At the end of the Cretaceous, a bizarre dinosaur roamed the shores of the Argentinean Rio Colorado. The 10-meter predator had “pneumatic” bones, and air sacs, which now only birds have, helped to breathe. Apparently, nature continued experiments with the structure of bones and the respiratory system on the branches of evolution, parallel to the avian
Light and Air Sacks of Arosteon // Todd Marshall / Project Exploration
Fossilized dinosaur bones in our time no longer surprise anyone, and the storerooms and collections of most museums can allow paleontological studies to be carried out without leaving home. Now scientists have focused on the analysis of only the most unique samples, and of particular interest are either giant predators, always causing genuine fear and attraction to the public, or transitional and intermediate forms, a detailed analysis of the remains of which is the best way to earn the respect of specialists.
Paul Sereno and his South and North American colleagues decided to kill two birds with one stone, and the dinosaur, which became the object for their study, died altogether in the Late Cretaceous period.
The Aerosteon riocoloradensis described by them, that is, the Rio-Colorado Arosteon, had air sacs - a unique feature that is characteristic only of birds in our time.
Thanks to cross-finned fishes, all terrestrial vertebrates have lungs - a spongy tissue saturated with blood vessels, the main role of which is an intensive exchange of gases, primarily oxygen and carbon dioxide.
Naturally, the needs of the body and the intensity of gas exchange directly depend on the rate of metabolism, hence the structure of this very lung. For example, toads and frogs, who only occasionally decide on short swims, are content with a simple bag with sparse partitions, and the surface area of the lung in an adult due to the alveoli is 35 times greater than the surface area of the entire body. Largely thanks to these 70 square meters, we can afford marathon distances and intense brain activity.
Air bags
These are cavities connected to the respiratory tract or to the esophagus, capable of filling with air, but not performing the function of gas exchange in most vertebrates. In tailless amphibians, these are paired or unpaired outgrowths in the posterior part of the oral cavity, called vocal sacs. Reptiles (some turtles, lizards) have blind outgrowths of the lungs.
Birds have 5 pairs of bags: abdominal, extending from the main bronchi and located between the organs of the abdominal cavity; the remaining 4 pairs are extrapulmonary extensions of the secondary bronchi: cervical, lying along the esophagus; clavicular, often merging into one interclavicular; front chest - on the abdominal side of the chest; hind pectorals - on its dorsal side.
The main function of these bags in birds is to suck in air through the lungs, especially during flight, as well as thermoregulation of the body and change the specific gravity of birds during swimming and diving. Many bones of the skeleton of birds (thigh, shoulder, sternum, etc.) have cavities filled with outgrowths of sacs. Birds also have sacs that are not associated with the bronchi; the outgrowths of these pharyngeal-nasal sacs in some birds penetrate into the bones of the skull, under the skin and into the forelimbs.
Mammals also have several types of air sacs: 1) arising as paired outgrowths of the mucous membrane of the Eustachian tubes (in horses, donkeys, zebras) and located on the neck, in the Atlantean region; 2) paired and unpaired formations that arise in the larynx and serve to amplify sound (vocal sacs); 3) extending from the posterior widened end of the trachea (in male striped seals) or from the esophagus (in male walruses), serves to change the specific weight of the body; 4) in the sperm whale, a blind bag with a volume of up to 1 m 3 that opens into the blowhole, where it draws air before diving.
But the birds' lungs were not enough, so they got air bags - special containers located behind the lungs and allowing them to breathe not only on inhalation, but also on exhalation.
And unlike all other terrestrial animals, the lung tissue of birds itself does not "breathe" - in the sense that it does not expand or contract: the difference in pressure required to fill and release the air is taken over by air sacs. They fill up with the lungs during inhalation, and when the bird exhales, "waste" air comes out of the lungs and is replaced by "clean" air from the bags. In addition, the latter are very useful for diving, and the corresponding cavities next to the organs allow especially zealous birds to avoid overheating.
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The ten-meter predator, whose remains were discovered back in 1996 on the shores of the Argentine Rio Colorado, had a similar "outfit".
Perhaps it was the equipment that allowed the isolated South American population to survive until the Late Cretaceous, while its closest relative, the Allosaurus, in North America, then separated by the ocean, suffered under the onslaught of the Tyrannosaurus.
The aerostéon described in PLos ONE did not suffer from scientists: in order to find air sacs, they did not even have to saw the bones, the work was done for them by a CT scanner.
As in birds, the skeleton of aerostheon, to match the name, turned out to be "pneumatized". According to Sereno, the cavities contained in the bones confirm the existence of air sacs. And although the arosteon did not possess any key features of the bird's skeleton, like a keel or a special rib cage, it had a "fork" that those who like to make a wish love to break, and its lungs worked exactly like birds.
It is difficult to imagine that this predator, larger than an elephant, had very light and airy bones.
Birds as a class arose earlier than this specimen died on the shores of the Rio Colorado, but scientists are sure that their find confirms the long preliminary preparation of evolution for flight.
In addition, these elongated cavities within the body, in addition to breathing, were very effective in redistributing body weight in bipedal runners and in dissipating heat from powerful muscles. And, although it is difficult to judge the fine structure of the structure from the remains of 85 million years old, Sereno found bags under the skin of his ward, so that the victim could boldly surrender at the mere sight of a swollen and even slightly pulsating formidable predator.