Belemnites were ancient marine cephalopods that lived during the Mesozoic Era, particularly from the late Triassic to the end of the Cretaceous Period. They resembled modern-day squids and had elongated, bullet-shaped shells, which are often found as fossils. These creatures were part of the group called ammonites, but unlike ammonites, belemnites had a more streamlined body. Fossils of belemnites are commonly discovered in sedimentary rock formations, providing valuable information about prehistoric marine environments. They played a significant role in the ocean ecosystem, serving as both predators and prey. Their extinction coincided with the mass extinction event that wiped out the dinosaurs around 66 million years ago.