page_title: Belemnitida
section_title: Development
hierarchical_section_title: Belemnitida / Description / Development
caption_attribution_description: English: A) Ammonoidea; (B) Sepiida (C) Belemnoidea (D) Spirulida (E) Orthocerida (F) Nautilida (G) Oncocerida (H) Pseudorthocerida (I) Oegopsina (J) Bactritida. IC = Initial Chamber
context_page_description: Belemnitida 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, though a belemnite could have had between 100 and 800 hooks in total, using them to stab and hold onto prey.
  Belemnites were an important food source for many Mesozoic marine creatures, 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 littoral zone and fed off the seafloor. The largest belemnite known, Megateuthis elliptica, had guards of 60 to 70 cm.
context_section_description: Like other cephalopods, belemnites may have laid floating-egg masses, and single female may have laid between 100 and 1,000 eggs. 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. 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.
  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).
  The guards of Megateuthis elliptica are the largest among belemnites, measuring 60 to 70 cm (24 to 28 in) in length and up to 50 mm (2.0 in) in diameter. The Cretaceous Neohibolites is one of the smallest known with a guard length of around 3 cm (1.2 in). 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. The mesohibolitid belemnites, using the same methods, had a lifespan of about a year. In Megateuthis, the guard was demonstrated to have fully developed after one or two years, and growth spurts followed the lunar cycle.
caption: Cephalopod embryonic shells. A) Ammonoidea (B) Sepiida (C) Belemnoidea (D) Spirulida (E) Orthocerida (F) Nautilida (G) Oncocerida (H) Pseudorthocerida (I) Oegopsina (J) Bactritida. IC indicates the initial chamber.
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