Here's why:
* Evolutionary History: Monotremes (like the platypus and echidnas) are the most ancient group of mammals, branching off from the common ancestor of all mammals very early on. This makes them the closest living relatives to reptiles.
* Shared Features: Monotremes share some features with reptiles, like:
* Laying eggs instead of giving birth to live young.
* Having a cloaca, a single opening for waste and reproduction.
* Possessing a reptilian-like skull and shoulder girdle.
* Fossil Record: Fossil evidence shows that early mammals were more reptilian-like than modern mammals.
It's important to remember that mammals and reptiles are both distinct classes of vertebrates with their own unique evolutionary paths. While monotremes are the closest living relatives to reptiles, they are still very much mammals.