Hugo Ashley, from Poulton, and his grandfather also found an ammonite cadoceras, and another Rieneckia ammonite.

BBC: Children discover 'rare' fossils at Cotswold Water Park

She said Dr Hollingworth found another Rieneckia ammonite in a similar quarry in the park several years ago.

BBC: Children discover 'rare' fossils at 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.

BBC: Children discover 'rare' fossils at Cotswold Water Park

The largest invertebrate on the planet, it is a member of the class Cephalopoda, which includes octopus, nautilus, and extinct ammonite species.

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His project report titled: "A baseline study of Monmouth Beach's Ammonite Pavement" has already been published by the UK mapping agency, the Ordnance Survey.

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If plankton provided the major part of the ammonite diet then this could help explain their extinction 65 million years ago, the team believes.

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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.

BBC: Children discover 'rare' fossils at Cotswold Water Park