According to this British web site on Ammonites "The first Ammonite appeared 300 million years ago, in the Devonian age. Their ancestors were an archaic form named Batridtide. After this, they continued to develop throughout the Paleozoic and Mesozoic eras (except during some biological crises). There were three broad groups: the goniatites, living at the Devonian and Carboriferous periods, the ceratites of the permian and triassic periods and the ammonites flourishing during the Jurassic and Cretaceous ages. But the number of genera and species considerably decreased from the Cretaceous period onwards. They finally died out 65 million years ago, at the Maastrichtian stage, at the same time as several other animals of the Cretaceous fauna."

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