Unlike most other days when there are strict restrictions on even touching the glass cases enclosing the delicate specimen on display, this week, visitors at the Government Museum Egmore will have a different experience.

The ammonite fossil on display as the ‘Exhibit of the week,' a somewhat greyish spiral rock of two feet diameter is from Ariyalur, Tiruchi, and believed to be one of the Cephalopods that first appeared in the seas 415 million years ago.

Ammonite fossils are found in every continent, and thus are an excellent tool for indexing and dating rocks, says R.D. Thulasi Brinda, curator, Geology Section. Any animal or plant may become a fossil only when it has hard structures such as bones and is covered by some deposits, she added.

During medieval times, ammonites were believed to be snakes and were fitted in boxes and sold to pilgrims, she said, adding: “While Greeks used discus made of fossilised ammonite, in these areas, ammonite fossils used to be identified with Lord Vishnu and are still used in rituals.”

The fossil is open for public viewing till Sunday. S.S. Jawahar, Commissioner of Museums participated in the exhibition.