Ever hear of Extant Phylogenetic Bracketing?
I recently had the pleasure of listening to a Ted Talk with evolutionary biologist Dr. Emma Schachner and the compelling new theory of why Dinosaurs were able to become the dominant species on the planet in the late Triassic.
Dr. Schachner explains that the most popular theory has to do with the early dinosaurs lungs and how they breathe. Both ancient and modern mammals have a set of lungs that expand when taking in oxygen and compress when expelling it. Further, the lungs are pressed up against the spine and if you inhale deep enough you can literally feel your lungs pressing up against your rib-cage and your lower back. Because of this your lungs are only able to take in a certain amount of air at a time.
The lungs of a sauropods and theoapods are much different. Through Extant Phylogenetic bracketing paleontologists are able to determine that these animals had a rib-cage shaped like modern day birds as well as air sacks and a thin membrane around the lungs called the blood-gas barrier.
In birds the lungs do not expand or contact. They are solid with the texture of a dense sponge and is held in place by the spine and rib-cage. Meaning these lungs are not moving at all. The lungs are unidirectionally ventilated by air-sacks positioned on opposite ends of the lungs. This entire system is protected and held in place by a series of forked ribs.
Now the blood-gas barrier in birds is much much thinner than a mammal. Oxygen is more easily diffused across thin membranes than thick ones. Also, a thin membrane is one way of enhancing respiration in a low oxygen environment.
The Triassic environment was extremely harsh. No grass, high temperatures and a low oxygen level, as low as 15% compared to today's 21%. Quite the opposite of the Carboniferous period where the oxygen levels were so high that insects were able to grow to enormous sizes.
This study shows that dinosaurs were able to survive and thrive in low oxygen conditions because they developed a respiratory system with a thin blood-gas barrier and air sack enabling them to take as much oxygen out of the air as needed. Thus propelling them to the top of the food chain for over 150 million years.
to hear the entire talk go to this link. It's quite interesting.