Ammonites are extinct cephalopods that lived in shells. That means their closest relatives are the modern day Nautiluses, Octopi, Squid, and Cuttlefish. Like the Nautilus, Ammonites gradually add onto their shell to accommodate their increasing body mass. As they extend their shell they build a wall, closing up the now too narrow portion of the shell behind them as they move into the larger portion of the spiral.
On the Metaphysical Side Ammonite Fossils are said to be perfect for protection. It provides insight and it helps to assist one in seeing the ‘whole picture.’ Since it is associated with the Root Chakra, it encourages ones survival instincts and is grounding.