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.
For decades, paleontologists only fossil link between birds and dinosaurs was archaeopteryx, a hybrid creature with feathered wings but with the teeth and long bony tail of a dinosaur. These animals appeared to have acquired their birdlike features feathers, wings and flight in just 10 million years, a mere flash in evolutionary time. Archaeopteryx seemed to emerge fully fledged with the characteristics of modern birds, said Michael Benton, a paleontologist at the University of Bristol in England.
Over time, they discovered, the face collapsed and the eyes, brain and beak grew. The first birds were almost identical to the late embryo from velociraptors, Abzhanov said. Modern birds became even more babylike and change even less from their embryonic form. In short, birds resemble tiny, infantile dinosaurs that can reproduce.
Given that paedomorphosis may have aided in the drive toward miniaturization or vice versa, why would it be significant for the evolution of birds? Selection for small size may have halted the development of the adult form because changes in size are frequently associated with changes in development. “Stop growing at smaller size is a neat way to cut short a developmental sequence,” Benton stated. Since babies typically have larger heads than adults do in relation to their bodies, an adult bird’s babylike skull may also help explain why birds have larger brains than adults. He claimed that maintaining child size into adulthood is a fantastic strategy to increase brain size.
Sophisticated new analyses of these fossils, which track structural changes and map how the specimens are related to each other, support the idea that avian features evolved over long stretches of time. In research published in Current Biology last fall, Stephen Brusatte, a paleontologist at the University of Edinburgh in Scotland, and collaborators examined fossils from coelurosaurs, the subgroup of theropods that produced archaeopteryx and modern birds. They tracked changes in a number of skeletal properties over time and found that there was no great jump that distinguished birds from other coelurosaurs.
Baby Face In 2008, Arkhat Abzhanov, a biologist at Harvard University, was elbow deep in alligator eggs. Since alligators descend from a common ancestor with dinosaurs, they can provide a useful evolutionary comparison to birds. (Despite their appearance, birds are more closely related to alligators than lizards are.) Abzhanov was studying alligators vertebrae, but what struck him most was the birdlike shape of their heads; alligator embryos looked quite similar to chickens. Fossilized skulls of baby dinosaurs show the same patternthey resemble adult birds. With those two observations in mind, Abzhanov had an idea. Perhaps birds evolved from dinosaurs by arresting their pattern of development early on in life.
Paleontologists discovered in the 1970s that Archaeopteryx and theropods, a small class of carnivorous dinosaurs, shared certain distinctive characteristics. With the exception of the ornithischian dinosaurs, every group of dinosaurs on this evogram is a theropod. Scientists hypothesized that birds could have descended from theropods based on the similarities between their features. Paleontologists were even more persuaded when they constructed evolutionary trees to investigate the issue. On the dinosaurs’ branch of the tree of life, the birds are merely a twig.
Following Archaeopteryx, birds continued to evolve in some ways similar to those of their theropod ancestors. Their reduced and fused bones may have contributed to their increased flight efficiency. Likewise, the feathers grew longer and their vanes asymmetrical, and the bone walls became even thinner, all of which likely improved flight. After the bony tail became a stump, a spray of feathers at the tail began to serve as a stabilizing and maneuvering aid. The wishbone, which was present in dinosaurs other than birds, developed into a more robust and complex structure, and the shoulder girdle’s bones changed to join the breastbone, stabilizing the forelimb’s flight mechanism. The breastbone grew in size and developed a central keel along the breast’s midline, which supported the muscles used for flight. As the primary means of locomotion changed from running to flying, the arms became longer than the legs, and in different lineages of early birds, teeth were repeatedly lost. The Late Cretaceous is when the ancestor of all living birds lived. In the 65 million years that have passed since the other dinosaurs went extinct, this ancestral lineage has diversified into the main species of birds that are still alive today.
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.
The finding that birds descended from Late Jurassic small carnivorous dinosaurs was made possible by newly found fossils from China, South America, and other regions, as well as by using innovative techniques and new perspectives to examine historical museum specimens. An early 1860s discovery of a specimen of Archaeopteryx, the first known bird, marked the start of the search for the ancestors of modern birds. It had feathers on its arms and tail, just like birds do, but it also had teeth and a long, bony tail, unlike living birds. Moreover, unlike many bones in living birds, many of the bones in Archaeopteryx’s hands, shoulder girdles, pelvis, and feet were distinct rather than fused and reduced. These traits led to the recognition of Archaeopteryx as a transitional species between birds and reptiles, but which reptiles?
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.
FAQ
Which group of dinosaurs did birds likely evolve from?
What is the origin of birds from dinosaurs?
Did birds evolve from Raptors?
Did birds coexist with dinosaurs?