The largest known shark known to have lived, Megalodon, had teeth that were unlike those of any living animal, according to new research.

Large and powerful Megalodon, which grew up to 66 feet long, flashed teeth that were heavily comprised of fluoride, the new study determined. Modern sharks only have fluoride on the surface, or enamel, of their teeth. The findings are published in the journal RSC Advances.

“You might say that they also used ‘built-in toothpaste,’” senior author Matthias Epple, a professor at the University of Duisburg-Essen’s Institute of Inorganic Chemistry, told Discovery News.

“The ‘built-in toothpaste’ refers to the fluoride content,” he said, adding that Megalodon never suffered from cavities.

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This, however, was primarily because the enormous shark had a “revolving jaw” that regularly replaced any teeth that went missing, maybe left behind in prey. Today’s great whites and other sharks benefit from such a replacement system too.

Megalodon teeth were almost 8 inches long, making them much larger than those of any living shark. Now extinct, Megalodon is thought to have terrorized ocean dwellers from around 16 to 2 million years ago.

For the study, Epple and his colleagues compared Megalodon teeth with teeth from five other extinct shark species, three living sharks (great white, tiger and mako), as well as teeth from two extinct marine reptiles and two dinosaurs: Spinosaurus marocannus (“Spine Lizard”) and Carcharodontosaurus saharicus. Both of the dinosaurs were large carnivores.

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The researchers focused on the chemical composition and microstructure of the teeth from these animals and determined that the now-extinct sharks and dinosaurs had fluoride on and within both the surface and interiors (dentin) of their teeth. This unique chemical composition occurred in these animals and related species for more than 100 million years.

A few million years ago, sharks evolved teeth that only had fluorapatite (the mineral associated with the compound fluoride) on the surface. The dentin mineral is now primarily hydroxyapatite, which comprises about 96 percent of human tooth enamel.

Why the switch happened remains a mystery, but Epple and his team suspect that fluorapatite must have been more common in prehistoric times than it is today. He explained that now “fluoride is not very common in seawater, (so) it is difficult for animals to collect fluoride, especially for a revolving jaw in sharks where the teeth are replaced every few weeks or months.”

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According to a University of California at Santa Barbara fact sheet, “Fluorapatite is more resistant to decay than is hydroxyapatite.” Epple also said that “fluoroapatite as a mineral is slightly harder than hydroxyapatite.”

That could be that Megalodon and the other prehistoric animals had sturdier teeth than any living creature today.

On the other hand, Epple said that fluorapatite releases a damaging acid -- hydrofluoric acid -- on contact with acidic fruits and certain other edibles, like meat, which are found on land.

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He does not think dinosaurs suffered from tooth decay much, though.

“They used fluoride in teeth for several millions of years without apparent problems,” he said, adding that the carnivorous dinosaurs that he and his colleagues studied probably rarely, if ever, ate fruit.

Diet also helps to explain why humans did not evolve continuously replacing teeth, like those of sharks.

Barry Berkovitz of King’s College London’s School of Biomedical Sciences, explained, “Continuous tooth replacement is particularly inefficient in mammals where teeth bite together to masticate food. They have evolved different strategies to cope with having only two sets of teeth during their lives.”

He added, “In humans, the ultimate solution is to get a third set from the dentist!”