Study shows Dinosaurs were Super Efficient Breathers
A recent study carried out by a team of scientists from the Faculty of Life Sciences at the University of Manchester (England) has added further evidence to the theory that dinosaurs and birds are closely related.
One view of dinosaurs is that many of them were active, highly mobile and busy animals rather than the very slow, cumbersome leviathans depicted by earlier scientists and illustrators. To be able to move quickly animals need efficient lungs to provide enough oxygen to muscles in their bodies. For the real heavyweights of the dinosaur world – the Sauropods and Titanosaurs, their sheer size led to their own set of anatomical problems.
The long necks on the likes of Diplodocus, and Brachiosaurus were to long for air to reach their lungs on the first breath, without super efficient lungs to process air and a large and powerful heart to pump blood round their enormous bodies, they would not have been able to function properly. In the case of a Brachiosaur, having to pump oxygen rich air from the centre of the body 40 feet up into the air to reach its head (and the tiny brain that perched on top of a fifty tonne body), would have been a momentous task without some sort of super efficient breathing system. If a sauropod did not have a very efficient heart and lungs then its blood pressure would have been very difficult to manage. If it dropped its head suddenly, the dramatic change in blood pressure could have caused the animal to lose consciousness; had they been burdened by a mammalian type set of lungs.
The good news for dinosaurs, revealed in this new study from the Manchester team, is that it looks like many of them had super efficient breathing systems, much superior to our own lungs and hearts.
Mammalian Breathing – the Downside
I am not one to put a downer onto our own mammalian anatomy, after all, if my lungs and heart did not work I certainly would not be able to finish this article. However, our breathing processes although perfectly adequate for our needs are not very efficient. In order for us to be active our blood needs to supply oxygen to our organs and muscles, and then take the waste products away. When we breathe in, air enters the two sack like objects in our chest (the lungs), through capillaries and other tiny vessels the oxygen seeps through into the blood stream and is carried away to all parts of the body by the blood cells. Waste products such as carbon dioxide is dumped back into the lungs by the blood stream and this is exhaled. A strong heart enables more oxygen to be pushed through our bodies. Although this is something of a very simplified explanation, I am sure you get the idea.
The mammalian system does have a major drawback, the lungs have only one entrance/exit point for the air. We mix up the exhaled air with new air being breathed in. We never get rid of all the used air inside us, every time we inhale we just mix fresh air with oxygen extracted air we are going to breathe out. This is not a very efficient process and as a result this contributes to the physical limitations of our bodies.
Bird, in contrast, are much more efficient breathers. Instead of just one entrance/exit point in the lungs they have openings at both ends, plus a series of air sacs in front and behind the lungs. It is these air sacs, not the lungs that inflate and deflate with each breathe. Acting like bellows they pump the air through the lungs and out a different tube than it went in. This is a one-way system with old, stale air never mixing with fresh oxygen rich air and as a result is a very effective system. For their size, birds have disproportionately large hearts, these provide the pumps to enable the whole system to function.
A Comparison of Mammalian and Avian Breathing
Picture Credit: Dorothy Sigler Norton reproduced from Bones Rock (Larson and Donnan)
The diagram above shows the air sacs and one-way breathing system in a bird compared to the less efficient lungs of a typical mammal (us).
The trouble with air sacs is that you have to make room for them inside your body. Birds allow for the air sacs by having hollow bones which can accommodate these specialised features. Hollow bones are found in the fossil record in Pterosaurs, Sauropods and the Theropods – as well as certain other groups such as Sphenodonts.
Taking the Pterosaurs first, although they are not closely related to birds, they may have evolved a similar solution for the need to get lots of oxygen into their bodies as they too were fliers. You need a super efficient oxygen transport system if you are going to do something as aerobically challenging as fly.
The Sauropods and Theropods (lizard-hipped dinosaurs – Saurischian dinosaurs) show honeycomb like structures linked by tiny passageways in their fossilised bones. These structures would have helped these animals by keeping their skeletons light to aid mobility and to assist with balance but could they also be indicators of a super efficient air-sac breathing system?
The Manchester University Team, led by Dr Jonathan Codd studied the breathing processes of modern birds and crocodiles (the closest living relatives to dinosaurs). They then compared these specimens to the fossils of small, bipedal Coelurosauria which are known to have hollow bones and the honey comb arrangement of structures. A specific group of Coelurosauria was studied – the Maniraptorians, animals such as Oviraptor, Velociraptor and Microraptor, believed by many palaeontologists to be the group of dinosaurs that actually gave rise to birds.
Dr Codd and his colleagues (including palaeontologist Dr Phil Manning) focused on tiny bones associated with ribs called uncinate processes. These tiny bones stretch between the ribs and are found in modern birds. Initially it was thought that these bones helped to strengthen the rib cage and enable birds to cope better with the physical stresses of flying. Now it is thought that they also act as levers helping to inflate the air sacs by pushing the rib cage outwards. Birds breathe with their beaks closed, air travels along the nasal cavity before filling the lungs and multiple air sacs. Air can flow in and out efficiently, thanks in part to the squeezing and pumping action of bones such as the uncinate processes.
The dinosaur species studied were found to have tiny L-shaped uncinate processes, could these bones have helped these animals expand their rib cages and sternums to enable the air sacs within their bodies to have been filled? In proportion to the birds; the dinosaurs have relatively larger uncinate processes, comparable to the scale seen on super efficient bird breathers such as penguins. As penguins dive for food they need to be able to keep a great deal of oxygenated blood inside their bodies. This new work, due to be published in the Proceedings of the Royal Society B: Biological Sciences, presents an explanation as to how groups of dinosaurs such as the Theropods and Sauropods were able cope with the problem of getting a lot of oxygen to their muscles.
So certain types of dinosaurs breathed like birds. How the Ornithischians such as the Hadrosaurs and the Thyreophorians managed is still open to debate, perhaps their extensive and highly complicated nasal passages seen in the skulls of certain genera may provide a clue.