era vs Age of Mammals

era and Age of Mammals both are nouns.

Word	Noun	Adjective	Verb	Adverb
era	Yes	No	No	No
Age of Mammals	Yes	No	No	No

era (noun)	Age of Mammals (noun)
a period marked by distinctive character or reckoned from a fixed point or event	approximately the last 63 million years
a major division of geological time; an era is usually divided into two or more periods