Polyphyodont

[1] Polyphyodonts include most toothed fishes, many reptiles such as crocodiles and geckos,[2][3][4] and most other vertebrates, mammals being the main exception, though not absolute.

[5] Young animals typically have a full set of teeth when they hatch; there is no tooth change in the egg.

During the evolution of Therapsida, there was a period during which mammals were so small and short-lived that wear on the teeth yielded no significant selection pressure to constantly replace them.

Instead, mammals evolved different types of teeth which formed a unit able to crack the exoskeleton of arthropods.

Mammals chew (masticate) their food which requires a set of firmly attached, strong teeth and a "full" tooth row without gaps.