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. The parts are, from the arms-most to the 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 and 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 were coleoids, a group that includes squid and octopuses, and are often grouped into the superorder Belemnoidea, though the higher classification of cephalopods is volatile and there is no clear consensus how belemnites are related to modern coleoids. 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.[2][3]
The belemnite cone is composed of three parts. Going from arms to tip, these are the tongue-shaped pro-ostracum; the conical, chambered phragmocone; and the spear-shaped guard at the very tip.[4][5] The guard attached to the phragmocone in a socket called the alveolus.[5][6] The cone, in life, would have been encased in muscle and connective tissue. They had calcite guards,[7] and aragonite pro-ostraca and phragmocones,[4] though a few belemnites also had aragonite guards,[8] and the alveolar side of the guards of belemnitellids may have also been of aragonite.[6] The pro-ostracum probably supported the soft parts of the belemnite, similar to the gladius of squid, and completely surrounded the phragmocone.[5][9]
The phragmocone was divided by septa into chambers, much like the shells of cuttlefish and nautiluses.[9] The chambered phragmocone was probably the center of buoyancy, and so was positioned directly above the center of mass for stability purposes. With regard 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.[4] At the tip of the phragmocone beneath the guard is a tiny, cup-like protoconch, the remains of the embryonic shell.[1][9]
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.[4][5] 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.[4]
The mantle cavity of cephalopods serves to contain the gills, gonads, and other organs; 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,[7] 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.[4] 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.[7]
Belemnites had 10 hooked arms of, more or less, equal length with suckers.[8] The hooks were rarely larger than 5 mm (0.20 in), and increased in size toward the midsection of the arm, possibly because the midsection is where maximum power could be exerted when grabbing, or bigger hooks on the extremities of the arm increased the risk of losing the arm. Having two rows of hooks covering the entire breadth of the arm, a belemnite could have had between 100 and 800 hooks in total.[11][12] Some hooks have a spur just above the base, but this may be a distortion from fossilization or preparation of the material.[13] 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.[12] Overall, they were fish-hook shaped, and probably only the uncinus was exposed.[11]
Like other cephalopods, belemnites may have laid floating-egg masses,[9] and single female may have laid between 100 and 1,000 eggs.[15] Hatchlings were either miniature forms of adults or went through a larval stage. According to the latter model, the egg was formed by the protoconch and a single-layered shell wall. During the larval stage, the protoconch became internal and the guard began to form. The embryo of Passaloteuthis, the most well-studied among belemnite embryos, had a protoconch, a developing guard, and a solid guard. The developing guard tightly surrounded the protoconch. The embryonic shell consisted of an ovoid protoconch and several chambers. The protoconch had two layers, and several compartments - called "protoconch pockets" - formed between the layers, which may have stored gas or liquid in life to stay buoyant. The protoconch and guard were probably made of chitin, a protective material which may have allowed the embryo to survive at greater depths and colder temperatures, develop into adults faster, and allow juveniles and adults to venture into deeper waters.[16] Further, the protoconch would have allowed them to form limbs before reaching the phragmocone stage, and thus inhabit the open ocean earlier. These may have allowed belemnites to colonize a range of habitats across the world.[16][17]
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 their life. Successive belemnite chambers tend to increase in size exponentially. Unlike other cephalopods, there is no decreasing trend of chamber size in the earliest stages. The decreasing trend generally coincides with hatching, meaning embryonic belemnites had no or few chambers and hatched only with a protoconch. The phragmocone, thus, developed after hatching. Ammonites are thought to have done the same, implying a similar reproductive strategy, and, considering both reached cosmopolitan distributions, a rather efficient one. Belemnite hatchling protoconches are estimated to have been generally around 1.5 to 3 mm (0.059 to 0.118 in).[9]
The guards of Megateuthis elliptica are the largest among belemnites, measuring 60 to 70 cm (24 to 28 in) in length[18] and up to 50 mm (2.0 in) in diameter.[19] The Cretaceous Neohibolites is one of the smallest known with a guard length of around 3 cm (1.2 in).[20] 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.[21] The mesohibolitid belemnites, using the same methods, had a lifespan of about a year.[22] In Megateuthis, the guard was demonstrated to have fully developed after one or two years, and growth spurts followed the lunar cycle.[23]
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