It's like the compromise of getting a car that's very reliable but very difficult to get repaired when it does have trouble, versus driving a beater that needs frequent repairs but is a model that's cheap and easy for any mechanic to fix. Tusks are less durable than our enamel-coated teeth, but they grow continuously, even if they get damaged.
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.Īnimals 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. "Enamel-coated teeth are a different evolutionary strategy than dentine-coated tusks, it's a trade-off," says Whitney. The different makeup of teeth versus tusks also gives scientists insights into an animal's life. The researchers decided that for a tooth to be a tusk, it has to extend out past the mouth, it has to keep growing throughout the animal's life, and unlike most mammals' teeth (including ours), tusks' surfaces are made of dentine rather than hard enamel.Ī dicynodont skull still in the ground that is broken to reveal the roots of their tusks/teeth (the white circular structures). "For this paper, we had to define a tusk, because it's a surprisingly ambiguous term," says Whitney. "I remember Ken picking them up and asking how come they were called tusks, because they had features that tusks don't have."Īngielczyk had hit upon a crucial distinction: not all protruding teeth are technically tusks, and the teeth's makeup and growth patterns tell us whether they count. "We were sitting in the field in Zambia, and there were dicynodont teeth everywhere," recalls Whitney.
The researchers got the idea to study the origin of tusks while taking a lunch break on a paleontological dig. The name dicynodont even means "two canine teeth." And since their discovery 176 years ago, one of their defining features has been the pair of protruding tusks in their upper jaws. Modern mammals are their closest living relatives, but they looked more reptilian, with turtle-like beaks. The dicynodonts mostly lived before the time of the dinosaurs, from about 270 to 201 million years ago, and they ranged from rat-sized to elephant-sized. "We were able to show that the first tusks belonged to animals that came before modern mammals, called dicynodonts," says Ken Angielczyk, a curator at Chicago's Field Museum and an author of the paper.
"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," says Megan Whitney, a researcher at Harvard University and the lead author of the study.
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