Belemnitida

Belemnitida (or belemnites) is an extinct order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous. Unlike squid, belemnites had an internal skeleton that made up the cone–from arms-most to tip: the tongue-shaped pro-ostracum, the conical phragmocone, and the pointy guard. The calcitic guard is the most common belemnite remain. Belemnites, in life, are thought to have had 10 hooked arms, a pair of fins on the guard. The chitinous hooks were usually no bigger than 5 mm (0.20 in), though a belemnite could have had between 100 and 800 hooks in total, using them to stab and hold onto prey. Belemnites formed the basis of the Mesozoic marine food chain, both the adults and the planktonic juveniles, and likely played an important role in restructuring marine ecosystems after the Triassic–Jurassic extinction event. They may have laid between 100 and 1,000 eggs. Some species may have been adapted to speed and swam in the turbulent open ocean, whereas others resided in the calmer nearshore and fed off the seafloor. The largest belemnite known, Megateuthis elliptica, had guards of 60 to 70 cm (24 to 28 in). Belemnites are coleoids, a group that includes squid and octopuses, and belemnites are sometimes, though not always, classified as a stem-group of squid (Decapodiformes). Belemnoids may have been the forerunners of modern coleoid forms. Guards can give information on the climate, habitat, and the carbon cycle of the ancient waters they inhabited. Guards have been found since antiquity and have become part of folklore. The belemnite cone is composed of three parts, going from arms to tip: the tongue-shaped pro-ostracum; then the conical, chambered phragmocone; and finally the spear-shaped guard at the very tip. The guard attached to the phragmocone in a socket called the alveolus. The cone, in life, would have been encased in muscle and connective tissue. They had calcitic guards, and aragonitic pro-ostraca and phragmocones, though a few belemnites also had aragonitic guards, and the alveolar side of the guards of belemnitellids may have also been aragonitic. The pro-ostracum probably supported the soft parts of the belemnite, similar to the gladius of squid, and completely surrounded the phragmocone. The phragmocone was divided by septa into chambers, much like the shells of cuttlefish and nautiluses. The chambered phragmocone was probably the center of buoyancy, and so was positioned directly above the center of mass for stability purposes. In regards to buoyancy, belemnites may have behaved much like modern ram's horn squid, having the chambers of the phragmocone flooded and slowly releasing more seawater via the siphuncle tube as the animal increases in size and weight over its lifetime to maintain neutral buoyancy. At the tip of the phragmocone beneath the guard is a tiny, cup-like protoconch, the remains of the embryonic shell. 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. This would have allowed the animal to move horizontally through the water. The guard may have also served to cut through waves while swimming at the surface, though modern cephalopods generally stay completely submerged. Though unlikely, it is possible fossilization increased the perceived density of the guard, and it may have been up to 20% more porous in life. Fins may have been attached to the guard, or the guard may have lent support for large fins. Including arms, guards could have accounted for one fifth to one third of the total length of a belemnite. Belemnites had a radula–the mouth–embedded in the buccal mass–the first part of a gastropod digestive system–similar to open ocean predatory cephalopods. The radula had rows of seven teeth, consistent with modern predatory squid. 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. Like other cephalopods, the skin was likely thin and slippery. The eyeballs were likely thicker, stronger, and more convex than in other cephalopods. The mantle cavity of cephalopods serves to contain the gills, organs, and gonads; also, water is siphoned into and expelled out of the mantle cavity via a tube opening near the arms of the animal, the hyponome, for jet propulsion. Though the hyponome was well-developed in belemnites, the phragmocone was large, implying a small mantle cavity and thus less jet propulsion efficiency. Like some modern squid, belemnites may have mainly used large fins to coast along currents. Two Acanthoteuthis specimens with preserved soft anatomy elements had a pair of rhomboid fins near the top of their guards; however, the specimens had different sized fins, possibly owing to sexual dimorphism, age, or distortion during fossilization. These specimens appeared to have had similar adaptations to modern squid for speed, and may have been able to reach similar maximum speeds of 1.1 to 1.8 km/h (0.68 to 1.12 mph) like modern migrating Todarodes flying squid.