| Belemnoids | |
|---|---|
 | |
| Well preserved diplobelidClarkeiteuthis conocauda, showing arm hooks and outline of mantle | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Cephalopoda |
| Subclass: | Coleoidea |
| Superorder: | †Belemnoidea |
| Orders | |
Belemnoids are an extinct group of marinecephalopod, very similar in many ways to the modernsquid. Like them, the belemnoids possessed anink sac,[1] but, unlike the squid, they possessed tenarms of roughly equal length, and notentacles.[2] The name "belemnoid" comes from the Greek word βέλεμνον,belemnon meaning "a dart or arrow" and the Greek word είδος,eidos meaning "form".[3]
Belemnoids includebelemnites (which belong to theorderBelemnitida), aulacocerids (orderAulacocerida), phragmoteuthids (orderPhragmoteuthida), and diplobelids (orderDiplobelida). Belemnoidea has been suggested to be paraphyletic by some authors.[4]
Belemnoids were numerous during theJurassic andCretaceous periods, and theirfossils are abundant inMesozoic marine rocks, often accompanying their cousins theammonites. The belemnoids become extinct at the end of the Cretaceous period along with the ammonites. The belemnoids' origin lies within thebactritoid nautiloids, which date from theDevonian period; well-formed belemnoid guards can be found in rocks dating from theMississippian (or EarlyCarboniferous) onward through the Cretaceous. Other fossilcephalopods includebaculites,nautiloids andgoniatites.
Belemnoids possessed a central rostrom/guard made ofaragonite and with negative buoyancy.[5] To the rear of the creature was a heavycalcite guard whose main role appears to have been to counterbalance the front (towards the head) of the organism; it positions the centre of mass below the centre of buoyancy, increasing the stability of the swimming organism.[5] The guard would account for between a third and a fifth of the length of the complete organism, arms included.[5]
Like some modern squid, belemnoid arms carried a series of small hooks for grabbingprey. Belemnoids were efficientcarnivores that caught smallfish and other marine animals with their arms and ate them with theirbeak-like jaws. In turn, belemnites appear to have formed part of the diet of marinereptiles such asIchthyosaurs, whose fossilizedstomachs frequently containphosphatic hooks from the arms of cephalopods.
Belemnoids were effectively neutrally buoyant, and swam in near-shore to mid-shelf oceans.[5] 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 rapidly by jet propulsion they could be tucked in to the body for streamlining.[5]
Normally with fossil belemnoids only the back part of the shell (called theguard orrostrum) is found. The guard is usually elongated and bullet-shaped (though in some subgroups the rostrum may only exist as a thin layer coating the phragmocone). The hollow region at the front of the guard is termed thealveolus, and this houses a chambered conical-shaped part of the shell (called thephragmocone). The phragmocone is usually only found with the better preserved specimens. Projecting forwards from one side of the phragmocone is the thinpro-ostracum.
While belemnoid phragmocones arehomologous with the shells of othercephalopods and are similarly composed ofaragonite, belemnoid guards are evolutionarily novel and are composed ofcalcite oraragonite, thus tending to preserve well. Broken guards show a structure of radiating calcite fibers and may also display concentricgrowth rings.
Diagenetic modifications of the shells are complex.[6][7] Radiating calcitic crystals are thin, or very large, with a shape indicative of a strong alteration.[8][9] In other samples, the aragonite - calcite boundary is not dependent on growth lines.[10] In a given fossil sites, some specimens are calcite, others are aragonite.
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 belemnoidMegateuthis gigantea, which is found inEurope andAsia, can measure up to 46 centimetres (18 in) in length, giving the living animal an estimated length of 3 metres (10 ft).
Very exceptional belemnoid 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 moderncephalopods and othersemelparous creatures.
The name "thunderbolt" or "thunderstone" has also been traditionally applied to the fossilised rostra of belemnoids. The origin of these bullet-shaped stones was not understood, and thus a mythological explanation of stones created where lightning struck has arisen.[11]
Thestable isotope composition of a belemnoid rostrum from thePeedee Formation (Cretaceous, southeast USA) has long been used as a global standard (Peedee Belemnite, "PDB") against which otherisotope geochemistry samples are measured, for bothcarbon isotopes andoxygen isotopes.
Some belemnoids (such asBelemnites ofBelemnitida) serve asindex fossils, particularly in the CretaceousChalk Formation of Europe, enablinggeologists to date the age therocks in which they are found.
Note: all families extinct
Notable fossilcephalopods(listed by first occurrence) | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Cenozoic |
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| Mesozoic |
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| Palaeozoic |
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| Misidentified |
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