Fossil fish reveals early development of teeth

Teeth have been with us for hundreds of millions of years—but they have not always looked, functioned, or been replaced the way they do today.

Modern jawed vertebrates fall into two main groups:

• cartilaginous fishes (aka Chondrichthyes: sharks, rays and skates), whose teeth are produced in a conveyor-belt-like dental lamina and frequently shed.
• bony fishes (aka Osteichthyes: from tuna and lungfish to frogs, whales and humans), whose teeth are replaced at specific sites through a process called resorption—a mechanism absent in sharks.

This raised an evolutionary question: what was the ancestral condition for tooth replacement? To answer this, palaeontologists turned to placoderms, a group of extinct “armoured fishes” that lived in the Silurian and Devonian periods.

Our new study focuses on Bullerichthys, a small placoderm from the famous Gogo Formation in Western Australia (-375 million years old). Using high-resolution synchrotron X-ray imaging, researchers were able to follow how individual teeth developed, aged, and eventually disappeared.

The results were striking. Young teeth had large pulp cavities, which were gradually filled with dentine. As the teeth aged, specialised cells (osteoclasts) broke down this dentine (resorption), and bone grew in its place. Eventually, the oldest teeth were resorbed completely. Earlier palaeontologists, seeing only these bone-filled remains, understandably mistook them for non-teeth.

This discovery shows that the resorption-based replacement system—long considered a hallmark of bony fishes —was already present at the very base of jawed vertebrate evolution. It also suggests that placoderm dentitions changed through an individual’s lifetime, potentially supporting different diets for young and adult fishes and reducing competition (niche partitioning).