belemnite

noun Paleontology.

1. a conical fossil, several inches long, consisting of the internal calcareous rod of an extinct animal allied to the cuttlefish; a thunderstone.

noun

1. any extinct marine cephalopod mollusc of the order Belemnoidea, related to the cuttlefish
2. the long pointed conical internal shell of any of these animals: a common Mesozoic fossil

1. Any of various extinct cephalopod mollusks of the order Belemnoidea that lived from the Triassic into the Tertiary Period. Belemnites had a large, cone-shaped internal shell with a complex structure that served as a support for muscles and as a hydrostatic device. Belemnites were closely related to the present-day squids and cuttlefishes.
2. The fossilized internal shell of one of these cephalopods. Belemnites are used as index fossils.