Ammonites are probably the most widely known fossil. These creatures lived in the seas between 240 - 65 million years ago, when they became extinct along with the dinosaurs. The name ammonite comes from the Greek Ram-horned god called Ammon. Ammonites belong to a group of predators known as cephalopods, which includes their living relatives the octopus, squid, cuttlefish and nautilus.
Information provided by www.discoveringfossils.co.uk
Ammonites began life as tiny planktonic creatures less than 1mm in diameter. In their infancy they would have been vulnerable to attack from other predators, including fish; however, they quickly assumed a strong protective outer shell that shielded their soft interior from damage. Evidence suggests that they gained in size rapidly, with females growing up to 400% larger than the males.