Two children have found "rare" specimens of a fossilised sea creature at the Cotswold Water Park.
Emily Baldry, five, from Chippenham, discovered the Rieneckia ammonite during a fossil hunt organised by the Cotswold Water Park Society on Sunday.
Hugo Ashley, from Poulton, and his grandfather also found an ammonite cadoceras, and another Rieneckia ammonite.
A society spokeswoman said Rieneckia ammonites were "extremely rare".
Ammonites were free-swimming molluscs of the ancient oceans, living around the same time as dinosaurs.
Society spokeswoman Jill Bewley said: "The chances of finding something like this [Rieneckia ammonites] are really, really slim.
"It's the proverbial needle in a haystack so to hit upon something like this is quite phenomenal."
After Emily hit upon the fossil with a spade, her father and palaeontologist Dr Neville Hollingworth helped her dig out the block of mudstone the 162.8 million-year-old object, which had spikes to ward off predators, was encased in.
A range of other fossils, including many ammonites, were also found during the hunt, in a sand and gravel quarry within the water park.
Ms Bewley said that once work to expose the Rieneckia ammonite, which measures about 40cm (16in) in diameter is complete, it will go on display at the Gateway Information Centre along with a range of other fossils.
She said Dr Hollingworth found another Rieneckia ammonite in a similar quarry in the park several years ago.
The 42 sq mile site of the Cotswold Water Park, which has 150 lakes, is on the Gloucestershire-Wiltshire border.
During the Jurassic period, 165 million years ago, the area was a warm shallow sea.