It depends on how you want it to sound.
if you said
Seals are amazing animals.
It sounds like a plain statement
if you said
Seals are amazing animals!
It sounds more exciting.
Periods go at the end of declarative sentences, and exclamation points go at the end of exclamatory sentences. An exclamatory sentence is one that expresses a strong or forceful emotion, such as anger, surprise, or joy
