Remember losing your milk teeth? After a lot of wiggling the tooth finally fell out. But in your hand was only the enamel-covered crown; the entire root of the tooth had somehow disappeared.

In a paper published this month in Nature, researchers from Uppsala University and the European Synchrotron Radiation Facility (ESRF) in France describe how they applied synchrotron X-ray tomography to a tiny jawbone of an ancient fossil fish in order to illuminate the origin of this strange system of tooth replacement.

Teeth are subject to a lot of wear and tear, so it makes sense to be able to replace them during the lifetime of the animal. Surprisingly, however, the teeth of the earliest jawed vertebrates were fixed to the jaw bones and could not be shed.

Tooth shedding eventually evolved independently on two occasions, using two quite different processes.

In sharks and rays, the fibres that anchor the tooth to the skin of the jaw dissolve and the whole tooth simply falls out.

In bony fish and land vertebrates, the developing tooth becomes attached directly to the jaw bone by a special tissue known as ‘bone of attachment’. When it is time for the tooth to be shed, this attachment is severed—specialised cells come in and resorb the dentine and bone of attachment until the tooth comes loose. That is what causes the root of the baby tooth to disintegrate.

So when did the latter process evolve?

To answer the question, researchers examined a jaw bone of the 424 million-year-old fossil fish, Andreolepis, from Gotland in Sweden, close to the common ancestor of all living bony fish and land vertebrates. Specifically, they spent several years painstakingly dissecting scan data in order to create a three-dimensional map of the entire sequence of tooth addition and loss.

“Every time a tooth was shed, the resorption process created a hollow where it had been attached. When the succeeding replacement tooth was cemented in place by bone of attachment, the old resorption surface remained as a faint buried scar within the bone tissue. I found up to four of these buried resorption surfaces under each tooth, stacked on top of each other like plates in a cupboard. This shows that the teeth were replaced again and again during the life of the fish,” explained Donglei Chen, first author of the study, adding that this is the earliest known example of tooth shedding by basal resorption.