Temporal range: Triassic–Late Cretaceous
Belemnitida (commonly referred to as belemnites) is an extinct order of cephalopods which existed during the Mesozoic era, from the Hettangian age of the Lower Jurassic to the Maastrichtian age of the Upper Cretaceous.
Belemnites were superficially squid-like. They possessed ten arms of equal length studded with small inward-curving hooks used for grasping prey. However, they lacked the pair of specialized tentacles present in modern squid.
Belemnites (and other belemnoids) were distinct from modern squids by possessing hard internal skeletons. The internal skeleton was composed of the guard or rostrum (plural: rostra), a heavy solid structure at the posterior of the animals. They were composed of calcite or aragonite. The rostrum is usually bullet-shaped and projects prominently backward, but in the suborder Belemnotheutina, it is only present as a thin layer. The rostrum is oftentimes the only remains of the animals preserved (often in very large numbers in a given area).
The rostrum is in turn attached to a chambered conical shell known as the phragmocone. At the tip of the phragmocone beneath the rostrum is a tiny spherical or cuplike nodule known as the protoconch, the remains of the embryonic shell. The space between the phragmocone and the rostrum is known as the alveolus (plural: alveoli). At the forward part of the phragmocone is a thin very fragile structure known as the proostracum (plural: proostraca). It is usually spoon-like in shape. It extends over the dorsal part of the mantle.
Fossils which preserve the soft parts of belemnites indicate that like modern coleoids, they possessed an ink sac, hard beaks, tail fins that were either apical or lateral, and large eyes. Additionally they had pro-ostracum which is basically a tongue that would have projected from the main body. Well preserved specimens have even retained evidence of strong muscular fibers in the mantle, indicating that they were powerful swimmers like modern squids.
Fossils of belemnite have been used as an international standard to define the normal isotope abundance of C12 and C13. It is referred to as the PDB standard. This standard has 1.111% C13 with the rest being C12. The fossils used are from a Cretaceous limestone formation at Peedee in South Carolina, USA. This standard ceased to be used in 1994, as the original material was exhausted. The new standard is referred to as Vienna Peedee Belemnite, or VPDB. It is a "synthetic" standard, defined to have the same value as the original PDB, but calibrated with NBS19 marble.
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- Isotope Fractionation http://www.es.ucsc.edu/~pkoch/EART_229/10-0111%20Fractionation/REQUIRED/Schoeller%2099%20JAS%2026-667.pdf