The mass extinction event that occurred just before the rapid increase in speciation among mammal species was the cretaceous–tertiary extinction event.
The late Cretaceous mass extinction 66 million years ago was marked by a worldwide ecological catastrophe and rapid turnover of species.
Several arboreal mammals, including primate and marsupial ancestors or marsupial mammals, survived or survived extinction and so were able to adapt to extinction and survive. It led to an increase in the numbers of various species of mammals. The surviving mammals diversified into all the new ecological niches that opened up when the dinosaurs and other species went extinct.
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