The guard may also have had a defensive function, or perhaps was some sort of 'wave cutter' when the belemnite was swimming close to surface. Some writers have questioned whether the density of the guard is an artefact of preservation, and that it was originally less dense, perhaps organic in composition and only mineralized after death. Belemnites are very common in Jurassic and Cretaceous sediments. Though the phragmocone is rather delicate and often crushed, the guard is very robust. It is often reworked into younger sediments, and can be transported many miles by rivers, glaciers, etc.
Belemnite guards have some use in biostratigraphy.
All living cephalopods are predators, and fossil belemnites appear to have been predatory as well. Fossilized gut contents of ammonites are quite well known, and are good evidence for this, and include mostly small invertebrate fragments. Crustaceans such as shrimps and ostracods, pieces of echinoderms, and foramaniferans suggest that ammonites tended to eat small or inactive animals, perhaps scooping in mouthfuls of sediment and sorting the food from the sand in the buccal cavity. Open water, pelagic ammonites may have eaten small zooplankton. The hooks on the arms of belemnites and the various fossil squids indicate a more active predatory lifestyle, probably based on catching smaller fish and other cephalopods.