Why did dinosaurs become extinct?

Dinosaurs lived on the earth for 65.438+0.6 billion years, but at the end of Cretaceous, they suddenly disappeared all over the world. The extinction of dinosaurs is a big unsolved mystery in the history of life on earth. Since 1970s, various theories and hypotheses about dinosaur extinction have emerged one after another, and an unprecedented debate has been launched. Some dinosaurs became birds. The name, time, possible causes and consequences of the historical extinction event, the Ordovician extinction, 438 million years ago, climate change led to 50% species extinction, mainly Devonian marine extinction, 360 million years ago, 40% species extinction by gamma radiation, 245 million years ago, volcanic activities and other species extinction, 66 million years ago, more than 95% species extinction, and 45% species extinction by meteorite impact. He and his team found a high concentration of iridium in the K-T stratum in chicxulub crater 65.3483 million years ago in China, which was 232 times higher than the normal level. Such a high concentration of iridium can only be found in meteorites in space, and the earth itself cannot exist. According to the accurate determination of iridium content around the Gulf of Mexico, the material equivalent to an asteroid on Mount Everest at that time not only hit the Central American region of the earth, but also broke the earth's crust, and then a major earthquake never happened on the earth. The impact caused the molten slurry to be thrown into the air for thousands of meters, followed by flashover for dozens of days. High temperature may not be the most deadly. In the next month, tens of millions of tons of dust and toxic substances spread all over the world.

In the next four months, the sun was just a vague shadow, plants stopped growing, herbivores decreased greatly, polluted air, food shortage, and wanton diseases, all of which devastated the surviving dinosaurs. Covered with dust, the earth is facing a cold attack. But cold doesn't seem to be the most serious problem, but remember that the sex of some animals is determined by temperature, and dinosaurs are one of them. Led to this mass extinction.

In the past, the academic circles discussed the collision theory of alien celestial bodies and the volcanic eruption theory respectively, but both of them had considerable defects. Alien celestial bodies say that the single impact is not so serious, so long, so far away (global), while volcanoes say that there are many and intense volcanic activities on the earth, but it is not enough to cause such a large extinction, including the Yellowstone super volcano. The direction and evidence provided by Li Yang, a scholar in China, perfectly answered the long-standing questions of the international paleontology community. German scientists suggested that the extinction of dinosaurs was caused by the bad "space weather" at that time, that is to say, the strong particle flow from the universe broke into the earth's atmosphere, causing drastic changes in the earth's climate and leading to the extinction of dinosaurs.

According to the German magazine Science Illustrated, Joerg Farr, a professor from the Institute of Astrophysics in Bonn, said that the earth fell into a strong cosmic particle flow "storm" 60 million years ago. When encountering such a storm, various particles entering the earth's atmosphere at high speed will reach hundreds of times as usual, tearing the molecules in the atmosphere into condensation nuclei necessary for the formation of rain, which will eventually lead to the thickening of clouds in the earth's atmosphere, frequent rainfall and sudden drop in temperature.

The hypothesis that asteroids hit the earth put forward by American physicist Luis Alvarez has attracted much attention. He found that the content of trace element iridium in the upper layer of the late Cretaceous strata in gubbio suddenly increased by more than 30- 160 times compared with other strata, and then people came to the same conclusion from sampling tests around the world, and the abnormal increase of iridium content in the late Cretaceous strata was indeed universal. So Alvarez thinks that an asteroid with a diameter of about 10 km hit the earth at the end of Cretaceous, and the dust produced covered the sky. It caused great changes in the surface climate environment and led to the extinction of dinosaurs.

Scientists believe that the explosion of cosmic particle flow led to the drastic change of the earth's climate conditions, and dinosaurs that could not adapt to this climate change became extinct in a short time.

Doubt:

However, there are many doubts about explaining the increase of iridium content in rock formations and the extinction of dinosaurs caused by the impact of asteroids on the earth.

1. Asteroids are generally composed of silicon and iron, so it is impossible for such a huge asteroid to fall to the surface of the earth even after a long time, and such a large meteorite has never been found on the earth;

2. At the end of Cretaceous, most rock formations were igneous rocks formed by lava cooling, and sedimentary rocks formed by dust accumulation only occupied a small part of the surface. Can the dust raised by the asteroid impact bury most of the animals and plants on the earth in rocks thousands of meters deep?

3. Can iridium contained in asteroids be evenly distributed or even cover the whole earth's surface? Iridium also exists deep in the earth. Why only speculate that iridium comes from outside the earth and not from inside it?

As we know, thermonuclear reactions inside the earth will continuously accumulate enormous energy. Once the crust can't bear it, the internal pressure will break through the crust and suddenly release to form a big explosion. Iridium date, an element mainly existing in the core of the earth, was brought to the surface of the earth's crust by lava eruption during the Big Bang. It is recognized that the clay layer at the end of Cretaceous was formed by a large amount of volcanic dust accumulation. Therefore, the general increase of iridium content in the strata at the end of Cretaceous proves that the crust erupted violently at that time.

Fossil archives tell us that most dinosaurs died and most dinosaur eggs were produced at the end of Cretaceous, and all the dinosaurs and dinosaur eggs found were preserved in the stratum under the thin clay layer rich in iridium, which is consistent with the time of a series of global crustal structural changes such as large-scale orogeny at the end of Cretaceous recognized by geologists.

Hundreds of protoceratops and Ankylosaurus fossils unearthed in the stratum of Bayinmandu Lake in Inner Mongolia at the end of Cretaceous period, a large number of complete dinosaur bones are piled up in groups. Judging from the burial posture of the remains, they died in extreme pain, including the bones of a whole group of dinosaur cubs. This scene shows that they are catastrophic collective deaths.

A large number of animal and plant fossils reflecting the characteristics of the earth's environment at that time show that before the end of Cretaceous, the density and thickness of the earth's atmosphere far exceeded that of 2000, and the surface was relatively flat, with a very warm and humid climate all over the world. At that time, the temperature difference between the polar regions and the equator was very small. In the 1980s, Canadian geologists discovered a metasequoia-dominated fossil forest in ellesmere island in the Arctic Circle, where there were crocodiles and other animal fossils, indicating that the polar regions used to have a tropical climate.

Some animals that have evolved into primitive birds and mammals as early as Jurassic follow the law of natural selection and survival of the fittest. In a relatively harsh environment, after 70 million years of continuous evolution, most species have changed their original forms. Of course, after every large-scale species evolution, some species will remain intact. For example, after fish evolved into amphibians, fish continued to survive, and very few reptiles (crocodiles, lizards, etc. ) still maintains its original form 70 million years ago.

The biological remains in the earth's rock strata reveal that in the history of biological evolution, there will be a mass extinction every once in a while. The extinction of dinosaurs at the end of Cretaceous is not the only disaster in the history of biological evolution. In an earlier era, there was a "Cambrian Big Bang" phenomenon in which most invertebrates suddenly appeared in a short time. Just like the evolution of organisms from single cell to multi-cell, reptiles to mammals, it needs an evolutionary process (such as 1984, the discovery of Chengjiang Fossil Group in China).

The fact that the geological structure of the earth changes continuously and frequently shows that the environmental "catastrophe" caused by periodic crustal structure changes always plays a leading role in the process of biological evolution. Of course, the gradual evolution of small-scale species also runs through the whole process of life evolution. Periodic celestial explosion (such as nova explosion) is an indispensable link in the evolution of all planets, including the earth. Marine fossils and seabed minerals in those mountains are the best explanation for the end of the dinosaur era caused by the drastic changes in the earth's crust. We know that the extinction time of dinosaurs was about 65 million years ago, and the geological age was the end of Mesozoic Cretaceous or the beginning of Cenozoic Tertiary. Moreover, at that time, not only all kinds of dinosaurs that ruled the earth for more than 654.38 billion years were extinct, but also the tragic fate befell many other creatures on the earth. Other marine reptiles such as ichthyosaurs and plesiosaurs, flying reptiles such as pterosaurs, terrestrial reptile relatives of dinosaurs such as colorful lizards, marine invertebrates such as ammonites and arrow stones, micro-zooplankton in the ocean, calcareous planktonic foraminifera and calcareous micro-phytoplankton were almost swept away. After this catastrophe, about 50% of the biological genera and almost 75% of the biological species on the earth disappeared from the earth forever.

About 65 million years ago, this extinction made a fundamental change in the biological world on earth. This extinction marks the end of Mesozoic era, and the geological history of the earth has entered a new era-Cenozoic era.

Here are only some familiar sayings: starvation, killing each other, etc. , but there are more and more claims, but China scientists research, dinosaurs survived about 2 million years after the meteorite impact. 1980, American scientists affret and his son discovered high concentration of iridium in the stratum 65 million years ago, which was dozens or even hundreds of times higher than the normal content. This concentration of iridium can be found in meteorites, so scientists associate it with the extinction of dinosaurs. According to the content of iridium, it is also inferred that the impact object is an asteroid with a diameter of about 10 km. The impact of such a large meteorite on the earth is an unparalleled blow. According to the earthquake intensity, it is about Richter scale 10, and the diameter of the crater produced by the impact will exceed 100 km. Scientists used 10 years and finally got the preliminary results. They found this crater in the strata of Yucatan Peninsula in Central America and Mexico. It is estimated that the diameter of this pit is between 180-300 km. In 2000, scientists were still doing further research on this big pit.

Now that this crater has been discovered, scientists have mastered some relevant evidence, and it seems that the mystery of dinosaur extinction can be settled. Because of the eruption of the volcano, a large amount of carbon dioxide is ejected, which causes the greenhouse effect of the earth and leads to the death of plants. Moreover, the volcanic eruption released a lot of salt, which broke the ozone layer, and harmful ultraviolet rays irradiated the earth's surface, causing extinction. But this theory has a premise that volcanoes erupt on a large scale.

Antonio Cikic, a famous Italian physicist, suggested that the extinction of dinosaurs was probably caused by a large-scale submarine volcanic eruption. Professor Qi Jichi believes that at the end of Cretaceous, a series of large-scale volcanic eruptions occurred on the earth under the ocean, which affected the thermal balance of seawater, and then caused the change of land climate, thus affecting the survival of dinosaurs and other animals that needed a lot of food to survive. In the past, all scientists thought that dinosaurs were cold-blooded or warm-blooded like other reptiles, but with the increasing number of fossil data, people's understanding has also changed. Some people think that some dinosaurs may be warm-blooded animals. First of all, they think that some dinosaurs are extremely agile, and they don't crawl on the ground like snakes, but run on the ground with two hind legs, which can reach 20 to 90 kilometers per hour. This requires a strong heart and a high metabolism, which is obviously impossible for cold-blooded animals. Secondly, dinosaurs ate a lot. It is speculated that a 30-ton sauropod dinosaur may eat nearly 2 tons of food every day, and only warm-blooded animals need so much energy. From the point of view that carnivorous dinosaurs are far less than herbivorous dinosaurs, this is also reasonable. In addition, there are some smaller dinosaurs whose bodies are covered with a layer of feathers or hair, which is also to prevent the loss of body temperature. Other aspects, such as the study of bones, also preliminarily show that some dinosaurs were warm-blooded animals. As soon as the theory of warm-blooded dinosaurs was put forward, it was strongly criticized, but it is still difficult to reach a conclusion.

Some people think that dinosaurs were warm-blooded animals, so they could not bear the cold climate in the late Cretaceous and could not survive. Because even if dinosaurs were warm-blooded, their body temperature was still not high, probably similar to that of living sloths, and to maintain such a body temperature, they could only live in a tropical climate. At the same time, the dinosaur's respiratory organs were not perfect and could not fully supplement oxygen. The difference between warm-blooded animals and cold-blooded animals is that if the body temperature drops below a certain range, it will consume physical energy to raise the body temperature and the body will soon become weak. They are too big to go into caves to avoid the cold, so if the cold days last for a few days, they may freeze to death because of exhaustion. But there is a doubt in this theory, that is, not all dinosaurs were so huge, and not all of them could hide in caves to take refuge, so this theory also has some imperfections. People think that the dinosaur's body was too big because it flourished for 65.438+0.6 billion years. Moreover, bones such as horns are also extremely developed, which brings great inconvenience to life and eventually leads to extinction.

The most representative confusing dragon among dinosaurs, with a body length of 25 meters and a weight of 30 tons, lost its viability because of its large size. In addition, Triceratops and others also went to the road of self-destruction because of their expanding three horns and unusually developed bones to protect their heads. According to the data obtained from deep-sea geological drilling, some scientists believe that the climate on the earth changed abnormally 65 million years ago and the temperature suddenly rose. This change makes the temperature-changing animals with weak heat dissipation ability, such as dinosaurs, unable to adapt to the environment well, resulting in the disorder of endocrine system in their bodies, especially causing serious damage to the reproductive system of male individuals. As a result, dinosaurs could not reproduce and eventually went extinct.

There is also a theory that although sudden climate change led to the extinction of dinosaurs, the process of speculation is different. Scholars of this school believe that about 70 million years ago, the Arctic Ocean was completely separated from other oceans by land. In the last days, due to various factors, salty seawater gradually turned into fresh water. By 65 million years ago, the "bank" separating the Arctic Ocean from other oceans suddenly burst. A large amount of seawater from the Arctic Ocean is lightened by seawater desalination and flows into other oceans. Because the water temperature in the Arctic Ocean is very low, these "overflowing" cold water forms a layer of cold flow, which makes the seawater temperature in the oceans of the earth drop rapidly by about 20 degrees. The drop of ocean temperature has seriously affected the climate of the mainland, making the air over the mainland cold. At the same time, the water vapor content in the air has also decreased rapidly, resulting in a large area of drought on land.

One possible way for climate change to lead to dinosaur extinction is to seriously affect dinosaur eggs. Some scientists found that at the end of Cretaceous before the extinction of dinosaurs, the eggshells of dinosaur eggs became thinner, indicating that there was a role caused by rapid climate change before the extinction of dinosaurs. Some paleontologists in China have also found that among the dinosaur eggs produced in some fossil sites, there are fewer pores on the eggshells of dinosaur eggs near extinction than in other periods, which is probably related to the cold and dry climate. The extinction of dinosaurs at the end of Cretaceous is an eternal mystery in biological history. Scientists have put forward one theory after another to try to explain the reasons, but so far there is no acceptable conclusion. The popular saying is that the disaster caused by the asteroid hitting the earth led to the extinction of dinosaurs, but this theory is not perfect. Because dinosaurs were the most successful animals on earth at that time, their rich diversity was manifested in different sizes, shapes and lifestyles.

Modern scientific analysis tells us that in the distant age when the earth was just formed, there was basically no oxygen in the air, but the content of carbon dioxide was very high. Later, with the emergence of autotrophs, photosynthesis began the process of consuming carbon dioxide to produce oxygen, thus changing the atmospheric environment on the earth. At the same time, on the one hand, carbon dioxide is deposited in the stratum in the form of coal and oil through biological fixation, and on the other hand, it is deposited in the form of various carbonates through organic or inorganic processes. This kind of evidence collection has been going on.

There is evidence that the concentration of carbon dioxide in Mesozoic era in which dinosaurs lived was very high, but the concentration of carbon dioxide in Cenozoic era after that was very low.

As we all know, every living thing needs a suitable environment to survive normally, and environmental changes can often lead to the rise and fall of a species. When the environment is beneficial to this species, it will flourish; Otherwise, it will decline or even become extinct. Environmental factors include temperature, water and other factors, as well as atmospheric composition. So, will the change of atmospheric composition affect the life span of living things? The answer is yes.

In Mesozoic, when dinosaurs lived, the content of carbon dioxide in the atmosphere was relatively high, indicating that dinosaurs adapted well to the atmospheric environment with high carbon dioxide concentration. Perhaps only in such an atmospheric environment can they live well. At that time, although mammals had appeared, they had not developed greatly. Perhaps because the environment such as atmospheric composition was not favorable to them, they were in a weak position and developed slowly in Mesozoic. With the passage of time, at the end of Cretaceous, the atmospheric environment changed greatly, the content of carbon dioxide decreased and the content of oxygen increased. This unfavorable environment may be reflected in two aspects: 1, the dinosaur's body became uncomfortable, and it was easy to get sick in the new environment, and the disease would spread like a plague. 2. The new atmospheric environment is more suitable for the survival of mammals, and mammals have become more advanced and adaptable competitors. Under the action of these two factors, dinosaurs finally became extinct. And those reptiles that are left behind are a few reptile species that can adapt to both the old environment and the new environment.

The theory that the change of atmospheric composition led to the extinction of dinosaurs has two starting points. One is that the atmospheric composition of Mesozoic is different from that of modern times, and the other is that every living thing needs a suitable atmospheric environment to survive. There was almost no oxygen in the ancient atmosphere, but the content of carbon dioxide was high. Later, due to the appearance of organisms, the process of carbon dioxide content gradually decreasing and oxygen content gradually increasing in the atmosphere under photosynthesis may explain many phenomena in the history of biological evolution. For example, the explosion of life in CAMBRIAN is also a mystery in evolutionary history. The change of atmospheric composition can also explain this, because animals can't directly use inorganic substances for photosynthesis, and its origin lags behind that of plants, and it will only happen when the oxygen content in the atmosphere reaches a certain level. Therefore, the outbreak of CAMBRIAN life must be guaranteed by the fact that the oxygen content in the atmosphere reaches a certain level, which has been proved by science.

In addition to the famous saying 12, there are many little-known sayings (such as sunspot explosion, electromagnetic disturbance, changes in the direction and intensity of the earth's magnetic field).

But no matter what happened, there is no doubt that dinosaurs could not adapt to the influence or change brought about by what happened. The latest research by scientists shows that the time when an asteroid collided with the earth 65 million years ago may not necessarily lead to the extinction of dinosaurs sooner or later. The real cause of extinction was that dinosaurs were in a fragile ecosystem at that time, and environmental upheaval easily led to extinction.

Before the asteroid hit the earth 65 million years ago, dinosaurs had been affected by environmental changes, and a large number of volcanic activities and temperature changes made the dinosaur ecosystem fragile. If asteroids collided with the earth millions of years earlier, a large number of dinosaur species and a relatively stable food chain would enable dinosaurs to survive; If an asteroid collides with the earth millions of years later, dinosaurs will have a chance to improve their evolution, better adapt to harsh environmental conditions, and may escape extinction.

About 65 million years ago, when an asteroid with a diameter of 10 km collided in today's Mexico, the dinosaur world suffered drastic environmental changes, leaving them in a fragile state. The latest research is led by an international research team led by paleontologists from the University of Edinburgh in the UK. They studied and analyzed a large number of dinosaur fossil specimens, most of which were excavated from North America.

The research team found that volcanic activity, sea level change and temperature fluctuation, which are common all over the world, led to the crisis of dinosaur food chain. At this time, an asteroid with a diameter of 10 km hit the earth, adding insult to injury and accelerating the extinction of dinosaurs.

This latest research report was published in the recently published journal Biological Review. When an asteroid collides with the earth, it will cause tsunami, earthquake, wildfire, sudden change of temperature and other environmental factors, which will bring fatal blows to the fragile dinosaur ecosystem and make dinosaur species extinct one after another.

The only dinosaur species that survived was flying in the air, which gradually evolved into today's birds. Dr Steve Brussat, from the School of Geological Sciences, University of Edinburgh, UK, said: "Dinosaurs are victims of' bad luck'. At that time, the earth was hit by a huge asteroid, and the dinosaur ecosystem was in the worst state, so dinosaurs were vulnerable to attack. This latest research helps to clarify the mystery of dinosaur extinction. "