The mystery of the origin of birds has been worrying paleontologists for a very long time. And the more various fossils of extinct ancient birds and feathered dinosaurs are found, the more confusing the history of the appearance of the birds we are used to
One after another, various signs, seemingly purely avian, are found in bipedal dinosaurs - theropods. These are feathers, and beaks, and various features of the skeleton and even incubation of eggs. Not surprisingly, with the great similarity between birds and small two-legged dinosaurs, it was they, theropods, who began to be considered avian ancestors. Now this hypothesis has become dominant in the scientific world, in many popular science publications and films you can even come across a phrase like "if you want to see a living dinosaur - look at the pigeon outside your window - here it is!" Of course, this is an exaggeration, even if birds evolved from dinosaurs, they are definitely not dinosaurs anymore. But did birds actually descend from them?
Paleontologists Devon Quick and John Ruben of Oregon State University answer this question in the negative. They compared the respiratory systems of modern birds and the extinct theropod dinosaurs and concluded that these systems are completely different. The results of their research are published in the Journal of Morphology.
Birds, which require a lot of energy to fly, have a markedly different respiratory system from that of mammals. It is much more powerful and effective - after all, the lungs of birds do not work according to the principle of "inhaled, received oxygen - exhaled exhaust air", as we do. They have air going through the lungs in an almost constant flow. This is achieved thanks to special outgrowths - air sacs, front and back. The back, abdominal sac, serves as a reservoir into which air is injected when inhaling - when exhaling, this air goes through the lungs and there it gives up the oxygen contained in it.
It seems that the currently unique respiratory system with air sacs, in the past, in the Mesozoic, was much more widespread in the animal kingdom. "Ammonit.ru" wrote that traces of air sacs were found on the skeletons of pterosaurs and even large carnosaurs. The fact is that air sacs partially penetrate into the cavities of large bones and their traces can be recognized on the skeleton.
But how exactly these bags were located in the body cavities of extinct animals, paleontologists do not yet know. On the other hand, it is well known that in modern birds the posterior air sac, which occupies a rather large volume in the bird's body, is supported by the femur.
It was this anatomical feature that the researchers paid attention to. In general, a short femur, inactive and located in a position close to horizontal, is a feature of all modern birds, including flightless ones. The thigh bones limit the expansion of the bag on inspiration.
But if in birds the hip is practically not involved in walking, then how do they move along the ground? If you look at any bird, for example, an ordinary crow or a sparrow, you will notice that their knees in their paws are turned backward, and not forward, as in ours. In fact, what we see is not the knees, but the ankle joint, it is just that birds have an additional bone - the tarsus, which is not in the legs of mammals. It is formed by the fusion of the bones that we have in the foot. It is this anatomical feature that allows the birds to create an additional joint and "free" the thigh.
One of the study's authors, John Ruben, says that all terrestrial vertebrates, including dinosaurs, were involved in locomotion, hence, even with air sacs, their respiratory system was different from that of birds. Hence, dinosaurs could not have been the ancestors of birds. In addition, Ruben recalls, real birds existed even in the early Cretaceous, and possibly in the Jurassic, when bird-like bipedal dinosaurs just appeared.
But if birds did not come from dinosaurs, then from whom, then? Probably, the authors of the study say, from some early archosaurs (which were previously called thecodonts) back in the Triassic. In this case, the similarity of birds to dinosaurs is also understandable - after all, both dinosaurs and pterosaurs also descended from archosaurs, and they could inherit feathers and some other anatomical features from a common ancestor.
Will this study put an end to the debate about the origin of birds? Of course not. Long before him, many paleontologists, including the famous Russian expert on the origin of birds E. N. Kurochkin of the RAS Paleontological Institute did not share the "dinosaur" theory. But the complexity of this issue is not only in the fact that from ancient birds and dinosaurs only bones, eggs and body prints have reached us, and all this is not always in good condition, but also in the fact that in the Jurassic and Cretaceous periods, a variety of flying and birds there were significantly more creatures than now. There were real fan-tailed birds, there were extinct birds of Enanciornis - also birds, but clearly descended from other ancestors, there were Confuciusornis - another branch of birds, there were gliding dinosaurs - microraptors, and just running feathered dinosaurs. And figure it all outwho descended from whom, who descended from common ancestors, and who had similar traits as a result of convergence - development in similar conditions - it is very difficult to understand this. But paleontology does not stand still, so new interesting discoveries in this area are likely to await us.