Why are platypuses ornithorhynchidae?

The scientific name for the platypus is Ornithorhynchus anatinus. The genus name, Ornithorhynchus, is derived from the Greek words ὀρνις (ornis), meaning "bird", and ῥύγχος (rhynchos), meaning "beak", in reference to the platypus' unique duck-like bill. The species name, anatinus, is derived from the Latin word anas, meaning "duck", again referring to the platypus' resemblance to a duck.

Therefore, the platypus is classified as a member of the family Ornithorhynchidae based on its scientific name. The family Ornithorhynchidae is also considered the only extant family in the mammalian order Monotremata, making the platypus monotreme alongside the long-beaked echidna (Zaglossus) and the short-beaked echidna (Tachyglossus).