rodent vs placental mammal

rodent and placental mammal both are nouns.

Word	Noun	Adjective	Verb	Adverb
rodent	Yes	No	No	No
placental mammal	Yes	No	No	No

As nouns, placental mammal is a hypernym of rodent; that is, placental mammal is a word with a broader meaning than rodent:

rodent: relatively small placental mammals having a single pair of constantly growing incisor teeth specialized for gnawing
placental mammal: mammals having a placenta; all mammals except monotremes and marsupials

Other hypernyms of rodent include eutherian, eutherian mammal, placental.

rodent (noun)	placental mammal (noun)
relatively small placental mammals having a single pair of constantly growing incisor teeth specialized for gnawing	mammals having a placenta; all mammals except monotremes and marsupials