Elephants have them. Pigs have them. Narwhals and water deer have them. Tusks are among the most dramatic examples of mammal dentition: ever-growing, projecting teeth used for fighting, foraging, even flirting.
So why, across the broad sweep of geologic history, do such useful teeth only appear among mammals and no other surviving groups of animals? According to a study published Wednesday in the journal Proceedings of the Royal Society B, it takes two key adaptations to teeth to make a tusk — and the evolutionary pathway first appeared millions of years before the first true mammals.
Around 255 million years ago, a family of mammal relatives called dicynodonts — tusked, turtle-beaked herbivores ranging in stature from gopher-size burrowers to six-ton behemoths — wandered the forests of the supercontinent Pangea. A few lineages survived the devastating Permian extinction period, during which more than 90 percent of Earth’s species died out, before being replaced by herbivorous dinosaurs.
“They were really successful animals,” said Megan Whitney, a paleontologist at Harvard University and the lead author of the study. “They’re so abundant in South Africa that in some of these sites, you just get really sick of seeing them. You’ll look out over a field and there’ll just be skulls of these animals everywhere.”
To work out how these animals evolved their tusks, Dr. Whitney and her colleagues collected bone samples from 10 dicynodont species, among them the tiny, big-eyed Diictodon and the tank-like Lystrosaurus. They looked at how their canines attached to the jaw, whether they regularly regenerated lost teeth, like many reptiles do, and for indicators that their teeth grew continuously.
Many mammal families have evolved long, saber-toothed fangs or ever-growing incisors for gnawing. Several early dicynodonts also had a pair of long canine teeth poking from their beaks. But these teeth, like most animal teeth, are composed of a substance called dentine, capped by a hard, thin covering of enamel. Tusks have no enamel, Dr. Whitney said, and grow continuously even as the comparatively softer dentine gets worn away.
Examining the dicynodont skulls, the team found that a shift occurred midway through the group’s evolution: the appearance of soft tissue attachments supporting the teeth, akin to the ligaments present in modern mammals. And like modern mammals, dicynodonts didn’t continuously replace their teeth.
Both of these shifts laid the groundwork for the development of an ever-growing, well-supported tooth — a tusk. Afterward, Dr. Whitney said, late dicynodonts developed tusks in at least two different lineages, and possibly more.
This evolutionary pathway is reminiscent of another group of tusked animals: elephants. Early elephant relatives had enlarged canines that were covered with enamel, Dr. Whitney said. Later members of the family reduced the enamel to a thin band on one side of the tooth, like a rodent incisor, allowing the tooth to grow continuously. Finally, they ditched the enamel entirely.
“You’re providing the means for a tusk to evolve if you unlock the evolution of reduced tooth replacement and soft tissue attachments,” Dr. Whitney said. “Once you have a group that has both conditions, you can go a long time of animals playing with different tooth combinations, and you start to see these independent developments of tusks.”
The reason that tusks are currently limited to modern mammals, then, lies in a specific arrangement of teeth that mammals inherited from the broader family of synapsids, the group that includes mammal forerunners like dicynodonts.
Even with those prerequisites, Dr. Whitney said, an adaptation like tusks isn’t inevitable. But it is available, and multiple mammal groups — elephants, whales, deer, pigs and walruses — have found uses for them.
“Mammals are kind of stuck with our teeth, unlike something like a shark, which has a conveyor belt of terror,” Dr. Whitney said. “So an ever-growing tooth is pretty brilliant if you’re only replacing your tooth once.”