An artist's impression of Microsyops latidens

An artist's impression of Microsyops latidens, fossils of which contain the oldest known tooth cavities. © Ann Sanderson

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Oldest known mammal tooth cavities offer glimpse of the world 54 million years ago

Our ancient relatives had a sweet tooth, researchers have found, after uncovering the oldest-known mammalian cavities.

The tooth decay was found in Microsyops latidens, a small mammal that is part of an extinct group of relatives to all modern primates, including humans. 

These cavities offer a glimpse of the life of these animals over a 550,000 year period, giving us clues to find out more about their diet and evolution over this period.

The research was carried out by a team of Canadian scientists from the University of Toronto Scarborough, and was published in the journal Scientific Reports.

Dr Keegan Selig, the lead author of the study, says, 'It might be surprising to some that cavities are not a modern phenomenon, and they certainly are not only unique to humans.'  

'I think it's interesting that here we have evidence of cavities that are more than 54 million years old, and that its teeth can tell us so much about this ancient animal that we couldn't get anywhere else.'

Micro-CT reconstruction of a jaw fragment and the cross-section of a molar cavity.

Micro-CT scans were used to find cavities and examine them. © Keegan Selig

Long in the tooth

Microsyopids were a group of mammals known as stem primates. This means that while they are closely related to all living primates, they left no descendants themselves. Appearing around 58 million years ago, they lived for a period of over 20 million years before dying out. 

About the size of a racoon, M. latidens itself first evolved 54 million years ago and lived for about 544,000 years. It is presumed to have lived in trees, but while few fossils of its skeleton have been found palaeontologists have unearthed thousands of jawbones containing teeth. 

Researchers have examined these jawbones and the teeth within them on the hunt for cavities. A dentist would generally do so by looking for discolouration and a change in density, but the process of fossilisation changes both of these factors. Instead, researchers looked for cavities by their shape and positioning, as well as signs the cavity was being plugged by the body. 

'These fossils were sitting around for 54 million years and a lot can happen in that time,' says Keegan. 'I think most people assumed these holes were some kind of damage that happened over time, but they always occurred in the same part of the tooth and consistently had this smooth, rounded curve to them.'

Around 7.5% of the fossils were found with cavities, rising as high as 17.24% in one layer of the fossil record. This suggests that changes were taking place in the diet of M. latidens

The fossilised teeth of M. latidens

The structure of the molars changed as the number of cavities increased, suggesting a high-sugar diet. © Keegan Selig

Sugar rush

This rise in cavities opens a window into the life of the species millions of years ago and what its environment was like.

Around the period when the number of cavities begins to increase, the shape of the molars also changed. Both of these point towards M. latidens eating more fruit during this 20,000 year period.

'Eating fruit is considered one of the hallmarks of what makes early primates unique,' says Keegan. 'If you're a little primate scurrying around in the trees, you would want to eat food with a high energy value. They also likely weren't concerned about getting cavities.' 

A change in diet can often be a clue of other changes affecting an extinct animal. For instance, changes in the shape of fossilised teeth from other microsyopid species have been linked with climate cooling around 55 million years ago.

These other mammals saw their teeth structure become more generalist during the cooling event, before becoming more specialist again afterwards. With the cooling event seeing the diversity of species drop by about 25%, competition for food likely became more intense creating pressure to adapt to a wider range of food sources.

While the exact climate M. latidens lived in, and how it changed, is currently unknown the researchers suggest that its teeth may have adapted to eat more fruit as a result of similar competition for food during this period. While the species is thought to be an omnivore, changes in the availability of its preferred fruits and leaves may have seen it adapt to eat those that were higher in sugar.

Following the end of the change in the climate, this environmental pressure would have been removed, with the teeth then reverting to a form more similar to those before it began. 

The researchers hope that their method of examining cavities provides an alternative route to study the diet of extinct species and giving us a greater understanding of how these animals behaved in the distant past.