A small mammal that swam around 120 million years ago had a feature that has almost disappeared among its descendants: some of its teeth could be replaced several times.
The fossil discovered in northeastern China is remarkably complete. Researchers named the animal Dongoconodon platycauda. It belonged to an ancient group of mammals that is now extinct and displayed several adaptations to a life partly spent in water.
Photograph of the fossil skeleton of Dongoconodon platycauda and annotated drawing of its main bones. The specimen comes from the Lower Cretaceous of China.
Source: Nature Communications, study doi:10.1038/s41467-026-77339-w, figure 1. CC BY 4.0 Licence.
Its hands and feet were adapted for swimming, probably with skin connecting the digits. Its tail had flattened vertebrae, a shape reminiscent of that of modern aquatic animals that use their tails to propel themselves or steer.
But its dentition is even more surprising. Most modern mammals produce only two generations of teeth: baby teeth and permanent teeth. In this fossil, several tooth positions show traces of a third generation, or even more.
This feature is more reminiscent of reptiles, which can replace their teeth many times. Researchers believe that this ability may have reappeared in this particular branch, perhaps in connection with its carnivorous diet.
A broken tooth was probably a serious problem for a predator that had to seize hard or slippery prey. Being able to replace it therefore offered an advantage. However, this interpretation remains linked to the anatomy observed and will need to be compared with other fossils from the same group.
The skeleton also provides information about the evolution of the ear. In the distant ancestors of mammals, several small bones that are now used for hearing were still part of the jaw. Their gradual separation was one of the major transformations in this lineage.
In Dongoconodon, an element called Meckel’s cartilage was already almost detached from the jaw, but retained slight contact. This configuration supports the idea that separation from the mandible occurred before the complete isolation of the small bones of the middle ear.
Other Cretaceous mammal fossils will help determine whether multiple tooth replacement was limited to a few species adapted to particular lifestyles. More detailed analyses of their growth could also show how this dental system coexisted with an ear already very close to that of modern mammals.