Aulacocerida
Phragmoteuthida
Belemnitida
Diplobelida
Belemnoteuthina

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. Like them, the belemnites possessed an ink sac and ten arms (or tentacles).

Belemnites were numerous during the Jurassic and Cretaceous periods, and their fossils are abundant 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 baculites, 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 and prominent eyes. The guard would have been in place toward the rear of the creature, with the phragmocone behind the head and the pointed end of the guard facing backward. Belemnite guards are composed of calcite and tend to preserve well. Broken specimens show a structure of radiating calcite fibers and may also display 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.

Unlike the modern squid, whose arms have suckers, belemnite arms carried a series of small hooks for grabbing prey. Belemnites were efficient carnivores that caught small fish and other marine animals with their tentacles 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 many phosphatic hooks from the arms of cephalopods.

Classification

Note: all families extinct

• Cohort Belemnoidea
  • Order Aulacocerida
    • Family Aulacoceratidae
    • Family Dictyoconitidae
    • Family Palaeobelemnopseidae
    • Family Xiphoteuthididae
    • Family Hematitidae
  • Order Phragmoteuthida
    • Family Phragmoteuthididae
  • Order Belemnitida
    • Suborder Belemnitina
      • Family Passaloteuthididae
      • Family Salpingoteuthididae
      • Family Hastitidae
      • Family Cylindroteuthididae
      • Family Oxyteuthididae
    • Suborder Belemnopseina
      • Family Belemnopseidae
      • Family Dicoelitidae
      • Family Duvaliidae
      • Family Belemnitellidae
      • Family Dimitobelidae
  • Order Diplobelida
    • Family Diplobelidae
    • Family Chondroteuthididae
  • Order Belemnoteuthina
    • Family Belemnotheutididae
    • Family Chitinobelidae
    • Family Sueviteuthididae

See also

• Nautiloidea
• Ammonoidea

External link

• The Fossil Coleoidea Page