* Mammals: Platypuses are mammals, meaning they have fur, give birth to live young, and nurse their offspring.
* Marine: While platypuses live near water and are excellent swimmers, they are not marine animals. They inhabit freshwater rivers and streams.
* Reptiles: Platypuses are more closely related to reptiles in terms of their egg-laying ability.
Therefore, the duck-billed platypus is a semi-aquatic mammal, not a marine mammal.