Mesozoic era

The Mesozoic era is an era of middle life. It is named so because the flora and fauna of this era are transitional between the Paleozoic and Cenozoic. During the Mesozoic era, the modern outlines of continents and oceans, modern marine fauna and flora gradually formed. The Andes and Cordillera, the mountain ranges of China and East Asia, were formed. The depressions of the Atlantic and Indian oceans were formed. The formation of the Pacific Ocean depressions began.

The Mesozoic era is divided into three periods: Triassic, Jurassic and Cretaceous.

Triassic

The Triassic period got its name from the fact that its deposits include three different complexes of rocks: the lower - continental sandstone, the middle - limestone and the upper - Naper.

The most characteristic deposits of the Triassic period are: continental sandy-clayey rocks (often with lenses of coal); marine limestones, clays, shales; lagoonal anhydrites, salts, gypsum.

During the Triassic period, the northern continent of Laurasia united with the southern one - Gondwana. A large bay that began in the east of Gondwana extended all the way to the northern coast of modern Africa, then turned south, almost completely separating Africa from Gondwana. A long bay stretched from the west, separating the western part of Gondwana from Laurasia. Many depressions appeared on Gondwana, which were gradually filled with continental sediments.

During the Middle Triassic, volcanic activity intensified. Inland seas become shallow and numerous depressions form. The formation of the mountain ranges of Southern China and Indonesia begins. In the territory of the modern Mediterranean, the climate was warm and humid. It was cooler and wetter in the Pacific zone. Deserts dominated the territory of Gondwana and Laurasia. The climate of the northern half of Laurasia was cold and dry.

Along with changes in the distribution of sea and land, the formation of new mountain ranges and volcanic areas, there was an intensive replacement of some animal and plant forms by others. Only a few families moved from the Paleozoic era to the Mesozoic. This gave grounds for some researchers to claim about the great catastrophes that occurred at the boundary of the Paleozoic and Mesozoic. However, when studying the deposits of the Triassic period, one can easily verify that there is no sharp line between them and the Permian deposits; therefore, some forms of plants and animals were replaced by others, probably gradually. The main reason was not catastrophes, but the evolutionary process: more perfect forms gradually replaced less perfect ones.

The seasonal temperature changes of the Triassic period began to have a noticeable effect on plants and animals. Certain groups of reptiles have adapted to cold seasons. It was from these groups that mammals originated in the Triassic, and somewhat later, birds. At the end of the Mesozoic era, the climate became even colder. Deciduous woody plants appear, which partially or completely shed their leaves during cold seasons. This feature plants are an adaptation to colder climates.

The cooling during the Triassic period was insignificant. It manifested itself most strongly in northern latitudes. The rest of the area was warm. Therefore, reptiles felt quite well in the Triassic period. Their most diverse forms, with which small mammals were not yet able to compete, settled across the entire surface of the Earth. The rich vegetation of the Triassic period also contributed to the extraordinary flourishing of reptiles.

Gigantic forms of cephalopods developed in the seas. The diameter of the shells of some of them was up to 5 m. True, even now the seas are inhabited by gigantic cephalopods, for example squids, reaching 18 m in length, but in the Mesozoic era there were much more gigantic forms.

The composition of the atmosphere of the Triassic period changed little compared to the Permian. The climate became wetter, but deserts remained in the center of the continent. Some plants and animals of the Triassic period have survived to this day in the region of Central Africa and South Asia. This suggests that the composition of the atmosphere and the climate of individual land areas remained almost unchanged during the Mesozoic and Cenozoic eras.

And yet stegocephalians became extinct. They were replaced by reptiles. More perfect, mobile, well adapted to a variety of living conditions, they ate the same food as stegocephals, settled in the same places, ate the young of stegocephals and ultimately exterminated them.

Among the Triassic flora, calamites, seed ferns and cordaites were also occasionally found. True ferns, ginkgo ferns, bennetite ferns, cycads, and conifers predominated. Cycads still exist in the Malay Archipelago region. They are known as sago palms. In my own way appearance Cycads occupy an intermediate position between palms and ferns. The cycad trunk is quite thick and columnar. The crown consists of hard, feathery leaves arranged in a corolla. Plants reproduce using macro- and microspores.

Triassic ferns were coastal herbaceous plants that had wide, dissected leaves with reticulated venation. Volttsia has been well studied among coniferous plants. It had a thick crown and cones like those of a spruce.

Ginkgo trees were quite tall trees, their leaves formed dense crowns.

A special place among the Triassic gymnosperms was occupied by bennettites - trees with whorled large compound leaves, reminiscent of the leaves of cycads. The reproductive organs of bennetites occupy an intermediate place between the cones of cycads and the flowers of some flowering plants, in particular magnolias. Thus, it is probably the bennetites that should be considered the ancestors of flowering plants.

Of the invertebrates of the Triassic period, all types of animals that exist in our time are already known. The most characteristic marine invertebrates were reef-building animals and ammonites.

In the Paleozoic, animals already existed that covered the bottom of the sea in colonies, forming reefs, although not very powerful. During the Triassic period, when many colonial six-rayed corals appear instead of tabulata, the formation of reefs up to a thousand meters thick begins. The cups of six-rayed corals had six or twelve calcareous partitions. As a result of the massive development and rapid growth of corals, underwater forests were formed on the seabed, in which numerous representatives of other groups of organisms settled. Some of them took part in reef formation. Bivalves, algae, sea ​​urchins, starfish, sponges lived between the corals. Destroyed by waves, they formed coarse-grained or fine-grained sand, which filled all the voids of the corals. Washed out of these voids by waves, calcareous silt was deposited in bays and lagoons.

Some bivalves are quite characteristic of the Triassic period. Their paper-thin shells with fragile ribs form in some cases entire layers in the sediments of a given period. Bivalves lived in shallow muddy bays - lagoons, on reefs and between them. In the Upper Triassic period, many thick-shelled bivalves appeared, firmly attached to the limestone deposits of shallow basins.

At the end of the Triassic, due to increased volcanic activity, part of the limestone deposits was covered with ash and lavas. The steam rising from the bowels of the Earth brought with it many compounds from which deposits of non-ferrous metals were formed.

The most common of the gastropods were prosobranchs. Ammonites spread widely in the seas of the Triassic period, the shells of which accumulated in huge quantities in some places. Appearing in Silurian period, they have not yet played a major role among other invertebrates throughout the Paleozoic era. Ammonites could not successfully compete with the rather complex nautiloids. Ammonite shells were formed from calcareous plates that were the thickness of tissue paper and therefore did little to protect the soft body of the mollusk. Only when their partitions bent into numerous folds did the shells of ammonites acquire strength and turn into real shelter from predators. With the increasing complexity of the partitions, the shells became even more durable, and the external structure gave them the opportunity to adapt to a wide variety of living conditions.

Representatives of echinoderms were sea urchins, lilies and stars. At the upper end of the body of crinoids there was a flower-like main part. It distinguishes between a corolla and grasping organs - “hands”. Between the “hands” in the corolla there were the oral and anal openings. With its “hands” the sea lily scooped water into its mouth, and with it the sea animals that it fed on. The stem of many Triassic crinoids was spiral.

The Triassic seas were inhabited by calcareous sponges, bryozoans, leaf-footed crayfish, and ostracods.

Fish were represented by sharks that lived in fresh water bodies and molluscoids that inhabited the sea. The first primitive bony fishes appear. Powerful fins, well-developed dental apparatus, perfect shape, strong and light skeleton - all this contributed to the rapid spread of bony fish in the seas of our planet.

Amphibians were represented by stegocephalians from the labyrinthodont group. These were sedentary animals with a small body, small limbs and a large head. They lay in the water waiting for prey, and when the prey approached, they grabbed it. Their teeth had complex labyrinthine folded enamel, which is why they were called labyrinthodonts. The skin was moistened by mucous glands. Other amphibians came onto land to hunt insects. The most characteristic representatives of labyrinthodonts are mastodonosaurs. These animals, whose skulls reached one meter in length, resembled huge frogs in appearance. They hunted fish and therefore rarely left the aquatic environment.

Mastodonosaurus.

The swamps became smaller, and mastodonosaurs were forced to populate deeper and deeper places, often accumulating in large numbers. That is why many of their skeletons are now found in small areas.

Reptiles in the Triassic are characterized by significant diversity. New groups are appearing. Of the cotylosaurs, only procolophons remain - small animals that fed on insects. An extremely interesting group of reptiles was represented by archosaurs, which included thecodonts, crocodiles and dinosaurs. Representatives of thecodonts, ranging in size from a few centimeters to 6 m, were predators. They also differed in a number of primitive features and were similar to the Permian pelycosaurs. Some of them - pseudosuchia - had long limbs, a long tail and led a terrestrial lifestyle. Others, including the crocodyliform phytosaurs, lived in the water.

Crocodiles of the Triassic period - small primitive protosuchian animals - lived in fresh water bodies.

Among the dinosaurs, theropods and prosauropods appear. Theropods moved on well-developed hind limbs, had a heavy tail, powerful jaws, and small and weak forelimbs. These animals ranged in size from a few centimeters to 15 m. All of them were classified as predators.

Prosauropods typically ate plants. Some of them were omnivores. They walked on four legs. Prosauropods had a small head, long neck and tail.

Representatives of the subclass of synaptosaurs led a very diverse lifestyle. Trilophosaurus climbed trees and ate plant foods. In appearance he resembled a cat.

Seal-like reptiles lived near the coast, feeding mainly on mollusks. Plesiosaurs lived in the sea, but sometimes came ashore. They reached 15 m in length. They ate fish.

In some places, quite often they find footprints of a huge animal that walked on four legs. It was called chirotherium. Based on the preserved prints, one can imagine the structure of the foot of this animal. Four gangly toes surrounded a thick, fleshy sole. Three of them had claws. The forelimbs of Chirotherium are almost three times smaller than the hind limbs. The animal left deep footprints on the wet sand. As new layers were deposited, the traces gradually petrified. Later, the land was flooded by the sea, hiding the traces. They turned out to be covered with marine sediments. Consequently, the sea flooded repeatedly during that era. The islands sank below sea level, and the animals living on them were forced to adapt to new conditions. Many reptiles appear in the sea, which undoubtedly descended from continental ancestors. Turtles with a wide bony shell, dolphin-like ichthyosaurs - fish lizards and gigantic plesiosaurs with a small head on a long neck - quickly developed. Their vertebrae are transformed, their limbs change. The cervical vertebrae of an ichthyosaur fuse into one bone, and in turtles they grow, forming top part shell.

The ichthyosaur had a row of uniform teeth; in turtles the teeth disappear. The five-fingered limbs of ichthyosaurs turn into flippers well adapted for swimming, in which it is difficult to distinguish the shoulder, forearm, wrist and finger bones.

Starting from the Triassic period, reptiles, which moved to live in the sea, gradually populated increasingly vast areas of the ocean.

The oldest mammal found in the Triassic sediments of North Carolina is called dromaterium, which means “running beast.” This “beast” was only 12 cm in length. Dromatherium belonged to oviparous mammals. They, like the modern Australian echidna and platypus, did not give birth to young, but laid eggs, from which underdeveloped young hatched. Unlike reptiles, which did not care at all about their offspring, Dromatheriums fed their young with milk.

Deposits of the Triassic period are associated with deposits of oil, natural gases, brown and hard coal, iron and copper ores, and rock salt.

The Triassic period lasted 35 million years.

Jurassic period

For the first time, deposits of this period were found in the Jura (mountains in Switzerland and France), hence the name of the period. The Jurassic period is divided into three divisions: Leyas, Doger and Malm.

The deposits of the Jurassic period are quite diverse: limestones, clastic rocks, shales, igneous rocks, clays, sands, conglomerates, formed in a wide variety of conditions.

Sedimentary rocks containing many representatives of fauna and flora are widespread.

Intense tectonic movements at the end of the Triassic and the beginning of the Jurassic periods contributed to the deepening of large bays, which gradually separated Africa and Australia from Gondwanaland. The gulf between Africa and America has deepened. Depressions formed in Laurasia: German, Anglo-Paris, West Siberian. The Arctic Sea flooded the northern coast of Laurasia.

Intense volcanism and mountain-building processes determined the formation of the Verkhoyansk fold system. The formation of the Andes and Cordilleras continued. Warm sea currents reached Arctic latitudes. The climate became warm and humid. This is evidenced by the significant distribution of coral limestones and the remains of thermophilic fauna and flora. Very few deposits of dry climates are found: lagoonal gypsum, anhydrites, salts and red sandstones. The cold season already existed, but it was characterized only by a decrease in temperature. There was no snow or ice.

The climate of the Jurassic period depended not only on sunlight. Many volcanoes and magma outpourings onto the bottom of the oceans heated the water and atmosphere, saturating the air with water vapor, which then rained onto the land and flowed into lakes and oceans in stormy streams. This is evidenced by numerous freshwater deposits: white sandstones alternating with dark loams.

The warm and humid climate favored the flourishing of the plant world. Ferns, cycads, and conifers formed vast swampy forests. Araucarias, thujas, and cycads grew on the coast. Ferns and horsetails formed the undergrowth. In the Lower Jurassic, throughout the northern hemisphere, vegetation was quite monotonous. But starting from the Middle Jurassic, two plant zones can be identified: the northern, in which ginkgo and herbaceous ferns predominated, and the southern with bennetites, cycads, araucarias, and tree ferns.

The characteristic ferns of the Jurassic period were matonia, which are still preserved in the Malay Archipelago. Horsetails and mosses were almost no different from modern ones. The place of extinct seed ferns and cordaites is taken by cycads, which still grow in tropical forests.

Ginkgo plants were also widespread. Their leaves turned edge-on to the sun and resembled huge fans. From North America and New Zealand to Asia and Europe, dense forests of coniferous plants - araucarias and bennetites - grew. The first cypress and possibly spruce trees appear.

Representatives of the Jurassic conifers also include sequoia - the modern giant California pine. Currently, redwoods remain only on the Pacific coast of North America. Some forms of even more ancient plants, for example glassopteris, have been preserved. But there are few such plants, since they were replaced by more advanced ones.

The lush vegetation of the Jurassic period contributed to the widespread distribution of reptiles. Dinosaurs have evolved significantly. Among them, lizard-hatched and ornithischian are distinguished. Lizards moved on four legs, had five toes on their feet, and ate plants. Most of them had a long neck, small head and long tail. They had two brains: one small one in the head; the second is much larger in size - at the base of the tail.

The largest of Jurassic dinosaurs there was a brachiosaurus that reached a length of 26 m and weighed about 50 tons. It had columnar legs, a small head, and a thick long neck. Brachiosaurs lived on the shores of Jurassic lakes and fed on aquatic vegetation. Every day, the brachiosaurus needed at least half a ton of green mass.

Brachiosaurus.

Diplodocus is the oldest reptile, its length was 28 m. It had a long thin neck and a long thick tail. Like a brachiosaurus, Diplodocus walked on four legs, the hind legs being longer than the front ones. Diplodocus spent most of its life in swamps and lakes, where it grazed and escaped from predators.

Diplodocus.

Brontosaurus was relatively tall, had a large hump on its back and a thick tail. Its length was 18 m. The vertebrae of the brontosaurus were hollow. Chisel-shaped small teeth were densely located on the jaws of the small head. The brontosaurus lived in swamps and on the shores of lakes.

Brontosaurus.

Ornithischian dinosaurs are divided into bipeds and quadrupeds. Different in size and appearance, they fed mainly on vegetation, but predators are already appearing among them.

Stegosaurs are herbivores. They had two rows of large plates on their backs and paired spikes on their tails that protected them from predators. Many scaly lepidosaurs appear - small predators with beak-like jaws.

Flying lizards first appeared in the Jurassic period. They flew using a leathery shell stretched between the long finger of the hand and the bones of the forearm. Flying lizards were well adapted to flight. They had light tube-shaped bones. The extremely elongated outer fifth digit of the forelimbs consisted of four joints. The first finger looked like a small bone or was completely absent. The second, third and fourth fingers consisted of two, rarely three bones and had claws. The hind limbs were quite developed. There were sharp claws at their ends. The skull of flying lizards was relatively large, usually elongated and pointed. In old lizards, the cranial bones fused and the skulls became similar to the skulls of birds. The premaxillary bone sometimes grew into an elongated toothless beak. Toothed lizards had simple teeth and sat in recesses. The largest teeth were in the front. Sometimes they stuck out to the side. This helped the lizards catch and hold prey. The animals' spine consisted of 8 cervical, 10–15 dorsal, 4–10 sacral and 10–40 caudal vertebrae. The chest was wide and had a high keel. The shoulder blades were long, the pelvic bones were fused. The most typical representatives of flying lizards are pterodactyl and rhamphorhynchus.

Pterodactyl.

Pterodactyls in most cases were tailless, varying in size - from the size of a sparrow to a crow. They had wide wings and a narrow skull elongated forward with a small number of teeth in the front. Pterodactyls lived in large flocks on the shores of the lagoons of the Late Jurassic Sea. During the day they hunted, and at nightfall they hid in trees or rocks. The skin of pterodactyls was wrinkled and bare. They ate mainly fish, sometimes sea lilies, mollusks, and insects. In order to take off, pterodactyls were forced to jump from cliffs or trees.

Rhamphorhynchus had long tails, long narrow wings, and a large skull with numerous teeth. Long teeth of varying sizes curved forward. The lizard's tail ended in a blade that served as a rudder. Rhamphorhynchus could take off from the ground. They settled on the banks of rivers, lakes and seas, feeding on insects and fish.

Rhamphorhynchus.

Flying lizards lived only in the Mesozoic era, and their heyday occurred in the Late Jurassic period. Their ancestors were, apparently, extinct ancient reptiles pseudosuchians. Long-tailed forms appeared earlier than short-tailed ones. At the end of the Jurassic period they became extinct.

It should be noted that flying lizards were not the ancestors of birds and bats. Flying lizards, birds and the bats each originated and developed in its own way, and there are no close family ties between them. The only thing they have in common is the ability to fly. And although they all acquired this ability due to changes in the forelimbs, the differences in the structure of their wings convince us that they had completely different ancestors.

The seas of the Jurassic period were inhabited by dolphin-like reptiles - ichthyosaurs. They had long head, sharp teeth, large eyes surrounded by a bony ring. The length of the skull of some of them was 3 m, and the length of the body was 12 m. The limbs of ichthyosaurs consisted of bone plates. The elbow, metatarsus, hand and fingers differed little from each other in shape. About a hundred bone plates supported the wide flipper. The shoulder and pelvic girdles were poorly developed. There were several fins on the body. Ichthyosaurs were viviparous animals. Plesiosaurs lived alongside ichthyosaurs. They had a thick body with four flipper-like limbs, a long snake-like neck with a small head.

During the Jurassic period, new genera of fossil turtles appeared, and at the end of the period, modern turtles appeared.

Tailless frog-like amphibians lived in fresh water bodies. There were a lot of fish in the Jurassic seas: bony fish, stingrays, sharks, cartilaginous fish, and ganoid fish. They had an internal skeleton made of flexible cartilaginous tissue impregnated with calcium salts: a dense scaly bone cover that protected them well from enemies, and jaws with strong teeth.

Among the invertebrates in the Jurassic seas, there were ammonites, belemnites, and crinoids. However, in the Jurassic period there were much fewer ammonites than in the Triassic. Jurassic ammonites differ from Triassic ammonites in their structure, with the exception of phyloceras, which did not change at all during the transition from the Triassic to the Jurassic. Certain groups of ammonites have preserved mother-of-pearl to this day. Some animals lived in the open sea, others inhabited bays and shallow inland seas.

Cephalopods - belemnites - swam in whole schools in the Jurassic seas. Along with small specimens, there were real giants - up to 3 m long.

Remains of belemnite internal shells, known as “devil's fingers,” are found in Jurassic sediments.

In the seas of the Jurassic period, bivalves also developed significantly, especially those belonging to the oyster family. They begin to form oyster banks.

The sea urchins that settled on the reefs are undergoing significant changes. Along with the round forms that have survived to this day, there lived bilaterally symmetrical, irregularly shaped hedgehogs. Their body was stretched in one direction. Some of them had a jaw apparatus.

The Jurassic seas were relatively shallow. Rivers brought muddy water into them, delaying gas exchange. The deep bays were filled with rotting remains and silt containing a large number of hydrogen sulfide. That is why in such places the remains of animals carried by sea currents or waves are well preserved.

Sponges, starfish, and crinoids often overflow the Jurassic sediments. “Five-armed” crinoids became widespread during the Jurassic period. Many crustaceans appear: barnacles, decapods, phyllopods, freshwater sponges, among insects - dragonflies, beetles, cicadas, bugs.

During the Jurassic period, the first birds appeared. Their ancestors were the ancient reptiles pseudosuchians, which also gave rise to dinosaurs and crocodiles. Ornithosuchia is most similar to birds. She, like a bird, walked on her hind legs, had a strong pelvis and was covered with feather-like scales. Some pseudosuchians moved to live in trees. Their forelimbs were specialized for grasping branches with their fingers. The pseudosuchian skull had lateral depressions, which significantly reduced the mass of the head. Climbing trees and jumping on branches strengthened the hind limbs. Gradually expanding forelimbs supported the animals in the air and allowed them to glide. An example of such a reptile is Scleromochlusa. His long, thin legs indicate that he was a good jumper. Elongated forearms helped animals climb and cling to branches of trees and bushes. The most important moment in the process of transformation of reptiles into birds was the transformation of scales into feathers. The animals' hearts had four chambers, which ensured a constant body temperature.

In the Late Jurassic period, the first birds appeared - Archeopteryx, the size of a pigeon. In addition to short feathers, Archeopteryx had seventeen flight feathers on its wings. The tail feathers were located on all tail vertebrae and were directed back and down. Some researchers believe that the bird's feathers were bright, like those of modern tropical birds, others - that the feathers were gray or Brown, third - that they were motley. The mass of the bird reached 200 g. Many signs of Archeopteryx indicate its family ties with reptiles: three free fingers on the wings, a head covered with scales, strong conical teeth, a tail consisting of 20 vertebrae. The bird's vertebrae were biconcave, like those of fish. Archeopteryx lived in araucaria and cycad forests. They ate mainly insects and seeds.

Archeopteryx.

Predators appeared among mammals. Small in size, they lived in forests and dense bushes, hunting small lizards and other mammals. Some of them have adapted to life in trees.

Deposits of coal, gypsum, oil, salt, nickel and cobalt are associated with Jurassic deposits.

This period lasted 55 million years.

Cretaceous period

The Cretaceous period received this name because thick chalk deposits are associated with it. It is divided into two sections: lower and upper.

Mountain-building processes at the end of the Jurassic period significantly changed the outlines of continents and oceans. North America, previously separated from the vast Asian continent by a wide strait, connected with Europe. In the east, Asia merged with America. South America was completely separated from Africa. Australia was located where it is today, but was smaller in size. The formation of the Andes and Cordilleras, as well as individual ridges of the Far East, continues.

During the Upper Cretaceous period, the sea flooded vast areas of the northern continents. Western Siberia and Eastern Europe, most of Canada and Arabia were under water. Thick layers of chalk, sand, and marls accumulate.

At the end of the Cretaceous period, mountain-building processes were again activated, as a result of which the mountain ranges of Siberia, the Andes, the Cordillera and the mountain ranges of Mongolia were formed.

The climate has changed. In the high latitudes in the north, during the Cretaceous period there was already a real winter with snow. Within the boundaries of the modern temperate zone, some tree species (walnut, ash, beech) were no different from modern ones. The leaves of these trees fell for the winter. However, as before, the climate in general was much warmer than today. Ferns, cycads, ginkgos, bennetites, and conifers, in particular sequoias, yews, pines, cypresses, and spruces, were still common.

In the mid-Cretaceous period, flowering plants flourished. At the same time, they displace representatives of the most ancient flora - spore and gymnosperm plants. It is believed that flowering plants originated and developed in the northern regions, and subsequently they spread throughout the planet. Flowering plants are much younger than conifers, known to us since the Carboniferous period. The dense forests of giant tree ferns and horsetails had no flowers. They adapted well to the living conditions of that time. However, gradually the humid air of the primary forests became increasingly dry. There was very little rain, and the sun was unbearably hot. The soil in the areas of primary swamps dried out. Deserts appeared on the southern continents. Plants moved to areas with cooler, wetter climates in the north. And then the rains came again, saturating the damp soil. The climate of ancient Europe became tropical, and forests similar to modern jungles arose on its territory. The sea recedes again, and plants that inhabited the coast in a humid climate found themselves in a drier climate. Many of them died, but some adapted to new living conditions, forming fruits that protected the seeds from drying out. The descendants of such plants gradually populated the entire planet.

The soil also underwent changes. Silt and the remains of plants and animals enriched it with nutrients.

In primary forests, plant pollen was carried only by wind and water. However, the first plants appeared, the pollen of which insects fed on. Some of the pollen stuck to the wings and legs of the insects, and they transferred it from flower to flower, pollinating the plants. In pollinated plants, the seeds ripened. Plants that were not visited by insects did not reproduce. Therefore, only plants with fragrant flowers of various shapes and colors were distributed.

With the advent of flowers, insects also changed. Among them appear insects that cannot live without flowers at all: butterflies, bees. Fruits with seeds developed from pollinated flowers. Birds and mammals ate these fruits and carried the seeds over long distances, spreading the plants to new areas of the continents. Many herbaceous plants appeared and populated the steppes and meadows. The leaves of the trees fell off in the fall, and in summer heat curled up.

The plants spread to Greenland and the islands of the Arctic Ocean, where it was relatively warm. At the end of the Cretaceous period, with the cooling of the climate, many cold-resistant plants appeared: willow, poplar, birch, oak, viburnum, which are also characteristic of the flora of our time.

With the development of flowering plants, by the end of the Cretaceous period the bennetites became extinct, and the number of cycads, ginkgos, and ferns decreased significantly. Along with the change in vegetation, the fauna also changed.

Foraminifera spread significantly, the shells of which formed thick chalk deposits. The first nummulites appear. Corals formed reefs.

Ammonites of the Cretaceous seas had shells of a peculiar shape. If all the ammonites that existed before the Cretaceous period had shells wrapped in one plane, then the Cretaceous ammonites had elongated shells, bent in the form of a knee, and there were spherical and straight shells. The surface of the shells was covered with spines.

According to some researchers, the bizarre forms of Cretaceous ammonites are a sign of aging of the entire group. Although some representatives of ammonites still continued to reproduce at high speed, their vital energy almost dried up during the Cretaceous period.

According to other scientists, ammonites were exterminated by numerous fish, crustaceans, reptiles, and mammals, and the strange forms of Cretaceous ammonites are not a sign of aging, but mean an attempt to somehow protect themselves from excellent swimmers, which by that time had become bony fish and sharks.

The disappearance of ammonites was also facilitated by a sharp change in physical and geographical conditions in the Cretaceous period.

Belemnites, which appeared much later than ammonites, also completely died out during the Cretaceous period. Among the bivalves there were animals of different shapes and sizes that closed the valves with the help of denticles and pits. In oysters and other mollusks that are attached to the seabed, the valves become different. The lower flap looked like a deep bowl, and the upper one looked like a lid. Among the rudists, the lower valve turned into a large thick-walled glass, inside of which only a small chamber remained for the mollusk itself. The round, lid-like upper flap covered the lower one with strong teeth, with the help of which it could rise and fall. Rudists lived mainly in the southern seas.

In addition to bivalves, whose shells consisted of three layers (outer horny, prismatic and mother-of-pearl), there were mollusks with shells that had only a prismatic layer. These are mollusks of the genus Inoceramus, widely distributed in the seas of the Cretaceous period - animals that reached one meter in diameter.

During the Cretaceous period, many new species of gastropods appeared. Among sea urchins, the number of irregular heart-shaped forms especially increases. And among sea lilies, varieties appear that do not have a stem and float freely in the water with the help of long feathery “arms”.

Great changes have also occurred among fish. In the seas of the Cretaceous period, ganoid fish gradually became extinct. The number of bony fishes is increasing (many of them still exist today). Sharks are gradually acquiring a modern appearance.

Numerous reptiles still lived in the sea. The descendants of the ichthyosaurs that became extinct at the beginning of the Cretaceous reached 20 m in length and had two pairs of short flippers.

New forms of plesiosaurs and pliosaurs appear. They lived on the open sea. Crocodiles and turtles inhabited freshwater and saltwater basins. The territory of modern Europe was inhabited by large lizards with long spines on their backs and huge pythons.

Of the terrestrial reptiles, trachodons and horned lizards were especially characteristic of the Cretaceous period. Trachodons could move on both two and four legs. They had membranes between their fingers that helped them swim. Trachodons' jaws resembled a duck's beak. They had up to two thousand small teeth.

Triceratops had three horns on their heads and a huge bone shield that reliably protected the animals from predators. They lived mainly in dry places. They ate vegetation.

Triceratops.

Styracosaurs had nasal projections - horns and six horny spines on the posterior edge of the bony shield. Their heads reached two meters in length. The spines and horns made Styracosaurus dangerous to many predators.

The most terrible predatory lizard was the Tyrannosaurus. It reached a length of 14 m. Its skull, more than a meter long, had large sharp teeth. The tyrannosaurus moved on powerful hind legs, supported by a thick tail. Its front legs were small and weak. The tyrannosaurs left fossilized footprints 80 cm long. The tyrannosaurus's step was 4 m.

Tyrannosaur.

Ceratosaurus was a relatively small but fast predator. It had a small horn on its head and a bone crest on its back. The ceratosaurus walked on its hind legs, each of which had three toes with large claws.

Torbosaurus was rather clumsy and hunted mainly for sedentary scolosaurs, which resembled modern armadillos in appearance. Thanks to their powerful jaws and strong teeth, torbosaurs easily chewed through the thick bony shell of scolosaurs.

Scolosaurus.

Flying lizards still continued to exist. The huge pteranodon, whose wingspan was 10 m, had a large skull with a long bony crest on the back of its head and a long toothless beak. The animal's body was relatively small. Pteranodons ate fish. Like modern albatrosses, they spent most of their lives in the air. Their colonies were located by the sea. Recently, the remains of another pteranodon were found in the Cretaceous sediments of America. Its wingspan reached 18 m.

Pteranodon.

Birds appeared that could fly well. Archeopteryx became completely extinct. However, some birds had teeth.

In Hesperornis - waterfowl- a long finger of the hind limbs was connected to three others by a short swimming membrane. All the fingers had claws. All that remained of the forelimbs were slightly bent humerus bones in the form of a thin stick. Hesperornis had 96 teeth. Young teeth grew inside the old ones and replaced them as soon as they fell out. Hesperornis is very similar to the modern loon. It was very difficult for him to move on land. Raising the front part of the body and pushing off the ground with its feet, Hesperornis moved in small jumps. However, he felt free in the water. He dived well, and it was very difficult for fish to avoid his sharp teeth.

Hesperornis.

Ichthyornis, contemporaries of Hesperornis, was the size of a dove. They flew well. Their wings were highly developed, and the chest bone had a high keel, to which powerful pectoral muscles were attached. The beak of Ichthyornis had many small teeth curved back. The small brain of Ichthyornis resembled the brain of reptiles.

Ichthyornis.

In the Late Cretaceous period, toothless birds appeared, whose relatives - flamingos - still exist today.

Amphibians are no longer different from modern ones. And mammals are represented by carnivores and herbivores, marsupials and placentals. They do not yet play a significant role in nature. However, at the end of the Cretaceous - beginning Cenozoic era When the giant reptiles became extinct, mammals spread widely across the Earth, taking the place of dinosaurs.

There are many hypotheses regarding the reasons for the extinction of dinosaurs. Some researchers believe that the main reason for this was mammals, of which many appeared at the end of the Cretaceous period. Predatory mammals exterminated dinosaurs, and herbivores intercepted plant food from them. A large group of mammals ate dinosaur eggs. According to other researchers, the main reason for the mass death of dinosaurs was a sharp change in physical and geographical conditions at the end of the Cretaceous period. Cold temperatures and droughts led to a sharp decrease in the number of plants on Earth, as a result of which the giant dinosaurs began to feel a lack of food. They were dying. And the predators for whom dinosaurs served as prey also died, since they had nothing to eat. Perhaps the sun's heat was not enough for the embryos to mature in dinosaur eggs. In addition, cold temperatures also had a detrimental effect on adult dinosaurs. Not having a constant body temperature, they depended on the temperature of the environment. Like modern lizards and snakes, they were active in warm weather, but moved sluggishly in cold weather, could fall into winter torpor and became easy prey for predators. Dinosaurs' skin did not protect them from the cold. And they hardly cared about their offspring. Their parental functions were limited to laying eggs. Unlike dinosaurs, mammals had a constant body temperature, and therefore suffered less from cold snaps. In addition, they were protected by wool. And most importantly, they fed their cubs with milk and took care of them. Thus, mammals had certain advantages over dinosaurs.

The birds that had constant temperature bodies and were covered with feathers. They incubated eggs and fed chicks.

Among the reptiles that survived were those that took refuge from the cold in burrows and lived in warm areas. From them came modern lizards, snakes, turtles and crocodiles.

The deposits of the Cretaceous period are associated with large deposits of chalk, coal, oil and gas, marls, sandstones, and bauxites.

The Cretaceous period lasted 70 million years.