I have a question. I wonder if anyone has a good answer. Since the Cretaceous era, it seems reasonable to believe that there will freqently have been ecological niches in which greatest size offered greatest advantage -- as one assumes from the evidence was the case in the era of the great Sauropods. At the present time, one assumes that the Blue Whale has evolved within a niche favourable to greatest size. Yet, as I understand it, the largest land animals to evolve during the entire Cenozoic era have not been greatly more massive than the present day African Bush Elephant. So my problem is this: Why has nature never pulled out all the stops (so to speak) to create a land animal of, say, 30 or 40 tons or more since the dinosaurs? Surely the size incentive must sometimes (across several continents and 65m years) have been present! Is there something about the basic structure of land mammals that puts an upper limit on size? And what was it about the Sauropods that let them defy this limit?
Endothermic animals--warm-blooded animals, the mammals and birds--require much more food than exothermic animals. (Note that biologists use "endothermic" and "exothermic" differently than chemists.) A land mammal the size of a sauropod might just not be able to get enough to eat.
Thanks, guys, I really appreciate your answers! I was beginning to wonder whether there was going to be any response at all. I agree your point, Fraggle, that warm-blooded animals require more food, though it is also true that calory intake relative to body weight falls (indeed, falls precipitously) as body weight increases. You may know that there has been a growing belief among some paleontogists that dinosaurs were warm-blooded. It has been argued: "Dinosaur bones show evidence of endothermic metabolism. Microscopic analysis has shown that the bones of some dinosaurs grew at a rate comparable to modern mammals, and have more features in common with the bones of mammals and birds than they do with the bones of modern-day reptiles." "Many dinosaur fossils have been found at high latitudes. Cold-blooded creatures are much more likely to evolve in warm regions, where they can use the environment to maintain their body temperatures. Higher latitudes entail colder temperatures, so it’s unlikely that dinosaurs were cold-blooded." "Birds are endotherms, so dinosaurs must have been too. Many biologists consider birds to be 'living dinosaurs,' and reason that modern birds’ warm-bloodedness is direct evidence of the warm-blooded metabolism of their dino ancestors." "Dinosaurs’ circulatory systems required a warm-blooded metabolism. If (say) the Brachiosaurus kept its head in a vertical position, like a giraffe, that would have put enormous demands on its heart—and only an endothermic metabolism could fuel its circulatory system." You reply makes me think I might take a closer look at this debate over whether or not dinosaurs were warm-blooded or not (or course, they didn't all have to be). This thing about the amount of oxygen in the air, spidergoat ... my understanding is the same as Michael's, that this was an explanation for the massive size of insects -- and this was way back in pre-Jurassic times. However, if anyone has an answer to the question, "Would a higher oxygen content in the air permit the evolution of larger mammals?", it would be interesting to read.
The largest dinosaurs were probably not truly warm blooded. More like a marlin that has warm blood, but only as long as it keeps swimming vigorously. If it slows down, the internal temperature drops. Dinosaurs had various mechanisms for keeping warm, such as orientation to the sun during the day, vigorous exercise, and possibly resting in sheltered and warm areas. Much of this is speculation, of course. However, there is no indication that the non feathered dinosaurs were truly warm blooded. Just had warming behaviour. And for this, great size helps. The feathered ones, on the other hand, probably were truly warm blooded.
