Ammonite fossil From: National Museum Cardiff  This ammonite, Sphenodiscus lenticularis, lived about 70 million years ago, just before ammonites, together with the dinosaurs, became extinct. When this one was preserved, the shell was naturally replaced by beautiful minerals, giving an iridescent sheen. Its shape shows that it was a fast swimmer.  This image forms part of the monthly 'Curators Choice' series from Rhagor, the collections based website from Amgueddfa Cymru - National Museum Wales.  What will your favourite item be?