Ammonoids are an extinct group of marine animals related to modern squids and octopuses. They lived during the Paleozoic and Mesozoic eras, roughly 400 million to 66 million years ago. Ammonoids are known for their distinctive coiled shells, which often have intricate patterns. These shells provided buoyancy and protection from predators. Ammonoids are classified within the class Cephalopoda, which also includes cuttlefish and nautilus. Fossils of ammonoids are commonly found in sedimentary rocks, making them important for dating geological layers. Their diverse forms and widespread presence make them valuable indicators of past marine environments.