Belemnite
Belemnites (or belemnoids) are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related to the modern cuttlefish, and like them the belemnites possessed an ink sac and ten arms (or tentacles).
They were numerous during the Jurassic and Cretaceous periods, and their fossils occur abundantly in Mesozoic marine rocks, often accompanying their cousins the ammonites. The belemnites become extinct at the end of the Cretaceous period, along with the ammonites. Other cephalopods include nautiloids and goniatites.
Normally with fossil belemnites only the back part of the shell (called the guard or rostrum) is found. The guard is elongated and bullet-shaped, being cylindrical and either pointed or rounded at one end. 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 phragmocone is usually only found with the better preserved specimens. Projecting forward from one side of the phragmocone is the thin pro-ostracum.
The guard, phragmocone and pro-ostracum were all internal to the living creature, forming a skeleton which was enclosed entirely by soft muscular tissue. The original living creature would have been larger than the fossilized shell, with a long streamlined body, prominent eyes, and arms which carried a series of small hooks for grabbing prey. The guard would have been in place towards the rear of the creature, with the phragmocone behind the head, and the pointed end of the guard facing backwards. Belemnite guards are composed of calcite and tend to preserve well. Broken specimens show a structure of radiating calcite fibers and may also display some concentric growth rings.
The guard of the belemnite Megateuthis, which is found in Europe and Asia, measures 50 to 60 centimetres in length (20 to 24 inches), giving the living animal an estimated length of 3 metres (over 11 feet).
Very exceptional belemnite specimens have been found showing the preserved soft parts of the animal.
Some belemnites (such as Belemnites) 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.
Belemnites appear to have formed part of the diet of marine reptiles such as Ichthyosaurs whose fossilized stomachs frequently contain many phosphatic hooks from the arms of cephalopods.
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