Extinctions happen and sometimes they are bad. After the Permian, a duet of two rapidly-following disasters did in 95% of life on Earth. After the Cretaceous, the only dinos left were birds. One question remains, how come dino-ancestors came out better over the Triassic but mammal-ancestors came out better over the Palaeocene.
Gregory Funston has an answer: the placenta. From Permian-Triassic the mammal-ancestors were still so reptilian we've been calling them the Mammal Like Reptiles since I was a boy in the early 1980s. As of the late Cretaceous we had already several clades of mammal as nourished the nonviable infant via the placenta - not via the egg, not even the pouch. A baby who could be sheltered for seven months and then breast-fed (outside a pouch) for three had the advantage.
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