Not to mention no animal has been able to take there place are the water edge. Their nich in the food chain as been threatened several times by similarly adapted animals, including giant amphibans and phrehistoric whales, through out their history and they have managed to out compete every one of those animals and all of those animals are now extinct. They are perfect at what they do. Millions of years ago they were hunting dinosaurs, today they are hunting zebra, millions of years from now they will probably be hunting whatever is existing then. They arn't the losers, sure other animals evole quickly and diverse into many differnet species, but theres a simple reason why crocs haven't evoled while other animals must. The reason crocodile haven't evoled is simply because they don't have to!