Dactylioceras Ammonite Pair, England

Ammonites are an extinct group of marine animals in the Cephalopoda class. They are invertebrates and are more closely related to coleoids (squids, octopus, and cuttlefish) than they were to the chambered nautilus, even though they looked much more similar to the later. The name Ammonite was derived from "ammonis cornua," translated to mean Horns of Ammon. Ammon was an Egyptian god that was typically depicted wearing ram horns, whose spiraled shape is similar to that of an Ammonite. Ammonites first appeared in the oceans during the Devonian Period, some 400 million years ago. They died out around 65.5 million years ago along with the dinosaurs.

Between 145 and 65 million years ago these extinct marine creatures, Cleoniceras cleon, flourished in a warm, shallow sea which covered much of the earth. As the shells of the creatures accumulated on the sea floor, they were buried by sediment and, over the ages, transformed into stone by physical and chemical processes. The chambers of the Ammonite acted as tiny geodes, allowing calcite to form crystals throughout.