pinniped vs aquatic mammal

pinniped and aquatic mammal both are nouns.

Word	Noun	Adjective	Verb	Adverb
pinniped	Yes	No	No	No
aquatic mammal	Yes	No	No	No

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

pinniped: aquatic carnivorous mammal having a streamlined body specialized for swimming with limbs modified as flippers
aquatic mammal: whales and dolphins, manatees and dugongs, walruses, seals

pinniped (noun)	aquatic mammal (noun)
aquatic carnivorous mammal having a streamlined body specialized for swimming with limbs modified as flippers	whales and dolphins, manatees and dugongs, walruses, seals