Ammonites are an extinct group of marine mollusk animals that existed from the Devonian to the Cretaceous period. Their spiral shells are commonly found as fossils and are used as index fossils to date the age of the rock layers in which they are found. Ammonites are important in paleontology for understanding the evolution of marine life and the environmental conditions of ancient oceans.
Reference: Monks, N., & Palmer, P. (2002). Ammonites. Smithsonian Books.