Description
Perisphinctes is an extinct genus of ammonite cephalopod that lived during the Jurassic period and serve as an index fossil for that time period.
Ammonites are perhaps the most widely known fossil, possessing the typically ribbed spiral-form shell. These creatures lived in the seas between 240 - 65 million years ago, when they became extinct along with the dinosaurs. The name 'ammonite’ originates 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.
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 developed 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.
Source: http://www.discoveringfossils.co.uk/ammonites.htm