'Long in the Tooth' Swedish Fish Fossil Unveils Milk Teeth Mystery

The tiny jawbones of a fossilized 424-million-years-old fish from the Swedish island of Gotland could provide answers to why we humans lose our milk teeth. Using synchrotron X-ray tomography, researchers at Uppsala University, together with colleagues at the ESRF in France examined the jaw to learn more about the mystery of milk teeth.

Virtually everyone, without exception, sheds their baby teeth during childhood. After a little wobbling, the tiny teeth are dislodged and tooth-fairy ready, with the entire tooth root having somehow disappeared.

The researchers behind the Swedish study, published in Nature magazine, decided to investigate a jawbone from a 424-million-year-old fish fossil, Andreolepis, from Gotland. Andreolepis was very closely related to the common ancestor of all living bony fish and terrestrial vertebrates. The jaw itself is a tiny mechanism, less than a centimeter long. But therein lies a great secret: the internal microstructure is perfectly preserved and contains information about the jawbone's growth story.

In September this year, the very same Androlepis was used to prove that human tooth enamel originated in ancient fish. Strangely, the teeth of the earliest jawed vertebrates were fixated on cheekbones and could not be shed. Teeth-shedding eventually evolved independently, and twice, by two quite different processes. Sharks and rays dissolve the fibers that attach the teeth to the jaw. In bony fish and terrestrial vertebrates, however, the growing teeth are fixed directly into the jawbone via attachment tissue and when the time comes for them to be shed, this tissue loosens through dentin-dissolving cells, which also eliminate the roots of the teeth.

The extinct Andreolepis shed its teeth by basal resportion—the earliest example of this mode of tooth replacement https://t.co/sCnAM0hjDC pic.twitter.com/3kOD4pk1mn

— nature (@nature) 21 октября 2016 г.

​Until recently, the only way to discern the internal structures of fossils was to cut them into thin slices for further investigation under a microscope. This, however, destroys the sample and provides only a two-dimensional image that is difficult to interpret. At the European Synchrotron Radiation Facility (ERSF) in Grenoble, France, it is now possible to make 3D maps of the teeth-shedding process through tomographic studies without destroying the fossil.

"Each time a tooth was shed, a cavity was left where it previously had sat. Whenever a replacement tooth grew, a weak underlying scar inside the bone was felt. I found up to four such buried cavities for each tooth, stacked like plates in a cupboard. This shows that the teeth were replaced again and again during the fish's life," Donglei Chen, a doctoral student at the Department of Organism Biology at Uppsala University, told Swedish science magazine Forskning.

This is the oldest example of shedding teeth through basal resorption known to mankind. This process still occurs in primitive fish, such as gars and bichirs.

"The amount of biological information that we obtain from these surveys is completely amazing. We can follow the process of growth and resorption right down to the cellular level, almost as in a living animal. As we continue using this technique on earlier vertebrates, we will understand their life processes much better — and will certainly encounter substantial surprises," Per Ahlberg, a professor at the Department of Organism Biology, Uppsala University and one of the project leaders, told Forskning.