Why do only mammals have fangs?

Why are some elephants being born without tusks?

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(CNN) -

Tusks are a striking feature of many animals - elephants, hippos, wild boars, walruses - and are often desired by humans.

But today, no bird, fish, or reptile sports this extreme and ever-growing part of the anatomy.

Only mammals do, although they weren't the first fanged creatures.

It's an ancient trait that predates dinosaurs, a new study has found.

"We were able to show that the first tusks belonged to animals that came before modern mammals, called dicynodonts," said Ken Angielczyk, curator of the Field Museum in Chicago and author of the new study, in a press release.

"They are very rare animals."

This is an artist's reconstruction of the Dicynodon dicynodont.

Aside from the fangs on the upper jaw, most dicynodonts had a tortoise-like beak that they used to chew their food.

From the size of a rat to the size of an elephant, dicynodonts lived about 270 million to 201 million years ago.

While their closest living relatives are mammals, they were more like reptiles, with tortoise-shaped heads.

Dicynodonts were the most abundant and diverse vertebrates before the rise of the dinosaurs, and they all had a pair of fangs protruding from their upper jaws.

Fangs versus teeth

Before delving into how exactly fangs evolved, the researchers had to define exactly what a tusk is and how it differs from a tooth - something that had been ambiguous.

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They determined that a tusk must extend from the mouth, consist solely of a substance called dentin, and continue to grow throughout the animal's life, even if it is damaged.

The teeth are also made of dentin.

However, they are coated with enamel.

This, along with their shape, makes them durable, but once permanent teeth grow in, there's not much you can do if they break.

They do not grow back.

This is the left side of the skull of the Tanzanian dicynodont Dolichuranus.

The large tusk is visible on the lower left of the specimen.

"Enamel-coated teeth are a different evolutionary strategy than dentin-coated fangs; it is a trade-off," said Megan Whitney, a postdoctoral fellow in the department of organic and evolutionary biology at Harvard University.

She was the lead author of the study.

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 micro-computed tomography scanners to examine how the fossils were attached to the skull and whether their roots showed evidence of continued growth.

They found that while some of the dicynodonts studied had true fangs, without enamel, the rest had large teeth.

Evolution

The scientists also found that there was no strict progression from non-fangs to fangs.

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 was hoping that there would be only one moment in dicynodont evolutionary history where the tusks evolved because that's the simplest explanation. However, we found the convergent evolution of the tusks later in dicynodont evolution," Whitney said.

Convergent evolution occurs when similar features evolve independently in different species or at different time periods.

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

"It's all aimed at giving us a better understanding of the tusks that we see in mammals today," Angielczyk said, speaking of the research, which was published in the journal Proceedings of the Royal Society B Biological Sciences.

Mammals