Belemnitida

Belemnitida is an extinct order of marine cephalopods with internal shells, known from the Triassic through the Cretaceous. Its members, commonly called belemnites, left distinctive bullet-shaped fossils.

Coleoidea: Coleoidea is the cephalopod group that includes belemnites, squids, cuttlefish, and octopuses. Belemnitida belongs to this broad group of internally shelled cephalopods.
Belemnite soft anatomy: The reconstructed body structures of belemnites, inferred from rare fossils and comparisons with living cephalopods. Rare soft-body fossils show features such as fins, arms, and an ink sac.
Belemnitella: A genus of belemnites known especially from Late Cretaceous marine deposits. Its abundant guards are characteristic fossils of some Late Cretaceous seas.
Biostratigraphy: The dating and correlation of rock layers using their fossil content. Changes in belemnite species help distinguish and correlate Mesozoic marine strata.
Ammonitida: An extinct order of externally shelled cephalopods that flourished through much of the Mesozoic. Ammonites carried conspicuous external shells, unlike belemnites' internal shells.
Belemnite guard: The solid, commonly bullet-shaped posterior part of a belemnite's internal shell. This durable shell part is the fossil most often recognized as a belemnite.
Buoyancy: The upward force exerted by a fluid on an immersed object. Gas-filled shell chambers helped belemnites regulate their position in the water.
Belemnopsis: A genus of belemnites found in Jurassic and Cretaceous rocks. Its fossil record illustrates belemnite diversity beyond the familiar European chalk forms.
Belemnite δ18O paleothermometry: A method for estimating ancient seawater temperatures from oxygen-isotope ratios in belemnite calcite. Belemnite guards preserve isotope signals used to reconstruct Mesozoic ocean temperatures.
Nautilida: A cephalopod order whose living representative is the chambered nautilus. Nautilids retain an external chambered shell, contrasting with the belemnite body plan.

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