This allows the belemnite to maintain the right orientation. There are some other possible functions for the guard. Many palaeontologists believe that belemnites had muscular fins attached to the guard, as shown in the picture here. 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.