Many animals have tusks, from elephants to walruses to hyraxes. But one thing today’s tusked animals have in common is that they’re all mammals — no known fish, reptiles or birds have them. But that was not always the case. In a study published Oct. 27 in the Proceedings of the Royal Society B, a team of paleontologists at Harvard University, the Field Museum, the University of Washington and Idaho State University traced the first tusks back to ancient mammal relatives that lived before the dinosaurs.
“Tusks are this very famous anatomy, but until I started working on this study, I never really thought about how tusks are restricted to mammals,” said lead author Megan Whitney, a postdoctoral researcher at Harvard University and a UW doctoral alum.
“We were able to show that the first tusks belonged to animals that came before modern mammals, called dicynodonts,” said co-author Ken Angielczyk, a curator at the Field Museum in Chicago. “Despite being extremely weird animals, there are some things about dicynodonts — like the evolution of tusks — that inform us about the mammals around us today.”
Dicynodonts lived from about 270 to 201 million years ago, largely before the so-called “time of the dinosaurs.” They ranged from rat- to elephant-sized. Modern mammals are their closest living relatives, but they looked more reptilian, with turtle-like beaks. One of their defining features is a pair of protruding tusks in their upper jaws. The word dicynodont means “two canine teeth.”
Not all protruding teeth are tusks. Their composition and growth patterns reveal whether they count.
“For this paper, we had to define a tusk, because it’s a surprisingly ambiguous term,” said Whitney.
For a tooth to be a tusk, the researchers argued it must extend out past the mouth, keep growing throughout the animal’s life and, unlike most mammals’ teeth — including ours — tusks’ surfaces are made of dentine rather than hard enamel.
Under these parameters, elephants, walruses, warthogs and hyraxes have tusks. Other big teeth in the animal kingdom don’t make the cut, though. For instance, rodent teeth, even though they sometimes stick out and are ever-growing, have an enamel band on the front of the tooth, so they don’t count.
Some of the dicynodont tusks that the team observed in Zambia didn’t fit the definition of a tusk either: They were coated in enamel instead of dentine.
The different makeup of teeth versus tusks gives scientists insights into an animal’s life.
“Enamel-coated teeth are a different evolutionary strategy than dentine-coated tusks,” said Whitney. “It’s a trade-off.”
Enamel teeth are tougher than dentine. But because of the geometry of how teeth grow in the jaw, if you want teeth that keep growing throughout your life, you can’t have a complete enamel covering.
Animals like humans made an evolutionary investment in durable but hard-to-fix teeth — once our adult teeth grow in, we’re out of luck if they get broken. Tusks are less durable, but they grow continuously, even if they get damaged. It’s like the compromise of getting a car that’s very reliable but very difficult to get repaired, versus driving a beater that needs frequent repairs but is cheap and easy to fix.