Ancient fish illuminates childhood mystery

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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.

The childhood experience of losing baby teeth is a universal one, often met with excitement (tooth fairy!) and a bit of mystery (where did that tiny tooth go?). But a recent scientific discovery sheds light on this age-old question, thanks to a 424-million-year-old fossil fish!

Scientists studying the jawbone of an Andreolepis, an ancient relative of bony fish and land vertebrates, made a surprising finding. The bone, incredibly preserved, revealed microscopic details of how teeth were replaced in this early fish. Unlike modern sharks, which shed their entire tooth row when new teeth push through, the Andreolepis had a system remarkably similar to humans. New teeth formed beneath the old ones, gradually dissolving the roots of the baby teeth until they eventually fell 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.

This discovery helps us understand the evolution of tooth replacement in vertebrates. While scientists already knew that bony fish and land vertebrates shared this “resorption” method, they weren’t sure when it first appeared. The Andreolepis fossil pushes back the timeline significantly, suggesting that this tooth-swapping system is much older than previously thought.

The finding also has implications for understanding human dental development. Studies of the Andreolepis jawbone can provide insights into the mechanisms that control tooth replacement in humans, potentially leading to new treatments for dental problems like overcrowding or delayed tooth eruption.

So, the next time you lose a baby tooth, remember that you’re participating in a process millions of years old, shared by an ancient fish and countless generations of humans. And thanks to scientific curiosity and a well-preserved fossil, we now have a clearer picture of this fascinating biological mystery.

Here are some additional details about the Andreolepis and its significance:

  • The Andreolepis fossil was found on the island of Gotland in Sweden.
  • The study was published in the journal Nature.
  • The researchers used a technique called synchrotron X-ray tomography to study the jawbone in detail.
  • The discovery could help us develop new treatments for dental problems like overcrowding or delayed tooth eruption.

When did the 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.

In the evolutionary journey, tooth shedding independently evolved in different lineages. Sharks and rays employ a dissolution process where tooth anchoring fibers dissolve, causing the entire tooth to fall out. In contrast, bony fish and land vertebrates, including humans, use a unique process. The developing tooth becomes directly attached to the jaw bone through the ‘bone of attachment.’ When it’s time for a tooth to be shed, specialized cells come into play, resorbing the dentine and bone of attachment until the tooth loosens and detaches.

The examination of the jawbone involved synchrotron X-ray tomography, creating a three-dimensional map detailing the sequence of tooth addition and loss. The process revealed the earliest-known example of tooth shedding through basal resorption. Each shed tooth left a distinct mark—a buried resorption surface within the bone tissue. The stacked layers of these resorption surfaces showcased the repeated tooth replacement during the fish’s lifetime.

This intricate process of tooth shedding not only adds a crucial chapter to the evolutionary history of vertebrate dentition but also emphasizes the sophistication of nature’s adaptations over millions of years.

“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.