Dolphins Grow Teeth in Layers…
As diphyodonts, humans have permanent teeth. One of the main reasons we have two sets of teeth is because we eat solids long before our jaws are fully grown.
If our permanent teeth erupted when we started eating solids, they’d have to be very small and we’d therefore have lots of empty space in our mouths as we grew up.
Most mammals are diphyodonts just like us, but there are exceptions. Dolphins, for example, have one set of teeth to last their whole life. Their teeth erupt at around three months of age, and they grow in layers, just like trees.
Seals have milk teeth, but they often shed them in the womb and so are born with only permanent teeth.
Meanwhile, elephants have ‘conveyor belt’ teeth, with newly grown ‘marching molars’ moving forward from the back and pushing out older teeth. Elephants do begin life with milk teeth, though − and ‘milk tusks’, called tushes.
No non-mammalian species are diphyodonts. Most replace their teeth throughout their lives, and are therefore polyphyodonts.
For example sharks have several rows of exposed teeth lining both the upper and lower jaw. Additional rows of teeth form behind these and gradually move forward, replacing the front biting teeth as they wear away and fall out.
