One of the things that makes dinosaurs so appealing is their sheer size: plant eaters like Diplodocus and Brachiosaurus weighed well over 50 tons, and a well-toned T. Rex tipped the scales at 7 or 8 tons. From the fossil evidence, it's clear that dinosaurs were more massive--species by species, individual by individual--than any other group of animals that ever lived, including modern mammals.
However, what's fun for dinosaur enthusiasts can cause paleontologists and evolutionary biologists to tear their hair out. The giant size of dinosaurs demands an explanation, and one that's compatible with other theories--for example, it's impossible to discuss dino gigantism without paying close attention to the whole cold-blooded/warm-blooded debate.
So what's the current state of thinking on plus-sized dinos? Here are a few more-or-less interrelated theories.
Theory #1: Dino size was fueled by vegetation.
During the Mesozoic Era--which stretched from the beginning of the Triassic Era, 250 million years ago, to the extinction of the dinosaurs at the end of the Cretaceous Era, 65 million years ago--atmospheric levels of carbon dioxide were much higher than they are today. If you've been following the global warming debate, you'll know that increased carbon dioxide is directly correlated with temperature--meaning the global climate was much warmer millions of years ago than it is today.
This combination of high levels of carbon dioxide (which plants recycle as food) and high temperatures meant that the prehistoric world was matted with all kinds of vegetation--plants, trees, mosses, etc. Like kids at an all-day dessert buffet, dinosaurs may have evolved to giant sizes simply because there was a surplus of nourishment. This would also explain why the predatory dinosaurs got so big; a 50-pound carnivore wouldn't have had much of a chance against a ten-ton plant eater.
Theory #2: Hugeness in dinosaurs was a form of self-defense.
If Theory #1 strikes you as a bit simplistic, your instincts are correct: the mere availability of huge amounts of vegetation doesn't entail the evolution of giant creatures who can swallow it down to the last shoot. Evolution works along multiple paths, and the drawbacks of gigantism (such as slow speed and limited population size) can easily outweigh its benefits in terms of food-gathering.
That said, some paleontologists think gigantism conferred an evolutionary advantage on the dinosaurs who had it: specifically, a jumbo-sized herbivore would have been virtually immune to attacks by predators. (This theory also lends some credence to the idea that T. Rex scavenged for its food--say, by happening on the body of an Apatosaurus that died of disease or old age--rather than actively hunting it down.)
Theory #3: Dino gigantism was a byproduct of cold-bloodedness.
This is where things get a bit sticky. Many paleontologists who study giant herbivores believe that these dinosaurs were cold-blooded, for two compelling reasons: first, based on our current models of metabolism, a warm-blooded Diplodocus would have cooked itself from the inside, like a potato, and promptly expired; and second, no land-dwelling, warm-blooded mammals living today even approach the size of the large, herbivorous dinos (elephants top out at a couple of tons, max).
Here's where the gigantism comes in. If it evolved to a large-enough size, scientists believe, a cold-blooded plant-eater could have achieved "homeothermy"--that is, the ability to maintain its own temperature. This is because a house-sized, cold-blooded creature would warm up (in the sun) and cool down (at night) very slowly, giving it a fairly constant average temperature.
The problem is, these speculations about cold-blooded herbivores run counter to the current vogue for warm-blooded dinosaurs. Although it's not impossible that a warm-blooded T. Rex could have coexisted alongside a cold-blooded Brachiosaurus, evolutionary biologists would be much happier if all dinosaurs had uniform metabolisms--even if these were "intermediate" metabolisms that haven't yet been modeled.
