Most mammals — notably us — only get a couple of rounds of teeth in our life: the ones we're born with, our adult teeth, and then sometimes our wisdom teeth, all of which are replaced from beneath like an elevator when new ones come in. But mammals that spend their days burrowing and chewing have evolved a constant stream of new teeth that crowd forward from the back like a treadmill. Now a new distinct variant has been found, and it combines the best of both.
That hideous looking thing at the top of the page is an African Mole-rat, Heliophobius argenteocinereus, and it uses a system best described as an escalator. The teeth start low and at the back of the jaw, and work their way up and forward where they erupt, and are then worn down through constant use.
Currently, there are only four known mammal species that continuously replace their teeth, which are defined by having three characteristics: a forward movement of new teeth from the rear to the front of the jaw, the continued eruption of teeth after the age of sexual maturity, and the growth of extra teeth.
So why does that matter? Because humans partially match these prerequisites, where other mammals like mice do not — which means that using mice models for studying the genetic mechanics of how our teeth form may not be accurate, and we need to start getting mole-rats into our labs.