belemnoid

belemnoid

 or belemnite

Reconstruction of squidlike belemnoid cephalopods from the Cretaceous System, southern Tennessee

(credit: Courtesy of the American Museum of Natural History, New York)

Member of an extinct group of cephalopods, possessing a large internal shell, that first appeared c. 345 million years ago, in the Early Carboniferous Period, and became extinct during the Eocene Epoch, which ended c. 36.6 million years ago. The internal shell of most species is straight, but that of some species is loosely coiled. The shell served for support and muscle attachment and enabled the animal to compensate for depth and its own body weight. See also ammonoid.