Belemnoidea

Taxobox
name = Belemnites
fossil_range = fossil range|416|65.5Devonian-Cretaceous

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image_width = 250px
image_caption = Small Belemnite fossils
regnum = Animalia
phylum = Mollusca
classis = Cephalopoda
subclassis = Coleoidea
unranked_ordo = Cohort †Belemnoidea
subdivision_ranks = Extinct Orders
subdivision =
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, but, unlike the squid, they possessed ten arms of roughly equal length and no 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. The belemnites' origin lies within the bactritoid nautiloids, which date from the Devonian period; well-formed belemnite guards can be found in rocks dating from the Mississippian (or Early Carboniferous) onward through the Cretaceous. Other fossil 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 forwards from one side of the phragmocone is the thin "pro-ostracum".

While belemnite phragmocones are homologous with the shells of other cephalopods and are similarly composed of aragonite, belemnite guards are evolutionarily novel and are composed of calcite, thus tending to preserve well. Broken guards show a structure of radiating calcite fibers and may also display concentric growth rings. The bulk geochemical signature contained within belemnite guards of the Peedee Formation (Cretaceous, southeast USA) has long been used as a global standard ("PDB") against which all other geochemical samples are measured, for both carbon isotopes and oxygen isotopes.

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. The guard of the belemnite "Megateuthis gigantea", which is found in Europe and Asia, can measure up to 46 cm in length (18 inches), giving the living animal an estimated length of 3 metres (10 feet).

Very exceptional belemnite specimens have been found showing the preserved soft parts of the animal. Elsewhere in the fossil record, bullet-shaped belemnite guards are locally found in such profusion that such deposits are referred to semi-formally as "belemnite battlefields" (cf. "orthocone orgies"). It remains unclear whether these deposits represent post-mating mass death events as are common among modern cephalopods and other semelparous creatures.

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.

Like some modern squid, 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 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.

Classification

Note: all families extinct
*Cohort Belemnoidea
** Basal and unresolved
*** Genus "Jeletzkya"
** Order Aulacocerida
*** Family Aulacoceratidae
*** Family Dictyoconitidae
*** Family Hematitidae
*** Family Palaeobelemnopseidae
*** Family Xiphoteuthididae
** Order Belemnitida
*** Suborder Belemnitina
**** Family Cylindroteuthididae
**** Family Hastitidae
**** Family Oxyteuthididae
**** Family Passaloteuthididae
**** Family Salpingoteuthididae
*** Suborder Belemnopseina
**** Family Belemnitellidae
**** Family Belemnopseidae
**** Family Dicoelitidae
**** Family Dimitobelidae
**** Family Duvaliidae
** Order Belemnoteuthina
*** Family Belemnotheutididae
*** Family Chitinobelidae
*** Family Sueviteuthididae
** Order Diplobelida
*** Family Chondroteuthididae
*** Family Diplobelidae
** Order Phragmoteuthida
*** Family Phragmoteuthididae

See also

*Nautiloidea
*Ammonoidea

External links

* [http://www.tonmo.com/science/fossils/fossilsjump.php TONMO.com Cephalopod Fossils articles and discussion forums]