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Elephants have them. Pigs have them. Narwhals and water deer have them. Tusks are among the many most dramatic examples of mammal dentition: ever-growing, projecting enamel used for combating, foraging, even flirting.
So why, throughout the broad sweep of geologic historical past, do such helpful enamel solely seem amongst mammals and no different surviving teams of animals? In accordance with a examine printed Wednesday within the journal Proceedings of the Royal Society B, it takes two key diversifications to enamel to make a tusk — and the evolutionary pathway first appeared hundreds of thousands of years earlier than the primary true mammals.
Round 255 million years in the past, a household of mammal family known as dicynodonts — tusked, turtle-beaked herbivores ranging in stature from gopher-size burrowers to six-ton behemoths — wandered the forests of the supercontinent Pangea. Just a few lineages survived the devastating Permian extinction interval, throughout which greater than 90 p.c of Earth’s species died out, earlier than being changed by herbivorous dinosaurs.
“They have been actually profitable animals,” stated Megan Whitney, a paleontologist at Harvard College and the lead creator of the examine. “They’re so ample in South Africa that in a few of these websites, you simply get actually sick of seeing them. You’ll look out over a area and there’ll simply be skulls of those animals all over the place.”
To work out how these animals developed their tusks, Dr. Whitney and her colleagues collected bone samples from 10 dicynodont species, amongst them the tiny, big-eyed Diictodon and the tank-like Lystrosaurus. They checked out how their canines hooked up to the jaw, whether or not they often regenerated misplaced enamel, like many reptiles do, and for indicators that their enamel grew repeatedly.
Many mammal households have developed lengthy, saber-toothed fangs or ever-growing incisors for gnawing. A number of early dicynodonts additionally had a pair of lengthy canine enamel poking from their beaks. However these enamel, like most animal enamel, are composed of a substance known as dentine, capped by a tough, skinny overlaying of enamel. Tusks haven’t any enamel, Dr. Whitney stated, and develop repeatedly even because the comparatively softer dentine will get worn away.
Inspecting the dicynodont skulls, the crew discovered {that a} shift occurred halfway via the group’s evolution: the looks of soppy tissue attachments supporting the enamel, akin to the ligaments current in fashionable mammals. And like fashionable mammals, dicynodonts didn’t repeatedly exchange their enamel.
Each of those shifts laid the groundwork for the event of an ever-growing, well-supported tooth — a tusk. Afterward, Dr. Whitney stated, late dicynodonts developed tusks at in not less than two totally different lineages, and presumably extra.
This evolutionary pathway is harking back to one other group of tusked animals: elephants. Early elephant family had enlarged canines that have been coated with enamel, Dr. Whitney stated. Later family members lowered the enamel to a skinny band on one facet of the tooth, like a rodent incisor, permitting the tooth to develop repeatedly. Lastly, they ditched the enamel solely.
“You’re offering the means for a tusk to evolve in case you unlock the evolution of lowered tooth alternative and mushy tissue attachments,” Dr. Whitney stated. “After you have a bunch that has each situations, you’ll be able to go a very long time of animals enjoying with totally different tooth mixtures, and also you begin to see these impartial developments of tusks.”
The rationale that tusks are at the moment restricted to fashionable mammals, then, lies in a selected association of enamel that mammals inherited from the broader household of synapsids, the group that features mammal forerunners like dicynodonts.
Even with these stipulations, Dr. Whitney stated, an adaptation like tusks isn’t inevitable. However it’s obtainable, and a number of mammal teams — elephants, whales, deer, pigs and walruses — have discovered makes use of for them.
“Mammals are sort of caught with our enamel, in contrast to one thing like a shark, which has a conveyor belt of terror,” Dr. Whitney stated. “So an ever-growing tooth is fairly sensible in case you’re solely changing your tooth as soon as.”
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