Ammonite

I found an amazingly detailed ammonite over 30 years ago in North Yorkshire.  Using latex, I took an impression and created this pendant in .999 silver - approx 1.5 inches including bail.  Polishing and burnishing has softened the detail of the silver version, but the lines on the original are incredibly crisp.  I read somewhere that this little creature created about 13 chambers a year as it grew, and most lived around two years.  How old does this make mine?