For the generation raised on Jurassic Park and perhaps for posterity, Tyrannosaurus rex will endure as a household name. It is the dinosaur people most love to loathe, a gargantuan, dagger-toothed monster every bit as fearsome as the fire-breathing dragons of fairy tales. Less widely appreciated is that the tyrant lizard king had modest roots. Indeed, before T. rex hit the scene, tyrannosaurs were relatively petite. Weighing one to two metric tons and standing several meters tall, these were not animals to be met in a dark alley or kept as pets. But in fact, T. rex broke the tyrannosaur mold, nearly tripling in body mass over its predecessors.

Evolutionary biologists have long pondered the factors that might have led to gigantism among theropods, the bipedal and mostly carnivorous dinosaurs. (Besides T. rex, four species in two other groups--the carcharodontosaurs and the spinosaurs--managed to evolve similarly supersize proportions.) In recent years, a few prime-mover hypotheses have emerged, attributing the growth spurt to such things as increased levels of atmospheric carbon dioxide (leading to elevated plant productivity, which in turn could support more of the herbivorous dinosaurs that theropods preyed on). At the annual meeting of the Society of Vertebrate Paleontology in St. Paul, Minn., last October, Scott Sampson of the University of Utah and his colleagues outlined a more nuanced model, one that takes multiple influences into account.

Piggybacking on studies of living carnivores, the team identified some likely prerequisites that permitted the rise of gigantic theropodsthose that tipped the scales at three metric tons or more, as estimated from the circumference of the thigh bone. First, contrary to what many investigators have postulated, the beasts probably had to have been cold-blooded, because the costs of maintaining a constant body temperaturewhich is to say, being warm-bloodedat that size would have necessitated unrealistic hunting success (up to 10 times that of a lion) and a means of cooling down to avoid overheating.

The second requirement derives from the observations that meat-eating species have more extensive geographic ranges and lower population densities than vegetarians do, and big carnivores range over bigger areas than small ones do. For gigantic theropod species to succeed, the researchers argue, they would have needed continent-size landmasses to sustain populations large enough to avoid extinction. Third, titans-in-the-making had to be released from the ecological pressure of competing with other large species for foodthrough the extinction of rivals or the hunting of different prey, for example.

Critics find some of the conclusions hard to swallow. Kevin Padian of the University of California at Berkeley says studies of bone histology indirectly indicate that dinosaurs more closely resembled warm-blooded mammals than cold-blooded reptiles in their metabolism. Furthermore, he contends, it is impossible to determine the population sizes of extinct animals. Without living, breathing dinosaurs at hand, scientists may never know where on the metabolic spectrum they fell. But so far the fossil record upholds other predictions of the teams model. The five known heavyweights have turned up only on the vast landmasses of North America, South America and Africa. And all of them lived free from competition with other giants. (Of the two that overlapped in time and space, Spinosaurus and Carcharodontosaurus, both from Africa, the former appears to have fished for its supper, whereas the latter hunted on terra firma.)

The strongest support for this theory of theropod gigantism comes from what is known about the rise of T. rex, however. For more than 25 million years before the emergence of this colossus, the Western Interior Seaway divided North America, flowing from the present-day Gulf of Mexico to the Arctic. On both sides, three species of tyrannosaur, weighing in at just one to two tons, roamed in isolation. After the water retreated around 69 million years ago, thus doubling the available habitat area, only one lineage remained, and it produced the 5.5-ton T. rex. "There is food for thought here," Thomas R. Holtz, Jr., of the University of Maryland says of the model, "but we need some more test cases."