An EU team examined whether the post-dinosaur rise of mammals actually reflected vegetation changes. The study found that some mammals became tree-dwelling specialists simultaneously with an increase in vegetation habitat complexity.
Mammals and dinosaurs evolved around the same time, yet dinosaurs dominated the Earth for 200 million years while mammals remained small and innocuous. The conventional explanation is that the presence of dinosaurs suppressed mammalian evolution, through predation and competition for niches.
Further details: Flowers and the rise of mammals