Why do adult platypuses have hardened gum pads instead of true teeth?

Adult platypuses do not possess true teeth; instead, they have developed flat pads of hardened gum tissue. While hatchlings are born with vestigial teeth, these are shed shortly after the young platypus leaves the burrow to begin feeding on its own.1

The absence of teeth in adults is a specialized adaptation for their lifestyle, as they are semiaquatic carnivores that forage for bottom-dwelling invertebrates, such as larval insects and crustaceans. While some extinct ancestors of the modern platypus, such as Obdurodon, retained functional teeth, the living species (Ornithorhynchus anatinus) has evolved these hardened pads to facilitate its specific feeding habits.12