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A collection of fossilised fragments of skin are the oldest ever found. The 300-million-year-old fossils belonged to a crocodile-like reptile and could help us understand how skin evolved.

It is rare for skin to fossilise because it decays quickly after an animal’s death. But this early species of Palaeozoic reptile ended up in the right place to preserve its skin: it was buried in oily clay sediment in an ancient limestone cave in what is now Oklahoma. There was little oxygen in the cave sediment, which decelerated the decay process long enough for the tissue to fossilise, trapping a cast of its mummified skin.

The discovery came as a surprise to researchers who were examining the tiny black fossil fragments using microscopic 3D scans. “We had to be very gentle with them because they are so small and thin,” says at the University of Toronto in Canada. Their scans revealed a pebbled texture of scales resembling crocodile skin – specifically, the flatter, smaller scales on a croc’s flanks.

The researchers concluded that the skin belonged to a Palaeozoic reptile, but they aren’t sure which species. Based on nearby fossils, they suspect the skin came from an extinct lizard-like species called Captorhinus aguti. It isn’t clear how the reptile ended up in its final resting place – perhaps it fell down a vertical shaft into the cave, or was swept into the cave during a severe rainstorm.

The fossilised skin fragments are around 21 million years older than any found before, illuminating new clues about how and when skin evolved in vertebrates following their transition from living in the sea to living on land. “It’s an exceptional opportunity to look into the first chapter of higher vertebrate evolution,” says at the University of Toronto. By providing a water-tight barrier between sensitive organs and the outside world, skin helped aquatic animals make the leap to living on land.