Why did most dinosaurs perish in the mass extinction that ended the Cretaceous period, while many groups of mammals and one group of small dinosaurs (birds) survived?. Earth & Marine Sciences

The terrestrial survivors of the K/T extinction were for the most part small omnivores. The leading hypothesis for the K/T extinction is that it was caused by a meteorite impact. This would have thrown much dust into the atmosphere and resulted in a drastic reduction of light and global temperatures for 3-6 months. Plants and herbivores would have been most affected by this ‘long winter’. The fossil record also suggests that few (if any) strict carnivores survived the K/T extinction. The survivors appear to have been omnivores and scavengers who were able to adapt to rapidly changing food sources. Additionally, the K/T extinctions wiped out the largest animals (dinosaurs included), while the survivors were relatively smaller. Smaller animals require less food, and often have higher birth rates.