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I kridthavet levede milliarder af mikroskopiske alger kaldet kokkolithoforider.

De dannede små runde kalkplader, som de pakkede sig ind i, lidt som at forestille sig
en fodbold, man pakker ind i tallerkner, bare i mikroskopisk størrelse.

De enkelte kalkplader kaldes kokkolitter, og når algerne døde, sank kalkpladerne langsomt ned til havbunden.

Gennem de millioner af år, kridthavet eksisterede, blev der langsomt opbygget flere hundrede meter tykke lag af kridtslam på havbunden. Med tiden hærdnede dette til den bjergart, vi i dag kalder skrivekridt, og som har givet navn til hele den geologiske tidsperiode Kridttiden.

Skrivekridtet kan vi i dag se flere steder i Nordeuropa, blandt andet ved de enorme hvide klipper i Dover i det sydlige England, ved de spektakulære kystklinter på den nordtyske ø Rügen, og i flere kridtbrud i det sydlige Sverige, Holland og Belgien.

I Danmark kan skrivekridtet bedst ses ved de flotte kystklinter på Stevns og Møn i det østlige Danmark, men det kan også opleves ved flere mindre lokaliteter i det nordlige Nordjylland, dog mest i kridtbrud.

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Belemnitida (or the belemnite) 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.

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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 (nearshore) and fed off the seafloor. The largest belemnite known, Megateuthis elliptica, had guards of 60 to 70 cm (24 to 28 in).

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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.