In what geological era did mammals expand on earth

It is in the cenozoic era mammals began to expand on earth. cenozoic means new life, this is the era where enormous dinosaurs are not presents. this gives room for the mammals to roam freely and expand, cenozoic era is commonly known as the age of mammals.

Three reasons of the deaths of native americans: new illnesses that their immune systems couldn't cope with. genocides so the settlers could have the good land.starvation from lack of rations and poor infertile land on resevations