Leioceras Ammonite, from Lyme Regis, Jurassic Coast, England (No.73)

Ammonites are a form of ammonoid distinguished by their complex suture lines. They were abundant and diverse in the seas of the Mesozoic Era, and they evolved very rapidly to produce a number of species and genera. After a decline in diversity during the late Cretaceous period, ammonites become extinct at the same time as other marine groups, such as Belemnites, and terrestrial groups, such as dinosaurs.