A 12-year-old girl has put dinosaur hunters to shame by unearthing a 250-million-year-old fossil during a field trip to a quarry.

Stephanie Gomersall had almost given up hope of finding anything of interest during a visit to Lafarge Thrislington Quarry, near Ferryhill, County Durham, with the RSPB's Wildlife Explorers Club, arranged by her father, Richard.

However, after a tap with a hammer, one of the final pieces of stone she was examining fell apart to reveal the imprint of a fish, called coelacanthus.

Steve McLean, curator of the Hancock Museum, in Newcastle, was also on the field trip and sent the fossil for tests at the Natural History Museum in London, where it was subsequently authenticated.

Stephanie, of Ponteland, Northumberland, said: "I was amazed as it's the first time I've looked for fossils."

Her find has been donated for display at the Hancock.

Mr McLean said: "Coelacanthus is a rare fossil. We only have a few in our collections. It is a very interesting type of fossil because it was thought fish of this type were extinct until a modern coelacanth was caught by a fishing boat off the coast of South Africa in 1938 - and they are still being caught today. It's great to find this fossil specimen in Durham."

The Hancock Museum is also to exhibit another find from the Thrislington Quarry.

Shotfirer Keith Farley was preparing slate for a group of students and found the imprint of a fish called janassa, which fed on the bottom of the sea.

"It's also quite rare and, although this specimen is somewhat mangled, we can still see evidence of the skin and the mouth and teeth of the fish," said Mr McLean.

The fossil find follows the recent unearthing of a 40,000-year-old skeleton of a woolly rhino at a Lafarge quarry in Staffordshire.