Belemnoids are an extinct group of marine cephalopod, very similar in many ways to the modern squid. Like them, the belemnoids possessed an ink sac,[1] but, unlike the squid, they possessed ten arms of roughly equal length, and no tentacles.[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 include belemnites (which belong to the order Belemnitida), aulacocerids (order Aulacocerida), phragmoteuthids (order Phragmoteuthida), and diplobelids (order Diplobelida). Belemnoidea has been suggested to be paraphyletic by some authors.[4]
Belemnoids were numerous during the Jurassic and Cretaceous periods, and their fossils are abundant in Mesozoic marine rocks, often accompanying their cousins the ammonites. The belemnoids become extinct at the end of the Cretaceous period along with the ammonites. The belemnoids' origin lies within the bactritoid nautiloids, which date from the Devonian period; well-formed belemnoid 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.
Belemnoids possessed a central rostrom/guard made of aragonite and with negative buoyancy.[5] To the rear of the creature was a heavy calcite 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 grabbing prey. Belemnoids 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.
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]