Ten-year-old Bruno Debattista found a piece of shale rock containing what he thought might be a fossilised imprint while on holiday in Cornwall.
Later Bruno took it to an after-school club at Oxford University's Museum of Natural History.
University experts were astonished to find it appeared to contain trackways left by horseshoe crabs crawling up a shore around 320 million years ago.
Chris Jarvis, education officer at the museum and organiser of the after-school club, said: "Footprints of this age are incredibly rare and extremely hard to spot, so we were amazed when Bruno produced them.
"Still more impressive is the fact Bruno had a hunch they might be some kind of footprints."
The museum believes Bruno's fossil shows footprints of a pair of mating horseshoe crabs.
Bruno, who attends Windmill Primary School in Oxford, and his family have decided to donate the fossil to the museum's collection.