When did the first placental mammals likely diversify after the dinosaur extinction?

After the end-Cretaceous extinction 66 million years ago, placental mammals expanded into many ecological roles. Although the exact timing of their earliest origins is debated, the big radiation of living mammal groups happened in the Paleogene. With dinosaurs gone, mammals rapidly diversified into forms adapted for running, digging, climbing, and swimming. This post-extinction expansion helped shape modern mammal diversity.