The "overlord" soaring into the sky: How did dinosaurs "regain life" in the blue sky?

In the fourth-grade Chinese textbook published by the People's Education Press, there is a popular science article called "Dinosaurs Flying to the Blue Sky". This text shows how the dinosaur family evolved from the ground for hundreds of millions of years, gradually evolving various bird-like features, evolving flying organs and flying abilities, and finally flying to the blue sky and becoming colorful birds. Are dinosaurs extinct? What evidence can prove that birds are descendants of dinosaurs? What secrets are there in the excavation of dinosaur fossils?

▲People's Education Press Chinese textbook "Dinosaurs Flying to the Blue Sky" (Photo/Xu Xing)

Xu Xing, author of "Dinosaurs Flying to the Blue Sky" and researcher at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, reveals to us how dinosaurs evolved into birds.

Curiosity makes "newbies"

Became the person who discovered the most fossils

Many people like dinosaurs. The reason may be because of their huge size, strange appearance or remote existence. In addition, there are also dinosaur stars such as Tyrannosaurus Rex, Triceratops, and Mamenchisaurus.

▲Reconstruction of Mamenchisaurus (Photo/Xu Xing/Zhao Chuang)

In 2005, the American magazine Science listed 125 scientific problems, including: What makes up the universe? How did life originate? There are also some questions related to dinosaur research, such as how did dinosaurs become extinct? Why did dinosaurs grow so big?

▲Reconstruction of dinosaur stars (Photo/Xu Xing/Brian Choo)

Paleontology not only studies dinosaur fossils, but also other types of fossils. Why are dinosaur fossils so attractive? The reason is that, like other scientific fields, paleontology research requires us to use our brains to explore the world, but it also has a special feature, which is that it requires us to embrace nature and go out into the wild to find the mysteries of life evolution hidden in the earth.

Since the team members who had just joined the work were curious about every detail in the wild and would not give up any abnormal phenomena, it was curiosity that guided them to discover many important fossils.

Dinosaur fossil excavators are called "dinosaur fossil hunters". The most important step in their work is to discover dinosaur fossils. Surprisingly, the people who find the most fossils are often not experienced field workers, but those who have just joined the field work. This is because they are full of curiosity about every detail in the field and will not give up any abnormal phenomenon. It is curiosity that guides them to discover many important fossils.

▲Team members who have just joined the field work are able to find the most important fossils from clues due to their curiosity (Photo/Xu Xing)

My favorite dinosaur fossil area is the Wucaiwan area in the Junggar Basin in Xinjiang. The scenery here is very beautiful. The small tents where the expedition members live are right next to the rocks, enjoying the fun of outdoor life. Looking up in the distance is the Gobi Desert.

▲Wucaiwan area in the Junggar Basin of Xinjiang, with snow-capped mountains in the distance (Photo courtesy of Xu Xing)

In fact, the most common challenges encountered in field scientific research are sandstorms, floods and other harsh environments. The biggest challenges in finding fossils in the Gobi Desert are high temperatures, mosquitoes, poisonous insects and wild animals. We have made many important discoveries in field excavations in the Junggar Basin. For example, we found the fossils of the ancestor of Triceratops, Yinlong, in rocks formed 160 million years ago.

▲Hidden Dragon Fossil (Photo courtesy of Xu Xing)

▲Reconstruction of the era in which Yinlong lived (Photo provided by Xu Xing)

We also discovered fossils of the Jurassic ancestor of Tyrannosaurus Rex, which is very important for understanding the evolutionary history of the Tyrannosaurus family.

▲Fossils of Guanlong, the ancestor of Tyrannosaurus Rex in the Jurassic period (Photo courtesy of Xu Xing)

▲Reconstruction of Guanlong (Photo/Xu Xing/Zhang Zongda)

In the Cretaceous strata, we found very small crocodile remains near the neck of the Alvarezsaurus fossil (the red dot in the picture below). We guess that the dinosaur died after eating the crocodile. We named this fossil "The Last Supper".

▲Alvarezosaur fossils (Photo courtesy of Xu Xing)

The fossils are preserved layer by layer, with small fossils on the top layer, medium fossils in the middle layer, and larger fossils at the bottom. We call this a "dinosaur sandwich" preservation method. Through research, we have come up with this explanation: In this area, not only did relatively small dinosaurs live, but also Mamenchisaurus, which was nearly 40 meters long. They trampled the soft mud by the river into small pits, which were flooded by muddy water, forming small quagmires. Some small dinosaurs living in this area would accidentally fall into the quagmires and die, and then be buried by some mud and sand, forming fossils. This story happened again and again, and finally formed the "dinosaur sandwich" preservation method.

▲Sandwich preservation of dinosaur fossils, a small dinosaur trapped in a pit stepped on by a giant dinosaur (Photo courtesy of Xu Xing)

In the "Dinosaur Sandwich", we found some strange dinosaurs. The most strange thing was that we found that the dinosaurs had a stomach full of pebbles. We know that some dinosaurs have no teeth like modern birds, and pebbles can effectively grind food. Some birds eat pebbles to clean their intestines. We found that among the small dinosaurs of the Jurassic era 160 million years ago, they also had bird-like behavior. We named this dinosaur "Mire Dragon". Because this dinosaur was stuck in a quagmire and couldn't escape, and then died and formed a fossil.

▲Small stones were found in the fossils of the Inescapable Mire Dragon (in the red frame in the above picture, the picture on the right is an enlarged picture) (Photo courtesy of Xu Xing)

Through the study of fossils, we found that the dinosaur that lived 160 million years ago had teeth before the age of 3, and only became bird-like and had a beak when it became an adult. Through research, we found that this dinosaur liked to eat meat when it was young and was an omnivore, but it began to eat plants when it became an adult, which is related to the loss of its teeth and the formation of its beak.

▲Reconstruction of the dragon that cannot escape the mud (Photo/Xu Xing/Portia Sloan)

Behind the accidental discovery,

There must be a lot of effort

The world's first evidence of dinosaur sleeping behavior was found in a fossil in which the dinosaur's head and body were coiled in a circle. When we first found this fossil, we thought it was just a particularly beautifully preserved fossil.

▲The fossil of Meilong, which dates back 140 million years to the Early Cretaceous, was discovered in Beipiao City, Liaoning Province, China, proving the sleeping behavior of dinosaurs for the first time in the world (Photo/Xu Xing/Mick Ellison)

I accidentally noticed that the dinosaur's mouth was buried under its wings. We know that different animals have different ways of sleeping. Birds need to keep warm and often hide their mouths or heads under their wings. It was this detail that made me realize that dinosaurs may have sleeping behaviors like birds. Erlianhot, Inner Mongolia is a holy place for dinosaur fossils. In the 1920s, the American Museum of Natural History formed an expedition team from Beijing, heading north and west. They hoped to find fossils of human ancestors on the Mongolian Plateau. However, they did not find human fossils during their search, but unexpectedly found various fossils, including dinosaur fossils. More than 20 years ago, Chinese researchers began to find many dinosaur fossils in this area, even including some new dinosaur species. We named two dinosaur species, one called Inner Mongolia Dragon and the other called Erlian Dragon. At that time, CCTV came to Erlianhot, Inner Mongolia, and wanted to shoot a documentary to show the process of discovering the fossils of Inner Mongolia Dragon and Erlian Dragon. After the filming was completed, the director of CCTV proposed that in order to make the documentary more vivid, it was necessary to shoot a few more shots of researchers digging fossils. When selecting the excavation site, one of the team members threw his hat into the air and dug wherever the hat landed. When we opened the rock, we were all shocked to find new dinosaur fossils under the rock where the hat landed.

▲CCTV accidentally discovered sauropod dinosaur fossils during a documentary shoot (Photo courtesy of Xu Xing)

We took the fossils back to the laboratory for repair and research, and found that the fossils were sauropod dorsal vertebrae, and they were completely different from the fossils found in the area. Based on the fossils, we named a new dinosaur species, called Sunitosaurus, which is a small sauropod dinosaur.

▲Xu Xing (left) takes a photo with the fossil of the sauropod dinosaur - Sunitosaurus (Photo courtesy of Xu Xing) Generally, the body length of sauropod dinosaurs is at least 20 meters, or even 30 meters, but Sunitosaurus is a "dwarf" in the sauropod family, with a body length of less than 10 meters. This discovery made us very excited. A Japanese TV station also came to Erenhot, hoping to report the discovery process of Sunitosaurus, and the director also made the same request to shoot a few shots. A team member suggested that a piece of bone was exposed on the edge of a dry riverbed just a few dozen meters away. Judging from the size, it is very likely to be Sunitosaurus. When we exposed more fossils through the brush, we realized that this fossil was not a fossil of Sunitosaurus at all, but a fossil of a dinosaur species that had never been seen before.

▲A Japanese TV station accidentally discovered a large theropod dinosaur, the fossil of Gigantopsaurus (Photo courtesy of Xu Xing)

We took the fossil back to the laboratory for repair and research, and found that it was a large theropod Gigantoraptor fossil. This dinosaur looks very much like an ostrich and is closely related to birds, but it was so large that we estimated it weighed about 3,000 kilograms when it died. We also used CT to take a CT scan of the inside of its bones to see what kind of creature it was. The bones of this dinosaur showed a nest-like structure under CT. Its bones are light in weight, but very strong. This discovery was named one of the top ten scientific discoveries in the world in 2007 by Time magazine.

▲Giant raptor died at the age of 11, weighing ~3250 kg (Photo courtesy of Xu Xing)

▲The skeleton of the Gigantoraptor fossil appears to be a nest-like structure under CT scan. The skeleton is light in weight but very strong (Photo courtesy of Xu Xing)

Why do some of the great discoveries in the scientific community always involve some accidental elements? In fact, before the accidental discovery, there must be a lot of hard work and dedication!

"Last day luck"

From persistence and dedication

In addition to curiosity and chance, persistence is also crucial to the discovery of dinosaur fossils. Liaoning and its surrounding areas are completely different from Inner Mongolia. The vegetation here is very dense, and the fossils in this place help us answer the question of whether dinosaurs were covered with scales or armor in the evolution of dinosaurs. Previously, dinosaur fossils found in some parts of the world showed that dinosaurs had scales and armor on their bodies, such as the duck-billed dinosaurs and ceratopsians found in North America. So we think that dinosaurs should be covered with scales and armor. But is this actually the case? In 1996, the first dinosaur fossil with preserved feathers was discovered in China. Scientists named it Sinornithosaurus, and it also has another name, Sinorhinosaurus. This naming is a process of our understanding of fossils. This fossil not only has bones preserved like other fossils, but more importantly, it also has black primitive feathers preserved on the fossil.

▲The first dinosaur with preserved feathers discovered - Sinornithosaurus fossil (Photo courtesy of Xu Xing)

In fact, the idea that dinosaurs had primitive feathers is not new. As early as the 1960s, an Argentinian scholar proposed that some dinosaurs that were closely related to birds should have feathers on their bodies. In the 1970s and 1980s, some American artists had already put feathers on some dinosaurs, but no scientist has ever found evidence that dinosaurs had feathers. In 1997, shortly after the discovery of the Sinornithosaurus fossil, we also came to this area. It was winter at the time and it was very cold, and we were very much looking forward to discovering more dinosaur fossils with feathers.

We worked for more than 30 days and still found nothing. On the last day when we were about to leave, the locals donated a very broken fossil with hundreds of pieces, hoping that it would be helpful for scientific research. We pieced together, restored and studied the fossil and found that the fossil actually represented a new species. We named it Beipiaosaurus. It is a very important discovery for our understanding of the evolution of feathers and how dinosaurs became birds.

▲The Beipiaosaurus fossil was very broken when it was discovered. This fossil provides important evidence for understanding the evolution of feathers and how dinosaurs became birds (Photo courtesy of Xu Xing)

▲Original picture of Beipiao Longfa (Photo courtesy of National Geographic)

Why does "last day luck" exist? The reason is simple. It is the result of persistence. Every "last day luck" is usually found after 30, 40, 50, or even 60 days of persistence in the wild. Only with a lot of effort can you finally make a discovery.

In 1998, we experienced another "last day of luck". That time, we were in the field for more than 70 days, and at the end of the expedition, a colleague found an important fossil with many bones and many feather impressions preserved. This fossil represents a new species of dinosaur, which we named Sinornithosaurus. This dinosaur showed us that dinosaurs could actually flap their wings like birds, which is very important for learning to fly.

▲Sinosaur fossil (Photo courtesy of Xu Xing)

▲Reconstruction of Sinosauropteryx (Photo courtesy of National Geographic)

Many scientific discoveries are

From doubts and criticisms

Many important discoveries in science are actually the most common type of discoveries, which are discoveries that come from doubts and criticisms. In the traditional sense, dinosaurs are large animals that live on the ground and have very strong running abilities. By observing various dinosaurs, scientists have discovered that the body structure of dinosaurs is suitable for living on the ground and for running on the ground. Generally, the claws on the feet of dinosaurs are relatively straight, and only the claws on the hands are relatively curved.

▲The first Microraptor fossil (Photo courtesy of Xu Xing)

However, we found two special features of the Microraptor fossils: First, Microraptor was very small, weighing no more than one kilogram; second, the claws on its feet were more similar to the claws of birds living in trees. This means that Microraptor may have been a dinosaur living in trees. This involves a very important evolutionary event. Scientists believed long ago that dinosaurs turned into birds, but for dinosaurs to turn into birds, they had to evolve various bird characteristics, one of the most important of which was flight.

▲The length ratio of Microraptor's toes and the curvature of its claws indicate that it could live in trees (Photo courtesy of Xu Xing)

▲Reconstruction of Microraptor living in a tree (Photo courtesy of Xu Xing)

How did dinosaurs fly from the ground to the blue sky? According to modern biology, creatures that have learned to fly or have just a little flying ability all live in trees. For example, lizards that can glide, flying squirrels that can glide, and even frogs can glide. These gliding animals usually fly down from trees and high places, because it is easier to fly down from a high place. It is very difficult to fly directly from the ground to the sky, just like an airplane taking off, only when the speed is very fast can it fly into the blue sky. Therefore, many experts believe that since dinosaurs live on the ground, it is impossible for them to evolve the ability to fly. However, the appearance of Microraptor tells us that some dinosaurs may live in trees. It is possible that they jumped down from trees, glided, and eventually learned the ability to fly. Through more research on Microraptor fossils, we have discovered even more surprising phenomena. Birds have broad feathers on their wings and tails, which we call flight feathers. Typical flight feathers are asymmetrical, with the left and right feathers of different sizes, one wide and the other narrow. Scientists have found that the stronger the flying ability of birds, the stronger the asymmetry of their feathers. The flight feathers of birds with weaker or no flying ability are almost always symmetrical. But in the Microraptor fossils, we actually saw asymmetrical feathers, and what surprised us even more was that asymmetrical feathers were also distributed on the feet of Microraptor.

▲Asymmetrical feathers are not only distributed on the forelimbs and tail of the Microraptor fossil, but also on the hind limbs (red circle) (Photo courtesy of Xu Xing)

The asymmetrical distribution of feathers in the Microraptor fossils led us to the idea of ​​a four-winged dinosaur: it had four wings, and if the tail is also considered a wing, it would be an animal with five wings. In fact, we did not come up with this idea directly, but rather after continuous modification.

▲Asymmetrical feathers were found on the Microraptor fossils (left), and feathers of modern birds (right). The stronger the flying ability, the more asymmetrical the feathers. From the perspective of aerodynamics, the asymmetrical structure helps flight (Photo courtesy of Xu Xing)

At that time, many teams around the world were studying Microraptor fossils. In 2002, two other research teams had published new research results on Microraptor fossils before us. Their research showed that Microraptor had no flight feathers on its feet. Another research team observed that the feathers on Microraptor were symmetrical. The observations of the two research teams were completely opposite to ours. Why do the same fossils lead to different conclusions? I attribute it to the fact that the knowledge you have already mastered will imprison your thinking. Age helps to increase experience and knowledge, but it will also lead to the loss of free thinking. The scientific knowledge you have mastered will imprison your thinking. Scientists have more or less "biases" in their observations of the world. What we can do is to minimize our own "biases". The progress of science is slowly formed in the collision with various "biases", and will eventually reach the other side of the truth.

The scientific knowledge you have acquired will restrict your thinking, and scientists' observations are more or less "biased". What we can do is to reduce our own "bias" as much as possible.

Why did the first research team think there were no flight feathers on the feet? Because our understanding of modern birds is that birds now only have asymmetrical feathers, which have never been found on the feet. They are all grown on the wings and tails. So this team speculated that the feathers buried near the foot of the Microraptor fossil must be feathers on the wings or tail, which were rushed over during the fossil formation process, rather than feathers that Microraptor originally had.

▲The first team published a paper that Microraptor had no flight feathers on its feet (Photo courtesy of Xu Xing) Why did the second team think that the flight feathers were symmetrical? Because they were mainly studying dinosaurs and had a lot of experience. They had a fixed mindset and believed that dinosaurs had not yet learned to fly. So when they saw feathers in the Microraptor fossils, even though the feathers were actually asymmetrical, they still thought they were symmetrical. The limitations of thinking led to cognitive bias.

▲The first team published a paper that Microraptor's flight feathers were symmetrical (Photo courtesy of Xu Xing) In 2003, we announced to the scientific community that Microraptor had asymmetrical feathers, and that these asymmetrical feathers grew on its feet. The British journal Nature published our paper (see the cover for fossils and restoration images), which caused a huge sensation around the world. Through this discovery, we can infer that dinosaurs began to fly to the blue sky on the ground. One type of dinosaur may have grown into trees and eventually evolved four wings. As the front wings became more and more developed, the back wings gradually degenerated, so today's birds only use two wings to fly, and their feet are used for walking.

▲In 2003, the famous academic journal Nature published a paper by Xu Xing's team on the hypothesis of the evolution of four-winged dinosaurs flying, which caused a great sensation around the world (Photo/Xu Xing) After our paper was published, it was questioned by many scholars. Kevin Padian, a famous scholar from the University of California, Berkeley, wrote several papers questioning and criticizing our paper. He believes that it is impossible to have four-winged dinosaurs. One possibility is that the fossils are fake. He also proposed that even if it is a real fossil, it has nothing to do with the origin of flight. It should be a special phenomenon and an individual case, and it has nothing to do with birds learning to fly. Why Kevin Padian does not believe in four-winged dinosaurs, because he is a pioneer in studying the transformation of dinosaurs into birds, and at the same time he will be affected by some inherent thinking. He is very familiar with the fact that birds now fly with wings and walk on their hind legs. Therefore, he cannot imagine the phenomenon of dinosaurs having four wings. In fact, doubts and criticisms are a common phenomenon in the scientific community, and scientific progress is generated in the process of such doubts, criticisms and discussions. The best way for us to respond to doubts and criticisms is to find more scientific evidence. We spent more time looking for more evidence in the wild and in the laboratory. Later it was proved that the four-winged phenomenon of Microraptor fossils was not a fossil fake, but a real biological phenomenon. More importantly, we not only found evidence of four wings in Microraptor, but also in early fossils such as Podilypteryx, Anchiornis, Xiaotingsaurus, and Avicennia. The four-winged stage was an inevitable stage for dinosaurs to evolve into birds.

▲In the following decade, researchers searched for more four-winged dinosaurs and even four-winged bird fossils (Photo courtesy of Xu Xing)

Doubts and criticisms are common phenomena in the scientific community. The best way to respond to doubts and criticisms is to find more scientific evidence. Scientific progress is made in the process of doubts and criticisms.

▲On December 12, 2014, the American magazine Science published a review article written by Xu Xing's team on the origin of birds. The article pointed out that the evolution of dinosaurs into birds must go through the four-wing stage (Photo/Xu Xing)

It is gratifying that the research results on the evolution of dinosaurs into birds have been incorporated into the textbooks of universities, colleges and primary and secondary schools at home and abroad.

▲The study of the origin of birds was selected as one of the top ten scientific breakthroughs in the world in 2014 by Science magazine (Photo by Xu Xing)

We can say with certainty that dinosaurs have never become extinct because the 11,000 species of birds on Earth today are actually living dinosaurs, direct descendants of the Mesozoic dinosaurs.

▲Dinosaurs did not become extinct, but evolved into birds and flew into the blue sky (Photo by Xu Xing)

(The content of this article comes from the Capital Science Lecture Hall. The lecture hall is hosted by the Beijing Association for Science and Technology, organized by the Beijing Science Center, and co-organized by the Beijing Science and Technology News Agency. Academicians and experts are invited to give lectures every week to promote the scientific spirit, help the public establish scientific thinking, master basic scientific methods, and understand necessary scientific knowledge)