to be in the Campo period and have more favorable geographical conditions for the preservation of fossils; while in some areas, it was even changed from land to land during the Maastricht period. The marine environment makes it more difficult to find dinosaur fossils.
The difference in dinosaur fossil records between the Campanian and Maastrichtian stages may be attributed to the fact that the fossil collection locations are only focused on a few areas. Published earlier in 2021, the paper focused on the impact of Tyrannosaurus on the ecological habitat of carnivorous dinosaurs in the ecosystem. The sturdy adult body that they have grown up allows them to occupy two completely different ecological habitats, and thus exclude the emergence of other medium-sized carnivorous dinosaurs.
The species diversity of carnivorous dinosaurs of the Maschician stage. The lack of medium-sized carnivorous dinosaurs may be caused by the sub-adults of Tyrannosaurus preempting ecological habitat.
If juvenile and adult Divide the Tyrannosaurus into two groups for analysis, which can make up for the diversity reduction gap in terms of the number of species alone.
For the contradictory argument, the newly published paper also proposes some refutations. For example, Tyrannosaurus had no obvious crowding effect on the other two carnivorous dinosaur groups analyzed in the article; and compared to other periods in the Cretaceous, the last The two ages may have the highest fossil preservation rate, which can compensate for the deviation of fossil excavation sites and deposition conditions.
Scientific controversy is endless, and continuous corrections are made in an infinite number of solutions.
Perhaps the debate on how dinosaurs other than birds will become extinct will continue. However, in addition to dinosaurs, as mentioned at the beginning of the article, pterosaurs, plesiosaurs , mosasaur , ammonites, and many other groups of creatures have also disappeared in this mass extinction.
Fengshen pterosaurs
Pterosaurs including Quetzalcoatlus, the largest flying animal in the history of the earth,It was also wiped out in the Cretaceous-Paleogene extinction.
If we believe in the argument of the latest paper, that is, non-avian dinosaurs gradually declined during the climate change at the end of the Cretaceous, so they could not rise again after being hit by meteorites. Then, for other dinosaurs that died at the same time How should scientists explain the taxa?
Imagine from another angle. Suppose we can accurately verify with science that a certain taxa actually continued to prosper until the last moment of the Cretaceous. It was completely annihilated overnight due to meteorite impact and subsequent sudden changes; then this Can be used to prove that , no matter whether the dinosaurs have previously declined, they cannot escape the impact of meteorites, so this 10 million-year decline period, whether it exists or not, is not the driving force of extinction?
In 2018, there was an interesting hypothesis that the reason why the ancestors of modern birds did not die together at the end of the Cretaceous was because they were small and terrestrial, and accidentally escaped the tsunami and forest after the impact. The fire was devastated; on the contrary, some arboreal and high-flying birds at that time did not escape the catastrophe.
The ancestors of modern birds, maybe ground-dwelling birds, escaped extinction?
Regardless of the extinction or survival of the taxa, the reasons for this need to be repeatedly verified and discussed. The fossils preserved in the
strata are limited. For the entire Cretaceous-Paleogene extinction event, in addition to the consensus of "celestial body impact", there are reasons why the various groups will become extinct, which is difficult to describe in one stroke. Some seemingly related signs may not have a causal relationship with each other; some seemingly unrelated events that cannot be connected at this stage may also be the key points in the whole series of dominoes.
Scientific controversy may never end, nor can it accurately point to the correct answer; but with the accumulation of research, there will eventually be a chance to get closer to the truth.
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