Last week stunned John was told the results, which showed it was part of a Plesiosaur’s paddle bone from the Jurassic period in ”stunning condition”.
Experts described it as ”very rare”, as it has been so well preserved that blood vessels are still visible in the 12 by 8 inches (20 by 30cm) rock.
Retired British Gas meter reader John, who lives with wife Eileen, 70, in a two-bed bungalow in Downham Market, Norfolk, said it was lucky he never threw it away.
He said: ”When we moved in I thought it seemed different to any other rock I had seen but I didn’t know what it was so I just left it in the garden.
”But we were curious about it for a number of years and I thought I’m going to find out about it.