In the history of earthly life, scientists have counted up to 11 mass extinctions of flora and fauna, 5 of which greatly changed the appearance of our biosphere. The last of these "Great" extinctions that occurred 65 million years ago, destroyed 1/6 of all species that existed then (Cretaceous-Paleogenic extinction).
At the same time, along with sea and flying lizards, the most "promoted" order of animals in the fossil record of our world disappeared - all dinosaurs.
Photo: geektimes.ru
Modern science does not have exhaustive data on the reasons for the last major extinction of species (as well as the previous ones). Among the main suspects are asteroids, volcanoes and internal processes in the earth's biosphere. Below, I suggest that you familiarize yourself with the chronicle of earthly catastrophes 300 million years long and form your own opinion about the reasons for the death of this wonderful detachment of reptiles.
Mother of all extinctions
250 million years ago, the largest known extinction in the history of our planet occurred; during the Permian-Triassic catastrophe, 95% of all species of marine and land animals died. Almost all the therapsids that dominated the land at that time disappeared. Among the few surviving therapsids were the ancestors of the cynodonts, whose descendants are all mammals.
Promotional video:
The early Permian pelicosaurs (dimetrodon on the left) and their descendants therapsids (gorgonops on the right) are referred to as animal-like lizards (synapsids). In particular, gorgonops are the closest relatives of cynodonts.
Photo: geektimes.ru
The vacated ecological niches of therapsids were occupied by archosaurs, which after 20 million years will begin to dominate as land predators (dinosaurs and crurotarsi).
The main reason for this extinction is usually considered to be the outpouring of magmatic Siberian traps at the border of the Permian and Triassic periods. During the formation of traps, about 4 million km3 of rocks were ejected, covering an area of 2 million km2. The process of the outpouring of rocks triggered a cascading reaction of global climatic changes as a result, presumably, and caused a mass extinction.
Siberian traps eruption area superimposed on the map of modern Russia
Photo: geektimes.ru
"Mysterious" Triassic-Jurassic extinction
Already after 50 million years, the terrestrial biosphere had to face another series of mass extinctions. On the border of the Triassic and Jurassic periods, an unknown global cataclysm found the Crurotars dominating on land. Having forced out their "cousins" dinosaurs and mammals, by that time the crurotarsses became the main and largest land predators of the Late Triassic.
Some representatives of the Late Triassic carnivorous crurotars
Photo: geektimes.ru
As a result of the catastrophe, the Crurothars shared the fate of the therapsids, giving way to their "cousins" - the dinosaurs, who will dominate the land for 140 million years. One of the two surviving groups of crurotars, protosuchia, are the direct ancestors of modern crocodiles.
The main versions of this extinction are the fall of a large asteroid and volcanic activity (Central Atlantic igneous province, CAMP). In the first case, the impact of a 4 km asteroid, which formed the 100 km crater of Manicouagan in Canada, was considered as the cause, but geological dating attributes its fall by 14 Ma to the Triassic extinction.
Today, Manicouagan crater has a transverse diameter of 70 km (originally 100 km). Craters of this size usually occur when asteroids with a diameter of about 4-5 km fall, and do not have long-term consequences for terrestrial fauna and flora.
Photo: geektimes.ru
The combined hypothesis received the greatest support. According to her, the CAMP, which caused the outpouring of 2 million km3 of volcanic rock, including a huge amount of CO2, triggered the release of huge ocean bottom "pockets" of methane hydrates by global warming. Methane, being a stronger greenhouse gas than CO2, triggered a chain reaction of overheating of the earth's atmosphere, which, presumably, caused the mass extinctions.
"Stable" Mesozoic
The period of domination of dinosaurs on land (Jurassic and Cretaceous periods of the Mesozoic era) was not at all geologically "quieter" than other periods of earth's history.
183 million years ago, a large magmatic outpouring Karoo-Ferar occurred, comparable in scale to the CAMP (2.5 million km3 of igneous rocks). However, this event did not cause any catastrophic consequences for earthly life. The collision with the Earth of a large asteroid about 4 km in diameter 167 million years ago - in the middle of the Jurassic (destroyed Puchezh-Katunsky crater in the Nizhny Novgorod region of Russia) passed without serious consequences.
The second mass extinction in dinosaur history occurred at the border of the Jurassic and Cretaceous periods - 145 million years ago. One of the many hypotheses connects the formation of one of the largest shield volcanoes in the solar system - the Tamu massif in the Pacific Ocean - with this "small Jurassic" extinction. However, it is possible that the global effect of the formation of the volcano intensified the impact of 4 km of the asteroid in the same period of time (Morokweng crater, South Africa). By this time, scientists attribute the appearance of flying dinosaurs - the ancestors of modern birds.
The Tamu Massif in the Pacific Ocean is one of the largest extinct volcanoes in the solar system. The total mass of the rocks that make up this ancient volcano is 80% of the mass of the Martian Mount Olympus
Photo: geektimes.ru
About 12 million years later, already at the beginning of the Cretaceous period, the world's flora and fauna experienced a series of the largest explosive volcanic eruptions in earth's history. The eruption at the beginning of the Hauterivian stage of the Cretaceous period of 8 supervolcanoes released a total of 50,000 km3 of gases and rocks. For example, the eruption of each supervolcano was, on average, twice as powerful as the eruption of the Toba supervolcano, 70,000 years ago, which caused the bottleneck effect.
The fact is also notable for the fact that the "parade" of supervolcanoes was only part of the formation of the giant magma traps Parana-Etendeka in South America. The total volume of released rocks was 2.3 million km3. However, like 50 million years earlier, these processes did not cause significant fluctuations in the diversity of the earth's biosphere.
Basaltic flow ledges of the ancient magmatic traps of Parana, Brazil
Photo: geektimes.ru
By the end of their era, dinosaurs experienced 3 more large peaks of volcanic activity, erupting in total 12 million km3 of rocks. During the Cretaceous, the Earth also experienced a series of collisions with large asteroids (3 asteroids 1 km in diameter, three more 2 km each, and one 3 km in size).
The largest (after Chiksulub) impact crater of the Cretaceous period - Karsky is located in the Nenets Autonomous Okrug of Russia. The impact of a 3 km asteroid 70 million years ago formed a crater with a diameter of about 70 km. The beginning of the decline in dinosaur speciation is attributed to the same period, although the connection between these two events is a subject of discussion.
Photo: geektimes.ru
End of eternity
If we could get to the end of the Cretaceous period, then many of us would not believe that we were in an ancient and alien world. Angiosperms (flowering) dominated everywhere, mammals were busy underfoot, not much different from modern small animals.
They have already managed to divide into placentals and marsupials. The first primates also lived then. Snakes and lizards familiar to us appeared. Since the Jurassic period, the forests were teeming with real birds, and their relatives, crocodiles, ambushed animals that came to the river.
Bees are also thought to be partly responsible for the decline in dinosaur diversity in the Late Cretaceous. Evolving about 100 million years ago from wasps feeding on pollinating insects, bees, thanks to their high efficiency, made flowering plants dominant in the earth's flora. Herbivorous dinosaurs had to slowly change their diet from gymnosperms to flowering plants.
Photo: geektimes.ru
The similarities of our world with that ancient one are limited to the composition of the fauna at the mental watering hole, most of which were still dinosaurs: tyrannosaurids, ceratopsians, hadrosaurs, sauropods, etc. (a more detailed list of the fauna of the end of the dinosaur era).
By the end of the era of the domination of dinosaurs, on the border of the Cretaceous and Paleogene periods, volcanic activity in India (then still an island in the middle of the Indian Ocean) increased. The volume of the outpouring of the Deccan traps for several hundred thousand years was about 2 million km3, the peak fell on the lava eruption of the Mahabaleshwar-Rajamandri trap, when during a short (geologically) period the volume of emissions amounted to 9 thousand km3 of rocks.
Deccan traps near Mumbai and a map of the area of India they occupy (in blue)
Photo: geektimes.ru
However, according to previous precedents of colossal volcanic activity, we already know that such phenomena by themselves do not necessarily have a catastrophic effect on the earth's climate, and, accordingly, flora and fauna. Most likely, such activity must coincide with exceptional circumstances to trigger the "mechanism" of mass extinction.
Only 6 of 11 major extinctions coincided in time with active geological processes. Most modern paleontologists are of the opinion that such an "exceptional circumstance" was the impact of 10 km of an asteroid in Central America 65 million years ago, during the active phase of the formation of the Deccan traps.
The impact power was unprecedented in the history of the Mesozoic era. The released energy was 2 million times higher than the energy of the explosion of the largest thermonuclear charge - "King of the Bomb". The area of the 180 km Chicxulub crater formed was comparable to the total area of all impact craters formed in the previous 200 Ma.
According to some geological models, the seismic wave from the explosion could focus at the antipode of the impact crater and cause (or intensify) lava eruptions. By the way, at the point-antipode of the collision then there was a region of increased volcanic activity - the very Deccan traps.
The hypothesis does not at all claim that volcanism was provoked by an asteroid impact, since the formation of these traps was a purely autonomous process of the earth's lithosphere. We are talking exclusively about a possible short-term increase in volcanic activity, since the phenomenon of "seismic focusing" in the particular case of the Earth is very limited.
Chicxulub crater on the Yucatan Peninsula (Mexico). Left - a crater in the visible range, right - overlaid with a map of gravitational anomalies
Photo: geektimes.ru
Another important condition for the beginning of the mass extinction process is the state of flora and fauna by the time of "force majeure". As before the Permian-Triassic extinction, paleontologists record a decrease in the diversity of dinosaurs and other archosaurs in the Maastrichian stage of the Posdengo Cretaceous (the last 7 million years of the existence of dinosars).
This is associated with a change in the global climate, since the decline in diversity has spread to many other groups of animals and plants (including mammals, birds and flowering plants). This gave rise to many paleontologists to assume that these two catastrophic events (volcanoes and an asteroid) occurred at an "inconvenient" time for living fauna.
Plot of the frequency of magmatic eruptions (scale on the right) and asteroid impacts (scale on the left) over the past 300 Ma (from confirmed). The former have a relatively long-term effect on the climate (millions of years); the impact of asteroids is "experienced" by nature for several tens of thousands of years. As you can see, natural disasters do not always provoke mass extinctions (red dots at the top - large extinctions, black - small)
Photo: geektimes.ru
Graph of "short-term" volcanic eruptions for the last 140 million years. Unlike explosive eruptions, lava eruptions are not accompanied by significant explosive precipitation of molten rocks. The eruption process is relatively calm. The red circle marks the eruption of the supervolcano Toba, 70 thousand years ago
Photo: geektimes.ru
Great turning point
The last of the major extinctions and the fourth for mammals occurred at the border of the Eocene and Oligocene epochs of the Paleogene period 35-30 million years ago. The percentage of extinction of species several times exceeded the "background" level - more than 3% against 0.7% (an order of magnitude weaker than the Cretaceous extinction).
This is the longest of all extinctions of the past 300 million years, lasted 4 million years. The Eocene-Oligocene extinction is associated both with the fall of two large asteroids 35 million years ago (~ 5 and ~ 4 km in diameter, respectively), and with significant global volcanic activity 35-29 million years ago (North, Central and South America, Africa and Middle East, see graph above).
100 and 90 km craters Popigay (Russia) and Chesapeake (USA), formed with a small time interval 35 million years ago, and presumably became one of the reasons for the Eocene-Oligocene extinction and general cooling of the climate in the Oligocene
Photo: geektimes.ru
Leviathans
However, according to many modern biologists, the Eocene-Oligocene extinction was not the last one. Since the last Ice Age, 11,000 years ago, the Earth's biosphere began to experience another “Great Extinction” in its history (Holocene extinction).
It has already exceeded the scale of the Eocene extinction, and according to scientists, the species diversity of the fauna of our planet will decrease by 50% at the end of this century (more than 80% for the terrestrial flora). And the fault is not at all volcanoes or asteroids, but the emergence and development of a very unusual species of animals - Homo sapiens.
As you can see in the illustration below, the appearance of a person most often provokes a sharp decline in the number of large mammals (Megafauna). In Africa and South Asia, the effect was weaker, as the fauna gradually adapted to coexistence with gradually replacing human species. On the rest of the continents, where the appearance of the "super hunter" was relatively sharp, the reduction effect was much more significant
Photo: geektimes.ru
Unfortunately, we often forget that the intellectual superiority of man over the rest of living nature should be accompanied by great responsibility, and not by predatory and often irrational plunder and destruction of its benefits.
Let's hope that the matter will not come to the "Great Anthropogenic Extinction", and if it does, we will not perish in the same abyss into which we will sweep away most of the earth's biosphere …
