Since the 90s of the last century, paleontologists have been divided into two camps. Most of them believe that the theory of the origin of birds from dinosaurs is indisputable, while the smaller part doubts it more and more and provides new evidence to the contrary. The struggle has not subsided for almost 20 years. Two new discoveries once again put scientists on opposite sides of the barricade.
The point of view recognized by official science is that birds evolved from bipedal theropods about 150 million years ago.
Indeed, dinosaurs and birds have many similarities. For example, among the ancient fossils there are bird-like dynos with feathers and even four wings, with avian behavior, and so on.
The main argument of the supporters of the theory is precisely the similarities (skeletal, morphological and soft tissues) - the constantly found "transitional links", as well as the authority that the theory automatically acquired over the years.
A fossil of one of these "transitional links" was recently discovered in the Junggar Basin in northwestern China. The study was conducted by a group of paleontologists led by James Clark of George Washington University and Xing Xu of the Chinese Institute of Vertebrate Paleontology and Paleoanthropology.
The long-necked dinosaur, somewhat reminiscent of an ostrich, was named Limusaurus inextricabilis, which means something like the following: "swamp lizard that could not escape." It is from 156 to 161 million years old (at least that is the age of the deposits in which the fossil remains were found), the body length of the animal did not exceed 1.7 meters.
This dinosaur is a truly unique find, according to many scientists, as it shows how theropods' fingers could have evolved into wings.
The lizard belongs to the Jurassic ceratosaurs (early theropods). The largest representative of this genus is Ceratosaurus nasicornis, it reached a length of 8 meters, and also had a characteristic nasal horn (L inextricabilis does not have it). Many ceratosaurs also had short "arms" and knobby fingers without sharp claws.
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L. inextricabilis, a distant relative of the beloved Tyrannosaurus rex, was a rather unusual creature: it had no teeth (perhaps it had a beak instead), and in general it ate exclusively plant foods. Perhaps his torso was also covered with primitive feathers, but there is no reliable evidence of this fact. By the way, it is also the only ceratosaurus found in East Asia.
However, the most interesting thing about him is not this at all, but his “fingers”.
Let us explain: in the "hands" of theropods, as in the wings of birds, there are three fingers each, which they got from some common five-toed ancestor.
Once upon a time, the fifth finger disappeared (if we count from the analogue of our thumb). This was the next step in the development of the brush.
Further, of the remaining four, dinosaurs "took" from the ancestor, relatively speaking, the first, second and third fingers (this happened around the time of the Dilophosaurs). At the same time, in birds, the wings consist of the second, third and fourth fingers of the same four-toed ancestor (scientists judge by their shape). How could this have happened?
This strange regularity haunted paleontologists for a long time, who could not find a decent explanation for it. After all (if one adheres to the opinion that birds evolved from dinosaurs), it turns out that the birds somehow had to lose their first finger and re-grow themselves a fourth (then the set 2-3-4 will turn out).
And now L inextricabilis enters the scene, which has four fingers (1-4), but the first of them is greatly reduced, while the second, on the contrary, is unusually enlarged due to the metacarpal bone. This bone is very similar to that found in the first toe of the early five-toed ancestors.
These two facts indicate that earlier paleontologists around the world misinterpreted what they saw and took 2-3-4 fingers of lizards for 1-2-3 (that is, the second was mistakenly considered the first, and so on), scientists write in an article published in the journal Nature.
In this case, everything becomes much simpler: early theropods lost the first and fifth fingers and passed the set of the second, third and fourth to the birds.
“We've probably found some kind of transitional form that, among other things, fits within a certain time frame,” says Clark.
He believes that the find will not end the controversy around the origin of the fingers-wings, but it will still help to understand this issue.
However, for some scientists, what is is enough. "This is the best evidence we could get," says Dr. Jack Conrad of the American Museum of Natural History.
There are those who do not find the new findings valuable, among them Gunter Wagner from Yale. He completely believes that the observed 2-3-4 fingers of the bird are not such, and gives the following data.
During the development of the embryo, the shape of the fingers of birds is determined by many factors (tissue, location, influence of genes). If the tissue is ready to become the second finger, and the genes bombarding it require the first to grow, then it will be so (the first will grow in the place of the second). The experiment showed that this happens in modern birds all the time.
"The fossil of a ceratosaurus may be one of the transitional links, but whether its second, third and fourth fingers are such, it is impossible to say with complete certainty," adds Wagner. (After all, modern scientists are not able to trace the development of dinosaur fingers from an embryo.)
There is one more doubt. It is likely that the appearance of the unusual four-fingered brush L. inextricabilis was due to its lifestyle and was characteristic only of this species.
After all, it is not known why L inextricabilis needed such "hands". T. Rex grabbed prey with its forepaws, but L inextricabilis was a herbivore. Clar, Xu and their colleagues have not yet decided on this issue.
“From their morphology, they were not meant to be grasped like in other theropods. Perhaps he needed them to get upright when he was lying down,”says Xu.
Summing up a preliminary conclusion, we can say that not everything is as simple and clear as we would like even in this particular study. What can we say about the whole theory of the evolution of birds!
The arguments of opponents of the theory of the origin of birds from dinosaurs are so far isolated, but from this no less weighty. Some scientists argue that dinosaur feathers are not feathers at all, others that they could appear and disappear, and others that the differences in the anatomical structure of dino and birds are so strong that they cannot be considered one branch of evolution.
The latter includes a group of researchers from the University of Oregon (Oregon State University).
Scientists say that modern birds cannot be direct descendants of carnivorous dinosaurs, because, unlike almost all land animals, the femur of birds (as well as ribs and sternum) is fixed in place.
In their article published in the Journal of Morphology, the scientists write that this difference is key.
Paleontologists practically do not operate with new data, arguing mainly over the existing ones. It has long been known that the hip of birds cannot move, forcing the birds to run at the expense of their knees. Recently, it was also discovered that this position of the bones, as well as the muscles surrounding them and the lungs, saves birds from collapsing the main organ of the respiratory system and air sacs.
Warm-blooded birds need about 20 times more oxygen than cold-blooded reptiles. That is why their lungs were transformed (the respiratory system of birds is considered the most complex among all vertebrates), and gas exchange began to occur more efficiently.
“This is one of the fundamental components of bird physiology. This position of the femur and muscles determines the work of the respiratory system and the volume of their lungs, and therefore the ability to fly,”- said in a press release from the university, one of the study's authors, Devon Quick.
At the same time, other animals (be they people, elephants, dogs, lizards, as well as ancient dinosaurs) have a movable hip, which is actively involved in their movement on the ground.
And if so, then the birds could not have descended from theropods, according to American paleontologists.
Velociraptor couldn't at some point just spread its feathers and fly away into the sunset, they add.
“It's even strange that over hundreds of years of studying the biology of birds and their flight, we have understood so little about them. This discovery means that birds evolved in parallel with the "dire lizards." Perhaps this process began even before the appearance of the first species of dinosaurs,”says the second author of this study, professor of zoology John Ruben.
In this case, Ruben means the theory that the most ancient ancestor of birds is not Archeopteryx, but protoavis, which lived on the planet about 225-210 million years ago, that is, at least in parallel with dinosaurs.
“Scientists often point to the similarity of the lungs of birds and dinosaurs, but because of the moving thighbone, the abdominal air sac of the lizards, if such really existed, should have collapsed,” says Ruben.
(By the way, non-dinosaur pterosaurs also had a similar respiratory system.)
As for the rest of the similarities between dinosaurs and birds, Ruben and Quick explain them as follows: both branches of evolution had a common ancestor. For example, it could be a thecodon. Birds, dinosaurs, and crocodiles could have emerged from this group. By the way, in terms of lung structure and physiology, crocodiles are much more similar to ancient dinosaurs, paleontologists from Oregon note.
Ruben also says that despite all his efforts (the University of Oregon has challenged the dominant theory for the first time), he does not hope that scientists around the world will be able to convince them overnight.
First, it is not so easy to destroy a theory that is so firmly entrenched in the minds of the majority. Second, dinosaurs are too loved by humans (it's hard to find a more romanticized ancient animal). And thirdly, a significant role, according to John, is played by the policy of museums, for which a change in the provisions of the basic theory entails serious consequences (up to the dismissal of some of the staff).
“However, every year there is more and more evidence in favor of our opinion, and we hope that soon the whole world will know - not all scientists agree with the established point of view,” the professor sums up.
Interesting Mechanics # 8 2009
