Most of the major groups of mammals arose at the end of the Cretaceous period.This most likely occurred because

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The extinction of non-avian dinosaurs created vacancies in ecological niches that allowed mammals to diversify and occupy new roles in those habitats. This was aided by the differentiation of teeth in mammals that allowed them to exploit new food sources. While hair may have aided in colder habitats, it is not the main reason for the emergence of major mammal groups. Development of a marsupium is a feature of some mammals but not a defining characteristic of major groups. Increased oxygen levels may have allowed for the evolution of larger mammals, but this is not the main reason for the emergence of major groups.

Most of the Major Groups of Mammals Arose at the End