Three-toed Horse

Here's an image of an Early Miocene horse (Parahippus leonensis) palate in an interesting stage of development. In this specimen, the deciduous teeth are reduced to enamel caps as the permanent teeth grow in beneath them.

In this horse, the upper P1 seems to have been functional rather than vestigial, though it is considerably smaller that the other premolars.

There is a little wear on the P1 teeth, but it does not match the degree of wear on the deciduous premolars P2 and P3. This may indicate that, as with the vestigial Equus P1, these teeth emerge without a deciduous precursor. This tiny P1 may be the first or among the first permanent teeth to emerge.