In 1861, the remains of Archeopteryx, a crow-sized feathered creature that lived approximately 145 million years ago, were discovered in southern Bavaria. As many scientists believed, it was he who was the ancestor of modern birds. But for more than a century in paleontology there was a gap between him and real birds that was not filled by other finds. Only in the last 20 - 25 years, with the discovery of many new birds of the Mesozoic era, it became clear: 140 - 110 million years ago their world was rich and diverse. True, different scientists interpret these findings differently. Which of the hypotheses they put forward is closer to the truth, and therefore to an understanding of the paths and patterns of evolution?
In the minds of the general public, paleontologists are specialists who find and study mammoths and dinosaurs. Indeed, their huge skeletons in museums attract attention and amaze the imagination. But well-preserved specimens are rare. Much more often it is possible to discover individual bones, teeth and skulls in the layers of the earth; they are used to describe the appearance of extinct animals and study their family relationships. Fossil giants basically show us only the final, highly specialized results of evolution. The origins of most vertebrate groups are among small and inconspicuous creatures without much anatomical diversity. Moreover, in this state they replaced each other for many millions of years, then either died out or found another free niche of life, where their wide, as experts say, adaptive radiation began, i.e. Many new species appeared, adapted to changing environmental conditions.
The history of birds and their flight has remained a mystery for a long time. Although science posed these questions even before the appearance of Charles Darwin’s work on the origin of species. In the middle of the 19th century. Almost simultaneously with the publication of this famous work, Archeopteryx was discovered, which was perceived by natural scientists as a triumph of evolutionary theory. It seemed that this was the missing transitional link between reptiles and birds. Until now, in textbooks, from school to university, you can read that birds are flying creatures, covered with feathers, with one legitimate ancestor - Archeopteryx - a transitional form from reptiles.
Immediately after the discovery of the first specimen (to date, 10 of them are already known), some scientists expressed doubts that Archeopteryx is the ancestor of other birds. If we consider as such all those who have wings, feathers and are capable of flying, he is fit to be a great-great-grandfather of a sparrow. If you delve into anatomy, it does not turn out to be a sparrow: functionally it is like a bird, but structurally it is a pure reptile. Apart from the feather, it has nothing in common with real birds: the skull is structured differently, the vertebrae are not the same, the forelimbs, although they have become wings, but the details of the structure of their skeleton are different, the same applies to the legs.
Researchers' opinions were divided. Some argued: birds descended from ancient, lizard-like thecodont reptiles; others believed that Archeopteryx and all other birds after it trace their ancestry to predatory (theropod) dinosaurs.
In 1926, a solid book by the Dane Gerhard Heilman, “The Origin of Birds,” was published in English. The author’s conclusions are clear: birds were “born” from thecodont reptiles, and not from predatory dinosaurs. Theropods themselves, predatory dinosaurs, also originate from thecodonts, according to scientists.
Archeopteryx really has a lot in common with the latter. Their close relationship was analyzed in detail and confirmed in the 1970s by the American paleontologist John Ostrom. But he considered Archeopteryx to be the oldest bird. This hypothesis is still adhered to by both supporters of the origin of birds from theropods and their opponents, who believe that Archeopteryx descends from more ancient predatory reptiles than theropods - archosauromorphs. If this is so, then for both one and the other hypotheses we have to accept that evolution proceeds sequentially, in a straight line, from simple to complex. But in nature this does not happen. All the experience of paleontological and modern molecular genetic research of recent decades shows: evolution proceeds on a broad front, by trial, through achievements and errors, in bundles of parallel lines of development. And new data on the historical development of birds well illustrate precisely this nature of the laws of evolution. This is why the discussion of feathers and bones becomes key to understanding the main problems of her theory.
For nearly 150 years, hypotheses about the origins and relationships of birds were based almost exclusively on the study of Archeopteryx. The history of Cenozoic birds (which includes all their modern representatives) over the past 65 million years has also been well studied. From the Mesozoic era, from the creatures of interest to us, there were only isolated rare finds that did not add up to the overall picture. And suddenly there was a breakthrough.
Enantiornithes were first described from Argentina in 1981. Soon they began to be found on all continents in the deposits of the Cretaceous period, i.e. in the range from 145 to 65 million years ago. Outwardly similar to real birds - fully feathered, with well-developed wings, seemingly with the same paws and tails, but in terms of the details of the skeleton structure - completely different: they have much in common with Archeopteryx. Therefore, they can be classified as a group of so-called lizard-tails, in contrast to fan-tails, which include all modern birds.
Then they discovered completely unusual birds called Confuciusornithidae. Their skeleton has many primitive and original structural features, but in some respects they are similar to modern birds. In particular, their beak was covered with a horny sheath and had no teeth, and on the crest of the humerus there was a large hole of unknown purpose.
It was believed that true fantails appeared and lived almost exclusively in the Cenozoic. But unexpectedly they began to be discovered in sediments of the beginning of the Cretaceous period, the last in the Mezoic era, which lasted about 80 million years, i.e. longer than the entire Cenozoic. The first reliable such bird, called Ambiortus dementjevi, was found in the early 1980s in Mongolia. Then it seemed so unusual that some paleontologists did not believe in the reality of its existence.
And finally, a variety of theropod dinosaurs with feathers were found in China. Moreover, some had a semblance of downy cover, others had long feathers located only at the ends of the wings and tail, and others were completely covered with small feathers. And suddenly scientists “met” a small dinosaur the size of a pheasant, which had real wings with corresponding feathers, and in addition, legs were equipped with the same feathers! Four-winged flyer! Later it turned out that different plumage is characteristic of dinosaurs from five different theropod families (Oviraptoridae, Avimimidae, Dromeosauridae, Therizino-sauridae, Troodontidae), found on all continents except Antarctica. Moreover, even the crowning tyrannosaurs (Tyrannosauridae) that crowned numerous predatory dinosaurs were most likely covered with feathers. What does this mean? Here the positions of specialists are again categorically divided. Some argue that some of these dinosaurs are essentially not dinosaurs; in fact, they are birds that have lost the ability to fly, while in other dinosaurs collagen structures of the skin modified in the fossil state are mistaken for the downy cover. Other scientists believe that these finds prove the origin of real birds (along with Archeopteryx) from theropod dinosaurs.
But a different assessment of all new facts is also possible. Archaeopteryx and enantiornithis, having a large number of features in common with theropod dinosaurs, most likely descended from them, crowning one of the attempts of reptiles to master the air environment. Alas, it was not successful. Archaeopteryx disappeared in the Jurassic period, and enantiornithes lost in competition with real birds and died out without a trace along with dinosaurs at the end of the Cretaceous period.
It turns out that real fan-tailed birds existed simultaneously with these dinosaur birds for millions of years? And did they arise from some original archosauromorphs (a subclass of reptiles characteristic of the beginning of the Mesozoic) long before the dinosaurs took flight? This could have happened at the end of the Triassic period (about 220 million years ago).
The Mesozoic era began about 250 million years ago and lasted about 185 million years; is divided into three periods: Triassic (beginning 250 million years ago, duration about 35 million years), Jurassic (beginning 213 million years ago, duration about 70 million years) and Cretaceous (beginning 144 million years ago, duration about 80 million years).
Caudipteryx (Caudipteryx zoui Ji et al., 1998) from the Early Cretaceous of China is a theropod dinosaur, but some scientists consider it a flightless bird. Caudipteryxes are characterized by small feathers on the tail and the ends of the forelimbs (shown by red arrows). Many caudipteryxes retain accumulations of gastroliths in the abdominal cavity (red arrow); with magnification they are shown at the top right.
Jeholornis (above) and Confuciusornis (below) have claws (shown by blue arrows) on their wing fingers that are curved outward, which is no coincidence: they were most likely intended to cling to branches. (Photo from the book of Chinese paleontologist L. Hou.).
Sacred Confuciusornis (Confuciusornis sanctus Houetal., 1995) is one of the first sensational finds of Early Cretaceous birds in Liaoning Province, China. Now 6 species have been described.
Longirostravis (Longirostravis hani Hou et al., 2003) from the group of enantiornis birds. The size of a starling, with an elongated thin beak, the tip of which was armed with small teeth, which probably made it possible to pull out hidden prey, holding it securely.
There is indirect evidence of this - finds of Late Triassic and Early Jurassic small bird tracks in South America, Africa and Europe. We do not know bird skeletons for those millions of years. However, until quite recently, their presence in Early Cretaceous deposits was not known. Only traces and a large number of feathers were found, which allowed us to talk for a long time about the unknown evolution of birds throughout at least the Cretaceous period.
From individual finds, other lines of ancient birds are also known, which, upon careful study, do not correlate with either Enantiornis, Confuciusornis, or true fantails.
It turns out that evolution made many attempts to lift reptiles and their descendants into the sky. For various reasons, most experiments failed and ended in extinction. Only fantails, in the end, gave a powerful outbreak of adaptive radiation and mastered the air environment in all its tiers. It turns out that evolution acts not as an economical housewife, but as a most wasteful dreamer.
Most of the sensational paleontological discoveries in recent years are associated with the Liaoning province in northeast China. Cretaceous localities in this region have been known to specialists since the 1920s. Previously, only fossil fish, insects and plants were found there in large quantities. But at the end of the 20th century. unexpectedly discovered birds and feathered dinosaurs. Among the first are true fantails, and enantiornis, and Confuciusornis, and several other individual genera of birds, which scientists cannot yet attribute to any of their known groups. And almost all of the feathered dinosaurs mentioned are described based on materials from Liaoning. Now they have been found in Early Cretaceous and even Jurassic deposits in other provinces of the Celestial Empire. By the way, in addition to a variety of birds, unknown mammals, lizards, pterosaurs, dinosaurs, turtles, amphibians, various fish, a lot of insects and rich materials on flora, including ancient flowering plants, were discovered in this province. In total, more than 30 species of birds, the same number of dinosaurs and 6 species of mammals have already been described from Liaoning.
It is remarkable that many animals are represented there not just by skeletons, but also by imprints of soft tissues, external integuments (skin, scales, wool, plumage), internal organs and even the contents of their digestive system. Thus, in place of the stomachs of Caudipteryx, feathered dinosaurs from the oviraptorid family, and Sapeornis, an ancient bird of unclear kinship, accumulations of gastroliths (small pebbles) were preserved, which contributed to the grinding of plant food. In the oral cavity of Yanornis, an ancient fan-tailed bird, remains of a fish were found, in Sinosauropteryx, a small carnivorous dinosaur, mammal bones were found, and in the mammal Repenomamus, dinosaur remains were found. Thus, Chinese burials make it possible to understand the biota of this part of the world of that time in all its diversity, as well as the ecological characteristics of its individual representatives. It is called the Jehol biota.
The geological age of its deposits, established by isotopes of argon, uranium and lead, is determined to be 110 - 130 million years ago. They were formed in freshwater lakes and adjacent river beds and deltas. It is known that such deposits are distributed over a vast territory in many parts of China, Mongolia, southern Siberia, Korea and Japan. Why is it that only Liaoning is distinguished by its vast paleontological wealth? The fact is that in this region in the Early Cretaceous there was strong volcanic activity. Periodic eruptions with powerful emissions of ash and the release of poisonous gases destroyed all living things, and entire animals were buried under layers of ash in lake sediments. For example, many hundreds, even thousands of specimens of the so-called “sacred Confuciusornis” (Confuciusornis sanctus) have been collected. True, they say that most of them were sold to private collections.
According to our hypothesis, various birds evolved in parallel over tens of millions of years, originating among various groups of reptiles (of course, we inevitably simplify the evolutionary scenario, omitting the entire complex of factual evidence already published in special scientific works, but we try to correctly convey the essence of the hypothetical process) . All of the above is also supported by data from other areas of paleontology, because not only birds, but also other main classes of vertebrates evolved according to a similar scenario.
Back in the 1970s, an employee of our institute, Leonid Tatarinov (academician since 1981), showed at least 7 attempts by reptiles to become mammals, but only one or two of them were successful. For many millions of years, several evolutionary lines of mammals existed in parallel, of which, as shown in 2007 by an employee of our institute, Doctor of Biological Sciences Alexander Agadzhanyan, only three have survived to this day: placentals, marsupials and oviparous ones. According to research by Academician Emilia Vorobyova (A. N. Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences), conducted in the 1970s - 1990s, fish-like creatures repeatedly tried to reach land. However, such “aspirations” are not unique to vertebrates. Various invertebrates tried to become arthropods (arthropods), which is reflected in the recent works of Doctor of Biological Sciences Alexander Ponomarenko (A. A. Borisyak Paleontological Institute of the Russian Academy of Sciences). And even in the plant world, as another of our collaborators, Doctor of Geological and Mineralogical Sciences Valentin Krasilov, showed in 1989, there have been at least six experiments to transform proangiosperms into flowering plants, most of which surround us today.
My colleague Ponomarenko figuratively called this picture of evolution an evolutionary “lawn.” Many separate “stems” develop on it simultaneously and parallel to each other. And we added that most of them “mow down” ecological and evolutionary mechanisms. And only some evolutionary “stalks”, usually located at the edge of the evolving space, are preserved, mature, produce a “panicle of seeds” and continue further development.
But why, for example, did enantiornithians become extinct, while true fan-tailed birds continued to develop? Perhaps because enantiornithes were in a hurry to become birds. We know the skeletons of their embryos from the Upper Cretaceous deposits of Mongolia, about 70 million years old. So, their skeleton was completely formed already in the egg. And their chicks, obviously, were born as an absolute copy of the adults. All they had to do was grow to full size, and they grew throughout their entire subsequent lives. In true fan-tailed birds, the chicks, as in our time, hatched from eggs with a half-cartilaginous skeleton. Then it very quickly, for most in 2 - 4 months, completely ossified and stopped further growth. Thus, enantiornithes emerged from eggs as already mature birds and subsequently followed the bird's path, sometimes, probably, not achieving all the possibilities that flight provided them.
In the postembryonic period, real birds quickly achieved the perfection of flyers, maintaining it throughout their lives. Perhaps this was the main reason why the enantiornithes lost air space to the fantails? We recently described a unique discovery of a fossilized bird brain from Cenomanian deposits (about 93 million years old) in the Volgograd region. A specialist in the study of the central nervous system of animals, Doctor of Biological Sciences Sergei Savelyev (Institute of Human Morphology of the Russian Academy of Medical Sciences) believes: in this brain, the sections responsible for mobility, intelligence, etc. were less developed than in modern birds. According to indirect evidence, such a brain could belong to enantiornithes. Could this be another reason for their competitive loss to real birds?
Previously it was believed that the origin of the feather and flight as such were inextricably linked. Now, with the discovery of various feathered theropod dinosaurs and various ancient birds, this hypothesis has to be abandoned. It turns out that the acquisition of the feather cover was due to other circumstances.
Perhaps at first it had a heat-protective function or protected its owners from harsh ultraviolet sunlight. You cannot fly on short soft outer feathers. Then where did the hard and long feathers of the wings and tail come from? It is assumed that the primary elongation and increase in their size in the ancestors of birds and flying dinosaurs was caused by the formation of decorative structures associated with mating displays.
But how did they fly? Until now, two hypotheses have competed in this regard. According to one, the “arboreal” one, designated by the “top to bottom” direction, the first flights took place at the stage of the arboreal archosauromorphic ancestors of birds, who climbed up trees, grasping them with their forepaws, and then began to jump down, after which they flew. According to another, “terrestrial” one, closely related to the origin of birds from dinosaurs, the vector was different - “bottom up”: they ran and ran, faster and faster, jumped and finally flew. In any case, these were bipedal ancestors - bipedal, as we call them, moving only on their hind legs, with free forelimbs freed from the function of support. But both hypotheses left many questions and inconsistencies that did not allow them to be accepted in their pure form. For example, in Archeopteryx, Enantiornis, and Confuciusornis, the claws on the front legs (wings) are curved outward for some reason. How can you grasp the trunk with this orientation? To do this, they must be bent inward to cling to the trunk.
Together with Candidate of Biological Sciences Igor Bogdanovich (I.I. Shmalhausen Institute of Zoology of the National Academy of Sciences of Ukraine), we have developed a new compromise hypothesis for the origin of flight. We consider the structure of the paws of birds and theropod dinosaurs to be the key factor after bipedality. Already in the very first known Early Cretaceous true birds, the hind leg was constructed according to the anisodactyl type: its three front toes were directed forward, and the first inner one was completely opposed to them and aimed back. By the way, although no one has yet found the skeletons of birds in the Triassic and Jurassic, prints of their tracks from the Late Triassic of Argentina and the Early Jurassic of Africa and Europe show exactly this paw structure.
The sequence of hypothetical stages of acquisition of flight by real birds:
I - bipedal terrestrial archosauromorph;
II - the emergence of anisodactyly in the archosauromorphic ancestor of true birds;
III - jumping onto the lower branches of trees and bushes;
IV - reliable landing on perches during the final formation of anisodactyly and the initial reduction of the long tail;
V and VI - the appearance of feathers with symmetrical webs on the distal segments of the forelimbs and tail for mating displays;
VII - formation of asymmetric aerodynamic feathers on the wings and reduction of the long tail;
VIII - transition to real flapping flight.
Next. In the earliest real birds known to us from the Lower Cretaceous deposits, the tail was already fan-shaped - a short section of the spine with a short pygostyle (a series of fused last caudal vertebrae), on which feathers sat like a fan. What did this give them? With paws with this arrangement of toes, they could securely grasp branches and hold on to them, without balancing with a long tail. And gradually he disappeared, because... there was no need for a similar rear balancer, as it served in Archeopteryx and feathered theropod dinosaurs. They could not stay firmly on the branches. Their first finger never reached a completely opposed position. Therefore, they had to stay on the branches, balancing with their long tail. And they could not climb the trees along the trunks, clasping them with their front paws-wings, since the claws on the fingers were bent outward. What then were the claws oriented in this way used for? We believe, in order to hold on to the surrounding branches with unreliable support on the hind legs - after all, their toes did not completely clasp the branch. Note: in the earliest true fan-tailed birds, although the claws on the fingers of the wings were still preserved, they were already small and almost straight - not curved.
How then did these early birds and theropod dinosaurs, with their desire to fly, climb trees? Probably jumping onto the lower branches and moving higher and higher. Moreover, the former held securely on the branches with the help of their anisodactyl paws, while the latter helped themselves with long wing fingers with claws curved outward.
Why did both need trees? First of all, not in order to learn to fly. The flight began only later, as a consequence of the development of the above-ground layer of life. It can be assumed that they began to climb there in an effort to develop new food resources. Or escape from terrestrial predators during overnight roosts. Or to build nests there, thereby insuring their clutches of eggs and offspring, again, from predators. Or for this, that, and the third together.
Having a short, lightweight tail and a light skeleton, real birds, initially descending from the trees, fluttering the rudiments of feathered wings, eventually took off in real flapping flight. Feathered dinosaurs, although they received long feathers on their wings, “grew” the same feathers on their hind legs, most likely did not acquire real flight, although they became quite advanced “glider pilots.”
So all that remains is to wait for at least one discovery of a real bird with an anisodactyl foot and a fan-shaped tail in the Jurassic, and even better, in the Late Triassic deposits. And they were. It’s just that, for many reasons, we haven’t come across one yet. After all, quite recently we knew almost nothing about the Early Cretaceous stage in the history of birds. They didn't just not know. Some experts did not even believe that there were real fantails even then.
The study was supported by the Russian Foundation for Basic Research grant 07 - 04 - 00306.
Doctor of Biological Sciences Evgeniy KUROCHKIN, head of the laboratory of the Paleontological Institute named after. A. A. Borisyak RAS
For more than a hundred years, paleontologists have believed that some living creatures were the ancestors of others. The question is: from what group of organisms does this or that genus, family, order, class, type come from, how long did it take, how did the modification of the organism take place? Some taxa are “luckier” - the remains of their ancestral forms are found faster, while the ancestors of other groups remain undiscovered for a long time. As examples, we can recall the evolution of equines, which began to become clearer back in the 19th century, and the evolution of cetaceans, a definite idea of which paleontologists developed only in the 90s. XX century
There can be many reasons for this. Thus, a particular key episode of evolution may have occurred hundreds of millions of years ago, or it may have been very rapid, all of which reduces the possibility of preserving ancestral forms as fossils. On the other hand, paleontologists themselves may at first not pay attention to important specimens that have already been found...
One of the mysteries that causes heated debate among scientists is the origin of birds. Many anatomical features indicate the closeness of birds to reptiles. The remains of the oldest known birds, Archeopteryx, date back to the end of the Jurassic period. This means that birds arose no later than the middle of this period and descended from some reptiles that lived then. But from whom exactly? At the moment there are two points of view on this problem.
According to one, earlier one, from thecodonts, a group of primitive reptiles widespread on Earth between the Early Triassic and Early Jurassic, terrestrial and aquatic predators, reaching a length of 15 cm to 6 m. Outwardly, most thecodonts resembled lizards or crocodiles. The name of the group literally translates as “cellular-toothed”: the teeth of these animals were located in the alveoli. Thecodonts, together with crocodiles, saurian and ornithischian dinosaurs and pterosaurs (flying lizards), are classified as a subclass of archosaurs, but are considered a more primitive, ancestral group in relation to other archosaurs.
According to another point of view, birds descended from some kind of theropod dinosaurs. Theropods (literally “beast-footed”) are representatives of the order of lizard-hipped dinosaurs, reptiles ranging in size from 25 cm to 15 m, which led a predatory lifestyle and moved on two legs. Theropods appeared in the middle or at the end of the Triassic and existed until the end of the Cretaceous period.
The debate between scientists defending the first and second points of view has been going on for a long time and is considered one of the hottest in paleontology. It gets to the point where sometimes irony and ridicule begin to take the place of scientific impartiality. Authoritative proponents of the “thecodont” hypothesis have expressed, for example, that the idea of the origin of birds from theropod dinosaurs is “one of the greatest hoaxes of our era, the paleontological equivalent of cold thermonuclear fusion,” and that the evidence for the origin of birds from theropods is less compelling than evidence of "alien abductions" and that "some paleontologists who spent three decades saying birds evolved from dinosaurs are now forced to defend the idea for career reasons." At the V Meeting of the Society for Avian Paleontology and Evolution, supporters of the “thecodont” hypothesis of the origin of birds wore badges: “Birds are not dinosaurs.”
For their part, those who see theropod dinosaurs as the ancestors of birds talk about the “death knell” for the “thecodont” hypothesis, that its current developments are simply unscientific... What serious arguments, based on paleontological data, allow debaters to defend so emotionally your positions?...
Similarities between birds and reptiles
It is easy to see the similarities between the classes of birds and reptiles. In both, the skin is almost devoid of glands, but is protected by horny scales in reptiles and feathers in birds. Note that in birds scales are developed on the non-feathered parts of the skin (tarsus). Bird feathers are also horny structures that develop from scales. Both classes are oviparous, and eggs are structured in a similar way: shell, yolk and white. The embryos of birds and reptiles are similar in appearance.
In the search for immediate reptile ancestors, small primitive reptiles were selected pseudosuchia, who lived approximately 200 million years ago (Triassic period). In search of food, some of these creatures adapted to climbing trees and jumping from branch to branch. In the course of evolution, this method turned out to be promising and helped primitive birds avoid competition among related species and escape from predators. As the scales lengthened, they turned into feathers, which helped the ancient ancestors of birds acquire the ability to plan, and then to be active, i.e. flapping, flight, which most modern birds have.
Protoavia (1984) was found in Post, Texas, USA, with an estimated age of 225,000,000 years.
The development of the Earth is divided into five periods of time called eras. The first two eras, Archeozoic and Proterozoic, lasted 4 billion years, that is, almost 80% of all earth history. During the Archeozoic, the formation of the Earth occurred, water and oxygen appeared. About 3.5 billion years ago, the first tiny bacteria and algae appeared. During the Proterozoic era, about 700 years ago, the first animals appeared in the sea. These were primitive invertebrate creatures, such as worms and jellyfish. The Paleozoic era began 590 million years ago and lasted 342 million years. Then the Earth was covered with swamps. During the Paleozoic, large plants, fish and amphibians appeared. The Mesozoic era began 248 million years ago and lasted 183 million years. At this time, the Earth was inhabited by huge dinosaur lizards. The first mammals and birds also appeared. The Cenozoic era began 65 million years ago and continues to this day. At this time, the plants and animals that surround us today arose.
Living at the end of the Triassic and in the Jurassic period, small carnivorous dinosaurs from the group coelurosaurs were bipedal with long tails and small forelimbs of the grasping type. They did not need to climb trees and glide from branch to branch. The active flight of ancient birds could have arisen on the basis of the flapping movements of the forelimbs, which helped knock down flying insects, for which, by the way, predators had to jump high. Coelurosaurs survived the mass extinction of the dinosaurs at the end of the Mesozoic era.
The very first dinosaur birds
In the Mesozoic era, that is, 150 million years ago, the ancestors of birds constituted the main group of land animals in Argentina. They are called theropods (Argentavis magnificens), beast-footed lizards, and they already knew how to fly. Therapods moved on two legs, their front legs turned into short grasping limbs. It was no longer possible to rely on them, but it was convenient to fight with prey. The powerful jaws of theropods were densely lined with teeth and resembled a saw blade. In place of worn-down teeth, new ones grew, so even when they grew old, the lizards could still torment their prey with the same fervor. (Sharks also renew their teeth.) During the process of evolution, some theropods developed a horny beak. Analyzing the anatomical features of theropods, it is believed that birds originated from these animals.
Fossils found in Argentina in 1979 indicate that this huge vulture-like bird had a wingspan of more than 6 m, a height of 7.6 m, and a weight of 80 kg.
Beast-like predatory lizard ornitholestes, which had a body length of 2.5 m, gives an idea of ancient birds.
their wingspan was 7.5 m; they lived in Europe, Africa, North and South America and were carnivores (eating fish and aquatic invertebrates).
After studying the fossilized remains of a small bird found in Liaoning province in northern China, scientists came to the conclusion that "Confuciusornis sanctus" - as the ancient bird was dubbed - lived 120 million years ago. Judging by the structure of the beak, Confuciusornis resembled modern birds: the teeth were no longer there, but a horny sheath had appeared.
In the Jurassic period, birds acquired the ability to actively fly. Thanks to the swings of their forelimbs, they were able to overcome the effects of gravity and gained many advantages over their ground-based, climbing and gliding competitors. Flight allowed them to catch insects in the air, effectively avoid predators and choose the most favorable environmental conditions for life. Its development was accompanied by a shortening of the long tail, replacing it with a fan of long feathers, well suited for steering and braking. Most of the anatomical transformations necessary for active flight were completed by the end of the Early Cretaceous (about 100 million years ago), i.e. long before the extinction of the dinosaurs.
Found by American scientists from North Carolina State University. They compared the changes in theropod limbs with the evolution of the wings of chickens, ostriches and cormorants. In the process of evolution, both of them retained only three fingers from the original five. However, American biologists have found that birds lack both external fingers, that is, the first and fifth. The lizards have lost their fourth and fifth fingers.
long bird
The skeleton of the lizard Unenlagia comahuensis, the "long bird", found in May 1996 in Argentina, fills the gap separating the ancient theropod reptiles and the first bird, Arechaeopteryx.
The higher development of birds is evidenced by an enlarged brain (in particular, the large size of the cerebral hemispheres and cerebellum in birds), perfection of the respiratory and circulatory systems - double breathing and separation of arterial blood from venous blood, as well as constant body temperature. All these improvements in the organization of birds are absent in reptiles.
The debate over whether Archeopteryx could fly has continued since 1861, when the first fossil was found, until now. The answer was found only recently. The creature's fossilized brain was placed in an X-ray machine, allowing thin "slices" of the object to be obtained. These slices were combined in a computer into a three-dimensional model. It turned out that in its anatomy the brain of Archeopteryx is much closer to the brain of modern flying birds than to the brain of dinosaurs, as paleontologists previously assumed. The analysis revealed, in particular, semicircular canals in the inner ear used for balance, and enlarged lobes of the brain responsible for vision - features that affect flight efficiency. The “flying” brain developed simultaneously with wings, and the ability to fly itself developed in the process of evolution much faster than scientists previously thought.
When hunting, ancient lizard-bird theropods regulated their body temperature to avoid overheating. The function of the air conditioner was performed by hollow bones.
The remains of an extinct bird resembling a magpie that lived in the second half of the Jurassic period, i.e. 140 million years ago, were discovered in Europe. In the layers of the earth's crust, scientists discovered the fossilized bones of the skeleton of an unknown creature, and nearby the imprints of its feathers. The bird got a name archaeopteryx (Archaeopteryx litographica), What does "ancient bird" mean? This small bird had sharp, slotted teeth, a long lizard-like tail, and forelimbs with three toes bearing hooked claws.
The shape of the Archeopteryx skull with teeth in both jaws and a very long tail with 20 vertebrae resembled a reptile. In most features, Archeopteryx was more like a reptile than a bird, except for the real feathers on the forelimbs and tail.
How did Archeopteryx fly?
The entire body of this creature, except the head, was covered with feathers, and the forelimbs had all the basic features of bird wings with flight feathers. Only the wing fingers were longer than those of modern birds and had claws. The feet had four toes: the first toe faced back, the rest - forward, which helped to grasp the branches well with the fingers. Tail feathers were attached in pairs on each vertebra of the long tail, and not, as in modern birds, in a wide fan on the coccygeal bone. Features of Arechaeopteryx indicate that it was capable of flapping flight, but only over very short distances.
Rahonavis This crow-sized animal, which lived about 80 million years ago, belongs to the same group of dinosaurs as Velociraptor. True, the creature also has a lot in common with birds. Rahonavis had a retractable sickle-shaped claw on its middle toe, feather cover, and a long, clawed tail similar to Archeopteryx.
The first representatives of the class arose and began to master flight, living in the forest on tree branches, jumping and climbing branches, clinging to them with long fingers of the forelimbs with claws. Having spread their wings, they glided in the air from top to bottom along an inclined plane, and also flew short distances by flapping their wings. Only later did some birds begin to adapt to life in the steppes and deserts, on the banks of reservoirs and in other places.
Archeopteryx for a long time remained the only link between birds and reptiles known to science, but in 1986 the remains of another fossil creature were found that lived 75 million years earlier and combined the characteristics of dinosaurs and birds. Although this animal was named Protoavis (protobird), its evolutionary significance is controversial among scientists.
After Archeopteryx, there is a gap of about 20 million years in the fossil record of birds. The following finds date back to the Cretaceous period, when many species of birds appeared, adapted to different habitats. Among the approximately two dozen Cretaceous taxa known from fossils, two are particularly interesting: Ichthyornis And Hesperornis. Both were discovered in North America, in rocks formed on the site of a vast inland sea.
Ichthyornis was the same size as Archeopteryx, its body length was about 50 cm, and it weighed 5 kg. Outwardly, it resembled a seagull with well-developed wings, indicating the ability to fly powerfully. Like modern birds, it had no teeth, but its vertebrae were similar to those of a fish, hence its generic name, meaning “fish bird.” His remains were found in the USA. Ichthyornis lived 65-90 thousand years ago.
Hesperornis ("western bird") was 1.5–1.8 m long (up to 2 m) and almost wingless. His weight was 40 kg. With the help of huge flipper-like legs extending sideways at right angles at the very end of the body, it apparently swam and dived no worse than loons. It had teeth of a "reptilian" type, but the structure of the vertebrae was consistent with that typical of modern birds. The remains of Hesperornis were found in the USA. This bird lived 70 thousand years ago.
With the onset of the Tertiary period (65 million years ago), the number of bird species began to increase rapidly. The oldest fossils of penguins, loons, cormorants, ducks, hawks, cranes, owls and some songbirds date back to this period.
Huge flightless birds
In addition to the ancestors of modern species, several huge flightless birds appeared in the Tertiary period, apparently occupying the ecological niche of large dinosaurs. One of them was Diatryma, discovered in Wyoming, 1.8–2.1 m tall, with massive legs, a powerful beak and very small, underdeveloped wings.
Their wingspan was 7.5 m; they lived in Europe, Africa, North and South America and were carnivores (eating fish and aquatic invertebrates).
According to paleontological data, there were large, partially feathered lizards. In 1834, the French explorer Goudeau found half an eggshell in Madagascar so large that it could be used as a water container. Then several giant bones were found in the swamps of the island, which were initially mistaken for the remains of an elephant or rhinoceros. But the bones belonged to a bird that must have weighed at least half a ton. Madagascan ostriches epiornithes (Aepyornithes), reached a height of 5 m, laid eggs 32 cm long and 22 cm wide, containing 8.5 liters of liquid contents. The largest egg in the Epyornis clutch is considered to be 24 cm long and 11 liters in volume.
The Venetian traveler Marco Polo did not have a chance to visit Madagascar himself, but he also heard amazing stories: “They say that there is a vulture bird there, it appears at a certain time of the year, and in everything the vulture is not the same as we think and how it is depicted. We say that the vulture is half bird, half lion, and this is not true. Those who have seen him claim that he looks like an eagle, but only very large... On the island he is called Ruk.”
French zoologists discovered the remains of an apiornis in Madagascar with a bronze ring attached to the bird's leg. Experts came to the conclusion that the signs on the ring are nothing more than an impression of a seal from the era of the most ancient civilization of India - Mohenjo-Daro. made about five thousand years ago. Radiocarbon dating of the bird's bones helped establish its age: it is five thousand years old! In the 3rd millennium BC, the inhabitants of Hindustan made daring sea expeditions. By this time, they had accumulated centuries of experience in driving ships, and the Indians had also visited Madagascar. At that time, apiornis were found here in abundance. In the stories of the sailors who returned home, they received a lot of attention.
The eggs, which were found on sand dunes and swamps in the southern part of the island of Madagascar, looked suspiciously fresh. They seemed to have been demolished quite recently. Local residents are sure that giant birds still live in the deepest forests of the island, but it is not easy to see them. In Madagascar, there are still huge areas of protected jungle and untrodden swamps; there is enough space for apyornis.
Judging by fossil leg bones found in 1974 near Alice Springs, flightless Dromomis stirtoni, a giant, ostrich-like bird that lived in central Australia from about 15 million to 25,000 years ago, reaching a height of 3 m and weighing about 500 kg.
A giant bird that looks like an ostrich moa (Dinornis maximus), living on the islands of New Zealand, presumably until the beginning of the 19th century, was probably even greater in height - 3.7 m, and weighed about 230 kg.
Analysis of ancient eggshells suggests Australia's enormous flightless birds went extinct 45,000 to 55,000 years ago after humans scorched their habitat.
An international team of scientists has analyzed hundreds of eggshell fragments from an extinct flightless bird called Geniornis that lived 130,000 to 50,000 years ago. Carbon isotopes from eggshells reveal what the birds ate when they laid their eggs. It was discovered that Geniornis's diet was strict and always included grass.
At the end of the Tertiary period (1 million years ago) and throughout the early Pleistocene, or glacial era, the number and diversity of birds reached a maximum. Many of today's species emerged, as well as others that later became extinct. Teratornis incredibilis from Nevada (USA), a huge condor-like bird with a wingspan of 4.8–5.1 m; was probably the largest known bird capable of flight.
Characteristics specific to the class of birds are primarily associated with the ability of these animals to fly, although some of their species, such as ostriches and penguins, lost it during their later evolution. What makes them stand out even more is their feathers, which are not found on any other animal. They differ from most mammals in that they lay eggs.
The first documented case of this kind was the destruction of the dodo. Mauritian dodo Raphus cuculatus large flightless pigeons, resembling turkeys in appearance, three species of which lived on three islands of the Mascarene archipelago in the Indian Ocean (Mauritius, Reunion and Rodrigues). They were quickly destroyed by humanity almost immediately after their discovery: the archipelago was discovered in 1507, the last dodo was seen in Mauritius in 1681. In the 174 years after the discovery of Mauritius by Europeans in 1507, the entire population of these birds was exterminated by sailors and the animals they brought on their ships. On Reunion Island, the last bird was killed in 1750; on Rodrigues Island, the last bird also did not survive until the end of the 18th century.
Passenger pigeon
In 1914, Martha, the last representative of a previously large genus, died at the Cincinnati Zoo (North Carolina, USA). passenger pigeons (Ectopistes migratorius). Passenger pigeons were mercilessly exterminated for meat.
First North American species to become extinct at the hands of humans
...became great auk (Alca impennis), extinct in 1844. It also did not fly and nested in colonies on the Atlantic islands near the continent. Sailors and fishermen easily killed these birds for meat, fat and to make bait for cod.
Soon after the disappearance of the great auk, two species in the east of the North American continent became victims of humans. One of them was Carolina parrot (Conuropsis carolinensis). Farmers killed these flocking birds in large numbers as thousands of them regularly raided gardens.
100 species of birds have disappeared
Since 1600, perhaps 100 bird species have become extinct worldwide. Most of them were represented by small populations on sea islands. Often incapable of flight, like the dodo, and almost unafraid of man and the small predators brought by him, they became easy prey for them.
Many species of birds are also on the verge of extinction Currently, many species of birds are also on the verge of extinction or, at best, are under threat of extinction. In North America, the California condor, yellow-legged plover, whooping crane, Eskimo curlew and ivory-billed woodpecker (possibly already extinct) are in the worst position. In other regions, the Bermuda typhoon, the Philippine harpy, the kakapo (owl parrot) from New Zealand, a flightless nocturnal species, and the Australian ground parrot are in great danger.
In natural conditions, only one lives in our time blue macaw (Cyanopsittaspixii), however, approximately 30 of these birds are kept in captivity.
Hawaiian Warbler, lepidopteran mojo (Moxobracattus), considered completely extinct and rediscovered only in 1960, apparently, is represented by only 2 pairs of individuals.
There are less than 20 survivors in the world (mostly in captivity) Red-legged Ibis (Nipponia nippon), but they are all apparently too old to reproduce.
As a result of uncontrolled hunting New Zealand owl parrot (Strigops habroptilus) was on the verge of extinction. The second reason for its extinction is that this flightless bird cannot escape from predators, so only 10 specimens remain alive.
Nowadays, only a few exist in natural conditions. California condors, bred in captivity and released in 1992.
Labrador eider Camptorhynchus labradorius.
Samoan moorhen Gallinula pacifica.
White plume Porphyrio albus.
Mauritius blue pigeon Alectroenas nitidissima.
Norfolk ground pigeon Hemiphaga argetraca.
Slender-billed nestor Nestor productus.
Cuban macaw Ara tricolor.
Cayman bluebird Turdus ravidus.
The birds listed above found themselves in an unenviable position mainly due to the fault of humans, who brought their populations to the brink of extinction through uncontrolled hunting, ill-considered use of pesticides or radical transformation of natural habitats.
26 species of birds and 132 species of mammals are now on the verge of extinction.
So Darwin's theory received another, and very strong, confirmation. The first birds, the skeletons of which were found in the slate quarries of Bavaria, lived on Earth about 160 million years ago - during 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 sizes. The wingspan of some flying creatures - pteranodons, for example - was 6-7 meters. These were the largest flying animals that ever lived on Earth.
The first birds were relatively small in size. Archeopteryx was only slightly larger than the pigeon. He was a poor flyer and moved by hovering from tree to tree or from tree to ground. From the ground, he climbed up the tree trunk again, clinging to the bark with the claws of his toes and wings. Weak jaws lined with small teeth indicate that Archeopteryx was not a predator. Most likely, this bird (systematic zoologists firmly include Archeopteryx in the class of birds, classifying it, however, as a separate subclass of ancient birds) fed on fruits and berries, not disdaining small insects and worms. It is impossible to say from fossil remains what the color of the feathers of Archeopteryx was. However, there is reason to believe that it was multi-colored, camouflaging the bird against the background of vegetation.
The origin of the first birds from reptiles is undoubtedly. True, paleontologists have not yet managed to find all the steps along which she walked. But they came to the unanimous conclusion that the ancestors of birds were small reptiles from the group of pseudosuchians, which originally lived on flat, steppe-like spaces, in places covered with small rocks. They had enlarged hind limbs, large brain cavities that lightened the weight of the skull - these signs allow us to conclude that their body straightened and the animals tried to walk on their hind limbs. Subsequently, some of these reptiles adapted to life in 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 appearance of wings.
Fossil remains of the transitional form between reptiles and birds have not yet been found. But we can assume its existence. Paleontologists even imagined the appearance of this ancestral bird. At this stage of development, the scales had already turned into feathers, which helped the animal make parachute flights from branch to branch or from tree to ground.
It’s not far from the great bird to Archeopteryx. The cover of feathers not only lifted the most ancient 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.
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 itself. Judging by its appearance, it looked like a cross between a reptile and a bird, with a toothy beak, a long bony tail and distinct plumage. In recent years, remains of other feathered reptiles have been found.
Bird plumage serves two important functions: it keeps birds warm and helps them fly. Those feathers that serve for heating are usually shorter and softer, and those with which birds fly - the so-called flight feathers - are larger and curved in the shape 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 then, millions of years later, some of them acquired a different, completely special form, designed specifically for flight. When exactly the plumage appeared is unknown. According to some paleontologists, some semblance of feathers is already found in a reptile that lived in Russia, however, most scientists are not completely convinced of this. The most convincing evidence in this sense comes from the presence of plumage as such in small theropods, whose fossilized remains were recently discovered in China. One of them, Sinosauropteryx, retains clear signs of short fluffy plumage in the form of a long comb stretching along the neck and entire back. It was already a feathered dinosaur, but it quite obviously did not yet know how to fly.
Sinosauropteryxes appeared somewhat later than Archeopteryxes, which makes it 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 what they looked like long before full plumage. But what is much more important is not even this, but how the wings developed and, most importantly, why? According to one theory, wings developed among the ancestors of today's birds as a special adaptation for hunting insects and other small animals. So, according to this very theory, the first birds, trying to overtake the victim, took off from the ground and overtook it in a jump, already in the air. Then, following the same theory, a long time later, the front legs of the first birds began to grow feathers, which helped them maintain balance or, perhaps, hold prey. The feathers gradually stretched out, and the muscles on the front legs became stronger. This is probably how animals arose that one fine day had the strength to get off the ground. 
Avimim (left) was a feathered theropod, but it could not fly. Archeopteryx (center) was smaller and lighter, and also had well-developed flight feathers. Compared to Archeopteryx, modern birds like the pigeon (right) have no teeth or claws on their wings, with the possible exception of the gaocin, and their tails are noticeably shorter.
This “down-to-earth theory” was based on some characteristic features identified in Archeopteryx, such as the extraordinary strength of its paws. However, according to most paleontologists, modern birds descended from reptiles that lived not on land, but in trees. With the development of exceptionally long feathers, such animals gained the ability to float 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 - by flapping their wings. But it took the reptiles a lot of time to get the hang of soaring. This was achieved, for example, by coelurosaurus 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 with the same thing. 
Archeopteryx had asymmetrical, or rather curved, feathers on its wings, like modern birds. Feathers like these help birds fly when the wind blows over them, which in turn confirms that Archeopteryx could fly.
It doesn’t take much strength to soar, 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 began to differ strikingly from their ancestors - dinosaurs. In this sense, evolution took 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 lost excess weight. The bones of the first birds were mostly fused, due to which their skeleton was slightly lighter. Like their theropod ancestors, the bones of the first birds were hollow, 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 flight, became stronger, as well as a triangular fork, or arch, that supported the sternum during flight. Such anatomical changes turned out to be quite successful. In the Cretaceous period, birds literally filled the Earth, especially since the time of reptiles was nearing its fatal end. Therefore, birds were the only surviving descendants of dinosaurs. 
Archeopteryx closely resembled a small theropod. Archaeopteryx fossils found in the 1950s were thought to belong to Compsognathus until the faint outline of feathers was discovered next to them.
