The start of Cenozoic Era marked the dawn of the age of Mammals means that it is the begining of animal lived and terrestrial animals that dominated both hemispheres were majorly mammals. The eutherians (placentals) in the northern hemisphere and the metatherians found in the southern hemisphere.
Cenozoic Era is the third era of the Earth's history which start around 66 million years ago and it enlongated to the present. It was the time in which the continents began their modern configuration and geographic positions. It is also the time the Earth's flora and fauna came to being.
Therefore, Cenozoic Era is the begining of animal lived and terrestrial animals that dominated both hemispheres were majorly mammals.
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