The end of the Cretaceous was marked by some severe shennanigans: there was a meteorite impact, as well as a great deal of volcanic activity in the Deccan Traps: both of which contributed to sudden and catastrophic changes to conditions of Earth, and led to the famous K-T mass extinction event.
Larger animals were affected worse than smaller ones. All non-avian dinosaurs died out; as did mosasaurs and icthysaurs. The ancestors of the mammals tended to be smaller and fared slightly better in the immediate aftermath of the catastrophe.
The Earth returned to its previous greenhouse climate after only a few decades, and the survivors diversified and moved into new evolutionary niches throughout the paleogene.