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Belemnoidea

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Belemnites
Temporal range: Devonian–Cretaceous PreꞒ Ꞓ O S D C P T J K Pg N
Small Belemnite guards
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Subclass: Coleoidea
(unranked): Cohort †Belemnoidea
Extinct Orders

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. The name "Belemnoid" comes from the Greek word belmnon meaning "a dart or arrow" and the Greek word eidos meaning "form".

Occurrence

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.

Anatomy

Belemnites comprise a central phragmocone, made of aragonite and with negative buoyancy. To the rear of the creature is a heavy calcite guard, whose main role appears to have been to counterbalance the front of the organism; it positions the centre of mass below the centre of buoyancy, increasing the stability of the swimming organism. The guard would account for between a third and a fifth of the length of the complete organism, arms included.

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.

Ecology

Belemnites were effectively neutrally buoyant, and swum in near-shore to mid-shelf oceans. Their fins could be used to their advantage in all water speeds; in a gentle current they could be flapped for propulsion; in a stronger current they could be held erect to generate lift; and when swimming rapildy by jet propulsion they could be tucked in to the body for streamlining.

Preservation

A belemnite fossil from the Franconian Jura.

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 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.

Belemnites

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).

Belemnite fossils at Bristol City Museum, Bristol, England. Found in the Lower Lias strata, Gloucestershire, England.

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.

==Uses==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.

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.

Classification

Note: all families extinct

See also

References

  1. Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1111/j.0031-0239.2004.00395.x, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1111/j.0031-0239.2004.00395.x instead.
  2. Webster's New Universal Unabridged Dictionary. 2nd ed. 1979.
  3. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1007/BF02988082, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1007/BF02988082 instead.

External links

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