The era of middle life is called. The Mesozoic Era - the "Era of Middle Life", is divided into three periods: Triassic, Jurassic and Cretaceous. Asteroceras obtusum is one of the species of ammonites that lived in the seas of the Jurassic period. At that time their shells were covered with much

How life arose and developed on Earth Gremyatsky Mikhail Antonovich

XII. Mesozoic ("middle") era

The Paleozoic era ended with a whole revolution in the history of the Earth: a huge glaciation and the death of many animal and plant forms. In the Middle Era we no longer find very many of the organisms that existed hundreds of millions of years before. Huge crayfish - trilobites, which were rampant in the seas of the Paleozoic, are disappearing, as if swept off the face of the Earth. Many echinoderms, entire families of sea urchins, starfish, sea lilies, etc. share their fate. Other echinoderms, however, remain in subsequent times, but they change greatly and develop in a completely new direction. Many coral species are disappearing. Great changes are also taking place in shellfish and fish. The land population is experiencing even more changes.

The heyday of tree ferns and horsetails has ended. Most of them did not survive the Paleozoic. Those species that still existed at the beginning of the Mesozoic era retained faint traces of their former splendor. They are found much less frequently, do not reach great heights, and often turn out to be completely short in stature. But conifers and sago trees flourish, and after a while they are joined by numerous new species flowering plants: palm trees are widespread. By its nature, the Mesozoic forest differs sharply from the forest of the ancient era. There was monotonous vegetation of gloomy tall trees. Here, coniferous and sago trees, palm trees, and behind them flowering plants give the earth's vegetation bright colors and cheerful tones. The fields are full of flowers.

The Mesozoic era is divided into three parts: the initial time - Triassic period, average - Jurassic period and later - chalky period.

At the beginning of Mesozoic time, a dry but warm climate was established, then it became more humid, but continued to remain warm. The Mesozoic era lasted, according to many geologists, about 120 million years, with more than half of this time accounting for the last, Cretaceous period.

Already in the first of these periods, a change in the animal world was sharply noticeable. In place of the disappeared inhabitants of the seas, long-tailed crayfish arose in large numbers, similar to those that now live in the seas and rivers. On land, along with amphibians, many new animals appeared, developing from amphibians and called reptiles, or reptiles. We know that their origin from amphibians is connected with the need to conquer new land areas far from water.

In our time, very few of the reptiles, or scaly reptiles, as they are sometimes called, live. We can find relatively small lizards, turtles, snakes and crocodiles. In Mesozoic times, one could also see large and small lizards everywhere, similar to the inhabitants of our forests and rocks. Turtles also lived in those days; Most of them were found in the seas. But besides the rather harmless turtles and lizards, there was a terrible crocodile-like reptile, the distant descendant of which is the current crocodile. There were no snakes at all until almost the end of the Mesozoic.

In Mesozoic times there were many other breeds of reptiles that have now completely disappeared.

Of their remains, we are especially interested in strange skeletons in which the characteristics of reptiles are mixed with the characteristics of mammals, i.e. those fur-covered animals whose females feed their young with milk (such as, for example, cows, pigs, cats, dogs and, in general, all carnivores , ungulates, rodents, monkeys, etc.). Amazing bones of beast-like reptiles have reached us, the structure of their legs and teeth was very reminiscent of mammals, which at that time did not yet exist on Earth. For its resemblance to animals, this breed received the name “beast-like”.

Rice. 31. Pareiasaurus (reptile, close to amphibians) - below and foreigners (reptile, close to mammals) - above

Among them is the famous foreigner, which was armed with sharp claws and powerful fangs, similar to the fangs of such predators as the lion and the tiger.

One can imagine the devastation such predators caused among the population of Mesozoic forests and steppes. They contributed to the death of ancient amphibians, thereby clearing the way for the unprecedented development of reptiles, which we see in the Jurassic and Cretaceous times.

Jurassic period. Changes in the plant world.

Bony fish. Reptiles

The Jurassic time brought a lot of new things both to the plant world and to the development of animals. Jurassic forests are already very different from Carboniferous forests: fern thickets have thinned out, gymnosperms and cycads have multiplied greatly. Cycads are similar in appearance to both ferns and palm trees. These are small trees with straight trunks, decorated at the top with long feathery leaves. They are descendants of seed ferns and in turn reproduced by seeds. Very few of them have survived to this day.

In Jurassic times, another group appeared - close relatives of cycads, the so-called bennettites. But their heyday dates back to the Cretaceous period. Bennettites also reproduced by seeds, which were collected in cones.

Some of the most remarkable Jurassic plants - ginkgo. One species - ginkgo biloba - now lives on Earth (in China and Japan). The leaves of these plants look like a fan and are collected in beautiful wide domes at the top. Their seeds taste like almonds; wood is very durable. A variety of ginkgo trees were very common on Earth during Jurassic times.

All these numerous plants energetically absorbed carbon (from the air) and accumulated reserves of complex organic substances, continuing the work that plants began in previous periods. The luxurious development of vegetation prepared a hitherto unheard of flourishing of animal life.

With the advent of the Jurassic period, animal life on Earth was enriched with new forms. In the seas, the evolution of fish led to the emergence of new breeds of fish - bony fish. They were strong rivals to the ancient cartilaginous fish, all those sharks, sturgeons, lobefin ​​fishes and lungfishes. It is worth observing the movements of fast, nimble bony fish to understand what their main advantage is over sedentary and clumsy cartilaginous species. From the middle of the Mesozoic, bony fish began to develop rapidly. They form many families, genera and species that fill the oceans, seas, lakes, and rivers. Even the greatest depths of the sea, in which, it would seem, no life is possible, give shelter to certain breeds of bony fish. Even light cannot penetrate this enormous depth.

The constant calm of cool water is occasionally disturbed by the appearance of strange, unprecedented forms of deep-sea creatures. Some of the deep-sea fish are almost devoid of eyes - they only have small rudiments of these organs, like a mole; in some, the eyes have completely disappeared, but at the front end of the muzzle there are huge luminous spots. Others have outgrowths with light organs at the ends (Fig. 32). The light emitted by the fish attracts prey to it, which even in the depths of the sea uncontrollably strives for light, like moths to a burning candle. In these inaccessible depths, brutal war and mutual devouring reign. There are fish there with huge mouths, with a stomach stretchable like a rubber bladder, and with long sharp teeth. With a deep-sea net it happened that a voracious predator with a transparent body was pulled out from there, in whose huge stomach the luminous fish it had recently swallowed still flickered.

Rice. 32. Marine fish recently found at a depth of 750 meters

The struggle for life has driven some bony fish into these monstrous depths; there these fish adapted to conditions in which it would seem impossible for anyone to live. But the vast majority of fish of a new form - bony ones - settled throughout the seas and rivers, displacing almost entirely the former inhabitants - sharks and other cartilaginous fish.

Life on land also advanced during this period. Forests, steppes and swamps were enriched with many species of reptiles. These animals were even more adapted to life on land than amphibians. The reptiles could have finally broken with the water. They are real inhabitants of forests, fields, mountains and valleys.

We know that they evolved from amphibians. How did this happen?

We saw that some fish developed lungs in the struggle for existence, and these fish, starting from the Carboniferous period, gradually began to turn into amphibians, which then spread widely across the Earth. Being associated with water, amphibians could not settle somewhere inland, in some desert area where the bright sun shines during the day. Their skin must be constantly moist; they only do well in damp places. Remember the frog.

Let's go back for a moment to the end of the Paleozoic, when the climate began to change dramatically. Glaciation has set in. At the same time, there was an uplift of vast expanses of land. The oceans and seas were receding. The swamps have become incomparably smaller. Vast dry plains and, in some places, deserts appeared. The amphibians had a hard time in the new environment: there was little water for the development of eggs and to maintain skin moisture. The adaptations that amphibians possessed were now insufficient for life on land. Some of them, like our toads, have warts on their skin. There were also those covered with scales. This made it easier for them to move to dry places and give rise to new breeds. But big changes had to happen to them too. First of all, in the method of reproduction. Laying eggs became impossible. It was replaced by a different way of development. First of all, the eggs began to linger longer in the body, where they grew and became covered with a dense shell. But this alone was not enough.

It is also necessary to take into account that eggs amphibians are very numerous and the larvae that hatch from them breathe through gills. They swim in the water for a long time and feed there on the food that they find in the mud and on aquatic plants. With the transition to terrestrial life, such development became impossible. On land, these helpless fish-like larvae are doomed to death. But they survive if the eggs turn into eggs, and gill respiration is replaced by pulmonary respiration. Neither reptiles nor their descendants - birds and mammals - ever have gill breathing, either in adulthood or in embryonic life. Even if these animals return to life in the water, such as whales, they rise to the surface of the water and draw air into their lungs to breathe. This is an important change, which is inevitably followed by others. Thus, a huge advantage in the struggle for existence was the formation in reptiles of two special embryonic membranes, which remained in all birds and mammals. One of them is called the water shell ( amnion), the other is respiratory ( allantois, urinary sac). Both of these shells serve to ensure that the developing embryo can use atmospheric air.

The egg of a reptile or bird is very different from the egg of a fish or amphibian. The egg contains a nutritious yolk - a supply of food for the embryo, which cannot obtain food for itself, like a frog tadpole. This food is enough for the embryo for the entire period of its development until it becomes able to feed on its own (in reptiles).

Complex egg The reptile is covered with a protective shell - a shell - which is not nearly as hard as that of birds. The eggs are laid on the ground, where they develop. As soon as the embryo is formed, a double fold grows from its abdominal wall, which expands and surrounds the entire embryo. Liquid accumulates between both folds, which is why these folds are called “ water shell" This shell separates the embryo from the outside world with its dangers and surprises. If someone pushes or rolls the egg, the water shell, like good springs, will protect it from shaking. If the air gets very hot, the water shell will prevent the egg from overheating or drying out; if it suddenly becomes cold, as happens at night in places with a dry climate, the shell will come to the aid of the embryo: the cold will not reach it so quickly through the layer of water.

Another embryonic membrane is the respiratory, or urine bag, - arises in a similar way as the water shell, and also consists of two layers. It serves mainly for breathing air. In this regard, the urinary sac lies outside the water sac, i.e., between the latter and the egg shell. This position is quite understandable: after all, it must be as close as possible to the outside air in order to absorb from it the oxygen necessary for breathing and release the carbon dioxide that accumulates in the embryo. On the surface of the urinary sac there is a dense branching network of blood vessels associated with the vessels of the embryo. Oxygen from the urinary sac is transported through blood vessels to the embryo.

The eggshell is pierced with many small holes, which are clearly visible through a magnifying glass. Through these holes, oxygen constantly leaks into the egg, and carbon dioxide leaves it. While the embryo is developing, the egg breathes vigorously. If these holes are covered up by covering, for example, an egg with varnish, then the embryo will soon die from suffocation, like a person whose throat has been squeezed. Therefore, the urinary sac serves for breathing and works like lungs, and not like gills. An egg placed in water cannot develop, and the embryo suffocates, like any pulmonary animal immersed in water. Such eggs, equipped with a water shell and a urinary sac, are laid by reptiles in the sand or hidden in a secluded burrow, warmed by the sun. After a few weeks, mobile juveniles hatch from them. If reptiles sometimes have to live in water, such as crocodiles or sea turtles, then they still come to the shore to reproduce and lay eggs.

It is clear that reptiles with such habits and adaptations can easily live in very dry areas. Indeed, many of them live permanently in deserts. Amphibians, even if they can sometimes live as adults in a very dry place, then it is already difficult for them to reproduce there.

The Jurassic period can rightfully be called the age of reptiles. Their prosperity was helped by the warm, uniform climate of that time, without sudden changes of heat and cold. It was warm everywhere - both in those countries where the climate is now hot, and in those where we live, that is, in a temperate climate, and even in the cold regions of the far north. The summer weather was smooth all year round. In places that are now permanently covered with ice, like Greenland, a mild and warm climate then reigned. The structure of the earth's surface in the Jurassic also favored the reproduction and settlement of reptiles. Then there were few mountains and other hills on Earth that would impede the movement of animals. All this prepared the way for an unprecedented flourishing of life on land.

It is difficult for us to even imagine how great the dominance of reptiles was then. In our climate, reptiles are hardly noticeable. Occasionally a green or gray lizard will scurry into the dry grass, even less often you will come across a grass snake or a viper, and very rarely you will be able to see a turtle in the wild. We only know crocodiles from zoological gardens and books. True, in warm regions it is still possible to come face to face with a crocodile and terrible snakes - a boa constrictor, a rattlesnake, a spectacled one; and now you can see huge turtles there, which a person could ride on. But modern monsters are pitiful small fry compared to those who lived in the Jurassic period. Then they were widespread throughout the Earth. And most of all there were those who completely disappeared a long time ago and gave way to new winners in the struggle of life.

In Jurassic times, monstrous reptiles swarmed everywhere. Some of them slowly and noisily wandered through the forests, knocking down huge trees with their heavy bodies, gnawing them and leaving a trail behind them, like a windbreak. Others, even larger ones, lived in swamps and devastated entire thickets. These included the largest land animals that ever lived. One of these monsters - the brontosaurus - reached a length of almost 20 meters and a height of 5 meters (Fig. 33). And this lizard weighed about 40 tons! And this huge carcass of meat was controlled by a very small brain sitting in a small head! One must think that the brontosaurus was not distinguished by either intelligence or speed of movement. Yes, this is hardly what he needed. Who would dare to attack such a strong man and giant? There were no such brave predators in those days. And it was difficult to attack him. The brontosaurus spent its time in the water, where it enjoyed chewing soft aquatic plants all day long. In the water, his body was very stable, because his legs were thick, like logs, and heavy, and his fat back, strengthened by the empty inside, very light dorsal vertebrae, was not heavy. Where the brontosaurus was neck-deep in water, any predator would have had to swim its way. This position is not very convenient for attackers.

Rice. 33. Brontosaurus (length about 20 meters) from the Jurassic deposits of North America

The Brontosaurus could have been accompanied by other equally huge herbivorous lizards, such as Diplodocus, which was even longer than the Brontosaurus (Fig. 34). The huge carcass of Diplodocus was supported only on a plant diet: plant food could then be obtained in abundance, but it was already difficult to get enough animal food to feed this huge body. Both now and in Jurassic times, the largest land animals were herbivores. But no modern elephant can compare either in height or weight with the reptiles of that time. They were at least five times larger than elephants. The structure of Diplodocus's teeth directly indicates a plant-based method of feeding: its teeth are small and weak and could only serve to grab soft plants. The nostrils opened on the upper side of the head; this was very convenient for the animal, which breathed air but spent its time in fairly deep water.

Rice. 34. Diplodocus (length about 30 meters) from the Jurassic layers of North America

Next to these huge but peace-loving vegetarians lived fierce predatory reptiles that recognized only meat food. With their huge sharp teeth, they brought no less horror to the living world of that time than lions and tigers do now.

We have already talked about one of the oldest predatory reptiles, about the foreignness discovered within our Union. Then the number of predators increased. One of them, Megalosaurus, lived in Western Europe. The huge bones of his legs were empty inside, which made jumping easier; The voids in the vertebrae also served the same purpose. This animal probably lay in the high thicket waiting for prey or lay in wait for it, hiding under the bushes. The prey was, presumably, mainly small animals. If any unwary lizard, carelessly hunting for insects, approached a predator, it instantly jumped to its feet and overtook the victim in one or two jumps. The sharp claws with which its paws were armed pierced the skin of the victim, penetrating the spaces between the scales or tearing the skin. The predator carried its prey away from the battlefield the way a cat carries away its prey. And then he used his saber-like teeth.

His relative was a small lizard known as compsognathus. It reached a height of only 35–40 centimeters. Looking at its skeleton, it is easy to imagine that it jumped or ran in a semi-erect position on its two hind legs, like a bird.

The largest of all predatory lizards was tyrannosaurus, truly a “terrible lizard”, “dinosaur”, as this entire group of extinct reptiles is called in science (Fig. 35). It reached 12–14 meters in length and 5–6 meters in height. Now there is no such huge predator on Earth. However, he was not very heavy to lift. This is evidenced by the voids in his bones, which lightened the weight of his body. He apparently lived at the very end of the Jurassic and in the next, Cretaceous, period.

Rice. 35. Tyrannosaurus (was 14 meters long)

In Jurassic times in North America, among the many large and small “saurs”, i.e. lizards, there lived another monster, which is impossible to keep silent about. When its remains were dug out of the ground, the strangest feature that caught everyone's eye was the huge bone plates protruding from its back. The plates were of unequal shape and reached a meter in diameter. The skull was amazingly small for such a huge animal and had short, thick jaws. Having carefully looked at the structure of the skull, we find that this animal had rather large eyes and, apparently, a good sense of smell: this is indicated by large eye sockets and a large nasal cavity. The jaws contained a row of teeth. When they wore off, new ones grew in their place. They indicate that he ate soft plant foods. But the teeth were not the strongest point of this monster.

The dorsal vertebrae had huge processes, strong and forked at the end, which supported heavy bone shields, as can be seen in our fig. 36. The front legs were thick and short, with five toes, the hind legs were much longer and stronger. If we add to this that a strong tail stretched from behind, then it is not difficult to guess that the animal often stood on its hind legs, while leaning on its tail, like on a tripod, like today's kangaroos. The hind legs had only three toes, covered with hooves. The front legs could move quite freely in different directions, like the forelimbs of monkeys, and help grasp food, and, if necessary, defend the animal. But for this purpose a strong tail, armed with powerful sharp spikes, could serve better: with one swing it could knock it down, or even kill any predator that dared to attack stegosaurus, as scientists called the described animal. One of the amazing features of the stegosaurus was the structure of its spinal cord. We have already said that his brain was very small. But the spinal cord in the sacrum area expanded greatly and seemed to form an additional brain, which was much larger than the brain. This “brain” apparently served to regulate movements. Such a beast, apparently, really was “strong in hindsight.”

Rice. 36. Stegosaurus (6 meters long)

Having mastered the land, the lizards multiplied so much and populated the Earth so densely that they began to experience overcrowding. Some of them could find more space and food for themselves in the water. Many reptiles, having adapted to life away from water, return again to their native element, to the water! But it is impossible to turn back the wheel of history of both the human and animal worlds. Returning to the water, the reptiles retained all their main acquisitions and adaptations for life on land and did not turn back into amphibians. They remained pulmonary animals, breathing atmospheric air, they did not begin to spawn in water, they retained their developed, well-ossified skeleton. At the same time, they acquired some new features necessary for aquatic existence, and in appearance they became more or less similar to fish.

The most famous aquatic reptile of Mesozoic times is the fish lizard, or ichthyosaur. It was a strong swimmer, equipped with an excellent motor for moving quickly through the water in search of prey, which it grabbed with its powerful jaws. Its engine was its long, muscular tail; side flippers helped speed and accuracy of movements. The head was pointed at the end, and the whole body had a streamlined shape, like a spindle, which reduced the resistance of water during rapid movement. The ichthyosaur reached 8 meters in height and was so strong that the most powerful sharks retreated before it. He hunted for fish, although his huge mouth, lined with sharp teeth, could grab any prey. Huge eyes sparkled on the sides of the head, surrounded by a ring of bones that protected them. As for the internal structure, the famous Cuvier, the founder of the science of fossil animals, beautifully said about it: “In the ichthyosaur we find the face of a dolphin, the teeth of a crocodile, the head and chest bone of a lizard, the flippers of a whale and the vertebrae of a fish!” Such is the strange mixture of features combined in the skeleton of the ichthyosaur (Fig. 37).

Rice. 37. Ichthyosaur

If this animal has such mixed characteristics of different groups, then what right do we have to say that it breathed with lungs, like any reptile, and not with gills, like fish? After all, the lungs are not preserved in fossil form. To solve this issue, the way is this: the gills of fish are always supported by special bones, which are called gill arches. Not the slightest trace of these arches has been found, although a lot of ichthyosaur skeletons have been excavated. Some museums have long held several dozen of them. In addition, the structure of the nasal cavity and nostrils of an ichthyosaur is exactly the same as that of other reptiles: the nostrils end with openings not at the end of the upper jaw, like in fish, but in front of the eyes, and from them there are special passages in the skull through which air the nostrils penetrated into the windpipe and lungs. Needing air to breathe, ichthyosaurs were forced to rise to the surface of the water from time to time. The caudal fin of ichthyosaurs is similar to that of a fish; it stands vertically and is especially well adapted to fast and powerful movements in the water. It is interesting to compare the tail fin of an ichthyosaur with that of a whale. In a whale, the fin lies transversely - in a horizontal plane and contributes much less to the speed of movement in this plane. This position of the fin is beneficial for the whale, as it makes it possible to quickly rise from the depths of the water to the surface for breathing. The whale, as a warm-blooded mammal, needs fresh oxygen incomparably more than the ichthyosaur, which, thanks to its cold blood, has less need for oxygen. If the whale did not have a fin located in this way, it would not have the means to swim to the surface of the sea with the necessary speed, especially since the whale has only one pair of side fins - the front ones. The fish lizard has both pairs of fins - front and rear, and they, of course, helped it swim from the depths to the upper layers of water.

Ichthyosaurs swarmed in the seas of the early Jurassic time and devoured countless numbers of small and larger fish. We have direct evidence of this; next to their skeletons, fossilized excretions of these animals, the so-called coprolites, are found; these are accumulations of undigested scales of cartilaginous fish, which, as we know, were especially numerous in those days.

The remains of other animals found along with the bones of ichthyosaurs show that these animals swam at shallow depths, not very far from the sea shores. And really, could an air-breathing fish lizard descend into the real depths of the sea? After all, he would have to spend too much time and effort to rise up to breathe.

Have ichthyosaurs ever come ashore? Previously, scientists thought that ichthyosaurs had to do this to lay eggs. However, it is difficult to allow ichthyosaurs with their fins and bare skin to dare to get onto land. How did they reproduce? Small skeletons of ichthyosaurs were sometimes found inside the skeleton of an adult ichthyosaur. These little skeletons were always completely intact, even undamaged. If ichthyosaurs devoured their young, then the bones they swallowed would be torn from one another, crushed, eaten, etc. But it is impossible to assume that ichthyosaurs always swallowed their young in their entirety. Therefore, we must think that they were viviparous and that their eggs were not laid in the sand, but developed in the mother’s body until the time when the embryo could independently swim in the water and catch fish. That there is nothing impossible in this is proven by the fact that among modern lizards there are also viviparous animals.

In the life of nature at that time, ichthyosaurs occupied the same place as whales now occupy in it. They even resembled whales in some external features: they had bare skin, their nostrils sat close to their eyes, like those of whales, their jaws were very elongated. But this strange similarity cannot be explained by the fact that ichthyosaurs are related to whales and that whales descended from ichthyosaurs. This similarity only shows that similar living conditions lead to similarities in some characteristics. In exactly the same way, whales are similar to fish in some features, but, of course, they are not closely related to fish.

No matter how strong, no matter how numerous the ichthyosaurs were, the time came when their days began to come to an end. Reptiles had to give up their place on Earth to other animals that were better organized. At one time, reptiles achieved dominance, but having begun to lag behind in the struggle for life, they almost became extinct by the end of the Cretaceous period. Major events on Earth at this time led to the extinction of many other ancient breeds of animals and plants.

But how widely these now extinct organisms spread in their time! Their remains were found in Europe, India, North America, Africa, Australia, and even the Arctic.

The climate in those days in all these places was almost the same and, moreover, soft and warm, semi-tropical. And one might think that it was climate change that was the first strong blow that led to their extinction. The appearance of other sea animals that competed for their prey was another reason for the death. The disappearance of ichthyosaurs was, of course, also facilitated by the extinction of the prey itself - some invertebrate animals and cartilaginous fish.

At this time, there was an increased extinction of two more large groups of animals: ammonites and belemnites - invertebrate animals belonging to soft-bodied animals, or mollusks - died out. Both of these groups were very numerous from the first half of the Paleozoic and were found in the seas in a variety of rocks. Their countless shells, preserved in different layers of the Earth, primarily attract the attention of a geologist studying the fossil world.

Usually these shells serve as the best guides in determining the antiquity of one or another layer of the earth's crust. Each layer, each of its divisions - a layer or tier - is characterized by its own breeds of ammonites with their own characteristics in the structure of the shell, characteristics that are easy to notice and convenient to describe. Both ammonites and belemnites belong to that class of soft-bodied animals called “cephalopods.” These are exclusively marine animals. Not many cephalopods live in modern seas and oceans: octopuses, cuttlefish and boats with beautifully curled shells. The ship (Fig. 38) is a very ancient animal, preserved almost unchanged from the Paleozoic era. It is considered a close relative of ammonites and belemnites. In most ammonites, like in the boat, the shell was spirally twisted in one plane and was divided inside by many partitions into a number of chambers following each other. The mollusk itself sits in the room closest to the entrance to the shell, in the so-called living chamber, while all the other chambers lying behind the living chamber are filled with gas and are therefore called “air chambers.” Passing through the middle of the partitions, a special organ stretches along the entire shell - a siphon, in which blood vessels are located. The mollusk has a complex organization, possessing well-developed sensory organs, a nervous system, gills and a muscular leg. It is believed that ammonites (Fig. 39) were predatory animals, some that swam well, others that crawled along the seabed. Belemnites had an internal shell with a long finger-shaped beak, which is usually the only one preserved. This is the so-called “devil's finger” (Fig. 40).

Rice. 38. A ship whose shell is shown open

Rice. 39. Fossilized shells of two ammonites

Rice. 40. Preserved part of a belemnite shell

Conquest of water and air by reptiles

The fish lizard we described was not the only reptile that adapted to life in the seas. We must say a few words regarding other marine predators that competed for prey with the fish lizards. The first place among them belongs to serpentine plesiosaurs.

Looking at the image of a plesiosaur (Fig. 41), we will understand why previous scientists compared it to a turtle with a snake threaded through it. The long, flexible neck and relatively small head are the first to catch your eye. The fins of a plesiosaur are very different from those of an ichthyosaur. The plesiosaur has limbs in the form of flippers, which retain five fingers, while the number of fingers in the ichthyosaur has increased significantly. Therefore, the plesiosaur managed to change less, adapting to aquatic life.

The difference between it and the fish lizard is especially great in the structure of the skull. The head of the ichthyosaur sat on the body without any neck, while in the plesiosaur the neck is the longest part of the body, and the head is small, with long jaws. The jaws had numerous cells in which teeth sat, like those of crocodiles (other reptiles have teeth simply attached to the jaws, without any cells). Those plesiosaurs that lived in the Jurassic period were small, reaching a length of just over two meters; their descendants in the Cretaceous period became much larger - sometimes five meters long or more.

Rice. 41. Plesiosaurs from Jurassic deposits. In the background on the right are ichthyosaurs

How did these animals swim? The ichthyosaur was most helped when swimming by its tail, equipped with a large vertical fin. But the plesiosaur's tail was neither particularly large nor particularly strong. This means that this swimmer could not count on him. He had to use his flippers the most. They were the main organs of movement and, given their size and strength, could successfully play this role. They looked like wide oars, two on each side of the body. There was not a trace of claws on them, even such weak ones as on the paws of turtles; therefore, we can think that plesiosaurs were at home in the sea, and not on land. It was almost impossible for them to crawl on the ground. Plesiosaurs, like all reptiles, breathed through the lungs and therefore had to swim to the surface to stock up on air. Plesiosaurs had quite a few close and distant relatives who filled the seas and lakes. We won't talk about them. Let's talk about just one creature to put an end to aquatic reptiles, the largest and most ferocious predator of the Mesozoic seas - the mosasaurus.

Mosasaurs appeared and flourished towards the end of the Mesozoic. There were especially many of them in America during the Cretaceous period. Until now, in some places, researchers have found thousands of skeletons of these animals buried in layers of earth. Among such a multitude of bones, there are also completely intact skeletons. They reached a length of 14 meters, had an elongated body, like a snake, and a very long tail; their head was large, flattened and pointed at the end, and their eyes were directed upward. The body was equipped with two pairs of fins, resembling the flippers of a whale and always containing the bones of five-fingered limbs. With their help, with the assistance of the tail and thanks to the bends of their body, they could swim very quickly. The mouth was lined with several rows of teeth, and the jaws were designed in a special way to swallow even very large prey whole. If people had lived at that time, it would have been easy for a mosasaurus to swallow a whole person. The jaw bones did not fuse together, but were connected by tensile, rubber-like ligaments, and the mouth could expand as much as needed, depending on the size of the prey. The same jaw structure is found in modern snakes. Adapting to life in water, reptiles acquired features that greatly distinguished them from their land-based counterparts. Aquatic life puts a strong stamp on animals, as seen in whales, seals and other aquatic mammals.

Rice. 42. Mosasaurus

But the reptiles did not stop at conquering the earth's surface and waters. They began to take over the air. In the Mesozoic era, the great ocean of air was no longer inhabited only by insects, such as dragonflies, grasshoppers, butterflies and moths. Numerous finds of fossil bones show that during the Mesozoic, some reptiles acquired the ability to fly and, in turn, populated the air. While there were no birds, these flying lizards were masters of the situation in the air; flocks of them noisily roamed the sky in all directions, chasing each other or looking for prey. How did reptiles manage to become flyers?

There are two ways to fly in the air. Real flight can be called active: we see this kind of flight in birds and technically carry it out on airplanes. Another flight - passive - consists of gliding through the air, like on a parachute. During passive flight, the animal only delays and slows down its fall with the help of the flight membrane. During active flight, it can rise into the air and control its movement there. In living vertebrates, both active and passive flight can be observed.

Some fish in the hot zone of the Earth can, with the help of strong blows of their tail, jump out of the water and rush over its surface for a hundred to one and a half meters, using their front fins, which are greatly enlarged in these fish. Sometimes they rise so high above the water that they happen to fly onto the deck of the ship and land on it from fatigue. This kind of flying fish lived in earlier times, as we know from their fossil bones and prints.

Rice. 43. Flying frog

Using other examples, you can see that passive flight most often develops in jumping animals. Here in front of you in Fig. 43 flying frog. When making large jumps, this tree frog spreads its fingers, between which a particularly wide membrane is stretched. With its help, the frog arrests its fall to the ground and glides through the air. Of course, she is not able to take off from the ground. In the same countries where flying frogs live, there is also a so-called “dragon”, that is, a flying lizard. Her flight membrane is strengthened on the ribs that strongly protrude to the sides. This dragon reaches 25 centimeters in length.

Finally, there is a flying snake; she lives on the island of Borneo (south of the Asian mainland). Unwinding her elastic body, like a spiral, she throws herself from the tree obliquely downwards, and the concave abdominal surface, presenting significant resistance to the air, protects her from falling to the ground; the snake descends with a smooth movement.

The flying dinosaurs of the Mesozoic were completely different creatures. They appeared from the Triassic period, i.e. from the beginning of the Mesozoic era, and existed until the end of the Cretaceous period. Over this huge period of time they have changed relatively little; only their structure became more and more adapted to flight. The flying lizards varied greatly in size. Some were as tall as sparrows, others had a wingspan of up to 8 meters. Some of the earlier ones had long tails and sharp teeth, while the later ones had shorter tails and no longer developed teeth. One cannot help but see the similarity with birds in this, but this similarity does not prove the close relationship between birds and flying lizards. The similarity is caused by adaptations to flight, which developed completely independently in birds and in flying lizards.

When fossil bones of flying lizards were first discovered, scientists were divided: some said they were the bones of special birds, others considered them to be bat-like mammals. Indeed, flying lizards have some similarities with both. Finally, about 130 years ago, the famous French scientist Cuvier took up these wonderful bones. He became convinced that the bones belonged to reptiles that were capable of flight. Cuvier investigated how the wings of these animals were constructed. They consisted of a leathery membrane, like those of bats, but were not stretched, like those of those, between elongated fingers, but went from the hind legs to the front and were attached in front to a very elongated little finger. For this arrangement of wings, Cuvier named these animals digitigrade, or pterodactyls. They are still known under this name (Fig. 44).

Rice. 44. Pterodactyl from Jurassic deposits

Cuvier noticed the huge eye sockets of these animals and decided that they had very large eyes, like an owl, and that they were probably nocturnal. Later, other pterodactyls were discovered, with small eye sockets. This means that among them, some flew more during the day, others at night. Some of them could, folding their wings, crawl along the ground, clinging to it with sharp claws; others hung from trees or rocks like bats; many flew over the seas and hunted fish, as sea gulls, albatrosses and other birds do now. Small breeds ate insects, which they caught with their wide beaks. And those that had a wingspan of several meters had enormous strength and could probably drag away heavy prey in their claws. Among them there were also those who ate fruits, as some of today's bats do. Of course, all pterodactyls often had to sit on the ground to rest, and among them there were no such tireless flyers as there are among birds.

We have not named even a tenth of those monsters that the Earth bore on itself in the Jurassic and Cretaceous periods. We haven't even mentioned some of the biggest ones. The largest of them were the size of a two- or three-story house. The fossilized skeletons of such lizards are kept in museums, where one such skeleton sometimes occupies two huge floors.

It would seem that reptiles, so large and strong, giving rise to so many breeds and not knowing any rivals for many millions of years, should have remained masters on Earth forever. But precisely when reptiles occupied a dominant position among other animals, the tirelessly continuing struggle for life led to the appearance on Earth of the first small and insignificant mammals, which the huge lizards probably didn’t even notice at first. And yet the mammals turned out to be the gravediggers of the giant lizards.

Around the same time, another great event took place in the history of the Earth. The first ones arose birds. Their remains have reached us. From them it is possible to some extent reconstruct the history of the origin of these wonderful creatures.

Origin of birds

In ancient fairy tales and legends, people are endowed with supernatural powers and are often depicted as flying through the air. But it was only about 150 years ago that science first came close to this issue, and the fantasy began to become reality. The first flights began balloons. Aeronautics remained at this stage until the end of the 19th century, when a new and major step forward was taken in the development of aeronautical technology - an aircraft was created that lifted both the pilot, the engine, and the fuel supply. But even now, despite the enormous achievements of aviation, modern airplanes in some respects it is still far from the perfection that distinguishes the amazing “flying machine” - the bird. Achieving avian perfection in flight is the task of future technology.

The remains of an ancient representative of birds - first birds- amazingly preserved to this day.

It was in the Jurassic period. If a person could travel back to that time, he would see a huge shallow sea covered with countless islands and islets on the site of most of modern Europe. Rich life flourished in the warm waters of this sea. Variegated corals piled up their structures, and countless fish, crustaceans and worms found shelter in them. There were especially many soft-bodied animals with a variety of shells (ammonites, belemnites). From time to time, the crocodile-shaped head of an ichthyosaur protruded from the water and the long swan neck of a plesiosaur, those voracious predators of the then seas, rose.

The bottom of the sea was completely strewn with many shells, shells and skeletons of dead animals and consisted of the most delicate and minute calcareous silt. Gusts of wind often brought plant seeds from neighboring islands, which bordered the limestone shores with a green frame, and sometimes insects - large dragonflies, flying in the air after prey. Having fallen onto soft mud, these animals often left delicate imprints of their structure on it. The tides carried with them the bodies of other animals. They threw the remains of marine rocks onto land, and carried the land ones into the sea. These latter found a grave for themselves here in the soft limestone mud, in which from year to year, from century to century, more and more remains and imprints of living beings accumulated.

The silt of the seabed gradually turned into an underwater museum, preserving countless remains of plants and animals of that time. Even those of them that did not have hard skeletal parts, but consisted entirely of a soft gelatinous substance, sometimes left their traces on it. Their tender bodies were enveloped in a soft mass that gradually hardened; when there was nothing left of the animal, a kind of death mask made of hardened, often petrified silt was preserved at the place of its burial.

Millions of years passed slowly. If we could speed up their flow and observe, as in cinema, the changes that took place where the Central European plain now extends, we would notice how the seabed rose and the waves receded, how the earth’s crust moved, how mountains arose and grew, how some plants and animals were replaced by others, until finally a picture of modern Europe emerged.

Mesozoic era - the Middle Ages of the earth Life takes over land and air What changes and improves living beings? The collections of fossils collected in the geological and mineralogical museum have already told us a lot: about the depths of the Cambrian sea, where people similar to