Approximately 150 million years ago (in the middle of the Jurassic), a branch deviated from reptiles, which gave rise to birds.

A little later - mammals (animals) also originated from other branches of reptiles, although their ancestors - animal lizards - arose earlier than the ancestors of birds. Now one can quite accurately imagine how this development took place. In the ground, in layered shales Western Europe(GDR, FRG and other places) found fossilized remains of skulls, bones of ancient birds and their ancestors, whole skeletons with scales, prints of feathers and the entire wing.

How did it happen?

Some lizards began to run away when frightened and in danger. At the same time, they rose on their hind limbs (there are such lizards even now). Then they learned to generally run only on their hind legs (which took millions of years). Calculations and comparisons give us the right to assert that they already had a four-chambered heart, since the ancestors of crocodiles, which have the same heart, were close to this family (pseudosuchians). It was an aromorphosis, an evolution that gave a sharp increase in their entire organization.

The running lizards made jumps and acted on the beta with their forelimbs, like rudders. Their horny scales began to stretch out, forming combs along the edge from the hand to the elbow, scooping up more air when running. Further. Such running species gradually shifted to climbing rocks and trees. For prey, they began to climb branches, which is what modern chameleons and many iguanas do.

Among fossil lizards, some species had hollow bones of the skeleton, filled with air. Those who had such bones climbed the trees, as well as those whose bones were heavy. But when it was necessary to jump from branch to branch, and later from tree to tree, the light-boned lizards jumped further and did not break when falling. Soon (relatively) their scales began to lengthen along the sides of the body, as well as along the posterior edge of the front paws. The scales became longer and more split, it was very light and "scooped up" more air during the jump, keeping the body in the air. It was easier for such an animal to jump - its body flattened out. Recall that a sheet of paper falls slowly, and if it is crumpled, it falls much faster. The scales extended in all directions acted like a parachute. Each flake was split in different ways: “herringbone” along the edges from the thickened middle rod or along the radii, to one center. In the first case, a feather was obtained from the scales, and in the second, fluff. In other parts of the body, the scales remained unchanged for a long time (for example, on the legs, the horn cover of the beak).

Ancient lizards, with still underdeveloped feathers, climbed trees and rocks with the help of all four limbs, which had fingers and claws. Only on the front legs were expanded feather-like scales, which formed flat peaks along the posterior edge of the paw. Such animals (pseudosuchia) are also known in the fossil state. It was they who gradually turned into the first birds (Archaeopteryx). Their skeletons with fingerprints and even feathers are quite well preserved. In 1974, in Bavaria (Germany), in the quarries of Solengofen, a well-preserved skeleton of the fourth Archeopteryx, the size of a starling, was found. The previous three finds were the size of a dove. It is proved that the bones were hollow, like those of real birds. Consequently, the air sacs leaving the lungs entered the bones. They had similarities with both lizards and birds.

Let's make this comparison:

Signs of reptiles preserved in the first birds:

• The jaws, although narrow, do not form a beak.
• On the jaws - teeth
• Tail of 21-28 vertebrae (could be bent)
• On the forelimbs - three free fingers
• The ribs were attached to the vertebrae, like in lizards, at one point and did not have processes that turned backwards and extended to the next rib in birds.

"Bird" signs of reptiles:

• The body is covered with feathers.
• The bones (thighs and shoulders) are hollow. Hence, there were air sacs entering the bones.
• Shoulder and forearm became a wing.
• Dense feathers grew on the wing, overlapping each other, like those of real birds.
• Below the lower leg, from longitudinally fused bones, a tarsus was formed.

To this we add that the air sacs depart from the lungs in modern chameleons as well. Some dinosaur bones also had cavities. However, neither one nor the other did not fly and do not fly. Therefore, it is not necessary to say that these devices served to "facilitate" the flight. Moreover, the best modern flyers - swifts do not have hollow bones. They are "overgrown" with bone marrow.

So, the jaws of the first birds were still wide, with many small teeth. Long, like those of lizards, the tail consisted of many vertebrae and could bend in all directions. On the forelimbs, at least two fingers disappeared, the remaining three were still well developed, with claws and, apparently, helped with climbing. But behind the hand, the limb carried a rather well-developed wing of dense feathers. Such first birds, probably, still did not fly well, they could only fly from tree to tree. The tail began to shorten first. The long tail outweighed at. planning, although it was lined with feathers around the edges. Then the forelimbs, which worked both as a climbing paw and as a wing, gradually freed themselves from the previous load and began to work only as a wing, losing free fingers.

There are, however, even today birds that retain free fingers on the wing, even with claws. Hoatzin chicks climb branches in this way. The claw on the first finger is present in certain types carnivores, geese, among Haun's paladins. In other Palamedes, “spurs” protrude from the edge of the wing, undoubtedly of the same origin. On the second finger, claws are less common in modern birds. They are known from cassowaries, nandu, kiwi and toucans. Finally, at African ostrich claws grow on all three fingers of the wing.

The teeth of the first birds were still preserved for a long time: judging by the skulls from the later layers of the earth (Cretaceous period), for 50 million years. The teeth disappeared completely in birds about 70 million years ago. The remains of fingers in the wing have been preserved in all birds to this day. There are three of them, including a short front (“thumb”), which can still turn slightly. A separate bundle of feathers is attached to it - a “winglet” on the front edge of the wing. Fast-flying (predatory, etc.) birds, turning their wings, regulate the flight, slow down on the fly, etc. On the legs of the birds, the scales from the ancient ancestors, the lizards, have been preserved. The structure of the eggs and the development of the embryos of birds differ very little from the development of lizards. Body temperature became constant. The changed circulatory system with a four-chambered heart ensures more active blood oxidation (combination with oxygen), which increases body temperature, and a thick cover of feathers and down keeps you warm.

So climbing lizards, who learned to jump, and later to jump from tree to tree, later became birds.

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Many scientists believe that birds evolved from small theropod dinosaurs. The main thing here was the appearance of feathers = as a result, some running and climbing animals gained the ability to fly.
Scientists consider Archeopteryx, first discovered in 1861, to be the first bird proper. Judging by its appearance, it looked like a cross between a reptile and a bird with a toothy beak, a long bony tail and pronounced plumage. IN last years remains of other feathered reptiles have also been found.

The very first feathered birds.

Bird feathers perform two important functions: they keep birds warm and they help them fly. Those feathers that serve for heating are usually shorter and softer, and those with which birds fly - the so-called fly feathers - are larger and curved in the form of a fan. It is unlikely that the plumage of both species appeared in birds at the same time. They were almost certainly the first to grow heat-protective feathers, and after that, after millions of years, some of them acquired a different, very special form, designed just for flight. When exactly the plumage appeared is unknown. According to some paleontologists, some kind of feathers are already found in a reptile that lived in, however, most scientists are not completely convinced of this. The most convincing evidence in this sense is the presence of plumage as such in small theropods, whose fossils were recently discovered in China. On one of them, Sinosauropteryx, there are clear signs of short fluffy plumage in the form of a long comb that stretches along the neck and entire back. It was already a feathered dinosaur, but, quite obviously, it still did not know how to fly.

Getting off the ground.

Sinosauropteryx appeared somewhat later than Archeopteryx, from which it is clear that the former were not their direct descendants. Meanwhile, judging by the presence of fluffy plumage in the ancestors of flying birds, it is easy to imagine how they looked long before full plumage. But much more important is not even this, but how the wings developed and, most importantly, why? According to one theory, the wings developed among the ancestors of today's birds as a special device for hunting insects and other small animals. So, according to this very theory, the first birds, trying to overtake the victim, broke away from the ground and overtook it in a jump, already in the air. Then, following the same theory, long time later, the forepaws of the first birds began to grow feathers, which helped them to maintain balance or, perhaps, to hold prey. Feathers gradually stretched out, and the muscles on the front legs grew stronger. So, probably, animals arose, which one day had the strength to get off the ground.

Avimim (left) was a feathered theropod, but he could not fly. Archeopteryx (center) was smaller and lighter, and also had well-developed flight feathers. Compared to Archeopteryx, modern birds like the dove (right) have neither teeth nor claws on their wings, except perhaps the gaotsin, and their tails are noticeably shorter.

Tree flying squirrels.

This "mundane theory" was based on some of the characteristics identified in Archeopteryx, such as the extraordinary strength of the paws. However, according to most paleontologists, modern birds evolved from reptiles that lived not on land, but in trees. With the development of exceptionally long feathers, such animals gained the ability to hover in the air, which allowed them to easily move through wooded areas without descending to the ground. Well, over time, they learned to fly for real - waving their wings. But it took a lot of time for reptiles to get the hang of soaring. This was done, for example, by coelurosauruses and other arboreal reptiles; the same can be said about some modern lizards. And supporters of the "tree" theory consider this direct evidence that the first birds started from the same place.
On the wings of Archeopteryx were asymmetrical, or rather, twisted feathers, like those of modern birds. Feathers like these help birds fly when the wind blows over them, and this, in turn, confirms that Archeopteryx could fly.

Weight and flight.

To soar, you don’t need a lot of strength, but flapping your wings is not an easy task. Over time, serious changes occurred in the anatomy of the first birds, thanks to which they learned not only to stay in the air for a long time, but also became strikingly different from their ancestors - dinosaurs. In this sense, evolution has taken a completely different path. And as they developed, the first birds began to spend more and more time in the air. Thanks to the same changes, the birds got rid of excess weight. The bones of the first birds were mostly fused, due to which their skeleton was slightly lightened. Like their theropod ancestors, the bones of the first birds were hollow, not filled with air - over time, the air cavities expanded, especially towards the wings and paws. In addition, their sternum expanded and the pectoral muscles, which ensured the flight, became stronger, as well as the triangular fork, or bow, which supported the sternum during the flight. Such anatomical changes were quite successful. In the Cretaceous period, birds literally flooded the Earth, especially since the time of reptiles was approaching a fatal end. Therefore, birds were the only surviving descendants of dinosaurs.

Archeopteryx closely resembled a small theropod. Archeopteryx fossils found in the 1950s were thought to be Compsognathus until subtle outlines of feathers were found next to them.

Origin birds- the question is somewhat unclear and controversial; undoubted is the belonging of bird ancestors (and therefore the birds themselves) to reptiles, namely to the class Archosaurs(Archosauria), which also includes many extinct forms, primarily Dinosaurs(Dinosauria), and of the living - only crocodiles(Crocodilia). To name the specific group of archosaurs that gave rise to birds, scientists cannot; There are at least two hypotheses about this.

The first, and most common, suggests that birds are direct descendants of dinosaurs - or rather, not even descendants, but their only surviving branch, so the statement about the extinction of dinosaurs at the border of the Mesozoic and Cenozoic eras is not entirely true. Of the two known groups of dinosaurs, the ancestors of birds were by no means Ornithischians(Ornitischia), as might be expected, and lizards(Saurischia); their closest relatives, according to this hypothesis, are representatives of the clade Deinonychosaurus(Deinonychosauria), which, along with bird ancestors, the birds themselves and some other dinosaurs, belong to the clade Maniraptors(Maniraptora), one of the branches of the group Animal(Theropoda). These maniraptors have lived on Earth since the late jurassic (156 million years ago), and already six to ten million years later, the oldest known bird lived - Archeopteryx(Archaeopteryx lithographica). Of course, Arechopteryx cannot be the ancestor of the rest of the birds - this is just one of the branches of the bird's trunk, which did not give descendants and is known as the infraclass lizard-tailed(Archaeornithes). Other extinct infraclasses of birds are enantiornithes(Enanthiornithes), Hesperornisaceae(Hesperornithes) and Ichthyornisaceae(Ichthyornithes); living birds are classified as infraclass fantail(Neornithes), known since late chalk(70 Ma).

Another hypothesis traces the genealogy of birds to Protoavisu(Protoavis texensis), which lived in the early triassic(225-210 million years ago) and having, according to a number of paleontologists, much more similar to modern birds than Archeopteryx. Some (in particular, the Russian paleornithologist E.N. Kurochkin) believe that it is Protoavis that is the ancestor of modern birds; thus, birds are not descendants of dinosaurs, but a related branch descended from some common archosaur ancestor with them. Archeopteryx and enanciornis, in this situation, are still descended from beasts and have nothing to do with birds. Most paleontologists do not agree with this hypothesis, arguing that, firstly, the very fact of the existence of Protoavis is debatable and, in fact, the discovered remains belong not to one organism, but to several different creatures, each of which had some sign in structure, common with birds (see below); and, secondly, there is a huge time gap between Protoavis and his fantail descendants; in such a range of dates, a significant number of transitional forms should have existed - but none have been found. There are also contradictions in the hypothesis of origin from Animals - and these contradictions lie in the structure of the wings: all birds have only three fingers (II, III, IV) in the structure of the hand, while their supposed ancestors also have three, but others (I, II and III).

Birds differ from their ancestors and relatives only in that they were able to combine all these signs - and, of course, learn to fly, because it is this skill that is the determining factor in building the body of birds. There are even hypotheses about the origin of some dinosaurs from the first birds that lost this ability - about the same as modern ostriches lost it.

How exactly the birds learned to fly is difficult to answer unambiguously; all assumptions about this can be combined into two hypotheses. The first suggests that birds were originally small forest animals, incapable of active flapping flight, but able to climb trees and plan with the help of their forelimbs - as they do, for example, winged wings And flying squirrels(mammals), some snakes And lizards. The limbs made it possible to lengthen these jumps, holding them in the air for some time. In both cases, the end result was the emergence of active flight, characteristic of all living birds - even flightless penguins And ostriches descended from flying species, lost this ability in the course of evolution.

Evolution

In any case, by the time of extinction dinosaurs(end Cretaceous) birds already successfully existed, dividing into the now known superorders Ancient palate(Paleognathae) and new palatine(Neognathae), differing primarily in the structure of the skull. Further separation took place during Paleocene And Eocene; By the Oligocene, fossil remains of most of the existing orders are known. At the same time, the squadrons of the Ancient Sky, despite their name, are younger than the squadrons of the New Sky; it is assumed that their various detachments are of independent origin, and their separation happened even before the loss of the ability to fly, which fact determined their similarity in structure and way of life. A similar convergence explains the similarity in the structure of the genetically distant orders of the Novonebnye, obtained as a result of development in similar ecological niches. Falcons, for example, much closer relatives sparrows than similar to them both externally and in lifestyle hawks.

As for the Ancient Sky, the most ancient relies on a detachment Tinamouiformes(Tinamiformes), which have retained the ability to fly, albeit over very short distances, the keel of the sternum and some other structural features common with the birds of the New Palatines. All other units of the Ancient Sky are united in the hoard Beskilev(Ratitae), completely switched to a running mode of movement. The reason why they stopped flying is quite understandable, if we accept the hypothesis about the origin of bird flight as a way to escape from predators - after all, an offshoot ostriches and their relatives happened after the mass extinction of the dinosaurs, when there were no more large predators from which one would have to fly away. The divergence of the Novopalatine began already in the Cretaceous, when two groups, unequal in number of species, emerged: Chicken-like(Galloanserae) and Higher neopalates(Neoaves). The former is believed to be more ancient and is characterized by such features as high fecundity associated with a large number of eggs in a clutch, a predominantly brood type of development, and a polygamous lifestyle; This clade includes two orders - anseriformes(Anseriformes) and Galliformes(Galliformes), which diverged back in Cretaceous. All other orders of the New Palestine belong to the second group and are characterized by a smaller number of eggs in the clutch, a predominantly nestling type of development, and a monogamous way of life; the allocation of most of these detachments took place in the first half Cenozoic.

So Darwin's theory received one more, moreover, very weighty confirmation. The first birds, whose skeletons were found in the shale quarries of Bavaria, lived on Earth about 160 million years ago - in the Mesozoic era of its geological development, more precisely at the end of the Jurassic period. The Mesozoic era was the age of reptiles, sometimes the highest flowering of this class of vertebrates. They lived in water, on land, and in the air. They sometimes reached gigantic proportions. The wingspan of some flying - pteranodons, for example - was 6-7 meters. These were the largest flying animals that have ever lived on Earth.

The first birds were relatively small. Archeopteryx was only slightly larger than the dove. He did not fly well and moved by hovering flight from tree to tree or from tree to ground. From the ground, he again climbed up the tree trunk, clinging to the bark with the claws of his toes and wings. Weak jaws, studded with small teeth, indicate that Archeopteryx was not a predator. Most likely this bird (zoologists-systematists firmly include Archeopteryx in the class of birds, however, referring it to a separate subclass of ancient birds) ate fruits and berries, not disdaining also small insects and worms. From the fossil remains it is impossible to say what the color of the feathers of Archeopteryx was. However, there is reason to believe that it was multi-colored, masking the bird against the background of vegetation.

The origin of the first birds from reptiles is undoubted. True, paleontologists have not yet been able to find all the steps along which they walked. But they came to the unanimous conclusion that the ancestors of birds were small reptiles from the pseudosuchian group, which originally lived on flat, steppe-like spaces, sometimes covered with small rocks. They had enlarged hind limbs, large brain cavities, which lightened the weight of the skull - these signs allow us to conclude that their body straightened and the animals tended to walk on their hind limbs. Subsequently, some of these reptiles adapted to life on trees, such as scleromochlus.

If in steppe upright species the forelimbs gradually became unnecessary and decreased in size, arboreal reptiles needed them in order to climb branches. Thanks to this, they retained an important prerequisite for the emergence of wings.

Fossil remains of a transitional form between reptiles and birds have not yet been found. But we can assume its existence. Paleontologists even imagined appearance this great bird. At this stage of development, the scales had already turned into feathers, which helped the animal to make parachuting flights from branch to branch or from tree to ground.

It's not far from the pratitsa to Archeopteryx. A cover of feathers not only lifted the oldest birds into the air. It helped maintain a constant body temperature. For the first time in the evolution of the living world, warm-blooded animals appeared on earth. This is how scientists imagine the origin of birds.

Forest inhabitants of the Chinese province of Liaoning 130 million years ago. A small four-winged dinosaur hovers in the foreground - the gui microraptor. Cathayornis flying to the right are also not considered birds. But on the left, on a branch, Confuciusornis is sitting, representing one of the evolutionary lines close to birds. Obviously, various groups of feathered animals tried to master the air environment in the Cretaceous period.

Until recently early evolution birds was perhaps the darkest page in the chronicle of fossil creatures. And although recent paleontological discoveries have clarified a lot, it is not possible to read it completely. What is known is that birds evolved from reptiles. But from which ones? The direct ancestors of modern birds have never been found, and plumage and the ability to fly repeatedly arose in various animals of the Mesozoic era. There are more than enough hypothetical ancestors: among them are pseudosuchia, ornithosuchia, pterosaurs, dinosaurs and even crocodiles. But Archeopteryx, familiar to everyone from the picture in the school textbook, has to be deleted from this list.

Birds, along with insects, are the main inhabitants of the air spaces of the Earth. Several devices allow them to take to the skies and control their movements in flight. First, a special skeleton. A complexly arranged wing is capable of holding the entire weight of the body in the air. Its swing movements depend on the structure of the shoulder girdle, formed by the scapula, coracoid, sternum and clavicles fused into a fork. There, for example, there is a three-osseous hole through which the tendon of the muscle passes, which lifts the wing up after it is lowered. To hold the tail feathers, which serve as a rudder in flight, the end of the spine formed a short and wide bone - the pygostyle. Secondly, plumage helps birds fly. Controllability in flight is provided by well-defined feathers: flywheels and tail feathers. But there are also feathers, the purpose of which is different: they create a streamlined body shape for birds both when flying and when diving, serve as a heat-shielding cover and, being brightly colored, help in communication between relatives.

In addition to birds, among the vertebrates currently only able to fly the bats and wings. However, they have a fundamentally different wing structure and no feathers, which makes their flight unlike that of a bird. In the past, the variety of flying and feathered creatures was extremely large. In addition to the long-known pterosaurs and Archeopteryx, paleontologists have discovered a large number of unusual species, the existence of which was not even suspected. It seems, animal world did not experience a lack of those wishing to conquer the sky.

There are two main hypotheses for the acquisition of flapping flight by animals: from faster running on the ground or from jumping and gliding from some elevated places - trees, elevations in the mountains. The latter hypothesis received indirect confirmation after the finds in China, in the province of Liaoning, of various feathered dinosaurs. Now most scientists believe that the flying species came from the environment that lived in the forest, probably some very small, no larger than a dove, species of reptiles and birds. Their descendants quickly passed the primitive stage - gliding from high places - and learned to fly for real. How long did all this take, how many species changed before the birds gained flight? No one will say, since the flying creatures found by paleontologists could not be the first, and the very beginning of the evolution of birds is still hidden from us.

For a long time it was believed that the plumage of birds is reptile scales modified by millions of years of evolution. However, the results latest research make you doubt it. Both plumage and scales, as, indeed, all integumentary formations in vertebrates, originate from the cells of the outer layer of the skin - the epidermis. Reptile scales are composed of what is known as alpha-keratin, a protein with short peptide chains. It is formed from protruding sections of one outer layer of the epidermis. With the development of a feather in birds, a tubercle of the epidermis also first appears, but it is formed not by one, but by its two outer layers. Then this tubercle sinks into the skin, forming a kind of sac - a follicle, from which a feather grows. Moreover, the material for the feather is slightly different - beta-keratin, composed of long peptide chains, which means it is more elastic and strong, capable of supporting feather plates. Alpha-keratin is also present in birds, it goes to the formation of the cover of the beak, claws and scales on the tarsals. In addition, the feather of birds has a tubular structure, and the scales of reptiles are solid. Apparently, the pen is an evolutionary innovation that has proven its usefulness over time.

The plumage, which easily acquires various shapes and colors, has opened almost unlimited possibilities for birds to various types flight, the development of signal and identification structures, the development of many ecological niches. It was the plumage that helped the birds achieve that huge variety which we are currently observing. Almost ten thousand species is more than all other terrestrial vertebrates.

If most feathered dinosaurs couldn't fly, why would they need down or feathers? Clearly not for flying. In any case, not immediately for the flight. It is possible that various downy formations arose among predatory lizards as a thermal insulation cover, as evidenced by paleoclimate data. In the middle and end of the Triassic period (230-210 million years ago), when the first dinosaurs appeared, a cooling occurred on Earth. Along the outskirts of the huge continent of Pangea, the only one at that time, latitudinal climatic zones appeared with a cool humid climate. The animals living there adapted to the cold, including with the help of plumage. On the contrary, the center of Pangea was occupied by dry and desert regions with a high level of solar radiation, since clouds were rare in those parts. Down and feathers were again suitable for reptiles to protect against radiation. Over time, the feathers at the ends of the forelimbs, on the tail and on the head could turn into elongated feathers that served as decorations or identification signs. They became the basis for the appearance of flying feathers in some dinosaurs. Other reptiles, among which were the distant ancestors of birds, could acquire plumage in a similar way.

Does not appear in birds

For almost 150 years, since the first discovery, Archeopteryx was considered the progenitor of modern birds. In fact, apart from knowledge about this creature, scientists did not have any other data on the origin of birds for a long time. It would seem that such signs as plumage and wings indisputably indicated that Archeopteryx is the oldest bird. On the other hand, in the structure of the skull, spine and other parts of the skeleton, it was similar to carnivorous dinosaurs. These observations gave rise to the hypothesis of the origin of birds from ancient pangolins, which has now become especially popular.

As often happens in science, an alternative hypothesis also found support at the same time. Long-voiced doubts about the direct relationship between Archeopteryx and birds (they are too different anatomically) have turned into conviction, since since the early 1980s, paleontologists have found feathered dinosaurs, ancient birds, and their close relatives. They also found new skeletons of Archeopteryx. Ten are known today, all of Upper Jurassic age (145 million years ago) from the Altmühl River in Bavaria. The last specimen, which was better preserved than the others, described at the end of 2005, finally convinces that Archeopteryx comes from predatory dinosaurs, but has nothing to do with modern birds. He is something else: not a dinosaur, but not a bird either. I had to look for another candidate for the role of the ancestor of birds.

Dinosaur down jacket

The existence of feathered dinosaurs was suspected for a long time, but there was no evidence for this. They appeared in the 1990s in China, in the province of Liaoning. There, paleontologists discovered a whole cemetery of forest flora and fauna aged 130-120 million years. What makes the event unique is the excavated natural area. Usually, marine or near-aquatic communities of animals and plants are available for study due to better conditions burial. Forest, steppe or mountain inhabitants of the past most often are not preserved in a fossil state, because they are quickly processed into dust by bacteria. And here is a snapshot of the life of the mid-Cretaceous forest, captured by volcanic ash.

Now more than a dozen lizards have been described, feathered in surprisingly diverse ways: from short down to real asymmetrical feathers on the limbs, indicating the ability to fly. In addition, in the skeleton of these predatory dinosaurs, some features characteristic only of birds were found: a furcula, hook-shaped processes on the ribs, and a pygostyle. But still, these were not birds, but small predators that moved mainly by running. With long tails, toothy, covered with scaly skin, with shortened front paws and long clawed fingers. Judging by the structure of the skeleton, for the most part they could not really fly, that is, flap their wings. Only one species is known to have risen a notch. This microraptor gui (Microraptor gui) is an interesting specimen of a small dromaeosaurus found in the same place, in Liaoning. All in fine plumage, with a tuft on his head. Its forepaws were covered with asymmetrical (with narrow outer and wide inner webs) flight feathers exactly like those of birds. The hind legs were also covered in flying feathers, longer on the metatarsus and shortened on the lower leg. It turns out nothing more than a four-winged feathered dinosaur that could fly from tree to tree. The flyer from it, however, turned out to be unimportant. In the absence of binocular vision (when the field of view of both eyes overlaps), the microraptor could not accurately target the landing site and descended into the trees, apparently rather awkwardly.

It would seem that it can be assumed that birds evolved from predatory dinosaurs soaring among the trees. However, this is not allowed by too significant anatomical differences between them. So do not rush and write down feathered dinosaurs as the ancestors of birds.

Side by side with feathered dinosaurs lived enantiornithes, which in Greek means "counterbirds" - creatures especially important for understanding the evolution of birds. Judging by the finds, this was the most numerous and diverse group of flyers that lived in the Cretaceous period.

Outwardly, enantiornithes strongly resembled modern birds. Among them there were small and large species, toothless and toothy, running, waterfowl, arboreal, and, most importantly, they all flew beautifully. There was also a lot of familiarity in the skeleton: the same bones of the wing, torso and hind limbs. Only something differently articulates in the shoulder blade, something in the heel, lower leg and spine. Small differences at first glance. And in the end - a different wing lifting system and footwork. Most real birds can turn their paws in different directions: turn in, turn out. Predators, eagles and falcons, it helps to deftly grab and hold the prey. The legs of enanciornis (many of which, by the way, were also predators) are arranged differently, because of which they walked on the ground, rather, clumsily waddling from side to side, like geese. All this greatly separates the enantiornithes from real birds. It turns out that their external similarity is formal. Just as the tail of the aquatic lizards of ichthyosaurs is similar to the tail of fish, so the paws and wings of the enantiornithes are similar to the paws and wings of real birds.

The Ancestor That May Not Be

For a long time it was believed that real birds, or, as they are also called, fan-tailed birds, appeared at the beginning of the Cenozoic era, that is, not earlier than 65 million years ago. And suddenly finds aged 100, 130 million years from the territory of the USA, Mongolia and China rained down. At first, they didn’t even believe the age definitions, but subsequent works confirmed that yes, in the time of dinosaurs and enanciornis, fan-tailed birds were already found. They looked just like modern ones and even achieved some diversity. Where did they come from, if the feathered and flying creatures discussed above are not suitable for their progenitors? Now there is only one assumption.

In 1991, the American paleontologist Shankar Chatterjee described an unusual creature he found in the state of Texas, in many ways similar to birds. Its age is 225 million years, which is 80 million older than Archeopteryx. The creature was called Protoavis Texas (Protoavis texensis) - "protobird", and not without reason. Its voluminous skull contained a rather large brain with hemispheres and cerebellum, which in the late Triassic time, when it lived, was not found in other vertebrates. Judging by the structure of the skull, Protoavis had binocular vision and wide-set large eyes, which indicates his ability to deftly hunt and navigate well in the world around him, as is characteristic of birds. In the skeleton of the protoavis, in general, there are many features similar to those of fan-tailed birds, but the proportions of the body, short and powerful limbs, and the position of the center of gravity indicate that it could not fly. And apparently he had no plumage. Despite this, Protoavis is more like a real bird than Archeopteryx, and at this point it is Protoavis that can be considered the closest ancestor of modern birds. If this is so, then their evolution should be carried out not from dinosaurs, but from more ancient reptiles, united in the group of archosaurs.

The discovery of protoavis made it possible to find the answer to one more question: what is the difference between birds and dinosaurs? As the birds spend to fly great amount energy, their metabolic rate is much higher than that of reptiles. In birds, the oxygen consumption during metabolism per kilogram of weight is 3-4 times higher than in reptiles. Since the metabolic rate is high, therefore, toxins from the body should be removed quickly. This requires large powerful kidneys. Modern birds in the pelvic bones have three deep depressions in which these large kidneys are located. The same cavities for the kidneys are also found in the pelvic bones of the protoavis. Obviously, his body was distinguished by a high level of metabolism, unusual for reptiles.

Everything would be fine, but the reconstruction of Protoavis does not inspire confidence in many paleontologists. Its remains were interspersed with the bones of other reptiles, in such conditions it is not surprising to confuse and count parts of two or even several different animals as a single creature. In general, for final conclusions, we should wait for other finds, and modern birds will remain without direct ancestors for now.

However, like ancient birds without direct descendants. Because it is not possible to trace the evolution of birds consistently from beginning to end. There are still plenty of gaps. In particular, no intermediate links have been found between the ancient fantails, which still retained reptilian features - teeth growing from the alveoli, abdominal ribs and a long row of vertebrae in the tail - and modern groups of birds. Suddenly, as if from nowhere, at the end of the Mesozoic era, ancient geese, loons, albatrosses, cormorants and other aquatic birds appeared.

As a hypothesis

So, we saw a number of amazing feathered creatures that lived on Earth at least at the end of the Mesozoic, in the interval 145-65 million years ago. At that time, the world was full of animals seeking to master the airspace. In addition to the ubiquitous enanciornis, toothy, booby-like ichthyornis were found in the seas of North America. Hesperornis lived in the Late Cretaceous in the seas of ancient Eurasia. In Europe, there were gargantuavis - an incomprehensible kinship of a bird the size of a turkey. The forests of Mongolia and China were inhabited by arboreal ambiorthus, lyaoninghornis and feathered dinosaurs. And there are many more single forms, whose position on the evolutionary tree of birds is difficult to establish. Only two branches are clearly traced: from protoavis to fan-tailed birds and from feathered dinosaurs to Archeopteryx and further to enantiornithes.

A number of fossil forms are known that did not progress beyond planning. While real flapping flight was only possible for pterosaurs (we don't discuss them here, as they are not related to birds at all), gui microraptors, enanciornis, and true fan-tailed birds. All of them were successful in mastering the air environment. Pterosaurs ruled the air for 160 million years, enantiornithes for at least 80 million years. Both of them were probably surpassed in the competitive struggle by fan-tailed birds, which have widely settled around the planet in the last 65 million years.

Over the past couple of decades, paleontologists have shown that parallel evolution is a widespread path among living beings. There have been several attempts among invertebrates to become arthropods, among ancient fish to land and become amphibians, among reptiles to become mammals, among plants to acquire flowering and become angiosperms. But usually only one or two of them turned out to be successful in the future.