there fossil record goes as far a nothern Mexico, so in the sub tropics, for why there were less tropic penipeds. Iam not an expert on that topic. But the fact that the tropics have less food is the major reason. With the only tropic region with historic and living populations on a major scale was the west coast of south america (Peru,Equador). Mostly because it has cold, oxygen rich waters unlike most tropical regions. An other reason would be less competition. Not even that far ago much more sea creaturesm like more diverse whales, many diffrent sharks, sea cows, turtles and even marine crocodiles were verys succesfull. And the colder regions being far less competiton with many other kinds of animals.