Researchers say they've used ultraviolet light to reveal the stunning but previously hidden colored patterns on ancient fossilized seashells.

The shells, between 4.8 and 6.6 million years old, had been bleached to an overall pearly white by sand and sea, making it difficult for scientists to properly differentiate between similar but distinct species.

However, photos of the cone snail shells taken under UV light revealed distinctive patterns of swirls and colors, none of which are exactly the same as seen in cone snails today, the researchers report in the journal PLOS ONE.

The ultraviolet light allowed the minute traces of organic matter still contained within the shells to become visible and reveal their original coloration and patterns.

The researchers say that's allowed them to make a preliminary identification of 13 new species of the predatory mollusks, something that would have unlikely without the availability of the new technique.

There are currently around 800 known species of cone snails worldwide.

"This study allows comparisons to be made between cone snail diversity patterns on modern and fossil coral reef systems for the first time," says study author Jonathan Hendricks, a geologist from San Jose State University.

Some of the ancient species — like one covered in polka dots — have gone extinct, while others continued to evolve slowly, so that some modern species have coloration similar to the ancient examples — similar, but not identical.

Hendricks studied 350 shells from 28 different cone shell species originally collected from waters off the Dominican Republic in the 1970s, of which 13 were identified as new, previously unknown species.

The shells, despite the new findings, are still holding onto some of their secrets, he says, noting that while the UV light has revealed some things about them, "we still do not have a clear understanding of exactly what compounds are responsible for pigmentation in modern shells, much less what matter is actually fluorescing in the fossil shells."

Cone snails, named for the distinctive shape of their shells, are found mostly in tropical waters of the world's oceans. They are venomous, using their venom to paralyze their prey, and they are capable of delivering a sting if handled by humans.

The venom of the largest of cone snail species can be dangerous, even occasionally fatal, to humans, but cone snail venom is also being investigated as a possible source of new drugs.

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