An ammonite is a type of extinct marine mollusk that lived during the Mesozoic Era, approximately 240 to 65 million years ago. They are known for their distinctive spiral shells, which are often found as fossils. Ammonites belonged to the class Cephalopoda, which also includes modern-day squids and octopuses. These creatures were active predators, using their tentacles to catch prey in the ocean. Ammonites varied greatly in size, with some being just a few centimeters across, while others could reach over two meters. Their fossils are important for dating geological formations and understanding ancient marine environments.