(class Cephalopoda, subclass Coleoidea)
One order of extinct cephalopods in which the shell is internal and composed of a phragmocone, rostrum, and pro-ostracum (see skeletal material). Belemnites appear in the Jurassic, continue through the Cretaceous, and a few persist into the Eocene. The other order, Aulacocerida (Carboniferous to Jurassic) may have retained a body chamber, but in the Belemnitida this is reduced to the pro-ostracum. The most posterior portion of the shell is known as the ‘guard’ (rostrum). This is a bullet-shaped cylinder made up of radiating needles of calcite with a conical cavity (alveolus) in its anterior end into which fits the phragmocone, a conical, aragonitic (see aragonite), septate (see septum) structure, cut by a tiny siphuncle, that is homologous to the external shell of other cephalopods. The pro-ostracum is a tongue-like, anterior projection from the phragmocone and perhaps protected the anterior part of the body.
Subjects: Earth Sciences and Geography.