Dinosaurs were big, some of them were monstrous. But, how is that even possible? Aren’t there natural constraints on size that prevent our own big mammals from getting to the size of buildings?
There are a few key problems that have contributed to what is known as the Dinosaur Paradox.
First, the digestion …
Sauropods swallowed enormous amounts of low-quality food that simply composted in their enormous bodies. The food wasn´t processed in the complex ways seen in ruminants or rabbits – it just went in the thin end, down to the thick middle, and took a long time to digest. As every gardener knows, the best and most efficient compost heaps are also the largest, so large gut volume combined with an active microflora and long retention times means an emphasis on size. Sauropods were gigantic walking compost heaps. (And I bet they farted like anything).
But wait, there´s more.
Indeed there is. As I mentioned in an early article on whether dinosaurs were warm blooded, the lungs that these beasts had are quite different than ours (and other mammals). Basically, they have the lungs of birds – air sac lungs. These would have aided in the expelling of the enormous amount of heat generated by the fermentation process we talked about above.
Unlike mammals, which have a simple set of lungs that pulls air in and expels carbon dioxide, birds have a complex series of air sacs, accessory to the lungs, which penetrate many parts of the body, including the bones. At least some dinosaurs are known to have similar arrangements. The apposition of air sacs to the surfaces of the gut in sauropod dinosaurs would have allowed for the transfer of terrific amounts of excess heat, dumped through to the wet surfaces of air-sac membranes and converted into water vapour. Another constraint on size, lifted.
The last reason sited for their excessive size is reproduction. Unlike the modern form of large animals like elephants and hippos, dinosaurs laid eggs. This meant that it was easy to create new ones.
Sauropods were hard to kill not just because they were big, but because replacing them was relatively easy – just lay a pile more eggs and bury them, the work of a moment, rather than incurring the energetic and temporal costs and life-historical limitations of gestation. Another constraint lifted – sauropods could grow bigger in a given environment, because making more of them was easier; they fed full-time on low-quality browse which they took time to digest (another incentive to grow larger) without having to chew it (ditto) and because of their bird-like structure, they were good at dissipating excess heat (the same) and were relatively lightly constructed (the same again, with a bag of crisps, please).
Bada-bing, bada-boom! You get big-ole dino’s roaming the earth.
What he didn’t mention was that the earth had a far thicker atmosphere during this time. In fact, during the time of the Mesozoic it is often compared to the thickness of water! Sure, it would feel different because air is not water. But, the thickness would be similar. While this could not account for all of the size issues (as even with the buoyancy of water, the shear size of these guys was still ridiculous), when combined with the above data, it helps to explain how the biggest of the biggest got so darn big.