How dinosaurs became birds

Unusual fossil find of dinosaurs from which birds evolved reveals how evolution creates entirely new species

By Steven Brost, the article is published with the permission of Scientific American Israel and the Ort Israel Network 09.03.2017

• Scientists have long known that birds evolved from dinosaurs and that they are actually a subgroup of them. Rich finds of feathered dinosaur fossils discovered in China and elsewhere have chronicled the details of the dramatic transition from monstrous land-dwelling dinosaurs to small birds that could fly.
• New techniques for analyzing fossils allow researchers to reconstruct how the body structure unique to birds was formed. The results show that the hallmarks of this group appeared slowly over tens of millions of years, for purposes different from those they serve today.
• These findings add to a growing body of evidence that major evolutionary transitions occur gradually rather than rapidly.

At about six in the morning, well before dawn, on a cold November day in 2014, I made my way through the Beijing train station and struggled to board the crowded carriage. I went toJinjo, a city in northeastern China, about the size of Chicago. I tried to steal some more sleep as we crawled past concrete factories and smog-shrouded cornfields, but I was too excited to fall asleep. At the destination, something that was rumored to be unbelievable awaited me: a mysterious fossil that a farmer came across by chance while harvesting his crops.

Four hours later I went out to the platform in Jinzhou and followed my colleague Junchang Lu, a famous dinosaur hunter from the Chinese Academy of Geological Sciences in Beijing, who asked for my help in researching the fossil. A small group of local dignitaries welcomed us and took us to the municipal museum housed in a dilapidated building on the outskirts of town. With the seriousness of a Ramat Darg political summit, our entourage walked along a long corridor and entered a side room where a piece of rock was placed on a small table. So I found myself face to face with one of the most beautiful fossils I have ever seen: a skeleton about the size of a donkey, the chocolate-brown color of its bones contrasting against the gray limestone.

I had no doubt that it was a dinosaur. The creature had sharp teeth like a meat knife, pointed claws and a long tail that clearly testified to its family relationship toVelociraptor the evil one fromJurassic Park". But several important characteristics distinguished this item from China from ordinary similar dinosaurs. His bones are light and hollow, his legs are long and thin like those of a heron, and his body is covered with all kinds of feathers, including quill feathers large ones that were placed on top of each other across his arms and formed wings. This dinosaur is remarkably similar to a bird.

About a year later, he and I designated this skeleton as a new species: Zhenyuanlong. This fossil is the latest in a long list of feathered dinosaurs found in China's Liaoning Province over the past twenty years, an extraordinary series of fossils that illustrate, like a scrapbook, how the monstrous dinosaurs of the Jurassic age evolved into the birds of today.

The implications of these fossils are enormous. Since the days of Charles Darwin, scientists have wondered how evolution produces distinctly new groups of animals. Is it happening quickly, because of some unusual mutation that can turn land creatures into lords of the sky? Or do these new groups form more slowly, as organisms adapt to changing environments over millions of years? The Zhenyuanlong and other fossils from Liaoning and elsewhere are beginning to answer this.

transitional fossils

Birds are endowed with many characteristics that distinguish them from other modern animals. Apart from the features that allow them to fly, their metabolism is fast, which allows them to grow at an incredible speed; Their large minds give them high intelligence and sharp senses. The birds are so different, in fact, from the other animals, that researchers have been puzzled about their origin for a long time.

In the 60s, the British biologist Thomas Henry Huxley, one of Darwin's closest friends and most vocal supporters, began to solve the mystery of the origin of birds. Just a few years after Darwin published his book Origin of Species In 1859, quarry workers in Bavaria cracked a slab of limestone and discovered a 150-million-year-old skeleton of a Frankensteinian creature. It had sharp claws and a long tail like a reptile, but also feathers and wings like a bird. Huxley understood that the animal, which received the name Archaeopteryx There is a surprising similarity to the small carnivorous dinosaurs such as theCompsognathus, which also began to appear in those days. Huxley therefore came up with a revolutionary idea: the birds are the descendants of the dinosaurs. Some disputed him, and the polemic went this way and that for the next hundred years.

The dispute was eventually settled, as is usually the case in such debates, when new fossils were discovered. In the mid-60s of the 20th century, the paleontologist revealed John Ostrom from Yale University the dinosaur Deinonychus, which remarkably resembles a bird, in western North America. It had long arms that look almost like wings and a lean build that suggests it was an active and energetic animal. It's possible, Ostrom concluded, that Deynonychus even had feathers. For if birds are the offspring of dinosaurs, which was already accepted by many paleontologists at the time, then feathers must have developed at some point in this evolutionary lineage. But Ostrom wasn't sure about that because he only had bones of that animal. Unfortunately, soft parts like feathers hardly survive the rigors of death, decay and burial and are not appetizing.

Ostrom waited. He continued to search for that "holy grail" that would prove beyond any doubt the connection between birds and dinosaurs: a dinosaur skeleton preserved in such detail that you could see feathers. Then, in 1996, near the end of his career, Ostrom attended the annual meeting of the Association for the Study of Vertebrate Fossils in New York and there approached him Philip Kerry, currently serving as a professor at the University of Alberta. Kerry, who also studied fossils of bird-like dinosaurs, had recently returned from China, where he learned of an unusual fossil. He pulled out a picture and showed it to Ostrom. And here in the picture is a small dinosaur surrounded by a halo of feathery fluff. It was preserved in all its details because volcanic ash quickly buried it, like in Pompeii. Ostrom burst into tears. Someone finally found their feathered dinosaur.

The fossil shown by Kerry to Ostrom, who later received the name Sinosauropteryx, was the first in a flood of discoveries. The scientists rushed to the Liaoning region of China where the fossil was found like prospectors during the gold rush, although those who really knew where to look were the local farmers. Today, two decades after the discovery of Sinosauropteryx, fossil hunters have recovered more than 20 species of feathered dinosaurs from Liaoning. And their variety is wide: ancient cousins ​​of Tyrannosaurus rex, which are nine meters long and are covered with hair-like down, to vegetarian dinosaurs that are the size of a dog and are decorated with simple feathers that resemble porcupine needles, to creatures that are the size of crows with real wings that knew how to fly. These fossils are among the most celebrated in the world.

The feathered dinosaurs from Liaoning settled the matter: birds did indeed evolve from dinosaurs. But this statement may be a bit misleading because it implies that these two groups are fundamentally different. After all, birds are dinosaurs: they are one of the many subgroups that can be traced back to the common ancestor of all dinosaurs, and are therefore dinosaurs no less thanTriceratops Or eBrontosaurus. It can be described as follows: the birds are dinosaurs just as the bats are an unusual type of mammal that can fly.

The fossils from Liaoning also helped clarify thegenealogy of the birds. They show which branch of the dinosaur family tree they belong to. The birds are a type of a therapist, the same group whose members are formidable predators like T-Rex,Allosaurus וSpinosaurus. But the birds' closest relatives are a subgroup of small, flexible and much smarter theropods: the raptors, including theVelociraptor, Ostrom's Deinonychus and his Zhenyuanlong and I set up in Jinzhou, which does look very much like a modern bird. Somewhere in this flock of feathered species the line between what is a bird and what is not a bird passes.

We have so many feathered dinosaur fossils today from Liaoning and elsewhere that together they paint a picture of great evolutionary payoff. This image is the clearest example of its kind in the entire fossil record. I and other scientists study these fossils using a variety of advanced methods: computed tomography (CT) scans to simulate their anatomy, computerized data analysis to compile genealogies, computer models to simulate how these animals moved, and advanced statistical techniques to find out how evolution produces new species and anatomical structures. Insights obtained recently from these studies allow us to unfold the story of the transformation of dinosaurs into birds, a story that serves as central evidence in solving the old riddle of how large and new groups were formed.

The appearance is random

The formation of feathers is a central issue in the puzzle of the development of birds. The feathers are the calling card of the birds, like Elvis Presley's cheek wigs and straight black hair. One look at the spread wings of an eagle or the flamboyant tail of a peacock is enough to know exactly what we are seeing. We understand that it must be a type of chicken because unlike mammals, reptiles or any other contemporary group of animals, only birds have feathers. And they are a wonderful accessory. Feathers are nature's Swiss army knife: a versatile tool that allows one to fly, impress a mate or opponent, and also maintain body heat and allow eggs to be incubated in the nest. And indeed their uses are so many that it was difficult to find out what was the purpose for which they were developed in the first place.

Thanks to the Sinosauropteryx and other fossils from Liaoning, we know one thing for sure: feathers did not suddenly appear with the first birds, but appeared much earlier, in their dinosaur ancestors. It is even possible that the common ancestor of all dinosaurs was a feathered species. But these early feathers were very different from the quill feathers of modern birds. The plumage of Sinosauropteryx, and of many other dinosaurs, was more like a woolly plume made of thousands of hair-like fibers. There is no way these dinosaurs could fly. Their feathers were too simple to catch wind, nor did they have wings at all. If so, feathers initially evolved for a different purpose, probably to keep these little dinosaurs warm.

Most of them had enough of their bristling feathers. But one subgroup, the theropods eraptors, underwent a total upgrade. The hair-like strands grew longer and then began to branch, first into a few simple tufts and then into a much more ordered system of branches spreading out from a central reed. And so the quill feather was born. And these elaborate feathers, being arranged in overlapping rows upon the arms, formed the wings. For some of the dinosaurs of Liaoning, such as theMicroraptor, a dinosaur that grew like a crow and was defined by Shu Xing From the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, there were wings on the legs and tail as well, an arrangement not found in any modern chicken.

Why did these dinosaurs turn their feathers into wings? The obvious answer is to fly: the maniraptorans turned their body into an airplane, and the wings developed and became the lifting surfaces. But a closer examination of the fossil evidence shows that this was not the case. Although some of the small, winged creatures such as the microraptor were surely able to fly, this is shown by experiments in wind tunnels and computer simulations conducted by Gareth Dyke From Debrecen University in Hungary, others, such as Zhenyuanlong from Jinzhou, were short-armed and their heavy bodies stayed on the ground. And besides, all these winged dinosaurs lacked the large pectoral muscles that provide the power necessary for flight, and only a few had asymmetrical quill feathers, with a short and stiff leading edge (front flagellum) and a softer trailing edge (rear flagellum), optimally adapted to withstand the strong forces of air resistance in fast motion.

There was no moment when a dinosaur turned into a bird, no big bang when the T-Rex turned into a chicken. It was a journey.

The latest findings suggest that the wings evolved to fulfill a different, less well-known role: flamboyant display. One type of evidence comes from pioneering work he did Jacob Winter from the University of Bristol in England. He used powerful microscopes to identify the bodies containing the pigments, Melanosomes, in fossilized dinosaur feathers. It turns out that the feathers of the feathered dinosaurs, which could not fly, were very colorful. Some of them even reflected light in a colorful glow like feathers of bird species even today. These bright ornaments must have been the perfect tool for attracting mates and deterring rivals.

The iridescence that probably characterized the feathers of these dinosaurs gave birth to a completely new hypothesis regarding the formation of the wings: they were initially developed for the sake of advertising - billboards that rose from the arms, legs and tail. These ornamental winged dinosaurs therefore found themselves running around with large, wide surfaces that, among other things, due to the laws of physics, have aerodynamic qualities. Want to say, the flight developed by chance. And it is possible that it developed simultaneously many times, when different raptors began to feel the lifting power of their wings in their jumps on the ground, in fast running up trees, or in jumping between branches. Eventually, the descendants of one of these raptor lineages began to shrink, developed large chest muscles and very elongated arms, lost their long tails, and became the birds of today.

Evolution step by step

The development of feathers and wings demonstrates a much more general evolutionary pattern. The Liaoling dinosaurs show that many other characteristics supposedly unique to birds evolved millions of years before the birds themselves and for reasons completely unrelated to flight.

Straight, long legs and feet with three thin big toes, the hallmark of the modern bird, first appeared more than 230 million years ago in the earliest dinosaurs. Their appearance, it seems, was part of a redesign of the entire dinosaur's body and turning it into a fast running machine on two legs, a creature that ran faster and hunted better than its competitors. These characteristics of the hind limbs are part of the characteristics that distinguish the entire group of dinosaurs, and they are the ones that helped them rule the world for so long. In some of these dinosaurs, which were the earliest members of the theropod lineage, the left clavicle and the right clavicle fused to form a new structure: the fork bone. It's a seemingly small change that stabilized the shoulder girdle and allowed these nimble, dog-sized predators to better absorb the shocks involved in hunting. Later, in the birds, the fork bone underwent a transformation and is used by them as a spring that stores energy when they flap their wings.

The hollow bones and the rapid growth, which distinguish the birds and are important for flight, also have roots in the dinosaurs. Many dinosaurs had bones that contained air sacs that made them hollow. These sacs are a clear sign that they had highly efficient "flow lungs", taking in oxygen not only when inhaling but also when exhaling. In the birds, lungs of this type provide the fuel necessary to maintain their high-energy lifestyle, in addition to making the skeleton lighter and enabling flight. And also, the microscopic structure of the dinosaur bones indicates that the growth rate and physiology of these animals were in the middle, between those of the reptiles, that is, the slow to mature, and between those of today's birds, who are warm-blooded and fast growing. Researchers therefore understand today that flowing and fast-growing lungs appeared more than 100 million years before birds spread their wings, when the first fast, long-legged dinosaurs began to sustain themselves as energetic runners, quite different from their clumsy rivals, the amphibians, lizards and crocodiles.

From the modest size of the birds, delicate and much smaller than the T-Rex and his friends, they also preceded the birds themselves. Mike Lee from Flinders University in Australia andRoger Benson From the University of Oxford, they each found that the gradual evolutionary reduction of body size began in the raptors and lasted more than 50 million years. It is not clear what exactly caused this trend, but one possibility is that the smaller and smaller body structure of these feathered dinosaurs opened the door to new ecological niches for them: trees, bushes, maybe even underground caves or burrows that giants like the Brachiosaurus and the Stegosaurus could not enter.

The neural and behavioral features of the birds alive today are also rooted in the age of the dinosaurs. Much of the decisive evidence for the distant history of these features comes from the Gobi Desert in Mongolia, where a joint team with the American Museum of Natural History in New York (AMNH) and the Mongolian Academy of Sciences has been collecting fossils for 25 years. Conducted by Mark NorrellוMike Novacek From AMNH, the expeditions, which leave every year in the summer, collected a variety of samples from the period Upper Cretaceous, 84 to 66 million years ago, providing an unprecedented detailed look into the life of dinosaurs and ancient birds. They found, among other things, a treasure trove of well-preserved skulls of velociraptors and other feathered raptors. CT scans of these fossils, which she did Amy Blanouf from Stony Brook University, discovered that these species had a large brain and that the most anterior part of this organ was enlarged. It is a large forebrain that makes the birds so smart and serves as a flight computer that allows them to master the uncomplicated matter of flight and move in the complicated air world in its three dimensions. Scientists still don't know why these dinosaurs developed such sharp intelligence, but their fossils clearly show that their bird ancestors became intelligent before they started flying.

The bird's body was therefore not an orderly plan, but more like a structure made of Lego, assembled brick by brick in the course of evolution. The transition between dinosaur and bird did not happen all at once, but over tens of millions of years of gradual development.

Smooth transition

The transition from dinosaur to bird was so gradual, in fact, that there is no clear distinction in the genealogy of the family between creatures that are not birds and those that are birds, as I showed in 2014 based on statistical calculations. My research stemmed from the doctoral thesis I wrote under Norell's guidance. Aside from his 25-year quest in the Gobi Desert, Norell has spent the last twenty years working with generations of doctoral students to assemble the ever-growing family trees of the dinosaurs. He and I, with our colleagues Grimm Lloyd from the University of Leeds in England andSteve Wang From Swarthmore College, we have built a database of more than 850 skeletal features of about 150 theropods spanning the entire spectrum of the dinosaur-to-avian transition. Then we used multivariate statistics to place each species in a "morphological space" which is basically a map that groups species according to the proportion of their shared characteristics. Two species are very similar to each other and anatomically they will be close on a map, like Chicago and Indianapolis on a road map, while two species whose skeletons are different from each other will be far apart, like Chicago and Phoenix. If the birds evolved from the dinosaurs in a series of dramatic, rapid mutations that soon gave rise to a completely different type of animal, then the two groups would be from different places on the map. But the resulting morphological space was "messy": the birds were scattered within larger areas of dinosaurs. There was no clear separation between them, which indicates a very slow transition to the point that it is practically not noticeable.

If so, birds are nothing but a type of dinosaur. Had I made a search in Jinzhou some 125 million years ago, when Zhenyuanlong was still alive flapping his wings futilely as he fled the ash cloud that eventually suffocated him, he would surely have looked like some kind of large chicken to me. I would say that in general, dinosaurs and birds of prey are. The technical definition of the Zhenyuanlong as a dinosaur rather than a bird is a scientific convention and a matter of tradition. Paleontologists have long defined birds as descendants of the most recent common ancestor of Huxley's Archaeopteryx and the modern bird: basically an animal with fat wings that was capable of flight. Since the dromaeosaurs, including the Zhengyuanlong, are several branches outside of this part of the family tree, they are not defined as birds.

However, poultry should not be underestimated. They may be dinosaurs and not a separate class, but they are special. They built a whole new way of life for them, and today they thrive with more than 10,000 species in a rich and spectacular variety of forms, from the hummingbird to the ostrich. Moreover, the birds managed to survive when all the other dinosaurs died 66 million years ago.

The thought of all the random vicissitudes of fate that operated over tens of millions of years and produced this group of animals is amazing. Their ancestors did not know that they were becoming more and more like birds. Even we, if we had witnessed the process, would not have expected that many of the characteristics that evolved to help these dinosaurs warm up and attract mates would be repurposed and become fundamental components of an aviation system.

Evolution does not see the unborn; It operates only on what is available now, and what directs it are only the ever-changing factors that never end: environmental pressures and competition. There was no moment when a dinosaur turned into a bird, no big bang when the T-Rex turned into a chicken. It was a journey, and as scientists learn about other major evolutionary transitions, such as the transition from fish to tetrapods with limbs and fingers, from land mammals to whales, from tree-dwelling primates to bipedal humans, we are aware of a common factor in all these transitions: it is a marathon, not a sprint, And there is no finish line.

It is worth mentioning another aspect of the saga of the origin of birds. The statistical study that my colleagues and I have done may explain how the birds survived the catastrophe of extinction that claimed the lives of the other dinosaurs. In our work, we used the large database we built to measure evolutionary rates: how quickly the skeletal characteristics of birds and their dinosaur cousins ​​changed. This is a sign of evolutionary vitality. The results surprised us. These early birds that lived alongside their dinosaur ancestors evolved at an extremely rapid rate, faster than the velociraptor, the gentianlong, and other non-avian species. It seems that from the moment a small dinosaur capable of flight was created, from the moment its Lego set was assembled, the realization of an incredible evolutionary potential became possible. The flying dinosaurs now had access to ecological niches and new opportunities. And when their brothers could not cope with the apocalyptic impact of the nine and a half kilometer diameter asteroid that collided with the Earth at the end of the Cretaceous era, the birds simply flew past the destruction, as a new world unfolded before them to conquer.

about the writer

Stephen Brost - Paleontologist from the University of Edinburgh in Scotland. He studies the change of large animal families, such as the dinosaurs and birds, over long periods of time.

for further reading

• Gradual Assembly of Avian Body Plan Culminated in Rapid Rates of Evolution across the Dinosaur-Bird Transition. Stephen L. Brusatte et al. in Current Biology, Vol. 24, no. 20, pages 2386–2392; October 20, 2014
• A Large, Short-Armed, Winged Dromaeosaurid (Dinosauria: Theroposda) from the Early Cretaceous of China and Its Implications for Feather Evolution. Junchang Lü and Stephen L. Brusatte in Scientific Reports, Vol. 5, Article No. 11775; July 16, 2015
• The Origin of Birds and Their Flight. Kevin Padian and Luis M. Chiappe; Scientific American, February 1998
• The home page of the article's author, Steven Broste