Paleontologists are actively investigating what caused all non-avian dinosaurs to die out 65 million years ago. Most agree that the impact of a large asteroid or comet played a major role, while scientists continue to debate how volcanic activity or climate change caused by retreating sea levels might have contributed. Still, paleontologists can say for certain that some proposed explanations for the extinction of nonavian dinosaurs are simply not true.
Did Dinosaurs... ?
Get too big to survive?
No. Most of the largest dinosaurs lived millions of years before the mass extinction.
Go extinct because their eggshells got too thin?
No. Although some dinosaurs did have unusually thin eggshells, many others did not. Many survivors, including birds (living dinosaurs), crocodiles and turtles, all have thin eggshells.
Die out because mammals drove them to extinction? Did mammals eat all of the dinosaurs' eggs?
No. Dinosaurs coexisted with mammals for over 150 million years, so there's no reason to think that mammals would have suddenly caused dinosaurs to go extinct.
Go extinct because their babies hatched as all males or all females?
No. This theory--which is based on the fact that the sex of baby alligators and crocodiles is determined by outside temperature--can't be easily tested. But it cannot explain why other animals that rely on temperature-dependent sex determination survived.
Dome-headed dinosaurs like Stygimoloch were among the last of the nonavian dinosaurs.
Fossils of a species of duck-billed dinosaur have been found above the Arctic circle. But when this dinosaur was alive, the Arctic region would have been much warmer than it is today: No polar ice caps were present. But animals living in this region would have faced long, dark winters just as residents of the far north do today.