The Origin of Bird-Syle Breathing.

` Another ephemeral Nature article has spawned a dinosaur entry! The article itself is: Dinosaurs Breathed Like Birds by Tom Simonite, which announces the discovery that extinct theropod dinosaurs had turbo-charged lungs like the pigeons, crows and seagulls carrying on outside my window right now.

` First thing's first: a cursory education on the way birds breathe.

` Stop groaning!

` For one thing, we, as mammals, take in air by expanding the interior size of our lungs. The inhaled air inflates little balloon-like structures where the oxygen is extracted from the air and carbon dioxide dumped. Then your muscles force the waste air out.
` When you think about this, it's really not that efficient - you breathe in fresh air and then expel air only slightly less fresh than what you breathed in. (Most of it doesn't get used!) Kind of like the digestive systems of primitive animals which only have one opening for the taking-in of food and the expelling of waste.

' Be glad we don't eat the way we breathe!

` Birds' breathing is somewhat more complicated: As you may know, a lot of the bones of birds are packed with air-filled compartments.
` These are actually connected to the lungs and trachea, and as you also may know, there are other air sacs outside of the bones. Together, these create one set of air sacs in front of the lungs and another behind the lungs - it is because of these that birds need to take two breaths to move one inhallation of air through their respiratory system.

` How is that more efficient, you ask?

` Well, if you could see a bird's lungs at work, you would see that the air goes round and round rather than in and out: The rear (posterior) sacs inflate first, then the air from those is pumped into the lung itself and into the front (anterior) sacs. Why? As the front air sacs are being emptied, air that was already in the rear air sacs is again pumped through the lungs, creating a continuous flow of fresh air.
` In other words, no spent air can stagnate, as with our 'dead-end' lungs. Also, this means that the air only flows in one direction through a bird's lungs (similar to the way food only moves one way through the intestines) and it is processed by thin tubes conatining masses of blood vessels that release CO2 into the airstream while catching oxygen as it goes by.

` Apparently, they're finding that carnivorous dinosaurs may have had such a flow-through pulmonary system. This type of a discovery tells us that they must have had a very high metabolism!
` This article I speak of is mainly about a reanalysis of the 67 million-year old Majungatholus atopus, a splendidly ugly theropod dinosaur. It wasn't a maniraptor (or even a coelurosaur!) like Tyrannosaurus and Velociraptor were.
` Majungatholus seems to have been much more closely related to Carnotaurus - a very scary-looking beast that, as I recall, was the fearsome predator in the annoyingly innacurate little Disney movie where an Iguanodon is adopted by lemurs.
` Um...
` Anyway, since Majungatholus is not even that closely related to birds, it probably means that most, if not all, theropods had such air sacs. What's the last common ancestor between today's birds and this type of dinosaur? I'm not sure, but it probably lived in the Late Triassic Period, when dinosaurs were new.

` Here's what the article mainly has to say:

Birds have fast metabolic rates thanks to their efficient way of extracting oxygen from the air. They have two lungs, as mammals do, but the airflow through them is controlled by a complex system of air sacs throughout the body. Most birds have nine such sacs, which also extend through their hollow bones.

Patrick O'Connor, of Ohio University in Athens, and Leon Claessens, of Harvard University in Cambridge, Massachusetts, compared the structure of air sacs in M. atopus's vertebrae to those in more than 200 living birds. The structures were very similar, they report in this week's Nature.

"This study forms part of an increasingly robust story that says birds are essentially dinosaurs, but smaller," says Paul Barrett, a palaeontologist at the Natural History Museum in London. "Using functional work in live animals is a nice addition, and perhaps now you could go as far as saying dinosaurs had a bird-like metabolism."

The study shows that the efficient breathing system of birds is older than previously thought, but Barrett thinks there is more to come: "To me it seems that a breathing system like this is of more ancient origin, from nearer the base of the dinosaur family tree." He says that finding older dinosaur fossils would support this, and perhaps show that other bird-like characteristics are older than suspected.

` Isn't that just dandy? So yes, the origins of air sacs may go all the way back to the Triassic Period, before birds even evolved!
` That's pretty neat.