After the Cretaceous–Paleogene extinction event wiped out the non-avian dinosaurs (birds being the only surviving dinosaurs) and several mammalian groups, placental and marsupial mammals diversified into many new forms and ecological niches throughout the Paleogene and Neogene, by the end of which all modern orders had appeared.
When the dinosaurs went extinct, access to different foods and environments enabled mammals to flourish and diversify rapidly in their tooth anatomy and evolve larger body size.
Some grew larger, some began to walk on four legs, and others grew into different shapes. Each new feature, a long neck or sharp teeth, or bony plates, if it helped the animal survive, was passed on to later generations. These features became the hallmark of each new kind of dinosaur.
Thus, environments enabled mammals to flourish and diversify rapidly in their tooth anatomy and evolve larger body size.
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