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Hard, protective body scales, made of mineralized tissue such as dentine or enamel, could have migrated into the mouth—a hypothesis known as outside-in.

Maya dentists drilled holes into the enamel and dentine, then fit the stones and applied a sealant, usually as part of a rite of passage to adulthood.

But these teeth, like most animal teeth, are composed of a substance called dentine, capped by a hard, thin covering of enamel.

Enamel has no feeling, unlike the next layer dentine.

They saw there were similar growth patterns in dentine, but the ones from Antarctica had "closely-spaced, thick rings," which the researchers believe was due to prolonged stress.