Some of the earliest dinosaurs did not have bones that were hollow. This suggests that the air sacs independently evolved in three different dinosaur lineages, namely sauropodomorphs, theropods, and pterosaurs.
Air Sacs in Hollow Dinosaur Bones
Live Science reports that the air sacs inside the biggest dinosaur and pterosaur bones were so remarkably beneficial that they may have independently evolved at least three times in various lineages. This was according to the study published in Scientific Reports.
Phys notes that these air sacs made the dinosaur's skeleton less dense and lighter. With bones that are less dense and have more air, the pterosaurs and dinosaurs had more oxygen within their blood. They also had better agility for hunting, fleeing, fighting, or even flying. The creatures also used up lesser amounts of energy and were more efficient in cooling their body.
Common Ancestor or Convergent Evolution?
Researchers knew that these creatures had air sacs within their bones, but earlier evidence suggests that these bubbles may have evolved later through a process known as convergent evolution, which is a phenomenon wherein various organisms evolve similar traits on their own.
Meat-consuming theropods, long-necked sauropods, and pterosaurs are all part of the ancient reptile group known as avemetatarsalians. This group is more closely linked to birds than crocodiles today. All three lineages had air sacs evolve in their bones, which enabled their skeletons to be nimble and light. Without such structures, the beasts could not have become so big or stayed cool in the warm climates.
As part of the study, the researchers looked into the 233 million-year-old fossil remains of three old dinosaur species, which were among the earliest good material from meat-consuming herrerasaurid and sauropod dinosaurs, according to paleobiologist Paul Barrett from the Natural History Museum in London.
According to Barrett, it revealed that these intricate air sac systems started becoming invasive in the latter part of their evolutionary history compared to the other reptile groups that developed such systems.
The researchers conducted micro-CT scans of the fossils, which were uncovered from 2011 to 2019 in Rio Grande do Sul. They found small spaces within the vertebrae that may have had tons of bone marrow and blood vessels but no air sacs.
This study could help determine if the air sacs in theropods, pterosaurs, and sauropods share a common evolutionary ancestor or whether these systems resulted from convergent evolution.
However, Barett notes that the study suggests that the three dinosaur groups may have developed their air sac systems independently. This is because, when the researchers examined earlier members of at least one of the groups, there was no evidence of air sacs.
A study from 2021 also suggested that bird-hipped dinosaurs called ornithischians lacked air sacs as well, even if they shared ancestry with dinosaurs that had air sacs.
Barrett notes, however, that the evolutionary story of air sacs is still filled with uncertainty. Not every air sac can be found in the bone. Some air sacs are present in the muscles or around the organs. There is a possibility that these creatures share ancestry but did not leave any bone traces.
Live Science reports that the fact that the researchers were unable to find air sacs in the earliest avematatarsalians does not rule out the possibility that these creatures could have had one.
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