These were all big, but not real big. Animatronic dinosaur Perhaps the question should not be why theropods got so large, but why carnivorous land mammals did not. There have been plenty of elephants, ground sloths, rhino-like titanotheres, true rhinos, and other oversized mammalian herbivores around for giant Tertiary mammalian carnivores to have eaten. Differences in metabolic rates do not offer an explanation, since the biggest predatory lizards were about the same size as the biggest mammalian carnivores. Besides, giant warm-blooded predators should not have any special problems feeding themselves. After all, the grant theropods seem to have done fine (look over the food consumption and population calculations in Appendixes A and B), and orcas and sperm whales are gigantic high-metabolic-rate predators.
On the other hand, predatory dinosaurs never did what some predatory mammals did: return to the sea. And it is in the modern seas that one has to look to find predators both fishy and mammalian as big as dinosaurs. Whales and seals are the descendants of various formerly-land-dwelling, meat-eating mammals. In their new habitat, some manunals have become very large. Killer whales, or orcas, reach the size of TJnonnoscurus rex, some seven tonnes. Sperm whales can push fifty tonnes, the size of brontosaurs. No other predator of any land, sea, or time approaches this bulk. However, the conical-toothed orcas and sperm whales feed mainly on such relatively small g;lme as fish, seals, and squid. Even orcas do not make a regular job of killing mood. A recently extinct marine giant that could easily kiil the comparatively modest-sized whales of its time was the giant white shark Carcharodon megalodon, realistic dinosaur costume which weighed perhaps twenty tonnes. This shark dwarfed its close relative, the presentday one-to-two-tonne great white shark, which often attacks big elephant seals. Another mystery is why dinosaurs never returned to the sea, especially since a number of birds, like penguins, walking dinosaur suit have adopted marine ways. Theropods certainly looked like good candidates for conversion, since some species fished, and their tails could have readily been converted into propulsive sculling organs.