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According to experts, the Earth has already entered the sixth period of mass extinction species. Ashok Khosla, President of the International Union for Conservation of Nature, spoke on this issue in Moscow yesterday and stated so bluntly that humanity is becoming a witness and a direct accomplice mass murder life on the planet. In the near future, tens of thousands of representatives of the biosphere will die out, as happened to the dinosaurs sixty-five million years ago during the fifth mass extinction.
You might be thinking that as head of the International Union for Conservation of Nature, Ashok Khosla is exaggerating a bit and it's actually not all that bad. Of course, at certain moments we can be ineradicable optimists! It is a pity that this will not help much in solving the problem. Speaking in dry facts, today up to a hundred species of animals die out every day. About 20 thousand species of flora and fauna are on the verge of extinction, which once represented up to 30 percent of the entire biomass of the planet. In addition, about fifty hectares of forests and more than 20,000 hectares of agricultural land die every day from deforestation and fires. All these most fertile places turn into a desert due to lack of water and soil erosion.
most big problem humanity in the near future may become mass extinction fish. The pessimistic forecasts of the president of the International Union for Conservation of Nature say that in forty years the world's oceans will run out of fish resources, and there will simply be nothing to catch. The entire industry will come to a global end.
Continuation of the article, as well as short description the previous five mass extinctions of animals in the history of the earth, we look under the cut ...
I googled a little to check how true these apocalyptic predictions are, and found that they have common sense. Given the rate at which the volume of fish and seafood catches is decreasing, it is obvious that the living creatures in the world's oceans are becoming less and less. Thus, the Food and Agriculture Organization of the United Nations cites data that the peak of fish production was passed in the early 1900s. After that, there was a short stabilization of catch volumes, and since the middle of the 20th century, an accelerating decline in catch has been observed.
And the most dangerous thing, despite the fact that the fish in the oceans is becoming less and less - water, in turn, everything stays and stays. In the next hundred years, according to forecasts of the International Union for Conservation of Nature, the water level in the ocean will rise by several meters and flood some island states.
The biological history of the Earth has five waves of mass extinctions, destroying on average up to three-quarters of the living creatures inhabiting the Earth. At the same time, during the most massive extinction - the Great Permian - up to 95% of all organisms on Earth were destroyed.
Comparing current data on the number of species that are on the verge of extinction with the dynamics of mass extinctions over the past 540 million years, a group of American paleobiologists, led by University of Berkeley professor Anthony Barnosky, has estimated the rate at which species diversity will decrease in the historically foreseeable future.
group conclusions, published in the journal Nature, look apocalyptic, although some reasons for optimism remain.
“If we take only those mammals whose probability of extinction in the next three generations is at least 50%, and assume that they will completely disappear from the face of the Earth in the next thousand years, this already takes the situation beyond the norm and indicates that we are moving towards mass extinction,” says Professor Barnosky, curator at the Museum of Paleontology, lead researcher at the Museum of Vertebrate Zoology, and specialist in integrative biology, the discipline that studies the complex diversity of living systems.
If species now officially classified as "Critically Endangered", "Endangered" and "Vulnerable" really go extinct, and if the rate of extinction remains the same, the sixth wave of mass extinction will occur in 3-22 centuries.
According to current estimates, today in different groups of animals, 1–2% of species have already become extinct.
To announce the beginning of a mass extinction, this figure is actually not enough. But if you look at this process in dynamics, the situation will look much more serious. The current rate of extinction, according to American estimates, is higher than the rate at which species disappeared during previous extinctions.
The main difficulty in comparing rates was the construction of a unified data scale.
Comparing extinction rates between fossils and modern data is like comparing oranges with apples, Barnosky admits: formally they are similar, but typologically they can be quite different.
Thus, the fossil record spans vast time spans of several hundred million years, while current extinction rates are based on data spanning several thousand years. In other words, having found evidence of mass extinction in the fossil layers, it is difficult to say how long this process took, which could last hundreds of thousands or a million years.
To get around this difficulty, a weighted average method for estimating extinction rates was developed, taking into account big number intervals and proportions of species affected by extinction.
Thus calculated, the average rate of extinction of vertebrates was less than two extinct species per one million years.
Over the past 500 years, out of 5570 species of invertebrates, 80 have disappeared. With the appropriate recalculation, it turns out that the current extinction rates are very close to those when mass extinctions occurred, even if the bar for "mass" is set extremely high.
If in the next few hundred years all species of animals that are endangered according to the International Union for Conservation of Nature (IUCN) die out, the Earth's biocenosis will indeed enter a period of mass extinction.
Naturally, in order to cover the entire diversity of the biocenosis, such an assessment should be corrected. a large number data on the rate of extinction of species not only in mammals, but also in other groups of animals, as well as plant and other species of organisms. The scenario, according to which the approaching tipping point separating us from the next era of mass extinction, can be delayed and even stopped by trying to save the most critically endangered species of mammals and other animals from extinction, the Americans assess as quite realistic.
Fifth (Cretaceous-Paleogene) mass extinction -
For a long time, paleontologists have tried to understand why the dinosaurs became extinct. After all, dinosaurs dominated for over 100 million years. They were the most prosperous class of animals on our planet. And then in some time - maybe in a few thousand years, or maybe in a couple of days - they disappeared. So what happened?
Many explanations have been offered, ranging from the fantastic (the dinosaurs were exterminated by little green men in flying saucers that hunted them) to the highly plausible (climate change has destroyed their ecological niche). What I like the most is the explanation that links the extinction of the dinosaurs to the appearance of flowering plants believed to have taken place 65 million years ago, just when the dinosaurs disappeared. The point is that until then the dinosaurs had eaten mostly pine needles and similar foods rich in natural oils, and when they had to switch to grass, they all died of constipation!
Fourth (Triassic) extinction
Sadness happened about 200 million years ago. As a result of a ruthless natural genocide, at least half of the species that lived on Earth at that time died out.
Today in science there are several versions of the extinction that happened. Most of all I like the hypothesis about the so-called. "methane hydrate gun", which is the most plausible. Due to volcanism and the accumulation of carbon dioxide in the atmosphere, huge amounts of methane began to be released from bottom clathrates. The toxic emissions of this nasty greenhouse gas played a role trigger for a sharp global warming, which destabilized the climate on the planet and caused a total akhtung.
Third ("Great" Permian) extinction
There is no sadder extinction in the world than the "Great" Permian 250 million years ago somewhere ... This is the most ruthless meat grinder of life in the entire history of the planet. More than 95% of the species of all living beings fell victim to it. And the Permians have nothing to do with it!
It is one of the largest catastrophes of the biosphere in the history of the Earth, led to the extinction of 96% of all marine species and 70% of terrestrial vertebrate species. The catastrophe was the only known mass extinction of insects, which resulted in the extinction of about 57% of the genera and 83% of the species of the entire class of insects. Due to the loss of such a quantity and diversity of species, the restoration of the biosphere took a much longer period of time compared to other disasters leading to extinctions.
Second (Devonian) extinction
It happened 364 million years ago, knocking down the inhabitants of the oceans by exactly half.
The late Devonian extinction was one of the largest extinctions in the history of terrestrial flora and fauna. 19% of families and 50% of the entire gene pool died out everywhere. Extinctions were accompanied by widespread oceanic anoxia, that is, a lack of oxygen, which prevented the decay of organisms, and predisposed to the preservation and accumulation of organic matter. This effect, combined with the ability of spongy reef rocks to retain oil, has made the Devon rocks an important source of oil, especially in the United States.
First (Ordovician-Silurian) extinction
The very first mass extinction happened 440 million years ago. During the Ordovician-Silurian extinction, more than 60% of marine invertebrates disappeared from the face of the earth. I say - from the face of the earth, but, to tell the truth, the land in those distant times was empty and uncomfortable. Everyone "hung out" in the seas and oceans, for which they paid!
And all because Gondwana is a giant continent, from which both Africa and South America, and Australia, and Antarctica, - lay down in a drift and headed exactly to South Pole. Water boundaries have changed, and with them the usual ranges of all kinds of brachiopods and mollusks. It all ended with global cooling - water and land. What is today the Sahara desert was then a continuous glacier. The ice significantly changed the terrain: the water level in the ocean dropped sharply. In a word, 60% of marine invertebrates could not pass on their genes.
P.S. And yet, "man" - sounds proud! Our civilization has been able for negligible a short time create all conditions in order to hasten the approach of the Sixth Mass Extinction. Aren't you bursting with a sense of pride from belonging to Homo sapiens? Seriously, it looks like our swarming is taking its toll on environment and the ecology of the planet with the same strength as Mother Nature herself...
The Permian extinction was one of the biggest catastrophes that happened in the long history of the Earth. The biosphere of the planet has lost almost all marine animals and more than 70% of terrestrial representatives. Have scientists been able to understand the causes of extinction and assess its consequences? What theories have been put forward and can they be trusted?
Permian period
In order to roughly represent the sequence of such distant events, it is necessary to refer to the geochronological scale. In total, the Paleozoic has 6 periods. Perm - a period on the border of the Paleozoic and Mesozoic. Its duration is 47 million years (from 298 to 251 million years ago). Both eras, both the Paleozoic and the Mesozoic, are part of the Phanerozoic eon.
Every period Paleozoic era interesting and eventful in its own way. During the Permian period there was an evolutionary impetus that developed new forms of life, and the Permian extinction of species that destroyed most of the animals of the Earth.
What is the name of the period
"Perm" is a surprisingly familiar name, don't you think? Yes, you read that right, it has Russian roots. The fact is that in 1841 a tectonic structure corresponding to this period of the Paleozoic era was discovered. The find was located near the city of Perm. And the whole tectonic structure today is called the Cis-Ural marginal foredeep.
The concept of mass extinctions
The concept of mass extinctions was introduced into scientific circulation by scientists at the University of Chicago. The work was carried out by D. Sepkoski and D. Raup. According to statistical analysis, 5 mass extinctions and almost 20 smaller catastrophes were identified. Information for the last 540 million years was taken into consideration, since for more than early periods not enough data.
The largest extinctions include:
- Ordovician-Silurian;
- Devonian;
- the Permian extinction of species (the reasons for which we are considering);
- Triassic;
- Cretaceous-Paleogene.
All these events took place in the Paleozoic, Mesozoic and Cenozoic eras. Their periodicity is from 26 to 30 million years, but many scientists do not accept the established periodicity.
The Greatest Ecological Disaster
The Permian extinction is the most massive catastrophe in the history of our planet. The marine fauna died out almost completely, only 17% of the total number of terrestrial species survived. More than 80% of insect species died out, which did not happen during other mass extinctions. All these losses occurred in about 60 thousand years, although some scientists suggest that the period of mass pestilence lasted about 100 thousand years. The global losses brought about by the great Permian extinction have drawn the final line - having crossed it, the Earth's biosphere has begun evolution.
The restoration of the fauna after the greatest ecological catastrophe lasted a very long time. We can say that much longer than after other mass extinctions. Scientists are trying to recreate models that could have led to a mass pestilence, but so far they cannot agree even on the number of shocks within the process itself. Some scientists believe that the Great Permian Extinction 250 million years ago had 3 peak shocks, other scientific schools are inclined to believe that there were 8 of them.
One of the new theories
According to scientists, the Permian extinction was preceded by another massive catastrophe. It happened 8 million years before the main event and significantly undermined the Earth's ecosystem. The animal world became vulnerable, so the second extinction within the same period turned out to be the greatest tragedy. If it can be proved that there were two extinctions in the Permian period, then the concept of the periodicity of mass catastrophes will be in doubt. In fairness, let's clarify that this concept is disputed from many positions, even without taking into account the possible additional extinction. But this point of view still holds scientific positions.
Possible causes of the Permian disaster
The Permian extinction still causes a lot of controversy. A sharp controversy unfolds around the causes of the ecological cataclysm. All possible grounds are considered equivalent, including:
- external and internal catastrophic events;
- gradual changes in the environment.
Let's try to consider some of the components of both positions in more detail in order to understand how likely they are to influence the Permian extinction. Photos of confirming or refuting findings are provided by scientists from many universities as they study the issue.
Catastrophe as the cause of the Permian extinction
External and internal catastrophic events are usually considered as the most probable causes Great Dying:
- During this period, there was a significant increase in the activity of volcanoes on the territory of modern Siberia, which led to a large outpouring of traps. This means that there was a huge eruption of basalt in a short time in the geological concept. Basalt is weakly eroded, and the surrounding sedimentary rocks are easily destroyed. As evidence of trap magmatism, scientists cite vast territories in the form of flat stepped plains on a basalt base as an example. The largest trap area is the Siberian trap, formed at the end of the Permian period. Its area is more than 2 million km². Scientists from the Nanjing Institute of Geology (China) studied the isotopic composition of the rocks of the Siberian traps and found that the Permian extinction occurred precisely during their formation. It took no more than 100 thousand years (before that it was believed that it took a longer period of time - about 1 million years). The activity of volcanoes could provoke the greenhouse effect, volcanic winter and other processes that are detrimental to the biosphere.
- The reasons for the biospheric catastrophe could be the fall of one or more meteorites, with a large asteroid. As evidence, a crater with an area of more than 500 km (Wilks Land, Antarctica) is given. Also, evidence of impact events was found in Australia (Bedout structure, Northeast of the continent). Many of the resulting samples were later refuted in the process of deeper study.
- One of possible causes consider a sharp release of methane from the bottom of the seas, which could lead to the total death of marine species.
- One of the domains of living single-celled organisms (archaea) could lead to a catastrophe when it acquired the ability to process organic matter, releasing large volumes of methane.
Gradual changes in the environment
- Gradual composition changes sea water and the atmosphere, resulting in anoxia (lack of oxygen).
- Increasing dryness of the Earth's climate - animal world unable to adapt to change.
- Climate change has resulted in disruption of ocean currents and a decrease in sea level.
Most likely, a whole complex of reasons influenced, since the catastrophe was massive and occurred in a short period.
Consequences of the Great Dying
The great Permian extinction, the causes of which are trying to establish academia, had serious consequences. Entire units and classes have completely disappeared. Most of the parareptiles died out (only the ancestors of modern turtles remained). A huge number of species of arthropods and fish disappeared. The composition of microorganisms has changed. In fact, the planet was empty, being dominated by fungi that feed on carrion.
After the Permian extinction, species survived that were maximally adapted to overheating, low oxygen levels, lack of food and excess sulfur content.
A massive biospheric cataclysm opened the way for new animal species. Trias, the first to show the world of archosaurs (progenitors of dinosaurs, crocodiles and birds). After the Great Dying, the first species of mammals appeared on Earth. The restoration of the biosphere took from 5 to 30 million years.
In total, there are five stages of mass extinctions of animals on Earth. The most famous completely wiped out the dinosaurs, but that was not the biggest disaster. The largest extinction deprived the Earth of 95% of all animals, but made it possible for new species to appear, which formed the modern zoosphere.
Let's find out more about them...
The very first mass extinction of animals occurred about 450-440 million years ago. It is impossible to name the exact cause of extinction, but most scientists are inclined to believe that the movement of Gondwana, a huge supercontinent that included almost all of the Earth's land, was to blame.
And all because Gondwana - a giant continent, from which Africa, South America, Australia, and Antarctica later "hatched" - fell into a drift and headed exactly to the South Pole. Water boundaries have changed, and with them the usual ranges of all kinds of brachiopods and mollusks. It all ended with global cooling - water and land. What is today the Sahara desert was then a continuous glacier. The ice significantly changed the terrain: the water level in the ocean dropped sharply. In a word, 60% of marine invertebrates could not pass on their genes.
It happened 374 and 359 million years ago. The Devonian extinction consisted of two peaks, during which the Earth lost 50% of all existing genera and almost 20% of all families. During the Devonian extinction, almost all agnathans disappeared (only lampreys and hagfish have survived to this day).
Extinctions were accompanied by widespread oceanic anoxia, that is, a lack of oxygen, which prevented the decay of organisms, and predisposed to the preservation and accumulation of organic matter. This effect, combined with the ability of spongy reef rocks to retain oil, has made the Devon rocks an important source of oil, especially in the United States.
The same mass extinction of animals that has ever happened on our planet. Some scientists call the Permian extinction the greatest mass extinction of all time. About 250 million years ago, 70% of all land animals disappeared. In the ocean, things were even worse - 96% of marine species died. During the Great Permian extinction, more than 57% of genera and 85% of insect species died. This is the only known extinction that affected insects.
Due to the loss of such a quantity and diversity of species, the restoration of the biosphere took a much longer period of time compared to other disasters leading to extinctions.
After the Permian extinction, the animal world was restored for 30 million years (some scientists believe that the restoration of the biosphere lasted 5 million years). Animals that had previously been in the shade more widely spread widely. strong species. So, this time is considered the period of formation of archosaurs (ancestors of modern crocodiles and extinct dinosaurs). Birds also originated from them, which could not have existed if not for the Great Permian extinction.
The Triassic extinction happened 200 million years ago. About 20% of all marine animals died, many archosaurs (which became widespread after the Permian extinction), and most species of amphibians. Scientists have calculated that half of all animals known to us that lived at that time died during the Triassic extinction.
A feature of the Triassic extinction is considered to be transience. It happened within 10 thousand years, which is very fast on a planetary scale. At this time, the disintegration of the supercontinent Pangea into separate continents began. It is possible that the reason for the breakup was a large asteroid that changed the weather on the planet, causing extinction. But there is no evidence of this theory, so far not a single large crater of the Triaric period has been found.
Today in science there are several versions of the extinction that happened. The most common hypothesis is the so-called. "methane hydrate gun", which is the most plausible. Due to volcanism and the accumulation of carbon dioxide in the atmosphere, huge amounts of methane began to be released from bottom clathrates. The toxic emissions of this unpleasant greenhouse gas played the role of a trigger for a sharp global warming, which destabilized the climate on the planet and caused a total akhtung.
The most famous extinction occurred about 65 million years ago. It is famous for the fact that dinosaurs died out on Earth at that time. More than 15% of families of marine animals and 18% of families of land animals also died.
Many explanations have been offered, ranging from the fantastical (dinosaurs were exterminated by little green men in flying saucers that hunted them) to the highly plausible (climate change has destroyed their ecological niche). The most famous theories say that the Earth collided with a large asteroid or fell into the radiation zone from a supernova explosion.
Most interesting explanation, connects the extinction of the dinosaurs with the appearance of flowering plants, which is believed to have occurred 65 million years ago - just when the dinosaurs disappeared. The point is that until then the dinosaurs had eaten mostly pine needles and similar foods rich in natural oils, and when they had to switch to grass, they all died of constipation!
Another very interesting theory is that they were exterminated by the first mammals that destroyed the clutches of dinosaurs, preventing them from multiplying. This is supported by the fact that some dinosaurs lived for quite a long time on the territory of modern North America and India, where, perhaps, "dangerous" mammals appeared later.
Researchers from Stanford University claim that the Earth in this moment is experiencing one of the early stages of another mass extinction. They shared their observations in a review published in latest release scientific journal Science. Their publication caused a wide resonance and attracted the attention of many popular science portals, such as LiveScience.
According to scientists, the planet is now experiencing the peak of biodiversity in 3.5 billion years of existence of life on it.
However, warning signals began to arrive long before the start of the industrial revolution: since 1500, about 320 species of terrestrial vertebrates have become extinct. And the populations of the remaining species have declined by a quarter. Moreover, of the living vertebrates, about 33% of species are endangered. The situation is especially dangerous with representatives of megafauna - elephants, giraffes, hippos and other large animals.
Despite the fact that there are not so many large animals on the planet, their disappearance is accompanied by catastrophic consequences. So, in Kenya, in the territories where elephants, giraffes and zebras used to live, rodents found shelter. And this, in turn, contributes to the uncontrolled eating of grass and shrubs by them.
The result of this is soil compaction, and as a result, a decrease in productivity, and subsequently desertification of vast areas. And the population of rodents that has become huge carries many diseases. Since there are no predators, there is no one to control the number of rodents and destroy sick individuals.
Scientists have not forgotten about invertebrates: over the past 35 years, their number has decreased by 45%. This trend is especially dangerous in light of the fact that it is insects that pollinate about 75% of the world's food crops. In addition, insects play an important role in the cycle nutrients and decomposition of organic matter. The U.S. alone spends $4.5 billion a year to combat predators that destroy these insects, which helps to realize the scale of the problem.
“In a global sense, extinction represents the loss of a particular ecosystem. But now extinction certain types directly threatens people. While there is time to try to change something,” says Professor Rodolfo Dirzo, who is one of the authors of the review.
Let's remember what we were
Life is a struggle for survival. Animals live in constant stress to get enough food to be well adapted to their environment. Animals that are ill-adjusted starve in difficult times, fail to reproduce, and eventually die completely. Throughout the history of the Earth, life has constantly taken on new forms that are immediately tested by survival. When the climate and environment change dramatically, many animals that are ill-adapted to the new situation die out. These events have been happening since the first appearance of life on Earth. All animals living today are the descendants of creatures that were lucky enough to adapt to new conditions. In this article, we will consider the ten largest extinctions in the history of the Earth.
1. Ediacaran extinction
In the Ediacaran period, for the first time, complex life began to take shape on Earth. Tiny bacteria evolved into more complex ones and eukaryotes, some of which clustered together to increase their chances of finding food and not becoming food for others. Most of these strange creatures left no traces behind them because they had no skeletons. They were soft and tended to rot when they died rather than becoming fossils. Only in special cases did fossil forms, such as those left lying on the soft mud, harden and leave an imprint. These few fossils tell us of a host of strange and alien creatures that resembled modern worms and sponges. However, these creatures were dependent on oxygen, just like us. Oxygen levels began to drop and a worldwide extinction occurred 542 million years ago. More than 50% of all species died. Vast numbers of dead creatures decompose and form some of today's fossil fuels. The exact reason for the decrease in oxygen levels is unknown.
2. Cambrian-Ordovician extinction
During the Cambrian period, life flourished. Life remained virtually unchanged for millions of years, but suddenly new forms began to appear in the Cambrian period. Exotic crustaceans and trilobites have become the dominant life form in great numbers and variety. Mollusks and giant insect-like aquatic arthropods filled the sea. These creatures had a rigid exoskeleton. Life flourished until over 40% of all species suddenly disappeared 488 million years ago. Those that remain have undergone changes due to changes in the harsh environment. What that change was, we do not know. One theory says that there was ice Age. Extreme changes in temperature can easily lead to the extinction of vast amounts of life. This event marked the disappearance of the boundaries between the Cambrian and Ordovician periods.
3. Ordovician-Silurian extinction.
Life began to flourish once more within Ordovician period. Nautiloids (primitive octopuses), trilobites, corals, sea stars, eels and jawed fish filled the sea. Plants are trying to take over the earth. Life gradually becomes more and more complicated. 443 million years ago, more than 60% of life died. This is considered the second largest extinction in history. This was due to the rapid decline in carbon dioxide levels. Much of the water that was home to life froze, which in turn caused a decrease in oxygen. A burst of gamma radiation from space is thought to have destroyed the ozone layer and the sun's unfiltered ultraviolet radiation has wiped out most of the plants. Although some species survived and life continued. It took more than 300 million years for the Earth to recover from this event.
4. Lauska event
After the disappearance of the Ordovician, the Silurian period began. Life has recovered from the last mass extinction, and this period was marked by the development of a true species of sharks and bony fish, most of which turned out to be quite modern. Some arthropods evolved into spiders and centipedes, which were adapted to dry air and lived alongside land plants. Huge scorpions became numerous, and trilobites continued to dominate. 420 million years ago there was a sudden climate change that caused the extinction of perhaps 30% of all species. Atmospheric gases have changed in proportion. The reason for these changes is unknown. This period ended and the Devonian began, when evolution produced a different pattern of life that flourished.
5. Devonian extinction
During the Devonian period, some fish evolved to have strong fins that allowed them to crawl onto land, becoming animals such as reptiles and amphibians. Vast coral reefs, fish and sharks appeared in the seas, some of which ate trilobites. The trilobites have lost their dominance as the dominant sea creatures. Some modern sharks look almost the same as their predecessors. Plants appeared on the earth. More complex land plants appeared for the first time in history. 374 million years ago 75% of all this amazing life died out. This was due to changes in atmospheric gases, possibly due to massive volcanic activity or a meteorite.
6. Extinction during the Carboniferous period
After Devonian period the Carboniferous began. Several land animals began to live almost anywhere on earth, and not be limited to the coast, where they could lay their eggs. Winged insects appeared. Sharks have survived their golden age, and a few trilobites have become rare. Appeared giant trees and vast rainforests covered much of the land, increasing the oxygen content of the air by up to 35%. For comparison, today 21% of the air is filled with oxygen. Coniferous trees from the Carboniferous period remain virtually unchanged today. 305,000,000 years ago, a sudden short ice age caused carbon dioxide levels to rise. The forests died out, and with them many of the land animals. Almost 10% of all species on Earth disappeared at that time.
7. Permian-Triassic extinction
After the rainforests disappeared, the most successful animals remained on earth. These were the ones who laid their eggs on land. They quickly dominated other species. 252,000,000 years ago there was a catastrophe the Earth had never seen before. It was caused by a meteorite or volcanic activity that changed the composition of the air at the root. Approximately 90% of all life has died out. This is the largest mass extinction in history.
8. Triassic-Jurassic extinction.
After the devastation of the Earth towards the end of the Permian period, reptiles again became dominant, and dinosaurs appeared. Dinosaurs were not dominant over other reptiles, and at this stage they were not much larger than horses. It is they who are the descendants of those who have become famous and scary creatures that we know so well. More and more dinosaurs, tyrannosaurs, stegosaurus, triceratops came in the Jurassic and Cretaceous periods. 205,000,000 years ago 65% of the Triassic died out, including all large land animals. Many dinosaurs have been saved due to their small size. This was probably caused by massive volcanic eruptions, eruption of huge amounts of carbon dioxide and sulfur dioxide, as a result of which the climate suddenly changed.
9. Jurassic extinction.
During the Jurassic, giant marine reptiles, such as the famous Plesiosaur, dominate the oceans. Pterosaurs rule the sky and dinosaurs rule the earth. Stegosaurus, long diplodocus and great hunters allosaurus became commonplace. Coniferous trees, cycads, ginkgo biloba and ferns "populated" dense forests. Smaller dinosaurs evolved into birds. 200 million years ago, 20% of all life suddenly disappears, mostly marine species. Shellfish and corals were widespread, but they have almost completely disappeared. The few who survived were able to populate the seas gradually over the next million years. This extinction does not greatly affect the life of animals, only some species of dinosaurs became extinct. The reason for this was that the oceanic tectonic plates descended and formed a deep ocean. Majority marine life adapted to shallow water.
10. Cretaceous extinction.
This is the most famous animal extinction. After it's over Jurassic period, dinosaurs continued to multiply and develop throughout the subsequent Cretaceous. They had the forms that are familiar to many children today. The number of species in the last period corresponds to and exceeds the number for the period since the Ordovician. Finally, small rodents appeared, creatures that were the first true mammals. 65 million years ago, a huge meteorite fell to Earth in present-day Mexico, disrupted the atmosphere and caused global warming, killing 75% of all species. This meteorite contained a high concentration of iridium, which is generally rare on Earth.
Major Changes During the Great Dying
Recent centuries mesozoic era were a time of dramatic events, the essence of which is still not entirely clear. It is possible that these events were to some extent prepared by the changes in flora we have just considered. Following the "victorious march" of angiosperms during the Late Cretaceous, their predecessors - bennettites and proangiosperms - die out, and the distribution and diversity of cycad ferns are greatly reduced. The general appearance of the flora of the Late Cretaceous is already entirely determined by angiosperms; Of the gymnosperms, only conifers have retained their positions.
Changes in flora primarily affected insects. During the Late Cretaceous, the entomofauna was gradually updated: a number of archaic families disappeared and groups appeared that still exist today. However, various dinosaurs still dominated the broad-leaved and coniferous forests and open plains of the Late Cretaceous, giant flying lizards soared in the air, various marine reptiles (plesiosaurs and mosasaurs, and in the Late Cretaceous, according to new data, the last ichthyosaurs) were abundant in the seas. , sea turtles), in fresh water - numerous crocodiles. At that time, the largest known crocodiles existed - Deinosuchus, Deinosuchus, whose skull length reached 2 m, and the total length was about 16 m. In the second half of the Cretaceous, for more than 45 million years after the widespread distribution of angiosperms, the general appearance of the fauna remained in generally the same, typical of the age of the dinosaurs.
But at the end of the Cretaceous period, in a relatively short (on a geological scale) period, many groups of vertebrates and invertebrates, terrestrial, aquatic and flying, became extinct. Gigantic forms, and animals of small sizes, both herbivorous and predatory, are dying out.
By the beginning of the Cenozoic, dinosaurs, flying lizards, plesiosaurs, mosasaurs, the last ichthyosaurs, 8 out of 10 Late Cretaceous families of crocodilians, archaic groups of ornithurians, and all enanciornis birds became extinct in most regions. Among invertebrates, bivalve mollusks, widespread in the Jurassic and Cretaceous, such as rudists, ammonites, belemnites, and many nautiloid cephalopods, became extinct, and many species of sea lilies became extinct. Significant was the extinction of marine phyto- and zooplankton.
The Great Dying was not accompanied by a simultaneous increase in the abundance and diversity of species of some other groups. As in the Permian period, there was a significant general depletion of the fauna. Only in the Cenozoic does the expansion of groups less affected by extinction (mammals, birds, terrestrial scaly reptiles, tailless amphibians) begin. On the other hand, as during the Permian extinction, and at the turn of the Mesozoic and Cenozoic, some groups of animals, as it were, "stayed aside" from the events taking place: their diversity and abundance did not undergo significant changes. Among vertebrates, these are various groups of fish, tailed amphibians, and turtles.
As in the Permian period, the great extinction at the end of the Cretaceous did not have the character of a "world catastrophe": the physical and geographical conditions at the turn of the Cretaceous and Paleogene did not undergo any sudden and drastic changes. With sufficient certainty, we can only talk about a certain cooling of the climate by the end of the Cretaceous, which occurred gradually and affected plant communities: in areas where it is possible to trace the entire sequence of deposits at the turn of the Cretaceous and Paleogene, a gradual replacement of heat-loving plant species by species adapted to a cooler temperature is found. climate (eg. North America subtropical forests have been replaced by temperate forests). However, in the tropical zone, significant changes in vegetation and, probably, climate did not occur.
The process of extinction was brief only in the geological sense: it continued for millions of years, when the endangered phyletic lineages gradually died out. It remains unclear to what extent these processes occurred simultaneously on different continents and in different oceans and seas. For example, according to R. Sloan, in the west of North America, dinosaurs (Triceratops, theropods, etc.) existed for several million more years at the beginning of the Paleogene, after their extinction in other places. Similar data are also available for India and some other regions. But, one way or another, the result was the same for everything the globe, which, in fact, gives this extinction, like other mass extinctions, a mysterious character.
Hypotheses about the causes of extinction
The hypothesis about the causes of extinction - this exciting problem has always attracted the attention of researchers. Enough detailed overview numerous hypotheses would require a separate book and far beyond the possibilities. Since the extinct groups of organisms eventually disappeared everywhere, many scientists assumed that the causes of such phenomena must have had the character of worldwide catastrophes.
The first of the catastrophic hypotheses was put forward by J. Cuvier, who considered the cause of the great extinction at the end of the Cretaceous to be volcanic activity associated with the Alpine phase of mountain building. Undoubtedly, the intensification of volcanism affects the organic world not only directly (the outpouring of lavas covering large areas that become uninhabitable for a long time, and other factors of volcanic eruptions that are detrimental to organisms), but also indirectly.
Significant landscape changes are taking place; Huge amounts of volcanic dust and carbon dioxide are emitted into the atmosphere, reducing the transparency of the air; All this affects the climate. However, in the Phanerozoic, manifestations of volcanism always had a local character, and the direct effect of volcanic activity could affect only a relatively small part of the earth's surface. On the other hand, mountain-building processes accompanied by volcanism took place in different regions of the globe both in the Jurassic and in the Cretaceous long before the era of the great extinction, without leading to catastrophic consequences for dinosaurs and their contemporaries. Therefore, volcanism in itself could not be the cause of the great extinction, although it probably played a significant role in climate change.
AT last decade a lively discussion was caused by the hypothesis of L. and U. Alvaretsov, according to which the cause of the catastrophe that caused the great extinction at the turn of the Cretaceous and Paleogene was a collision with the Earth of one or more asteroids. In a newer version of this hypothesis, called "impact" (from the English impact - impact, push), the Earth is supposed to collide not with an asteroid, but with a giant comet or with several comets.
As evidence for this space disaster indicate an increased (about 30 times) content of iridium (which is attributed to asteroid or cometary origin) in a layer of clay deposits at the boundary of the Cretaceous and Paleogene, the presence of solidified melt drops, shock metamorphosed quartz crystals, and also a large number of particles of coal soot in the boundary deposits , which are believed to have been formed during hurricane fires that arose after a cosmic catastrophe.
When large asteroids fell to the Earth, giant craters should have appeared (usually the diameter of the crater is about 10 times the diameter of the fallen meteorite). Craters of the "right size", formed at the end of the Cretaceous, have not yet been found on Earth; the largest crater known today, Chickshulub, located in the north of the Yucatan in Mexico and formed about 65 million years ago, has a diameter of about 180 km. supporters of the impact hypothesis admit that the asteroid fell into the ocean.
The mechanism of the impact of such a cosmic impact on the Earth's biosphere is understood by different scientists in different ways. Some researchers believe that the mass extinction was caused by a sharp increase in air and ocean temperatures (with possible poisoning of the waters with cyanide compounds) and the occurrence of hurricane fires on land.
Other scientists (among them the authors of Alvarezza's impact hypothesis) consider the development of events according to the so-called "nuclear winter scenario", developed in the analysis of the probable consequences of a thermonuclear war, to be more probable. The clogging of the atmosphere with meteorite dust and soot particles from hurricane fires should have led to a significant decrease in air transparency, as a result of which the temperature of the lower layers of the atmosphere, ocean and soil should have dropped significantly and photosynthesis should have sharply decreased. This could lead to the destruction of biocenoses and the mass extinction of plants and animals both on land and in the ocean.
Some paleoclimatological data do point to a decrease in average annual temperatures by 5-6°C by the end of the Cretaceous, which was especially noticeable in the subpolar and middle latitudes, where subtropical vegetation was replaced by forests characteristic of a temperate climate. However, there were no significant changes in the temperature regime in the tropical belt, and the general course of these climate change does not at all correspond to the "nuclear winter scenario" and the impact hypothesis, since these processes developed gradually over several million years.
Further extensive studies of sediments at the boundary of the Cretaceous and Paleogene showed that in some areas the layers containing soot residues are located significantly below the layer enriched in iridium and, obviously, arose much earlier than the latter. Moreover, the unity of the "iridium layer" was not confirmed either - in different areas, the corresponding deposits have different ages and could not have arisen as a result of one cosmic catastrophe. Finally, everything anomalous phenomena, used as arguments in favor of the impact hypothesis, could arise under the influence of purely "terrestrial" causes - for example, as a result of volcanic activity, which, as we have already noted, increased significantly by the end of the Cretaceous in India, North America, and some other areas.
In addition, the very process of extinction of organisms at the end of the Cretaceous, as already emphasized, was sufficiently extended in time (the gradual depletion of the fauna took place over more than 7 million years). This process was neither sudden nor strictly simultaneous throughout the Earth and for all groups of organisms, and the extinction itself began long before the time of the formation of the iridium layer, and was by no means universal, but selective, and some organisms turned out to be practically unaffected by it. It should be taken into account that, in general, large craters are also known on Earth, which probably arose during the fall of fireballs (for example, the Montana crater on the Atlantic shelf off the coast of Canada with a diameter of about 45 km, formed at the end of the Early Eocene, or the Middle Oligocene Popigay crater on Taimyr - diameter of about 100 km However, the fall of these large celestial bodies did not lead to noticeable changes in the biosphere and did not have the consequences of mass extinction processes.
Thus, the totality of the currently available data speaks generally against the catastrophic hypotheses of extinction at the end of the Cretaceous (as well as in other geological epochs).
There have been suggestions about the connection between the extinction of dinosaurs and changes in biotic factors, which were called, in particular, competition from mammals or flora transformations associated with the wide distribution of angiosperms in the middle Cretaceous. However, mammals arose as early as the Late Triassic, and for about 130 million years that passed until the end of the Mesozoic, they remained a relatively inconspicuous and insignificant group of animals.
There is a hypothesis that the predominance of angiosperms in Late Cretaceous plant communities could play an important role in the extinction of dinosaurs, since angiosperms biochemically differ significantly from those groups of plants that served as food for herbivorous animals until the middle Cretaceous. However, dinosaurs coexisted with angiosperms for about 70 million years, and the dinosaur fauna, which included numerous and diverse herbivorous species, flourished for at least 45 million years after the wide expansion of angiosperms.
We should also not forget about other groups of animals (in particular, marine ones) that became extinct at the end of the Mesozoic: obviously, these biotic factors alone cannot explain the extinction of plesiosaurs, mosasaurs, russists, sea lilies, etc. Since extinction affected some groups of animals and almost or did not affect others at all, the key to understanding the events that took place at the turn of the Mesozoic and Cenozoic, apparently, should be sought not only in changes in external factors, but also in the features of the organization and biology of animals that underwent extinction.
A particular difficulty lies in the fact that at the end of the Cretaceous, groups of animals that differed significantly from each other ecologically and lived in different environments (terrestrial, amphibious, freshwater and marine) became extinct. And while it remains unclear whether the extinction of such diverse animals as all kinds of dinosaurs, flying lizards, ammonites, rudists, etc. was caused. some one external cause (at least indirectly) or the simultaneous action of various factors that are not causally related to each other.
Dinosaur extinction
Since the dinosaurs attracted the most attention, most hypotheses discuss the extinction of these animals in the first place. In search of a "weak point" in the organization of dinosaurs, which could contribute to their extinction under certain changes in external conditions, many scientists dwelled on the features of the heat exchange of these reptiles. As already mentioned, most likely, dinosaurs remained physiologically cold-blooded animals, like all modern reptiles. However, using heliothermy, dinosaurs (especially large forms) in the even and warm climate of the Jurassic and Cretaceous could maintain their body temperature at a practically constant level, optimal for body functions. In the absence of significant seasonal climate changes, such as, for example, modern ones in middle latitudes, dinosaurs could not have developed any physiological or behavioral mechanisms for successful wintering.
In search of those changes in external conditions that caused the extinction of dinosaurs, D. Axelrod and G. Bailey again turned to the processes of mountain building and volcanism that took place at the end of the Cretaceous, the consequences of which could be important, although they were not of the nature of a catastrophe. The Mesozoic was generally an era of low standing of the continents. The Alpine phase of mountain building, which gradually developed in the Jurassic and Cretaceous, was accompanied by a significant general uplift of the land towards the end of the Mesozoic. The result of this, as well as a decrease in the transparency of the atmosphere due to volcanic activity, was a gradual decrease in the average annual temperature over 20 million years by about 5 °C.
But, probably, an even more important factor was the increase in the unevenness of temperature conditions in temperate zone with the development of an increasingly pronounced seasonality of the climate and a significant increase in the difference between maximum and minimum temperatures. This, in particular, is indicated by changes in vegetation: the appearance in the Late Cretaceous in the middle latitudes of deciduous flora instead of subtropical forests, and at the beginning of the Paleogene, deciduous flora was to some extent replaced by more cold-loving flora. coniferous forests. Dinosaurs were adapted to this direction of climate change much worse than mammals and birds, which had already formed true homoiothermy, and also than reptiles, which could survive the unfavorable seasons of the year in an inactive state (lizards, snakes, turtles). mass extinction catastrophe dinosaur
The last adaptation path for dinosaurs was difficult due to their large size (which was so advantageous in terms of energy throughout the Jurassic and Cretaceous), as well as the specifics of their heat transfer: not being homoiothermal, dinosaurs were adapted to almost constant optimal temperatures. Note that, speaking here of large sizes, we do not mean gigantic forms, but generally large ones - more than 1 m, namely, small dinosaurs were like that. We also note that now in the temperate zone reptiles are represented only by small species, as a rule, less than 1 m, which can successfully survive the winter in various shelters. All major modern views reptiles (crocodiles, large species snakes, lizards and turtles) are tropical animals.
This hypothesis can be consistent with the observations of French paleontologists about anomalies in the egg shell, often found in fossil dinosaur clutches from the Upper Cretaceous deposits of Provence. It has been suggested that these anomalies were the result of repeated intravital suspensions of the shell formation process during egg development in the oviducts of female dinosaurs, which could be caused by a cold snap.
The advantages of the hypothesis under consideration are, firstly, the coordination of a number of sufficiently diverse and reliable data, and secondly, the recognition of the gradualness of the changes taking place on Earth and the process of extinction itself. However, this hypothesis also leaves open a number of serious questions: why did dinosaurs and flying pangolins not survive in the tropics, where even with a slight decrease in average temperature generally warm and even climates were maintained throughout the Phanerozoic (and where, for example, crocodiles survived, which were probably physiologically close to dinosaurs); why marine reptiles and a number of other groups have become extinct everywhere marine organisms, because in the ocean, especially in low latitudes, there could not be a temperature unevenness comparable to that on land.
The hypothesis of the French paleontologist L. Ginzburg, also based on the geological fact of the rise of the continents towards the end of the Cretaceous, which was associated with a significant marine regression. In the course of this regression, by the end of the Cretaceous, compared with its middle, the sea level dropped by 180–200 m. At the same time, the water area of the epicontinental seas (i.e., parts of continental platforms covered by the sea) decreased by about 50 times. Cretaceous warm epicontinental seas are the most favorable zone of the world ocean for life, the most abundant in species of organisms. Probably, such a significant reduction in their water area could not but affect the most diverse groups of marine organisms. However, the selectivity of extinction remains unclear: why marine reptiles, many groups of mollusks, etc., died out, but teleost fish, for example, remained practically unaffected.
Mass extinctions of various scales occurred repeatedly in the Phanerozoic, the largest of them in the Early Cambrian, Late Ordovician, Late Permian, and at the end of the Cretaceous. Many attempts have been made to catch some kind of periodicity of mass extinctions, but the intervals between them vary greatly, amounting to 20-60 million years. Mass extinctions of a smaller scale took place in the Triassic and in the first half of the Jurassic. In general, there is a smooth and gradual transition in the scale of extinction from the background extinction that occurred in all epochs to mass extinctions, and the latter - for all their spectacularity - cover only about 5% of all extinction phenomena in the history of the Earth, the remaining 95% fall on a less noticeable background extinction.
The total body of evidence speaks against catastrophic causes of mass extinctions. Probably, in most cases, the scale and specifics of extinction processes in specific epochs of the history of the biosphere were more determined by the state of ecosystems than by changes in abiotic factors. The latter, on the other hand, play the role of a stress mechanism that "tests the strength" of the stability mechanisms of biocenoses, leading to the decline and extinction of some species of organisms. As already mentioned, the stability of biocenoses has its limits: if violations of the structure of the biocenosis go beyond these limits, the collapse of the entire ecosystem begins. At the same time, the previously established pathways for the transfer of organic substances and energy in the biosphere are violated. Then new species are subject to extinction, which in themselves have not yet been directly affected by changes in abiotic factors. This process will grow like an avalanche until a new balance is reached in one way or another between biosynthesis and destruction of organic substances, between plant species, herbivorous animals, predators and microorganisms, i.e. until new sustainable and capable of self-regulation ecosystems - biocenoses - are formed.
