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The Wikipedia article for polyphyodont says this:

Although most extant mammals are not polyphyodont, mammalian ancestors were. 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.

...which was a bit surprising at first, but makes sense. So I'm wondering - have any species evolved in the opposite direction? In other words, have there been any non-polyphyodont species which have teeth, and evolved into polyphyodonts?

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Not true polydonty, True polydonty is an animals that grows endless numbers of teeth like reptiles and fish do. Elephants, kangaroos, and manatees are sometimes called polydont but they are doing something very different, they still only grow a set number of teeth, but instead of growing them in two sets they spread them out growing them a few at a time. the total number of teeth is still the same as other mammals and once that last tooth falls out they can't make anymore. It is worth noting the teeth migrate forward in the jaw as they wear out unlike true polydont in which new teeth grow from below endlessly.In essence they are rationing that limited number of teeth.

True polydonty has never evolved from a non-polydont ancestor.

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  • $\begingroup$ I might not be understanding correctly, but it seems like this isn't really answering the question $\endgroup$
    – Josh Withee
    Commented May 21, 2018 at 16:20
  • $\begingroup$ Polydont is a difficult term there is ture polydonts and mammals that get called poly dont but aren't. $\endgroup$
    – John
    Commented May 21, 2018 at 16:27

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