Dinosaurs, died out about 65 million years ago, they were certainly gloomy creatures - thick-skinned, armored, solid teeth and claws. For example, Tyrannosaurus rex, the largest land predator of all time, could easily bite a rhino or an elephant in half with one elusive movement of its terrible jaws. And the weight of herbivorous lizards with columnar legs reached 30 and even 50 tons. And it is no coincidence that paleontologists, having discovered the unmanageable bones of another antediluvian creature, called it a seismosaur, that is, a lizard that shakes the earth. The length of this monster, according to the conservative estimates of researchers, was 48-50 meters.
For nearly 200 million years, the majestic reptiles were the sovereign masters of all three elements: nimble ichthyosaurs, resembling modern dolphins, swam in the primeval sea, multi-ton diplodocus walked on the ground, and toothy pterodactyls looked out for prey in the sky. (Incidentally, the wingspan of these flying monsters sometimes reached 16 meters, which is quite comparable to the dimensions of a combat fighter of our time.)
And then suddenly the dinosaurs began to die out rapidly, they were replaced by nondescript, small and unremarkable creatures, leading mostly nocturnal. Scientists already knew about unexpected and catastrophic changes in the composition of planetary biota at the end of the Cretaceous period in the 18th century, and after that this mysterious phenomenon is often called the “Great Extinction”.
Why are dinosaurs extinct? What could have happened? As a rule, textbooks paint such an unpretentious picture. A large and prosperous group of reptiles (both carnivorous and herbivorous), which populated all ecological niches of the planet, suddenly died suddenly - instantly and everywhere. And because these giants did not have any serious competitors at that time (mammals huddled on the outskirts of evolution and subsequently simply occupied the empty house), it is logical to look for some external reason. For example, a climatic cataclysm (a sharp cooling or, on the contrary, a warming), a supernova explosion accompanied by lethal fluctuations in the gamma background, or a change in magnetic poles, which temporarily deprived the planet of its protective shell.
Asteroid hypothesis
For some time now the asteroid hypothesis has become quite popular. Say, at the end of the Cretaceous period, a huge meteorite collapsed on our planet, throwing billions of tons of dust into the stratosphere, which screened the earth's surface, which led to the death of green plants, and after them - the rest of the fauna. In addition, the fall of such a meteorite could provoke a revival of terrestrial volcanism, which could greatly aggravate the situation. It should be noted that serious paleontologists do not particularly support this point of view.
Where did the asteroid hypothesis come from? In the mid-60s of the 20th century, in geological deposits dating from the Cretaceous and Cenozoic boundary (about 67 million years ago), scientists discovered a layer of blue clay with an abnormally high content of the rare metal iridium (20 times more than the average in the earth's crust) … Later, many similar anomalies were found (in some of them the iridium concentration exceeded the background by 120 times), while all of them were of the same age - they lay on the border of the Cretaceous and Cenozoic.
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Because there is very little iridium in the earth's crust, and in meteorite matter (primarily in iron meteorites, which are considered fragments of planetary nuclei), it is found in excess, a physicist from the United States Alvarez linked the iridium anomaly with the fall of an asteroid. He estimated its diameter at 10-12 km and even indicated the place of the catastrophe - the Yucatan Peninsula, where a crater of impressive size about 150 km in diameter was found.
The fall of such an asteroid would strongly shake the Earth: a tsunami wave of monstrous strength and height would devastate the coasts tens and hundreds of kilometers inland, and a grandiose dust cloud would eclipse the sun for a long time. A six-month absence of sunlight would kill green plants (photosynthesis processes would stop), and then (along the food chains) and animals - both land and sea.
It has been a long time since Alvarez put forward his impact hypothesis in 1980 (from the English impact - "blow"). Several dozen iridium anomalies are now known, while in geological deposits of various ages, but it is not possible to connect them with the mass death of flora and fauna. Moreover, geologists have at their disposal a number of craters much more impressive than the notorious Yucatan. The diameter of some of them reaches 300 km, but absolutely nothing serious has happened to the planetary biota (and this has been established reliably). This is quite natural, because the biosphere is by no means a children's designer, whose elements can be shuffled and folded at random, but a stable homeostat that can effectively resist various kinds of disturbances.
The famous Russian paleontologist K. Yu. Eskov noted:
In this sense, the situation with the Eltaninsky asteroid (about 4 km across), which fell in the Late Pliocene, about 2.5 million years ago, on the shelf between South America and Antarctica is quite indicative; the remains of the asteroid were relatively recently raised from a crater formed on the seabed. The consequences of this fall look quite catastrophic: kilometer tsunamis threw marine fauna deep into the land; just at that time very strange burials of fauna with a mixture of marine and terrestrial forms appeared on the Andean coast, and purely marine diatoms suddenly appeared in the Antarctic lakes. As for the distant, evolutionarily significant consequences, they simply did not exist (traces of this impact are enclosed within one stratigraphic zone), i.e.absolutely no extinction followed all these terrible perturbations.
Thus, the picture is quite interesting. As soon as the iridium anomalies began to purposefully search, it immediately became clear that their rigid connection to the mass extinction of dinosaurs (or any other organisms) is nothing more than an illusion. The fossil remains of the Mesozoic lizards unambiguously testify: the catastrophic scenario of the Mel-Paleogene extinction is worthless, because some groups of dinosaurs disappeared long before the iridium anomaly, while others sank into oblivion much later. The process stretched over hundreds of thousands and millions of years, so there can be no question of any rapidity of extinction of dinosaurs.
Therefore, the asteroid hypothesis, as well as all other scenarios of "shock impact", can be sent to the archive with peace of mind, because they imply the simultaneous destruction of flora and fauna. Meanwhile, even the mass death of marine organisms at the end of the Cretaceous period (much more hasty than the extinction of dinosaurs) was instantaneous only by geological standards and stretched out for a fair amount of time - according to various estimates, from 10 to 100,000 years. As for the reptiles, they didn't become extinct overnight.
K. Yu. Eskov wrote:
How so?! And it is very simple: the extinction of dinosaurs goes through the entire Late Cretaceous with a more or less constant speed, but from a certain moment this decrease ceases to compensate for the emergence of new species; old species died out - and new ones did not appear to replace them, and so on until the complete destruction of the group. In other words, at the end of the Cretaceous there was not a catastrophic extinction of dinosaurs, but a failure to replace them with new ones (this, you see, noticeably changes the picture). This means that we can talk about a rather lengthy natural process.
Changing the Earth's magnetic poles
Alternative versions are no more convincing - for example, the hypothesis of a sudden change of the Earth's poles or a supernova explosion near the solar system. Of course, the magnetic polarity reversal is a rather unpleasant thing, since the streams of high-energy charged particles flying from the Sun deflect in the lines of force of the magnetic field, forming the onion scales of the radiation belts. If you tear off our planet's thick magnetic "coat", then hard radiation will freely reach the surface of the planet.
But, firstly, the leapfrog of the magnetic poles is by no means an exotic, but a regular periodic process, and the data of special studies, usually, do not reveal a relationship between global biospheric crises and changes in terrestrial magnetism. And secondly, the biosphere as a whole is an impeccably debugged homeostat that easily resists any outside interference.
Supernova explosion
A supernova explosion is a galactic cataclysm. If such an event occurs in the vicinity of the solar system (according to astronomers, this happens once every 50-100 million years), then the flux of X-ray and gamma radiation will not only destroy the ozone layer, but also sweep away part of the earth's atmosphere, provoking the so-called “effect highlands”, which not all organisms can survive.
But even in this case, the extinction will most likely not be sudden, but will stretch over tens and hundreds of millennia. In addition, the harsh radiation and the effect of high mountains should primarily affect the population of land and shallow water, but in fact, as we know, the situation was exactly the opposite: the flora and fauna of the open sea, including microscopic ones, suffered the most. sushi, for some reason, only dinosaurs became victims of the Great Extinction.
This incredible selectivity is generally the most vulnerable point of all shock hypotheses: in fact, why did the dinosaurs become extinct, and the crocodiles survived and live happily now? Perhaps the unprecedented popularity of various kinds of "shock" versions is mainly due to the successes of observational astronomy over the past 20-30 years.
Climate Change or "Natural" Causes?
So why are dinosaurs extinct? One of two things: either climatic changes at the Cretaceous-Cenozoic boundary, or purely "natural" reasons - a radical restructuring within ecosystems and a change in communities.
Let's figure it out in order. We are accustomed to the fact that the planetary climate is characterized by a pronounced latitudinal zonality: rainforests grow at the equator, savannahs lie to the south and north of them, periodically moistened, where countless herds of ungulates graze, and even further to the north and south there is a strip of sun-scorched deserts and semi-desert. The subtropics give way to temperate forests - deciduous and coniferous, and those gradually give up their positions on the cold tundra, where almost nothing grows. Well, at the poles, eternal frost and eternal ice reign.
However, this was not always the case. The Mesozoic is a classic example of the thermoera, when latitudinal zoning was absent, and the global climate resembled the current subtropical Mediterranean type. In high latitudes and even at the Pole it was warm and quite comfortable, but at the same time it was not too hot at the equator. In other words, the temperature gradient - both seasonal and diurnal - was barely perceptible. But at the end of the Cretaceous, the thermoer was replaced by a cryoer with a latitudinal temperature difference.
Dinosaurs were cold-blooded (poikilothermic) animals. Not being able to regulate body temperature "from the inside", they completely depended on their environment, but in the even climate of the Mesozoic, this could not give them much trouble. If outside heat enters in excess, and the impressive dimensions do not allow to cool down overnight (for the most part, dinosaurs were large creatures), then maintaining a high body temperature will not be difficult. And all this without any participation of their own metabolism, for which mammals spend 90% of the energy they consume with food.
This interesting phenomenon is called inertial homeothermy (warm-bloodedness), and many scientists believe that thanks to this valuable quality, dinosaurs became the rulers of the Mesozoic. And when the climate changed radically at the end of the Cretaceous, the giant lizards disappeared.
It would seem that we have found the answer, but again something does not converge. For what reason did dinosaurs become extinct, and other reptiles - also cold-blooded - continue to exist to this day? Why did the Cretaceous crisis affect mainly marine life, and the land creatures survived it calmly? Why did some groups of dinosaurs begin to actively die out long before the fatal calendar date, while others leisurely lived out their days in the Paleogene?
Perhaps it makes sense to look for the answer elsewhere - in the structure of ecosystems? Let us remind the reader of the nondescript Mesozoic mammals, which for 120 million years lived side by side with lizards, without interfering with them in any way. These small insectivorous creatures, similar to modern opossums or hedgehogs, occupied their own ecological niche, which no one encroached on. However, in the Cretaceous, the situation changed radically.
K. Yu. Eskov described these events as follows: evolution spurred on the sluggish exchange of primitive mammals and made a "phytophage in a small size class" on this new metabolic basis. (Herbivorous dinosaurs were very large animals.) And if a small phytophagous species appeared, then a predator will certainly appear, which will not limit itself to hunting close relatives, but will suffice everyone who is within his power. Therefore, a baby dinosaur - a small defenseless lizard that does not possess inertial homeothermy - will instantly become a tasty prey for such a 24-hour active predator.
The version, no doubt, is curious, but it also does not answer all the tricky questions. And here genetics will come to our aid, understood in the broad sense of the word. Let's talk about marginality as the antipode of narrow specialization, because the organic world develops in this way.
Let us recall the Mesozoic mammals, who voluntarily surrendered the world to magnificent reptiles and vegetated on the sidelines of evolution. Huddling in remote corners, they were the most real marginal, because they occupied those few ecological niches that the ruling class ignored with majestic negligence.
The food base for herbivorous dinosaurs was gymnosperms and ferns, which were widespread in the Devonian. The angiosperms, or flowering, flora, which appeared at the beginning of the Cretaceous period, were forced to settle in the backyards, because gymnosperms prevailed. Thus, flowering plants were just as marginal as the small Mesozoic mammals. They had no choice but to occupy empty lands, where there were no established communities of gymnosperms: landslides, burnt-out areas, river banks, that is, such biotopes that are usually called “disturbed”. And the very species that settle in such conditions are called by biologists "cenophobic", that is, they are afraid of communities that prefer to exist separately.
But the tactical loss ultimately turned out to be an important strategic advantage. Firstly, the flowering plants that had settled on "bad" lands no longer allowed gymnosperms there, and secondly, they had a flower, which played a decisive role in the struggle for existence. If gymnosperms for the reproduction of their own kind entirely and completely relied on the wind, passively carrying their pollen, and therefore were forced to settle in heaps, then flowering ones actively attracted insects, which greatly increased their viability.
The existence of flowering plants did not depend on the elements, and the angiosperms could afford the luxury of dwelling in scattered wastelands. In addition, the flora of a new type has learned to form herbaceous forms that not only effectively resist erosion, but also quickly capture vacant land.
The change in plant communities turned into a real disaster. Contrary to popular belief, not only dinosaurs died out, but also 25% of Mesozoic invertebrate families - cephalopods and bivalves, unicellular radiolarians, diatoms, foraminifera and other representatives of planktonic organisms. Their calcium shells formed gigantic deposits, which is why this period of the geological record was named Cretaceous.
So yesterday's inconspicuous marginals - flowering plants and mammals - crushed the dominant fauna and flora of the Mesozoic.
The onset of flowering plants is now called the great angiospermization (from the Latin angiospermae - "angiosperms"). When the flora of the new type began to dominate decisively, something happened that always happens when a foundation collapses: the building simply collapsed. After all, the plant kingdom is exactly the foundation on which the floors of herbivorous animals and predators stand, and they are connected with each other not only by food chains, but also by more complex relationships.
Dinosaurs tried to master a new diet - they got beaks and powerful dental batteries for grinding highly abrasive food. But it didn’t matter for them, especially in cereal grazing systems, where they obviously lost to ungulates. In addition, herbaceous forms of flowering plants form turf, which reduces erosion and organic runoff into fresh waters and the oceans, which has dealt a severe blow to the communities of marine invertebrates.
This is because the vast majority of the creatures that inhabited the Earth in the Late Cretaceous have advanced too far along the path of narrow specialization. For the time being, this gave them excellent chances of survival, but any dignity sooner or later turns into a disadvantage. Attachment to the communities of gymnosperms eventually played a cruel joke with the lizards: when the flowering plants moved on the offensive, taking away one territory after another from the previous owners of life, mammals easily joined the newly formed communities. But dinosaurs could not do this and found themselves in an evolutionary dead end, because their adaptive resources were wasted long ago. And for mammals-marginalized, such a turn of events was only on hand. Having survived an explosion of speciation under the new conditions, they populated the entire planet.
Of course, it is not only such large taxa as a class of animals or a type of plant that can be marginalized. Separate biological species also, as a rule, do not sin with complete uniformity across the entire set of traits. Moreover, the higher the genetic diversity of a species or population, the greater their adaptive potential. Such a community will almost always find a way to prolong its existence in a changed environment. And even with a stable and measured life, intraspecific marginals can play an important role.
For example, in populations of wingless water striders, winged individuals are occasionally found. There are very few of them - only 4%. They have genetic differences, but at the same time they can interbreed with their wingless companions and give offspring. It turned out that these volatile geeks are capable of migrating over very long distances, thus ensuring genetic continuity between the water-loving population of all water bodies. Four percent of the marginalized is more than enough to accomplish this task.
I must say that almost every biological species has, just in case, such an emergency reserve in the form of a rare genotype or an unusual shape, which allows it to survive difficult times. We repeat once again: the genetic diversity of a species or population is the key to their evolutionary success, so the marginal should be treated not only with respect, but also with care.
So, the emergence and widespread distribution of flowering plants at the end of the Early Cretaceous (about 30 million years before the death of the dinosaurs) not only radically changed the structure of continental communities, but also destroyed the dinosaurs that had lost their plasticity, hopelessly stuck in the dead ends of evolution. Of course, climatic perturbations could also have played a role, but the key event, the starting point was almost certainly precisely this fact - the onset of angiosperms.
V. Levitin