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Why were dinosaurs so big? And why after they disappeared, no reptile has evolved into dinosaurs again?
We're not experts in this sort of thing, but can share some amateur thoughts.
Although dinosaurs were probably not cold-blooded like modern reptiles, they probably had somewhat different metabolic rates and thermal regulation than do mammals. Thus obtaining enough food and avoiding overheating might have been less of a problem for a huge dinosaur than for a huge land mammal.
As for why new dinosaurs didn't evolve after the old ones were wiped out, it's almost certainly because there were new competitors (modern mammals) to deal with. Why did mammals make it through that big extinction but dinosaurs (not counting birds) didn't? Probably being a little animal living in burrows and able to eat old rotten food was a big advantage during the global environmental catastrophe brought on by the asteroid (or comet) strike or strikes. We come from a distinguished ancestry.
(published on 11/22/10)
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