Home Biological science Why do only mammals have tusks? Study sheds light on their surprising origins

Why do only mammals have tusks? Study sheds light on their surprising origins

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But today, no bird, fish or reptile sports this extreme and ever-growing part of the anatomy. Only mammals do, even though they weren’t the first tusked creatures. It’s an ancient trait that predates dinosaurs, according to a new study.

“We were able to show that the earliest tusks were from animals that predate modern mammals, called dicynodonts,” Ken Angielczyk, curator at the Field Museum in Chicago and author of the new study, said in a press release. “They are very strange animals.”

Ranging from the size of a rat to an elephantine, dicynodonts lived around 270 million to 201 million years ago. While their closest living relatives are mammals, they looked more reptilian, with turtle-shaped heads.

The dicynodonts were the most abundant and diverse vertebrates before the advent of the dinosaurs, and they all had a pair of tusks protruding from their upper jaws.

Defenses against teeth

Before delving into the exact evolution of tusks, researchers had to define exactly what a tusk is and how it differs from a tooth, which was ambiguous.

This is the left side of the skull of the dicynodont Dolichuranus from Tanzania.  The large tusk is visible at the bottom left of the specimen.

They determined that a tusk must extend from the mouth, consist only of a substance called dentin, and continue to grow throughout an animal’s life, even if damaged. Teeth are also made of dentin. However, they are covered with enamel. This, along with their shape, makes them durable, but once the adult teeth grow there isn’t much you can do if they snap. They don’t grow back.

“Enamel-coated teeth are a different evolutionary strategy than dentin-coated defenses – it’s a compromise,” said Megan Whitney, postdoctoral researcher in the Department of Biological and Evolutionary Biology at Harvard University. She was the lead author of the study.

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The researchers then analyzed thin sections of 19 fossilized dicynodont tusks, representing 10 different species found in South Africa, Antarctica, Zambia and Tanzania. They also used microcomputer tomograms to examine how the fossils were attached to the skull, and if their roots showed signs of continued growth. They found that while a few of the dicynodonts studied had real tusks, without enamel, the others had large teeth.

Scientists also found that there was no strict progression from non-defenses to defenses. Different members of the dicynodont family developed tusks independently at different times, and some never developed true tusks.

Field teams found isolated tusk fragments in Zambia in 2018.

“I expected there would be a single point in the history of dicynodont evolution where tusks evolved because that’s the simplest explanation. However, we found a convergent evolution. tusks later in the evolution of dicynodonts, ”Whitney said. Convergent evolution occurs when similar characteristics evolve independently in different species or at different times.

For defenses to evolve, they found that a flexible ligament securing the tooth to the jawbone was needed, as well as reduced rates of tooth replacement – a combination of characteristics only found in mammals today. modern.

“All of this allows us to better understand the defenses we see in mammals today,” said Angielczyk, speaking of the research, published in the journal Proceedings of the Royal Society B Biological Sciences.


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