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

The researchers then analyzed thin sections of 19 fossilized tusks of dicynodonts, representing 10 different species found in South Africa, Antarctica, Zambia and Tanzania.

Different members of the dicynodont family evolved tusks independently at different times, and some never evolved true tusks.

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

For tusks to evolve, they found that a flexible ligament attaching the tooth to the jaw was needed, as well as reduced rates of teeth replacement -- a combination of features that today is uniquely found in modern mammals.

"It all ladders up to giving us a better understanding of the tusks we see in mammals today," said Angielczyk, speaking of the research, which published in the journal Proceedings of the Royal Society B Biological Sciences.