The last of the first five mass extinctions was at the end of the Cretaceous and almost 50 percent of the organisms living then went extinct. This is when all non-flying dinosaurs died. The major theory for this extinction was an asteroid or comet hitting the Earth. After this extinction, the dominant organisms in this new time period were mammals. In fact, many of the most common mammals that we know today originated in this period such as primates, rodents, and hoofed- herbivores like cows and deer. There were also a lot of birds in this time period including many large, flightless birds that are now extinct.
