In biogeography, the region or zone characterized by the presence of atherian mammals (placental mammals), particularly in continental distributions.
From Greek atheria (mammals of a certain type) + gaea (earth), combining zoological and geographical terminology to describe a biogeographic realm.
This term comes from 19th-century attempts to map where different types of mammals lived, before we fully understood evolution and continental drift—it's a historical artifact of how scientists first tried to organize life geographically.
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