The Cretaceous also saw the first radiation of the diatoms in the oceans (freshwater diatoms did not appear until the Miocene). The Cretaceous-Tertiary extinction. The most famous of all mass extinctions marks the end of the Cretaceous Period, about 65 million years ago.As everyone knows, this was the great extinction in which the dinosaurs died out, except for the birds, of course.he Cretaceous also saw the first radiation of the diatoms in the oceans (freshwater diatoms did not appear until the Miocene). The Cretaceous-Tertiary extinction. The most famous of all mass extinctions marks the end of the Cretaceous Period, about 65 million years ago.