How did the dinosaur kingdom fall?
A philosopher has a famous saying: "The emergence of species on earth is accidental, but extinction is inevitable." The dinosaurs that dominated the earth for 150 million years ended in the Mesozoic Era 65 million years ago. Suddenly disappeared from the earth. Also disappearing with the dinosaurs were ichthyosaurs, plesiosaurs in the ocean, pterosaurs flying in the sky, ammonites that occupied the ocean, and a large number of plankton. According to paleontologist statistics, about 70% of species disappeared at that time. Extinct. At the end of the Mesozoic Era, there was undoubtedly an unprecedented catastrophe. The extinction of living things cannot be explained by the normal laws of biological change. The result of extinction is often manifested in the obvious scarcity of biological species surviving in a short period of time and space. This is reflected in the sudden scarcity of animal and plant fossils in the earth's sedimentary strata. At that time, some species perished and disappeared forever, while the remaining species - the survivors, after a short period of geological history, quickly adapted to the new environment, accelerated evolution, developed on a large scale, and formed New prosperity.
As for the extinction of dinosaurs, no matter how much space scientists use to explain the extinction theory, it is always just an uncertain hypothesis. Someone once made statistics and found that there are more than 60 such hypotheses, but none of them have been able to prove it. to a completely satisfying conclusion. At 7:14 in the morning on June 30, 1908, an alien object fell from the sky in the Tunguska region of Siberia, Russia. The explosion created a huge crater and hundreds of square kilometers of trees fell down. The dust in the sky covered the sun and the moon and remained there for dozens of days. The dust storm spread across the sea and reached North America. This incident is reminiscent of the extinction of life on earth. Is it related to celestial bodies?
The Mystery of Tunguska (Cheng Yannian: "The Great Extinction and Recovery of the Dinosaurs")
Since the 1980s, a popular topic in dinosaurology has been "misfortune fell from the sky". The doctrine of catastrophe prevailed. Nobel Prize winner and physicist Alfred Alfredo put forward the theory that an asteroid impact disaster caused the extinction of the dinosaurs. This is a hypothesis that attracts the most attention from the public.
The moment a presumed asteroid hits the earth
The diameter of the Arizona crater is 1.2 kilometers (according to Cheng Yannian: "The Secret of the Dinosaurs")
1977 In 2007, Alfred Alfred Jr., a geologist at the University of California, Berkeley, and his colleagues at the Lawrence Laboratory of the University of California, tested rock samples they collected from the Obigo region in Italy. The sample was collected from a thin layer of clay at the junction of the top of the Cretaceous and the Tertiary. Geologists call this boundary T/K (K is the code for the Cretaceous, and T is the code for the Tertiary in the Cenozoic. ), the event that occurred is called the T/K extinction event, and the age of the boundary was determined to be 65 million years old by paleomagnetic measurements. The Lawrence Laboratory's measurements surprised geophysicists because the samples contained surprisingly high concentrations of iridium.
Rocks collected from the T/K boundary in Alberta, Canada
Iridium belongs to the platinum group elements (platinum family) and is very rare on the surface of the earth. It is mostly concentrated in the core of the earth. When comets, meteorites and asteroids hit the earth, they can bring large amounts of iridium contained in external space materials. So how to explain the enrichment of iridium at the T/K interface? The Alfreds and his sons put forward a theory that in the late Cretaceous period, a small asteroid hit the earth. The diameter of this asteroid was about 10 kilometers. The impact caused the earth to suffer a disaster. The surface of the earth after the impact A "nuclear winter effect" formed, killing the dinosaurs. After the introduction of this theory, the reaction of the paleontological community was not enthusiastic, because the catastrophic hypothesis had been mentioned a long time ago, but Alfred's evidence was more sufficient and more convincing. The biggest difficulty with this theory is why only dinosaurs became extinct, while other reptiles such as crocodiles, turtles, and lizards that were contemporary with dinosaurs did not become extinct? But soon, controversy ensued.
In order to increase their evidence, scholars who hold the impact theory have sent people to various continents around the world to search for clay at the T/K boundary and test for abnormal events in iridium metal. At present, more than 60 places have been found in Spain, Denmark, North America and other places. They think the evidence is sufficient. Their view is that a planetary impact caused a worldwide disaster.
One problem with the hypothesis of mass extinction caused by a planet impacting the earth is that large craters must be found 65 million years ago, and the diameter of such craters must not be less than 100 kilometers. However, so far there is no crater that matches the age and size required by this hypothesis. In 1990, scholars from the former Soviet Union claimed to have found a crater in the town of Popigai in the Siberian Arctic Circle. From the quartz glass particles produced after the collision, the age of this crater was measured to be more than 66 million years ago, close to the age when the dinosaurs became extinct. The diameter is more than 100 kilometers. Planetary impact theorists explain that the reason why it is difficult to find such craters is that the meteorites probably fell into the ocean and disappeared due to the subduction and compression of continental plates. The search for the impact site continues, and even NASA has joined the effort. Many craters were discovered at one time, and the largest one was found in the Yucatan Peninsula of Mexico. The impact time coincided with the extinction of the dinosaurs - 65.01 million years. This discovery increased the confidence of supporters of the impact theory. However, other scholars abandoned Alfred's theory of an asteroid impacting the earth and favored meteor showers and comets. Miller, Alfred's colleague, proposed that mass extinctions caused by comets are cyclical because the solar system has a companion star with an orbital period of 28.4 million years.
Whenever it approaches the Earth, it will throw fragments to the Earth, forming a meteorite rain. Such meteorites are mostly composed of ice and other elements, including iridium. Once a meteor shower occurs, it will immediately trigger a disaster and lead to the extinction of living things. Miller called the comet "Nemesis," but astronomers have yet to conclude that the "Nemesis" comet actually exists.
The leading figure in the comet collision theory is Mr. Xu Jinghua, a Chinese-Swiss geologist. Mr. Xu studied sedimentology in the United States in his early years. He is an authority on sedimentology and marine geology and has led international deep-sea drilling programs many times. Since the 1980s, Mr. Xu has discussed the T/K extinction event many times. He suggested that the solar system may have encountered a swarm of comets as it traveled through the Milky Way, and the resulting comet rain fell on the earth. If it falls into the ocean, it will cause cyanide pollution in the ocean, thus extinguishing a large number of marine life; if a star hits the earth, if the star rushes towards the earth at a speed of 20 kilometers per second, it will cause a big explosion on the earth. Dust thrown up after the explosion rose into the sky, causing a "nuclear winter effect." The sky is filled with nuclear dust, and the sky is dark, which can last for 2 to 6 months, or even longer. Fires followed, triggering volcanoes. At this time, the impact of high temperature caused cyanide to dissolve into rainwater to form acid rain. The earth was in a hell. Most animals were doomed. Plants lost photosynthesis and died, leaving the earth in a mess. This is the consequence of the T/K event described by the collision theory.
Chinese geologists are also engaged in research on T/K events. This kind of research will first look for continuous strata at the transition from the end of the Cretaceous to the Tertiary Period, and the interface of the strata must be clear. Since 1986, Chinese and German stratigraphic paleontologists have conducted long-term cooperation in the Nanxiong Basin of Guangdong Province. They tested the age of paleomagnetic and unearthed many dinosaur fossils from the end of the Cretaceous period and a large number of well-preserved egg fossils. Although no iridium anomalies were found in their tests, numerous chemical element content anomalies were found at the interface. Scientists believe that abnormalities in chemical elements triggered and changed the development mechanism of dinosaur eggs, causing the eggshell structure to mutate and the eggshell layer to become thinner, making it unable to hatch, thus causing the extinction of dinosaurs.
In recent years, the Geology and Paleontology Research Center of Jilin University, under the leadership of Professor Sun Ge, has organized international T/K experts from Germany, the United Kingdom, Russia, South Korea, Japan, the United States and China to conduct multidisciplinary research in the Heilongjiang Basin. A comprehensive survey was carried out to divide, measure and compare T/K boundary profiles in the Baishantou area on the bank of Heilongjiang River in Jiayin County, and conduct sampling work for exploratory well operations. Procedures have been established for the analysis of pollen samples, and geochemical analysis and comparison work is now being done. This is currently the most promising area in China to establish a T/K sequence.
In 1983, Chinese scholars found strata with a T/K interface in the Gangba area of the Tibet Autonomous Region. There was a layer of clay 1 to 2 centimeters thick at the interface, in which they found that the precious metals platinum and iridium were unusually rich. set, and the richness ratios of many elements to each other are not the normal ratios on Earth. They also found microspheres formed by melting after the impact. Changes in the composition of oxygen and carbon isotopes indicate that climate and environmental changes have occurred. They also found minerals produced by the impact and charcoal, a product of forest fires. These facts indicate that the earth was hit by a small celestial body in the Gangba area.
The emblem of the 2003 International Symposium on the T/K Boundary in the Heilongjiang Basin
However, opponents believe that the real impact craters and debris formed by celestial collisions Not found yet. Regarding the enrichment of iridium, some people also say that the iridium element can come from volcanic eruptive rocks. The hot magma can bring iridium out from the depths of the earth and emit it in the atmosphere. For example, it is found in the gas emitted from the active volcano Kiloa in Hawaii. traces of iridium.
Most paleontologists deny mass extinction events. They believe that there was no meteorite impact disaster at the end of the Cretaceous. Even if there was, the extinction of species was at best a "final kick". They believe that the extinction of species occurs gradually. In fact, extinction began after the appearance of dinosaurs. For example, in the Early Jurassic (180 million years ago), the prosauropod dinosaurs became extinct, and in the Early Cretaceous At the end of the world, the Stegosaurus also disappeared. This gradual extinction reached its climax at the turn of the Cretaceous and Tertiary periods, and then transitioned to a stable state. The evidence they hold is: dinosaur fossils were discovered in Montana, USA. These fossils show that in the millions of years before the turn of the Cretaceous and Tertiary periods, the types and numbers of dinosaurs decreased. This great change Occurred about 10 million years before the hypothesized mass extinction event. Judging from fossil records, half or even more than two-thirds of the dinosaur species became extinct in the years before and after the mass extinction event.
Recently, dinosaur fossils were discovered on Bilot Island in the Canadian Arctic. This discovery shows that dinosaurs must have had strong overwintering adaptability and could safely survive severe cold weather in high latitudes. Perhaps dinosaurs had the ability to migrate long distances. If this were the case, dinosaurs would have been able to survive the disaster caused by the meteorite impact. Several paleontologists believe that birds are the descendants of dinosaurs and are still active on the earth today, claiming that dinosaurs are not extinct.
The debate continues, and the outcome is still far away. We can only say that the mystery of the extinction of the dinosaurs is gradually being revealed.
Imagination of dinosaur extinction