Why do only mammals have tusks? Study traces their surprising origins

For tusks to evolve, researchers 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.

But no birds, fish or reptiles today sport this extreme and ever-growing bit of anatomy. Only mammals do, even though they weren't the first tusked creatures. It is 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, a curator at Chicago's Field Museum and an author of the new study, in a news release. "They're very weird animals."

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

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