The discovery that birds evolved from small carnivorous dinosaurs of the Late Jurassic was made possible by recently discovered fossils from China, South America, and other countries, as well as by looking at old museum specimens from new perspectives and with new methods. The hunt for the ancestors of living birds began with a specimen of Archaeopteryx, the first known bird, discovered in the early 1860s. Like birds, it had feathers along its arms and tail, but unlike living birds, it also had teeth and a long bony tail. Furthermore, many of the bones in Archaeopteryxs hands, shoulder girdles, pelvis, and feet were distinct, not fused and reduced as they are in living birds. Based on these characteristics, Archaeopteryx was recognized as an intermediate between birds and reptiles; but which reptiles?
In the 1970s, paleontologists noticed that Archaeopteryx shared unique features with small carnivorous dinosaurs called theropods. All the dinosaur groups on this evogram, except the ornithischian dinosaurs, are theropods. Based on their shared features, scientists reasoned that perhaps the theropods were the ancestors of birds. When paleontologists built evolutionary trees to study the question, they were even more convinced. The birds are simply a twig on the dinosaurs branch of the tree of life.
As birds evolved from these theropod dinosaurs, many of their features were modified. However, its important to remember that the animals were not trying to be birds in any sense. In fact, the more closely we look, the more obvious it is that the suite of features that characterize birds evolved through a complex series of steps and served different functions along the way.
Take feathers, for example. Small theropods related to Compsognathus (e.g., Sinosauropteryx) probably evolved the first feathers. These short, hair-like feathers grew on their heads, necks, and bodies and provided insulation. The feathers seem to have had different color patterns as well, although whether these were for display, camouflage, species recognition, or another function is difficult to tell.
In theropods even more closely related to birds, like the oviraptorosaurs, we find several new types of feathers. One is branched and downy, as pictured below. Others have evolved a central stalk, with unstructured branches coming off it and its base. Still others (like the dromaeosaurids and Archaeopteryx) have a vane-like structure in which the barbs are well-organized and locked together by barbules. This is identical to the feather structure of living birds.
Another line of evidence comes from changes in the digits of the dinosaurs leading to birds. The first theropod dinosaurs had hands with small fifth and fourth digits and a long second digit. As the evogram shows, in the theropod lineage that would eventually lead to birds, the fifth digit (e.g., as seen in Coelophysoids) and then the fourth (e.g., as seen in Allosaurids) were completely lost. The wrist bones underlying the first and second digits consolidated and took on a semicircular form that allowed the hand to rotate sideways against the forearm. This eventually allowed birds wing joints to move in a way that creates thrust for flight.
The functions of feathers as they evolved have long been debated. As we have seen, the first, simplest, hair-like feathers obviously served an insulatory function. But in later theropods, such as some oviraptorosaurs, the feathers on the arms and hands are long, even though the forelimbs themselves are short. What did these animals do with long feathers on short arms? One suggestion comes from some remarkable fossils of oviraptorosaurs preserved in the Cretaceous sediments of the Gobi Desert. The skeleton of the animal is hunched up on a nest of eggs, like a brooding chicken. The hands are spread out over the eggs as if to shelter them. So perhaps these feathers served the function of warming the eggs and shielding them from harm.
Birds after Archaeopteryx continued evolving in some of the same directions as their theropod ancestors. Many of their bones were reduced and fused, which may have helped increase the efficiency of flight. Similarly, the bone walls became even thinner, and the feathers became longer and their vanes asymmetrical, probably also improving flight. The bony tail was reduced to a stump, and a spray of feathers at the tail eventually took on the function of improving stability and maneuverability. The wishbone, which was present in non-bird dinosaurs, became stronger and more elaborate, and the bones of the shoulder girdle evolved to connect to the breastbone, anchoring the flight apparatus of the forelimb. The breastbone itself became larger, and evolved a central keel along the midline of the breast which served to anchor the flight muscles. The arms evolved to be longer than the legs, as the main form of locomotion switched from running to flight, and teeth were lost repeatedly in various lineages of early birds. The ancestor of all living birds lived sometime in the Late Cretaceous, and in the 65 million years since the extinction of the rest of the dinosaurs, this ancestral lineage diversified into the major groups of birds alive today.
A lot of the features that birds inherited from these theropod dinosaurs changed as they evolved. But it’s crucial to keep in mind that the animals weren’t in any way “trying” to be birds. In actuality, it becomes increasingly clear the closer we examine that the collection of traits that define birds evolved via a convoluted process and fulfilled various roles along the way.
Variations in the dinosaurs’ digits that led to the development of birds provide additional evidence. Small fifth and fourth digits and a long second digit were present in the hands of the earliest theropod dinosaurs. In the theropod lineage that would eventually give rise to birds, as the evogram demonstrates, the fifth digit (e g. , as seen in Coelophysoids) and then the fourth (e. g. , as seen in Allosaurids) were completely lost. The wrist bones that supported the first and second fingers coalesced into a semicircular shape, enabling sideways rotation of the hand against the forearm. Eventually, this made it possible for the wing joints of birds to move in a way that produces thrust for flight.
We find several new types of feathers in theropods even more closely related to birds, such as the oviraptorosaurs. One is branched and downy, as pictured below. Others have developed an unstructured base and a central stalk with branches branching off of it. Others (such as the Archaeopteryx and Dromaeosauridae) have a structure resembling a vane, with barbules holding the well-organized barbs together. This is identical to the feather structure of living birds.
Take feathers, for example. Small theropods related to Compsognathus (e. g. , Sinosauropteryx) probably evolved the first feathers. Their heads, necks, and bodies were covered in these short, hair-like feathers that served as insulation. The feathers appear to have had distinct color patterns as well, though it’s unclear if these were for show, camouflage, species identification, or some other purpose.
There has long been discussion regarding the evolutionary roles of feathers. It is evident that the initial, most basic feathers resembled hairs had an insulating purpose. However, even though the forelimbs of later theropods, like some oviraptorosaurs, are short, the arms and hands have long feathers. One theory regarding the function of these animals with long feathers on short arms comes from some amazing fossils of oviraptorosaurs found in the Cretaceous sediments of the Gobi Desert. The animal’s skeleton is hunched over an egg nest, resembling a protective chicken. The hands are positioned so that they appear to be protecting the eggs. It is possible that these feathers provided warmth and protection for the eggs.
Classification of modern species edit The diversity of modern birdsSee also:
The phylogenetic classification of birds is a contentious issue. Sibley The overwhelming body of data indicates that the majority of contemporary bird orders make up strong clades. Scientists disagree on the specific relationships between the major clades, though. Modern bird anatomy, fossils, and DNA evidence have all been used to address the issue, but no clear consensus has been reached.
Systematists have historically had sufficient information from structural traits and fossil records to formulate theories about the phylogenetic relationships among birds. The primary cause of the lack of precise knowledge regarding the orders and families of birds is the imprecisions in these methods. Although studying DNA data can only go so far and there are still unanswered questions, advancements in the fields of computer-generated DNA sequencing and computer-generated phylogenetics have produced a more accurate method for classifying bird species. [24].
Origins editMain article:
There is strong evidence that birds descended from theropod dinosaurs; more precisely, birds belong to the group Maniraptora, which also includes oviraptorids and dromaeosaurs. [4] There is a growing body of evidence suggesting that there is less of a clear line separating non-avian theropods from birds as they mature. Thomas Huxley made notice of this in the 19th century when he wrote:
Findings in the Liaoning Province of northeastern China show that numerous small theropod dinosaurs, including the microraptorian dromaeosaurid Sinornithosaurus and the compsognathid Sinosauropteryx, did in fact have feathers. This has made it more difficult to determine where to draw the boundary between reptiles and birds. [6] Cryptovolans was a dromaeosaurid that was discovered in 2002. It may be a junior synonym of Microraptor. It had uncinate processes on its ribs, a sternal keel, and the ability to fly. Compared to Archaeopteryx, which lacks some of these contemporary bird characteristics, Cryptovolans appears to make a better “bird.” Some paleontologists have proposed that dromaeosaurids are actually descended from a flying ancestor and that the larger members became secondarily flightless, mirroring the loss of flight in modern paleognaths like the ostrich, because some basal members of Dromaeosauridae, including Microraptor, were capable of powered flight. [7] More evidence for the theory that flight originated in the bird line by early dromaeosaurids rather than later by Aves, as was previously thought, has been provided by the discovery of additional basal dromaeosaurids that may be capable of powered flight, such as Xiaotingia. [8].
If the theory of dinosaur origin is right, then birds actually descended from the saurischian (lizard-hipped) dinosaurs, despite the fact that ornithischian (bird-hipped) dinosaurs and birds have the same hip structure. They thus arrived at their hip structure condition independently. In fact, among a peculiar group of theropods called the Therizinosauridae, a bird-like hip structure also developed a third time.
A different theory regarding the dinosaurian origin of birds is advocated by a few scientists, such as Larry Martin and Alan Feduccia. This theory suggests that early archosaurs like Longisquama gave rise to birds, including maniraptoran “dinosaurs.” The majority of other paleontologists and specialists in the evolution and development of feathers disagree with this theory. [10].
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