Early Cretaceous. Mesozoic period
The duration of which is approximately determined to be ~80 million years (began ~145 million years ago and ended ~65 million years ago).
Flora and fauna
The fauna of the Cretaceous period had an appearance characteristic of the Mesozoic era, but at the same time was sharply different from the fauna of the Jurassic period. Among the invertebrates, new forms of belemnites and ammonites appeared in large numbers, and among the latter there were many representatives with an anomalous shell: stick-shaped, turret-shaped, etc. Some groups of elasmobranchs (rudists, inocerams, trigonians) and gastropods (nerineids) developed luxuriantly. Irregular sea urchins acquired significant development, and large foraminifera (orbitolins, orbitoids) appeared. Among vertebrates, the development of reptiles reached its culmination point, many of which acquired gigantic sizes. There was a flourishing of bony fish, which took a dominant position. Of the birds, only toothy ones existed. Mammals still played a modest role and did not reach great size. Among them, primitive placental forms appeared. Among fossil vertebrates, reptiles still occupy the leading place. Many large dinosaurs appeared on land. Of the aquatic lizards, plesiosaurs and snake-like mosasaurs were widespread, and to a lesser extent, ichthyosaurs, flying lizards, etc. Snakes appeared in the group of terrestrial reptiles. Cretaceous birds are represented by forms that still had teeth in their mouths, but had already lost features reminiscent of reptiles. The heyday of bony fish has arrived.
In the Early Cretaceous era, the flora resembled the Jurassic: conifers, ginkgo trees, cycads, and ferns continued to exist. At the same time, the first angiosperms (flowering plants) appeared, which quickly evolved and spread on the Cretaceous land. By the beginning of the Late Cretaceous era, angiosperms began to occupy a dominant position, and gymnosperms were relegated to the background. In the Cretaceous period, angiosperms - flowering plants - appeared. This resulted in an increase in the diversity of insects that became flower pollinators. The vegetation, preserving the Mesozoic appearance from the beginning of the period, from the Cenomanian century is characterized by the predominance of angiosperms flowering plants, the first signs of which were found in deposits of Hauterivian or even Valanginian age. All classes of plants of the Cretaceous period continue to exist to this day, but the ratio of angiosperm families has changed significantly.
At the end of the Cretaceous period, major changes occurred in the fauna: aquatic reptiles, dinosaurs, flying dinosaurs, toothed birds, ammonites, almost all belemnites and a number of genera and families of invertebrates became extinct. At this time, the most famous and very large extinction of many groups of plants and animals occurred. Many gymnosperms, all dinosaurs, pterosaurs, and aquatic reptiles became extinct. Ammonites, many brachiopods, and almost all belemnites disappeared. In the surviving groups, 30-50% of species became extinct. Whether this was caused by a planetary catastrophe, and if so, what were its causes and scale, remains unclear.
Tectonics and magmatism
During the Cretaceous period, the Mesozoic tectonic stage of development ends, which manifested itself especially rapidly along the outskirts of the Pacific segment of the earth's crust. The result of this was, first of all, the complete formation of Mesozoic mountain-folded structures (mesozoid) on the site of the Verkhoyansk-Chukotka and Sikhote-Alin geosynclinal regions in the Western Pacific geosynclinal belt, almost entirely in the Cordilleran geosynclinal region of the East Pacific belt and within the Tibetan geosynclinal region in eastern Mediterranean geosynclinal belt.
Extra-geosynclinal depressions complete their active tectonic development and platform granitoid magmatism ceases.
At the border of the Pacific geosynclinal belts and adjacent platforms, a structural zone appears in the form of linear large splits, along which the intrusion and outpouring of acidic magma occurs. This volcanic belt is called the Chukotka-Kathasian belt.
The orogenic stage of mesozoid development was accompanied by the formation of large marginal troughs (Pre-Verkhoyansk trough) at the border with the platforms.
Mountain building processes were accompanied by the intensive introduction of granitoid intrusions.
Intense tectonic activity in the Cretaceous period is not limited only to folding and magmatism. New major faults are being formed. They lead to the subsidence of vast areas of Gondwana. As a result, the Gondwanan continent breaks up into separate large blocks - South American, African, Indian, Australian and Antarctic, and between them the depressions of the Indian and South Atlantic oceans are completely formed. Similar processes occur on Angarid, which splits into two parts: Eurasian and North American; a depression in the northern part of the Atlantic Ocean is formed between them. Obviously, the formation of the Arctic Ocean depression is associated with the same time.
On the African and Hindustan
Cretaceous period
In the lower reaches of the Volga, in Ukraine near Kharkov and in other places, there are thick layers of white writing chalk.
Look at a grain of chalk under a microscope. You will see that half of it consists of tiny shells covered with holes and their fragments. Shell-dwelling foraminifera (“hole makers”) lived in the sea that covered these places 70–80 million years ago. And they inhabited the sea in such innumerable numbers that from the myriads of their shells, over time, the main sedimentary rock of this period was formed - chalk.
What do Cretaceous fossils tell us?
New species of ray-finned and bony fish appeared in the Cretaceous seas. Ammonites and belemnites lived in the same abundance as in the Jurassic period. But at the end of the period they began to die out.
A mosasaurus appeared in the seas.
Its snake-like body with two pairs of fins and a crocodile head reached a length of 13–15 meters. The fossilized remains of this sea lizard were found near the Meuse River in Western Europe. The Latin name of this river is Mosa. Mosasaurus is the "lizard from the Mosa River".
Like the ichthyosaur, this reptile hunted for fish.
Snakes first appeared during the Cretaceous period. Their flexible scaly bodies slid through the thickets with a slight rustle. Huge turtles basked in the sun on the sandbanks.
Dinosaurs were still the rulers of the land. New giants appeared among them. We see them in a painting hanging in one of the halls of the museum. It depicts the sea coast of the North Atlantic continent during the Cretaceous.
Cretaceous period. A tyrannosaurus tears apart a herbivorous lizard.
...It's getting evening. The sand, the edge of the grove, light clouds floating in the darkening sky - everything is illuminated by sunset fire.
A dying trachodont lies on the sand, its long neck stretched out, its duck-shaped mouth half-open. The last trembling runs through his 10-meter torso, pressed to the ground by the monster standing on him. This is a tyrannosaurus - a “killer lizard”. Indestructible power emanates from his 14-meter, massive, rock-like body. The fury of the fight still sparkles in the eyes. Huge half-meter claws dug into the body of the prey.
In the distance, at the edge of the forest, a herbivorous Triceratops (“three-horned lizard”), the size of a large elephant, froze in indecision. True, the predator is busy with its prey and the Triceratops itself is well armed: it has three huge, forward-pointing horns on its head and its neck - the most vulnerable place - is protected by a bone collar. But still, it’s better to quickly get away from the dangerous predator...
Triceratops.
The flying pteranodon (“winged toothless lizard”) flies over the clearing on huge leathery wings reaching 8 meters in span. These tailless flying lizards are already becoming extinct. Soon the last flying dragons will disappear and they will be replaced by various breeds of birds.
Pteranodon.
We see one of these ancient birds in the picture. This is a toothy ichthyornis, already reminiscent of modern birds in its structure.
Ichthyornis.
The dominance of lizards on Earth lasts for tens of millions of years. There seems to be no force that could resist their power, crush their powerful bodies. Dinosaurs felt equally at home on the sandy shallows of the seashore, in the swamps, and in the thickets of forests. But their bodies had one significant drawback: they were cold-blooded animals that could only live in warm climates. The cooling that occurred on Earth at the end of the Cretaceous period played a fatal role in the life of lizards.
In the Cretaceous period, a new mountain-building cycle began, the so-called Alpine orogeny. The lights of volcanoes sparkled on the coasts of the oceans, where new mountain ranges grew. The rising chains of mountains deprived the land of the beneficial influence of the sea winds.
The warm and humid climate, so favorable for cold-blooded reptiles, became increasingly cooler.
The cooling of the climate had a detrimental effect on the lizards. After all, reptiles, like fish and amphibians, do not have a constant body temperature. It depends on the ambient temperature. At low temperatures, reptiles become lethargic and fall into torpor.
The movement of the seas also played a significant role in the beginning of the extinction of lizards.
At the end of the Cretaceous period, the internal forces of the Earth raised the land in many places, forcing the sea to retreat.
The drainage of the swampy lowlands that stretched along the sea coasts sharply worsened the living conditions of the lizards. The sea retreated hundreds and thousands of kilometers, and wetlands began to dry out quickly.
The giant herbivorous lizards who lived here lost shelter and food. With difficulty moving their heavy bodies across the parched ground, exhausted by hunger, they easily became prey for predatory dinosaurs. Their mass death, in turn, led to the rapid extinction of the predators that fed on them.
By the beginning of the new, Cenozoic era, dinosaurs were no longer on Earth. But life did not stop in its development, it manifested itself in new, more perfect forms, and took new paths in its development.
Once again, a qualitative restructuring of the entire organic world of the Earth has begun in nature. Mammals became the winners in the struggle for existence.
While the giant reptiles flourished, these small animals, reminiscent of modern marsupial rats, shrews and hedgehogs, were few and far between and led an inconspicuous lifestyle. But now their time has come - the time of warm-blooded animals.
The changed living conditions immediately revealed the enormous advantages of mammals over cold-blooded reptiles.
The mole, fox, bear and other mammals have a constant body temperature: an average of plus 39 degrees, and is maintained at the same level by a number of devices. The lungs of mammals have a large volume and a large respiratory surface. Therefore, oxygen entering the lungs during breathing is immediately absorbed by the blood. Oxygenated blood moves quickly through the vessels, ensuring vigorous metabolic processes and generating large amounts of heat in the body. The hair and subcutaneous layer of fat protect the animal from excessive heat loss during the cold season.
The brain of mammals is more developed than that of lizards; teeth serve not only for capturing food, but also for chewing it. They give birth to live young and feed them with milk, caring for and protecting their offspring.
At the end of the Cretaceous period, a variety of mammals began to rapidly spread across the Earth.
An equally profound qualitative restructuring took place in the plant world.
Angiosperms, or flowering plants, the first forms of which appeared in the Jurassic period, developed rapidly and everywhere.
In angiosperms, the seeds are located inside the fruit, and the reproductive organ is the flower.
Among flowering plants, there are the largest number of species and amazing diversity of species. Angiosperms have extraordinary endurance and adaptability to environmental conditions. They penetrate deserts, where not a drop of rain falls for months, grow on soils saturated with salt, and inhabit cold tundras and the shores of northern seas, where frosts in winter reach 50 degrees. By the end of the period, forests of palm trees, magnolias, laurels, plane trees, oaks, and maples gradually covered the territory of the continents. Forests interspersed with grassy meadows.
The plains and mountains were full of flowers. Insects appeared in abundance. For the first time in its long history, wildlife dressed in a bright, flowery outfit.
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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 seeds, however, the presence of a shell surrounding the seeds is the main characteristic of a 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 coniferous plant species in the local flora was relatively constant. 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 period, the role of modern cattle, whose grazing sometimes almost completely destroys plant cover, was played by large herbivorous dinosaurs that consumed huge amounts of 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. First, the distribution of angiosperms did not coincide in time with the peak numbers of herbivorous dinosaurs feeding on low-growing plants, and second, the geographical distribution of these tank-like and 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 dinosaurs - four-legged ankylosaurs covered with a strong shell, reaching 6 m in length and weighing, it is estimated, 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). They reached much larger sizes at this time than their Jurassic predecessors (eg Camptosaurus), and probably therefore foraged at higher levels.
Tyrannosaurus rex catching 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 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. During the mating season, males used their heads as weapons in fights with rivals. Their descendants, for example, the huge Triceratops, are typical dinosaurs of the last days of the 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 a 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.
The Cretaceous period is a geological period. The Cretaceous is the last period of the Mesozoic era, began 145 million years ago and ended 65 million years ago. It lasted about 80 million years.
During the Cretaceous period, angiosperms—flowering plants—appeared. This resulted in an increase in the diversity of insects that became flower pollinators. So the plant cover of the Earth in the Cretaceous period would no longer surprise modern people. The same cannot be said about the animal world of that time.
Among the land animals, a variety of dinosaurs reigned. Dinosaurs are divided into two groups - lizard-hatched, which included both predators and herbivorous forms, and ornithischian, exclusively herbivorous. The most famous lizard-hipped dinosaurs are tyrannosaurs, tarbosaurs, and brontosaurs. Among ornithischian lizards, ceratopsians, iguanodons, and stegosaurs are known. 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.
Modern lizards and snakes evolved, so snakes are a relatively young group.
There were no mammals in the seas, and the niche of large predators was occupied by reptiles - ichthyosaurs, plesiosaurs, mososaurs, sometimes reaching 20 meters in length.
During the Cretaceous period, the continental breakup continued. Laurasia and Gondwana were falling apart. South America and Africa moved away from each other, and the Atlantic Ocean became wider and wider. Africa, India and Australia also began to diverge in different directions, and giant islands eventually formed south of the equator.
The causes of the Cretaceous disaster are not fully understood. Now the most popular theory has become the asteroid theory, which explains the extinction of dinosaurs and other organisms by the fall of a giant asteroid and the subsequent “asteroid winter.” On the surface of the Earth there really is a crater from a meteorite fall, formed about 65 million years ago at the end of the Cretaceous period as a result of a meteorite impact with a diameter of about 10 km - this is the Chicxulub crater. But the asteroid theory cannot explain why certain organisms survived when others died. In addition, many groups of animals clearly began to die out long before the end of the Cretaceous. The transition of the same ammonites to heteromorphic forms also clearly indicates some kind of instability. It may very well be that many species had already been undermined by some long-term processes and were on the path to extinction, and a catastrophe - an asteroid, increased volcanism or climate change due to the movement of continents - simply accelerated the process.
The Cretaceous period began 144 million years ago; it lasted 80 million years and was the link between the early Mesozoic and the Cenozoic era, the era of mammals.
By the beginning of the Cretaceous period, the Earth began to acquire many of the features we know. Animals and plants became characterized by regional characteristics as the division of continents continued. The division of continents also influenced the climate. Throughout the Cretaceous period, the world's climate became increasingly seasonal, with annual fluctuations in precipitation and air temperature becoming more pronounced.
The Cretaceous period received this name because thick chalk deposits are associated with it. It is divided into two sections: lower and upper.
The Cretaceous period is the final part of the Mesozoic era. It is famous for its tragic continental voyages, the explosion of life, the one that ended tragically and catastrophically with high sea levels.
The Cretaceous period follows the Jurassic and began approximately 144 million years ago. During this period, the supercontinent “Pangaea” split into two large parts, one – Laurasia, and the second – Gondwana. Laurasia went to the north, and Gondwana, respectively, to the south. But these continents also did not remain in this state for long and began to divide in turn into smaller parts. This is how the continents on which humanity now lives were formed.
These changes caused very strong changes in the Earth's climate, this was reflected in the water level in the ocean, at that time it was 200 meters higher than now. The name of this period was formed due to the fact that shells in shallow water covered the bottom of the shallow water in several layers, and as a result turned into chalk.
During the Cretaceous period, the first angiosperms—flowering plants—appeared.
Cretaceous period, or Cretaceous (145-66 million years ago)
This resulted in an increase in the diversity of insects that became flower pollinators. The evolution of the plant world gave impetus to the rapid development of the animal world, including dinosaurs. The diversity of dinosaur species reached its peak during the Cretaceous period.
Tyrannosaurus Photo: Martin Belam
Dinosaurs are divided into two groups - lizard-hatched, which included both predators and herbivorous forms, and ornithischian, exclusively herbivorous. The most famous lizard-hipped dinosaurs are tyrannosaurs, tarbosaurs, and brontosaurs. Among ornithischian lizards, ceratopsians, iguanodons, and stegosaurs are known. 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.
In the Cretaceous period, the first placental mammals appeared, and groups of ungulates, insectivores, predators and primates had already emerged.
Modern lizards and snakes evolved, so snakes are a relatively young group.
There were no mammals in the seas, and the niche of large predators was occupied by reptiles - ichthyosaurs, plesiosaurs, mososaurs, sometimes reaching 20 meters in length.
The diversity of marine invertebrates was very large. As in the Jurassic period, ammonites and belemnites, brachiopods, bivalves and sea urchins were very common. Among bivalves, a major role in marine ecosystems was played by rudists that appeared at the end of the Jurassic - mollusks that looked like solitary 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 forms 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. It could be a spiral 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.
Oddly enough, orthoceratids were still found in the seas extremely rarely, but still - relics of the long-past Paleozoic era. Small shells of these straight-shelled cephalopods are found in the Caucasus.
The Cretaceous system occupies one of the first places among the Phanerozoic divisions in terms of diversity and quantity of minerals. One of the most significant ore formations in the history of the Earth is associated with the powerful magmatism of the Cretaceous period. The predominant part of ore minerals gravitates to the Pacific mobile belt, within which there are deposits of non-ferrous metal ores. In East Asia, the largest tin-bearing province stretches from north to south. Since the end of the Late Cretaceous, porphyry copper deposits have been formed around the Pacific Ocean, most of which are confined to the eastern branch of the belt from Alaska in the north to Chile in the south. Copper and accompanying molybdenum ore occurrences are also known in the western branch in Chukotka, Kamchatka and Primorsky Krai. In the Mediterranean belt, porphyry copper deposits of Late Cretaceous - Paleogene age are found in Yugoslavia and Bulgaria. In the Caucasus, sulfur and copper pyrite ores of the Somkheto-Karabakh zone are associated with volcanogenic rocks of the Upper Cretaceous; skarns with iron and cobalt of Dashkesan, as well as copper-molybdenum deposits of the Miskhano-Zangezur zone, are associated with the pre-Cenomanian magmatic series. In the Cretaceous deposits in Ukraine and Siberia there are zirconilmenite coastal-marine placers, which also contain the gold placers of Zeya, Khingan, Kuznetsk Alatau and eastern Transbaikalia.
The Cretaceous system contains rich deposits of combustible minerals. In terms of total oil reserves, they are in 2nd place after the Cenozoic; about 1/2 of the gas reserves of the world's main fields are confined to them. The main oil and gas basins and provinces associated with the Cretaceous system are located along the Rocky Mountains of North America, in Alaska and California, in the Gulf of Mexico, in many countries of South America, in West Africa, on the northern and northeastern frame of the African-Arabian Platform from Libya to the Persian Gulf, in Central Asia, Western Siberia and other areas.
One of the largest oil and gas basins in the world is the Persian Gulf, in which 1/3 of the oil reserves are confined to Cretaceous reservoirs. Lower Cretaceous sandstones in the Lake Athabasca basin (Canada) contain large accumulations of semi-solid bitumen. In the territory of the former CCCP, Cretaceous deposits occupy 1st place in terms of oil and gas reserves. The greatest concentration of deposits is found on the West Siberian Plate, where the main oil deposits are concentrated in Neocomian and partially Aptian rocks, and natural gas deposits in the Aptesenomanian. Many deposits of the North Caucasus and Central Asia belong to the Lower and Upper Cretaceous. The Cretaceous period and especially its later era were favorable times for phosphate deposition.
At the end of the Cretaceous period, the most famous and very large extinction of many groups of plants and animals occurred. Many gymnosperms, all dinosaurs, pterosaurs, and aquatic reptiles became extinct. 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. Now the most popular theory has become the asteroid theory, which explains the extinction of dinosaurs and other organisms by the fall of a giant asteroid and the subsequent “asteroid winter.” On the surface of the Earth there really is a crater from a meteorite fall, formed about 65 million years ago at the end of the Cretaceous period as a result of a meteorite impact with a diameter of about 10 km - this is the Chicxulub crater. But the asteroid theory cannot explain why certain organisms survived when others died. In addition, many groups of animals clearly began to die out long before the end of the Cretaceous. The transition of the same ammonites to heteromorphic forms also clearly indicates some kind of instability. It is very possible that many species had already been undermined by some long-term processes and were on the path to extinction, and a catastrophe - an asteroid, increased volcanism or climate change due to the movement of continents - simply accelerated the process.
V.V. Arkadyev. Russian Geological Encyclopedia, 2011.
Cretaceous system/period(eng. Cretaceous System)– upper system of the Mesozoic erathema. The exact position of the lower boundary of the system is debatable. The system was isolated in 1822 by the Belgian geologist J.B. d'Aumalius d'Allois in the Anglo-Paris Basin. The name of the system comes from the widespread in Europe, Western Asia and North America strata of writing chalk that make up its upper part. It is divided into lower and upper sections, which combine six tiers (see table).
The four lower tiers were sometimes combined into a supertier neocom, and the four upper ones are in the overtier Senon.
There was a variant of the three-member division of the Cretaceous, in which the Albian, Cenomanian, Turonian and Coniacian were usually classified as the middle section. The tiered scale was developed in Western Europe. The Valanginian and Hauterivian stratotypes are found in Switzerland, the Maastrichtian - in the Netherlands, and the remaining stages - in France. The zonal division of Cretaceous deposits is based on the distribution of ammonites, and in a number of areas - bivalves (inocerams and buchia). In addition, belemnites, sea urchins and foraminifera are important for the stratigraphy of the Upper Cretaceous, and reptiles are important for continental sediments.
The Cretaceous period is the last period of the Mesozoic era, lasting 80 million years. It began 145.5 million years ago and ended 65.5 million years ago. The Cretaceous system is second only to the Quaternary in distribution. Marine facies of the Cretaceous are fully and diversely represented in the folded structures of the Alpine (Pyrenees, Alps, Atlas, Crimea, Caucasus, Kopet Dag, Central Iran, Himalayas) and Pacific (Far East and northeast Russia, Alaska and Cordillera) belts. Various continental sediments are widespread on the platforms: red-colored, gypsum-bearing and salt-bearing, freshwater lacustrine-deltaic and coal-bearing sediments. On the East European Platform in the Early Cretaceous era, there was a meridionally elongated sea basin that connected the northern seas with the seas of the Mediterranean belt. In it, under the conditions of a shallow cold sea, with currents and calm bays, sandy-clayey sediments of small thickness accumulated. Marine sedimentation conditions persisted throughout the Cretaceous period in the West Siberian Basin. Here the Cretaceous deposits are represented by a thick (several km) layer of sandy-clayey rocks with the remains of marine fauna. In North America, the Cretaceous system corresponded Comanche(lower section) and Gulf(upper section) of the system.
During the Cretaceous period, the process of opening of oceanic basins continued. In the Early Cretaceous, the South Atlantic formed, the Caribbean and the Tethys Ocean further expanded, and the ocean depth increased (black clays and turbidites accumulated in the Central and South Atlantic). The Indian Ocean was going through the initial stage of spreading (clay deposits were forming here) (Fig. 1).
In the Early Cretaceous, Cimmerian (Mesozoic) tectogenesis ends. There is a collision of Hyperborea with the northeastern edge of Eurasia, where the Verkhoyansk-Chukotka folded region is formed. At the end of the Early - beginning of the Late Cretaceous, in the space from Chukotka to Kalimantan, as a result of the collision of microcontinents with the margin of Eurasia, a powerful East Asian (Chukotka-Kathasian) volcano-plutonic belt was formed.
In the Late Cretaceous, the separation of Australia from Antarctica began, and in the north of the Atlantic, Greenland, along with Eurasia, from North America (formation of the North Atlantic) (Fig. 2).
As a result of the expansion of the Indian Ocean, Africa and Hindustan are moving north. The pressure of Africa on the western part of the Mediterranean belt is associated with deformations of Alpine tectogenesis in the Late Cretaceous, which manifested themselves in the Eastern Alps, Carpathians, Balkanids, Crimea, the Caucasus, Iran and Southern Afghanistan. On the active Pacific margin of the Americas, intense folding and thrust formation also occur (Laramie orogeny). In all collision zones of the Cretaceous period, folding was accompanied by powerful granitoid magmatism. Colossal outpourings of basalts on the bottom of the oceans and on the surface of the southern continents are confined to the Cretaceous period. hemispheres (Hindustan, South America).
Since the Albian, one of the largest transgressions in the history of the Earth has occurred.
CHALK SYSTEM (PERIOD)
A significant part of the territory of Eurasia, from England to Western Asia, at that time was covered by a relatively shallow sea, in which carbonates accumulated (formation of chalk). Late Cretaceous transgression was widespread in Africa and on the North American platform.
The Cretaceous period is characterized by the flourishing of two important groups of invertebrate animals - ammonites and belemnites. Large coral-like bivalves, rudists and nerineids (gastropods), were widespread in tropical seas. Irregular sea urchins, sea lilies, and, in the Late Cretaceous, inoceramides and sponges reached significant diversity. The main reef-building organisms were scleractinians and bryozoans. Among seaweeds, golden ones are very characteristic - coccolithophores and diatoms. They, together with small foraminifera, participated in the formation of white writing chalk in the Late Cretaceous. Of the vertebrates, reptiles dominated, conquering land, water and air space. There were a variety of herbivores and huge predatory dinosaurs (tyrannosaurs, tarbosaurs) (Fig. 3, 4). The Cretaceous period is characterized by the appearance of snakes. Bony fish have evolved significantly, toothy birds have spread, and placental mammals appear. The flora of the Early Cretaceous era is characterized by the dominance of gymnosperms and pteridophytes, but starting from the Albian age, angiosperms sharply predominate (the beginning of the Cenophytic stage in the development of vegetation). By the end of the Cretaceous period, at the turn of the Maastrichtian and Danian, coccolithophores, planktonic foraminifera, ammonites, belemnites, inoceramides, rudists, dinosaurs and a number of other groups disappeared. 50% of radiolarian families, 75% of brachiopod families disappeared, the number of sea urchins and crinoids decreased significantly, and the number of sharks decreased by 75%. In total, more than 100 families of marine invertebrates and about the same number of terrestrial animals and plants became extinct. This decline in fauna and flora is often called the “Great Mesozoic Extinction.” One of the most widely accepted ideas about the causes of this extinction is the collision of the Earth with an asteroid, the diameter of which could be 10-15 km. Traces of such a collision are recorded in the form of an “iridium anomaly” in the boundary layers of the Cretaceous and Paleogene in a number of sections in the West. Europe. The Chicxulub crater on the Yucatan Peninsula in Mexico is currently considered the most likely candidate for large craters formed on Earth by an asteroid impact at the Cretaceous-Paleogene boundary. The “asteroid winter” that occurred after the explosion could have caused a number of processes negative for the life of organisms - a reduction in food resources, disruption of food connections, a decrease in temperature, etc.
In the Cretaceous period, climatic zonation was clearly expressed. In particular, the Boreal, Mediterranean (Tethyan), Southern and Pacific paleobiogeographic regions, which differed in the nature of sediments and the development of groups of the organic world, were clearly distinguished.
The Cretaceous system is rich in various minerals. More than 20% of the world's coal reserves are associated with continental deposits (Lensky, Zyryansky coal basins in Russia, coal basins in western North America). Large deposits of bauxite are known in the Turgai trough, on the Yenisei Ridge, the Southern Urals, the Ukrainian Shield and in the Mediterranean. A belt rich in phosphorites stretches from Morocco to Syria; deposits of phosphorites are known on the East European Platform. There are salt deposits confined to lagoonal deposits in Turkmenistan and North America. The Upper Cretaceous is associated with large reserves of writing chalk and raw materials for the cement industry on the territory of the North American and East European platforms. Many oil and gas fields in Western Siberia, western Central Asia, Libya, Kuwait, Nigeria, Gabon, Canada and the Gulf of Mexico are Cretaceous in age.
Deposits of tin, lead and gold associated with Cretaceous acid intrusions are known in northeastern Russia and western North America. The largest tin belt can be traced in the area. Malaysia, Thailand and Indonesia. Large deposits of tin, tungsten, antimony and mercury are known in southeast China and South Korea. Diamond deposits in Cretaceous kimberlite pipes are being developed in South Africa and India.
Bibliography::
Biske Yu.S., Prozorovsky V.A. General stratigraphic scale of the Phanerozoic. Vendian, Paleozoic and Mesozoic.Textbook. allowance. St. Petersburg: St. Petersburg State University Publishing House, 2001.
Fedorov P.V. History of the earth's crust. Atlas of illustrations for the course of historical geology: Textbook. - St. Petersburg State University, 2006, 16.
Khain V.E., Koronovsky N.V., Yasamanov N.A. Historical geology. M.: Moscow State University Publishing House, 1997.
Not everywhere the coastline consists of sandy beaches that gradually descend to the sea. In some places on the coast there are rocky cliffs, and sometimes they are not brown, but white.
The White Cliffs dominate the coastline near Dover on the southwest coast of England, and also around Calais in northeast France.
There are no beaches in these parts of the coast.
Cretaceous period
It is very difficult to land on the rocky shore. All this makes navigation here very dangerous.
Why are these rocks white?
The rocks are made of chalk - the fossilized remains of single-celled animals that once lived in the sea. They were very small, and today the remains of the animals can only be seen under a microscope.
Many centuries ago they died, their remains sank to the bottom, and chalk formed from them.
Its white color is due to the fact that calcium contained in fossil animals turned into limestone over time. And limestone, as you know, is a white mineral.
The stone bordering these coasts can be white, gray or bluish. The more chalk the stone contains, the lighter it is.
Chalk is a very fragile mineral, so the rocks consisting of it are gradually eroded by the sea and destroyed by the wind. Floods have an equally destructive effect on chalk rocks.
You can see this for yourself if you simply put a piece of chalk in water. You will see how it is saturated with water and becomes completely soft.
When water constantly rolls into the same place, huge caves are formed in the rock.
If the cave becomes too large, the upper layers of chalk collapse and water flows into the cave. Such a cave is called a grotto. The sound of waves and wind fills the grottoes with bizarre sounds. Therefore, popular fantasy populated them with underwater inhabitants - mermaids and Morgans.
The Cretaceous period is the last era that ends 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 there were no catastrophes of a planetary scale on Earth and, consequently, the extinction of a large number of species of plants and animals, the movement of tectonic plates, changes in the level of the world ocean and climate change made their own 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 vast areas of Eastern Europe, Western Siberia, all of Arabia and almost all of modern Canada found themselves 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, have survived to this day 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. The fish kingdom is dominated by sharks and Animals of the Mesozoic era - primarily dinosaurs and the first mammals - safely “migrated” from the Jurassic 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 primacy 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 marsupial animals, weighing up to 500 g. But they were the future.