Researchers in the US have discovered "beautifully preserved" 15 million-year-old thin protein sheets in fossilised shells of a snail-like, shallow marine-dwelling mollusc called Ecphora.
The now extinct Ecphora species lived in the mid-Miocene era - between eight and 18 million years ago.
"We were amazed to find that the shells, once dissolved in dilute acid, released intact thin sheets of shell proteins more than a centimeter across," said main author John Nance and his colleagues from Carnegie Institution for Science in Washington.
Chemical analysis including spectroscopy and electron microscopy of these sheets revealed that they are indeed shell proteins that were preserved for up to 15 million years.
"These are some of the oldest and best-preserved examples of a protein ever observed in a fossil shell," one of the authors Robert Hazen said.
Remarkably, the proteins share characteristics with modern mollusc shell proteins. They both produce thin, flexible sheets of residue that is the same colour as the original shell after being dissolved in acid.
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Ecphora is known for an unusual reddish-brown shell colour, making it one of the most distinctive North American molluscs of its era.
The study appeared in the journal Geochemical Perspectives Letters.