Ammonites with a name coming from the Greek ram-horned God called Ammon are the most widely known fossil; they are cephalopods and first appeared in the seas 415 million years ago, in the form of a straight shelled creature known as Bacrites. During their evolution three catastrophic events occurred.
The first during the Permian period (250million years ago), only 10% survived. They went on to flourish throughout the Triassic period, but at the end of this period (206 million years ago) all but one species died. Then they began to thrive from the Jurassic period until the end of the Cretaceous period when all species of ammonites became extinct.
Ammonite fossils are found on every continent. And come in every naturally occurring color. Because of their rapid evolution and wide spread distribution they are an excellent tool for indexing and dating rocks. If an ammonite is found in clay the clay will preserve the Ammonites mother-of-pearl luster.
Ammonites began life very tiny, less then 1mm in diameter, and were vulnerable to attack from predators. They fed on plankton and quickly assumed a strong protective outer shell. They also grew quickly with the females growing up to 400% larger then the males; because they needed the larger shell for egg production. Most ammonites only lived for two years. Some lived longer becoming very large.
Ammonites belong to the mollusca phylum in a class known as cephalopods – “head-footed” creatures such as octopus and squid. They swam in ancient oceans from the Devonian Period, about 400 million years ago, to their extinction along with the dinosaurs in the Cretaceous Period, 65 million years ago.
Ammonites were free floating invertebrates that were attacked by plesiosaurs and mosasaurs, two groups of gigantic marine reptiles. One way that ammonites could avoid an attack was to quickly change their buoyancy levels, zig zagging and sinking rapidly.