Mass extinctions of species in the history of the earth. Humanity is on the verge of extinction: the sixth mass extinction of species on Earth has begun. Great Permian extinction

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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 Difficult 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 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 an 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 recovered from the last mass extinction, and this period was marked by the development of the 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. Trilobites have lost their dominance as 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 the Devonian period, the Carboniferous period 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 terrible 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 oceanic tectonic plates sank 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 evolve throughout the subsequent Cretaceous period. 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 hit 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.

Extinction is a phenomenon in biology and ecology, which consists in the disappearance (death) of all representatives of a certain biological species or taxon. Extinction can have natural or anthropogenic causes. With especially frequent cases of extinction of biological species in a short period of time, they usually talk about mass extinction.
The largest extinctions in the history of the Earth
440 million years ago- Ordovician-Silurian extinction - more than 60% of marine invertebrate species disappeared;
364 million years ago- Devonian extinction - the number of species of marine organisms decreased by 50%;
251.4 Ma- the "great" Permian extinction, the most massive extinction of all, which led to the extinction of more than 95% of the species of all living beings;
199.6 mya- Triassic extinction - as a result of which at least half of the now known species that lived on Earth at that time died out;
65.5 Ma- Cretaceous-Paleogene extinction - the last mass extinction, which destroyed a sixth of all species, including dinosaurs.
33.9 mya- Eocene-Oligocene extinction.

Ordovician-Silurian extinction
About 440 million years ago, at the end of the Ordovician period, the Earth experienced the first mass extinction and the second largest mass extinction: more than 75% of marine species. The exact cause of the disaster is unknown, but Seth Finnegan of the California Institute of Technology (USA) and his colleagues found new evidence that this event was associated with climate cooling.
At that time, we recall, North America was on the equator, and the main part of the rest of the land was the Gondwana supercontinent, which stretched from the equator to the South Pole.
Using a new method to measure ancient temperature fluctuations, researchers have been able to find clues to the timing and extent of glaciation and its impact on ocean temperatures near the equator.
The fact that the extinction occurred during the Ice Age, when huge glaciers covered most of what is now Africa and South America, greatly complicates the assessment of the role of climate. It is very difficult to distinguish between changes in temperature and the size of the continental ice sheet. Both factors could have caused the mass extinction: the drop in water temperature is inconsistent with the habits of many species, and the freezing of large volumes of water is drying up the oceans.
The usual method for determining ancient temperature involves measuring the ratio of oxygen isotopes in minerals found in marine sediments. The ratio depends on temperature and the concentration of isotopes in the ocean, so you can know about the temperature only if the concentration of isotopes is known. But glaciers preferentially capture one of the isotopes, which reduces its concentration in the ocean. No one knows how big the ancient glaciers were, and it is extremely difficult to determine the concentration of isotopes. Therefore, until now there was no reliable way to know the temperature of the water during ice ages late Ordovician.
364 million years ago. Devonian extinction.
The Devonian extinction, a late Devonian extinction, was one of the largest extinctions in the history of terrestrial flora and fauna. The main extinction occurred at the boundary that marks the beginning of the last phase of the Devonian period, about 364 million years ago, when almost all of the jawless fish fossils suddenly disappeared. The second strong devastating impulse ended the Devonian period. Everywhere died out, 19% of the families and 50% of the entire gene pool.

While it is clear that there was a massive decline in biodiversity towards the end of the Devonian, the time interval over which this event took place is unclear: estimates vary from 500,000 to 15 million years.

It is not entirely clear whether this event was represented by two mass extinction peaks or a series of smaller extinctions, but the results of the latest study rather indicate a multi-stage development of the extinction, from a series of individual extinction pulses over a time interval of about three million years. Some suggest that the extinction consisted of at least seven separate events occurring over a period of 25 million years. Some cite a 250 million year range during which the extinctions took place.
By the late Devonian, the land was fully developed and inhabited by plants, insects and amphibians, and the seas and oceans were full of fish. In addition, giant reefs formed by corals and stromatoporoids already existed during this period. The Euroamerican continent and Gondwana have just begun moving towards each other to form the future supercontinent Pangea. It is likely that the extinction mainly affected marine life. Reef-building organisms were almost completely destroyed, as a result, coral reefs were revived only with the development of modern corals in the Mesozoic. Brachiopods (brachiopods), trilobites and other families have also been heavily affected. The reasons for this extinction are still unclear. The underlying theory suggests that changes in ocean levels and the depletion of oxygen in ocean waters served to main reason extinction of life in the oceans. It is possible that global cooling or extensive oceanic volcanism acted as an activator for these events, although the fall of an extraterrestrial body, such as a comet, is also quite possible. Some statistical studies of the marine life of that time suggest that the decline in diversity was due to a decline in the rate of speciation rather than an increase in extinction.

Late Devon world
In late Devon, the world was very different from today. The continents were located differently than they are now. The supercontinent Gondwana occupied more than half of the southern hemisphere. The Siberian continent occupied the northern hemisphere while the equatorial continent, Laurasia (formed by the collision of Baltica and Laurentia (North American Platform (Laurence)) drifted towards Gondwana. The Calydonian Mountains (Caledonia - Latin name, given by the Romans of the northern part of the island of Great Britain) still grew across what is now known as the Scottish Highlands and Scandinavia, while the Appalachians grew in North America. At one time, the Appalachians brought a lot of mysteries to geologists, because the northeastern Appalachians suddenly break off right into the ocean. But this mystery was solved after the creation of the theory of lithospheric plate tectonics, which explained that the continuation of this mountain range is on the other side Atlantic Ocean are the Caledonian Mountains in Ireland and Scotland. These mountain belts were the Devonian equivalent of today's Himalayas.
The flora and fauna of that period differed from the modern one. Plants that existed on land in the form of mosses and lichens since the Ordovician had by this time developed root systems, spore reproduction and a vascular system (to carry water and nutrients to all parts of the plant), which allowed them to survive not only in constantly wet places, but spread further, and as a result form huge forests in mountainous areas. By the late Givetian Stage, several plant clades already exhibited features characteristic of shrubs or trees, including: ferns, lycopsids, and primary gymnosperms Tiktaaliki, primary tetrapods, appeared on land.

Duration and dating of extinction periods

For a broad period spanning the last 20 to 25 million years of the Devonian, the species extinction rate was found to be higher than the background rate of extinction. During this period, 8 to 10 separate events can be identified, of which two stand out as the largest and most severe. Each of these major events was a prelude to the subsequent long period of biodiversity loss.
Kellwasser Event
The Kellwasser event is the term given to an extinction impulse that took place near the Frasnian-Famennian border. Although in fact, there could be two closely spaced events.
Hangenberg event
The Hangenberg event occurred at or just below the Devonian-Carboniferous boundary and marks the final peak in the overall extinction period.
Consequences of extinction events

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.
biological shock
The Devonian crisis primarily affected the marine community, and selectively affected shallow-water heat-loving organisms much more than organisms that preferred cold water for habitat. The most important class affected by the extinction were the reef-building organisms of the great Devonian reef systems, including stromatoporoids, folded and plate corals. Late Devonian reefs dominated sponges and calcareous bacteria, producing structures similar to those produced by oncolites and stromatolites. The collapse of the reef system was so abrupt and severe that the main reef-building organisms (represented by new families of carbonate-producing organisms, modern scleractin or "stony" corals) did not recover until the Mesozoic era. Kolihapeltis sp., Devonian, Morocco.

In addition, the following classes were greatly affected by the extinction; brachiopods, trilobites, ammonites, conodonts, and acrytarxes as well as jawless fish and all armored fish (placoderms). At the same time, many freshwater species, including our four-legged ancestors, and land plants remained relatively unscathed.
The surviving classes during the extinction show the morphological evolutionary trends that took place during the extinction event. At the peak of the Kellwasser event, trilobites evolve smaller eyes, although they are subsequently seen to enlarge again. This suggests that vision became less important during the extinction event, perhaps due to increased habitat depth or water turbidity. In addition, the size of the whisker-like processes on the heads of trilobites also increased during this period, both in size and length.
It is believed that these processes served for the purposes of respiration and that it was the increasing anoxia (depletion of water with oxygen) that led to an increase in their area.
The form oral apparatus conodonts varied at different levels of the ?18O isotope and, consequently, temperature sea water. This may be due to their occupation of different trophic levels as a result of changing the basic diet.
As with other extinctions, the specialized classes that occupied narrow ecological niches suffered significantly more than the generalists.

The absolute value of the event
The late Devonian decline in biodiversity was more devastating than the similar extinction that ended the Cretaceous (the dinosaur extinction). A recent study (McGhee 1996) estimates that 22 percent of all marine animal families (mostly invertebrates) have gone extinct. It should be borne in mind that the family is a very large countable unit, and that the loss of such a large number creatures means the complete destruction of the diversity of ecosystems. On a smaller scale, the loss is even greater, accounting for 57% of the genera and at least 75% of the species that did not pass into the Carboniferous. The latter estimates need to be taken with a certain degree of caution, as estimates of the number of species lost depend on the breadth of the survey Devonian marine classes, some of which may not be known. Thus, it is still difficult to fully assess the effect of the event that took place during Devon.

Causes of extinction
Since the "extinction" occurred over a long period, it is very difficult to single out a single cause that led to extinction and even to separate the cause from the effect. Sedimentary deposits show that the Late Devonian was a time of change environment, which directly affected living organisms, causing extinction. What directly caused these changes is, in part, a more open topic for debate.
Major environmental changes
From the end of the middle Devonian, several environmental changes can be identified from the study of sedimentary rocks, which continued until the late Devonian. There is evidence of widespread anoxia (depletion of oxygen in the waters) in the bottom ocean waters, while the rate of carbon deposition has jumped, and benthic organisms (flora and fauna on the bottom of the ocean or other water body) have been destroyed, especially in the tropics and especially reef communities. There is strong evidence of high-frequency fluctuations in global sea level near the Frasnian-Famennian (Frasnian/Famennian) boundary, with sea level rise clearly associated with the formation of anoxic sediments.
Possible initiators
Meteor falling
Meteor impacts can certainly be dramatic causes of mass extinctions. It is claimed that it was the fall of a meteorite that was the primary cause of the Devonian extinction [, but reliable evidence of a specific extraterrestrial impact has not been identified in this case. Impact craters such as the Alamo and Woodleigh cannot be dated with sufficient accuracy to tie them to this event. and microspheres (microscopic balls of melted rock)), but perhaps the formation of these anomalies is caused by other reasons.
plant evolution
During the Devonian, land plants made a significant leap in evolution. Them maximum height increased from 30 centimeters [source not specified 348 days], at the beginning of Devon, to 30 meters by the end of the Devonian period. Such a huge increase in size was made possible by the evolution of an advanced vascular system that allowed the cultivation of extensive crowns and root systems. At the same time, the development of seeds made it possible to successfully multiply and settle not only in wetlands, thereby allowing plants to colonize previously uninhabited inland and mountainous lands. Two factors combined, developed vascular system and propagation by seeds to greatly increase the role of plants on the world scale of life. This is especially true for the Archeopteris forests, which expanded rapidly during the final stage of the Devonian.
erosion effect
Newly evolved tall trees needed deep root systems to reach water and nutrients and ensure their resilience. These systems cracked the top layer of bedrock and stabilized a deep layer of soil that was probably on the order of a meter thick. For comparison, early Devon plants had only rhizoids and rhizomes that could not penetrate the soil deeper than a couple of centimeters. The movement of large areas of soil would have enormous consequences. Accelerated soil erosion, chemical breakdown of cameos and the resulting release of ions that act as nutrients for plants and algae. A relatively sudden influx of nutrients into the river water could serve as a source of eutrophication and subsequent anoxia (depletion of oxygen in the waters). For example, during a period of abundant algal blooms, organic material that has formed on the surface can sink at such a rate that rotting organisms use up all available oxygen to decompose, creating anoxic conditions and thereby suffocating demersal fish. The Frasnian fossil reefs were dominated by stromatolites and (to a lesser extent) corals, which thrived only under nutrient-poor conditions. The postulate that high levels of nutrients in the water can cause extinction is supported by the phosphates that annually wash off the fields of Australian farmers and which today cause immeasurable damage to the Great Barrier Reef. that anoxia may have played a dominant role in the extinction.
Other assumptions
Other mechanisms have been proposed to explain the extinction, including: climate change as a result of tectonic processes, changes in ocean levels, and reversal of ocean currents. But these assumptions are usually not taken into account, since they cannot explain the duration, selectivity and frequency of extinctions.

251 million years ago. Permian extinction.

The Permian mass extinction (informally referred to as The Great Dying or The Greatest Mass Extinction of All Time) - one of the five mass extinctions - formed a boundary dividing the Permian and Triassic geological periods, that is, the Paleozoic and Mesozoic, approximately 251.4 million years ago.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% terrestrial species vertebrates. 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. The models by which the extinction proceeded are under discussion. Various scientific schools suggest from one to three extinction shocks.
Causes of the disaster

Currently, there is no generally accepted opinion among specialists about the causes of extinction. A number of possible reasons are considered:
gradual environmental changes:
anoxia - changes chemical composition sea water and atmosphere, in particular oxygen deficiency;
increasing dryness of the climate;
changes in ocean currents and/or sea levels due to climate change;
catastrophic events:
the fall of one or many meteorites, or the collision of the Earth with an asteroid with a diameter of several tens of kilometers (one of the evidence for this theory is the presence of a 500-kilometer crater in the area of Wilkes Land;
increased volcanic activity;
sudden release of methane from the bottom of the sea.
The most common hypothesis is that the catastrophe was caused by the outpouring of traps (at first, the relatively small Emeishan traps about 260 million years ago, then the colossal Siberian traps 251 million years ago). Volcanic winter, the greenhouse effect due to the release of volcanic gases and other climatic changes that affected the biosphere could be associated with this;

Consequences of extinction
As a result of the mass extinction, many species have disappeared from the face of the Earth, entire orders and even classes have become a thing of the past; most of the parareptiles (except for the ancestors of modern turtles), many species of fish and arthropods (including the famous trilobites). The cataclysm also hit the world of microorganisms hard.
The extinction of old forms opened the way for many animals that remained in the shadows for a long time: the beginning and middle of the next Permian, Triassic period was marked by the formation of archosaurs, from which dinosaurs and crocodiles descended, and later birds. In addition, it was in the Triassic that the first mammals appeared.

33.9 million years ago. Eocene-Oligocene extinction (Cenozoic extinction).

The Cretaceous-Paleogene extinction (Cretaceous-Tertiary, Cretaceous-Cenozoic, K-T extinction) is one of five so-called. "great mass extinctions", on the border of the Cretaceous and Paleogene period, about 65 million years ago. There is no consensus on whether this extinction was gradual or sudden, which is currently the subject of research.
Part of this mass extinction was the extinction of the dinosaurs. Along with the dinosaurs, marine reptiles (mosasaurs and plesiosaurs) and flying pangolins, many mollusks, including ammonites, belemnites and many small algae, died out. In total, 16% of families of marine animals (47% of genera of marine animals) and 18% of families of terrestrial vertebrates perished.
However, most of the plants and animals survived this period. For example, land reptiles such as snakes, turtles, lizards, and aquatic reptiles such as crocodiles have not died out. The closest relatives of the ammonites, the nautilus, survived, as did birds, mammals, corals, and land plants.
Presumably some dinosaurs (triceratops, theropods, etc.) existed in the west North America and in India a few more million years at the beginning of the Paleogene, after their extinction in other places.

The most famous versions of extinction.
extraterrestrial
The fall of an asteroid is one of the most common versions (the so-called "Alvarez hypothesis"). It is based mainly on the approximate timing of the formation of the Chicxulub crater (which is the aftermath of a 10 km asteroid impact about 65 Ma ago) in Mexico's Yucatán Peninsula and the extinction of most of the extinct dinosaur species. In addition, astrophysical calculations (based on observations of currently existing asteroids) show that asteroids larger than 10 km collide with the Earth on average about once every 100 million years, which corresponds in order of magnitude, on the one hand, to the dates of known craters left by such meteorites, and, on the other hand, the time intervals between the extinction peaks of biological species in the Phanerozoic. It should be noted that the authors and supporters of this hypothesis in scientific environment, for the most part, are not paleontologists, but representatives of other scientific directions(physicists, astronomers, geologists, etc.) The theory is confirmed by the increased content of platinoids in the layer on the border of the Cretaceous and Paleogene. Increased content platinoids are noted on the border of the Mesozoic and Cenozoic everywhere in earth's crust. These elements, in particular the Os-187 isotope, could not have formed in such a concentration for some other reason and have a clearly meteorite genesis.
A "multiple impact event" version that involves several
successive strikes. It is invoked, in particular, to explain that the extinction did not occur all at once (see section Disadvantages of Hypotheses). Indirectly in her favor is the fact that the asteroid that created the Chickshulub crater was one of the fragments of a larger celestial body. Some geologists believe that Shiva Crater is at the bottom indian ocean, dating from about the same time, is a trace of the fall of the second giant meteorite, but this point of view is debatable.
Supernova explosion or nearby gamma-ray burst.
Collision of the Earth with a comet.

Terrestrial abiotic
Increased volcanic activity, which is associated with a number of effects that could affect the biosphere: changes in the gas composition of the atmosphere; the greenhouse effect caused by the release of carbon dioxide during eruptions; change in the illumination of the Earth due to emissions of volcanic ash (volcanic winter). This hypothesis is supported by geological evidence of a giant outpouring of magma between 68 and 60 million years ago in the territory of Hindustan, as a result of which the Deccan traps were formed.
A sharp drop in sea level that occurred in the last (Maastrichtian) phase of the Cretaceous period ("Maastrichtian regression").
Changes in average annual and seasonal temperatures, despite the fact that inertial homeothermy large dinosaurs, requires an even warm climate Extinction, however, does not coincide with significant climate change
A sharp jump in the Earth's magnetic field.
Too much oxygen in the Earth's atmosphere.
Rapid cooling of the ocean.
Changes in the composition of sea water.
33.9 Ma - Eocene-Oligocene extinction event
At the end of the Eocene, the African lithospheric plate began to run into the European and Asian ones, the large and deep Tethys Sea began to turn into a shallow Mediterranean Sea. And the Indian lithospheric plate, which came into contact with the Asian one at the beginning of the Eocene, began to noticeably push up the Tibetan-Himalayan mountain system. As a result, the ways of circulation of water and air masses have changed greatly, it has become noticeably colder on Earth, and a glacier has begun to form in Antarctica. All of the above led to a moderately large extinction, which marks the end of the Eocene. However, this extinction can be called moderately large only by Cenozoic standards, compared with the extinction of dinosaurs, it was sheer nonsense, and by the standards of the Cambrian, this is not an extinction at all, but normal everyday life.

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; many species became extinct. sea lilies. 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 (for example, in 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 population. 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 of this cosmic catastrophe, they point to an increased (about 30 times) content of iridium (which is attributed to asteroid or cometary origin) in the layer of clay deposits at the Cretaceous-Paleogene boundary, the presence of frozen melt drops, shock-metamorphosed quartz crystals, and also a large number of particles of coal soot, which are believed to have been formed during hurricane fires that arose after a space 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 changes climate, similar, for example, to 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 more an important factor there was an 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 of marine organisms died out everywhere, because in the ocean, especially in low latitudes, temperature unevenness comparable to that on land could not arise.

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 different scale occurred repeatedly in the Phanerozoic, the largest of them - in the early Cambrian, late Ordovician, late Permian and at the end of the Cretaceous period. 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.

The rapid extinction of many mammal species today may be a sign of the impending sixth mass extinction in the history of the Earth, but the situation is not too late to "turn around", scientists say in an article published in the journal Nature. However, before that, the world had already experienced as many as five such catastrophes. Let's see how and when it was.

1. Ordovician-Silurian extinction

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 is a giant continent, from which both Africa and South America, and Australia, and Antarctica, - lay down in 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.

2. Devonian extinction

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.

3. Great Permian extinction

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 shadow of stronger species spread widely. 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.

4. Triassic 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 nasty greenhouse gas acted as a trigger for a dramatic global warming, which destabilized the climate on the planet and became the cause of total Achtung.

5. Cretaceous-Paleogene extinction

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 (the dinosaurs were exterminated by little green men in flying saucers that hunted them) to the highly plausible (climate change destroyed them). 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.
The most interesting explanation links the extinction of the dinosaurs to 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.

About 60% of all marine invertebrates died out

Most of the animals at that time lived in the water, and the fall in the level of the world's oceans destroyed or damaged the habitats of most animal species of the Ordovician and Silurian period.

Devonian extinction

About 50% of marine animals died out

It happened 374 and 359 million years ago. 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).

It is not at all clear what caused this mass extinction. The main version of what happened is a change in the level of the world's oceans and oxygen depletion of the ocean. This was probably caused by the high volcanic activity of the Earth. Some scientists also do not rule out the fall of a large extraterrestrial body, such as a comet.

Great Permian extinction

Extinction of 95% of all animal species

This is the most mass extinction of animals that has ever happened on our planet. Some scientists call 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 insect genera died. This is the only known extinction that affected insects.

Extinction even affected microorganisms, which, it would seem, could do little harm.

Scientists do not have one opinion why such a large-scale extinction happened. Some are inclined to believe that the whole cause was increased volcanic activity. Some suggest that a lot of methane was released from the ocean floor (see frozen methane at the bottom of the ocean), which led to disastrous climate change. A number of scientists believe that at this time the Earth collided with a huge asteroid. The proof of the latter theory is a huge crater in Antarctica (located on Wilkes Land).

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 shadow of stronger species spread widely. 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.

Triassic extinction

50% of animals died out

feature Triassic extinction shortness is considered. 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.

Some scientists believe that the cause of the Triassic extinction, like all other mass extinctions of animals, was the increased volcanic activity of the Earth at that time.

Cretaceous-Paleogene extinction event

More than 15% of all animals died out

It is not entirely clear what led to this mass extinction. Scientists continue to study the Cretaceous and Paleogene period of the Earth to find the cause of the disaster. The most famous theories say that the Earth collided with a large asteroid or fell into the radiation zone from a supernova explosion.

But besides the "cosmic" reasons, there are suggestions that dinosaurs (as well as some other animal species) simply could not adapt to the new vegetation, the violent development that was observed at that time, and simply "poisoned" with inedible leaves. Or they were exterminated by the first mammals that destroyed the masonry of dinosaurs, preventing them from multiplying. The latter theory is supported by the fact that some dinosaurs lived for quite a long time in the territory of modern North America and India, where, perhaps, "dangerous" mammals appeared later.