When one reads about Tyrannosaurus and Brontosaurus, one is not dealing with species, like lions or African elephants. Instead, these are genera, a g… - Gregory S. Paul

By the Cretaceous crocodilians of essentially modern form were the theropods [sic] main competitors. Yet crocodilians appear to be less abundant in most Mesozoic deposits than they are later in the mammal-dominated Cenozoic. Not only that, but they tended to be small-bodied: few specimens were as big as American alligators or Nile crocodiles. It is possible that theropods were eating the crocs. Even today, big cats once in a while kill a fairly large crocodilian. A tyrannosaur could have swallowed one whole, and gone into the water after them. Constant attacks could have suppressed croc populations, and favored the smaller, harder to catch species.

The culmination of tyrannosaur evolution, T. rex was one of the very last North American dinosaurs. Nothing else combined its size, speed, and power. Since its demise we have had to make do with lions and tigers and bears, and other "little" mammalian carnivores.

The biggest living terrestrial predator, the Siberian tiger, at about a third of a metric ton (300 kg) pales in comparison to the biggest of the meat-eating dinosaurs, which reached 5 to perhaps 20 metric tons - the size of elephants and bigger. But while elephants cannot run, the biggest predatory dinosaurs probably ran as fast as horses, and they hunted herbivores that themselves were as big as or bigger than elephants.