Aulacocerida Phragmoteuthida Belemnitida Diplobelida Belemnoids are an extinct group of marine cephalopod, very similar in many ways to the modern squid.

[4] Belemnoids were numerous during the Jurassic and Cretaceous periods, and their fossils are abundant in Mesozoic marine rocks, often accompanying their cousins the ammonites.

Belemnoids were efficient carnivores that caught small fish and other marine animals with their arms and ate them with their beak-like jaws.

In turn, belemnites appear to have formed part of the diet of marine reptiles such as Ichthyosaurs, whose fossilized stomachs frequently contain phosphatic hooks from the arms of cephalopods.

The hollow region at the front of the guard is termed the alveolus, and this houses a chambered conical-shaped part of the shell (called the phragmocone).

The guard, phragmocone and pro-ostracum were all internal to the living creature, forming a skeleton which was enclosed entirely by soft muscular tissue.

It remains unclear whether these deposits represent post-mating mass death events, as are common among modern cephalopods and other semelparous creatures.

Some belemnoids (such as Belemnites of Belemnitida) serve as index fossils, particularly in the Cretaceous Chalk Formation of Europe, enabling geologists to date the age the rocks in which they are found.