Study of marsupial’s teeth sheds light on dental evolution

Australia: Researchers from Monash University and the University of Melbourne conducted a study of the teeth of tammar wallabies from the embryo through to adulthood, and found “important clues” that shed light on the evolution of humans and other mammals.

The study titled “From Embryo to Adult: The Complete Development and Unusual Replacement of the Dentition of the Tammar Wallaby (Macropus eugenii)” was published in the recent issue of the Journal of Mammalian Evolution.

Strange tooth-replacement habits

The scientists discovered strange tooth-replacement habits of the tammar wallaby, a small Australian marsupial.

“It’s been long assumed modern mammals all replace their teeth the same way,” wrote Qamirya Nasrulla, in an article published in the Science section of the Monash University website.

“However, advances in 3D scanning and modelling have revealed mammals with unusual tooth replacement, such as the tammar wallaby (Macropus eugenii) and the fruit bat (Eidolon helvum).

“These mammals have given us important clues as to how humans and other mammals have evolved from ancestors with continuous tooth replacement.”

Nasrulla is a Post-doctoral Research Associate in Evolution Morphology, Evans EvoMorph Lab, School of Biological Sciences, Monash University.

Nasrulla co-authored the paper with her colleagues Marilyn B Renfree, University Laureate Professor and Ian Potter Chair of Zoology; and Alistair R Evans, Associate Professor, School of Biological Sciences, Monash University.

Countering long-held assumptions

Like humans, Nasrulla explained that most mammals replace their teeth once in the course of their lives – which are classified as being “diphyodonty” (two sets of teeth).

Their teeth development differs from that of other groups, for example rodents that do not replace their teeth at all – classified as “monophyodonts”.

Since the tammar wallaby is also a diphyodont, scientists have long assumed it replaced its teeth in the same way humans do, although historical notes dating as far back as 1893 recorded unusual things about the marsupial’s tooth development.

“For starters, while we replace our incisors, canines and premolars, tammar wallabies only replace their premolars,” Nasrullah noted.

What exactly are these teeth?

Using a technique called diceCT, which combines staining and CT scanning, the researchers found that the tammar wallaby had an unusual teeth development pattern

“Instead of replacement premolar teeth developing from the successional lamina, they were in fact delayed baby teeth developing from the primary dental lamina,” she added.

“This means the tammar wallaby does not have any traditional tooth replacement. This discovery opens up a huge set of new questions. What exactly are these teeth?

“One explanation for these delayed baby teeth could be a link to our ancestry of continuous tooth replacement.”

According to the researchers, the discovery “provides a clue about how we’ve evolved from ancestors with continuous tooth replacement – by modifying and reducing a system that’s hundreds of millions of years old.”

“This is exciting because, together with the tammar, it shows there may well be a wealth of tooth replacement diversity across mammals happening right under our noses – or our gums,” Nasrullah concluded.

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