Some species may have been adapted to speed and swam in the turbulent open ocean, whereas others resided in the calmer littoral zone (nearshore) and fed off the seafloor.

[11] The pro-ostracum probably supported the soft parts of the belemnite, similar to the gladius of squid, and completely surrounded the phragmocone.

[1][14] The dense guard probably served to counterbalance the weight of the soft parts in the mantle cavity near the arms on the opposite end of the animal, analogous to the camera of nautiloids.

[9][10] The guard may have also served to cut through waves while swimming at the surface, though modern cephalopods generally stay completely submerged.

[9] Belemnites had a radula – the "tongue" embedded in the buccal mass, the first part of a gastropod digestive system – similar to open-ocean predatory cephalopods.

The statocysts – which give a sense of balance and function much like the cochlea of the ear – were large, much like in modern fast-moving squid.

Though the hyponome was well-developed in belemnites,[12] the phragmocone was large, implying a small mantle cavity and thus less efficient jet propulsion.

[18] The chitinous hooks are subdivided into three sections: The base - which can be either flat or concave - the shaft - which projects either upward at an incline either straight or bent - and the uncinus - which can be hook- or saber-like.

[23] Further, the protoconch would have allowed them to form limbs before reaching the phragmocone stage, and thus inhabit the open ocean earlier.

[23][24] Much like in cuttlefish, nautiluses, and ammonites, the number and successive size of the chambers of the phragmocone are used to analyze the growth of an individual over its life.

Ammonites are thought to have done the same, implying a similar reproductive strategy, and, considering both reached cosmopolitan distributions, a rather efficient one.

[27] In the New Zealand Belemnopsis, four major annual growth stages were preserved in the guard, giving belemnites a lifespan of about three to four years.

One belemnite guard also presents a double-pointed tip, with one of the points projecting higher than the other, probably a sign of an infection or settlement of a parasite.

However, the 2012 discovery of early Asian forms classified into the family Sinobelemnitidae[1]—now moves this to around 234 mya in the Carnian stage of the Late Triassic.

Belemnites probably originated in the Asian part of the Panthalassic Ocean around the eastern coasts of the ancient continent of Laurasia in a cephalopod radiation, alongside the octopus-like Prototeuthina and the belemnoid Phragmoteuthida.

[37] By the Early Jurassic, belemnites were probably quite common, having spread out into the western Laurasian coasts and Gondwanan waters to the south.

[39] The first mention of belemnites in writing comes from the Greek philosopher Theophrastus, who lived in the 4th and 3rd century BCE, in his book De Animalibus Quæ Dicuntur Invidere who described it as lyngurium, lynx urine which had been buried and solidified.

Pliny the Elder, in the first century CE, did not believe in lyngurium and called the gemstone a belemnite for the first time—though not recognizing it as a fossil.

[40] In 1823, English naturalist John Samuel Miller classified belemnites as cephalopods,[40] comparing the newly discovered phragmocone remains to that of a nautilus, and concluding a resemblance to Sepia cuttlefish.

[41] This classification was confirmed when the first impressions of belemnite soft body anatomy were described by English paleontologist Richard Owen in 1844.

[15] In 1895, German paleontologist Karl Alfred Ritter von Zittel organized the clade Belemnoidea and included the families Belemnitidae, Asteroconites, and Xiphoteuthis.

[9][35] Guards are difficult to distinguish at the species level, and, consequently, synonyms are common and inflate the group's apparent diversity.

[18] Preserved fossil guards are used to measure the ancient isotopic signature of the waters the individual inhabited in life, which gives information on the climate, habitat, and carbon cycle.

Also, the order Phragmoteuthida is sometimes believed to be a sister group to Belemnoidea, but Doyle considered it to be a stem-group to Decapodiformes and Octopodiformes.

[45] According to some authors, belemnites were a stem-group of Decapodiformes: Nautiloidea †Ammonoidea †Bactritida †Aulacocerida Octopodiformes †Phragmoteuthida †Belemnitida †Diplobelida Oegopsida Myopsida Sepiida Sepiolida Spirulida Nautilidae Loliginidae Ommastrephidae Spirulidae Sepiidae Sepiolidae †Belemnitida Octopodiformes According to the "belemnoid root-stock theory", belemnoids gave rise to modern coleoids sometime in the Mesozoic, with octopuses deriving from Phragmoteuthida and squid from Diplobelida, making Belemnoidea paraphyletic.

[28] Broadly speaking, they may have preferred temperatures of 12–25 °C (54–77 °F), and, like modern squid, warmer waters may have heightened their metabolism, increasing birth and growth rates, but also decreasing lifespan.

Giant pachycormid fish are thought to have been the main filter feeders of the time, occupying the same niche as modern baleen whales.

The most quoted explanation is that belemnites were semelparous and died shortly after spawning, much like modern coleoids which migrate from the ocean to the shelf area.

In battlefields comprising both adults and juveniles—as the former model would consist entirely of adults—large groups of belemnites may have been killed by volcanism, changes in salinity or temperature, harmful algal blooms (and, thereby, anoxia), or mass stranding.

[50][52] Belemnites declined through the Late Cretaceous, and their range became more restricted to the polar regions; the southern populations became extinct in the early Maastrichtian, and the last belemnites—of the family Belemnitellidae—inhabited what is now northern Europe.