Cretaceous period time. Cretaceous period, Cretaceous era, Cretaceous period of the Mesozoic era, Cretaceous dinosaurs, Cretaceous dinosaurs. Divisions of the Cretaceous period, geographical features and climatic changes

The Cretaceous period is the last era, ending the Mesozoic era. It replaced the Jurassic, according to geologists, somewhere around 145 million years ago and lasted approximately eighty million years, after which another Tertiary period began, the “era of new life.” This rather long stage of the Earth’s development received its name due to the fact that it left us a legacy of powerful deposits of chalk, marl and sand. Although during these eighty million years no planetary-scale catastrophes and, consequently, extinctions occurred on Earth large quantity species of plants and animals, yet the movement of tectonic plates, changes in the level of the world's oceans and climate change have made amendments to the process of evolution of living beings.

The Cretaceous period is usually divided into subsections: Lower and Upper Cretaceous. To understand how life developed in the seas, on land and in the air of that time, it is necessary to briefly characterize the tectonic mountain-building processes that took place, starting from the Jurassic stage. During the Lower Cretaceous, Gondwana and Laurasia continued to move away from each other. Exactly the same process occurred with Africa and South America. Thus, it increasingly took on the outlines familiar to us now. But in the east, Gondwana connected with Laurasia. Australia was where it is today, but only a third of its current territory rose above the water.

The Upper Cretaceous is characterized by the fact that the level of the world's oceans began to rise, and huge areas of Eastern Europe, Western Siberia, all of Arabia and almost all of modern Canada were under water. However, by the end of the Cretaceous period, the Earth began to resemble the modern globe in its outline.

During the Cretaceous period, the climate also underwent changes. It was, of course, much warmer than the modern one. The spaces of today's Europe were covered by real tropical jungles. However, in high latitudes The seasons were already changing, and snow was falling in winter. This gave impetus to the fact that, along with spores and gymnosperms, angiosperms appeared. Trees such as beech, birch, ash and walnut, having appeared in the Cretaceous era, survived until today without changes. The earth acquired its first flowering plants - first magnolias, then roses. Flowering plants had the advantage that their pollen was carried not only by the wind, but also by insects. Fruit plants, hiding the seed in the fruit, spread with the help of animals that ate the fruit. Thus, fruit and flowering plants filled the entire planet.

Changes in the flora during the Cretaceous period also led to the emergence of new species of fauna. The first butterflies began to flutter in the air and bees began to fly, feeding on the nectar of flowers. The sea is dominated by foraminifera, whose dead and crumbled shells gave the name to this entire geological time. Along with them, other ammonite mollusks appear. Sharks and Animals dominate the fish kingdom Mesozoic era- first of all, dinosaurs and the first mammals - safely “migrated” from the Jurassic period to the Cretaceous. But throughout the Cretaceous, several dead-end branches of bird-like dinosaurs died out, for example, Archeopteryx. But birds appear - the ancestors of modern geese, plovers, ducks and loons.

(the Jurassic period in particular), judging by the famous film, is also known as the era of dinosaurs. In general, the dominance of ancient lizards continues in the Cretaceous. But during the last period, the stegosaurus disappeared from the face of the earth, and its niche was occupied by the tyrannosaurus. The rich flora contributed to the emergence of new species of Triceratops, Iguanodons, Ankylosaurs and others. We can say that during the Cretaceous era, the species diversity of dinosaurs reached its peak. And at this time, hiding from the giants in their burrows, the future rulers of the Earth - mammals - lived. These rat-like animals rarely reached one meter in length; most species were small oviparous, armored or marsupials, up to 500 g in weight. But they were the future.

During the transition to the Middle Cretaceous period, the most important changes occurred in the plant world - the first flowering plants appeared. At the same time, the evolution of huge herbivorous dinosaurs continued.

The first flowering plant, Archaefmctus ("ancient fruit"), is known from rocks of the Lower Cretaceous. Its fossils were found in the Chinese province of Liaodun (in honor of which it received its name - Archaefruclus liaoningensis) 400 km north of Beijing, in an area that 140 million years ago was covered with swampy forest. The fruits of Arcbaefructus bore little resemblance to the fruits of modern plants; they looked more like a pair of leaves wrapped around the seeds, however, the presence of a shell surrounding the seeds - main feature flowering (angiosperm) plant. Determining the age of the rocks containing these fossils causes some difficulties. While some paleontologists believe that they are no more than 120 million years old, others estimate their age at 140 million years. In any case, Archaefruclus is the oldest known flowering plant.

Among the finds of plant fossils from the Late Cretaceous period, especially in regions located at high latitudes with a temperate climate, flowering plants account for 50 to 80%.

A fossilized magnolia leaf found in Upper Cretaceous rocks in Saxony, Germany. Reconstruction of the plant shows that it was very similar to Magnolia (Magnolia grandiflora), a favorite of gardeners.

The increase in the number of flowering species was accompanied by a decrease in the diversity of cycads and ferns, while the proportion of species coniferous plants was relatively constant in the local flora. However, in terms of biomass produced, the main components of terrestrial plant ecosystems at this time remained conifers, ferns and cycads.

Co-evolution?

In the 1970-80s. theories appeared in which the flourishing of angiosperms was associated with an increase in the number of herbivorous dinosaurs. It was stated that "flowering plants were spread by dinosaurs." The idea was that damaged flowering plants nowadays recover faster and more efficiently than gymnosperms (conifers and ferns). In the Cretaceous, the role of modern large cattle, the grazing of which sometimes almost completely destroys the vegetation cover, played by large herbivorous dinosaurs that consumed great amount plant food. Under such conditions, the increased resistance of flowering plants to damage gave them great advantages over gymnosperms.

However, recent studies in England have shown that the basic assumptions of these theories are unfounded. Firstly, the distribution of angiosperms did not coincide in time with maximum number herbivorous dinosaurs feeding on low-growing plants, and secondly, the geographical distribution of these tank-like or bulldozer-like animals did not coincide with the zones of origin and species diversity of flowering plants. Moreover, these theories assumed the dominant position of angiosperms in the plant world of the beginning of the Late Cretaceous period, which is also not true.

The Triceratops depicted in the picture fed on young shoots of plants and most likely led a gregarious lifestyle. Its terrifying horns and bone collar covering its neck provided reliable protection from any predator. These animals reached 7 m in length.

A large diversity of species of a group of plants does not automatically mean its important role in the flora of a given region. For example, the orchid family is now unusually diverse. But in any region where orchids grow, they are found as individual plants and constitute a negligibly small part of the biomass of the local ecosystem. Therefore, it is unlikely that during the Cretaceous period any species of herbivorous dinosaurs, let alone their entire community, fed only on a variety of but rarely found angiosperms.

Social insects

The oldest fossilized remains of termites and ants date back to the late Cretaceous period. The appearance of these insects should have significantly influenced the development of both flora and fauna. This is an important and interesting point in evolution, since it is believed that the body structure of some fossil animals, including small dinosaurs, allowed them to tear apart termite mounds in search of food. But, firstly, some of these animals existed before the appearance of social insects. And, secondly, the fossilized remains of the first social insects do not indicate their life in large communities immediately after their emergence. They became a significant source of food for large animals only after they began to create huge colonies. Nowadays, large animals such as anteaters, aardvarks and aardwolves feed on them.

The emergence of flowering plants undoubtedly accelerated the evolution and complicated the organization of communities of such social insects as bees, although finding out the details of the evolution of these small and fragile creatures is a rather difficult task.

Beginning of separation

By the beginning of the Cretaceous period, fossil remains of tetrapods (which include all vertebrates except fish) begin to show increasing differences between the fauna of the Northern and Southern Hemispheres, although limited exchange of land animals between them continued. The major change in the fauna of the Northern Hemisphere of this era was a decrease in the number and number of species of giant herbivorous sauropods that fed on the leaves and shoots of tall plants.

Along with these herbivorous giants, in the Early Cretaceous period the number of stegosaurs also decreased significantly, which, judging by their structure, were also herbivores and fed on shoots and leaves growing at low and medium altitudes. The slow decline in their numbers was accompanied by the spread of another type of large herbivorous dinosaur - the covered dinosaur. strong shell quadrupedal ankylosaurs that reached 6 m in length and weighed, according to estimates, up to 3 tons.

Although they, like stegosaurs, occupied the ecological niche of "herbivores that feed on short plants", their wide, massive skulls were radically different from the long, low, small-toothed skulls of stegosaurs. The heads of ankylosaurs were almost completely covered (even the eyelids) by a shell. But, despite the complex structure of the skull, the teeth of the ankylosaur differed little from the teeth of the stegosaurus. The peculiarities of their abrasion made it possible to establish how ankylosaurs crushed food, and to conclude that, most likely, they ate roots, tubers and the core of plants. Differences in feeding habits explain why these two species of herbivorous dinosaurs, occupying almost the same ecological niche, could coexist for a long time. It is also possible that they ate different types of plants.

The gentle giants of the iguanodon settle down for the night. They reached 9 m in length and up to 5 m in height. Next to them was a group of small reptiles called Hypsilophodon. Speed and dexterity helped the “little ones” (their size did not exceed 70 cm) survive.

North and South

Giant sauropods continued to dominate the south during this time, and the dominant herbivorous ornithopods in the Northern Hemisphere, such as hadrosaurs ("duck-billed dinosaurs"), were quite rare here.

A feature of the Cretaceous period was the very rapid spread of herbivorous dinosaurs from the suborder of orthithopods in the Northern Hemisphere: hadrosaurs, iguanodons (Iguanodon) and tenontosaurs (Tenontosaurus). At this time they achieved significantly large sizes than their Jurassic predecessors (e.g. Camptosaurus), and probably therefore foraged at higher levels.

Tyrannosaurus ( Tyrannosaurus rex), overtaking prey. The largest of all terrestrial predators that ever existed, it reached 13 m in length and rose 5 m above the ground. It probably used its disproportionately short forelimbs to rise from a lying position. Remains of a Tyrannosaurus rex discovered in the USA. Similar creatures They also lived in Canada and China.

In these ornithopods, there is a clear evolutionary tendency towards a more complex mechanism for chewing food. Their teeth interlocked with their bite, allowing for efficient grinding of hard plant foods. The peculiarities of the connection of the cranial bones of iguanodons allowed the upper jaw to move slightly forward under the pressure of the teeth of the lower jaw. Unlike mammals (such as the camel), reptiles could not chew because they did not have jaw muscles that move the lower jaw laterally. However, the described structural features of ornithopods allowed them to grind food quite well with a longitudinal displacement of the jaws, which probably became one of the main reasons for their wide distribution throughout the Cretaceous period.

Other advanced herbivorous dinosaurs (not belonging to the suborder Ornithopod) appeared in the Late Cretaceous, and in many respects their jaws were even more developed than those of Iguanodons. These were the so-called. horned dinosaurs, or ceratopsians. The first ceratopsians were apparently the bipedal psittacosaurs from the Early Cretaceous of Mongolia and the massive, pig-like protoceratopsians from slightly later rocks. These were massive animals with short limbs and a protective collar around the neck formed by overgrown skull bones (such a collar was absent in psittacosaurs).

Closely related to them were the pachycephalosaurs (“thick-skull lizards”) with massive and durable skulls. IN mating season males used their heads as weapons in fights with rivals. Their descendants, for example, the huge Triceratops, are typical dinosaurs last days prosperity of these huge reptiles.

During the Late Cretaceous period, there was an established and extremely diverse community of herbivorous dinosaurs of all shapes and sizes, which served as prey for the numerous predators of the era. Among the latter there were those who could hunt the largest herbivores.

Animals such as Trodden weighed no more than modern dog, while the mass of the largest of the carnivorous dinosaurs, the giant tyrannosaurus (Tyrannosaurus rex), according to most scientists, reached 7 tons (according to other estimates, 4 tons). The diversity of dinosaurs' feeding habits and the way they obtained food in this era is amazing. In the late Cretaceous period, at the last stage of dinosaur development, their most progressive forms arose.

Cretaceous tectonics:

During Cretaceous period continental movement continued. Laurasia and Gondwana were falling apart. Africa, India and Australia also began to diverge different sides, and giant islands eventually formed south of the equator. South America and Africa moved away from each other, and the Atlantic Ocean became wider and wider. Some obvious disasters in Cretaceous period there was none, so the process of evolution proceeded naturally. The earth took on shapes very close to those known to us.

Climate Cretaceous period:

The climate has changed compared to the Jurassic period. Due to the changing position of the continents, the change of seasons became more and more noticeable. Snow began to fall at the poles, although there were no such ice caps on Earth as there are now. On different continents the climate varied. This caused differences in the development of flora and fauna in various parts Sveta.

Flora Cretaceous period:

Flora Cretaceous period was rich and varied. In addition to the plant species carried over from the Jurassic period, a new, revolutionary branch of flowering plants appears. Flowering plants, having entered into an “alliance” with insects, had advantages over their predecessors. Thanks to this partnership, flowering plants spread much faster. Gradually populating the land, new groups of plants began to form vast forests. There, a wide variety of leaves and other edible vegetation were available to land animals. Thanks to the emergence of flowering plants in Cretaceous period the amount of plant biomass has increased.
The reverse process took place at sea. This was again facilitated by the development of flowering plants. Dense roots prevented soil erosion and therefore less minerals entered the sea. The amount of phytoplankton has decreased.

Fauna Cretaceous period:

Insects:

Growth of flowering plants Cretaceous period contributed to the increase in species of insects feeding on nectar and spreading pollen. Exactly at Cretaceous period. Insects have appeared whose life is completely dependent on flowering plants. These are bees and butterflies. Insects collected pollen and transported it to its destination. The brightly colored petals and attractive aroma of flowers became a bait for insects. In turn, the sweet sugary nectar, and the pollen itself, supplied the insects with everything they needed. nutrients. Cretaceous period marked the beginning of an era of close interaction between plants and insects.

Dinosaurs:

Among the land animals, a variety of dinosaurs reigned. During the Cretaceous period The variety of dinosaur species was especially great. The development of the plant world and the increase in plant biomass gave impetus to the emergence of new species of herbivorous dinosaurs.
Of the lizard-hipped dinosaurs, the most famous of which was tyrannosaur, were widespread Tarbosaurus, spinosaurus, Deinonychus and others.
The diversity of ornithischian dinosaurs was particularly high during the Cretaceous period. Widely known in Jurassic period, stegosaurs, will disappear from the face of the planet. Their place will be taken by such famous herbivorous dinosaurs as iguanodons, Triceratops, ankylosaurs, pachycephalosaurs and many other types.

First mammals:

The first beast-like animals appeared in Triassic, about 220 million years ago. These animals belong to the so-called synapsid group.
In the first half Cretaceous period, among these inconspicuous mammals, against the backdrop of dinosaurs, serious things began to happen evolutionary processes. As a result, these processes resulted in the appearance of monotreme marsupials and placental mammals. It is to these groups of animals that in the end Cretaceous period and the beginning of the Cenozoic era were destined to become the successors of dinosaurs.

The vast majority of Cretaceous synapsids are mammals. Primitive dicynodonts and cynodonts have not yet become extinct, but are already close to it. Almost all mammals Cretaceous period belonged to the primitive subclass of allotheria and differed little from their Jurassic predecessors. These were small creatures weighing 20-500 g, similar to mice. Among them there were repenomamas reaching 1 m in length and weighing up to 14 kg, but most were as small as other mammals of the Cretaceous period.

At first Cretaceous period True animals, the ancestors of modern mammals, separated from allotheria. They quickly divided into three main branches: oviparous, marsupial and placental mammals, with placentals already divided into Laurasiatherians, Gondwanatherians, and the latter divided into rodents and primates. The marsupial branch gave rise to almost modern opossums, and the oviparous branch gave rise to almost modern platypuses. The first known primate-like mammal was Purgatorius.

Flying:

Winged reptiles - pterodactyls - occupied almost all niches of aerial predators. Cretaceous period gave birth to the largest flying creatures that ever lived on Earth. These are the giant Orcheopteryx and Quetzatcoatlus. To date, the question of which of them was larger has not been finally resolved.

But during the Cretaceous period, pterosaurs had competitors - birds. And although the first birds appeared in the Jurassic period, Cretaceous period, the diversity of their species has increased. A transitional species between pterosaurs and birds, Archiopteryx became extinct. Thus, flying lizards and birds existed in parallel.
Some Cretaceous birds are ancestors modern birds. Already in Cretaceous period ducks, geese, loons and plovers appeared, almost no different from the modern versions of these birds. Many of the birds Cretaceous period were a dead-end branch of evolution and subsequently became extinct. Bird classification Cretaceous period very vague and contradictory.
The size of the Cretaceous birds varied from 4 cm to 1.5 m in length, and weight - from several grams to several kilograms.

Marine fauna:

There were no mammals in the seas, but a niche large predators occupied by reptiles - ichthyosaurs, plesiosaurs, mososaurs, sometimes reaching 20 meters in length.
Among the inhabitants of the Cretaceous seas, the majority were plesiosaurs with long necks and small heads that fed small fish and shellfish. They could not swim quickly, but were very maneuverable, and their small head on a very long neck made it difficult for them to be detected in a timely manner by a school of prey - the fish saw only a small head, and huge body was lost in the distance. A bright representative This species was an elasmosaurus up to 20 m long and weighing 14 tons.

Another species that lived in the seas Cretaceous period there were mosasaurs. Mosasaurs are very large predatory, sea lizards that reigned in the Cretaceous seas. They replaced the saltwater crocodiles of the Jurassic period. These were very aggressive animals - many mosasaurs have traces of healed fractures and bites on their bones, apparently received in fights with their own kind.

Turtles Cretaceous period practically no different from modern ones. The sizes of Cretaceous turtles varied from 20 cm to 4.6 m, weight reached 2 tons. Most species led an aquatic lifestyle.

Other reptiles:

IN Cretaceous period The first lizards and snakes arose, so snakes. They have survived to this day practically unchanged. This is a relatively young group of animals.

All dinosaurs of the Cretaceous period

Herbivorous dinosaurs:

Sauropods: abydosaurus ... Agustinia ... alamosaurus ... amargosaurus ...

ampelosaurus ... aragosaurus ... argentinosaurus ... Egyptosaurus ... lapatasaurus ...

maxcalisaurus ... Nigersaurus ... paralititanium ... saltasaurus ... seismosaur ...

Thierophorans, ankylosaurids: acanthopholis ... aletopelta ... ankylosaur ...

minmi ... nodosaur ... Scolosaurus ... styracosaurus ... talarurus ... euplacecephalic

Ceropods: avaceratops ... agathamus ... adasaurus ... adamantosaurus ...

ankyceratops ... barylium... hypselospin ... Gypselophodon ... zalmoxis ...

iguanodon ... Zuniceratops ... Coahuilaceratops ... leptoceratops ...

Medusaceratops ... monoclone ... muttaburrasaurus ... ochoceratops ...

Pachyrhinosaurus ... protoceratops ... Psitaccosaurus ...Stegoceras ... Torosaurus ...

Treceratops ... chasmosaurus ...

Hadrosaurs: anatotitan (anatosaurus)... brachylophosaurus ... hadrosaurus ...

saurolophus ... Corythosaurus ... lambeosaurus ... Mayasaurus ... parasaurolophus ...

probactosaurus ... tenodontosaurus ... Ouranosaurus ... Edmontosaurus ...

Pachycephalosaurs: Dracorex ... pachycephalosaurus ... Stegoceras ... techacephalus

Carnivorous dinosaurs:

Theropods: abelisaurus ... avimim ... australovenator ...

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Cretaceous period is the last of the three periods that make up the Mesozoic era. Beginning 144 million years ago, it lasted almost 80 million years and ended 65 million years before the present. Its name comes from the abundance of writing chalk, formed from dying invertebrate organisms, in its sediments. The Cretaceous period is significant for the second largest worldwide extinction of species (after the Permian).

Divisions of the Cretaceous period, geographical features and climatic changes

In 2016 International Union geological sciences accepted the following Cretaceous division:

The lower section is divided into the Berriasian, Valanginian, Hauterivian, Barremian, Altian and Altian stages;
The upper section is divided into the Cenomanian, Turonian, Cognacian, Santonian, Campanian and Maastrichtian stages.

Cretaceous period (Cretaceous)	Departments	tiers
	Lower	Berriasian
		Valanginian
		Goterivsky
		Barremsky
		Altsky
		Albian
	Upper	Cenomanian
		Turonian
		Cognac
		Santonsky
		Campanian
		Maastrichtian

During the Cretaceous period, the division of Laurasia into the North American continent and the Euro-Asian continent continued. Gondwanaland finally split into the South American continent, the African, Indian segments, Antarctica and Australia. Throughout the Cretaceous, these gigantic land areas diverged more and more from each other, the southern and northern parts of the Atlantic Ocean were no longer connected by narrow straits, but acquired a solid oceanic structure. But despite this, a significant part of Europe, the Middle East, the Caucasus and northern Africa were still under water until the very end of the Cretaceous period.

Cretaceous climate compared to the previous Jurassic it became noticeably colder. At first, its average temperature across the planet dropped by 5 degrees, which led to the formation of polar ice caps, but after some time the climate warmed again, and in general the entire planet was relatively warm, with winter temperatures even in the coldest zones globe on average fluctuated within +4°C. By the end of the period, the greenhouse effect caused by side factors led to an even greater and sharp increase in temperature.

Sedimentation

The Cretaceous period is characterized by the maximum flysch accumulation in geosyncal areas in the entire history of the Earth. As a result of violent magmatism caused by the splitting of continental regions, siliceous and split-dibasic formations were formed, and granitoid emissions were extensive and colossal. In general, the accumulation of trigenic and volcanogenic strata was widespread during the Cretaceous period. Such rift zones appeared in Africa and Brazil. Huge layers of writing chalk accumulate in the depths of the sea.

Animals of the Cretaceous period

The most significant marine invertebrates during the Cretaceous period were cephalopods. In the Upper Cretaceous, the role of outer shells (ammonoids) slightly decreased, but intrashells (belemnites) were fundamental until the very end of the period. Closer to the middle, some ammonoids, for example, such as ammotoceras, reached 2 meters in size.

Molluscs such as pelecypods (bivalves) and gastropods (gastropods) also developed widely. Most bivalves would become completely extinct by the end of the Cretaceous. Incorrect ones have also developed sea urchins along with large foraminifera.

Felt great and Cretaceous insects. Most of them, having adapted to the current flowering plants, were forced to change themselves due to biological changes in vegetation, but in general, the species of both flying and crawling insects steadily progressed. All kinds of worms also felt great.

IN coastal seas and oceanic zones, the first lobsters and other predatory crustaceans such as crabs and shrimp appeared.

Rice. 1 - Dinosaurs of the Cretaceous period

Vertebrates animals of the Cretaceous period stood out in that among them, as in the Jurassic period, reptiles reigned supreme (Fig. 1). Among them were crawling creatures, walking on four limbs, moving only on two hind limbs, waterfowl and, of course, flying Hymenoptera. The richness of their diversity and forms was truly amazing. This entire numerous army of reptiles constantly devoured both huge masses of green space and themselves, meanwhile increasing in numbers until, in an incomprehensible way, in the upper Maastrichtian stage of the Cretaceous period it became almost completely and universally extinct.

The first snakes appeared (Fig. 2). Some grew to truly gigantic size and hunted mainly in the aquatic environment, in coastal or river basins. It was not difficult for some of them to wrap around and crush or strangle a gaping one and a half meter raptor.

Rice. 2 - Cretaceous Snake

The variety of flying dinosaurs was also great. The real giant was the pteradon, whose wingspan averaged 8 meters. These giant reptiles hunted primarily over the sea, easily diving into air currents and every now and then, snatching fish and other representatives of marine fauna from the water.

Birds also developed widely, the first varieties of which appeared in the Jurassic period. In the Cretaceous period, highly organized and specialized formations appeared among them.

And in the depths of the sea, fish with a hard-boned skeleton developed further. The ray-finned offspring of the Triassic and Jurassic multiplied unusually, a huge number of new varieties appeared both among the inhabitants of freshwater and inland basins, and among salty marine and oceanic species (Fig. 3).

Rice. 3 - Marine animals of the Cretaceous period

Despite the undivided dominance of reptiles, mammals still advanced in their evolutionary development in the Cretaceous period. Having appeared on the threshold of the Mesozoic, these beast-like animals (synapsids) slowly but surely waited in the wings throughout the entire era, increasingly adapting to the difficult life in the background. Synapsids often settled in cold areas of continents, where predatory but heat-loving reptiles were rare guests. Those who were forced to live among reptiles in hot areas went out to hunt mainly at night. All this greatly contributed to their adaptation to difficult conditions, which determined the mammals’ survival in the difficult conditions of the asteroid winter that struck the Earth at the end of the Cretaceous.

All synapsids were divided into three main varieties - dicynodonts, cynodonts and allotherians. Dicyodonts and cynodonts almost completely went extinct during the Cretaceous period, and allodonts developed into mammals. During the Late Jurassic and subsequent Cretaceous period, they clearly divided into three branches - oviparous, marsupial and placental. Oviparous animals, unable to withstand competition with marsupials and placentals, soon also disappeared; today marsupials survive only in Australia, and it is from placentals that all subsequent species of modern mammals developed. Placentals at that time were divided into Laurasiatherians and Gondwanatherians. It was Gondwanotheria that were the ancestors of modern rodents and primates.

From the marsupial branch, possum-like animals evolved, and from the oviparous branch, only platypuses remain today. The ancestor of primates is considered ancient mammal purgatorius.

Mostly mammals of the Cretaceous period(Fig. 4) weighed no more than half a kilogram and rarely exceeded the size of a modern rat. There were, of course, such rare specimens as meter-long and fourteen-kilogram repenomamas, but they were too few in number.

Rice. 4 - Mammals of the Cretaceous period

For the most part, reptiles owe their extinction to these small creatures, which, having multiplied unusually towards the end of the Cretaceous period, fed mainly on insects, but did not disdain the eggs of reptiles.

Despite the fact that the first flowering plants began to appear long before the Cretaceous period, it was at this time that the formation of flowering vegetation entered a real boom stage. It is no coincidence that half of all currently existing known plants are flowering plants. And this is connected with this.

By spreading spores in the wind, primitive plants took a great risk. And not in vain, since the bulk of the disputes never achieved their intended goal. And many plants of that era had not yet acquired at least some varieties of spore spraying mechanisms. Their spores were forced to fall to the ground, right in the same place where the plants themselves grew. It is clear that with such reproduction it was not possible to achieve a more or less reliable result. Hence the vital need to develop new, more effective techniques pollen spread. And insects came to the aid of plants.

A kind of union began to develop and become stronger between the flower groups. While insects carried pollen from plants, plants produced nectar for them so that they would work more intensively on pollination. In the process of evolution, it turned out that many insects simply could no longer do without flowering plants, since their entire lives and body biology were inextricably linked and aimed at a life associated with such plants. And the plants, with the help of their insect helpers, began to multiply many times faster, and soon dense vegetation spread even to those areas of land where it had never existed before. This type of partnership between plants and insects continues to this day.

Rice. 5 - Plants of the Cretaceous period

Underwater Cretaceous plants were in many ways similar to plants of previous periods of the Mesozoic. The only difference was that microscopic algae such as nanoplankton (for example, golden coccolithophores) and diatoms multiplied unusually. It is nanoplankton and small foramnifera that are responsible for the formation of such thick layers of writing chalk.

By the end of the Mesozoic era, the flora of the land had undergone a number of significant changes. From the middle of the Cretaceous period, the first angiosperms began to appear, which towards the end of the Cretaceous already constituted the overwhelming majority among terrestrial plants. The first varieties of plants with foliage of increased succulence began to appear. This most applies to places where the climate has become more arid and hot.

What happened at the boundary of the Mesozoic and Cenozoic, or more precisely, in the Maastrichtian - the final stage of the upper section, Cretaceous species extinction was the second largest after the Permian. Coccolithophores ceased to exist overnight, and Cretaceous planktonic foramonifers, ammonites, belemnites, and coral-like bivalves- rudists. Dinosaurs and many other species of reptiles disappeared from the face of the earth. Many species of birds and insects, both above and underwater, ceased to exist. In particular, the total number of all kinds of rliolarians decreased by 50%, 75% of all brachiopods, from 30 to 75% of bivalves and gastropods, sea lilies and urchins. From total number Only 25% of sharks remain. More than 100 different families of marine invertebrates have become extinct. In general, the damage suffered by flora and fauna was truly enormous.

What was the reason for such massive extinction of species during the Cretaceous period still not known. Scientists' opinions on this matter are divided. Opinions have also been expressed that powerful cosmic radiation generated as a result of a supernova explosion reached the Earth. Some people talk about a strong greenhouse effect associated with an extremely aggravated volcanic activity. But most are in favor of the version that is based on falling to the ground giant asteroid(Fig. 6). This version is confirmed by the presence of iridium inclusions in the strata of this era, which is constantly found in places where meteorites fall.

Rice. 6 - Asteroid impact

It is alleged that an asteroid with a magnitude of 10 to 15 km entered the earth's atmosphere, split into several segments, which collided with the earth's surface. The explosive energy, amounting to approximately 10 to the 30th erg, raised a lot of pollutants from the earth's crust, which for a long time blocked plants and animals from accessing sunlight. Thus, as a result of the unique “asteroid winter” that was created, most terrestrial animals became extinct. Apparently, this did not have such an impact on the plant world because the atmosphere cleared in a relatively short period of time. And if the plant seeds were able to safely survive this catastrophe in the soil and soon simply sprouted as if nothing had happened, then animal world The Cretaceous period could not endure this global catastrophe with such ease. And as a result, only the most adapted and more tenacious species survived, such as, for example, mammals.

Minerals of the Cretaceous period

The Cretaceous period is unusually prolific in various types of minerals, most of which arose as a result of intrusive magmatism and volcanism, which accompanied the worldwide division of Pangea into smaller components. About 20% of coal deposits were accumulated during this time. The largest coal basins of this time are the Lensky and Zyryansky, as well as a number of North American coal basins.

Also associated with the Cretaceous period are most Russian, French and Spanish bauxite deposits, Western Siberian oil and gas fields, and oil and gas fields of Kuwait and Canada. Vast oolitic deposits were discovered on the territory of Western Siberia. iron ores. There are also numerous phosphate deposits in the territories of Russia, Morocco, and Syria. Extensive salt deposits have been found on the territory of Turkmenistan and in some North American regions. In the northeast of Russia, in the territory North America deposits of tin, lead and gold were discovered. The famous Indian and South African diamond deposits also date back to this period.

Writer's chalk was found almost everywhere in Cretaceous sediments.

Which is extracted from sedimentary deposits of this period, formed by rich accumulations of fossilized invertebrate marine organisms.

Cretaceous divisions

system	Department	tier	Age, million years ago
Paleogene	Paleocene	Danish	less
Chalk	Upper	Maastrichtian	72,1-66,0
		Campanian	83,6-72,1
		Santonsky	86,3-83,6
		Cognac	89,8-86,3
		Turonian	93,9-89,8
		Cenomanian	100,5-93,9
	Lower	Albian	113,0-100,5
		Aptian	125,0-113,0
		Barremsky	129,4-125,0
		Goterivsky	132,9-129,4
		Valanginian	139,8-132,9
		Berriasian	145,0-139,8
Yura	Upper	Titonian	more
Divisions are given according to IUGS as of December 2016

The Aptian anoxic event (Selli Event or OAE 1a) occurred approximately 120 million years ago. About 116 million years ago, the average temperature on the planet dropped by 5 °C, and global cooling lasted more than a million years. Then warming began again - the volcanoes of the Indian Ocean began to pump carbon into the atmosphere. Global warming led to the depletion of oxygen in ocean waters, which 94 million years ago led to the “anoxic catastrophe” and the extinction of ichthyosaurs that did not adapt to climate change.

Approximately 91.5 ± 8.6 million years ago, the Cenomanian-Turonian biotic boundary event occurred, leading to the complete extinction of ichthyosaurs and pliosaurs, the families of Megalosauridae and Stegosauridae, and greatly reduced the species diversity of other groups of animals.

70 million years ago the Earth was cooling. Ice caps formed at the poles. The winters became harsher. The temperature dropped in some places below −10 degrees, and in Alaska - down to −45. For dinosaurs of the Cretaceous period, this difference was sharp and very noticeable; more cold-loving species became more widespread. Such temperature fluctuations were caused by the breakup of Pangea, and then Gondwana and Laurasia. Sea levels have risen and fallen. The jet streams in the atmosphere have changed, causing ocean currents to change.

Vegetation

In the Cretaceous period, angiosperms - flowering plants - appeared. This led to an increase in the diversity of insects, which became flower pollinators. As temperatures rose during the late Cretaceous period, plants with richer foliage evolved.

Animal world

Among the land animals, a variety of large reptiles reigned. This was the heyday of giant lizards - many dinosaurs reached 5-8 meters in height and 20 meters in length. Winged reptiles - pterodactyls - occupied almost all the niches of aerial predators, although real birds had already appeared. Thus, flying lizards, lizard-tailed birds such as Archeopteryx, and true fan-tailed birds existed in parallel.

At the end of the period, snakes spread.

There were no mammals in the seas, and the niche of large predators was occupied by reptiles with a comparable level of metabolism - ichthyosaurs, plesiosaurs, mosasaurs, sometimes reaching 20 meters in length.

The diversity of marine invertebrates was very large. As in the Jurassic, ammonites and belemnites, brachiopods, bivalves and sea urchins were very common. Among bivalves, an important role in marine ecosystems played by rudists that appeared at the end of the Jurassic - mollusks similar to single corals, in which one valve looked like a cup, and the second covered it like a kind of lid.

By the end of the Cretaceous period, many heteromorphic ones appeared among ammonites. Heteromorphs arose earlier, in the Triassic, but the end of the Cretaceous became the time of their mass appearance. The shells of heteromorphs were not similar to the classic spiral-twisted shells of monomorphic ammonites. These could be spirals with a hook at the end, various balls, knots, unfolded spirals. Paleontologists have not yet come to a common explanation of the reasons for the emergence of such forms and their way of life.

Orthoceras were still found in the seas - relics of the long-past Paleozoic era. Small shells of these straight-shelled cephalopods are found in the Caucasus.

Cretaceous disaster

At the end of the Cretaceous period, the most famous and very large extinction of many groups of plants and animals occurred. Many gymnosperms, aquatic reptiles, pterosaurs, and all dinosaurs became extinct (but birds survived). Ammonites, many brachiopods, and almost all belemnites disappeared. In the surviving groups, 30-50% of species became extinct. The causes of the Cretaceous disaster are not fully understood.

Notes

International Chronostratigraphic Chart v. 2019-05 (undefined) . International Commission on Stratigraphy (2019). Archived from the original on August 13, 2019.
N. M. Chumakov. Climate in eras of major biosphere restructuring. M: Nauka, 2004. - 299 p. Ch. 5. Climatic zonation and climate of the Cretaceous period.
Li, Yong-Xiang; Bralower, Timothy J.; Montañez, Isabel P.; Osleger, David A.; Arthur, Michael A.; Bice, David M.; Herbert, Timothy D.; Erba, Elisabetta; Premoli Silva, Isabella. Toward an orbital chronology for the early Aptian Oceanic Anoxic Event (OAE1a, ~ 120 Ma) // Earth and Planetary Science Letters (English) Russian: journal. - 2008. - July 15 (vol. 271, no. 1-4). - P. 88-100. - DOI:10.1016/j.epsl.2008.03.055.
Mid-Cretaceous “global cooling” studied, June 18th, 2013
The extinction of ichthyosaurs was explained by the slowness of their evolution, March 11, 2016
Head J.J. Fossil calibration dates for molecular phylogenetic analysis of snakes 1: Serpentes, Alethinophidia, Boidae, Pythonidae // Palaeontologia Electronica (English) Russian: journal. - 2015.
Caldwell M. W., Nydam R. L., Palci A., Apesteguía S.