Ammonites were marine mollusks that lived during the Mesozoic Era, approximately 240 to 65 million years ago. They are known for their distinctive spiral shells, which often have intricate patterns. Ammonites are closely related to modern squids and octopuses, belonging to the class Cephalopoda. These creatures thrived in oceans worldwide and are important index fossils, helping scientists date the layers of rock in which they are found. Ammonites went extinct during the Cretaceous-Paleogene extinction event, which also wiped out the dinosaurs. Their fossilized shells provide valuable insights into ancient marine environments.