BIRDS EVOLVED from carnivorous theropod dinosaurs. This is no longer a matter of debate, as the famous ‘feathered dinosaurs’, and other fossils, clearly show that birds and dinosaurs share a number of features in common that could only be the result of a direct evolutionary relationship.
The understanding that birds evolved from dinosaurs is probably the single greatest fact ever discovered by dinosaur palaeontologists. And not only is it a cool factoid about the history of life, but it also raises several intriguing questions about dinosaurs.
For example, did some dinosaurs behave like birds, grow like birds, move like birds, breathe like birds?
Let's look a little deeper at that last question. Living birds have a very unusual system of respiration, which sets them apart from all other animals: their lungs are unidirectional. This means that air can travel in only one direction across the gas exchange tissues of the lung.
This is different from the way we breathe, with air travelling back and forth across the lungs. This means that air with oxygen passes through our lungs only when we inhale. When we exhale, air with carbon dioxide passes through the lungs. So during an exhale, our lungs take in no oxygen.
In birds, the unidirectional nature of the lung means that air with oxygen passes through the lungs during inhalation and exhalation. This is very difficult to imagine visually, I know. It is a true marvel of engineering.
Birds can do this because they have a system of air sacs that connect to the lungs, which store air during the entire breathing cycle. During inhalation, oxygen-rich air passes through the lungs directly from the trachea, but during exhalation other oxygen-rich air that had been stored in air sacs passes across the lungs.
Why do birds have such a strange lung? Their lung is remarkably efficient at getting lots of oxygen into the blood. This is important because birds fly, and flight is an activity that requires a lot of energy and an elevated metabolism, both of which require a large quantity of oxygen.
So did dinosaurs have this type of lung? Some dinosaurs definitely did. We know that because the air sacs often leave characteristic traces on the bones, which fossilise.
In many birds and dinosaurs the air sacs are actually inside the bones themselves, hollowing out the centre. We can see these internal sinuses, as well as the circular or oval-shaped openings that lead into them, in many bones of theropod and sauropod dinosaurs. This is clear evidence that they had a bird-like lung.
We are not so sure about ornithischians, such as the horned dinosaurs and duck-billed dinosaurs. Their bones do not have the hollow centres and circular or ovoid ‘foramina’ that are so often seen in the bones of theropods and sauropods. This may mean that ornithischians breathed very differently compared to other dinosaurs.
This is an important question about dinosaur biology that is currently the subject of much research by palaeontologists, so keep your eyes peeled!
- Steve Brusatte, resident palaeontologist