Belemnites cylindricus

Belemnites cylindricus is an extinct species of belemnite, a group of marine cephalopods related to modern squids, octopuses, and cuttlefish. Belemnites, part of the subclass Coleoidea, bridge the gap between ancient, externally shelled ammonites and nautiloids, and modern, soft-bodied coleoid cephalopods.

The anatomy of Belemnites cylindricus was adapted for life in the open seas. Its fossilised guard, or rostrum, represents the posterior portion of the internal skeleton. The guard is dense and bullet-shaped, providing ballast to aid in stabilisation during swimming. Another important feature of the skeleton is the phragmacone, a conical, chambered structure located at the anterior end of the guard. The phragmacone housed gas-filled chambers that helped regulate buoyancy, enabling the animal to maintain its position in the water column.

Belemnites cylindricus is characterised by an elongated, cylindrical guard with a smooth surface and a tapering apex, typically ranging between 5 and 15 centimetres in length. Fossils of this species often show both the guard and the phragmacone, though occasional tentacle impressions and more are found.

Belemnites cylindricus was a skilled predator, preying on small fish and crustaceans. It possessed 10 tentacles, two of which were longer and equipped with hooks for grasping prey, much like modern squids have. Like modern cephalopods, it used jet propulsion, expelling water through a siphon for rapid movement, and had an ink sac for defence, releasing clouds of ink to confuse predators.

From an evolutionary perspective, belemnites represent an intermediate step in the development of coleoid cephalopods. Unlike their ammonite and nautiloid relatives, they had completely internalised their shell, a feature retained in modified forms in modern squids and cuttlefish. This adaptation reduced drag and made them more agile swimmers. Belemnites cylindricus was part of the family Belemnitidae, which thrived during the Jurassic and Cretaceous periods before becoming extinct in the Late Cretaceous mass extinction.

The widespread distribution of Belemnites cylindricus and its abundance in Jurassic sediments make it a valuable biostratigraphic marker, aiding geologists in dating and correlating rock layers.

A cephalopod is a member of the molluscan class Cephalopoda, which includes creatures like squid, octopuses, cuttlefish, nautiluses, ammonites, orthocones, and belemnites. These exclusively marine animals are known for their bilateral body symmetry, prominent heads, and arms or tentacles—muscular hydrostats derived from the primitive molluscan foot.

Cephalopods first became dominant during the Ordovician period, primarily represented by primitive nautiloids. Today, the class includes two distantly related extant subclasses: Coleoidea (octopuses, squids, and cuttlefish) and Nautiloidea (Nautilus and Allonautilus). In coleoids, the shell is internalized or absent, while nautiloids retain their external shells. There are approximately 800 living species of cephalopods, with an estimated 11,000 extinct species. Some well-known extinct groups include Ammonoidea (ammonites) and Belemnoidea (belemnites). Because many cephalopods are soft-bodied, they are not easily fossilized.

Cephalopods are exclusively marine and have never adapted to freshwater habitats, likely due to biochemical constraints. They are widely regarded as the most intelligent of all invertebrates, possessing highly developed senses and large brains—larger than those of their molluscan relatives, the gastropods. The cephalopod nervous system is the most complex among invertebrates, and their brain-to-body mass ratio falls between those of endothermic and ectothermic vertebrates.

With the exception of the Nautilidae and certain deep-sea octopuses (suborder Cirrina), all known cephalopods possess an ink sac, which they use to expel a cloud of dark ink to confuse predators. This sac is a muscular extension of the hindgut, releasing almost pure melanin mixed with mucus to form a thick, smokescreen-like cloud. The ink is ejected via their funnel, using the same water jet propulsion system employed for locomotion. Early cephalopods likely produced jets by retracting their bodies into their shells, as Nautilus still does today.

The evolution of cephalopods is believed to have begun in the Late Cambrian, likely from a monoplacophoran-like ancestor with a curved, tapering shell, closely related to gastropods. The development of the siphuncle, a tube-like structure, allowed early cephalopods to fill their shells with gas, achieving buoyancy and differentiating them from putative ancestors like Knightoconus, which lacked a siphuncle. This buoyancy enabled cephalopods to rise off the seafloor and eventually develop jet propulsion, which furthered their evolution as top predators.

After the late Cambrian extinction, cephalopods diversified significantly during the Ordovician, filling newly available predatory niches and becoming a dominant presence in Paleozoic and Mesozoic seas. Initially, their range was limited to shallow, sublittoral regions in the low latitudes, often in association with thrombolites. Over time, they adopted a more pelagic lifestyle. By the mid-Ordovician, some cephalopods developed septa strong enough to withstand deeper water pressures, allowing them to inhabit depths greater than 100–200 meters.

The direction of shell coiling became a crucial evolutionary trait. Endogastric coiling, in which the ventral (lower) side is concave, limited size expansion, while exogastric coiling, where the ventral side is convex, allowed the large spiral shells familiar in fossil records. Ancient cephalopods, unlike most modern species, had protective shells. These early shells were straight and conical but later evolved into curved nautiloid shapes similar to modern Nautilus.

Competition from fish during evolutionary history is thought to have driven shelled cephalopods into deeper waters, exerting pressure towards shell loss. This adaptation gave rise to modern coleoids, which, despite losing buoyancy, gained greater maneuverability, allowing them to re-colonize shallow waters. Some straight-shelled nautiloids eventually evolved into belemnites, which in turn evolved into squid and cuttlefish. The loss of the shell may also have been a response to evolutionary pressures for increased mobility, giving cephalopods a more fish-like behavior.