Origin of amphibians. Landfall Reduction of skin mucus glands and appearance of horn formations

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• Introduction
• 6. Occurrence of the amnion
• 9. Live birth
• Conclusion

Introduction

The emergence of vertebrates from water to land was a major step in the history of the development of the animal world, and therefore the discussion of the origin of amphibians is of particular interest. Amphibians were the first vertebrates to have dissected and finger-bearing limbs, switched to pulmonary respiration and thus began the development of the terrestrial environment.

The arid climate of the continental regions, characteristic of the Devonian period, put the inhabitants of drying up reservoirs or reservoirs with oxygen-poor water in the most unfavorable conditions. Under such conditions, the vital advantage remained with those fish that could use their swim bladder as a respiratory organ and thus endure temporary drying up and survive until a new rainy period in order to return to the fish lifestyle again.

This was the first step on the way out of aquatic environment. But it was still far from the real development of the conditions of terrestrial life. The most that lungfish could then achieve was the ability to passively survive the unfavorable season, hiding in the silt.

But the Devonian period was replaced by the Carboniferous period. Its very name speaks of a huge mass of plant remains that formed layers of coal in shallow water conditions. And the magnificent development of tree-like spore plants, and the fact that these plants did not decay on the surface, but were charred under water - all this testifies to the damp and hot climate that prevailed at that time over vast areas of the Earth.

The changed climate also created new conditions for the descendants of the Devonian lungfish. One of them, the ability to breathe air came in handy in connection with life in warm swampy reservoirs with decaying vegetation (these are approximately the same conditions in which the Amazonian flake lives now); others, in whom the internal changes in the process of metabolism and the action of natural selection have developed the ability to temporarily do without water, in the damp atmosphere of the coal forests could already lead a more active life - move and get their own food.

The emergence of vertebrates on land occurred in the late Devonian era, approximately 50 million years after the first land conquerors - psilophytes. At this time, the air was already mastered by insects, and the descendants of lobe-finned fish began to spread across the Earth. The new mode of transportation allowed them to move away from the water for some time. This led to the emergence of vertebrates with a new way of life - amphibians. Their most ancient representatives - Ichthyostegs - were found in Greenland in Devonian sedimentary rocks. The short five-fingered paws of the ichthyosteg, thanks to which they could crawl on land, looked more like flippers. The presence of a caudal fin, a body covered with scales, indicates the aquatic lifestyle of these animals.

The heyday of ancient amphibians is dated to the Carboniferous. It was during this period that stegocephals (shell-headed) were widely developed. Their body shape resembled newts and salamanders. The reproduction of stegocephalians, like modern amphibians, occurred with the help of caviar, which they spawned into the water. In the water, larvae developed, which had gill breathing. Because of this feature of reproduction, amphibians have forever remained connected with their cradle - water. They, like the first land plants, lived only in the coastal part of the land and could not conquer inland massifs located far from water bodies.

vertebral land air breathing

1. Prerequisites for the emergence of vertebrates on land

A dense "brush" of helophytes (one might call it "rhinophyte reeds") that appeared in coastal amphibiotic landscapes begins to act as a filter that regulates mantle runoff: it intensively filters (and precipitates) detrital material carried from the land and thereby forms a stable coastline. Some analogue of this process can be the formation of "alligator ponds" by crocodiles: animals constantly deepen and expand the swamp reservoirs inhabited by them, throwing soil ashore. As a result of their many years of "irrigation activity", the swamp turns into a system of clean deep ponds, separated by wide forested "dams". So the vascular vegetation in the Devonian divided the notorious amphibious landscapes into "real land" and "real freshwater reservoirs."

It is precisely with the newly emerged freshwater bodies that the appearance in the Late Devonian of the first tetrapods (quadrupeds) is associated - a group of vertebrates with two pairs of limbs; it combines in its composition amphibians, reptiles, mammals and birds (simply speaking, tetrapods are all vertebrates, except fish and fish-like). It is now generally accepted that tetrapods are descended from lobe-finned fishes (Rhipidistia); this relict group now has the only living representative, the coelacanth. The once popular hypothesis of the origin of tetrapods from another relict group of fish - lungfish (Dipnoi), now has practically no supporters.

The Devonian period, in which stegocephalians arose, was apparently characterized by seasonal droughts, during which life in many fresh water bodies was difficult for fish. The depletion of water with oxygen and the difficulty of swimming in it was facilitated by abundant vegetation that grew in the Carboniferous time in swamps and on the banks of reservoirs. Plants fell into the water. Under these conditions, adaptations of fish to additional breathing with lung sacs could arise. In itself, the depletion of water with oxygen was not yet a prerequisite for landfall. Under these conditions, lobe-finned fish could rise to the surface and swallow air. But with a strong drying up of reservoirs, life for fish became already impossible. Unable to move on land, they perished. Only those of the aquatic vertebrates, which, along with the ability to pulmonary respiration, acquired limbs capable of providing movement on land, could survive these conditions. They crawled out onto land and crossed into neighboring reservoirs, where water was still preserved.

At the same time, movement on land for animals covered with a thick layer of heavy bone scales was difficult, and the bony scaly shell on the body did not provide the possibility of skin respiration, which is so characteristic of all amphibians. These circumstances, apparently, were a prerequisite for the reduction of the bone armor on most of the body. In separate groups of ancient amphibians, it was preserved (not counting the shell of the skull) only on the belly.

2. The appearance of a five-fingered limb

In most fish, in the skeleton of paired fins, a proximal section is distinguished, consisting of a small number of cartilaginous or bony plates, and a distal section, which includes a large number of radially segmented rays. The fins are inactively connected to the limb girdle. They cannot serve as a support for the body when moving along the bottom or on land. In lobe-finned fish, the skeleton of paired limbs has a different structure. Total their bone elements are reduced and they are larger. The proximal section consists of only one large bone element corresponding to the humerus or femur of the forelimbs or hind limbs. This is followed by two smaller bones, homologous to the ulna and radius or the tibia and tibia. They are supported by 7-12 radially arranged beams. Only homologues of the humerus or femur are involved in connection with the limb belts in such a fin, so the fins of the lobe-finned fish are actively mobile (Fig. 1 A, B) and can be used not only to change the direction of movement in the water, but also to move along a solid substrate . The life of these fish in shallow, drying up reservoirs in the Devonian period contributed to the selection of forms with more developed and mobile limbs. The first representatives of the Tetrapoda - stegocephals - had seven- and five-fingered limbs, resembling the fins of lobe-finned fish (Fig. 1, B)

Rice. Fig. 1. Skeleton of the limb of a lobe-finned fish (A), its base (B) and the skeleton of the forepaw of a stegocephalus (C): I-humerus, 2-ulna, 3-radius.

In the skeleton of the wrist, the correct radial arrangement of bone elements in 3-4 rows is preserved, 7-5 bones are located in the wrist, and then the phalanges of 7-5 fingers also lie radially. In modern amphibians, the number of fingers in the limbs is five or their oligomerization to four occurs. Further progressive transformation of the limbs is expressed in an increase in the degree of mobility of bone joints, in a decrease in the number of bones in the wrist, first to three rows in amphibians and then to two in reptiles and mammals. In parallel, the number of phalanges of the fingers also decreases. Also characteristic is the lengthening of the proximal limbs and the shortening of the distal ones.

The location of the limbs also changes during evolution. If in fish the pectoral fins are at the level of the first vertebra and are turned to the sides, then in terrestrial vertebrates, as a result of the complication of orientation in space, a neck appears and mobility of the head occurs, and in reptiles and especially in mammals, in connection with raising the body above the ground, the forelimbs move backwards and are oriented vertically rather than horizontally. The same applies to the hind limbs. The variety of habitat conditions provided by the terrestrial lifestyle provides a variety of forms of movement: jumping, running, crawling, flying, digging, climbing rocks and trees, and when returning to the aquatic environment, swimming. Therefore, in terrestrial vertebrates, one can find both an almost unlimited variety of limbs and their complete secondary reduction, and many similar adaptations of limbs in various environments repeatedly arose convergently (Fig. 2).

However, in the process of ontogenesis in most terrestrial vertebrates, common features in the development of the limbs: the laying of their rudiments in the form of poorly differentiated folds, the formation of six or seven rudiments of fingers in the hand and foot, the extreme of which are soon reduced and only five develop in the future.

Rice. 2 Skeleton of the forelimb of terrestrial vertebrates. A-frog - B-salamander; B-crocodile; G-bat; D-person: 1-humerus, 2-radius, 3-carpus, 4-pascarpus, 5-phalanges, 6-ulna

3. Reduction of skin mucus glands and the appearance of horn formations

In amphibian larvae, the epidermis also contains a large number of glandular cells, but in adult animals the latter disappear and are replaced by multicellular glands.

In legless amphibians, in the anterior half of each segment of their annular body, in addition to glands of the usual type, there are also special giant skin glands.

In reptiles, the skin is devoid of glands. As an exception, they have only individual large glands that carry special functions. So, crocodiles have a pair of musk glands on the sides of the lower jaw. Turtles have similar glands at the junction of the dorsal and ventral shields. In lizards, special femoral pores are also observed, but they push out of themselves in the form of a papilla only a mass of keratinized cells and therefore can hardly be attributed to glands (some authors compare these formations with hair).

The skin of reptiles, freed from the function of respiration, undergoes significant changes aimed at protecting the body from drying out. There are no skin glands in reptiles, since the need for wetting the skin has disappeared. Evaporation of moisture from the surface of the body has decreased, since the entire body of these animals is covered with horny scales. A complete break with the aquatic environment leads to the fact that the osmotic pressure in the body of reptiles becomes independent of environment. The keratinization of the skin, which makes it impermeable to water, removes the threat of changes in osmotic pressure even when the reptiles switch to an aquatic lifestyle for the second time. Since water enters the body of reptiles only voluntarily along with food, the osmoregulatory function of the kidneys almost completely disappears. Reptiles do not need, like amphibians, to remove the constantly arising excess water from the body. On the contrary, they, like land animals, need to economically use the water in the body. Trunk kidneys (mesonephros) of amphibians are replaced in reptiles by pelvic kidneys (metanephros).

Birds also do not have skin glands, with the exception of only one paired gland that has a special function. This is the coccygeal, which usually opens with a pair of holes above the last vertebrae. It has a rather complex structure, consists of numerous tubes that converge radially to the excretory channel, and releases an oily secret that serves to lubricate the feathers.

Mammals adjoin the abundance of skin glands to amphibians. In the skin of mammals, there are multicellular glands of both main types - tubular and alveolar. The first include sweat glands, which look like a long tube, the end of which is often folded into a ball, and the rest is usually curved in the form of a corkscrew. In some lower mammals, these glands are almost sac-shaped.

4. Appearance of air breathing organs

The similarity of the lungs of lower terrestrial vertebrates with the swim bladder for which fish have long led researchers to the idea of ​​the homology of these formations. In this general form, however, this widespread opinion encounters considerable difficulties. The swim bladder of most fish is an unpaired organ that develops in the dorsal mesentery. It is supplied with intestinal arterial blood and gives venous blood partly to the cardinal, partly to the portal vein of the liver. These facts undoubtedly speak against this theory. However, in some fish, a paired swim bladder is observed, which communicates with the abdominal wall of the esophagus and, moreover, further ahead. This organ is supplied, like the lungs of terrestrial vertebrates, with blood from the fourth pair of branchial arteries and gives it directly to the heart (in the venous sinus in lungfish and in the adjacent part of the hepatic vein in Polyptorus). It is quite clear that we are dealing here with formations of the same kind as the lungs.

Thus, the above hypothesis about the origin of the lungs can be accepted with certain limitations - the lungs of terrestrial vertebrates are the result of further specialization (as a respiratory organ) of the pulmonary bladder.

Based on the fact that the lungs in amphibians are formed in the form of paired sac-like outgrowths behind the last pair of gill sacs, Goethe suggested that the lungs are the result of the transformation of a pair of gill sacs. This theory can be brought closer to the first, if we assume that the swim bladder has the same origin. Thus, some authors believe that the swim bladder of fish and the lungs of terrestrial vertebrates developed independently (divergently) from the last pair of gill sacs.

At present, it can be considered that Goethe's theory of the origin of the lungs is most consistent with the facts. As regards the question of the origin of the swim bladder of fishes, we can only accept the same origin for the paired bladder of multifinned ganoids and lungfish as for the lungs. In such a case, there is also no need to accept a completely independent development of these organs. The lungs of terrestrial vertebrates are specialized paired swim bladders. The latter arose by transformation from a pair of gill sacs.

5. Occurrence of homoiothermia

Homeothermy is a fundamentally different way of temperature adaptations, which arose on the basis of a sharp increase in the level of oxidative processes in birds and mammals as a result of evolutionary improvement of the circulatory, respiratory and other organ systems. Oxygen consumption per 1 g of body weight in warm-blooded animals is tens and hundreds of times greater than in poikilothermic animals.

The main differences between homoiothermic animals and poikilothermic organisms:

1) a powerful flow of internal, endogenous heat;

2) the development of an integral system of efficiently operating thermoregulatory mechanisms, and as a result 3) the constant flow of all physiological processes in the optimal temperature regime.

Homeothermal keep a constant heat balance between heat production and heat transfer and, accordingly, maintain a constant high temperature body. The body of a warm-blooded animal cannot be temporarily "suspended" in the way that occurs during hypobiosis or cryptobiosis in poikilotherms.

Homeothermic animals always produce a certain minimum of heat production, which ensures the functioning of the circulatory system, respiratory system, excretion, and others, even when at rest. This minimum is called the basal metabolic rate. The transition to activity increases the production of heat and, accordingly, requires an increase in heat transfer.

Warm-blooded animals are characterized by chemical thermoregulation - a reflex increase in heat production in response to a decrease in environmental temperature. Chemical thermoregulation is completely absent in poikilotherms, in which, if additional heat is released, it is generated due to the direct motor activity of animals.

In contrast to poikilothermic processes, under the action of cold in the body of warm-blooded animals, oxidative processes do not weaken, but intensify, especially in skeletal muscles. In many animals, tremors are first observed - uncoordinated muscle contraction, leading to the release of thermal energy. In addition, the cells of muscle and many other tissues emit heat even without the implementation of working functions, coming into a state of a special thermoregulatory tone. With a further decrease in the temperature of the medium, the thermal effect of thermoregulatory tone increases.

When additional heat is produced, lipid metabolism is especially enhanced, since neutral fats contain the main supply of chemical energy. Therefore, the fat reserves of animals provide better thermoregulation. Mammals even have specialized brown adipose tissue, in which all the released chemical energy is dissipated in the form of heat, i.e. goes to warm the body. Brown adipose tissue is most developed in animals living in cold climates.

Maintaining the temperature due to the increase in heat production requires a large expenditure of energy, therefore, with an increase in chemical thermoregulation, animals either need a large amount of food or spend a lot of fat reserves accumulated earlier. For example, the tiny shrew has an exceptionally high metabolic rate. Alternating very short periods sleep and activity, it is active at any hour of the day and eats food 4 times more than its own weight per day. The heart rate in shrews is up to 1000 per minute. Also, birds that stay for the winter need a lot of food: they are afraid not so much of frost as of starvation. So, with a good harvest of spruce and pine seeds, crossbills even breed chicks in winter.

Strengthening chemical thermoregulation, therefore, has its limits, due to the possibility of obtaining food. With a lack of food in winter, this way of thermoregulation is ecologically unfavorable. For example, it is poorly developed in all animals living beyond the Arctic Circle: arctic foxes, walruses, seals, polar bears, reindeer and others. For the inhabitants of the tropics, chemical thermoregulation is also little characteristic, since they practically do not need additional heat production.

Within a certain range of external temperatures, homoiotherms maintain body temperature without spending additional energy on it, but using effective mechanisms of physical thermoregulation, which make it possible to better retain or remove the heat of basal metabolism. This temperature range, within which animals feel most comfortable, is called the thermoneutral zone. Beyond the lower threshold of this zone, chemical thermoregulation begins, and beyond the upper threshold, energy is spent on evaporation.

Physical thermoregulation is environmentally beneficial, since adaptation to cold is carried out not due to additional heat production, but due to its preservation in the body of the animal. In addition, it is possible to protect against overheating by enhancing heat transfer to the external environment.

There are many ways of physical thermoregulation. In the phylogenetic series of mammals - from insectivores to bats, rodents and predators, the mechanisms of physical thermoregulation become more and more perfect and diverse. These include reflex constriction and expansion of the blood vessels of the skin, which changes its thermal conductivity, a change in the heat-insulating properties of fur and feather cover, countercurrent heat exchange by contacting blood vessels during the blood supply to individual organs, and regulation of evaporative heat transfer.

The thick fur of mammals, feathers and especially the down cover of birds make it possible to keep a layer of air around the body with a temperature close to that of the animal's body, and thereby reduce heat radiation to the external environment. Heat dissipation is regulated by the slope of the hair and feathers, seasonal change fur and plumage. The exceptionally warm winter fur of Arctic mammals allows them to do without a significant increase in metabolism in cold weather and reduces the need for food. For example, Arctic foxes on the coast of the Arctic Ocean consume even less food in winter than in summer.

In marine mammals - pinnipeds and whales - a layer of subcutaneous adipose tissue is distributed throughout the body. The thickness of subcutaneous fat in some species of seals reaches 7-9 cm, and its total mass is up to 40-50% of body weight. The heat-insulating effect of such a "fat stocking" is so high that the snow does not melt under the seals lying on the snow for hours, although the animal's body temperature is maintained at 38°C. In animals of a hot climate, such a distribution of fat reserves would lead to death from overheating due to the impossibility of removing excess heat, so fat is stored locally in them, in separate parts of the body, without interfering with heat radiation from a common surface (camels, fat-tailed sheep, zebu, etc.). ).

Countercurrent heat exchange systems that help maintain a constant temperature of internal organs are found in the paws and tails of marsupials, sloths, anteaters, prosimians, pinnipeds, whales, penguins, cranes, etc. At the same time, the vessels through which heated blood moves from the center of the body are in close contact with the walls of the vessels that direct the cooled blood from the periphery to the center, and give them their heat.

Of no small importance for maintaining the temperature balance is the ratio of the surface of the body to its volume, since in the final analysis the scale of heat production depends on the mass of the animal, and heat exchange occurs through its integuments.

6. Occurrence of the amnion

All vertebrates are divided into primary aquatic - Anamnia and primary terrestrial - Amniota, depending on the conditions under which their embryonic development occurs. The evolutionary process in animals was associated with the development of a new habitat - land. This can be seen both in invertebrates, where the highest class of arthropods (insects) became an inhabitant of the terrestrial environment, and in vertebrates, where the land was mastered by higher vertebrates: reptiles, birds and mammals. Landing was accompanied by adaptive changes at all levels of organization - from biochemical to morphological. From the standpoint of developmental biology, adaptations to a new environment were expressed in the appearance of adaptations that preserve the living conditions of their ancestors for the developing embryo, i.e. aquatic environment. This applies both to ensuring the development of insects and higher vertebrates. In both cases, the egg, if development occurs outside the mother's body, is dressed in shells that provide protection and preservation of the macrostructure of the semi-liquid contents of the egg in the air. Around the embryo itself, which develops inside the egg membranes, a system of embryonic membranes is formed - amnion, serosa, allantois. The embryonic membranes in all Amniota are homologous and develop in a similar way. Development until the exit from the egg takes place in an aquatic environment, which is preserved around the embryo with the help of the amniotic membrane, by the name of which the entire group of higher vertebrates is called Amniota. Insects also have a functional analogue of the vertebrate amnion. Thus, the problems find a common solution for two such different groups animals, each of which can be considered the highest in its evolutionary branch. The amniotic membrane forms an amniotic cavity around the embryo, filled with a fluid whose salt composition is close to that of the cell plasma. In reptiles and birds, the embryo rising above the yolk is gradually covered in front, on the sides and behind by a double fold formed by the ectoderm and parietal mesoderm. The folds close over the embryo and grow together in layers: the outer ectoderm with outer ectoderm, the parietal mesoderm underlying it with the parietal mesoderm of the opposite fold. At the same time, the entire embryo and its yolk sac are covered from above by the ectoderm and the underlying parietal mesoderm, which together form the outer shell - serosa. The ectoderm of the inner part of the folds, facing the embryo, and the parietal mesoderm covering it, close over the embryo, forming the amniotic membrane, in the cavity of which it develops. Later, an outgrowth of its ventral wall (endoderm with visceral mesoderm) appears in the embryo in the region of the hindgut, which increases and occupies the exocel between the serosa, amnion, and yolk sac.

This outgrowth is the third germinal membrane, called the allantois. In the visceral mesoderm of the allantois, a network of vessels develops, which, together with the vessels of the serous membrane, come close to the subshell membranes and the shell membrane of the egg, which is equipped with pores, providing gas exchange of the developing embryo.

The preadaptations preceding the formation of the Amniota embryonic membranes (their common "promising standard") during evolution can be illustrated by two examples.

1. Notobranchius (Notobranchius) and Aphiosemion (Aphiosemion) fish in Africa and Cynolebias (Cynolebias) in South America live in drying up water bodies. Caviar is laid back in water, and its development occurs during a drought. Many adult fish die during drought, but the laid eggs continue to develop. In the rainy season, fry are hatched from the eggs, immediately capable of active feeding. The fish grow rapidly and at the age of 2 - 3 months already lay eggs themselves. At the same time, at first there are only a few eggs in the clutch, but with age and growth, the size of the clutches increases. It is interesting that the adaptation to reproduction in periodically drying up water bodies led to the dependence of development on this factor: without preliminary drying, the eggs lose their ability to develop. So, for the development of the golden-striped afiosemion, its caviar must go through a six-month drying in the sand. In the eggs of these fish, the yolk under the embryo liquefies and the embryo begins to sink into it, dragging along the upper wall of the yolk sac. As a result, folds from the outer walls of the yolk sac close around the embryo, forming a chamber that retains moisture and in which the embryo experiences drought. This example shows how the embryonic membranes of Amniota could arise, and it seems to imitate and anticipate the way and way of formation of the amnion and serosa in higher vertebrates.

2. The embryo of primitive reptiles, whose eggs are devoid of protein, increases in the process of development, separates from the yolk and rests on the shell. Unable to change the shape of the shell, the embryo sinks into the yolk, and the extraembryonic ectoderm (according to actual data, it was the first one) closes in double folds above the sinking embryo. Later, the parietal mesoderm grows into the folds.

A comparison of these two examples suggests a possible scheme for the evolutionary origin of two of the three germinal membranes - serosa and amnion.

The origin of allantois was originally associated with the excretion of nitrogen metabolism products in the embryogenesis of higher vertebrates. In all amniotes, the allantois performs one common function - the function of a kind of embryonic bladder. In connection with the early functioning of the kidney of the embryo, it is believed that allantois arose as a result of the "premature" development of the bladder. The bladder is also present in adult amphibians, but is not developed in any noticeable way in their embryos (A. Romer, T. Parsons, 1992). In addition, allantois performs a respiratory function. Connecting with the chorion, the vascularized chorioallantois acts as a respiratory system, absorbing oxygen entering through the shell and removing carbon dioxide. In most mammals, allantois is also located under the chorion, but already as an integral part of the placenta. Here, the allantois vessels also deliver oxygen and nutrients to the fetus and carry carbon dioxide and end products of metabolism to the mother's bloodstream. In various manuals, allantois is called a derivative of the visceral mesoderm and ectoderm or endoderm. The discrepancy is explained by the fact that anatomically it is close to the cloaca, which, according to G. J. Romeis, is the primary feature of vertebrates. The cloaca itself in embryogenesis has a dual origin. In the embryos of all vertebrates, it is formed by an expansion of the posterior end of the endodermal hindgut. Until relatively late stages of development, it is fenced off from the external environment by a membrane, outside of which there is an invagination of the ectoderm (proctodeum) - the hindgut. With the disappearance of the membrane, the ectoderm is incorporated into the cloaca, and it becomes difficult to distinguish which part of the cloacal lining comes from the ectoderm and which part from the endoderm.

In all reptiles and birds, the eggs are large, polylecital, telolecital with a meroblastic type of crushing. A large amount of yolk in the eggs of animals of these classes serves as the basis for the lengthening of embryogenesis. Postembryonic development they are direct and not accompanied by metamorphosis.

7. Changes in the nervous system

The role of the nervous system became especially significant after the emergence of vertebrates on land, which put the former primary aquatic in an extremely difficult situation. They perfectly adapted to life in the aquatic environment, which bore little resemblance to terrestrial habitat conditions. New requirements for the nervous system were dictated by low environmental resistance, an increase in body weight, good distribution in the air smells, sounds and electromagnetic waves. The gravitational field imposed extremely stringent requirements on the system of somatic receptors and on the vestibular apparatus. If it is impossible to fall in water, then such troubles are inevitable on the surface of the Earth. At the boundary of the media, specific organs of movement, the limbs, were formed. A sharp increase in the requirements for coordinating the work of the muscles of the body led to the intensive development of the sensorimotor parts of the spinal, hindbrain, and medulla oblongata. Breathing in the air, changes in the water-salt balance and mechanisms of digestion led to the development of specific systems for controlling these functions from the brain and peripheral nervous system.

The main structural levels of the organization of the nervous system

As a result, the total mass of the peripheral nervous system increased due to the innervation of the limbs, the formation of skin sensitivity and cranial nerves, and control over the respiratory organs. In addition, there was an increase in the size of the control center of the peripheral nervous system - the spinal cord. Special spinal thickenings and specialized limb movement control centers were formed in the hindbrain and medulla oblongata. In large dinosaurs, these sections exceeded the size of the brain. It is also important that the brain itself has become larger. An increase in its size is caused by an increase in the representation of analyzers in the brain various types. First of all, these are motor, sensorimotor, visual, auditory and olfactory centers. The system of connections between different parts of the brain was further developed. They became the basis for quick comparison information coming from specialized analyzers. In parallel, an internal receptor complex and a complex effector apparatus developed. To synchronize the control of receptors, complex muscles and internal organs, in the process of evolution, associative centers arose on the basis of various parts of the brain.

The main centers of the nervous system of vertebrates on the example of a frog.

Important evolutionary events leading to a change in habitat required qualitative changes in the nervous system.

Detailed description of illustrations

In animals of different groups, the comparative sizes of the spinal cord and brain vary greatly. In a frog (A), both the brain and spinal cord are almost equal, in a green monkey (B) and a marmoset (C), the mass of the brain is much greater than the mass of the spinal cord, and the spinal cord of a snake (D) is many times larger than the brain in size and mass.

Three dynamic processes can be distinguished in the metabolism of the brain: the exchange of oxygen and carbon dioxide, the consumption of organic substances, and the exchange of solutions. The lower part of the figure shows the share of consumption of these components in the brain of primates: the upper line is in a passive state, the lower line is during hard work. Consumption of aqueous solutions is calculated as the time it takes for all body water to pass through the brain.

The main structural levels of the organization of the nervous system. The simplest level is a single cell that receives and generates signals. A more complex option is the accumulation of bodies of nerve cells - ganglia. The formation of nuclei or layered cellular structures is the highest level of cellular organization of the nervous system.

The main centers of the nervous system of vertebrates on the example of a frog. The brain is colored red and the spinal cord is blue. Together they make up the central nervous system. The peripheral ganglia are green, the cephalic ganglia are orange, and the spinal ganglia are blue. There is a constant exchange of information between the centers. Generalization and comparison of information, control of effector organs occur in the brain.

Important evolutionary events leading to a change in habitat required qualitative changes in the nervous system. The first event of this kind was the emergence of chordates, the second - the emergence of vertebrates on land, the third - the formation of the associative part of the brain in archaic reptiles. The emergence of the brain of birds cannot be considered a fundamental evolutionary event, but mammals went much further than reptiles - the associative center began to perform the functions of controlling the operation of sensory systems. The ability to predict events has become a tool for mammals to dominate the planet. A-D - the origin of chordates in muddy shallow waters; D-Z - landfall; Z, P - the emergence of amphibians and reptiles; K-N - the formation of birds in the aquatic environment; P-T - the appearance of mammals in the crowns of trees; I-O - specialization of reptiles.

8. Changes in water-salt metabolism

Amphibians have developed trunk (mesonephric) kidneys. These are elongated compact reddish-brown bodies lying on the sides of the spinal column in the region of the sacral vertebra (Fig. 3). From each kidney stretches the ureter (Wolf's canal) and each independently flows into the cloaca. An opening at the bottom of the cloaca leads to the bladder, into which urine enters and where water is reabsorbed, and concentrated urine is excreted from the body. Absorption of water, sugars, vitamins, sodium ions (reabsorption or reabsorption) also occurs in the renal tubules, some of the decay products are excreted through the skin. Amphibian embryos have functioning head kidneys.

Rice. 3. Urogenital system of a male frog: 1 - kidney; 2 - ureter (aka vas deferens); 3 - cavity of the cloaca; 4 - urogenital opening; 5 - bladder; 6 - opening of the bladder; 7 - testis; 8 - vas deferens; 9 - seminal vesicle; 10 - fat body; 11 - adrenal gland

At the front edge of each kidney in both sexes lie finger-shaped yellowish-orange fat bodies that serve as a reserve of nutrients for the sex glands during the reproductive period. A narrow, barely noticeable yellowish strip stretches along the surface of each kidney - the adrenal gland - the endocrine gland (Fig. 3).

In reptiles, the kidneys have no connection with the wolfian duct, they have developed their own ureters connected to the cloaca. The Wolf canal is reduced in females, and in males it performs the function of the vas deferens. In reptiles, the total filtration area of ​​the glomeruli is smaller, and the length of the tubules is greater. With a decrease in the area of ​​the glomeruli, the intensity of water filtration from the body decreases, and in the tubules, most of the water filtered in the glomeruli is absorbed back. Thus, a minimum of water is excreted from the body of reptiles. In the bladder, water is still additionally absorbed, which cannot be removed. In sea turtles and some other reptiles, forced to use salt water for drinking, there are special salt glands to remove excess salts from the body. In turtles, they are located in the orbit of the eyes. sea ​​turtles they really "cry bitter tears", freeing themselves from excess salts. At marine iguanas there are salt glands in the form of the so-called "nasal glands" that open into the nasal cavity. Crocodiles do not have a bladder, and salt glands are located near their eyes. When a crocodile grabs prey, the muscles of the visceral skeleton work and the lacrimal glands open, so there is an expression "crocodile tears" - the crocodile swallows the victim and "sheds tears": this is how salts are released from the body.

Rice. 4.1 The genitourinary system of the female Caucasian agama: 1 - kidney; 2 - bladder; 3 - urinary opening; 4 - ovary; 5 - oviduct; 6 - funnel of the oviduct; 7 - sexual opening; 8 - cavity of the cloaca; 9 - rectum

Rice. 4.2 The genitourinary system of the male Caucasian agama: 1 - kidney; 2 - bladder; 3 - testis; 4 - appendage of the testis; 5 - seed tube; 6 - urogenital opening; 7 - copulatory sac; 8 - cavity of the cloaca; 9 - rectum

The development of reptiles is not associated with the aquatic environment, the testes and ovaries are paired and lie in the body cavity on the sides of the spine (Fig. 4.1 - 4.2). Fertilization of eggs is carried out in the body of the female, development occurs in the egg. The secretions of the secretory glands of the oviduct form around the egg (yolk) a protein shell, underdeveloped in snakes and lizards and powerful in turtles and crocodiles, then the outer shells are formed. During embryonic development, embryonic membranes are formed - serous and amnion, allantois develops. A relatively small number of reptile species have ovoviviparity ( common viper, viviparous lizard, spindle, etc.). Real live birth is known in some skinks and snakes: they form a real placenta. Parthenogenetic reproduction is assumed in a number of lizards. A case of hermaphroditism was found in a snake - island botrops.

The excretion of metabolic products and the regulation of water balance in birds are carried out mainly by the kidneys. In birds, metanephric (pelvic) kidneys are located in the recesses of the pelvic girdle, the ureters open into the cloaca, there is no bladder (one of the adaptations for flight). Uric acid (the end product of excretion), which easily falls out of the solution with crystals, forms a mushy mass that does not linger in the cloaca and is quickly released to the outside. The nephrons of birds have a middle section - the loop of Henle, in which water is reabsorbed. In addition, water is absorbed in the cloaca. Thus, osmoregulation is carried out in the body of birds. All this allows you to remove decay products from the body with minimal loss of water. In addition, most birds have nasal (orbital) glands (especially sea ​​birds salt water drinkers), serving to remove excess salts from the body.

Water-salt metabolism in mammals is carried out mainly through the kidneys and is regulated by the hormones of the posterior pituitary gland. The skin with its sweat glands and intestines participate in the water-salt metabolism. Metanephric kidneys are bean-shaped and located on the sides of the spine. The ureters empty into the bladder. The duct of the bladder in males opens into the copulatory organ, and in the female - on the eve of the vagina. In oviparous (cloacal) ureters flow into the cloaca. The reabsorption of water and sodium ions occurs in the loop of Henle, the reverse absorption of substances useful for the body (sugar, vitamins, amino acids, salts, water) occurs through the walls of different sections of the nephron tubules. In the water balance, the rectum also plays a certain role, the walls of which absorb water from the feces (typical for semi-desert and desert animals). Some animals (for example, camels) during the feeding period are able to store fat consumed in low-feed and dry times: when fat is broken down, a certain amount of water is formed.

9. Live birth

Live birth is a way of reproducing offspring in which the embryo develops inside the mother's body and an individual is born that is already free of egg membranes. Viviparous some coelenterates, cladocerans, shellfish, many roundworms, some echinoderms, salps, fish (sharks, rays, and also aquarium fish- guppies, swordtails, mollies, etc.), some toads, worms, salamanders, turtles, lizards, snakes, almost all mammals (including humans).

Among reptiles, live birth is quite widely developed. It occurs only in forms with soft egg shells, thanks to which the eggs retain the possibility of water exchange with the environment. In turtles and crocodiles, whose eggs have a developed protein shell and shell, live birth is not observed. The first step to live birth is the retention of fertilized eggs in the oviducts, where partial development takes place. So, in a quick lizard, eggs can linger in the oviducts for 15-20 days. For 30 days, a delay can occur in an ordinary snake, so that a half-formed embryo is in the laid egg. Moreover, the further north the area, the longer the delay of eggs in the oviducts, as a rule, occurs. In other species, such as viviparous lizards, spindles, copperheads, etc., the eggs are retained in the oviducts until the embryos hatch. This phenomenon is called ovoviviparity, since development occurs due to reserve nutrients in the egg, and not due to the mother's body.

True live birth is often considered only the birth of individuals in placentals.

Fertilized eggs of lower vertebrates are retained in the oviducts of the female, and the embryo receives all the necessary nutrients from the egg reserves. In contrast, small mammalian eggs have negligible amounts of nutrients. Fertilization in mammals is internal. Mature egg cells enter the paired oviducts, where they are fertilized. Both oviducts open into a special organ of the female reproductive system - the uterus. The uterus is a muscular bag, the walls of which are capable of greatly stretching. The fertilized egg attaches to the wall of the uterus, where the fetus develops. At the site of attachment of the egg to the wall of the uterus, a placenta or child's place develops. The fetus is connected to the placenta by the umbilical cord, inside which its blood vessels pass. In the placenta, through the walls of blood vessels from the mother's blood, nutrients and oxygen enter the blood of the fetus, are removed carbon dioxide and other waste products harmful to the embryo. At the moment of birth in higher animals, the placenta separates from the wall of the uterus and is pushed outward in the form of an afterbirth.

The position of the fetus in the uterus

Features of reproduction and development of mammals allow us to divide them into three groups:

oviparous

marsupials

placental

egg-laying animals

Oviparous include the platypus and echidna that live in Australia. In the structure of the body of these animals, many features characteristic of reptiles have been preserved: they lay eggs, and their oviducts open into the cloaca, like the ureters and intestinal canal. Their eggs are large, containing a significant amount of nutritious yolk. In the oviduct, the egg is covered with another layer of protein and a thin parchment-shaped shell. In echidna, during the laying of eggs (up to 2 cm long), the skin on the ventral side forms a brood bag, where the ducts of the mammary glands open, without forming nipples. An egg is placed in this bag and hatched

marsupials

In marsupials, the embryo first develops in the uterus, but the connection between the embryo and the uterus is insufficient, since there is no placenta. As a result, the babies are born underdeveloped and very small. After birth, they are placed in a special bag on the mother's belly, where the nipples are located. The cubs are so weak that at first they are unable to suck milk themselves, and it is periodically injected into their mouths under the action of the muscles of the mammary glands. The cubs remain in the pouch until they are able to feed and move around on their own. Marsupials are animals that have a variety of adaptations to living conditions. For example, the Australian kangaroo moves by jumping, having greatly elongated hind limbs for this; others are adapted to climbing trees - the koala bear. The marsupials also include the marsupial wolf, marsupial anteaters and others.

These two groups of animals are classified as lower mammals, and taxonomists distinguish two subclasses: the oviparous subclass and the marsupial subclass.

placental animals

The most highly organized mammals belong to the subclass of placental animals, or real animals. Their development takes place entirely in the uterus, and the shell of the embryo fuses with the walls of the uterus, which leads to the formation of the placenta, hence the name of the subclass - placental. It is this method of development of the embryo that is the most perfect.

It should be noted that mammals have a well-developed care for offspring. Females feed their cubs with milk, warm them with their bodies, protect them from enemies, teach them to look for food, etc.

Conclusion

The emergence of vertebrates on land, like any major expansion of the adaptive zone, is accompanied by a transformation mainly of four morphofunctional systems: locomotion, orientation (sensory organs), nutrition, and respiration. Transformations of the locomotor system were associated with the need to move along the substrate under the condition of an increase in the action of gravity in the air. These transformations were expressed primarily in the formation of walking limbs, the strengthening of the limb belts, the reduction of the connection between the shoulder girdle and the skull, and also in the strengthening of the spine. Transformations of the food capture system were expressed in the formation of the autostyle of the skull, the development of head mobility (which was facilitated by posttemporale reduction), and also in the development of a movable tongue, which ensures the transport of food inside oral cavity. The most complex rearrangements were associated with adaptation to breathing air: the formation of the lungs, the pulmonary circulation and the three-chambered heart. Of the less significant changes in this system, it should be noted the reduction of gill slits and the separation of the digestive and respiratory tracts - the development of the choanae and laryngeal slit.

The whole range of adaptations associated with the use of air for breathing has developed in the lobe-finned fishes (and their ancestors) in the water (Schmalhausen, 1964). Breathing out of water entailed only the reduction of the gills and the ophthalmic apparatus. This reduction was associated with the release of the hyomandibulare and its transformation into stapes - with the development of the orientation system and the emergence of tongue mobility. The transformation of the orientation system was expressed in the formation of the middle ear, the reduction of the seismosensory system, and in the adaptation of vision and smell to functioning out of water.

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A lot of work had to be done in search of fossil traces of extinct creatures in order to clarify this issue.

Previously, the transition of animals to land was explained as follows: in the water, they say, there are many enemies, and so the fish, escaping from them, began to crawl out onto land from time to time, gradually developing the necessary adaptations and changing into other, more advanced forms of organisms.

This explanation cannot be accepted. After all, even now there are amazing fish, which at times crawl ashore, and then return to the sea. But they don’t throw water at all for the sake of salvation from enemies. Let us also recall frogs - amphibians, which, living on land, return to the water to produce offspring, where they spawn and where young frogs - tadpoles - develop. Add to this that the most ancient amphibians were by no means defenseless creatures suffering from enemies. They were chained in a thick hard shell and hunted other animals like cruel predators; it is unbelievable that they or others like them should be driven out of the water by danger from enemies.

They also expressed the opinion that aquatic, animals that overflowed the sea, as if suffocated in sea water, felt the need for fresh air, and they were attracted by the inexhaustible supply of oxygen in the atmosphere. Was it really so? Let's think about flying sea fish. They either swim near the surface of the sea, or rise out of the water with a strong splash and rush in the air. It would seem that it is easiest for them to start using the air of the atmosphere. But they just don't use it. They breathe with gills, i.e., respiratory organs adapted for life in water, and are quite content with this.

But among freshwater there are those that have special adaptations for air breathing. They are forced to use them when the water in the river or user becomes cloudy, clogged and poor in oxygen. If clogged sea ​​water some streams of mud flowing into the sea, then sea fish swim away to another place. sea ​​fish and do not need special devices for air breathing. Freshwater fish find themselves in a different position when the water around them becomes cloudy and rots. It is worth watching some tropical rivers to understand what happens.

Instead of our four seasons in the tropics, a hot and dry half of the year is replaced by a rainy and damp one. During stormy rains and frequent thunderstorms, the rivers overflow widely, the water rises high and is saturated with oxygen from the air. But here the picture is changing dramatically. The rain stops pouring. The waters are subsiding. The scorching sun dries up the rivers. Finally, instead of flowing water, there are chains of lakes and swamps in which stagnant water is overflowing with animals. They die in droves, the corpses quickly decompose, and rotting consumes oxygen, so that it becomes less and less in these reservoirs full of organisms. Who can survive in such a drastic change in living conditions? Of course, only one who has the appropriate adaptations: he can either hibernate, burrowing into the silt for the entire dry time, or switch to breathing atmospheric oxygen, or, finally, he can do both. All the rest are doomed to extermination.

Fish have two kinds of devices for air breathing: either their gills have spongy outgrowths that retain moisture, and as a result, air oxygen easily penetrates into the blood vessels washing them; or they have a modified swim bladder, which serves to keep the fish at a certain depth, but at the same time can also play the role of a respiratory organ.

The first adaptation is found in some bony fish, i.e., those that no longer have a cartilaginous, but a completely ossified skeleton. Their swim bladder is not involved in respiration. One of these fish - "creeping perch" - lives in tropical countries and now. Like some

other bony fish, it has the ability to leave the water and use its fins to crawl (or jump) along the shore; sometimes it even climbs trees in search of slugs or worms on which it feeds. Astonishing as the habits of these fish are, they cannot explain to us the origin of the changes that made it possible for aquatic animals to become inhabitants of the land. They breathe with the help of special devices 9 gill apparatus.

Let us turn to two very ancient groups of fish, those that lived on Earth already in the first half of ancient era the history of the earth. These are lobe-finned and lungfish. One of the wonderful cross-finned fish, called the polypter, still lives in the rivers of tropical Africa. During the day, this fish likes to hide in deep holes on the muddy bottom of the Nile, and at night it comes to life in search of food. She attacks both fish and crayfish, and does not disdain frogs. Lying in wait for prey, the polypter stands on the bottom, leaning on its wide pectoral fins. Sometimes he crawls along the bottom on them, as if on crutches. Pulled out of the water, this fish can live for three to four hours if kept in wet grass. At the same time, her breathing occurs with the help of a swim bladder, into which the fish now and then gains air. This bladder in lobe-finned fish is double and develops as an outgrowth of the esophagus from the ventral side.

We do not know of a polypter in a fossil state. Another lobe-finned fish, a close relative of the polypter, lived in very distant times and breathed with a well-developed swim bladder.

Lungfish, or lungfish, are remarkable in that their swim bladder has become a respiratory organ and works like lungs. Of these, only three genera have survived to our time. One of them - the horned tooth - lives in the slowly flowing rivers of Australia. In the silence of summer nights, the grunting sounds that this fish makes, floating to the surface of the water and releasing air from the swim bladder, are carried far. But usually this big fish lies motionless at the bottom or slowly swims among the water thickets, plucking them and looking for crustaceans, worms, mollusks and other food there.

She breathes in two ways: with both gills and a swim bladder. Both that, and other body works at the same time. When the river dries up in summer and small reservoirs remain, the cattail feels great in them, while the rest of the fish die in masses, their corpses rot and spoil the water, depriving it of oxygen. Travelers in Australia have seen these paintings many times. It is especially interesting that such pictures unfolded extremely often at the dawn of the Carboniferous Age across the face of the Earth; they give an idea of ​​how, as a result of the extinction of some and the victory of others, a great event in the history of life became possible - the emergence of aquatic vertebrates on land.

The modern horntooth is not inclined to move ashore to live. He spends the whole year in the water. Researchers have not yet been able to observe that he hibernates for a hot time.

His distant relative- ceratod, or fossil horntooth, lived on Earth in very remote times and was widespread. Its remains were found in Australia, Western Europe, India, Africa, North America.

Two other lung fish of our time - protopter and lepidosiren - differ from the horntooth in the structure of their swim bladder, which has turned into lungs. Namely, they have a double one, while the horntooth has an unpaired one. The protopter is quite widespread in the rivers of tropical Africa. Or rather, he does not live in the rivers themselves, but in the swamps that stretch next to the riverbed. It feeds on frogs, worms, insects, crayfish. On occasion, the protopters attack each other. Their fins are not suitable for swimming, but serve to support the bottom when crawling. They even have something like an elbow (and knee) joint approximately in the middle of the length of the fin. This remarkable feature shows that even before they left the water element, lung fish could develop adaptations that were very useful to them for life on land.

From time to time, the protopter rises to the surface of the water and draws air into the lungs. But this fish has a hard time in the dry season. There is almost no water left in the swamps, and the protopter is buried in the silt to a depth of about half a meter in a special kind of hole; here he lies, surrounded by hardened mucus secreted by his skin glands. This mucus forms a kind of shell around the protopter and does not allow it to dry out completely, keeping the skin moist. Through the entire crust there is a passage that ends at the mouth of the fish and through which it breathes atmospheric air. During this hibernation, the swim bladder serves as the only respiratory organ, since the gills then do not work. Due to what is the life in the body of the fish at this time? She is losing a lot of weight, losing not only her fat, but also part of her meat, just as she lives at the expense of accumulated fat and meat during hibernation and our animals - a bear, a marmot. Dry time in Africa lasts a good six months: in the homeland of the protopter - from August to December. When the rains come, life in the swamps will revive, the shell around the protopter dissolves, and it resumes its lively activity, now preparing for reproduction.

Young protopters hatched from eggs look more like salamanders than fish. They have long external gills, like those of tadpoles, and the skin is covered with multi-colored spots. At this time, there is no swim bladder yet. It develops when the external gills fall off, in exactly the same way as it happens in young frogs.

The third lung fish - lepidosiren - lives in South America. She spends her life almost the same as her African relative. And their offspring develop very similarly.

No more lungfish survived. Yes, and those that still remain - the horned tooth, the protopter and the lepidosiren - approached the sunset of their age. Their time is long past. But they give us an idea of ​​the distant past and are of particular interest to us.

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Now let's return from the Mesozoic to the Paleozoic - to the Devonian to where we left the descendants of the lobe-finned fish, which were the first of the vertebrates to come ashore.

However, you can't forget about it! - this feat, which I described before (traveling over land in search of water), is a very, very approximate simplified diagram of the motives that forced the fish to leave the drying up reservoirs.

It's easy to say: fish got out of the water and began to live on land . Centuries, thousands of thousands of years passed irrevocably, until the restless descendants of the lobe-finned fish slowly but surely, dying out and surviving in whole clans, adapted to everything that the land met them with, inhospitable as an alien world: sand, dust, stones. And emaciated psilophytes, primeval grasses, hesitantly surrounding damp hollows in some places.

So, shortening the tedious time spent by the ancestors of amphibians to conquer a new element, let's just say: they got out of the water and looked around. What did they see?

There is something, one might say, and nothing. Only near the shores of the seas and large lakes in rotting plants, thrown out by waves on land, crustaceans and worms swarm, and near the edge of fresh water - primitive wood lice and centipedes. Here and at a distance, along the sandy lowlands, various spiders and scorpions crawl. The first wingless insects also lived on land by the end of the Devonian. A little later, winged ones appeared.

It was scarce, but it was possible to feed on the shore.

Landing of half-fish, half-amphibians - ichthyostegs (the first stegocephalians ) - was accompanied by many radical changes in their body, which we will not delve into: this is too specific a question.

To breathe fully on land, you need lungs. They were in lobe-finned fish. In stagnant lakes and swamps full of decaying plants and depleted of oxygen, the lobe-feathers floated to the surface and swallowed air. Otherwise, they would have suffocated: in musty water, gills alone are not enough to saturate the body with oxygen necessary for life.

But here's the thing: as calculations showed, lobe-finned fish could not breathe with their lungs on land!

“In the resting position, when the animal is lying on the ground, the pressure of the entire body weight is transferred to the belly and floor of the oral cavity. In this position of the fish lung breathing is impossible. Sucking air into the mouth is possible only with difficulty. Suction and even forcing air into the lungs required great effort and could only be carried out by raising the front part of the body (with the lungs) on the forelimbs. In this case, pressure on the abdominal cavity stops, and air can be distilled from the oral cavity into the lungs under the action of the hyoid and intermaxillary muscles ”(Academician I. Schmalhausen).

And the limbs of the lobe-finned fish, although they were strong, however, in order to support the front part of the body for a long time, were not suitable. Indeed, on the shore, the pressure on the fins-paws is a thousand times greater than in the water, when the lobe-finned fish crawled along the bottom of the reservoir.

There is only one way out: skin breathing. Assimilation of oxygen by the entire surface of the body, as well as by the mucous lining of the mouth and pharynx. Obviously, it was the main one. Fish crawled out of the water, at least only half. Gas exchange - the consumption of oxygen and the release of carbon dioxide - went through the skin.

But here at ichthyostegov, the closest evolutionary descendants of the lobe-finned fishes, the paws were already real and so powerful that they could support the body above the ground for a long time. Ichthyostegs are called "four-legged" fish . They were inhabitants of two elements at once - water and air. In the first, they bred and mostly fed.

Amazing mosaic creatures ichthyostegi. They have a lot of fish and frogs. They look like scaly fish with legs! True, without fins and with a single-bladed tail. Some researchers consider the ichthyostegi to be a barren side branch of the amphibian family tree. Others, on the contrary, chose these "four-legged" fish as the ancestors of stegocephals, and, consequently, of all amphibians.

Stegocephalians (shell-headed ) were huge, similar to crocodiles (one skull is more than a meter long!) And small: ten centimeters the whole body. The head from above and from the sides was covered with a solid shell of skin bones. It has only five openings: in front - two nasal, behind them - eye, and on the top of the head one more - for the third, parietal, or parietal, eye. It apparently functioned in Devonian armored fish, as well as in Permian amphibians and reptiles. Then it atrophied and in modern mammals and humans turned into the pineal gland, or pineal gland, the purpose of which is not yet fully understood.

The back of the stegocephalians was bare, and the belly was protected by not very strong armor made of scales. Probably so that, while crawling on the ground, they would not injure their belly.

One of stegocephalians, labyrinthodonts (labyrinth-toothed: the enamel of their teeth was intricately folded), gave rise to modern tailless amphibians. Others, the lepospondyls (thin vertebrates), produced caudate and legless amphibians.

Stegocephalians lived on Earth "a little" - about a hundred million years - and in the Permian period they began to quickly die out. Almost all of them died for some reason. Only a few labyrinthodonts passed from the Paleozoic to the Mesozoic (namely, the Triassic). Soon they came to an end.

Landfall

The impulse to change the organism was always given by external conditions.

V. O. Kovalevsky.