This topic - the subject of a symposium yesterday at ICVM9, might remind those of more cynical and jadied mien than I of the famous Monty Python sketch featuring Anne Elk (Miss) and her Theory of the Brontosaurus, which you´ll recall was thin at one end, thick in the middle, and thin at the other end. Things have moved on since then, and we know a great deal more than we did in Miss Elk´s time of the biology of large dinosaurs. The histology of the bones of sauropods is now known in such detail that we can get a good idea of how these creatures grew (very rapidly), and the realization of the close relationships between dinosaurs and birds has allowed us to get a refreshingly different view of dinosaurs as gigantic and birdlike, rather than gigantic mammals such as elephants, only made more ... well, gigantic.
Much of the work in this area has been done by Prof. M. S. of Bonn and his group, who made a strong showing here in Uruguay, and a very recent paper by Prof. M. S. and his colleagues (available free here) summarizes much of the proceedings. Basically, the researchers looked at all the physiological needs and organ systems of large dinosaurs (feeding, locomotion, respiration and so on), linking them all together in a network whose almost inevitable result was that large size - for some animals - is terrifically advantageous.
It all starts with the mouth. Anne Elk was quite right in her breakthrough realization that brontosauruses are thin at one end - for sauropod dinosaurs did indeed have very tiny heads, at the end of very long necks. The teeth were small and weak, so the dinosaurs didn´t do any chewing. Browse was nipped off in tiny pieces and swallowed immediately. Having a long neck is an advantage for an animal that wants to stay more or less in one place and crop from a wide area - but for this to happen, the head must be small and light, to minimize load. Certainly, the skulls and vertebrae of sauropods are full of air sacs, and are very much less dense than any bones we know from large mammals, say.
Why didn´t sauropods chew their food? There´s the mechanical constraint on small head size just mentioned, but there´s another disadvantage - chewing takes an immense amount of energy, and also time. Time that might be spent cramming yet more food into one´s gob. Some animals replace mastication in the mouth with a crop in which food is stored and macerated by stones, and one might imagine that this would be true of sauropods. Much to my surprise I learned that the evidence for gizzard stones in large sauropods is extremely scanty and ambiguous. Instead, 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.
What happened to all that heat? Wouldn´t having an enormous gut, deep inside a large, voluminous creature, impose limits on size, or at least, shape? But small sauropods had much the same shape as larger ones, so there seems to have been no constraint on size imposed by internal heat generation. Why? The key is to think of sauropods not as large mammals but as large birds. 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.
Something we don´t often factor in to the evolution of size is the mode of reproduction of the creatures concerned. It´s a well known fact that larger animals are rarer than smaller ones. As a rule, large animals do not shed their gametes into the
But is this universally true? It´s true enough for the large animals we know about nowadays, such as elephants and rhinos. Such large animals reproduce rarely, and are thus more vulnerable to extinction because of it. However, just because all the large animals we know about go about their business like that doesn´t mean that it´s axiomatic. Consider: the same can´t have been true for sauropods - because they laid eggs. 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).
That, my friends, is why sauropods grew so huge - not for any one reason, but an interconnected interplay of many things connected with every part of their life and structure, from their phylogenetic heritage to the kind of food they ate, the way they collected and digested it, how they respired and handled heat, and how they reproduced, all of which taken together put the accent on large size. Now, if elephants laid eggs...