BELEMNOID

also called Belemnite, member of an extinct group of cephalopods, animals related to the modern squid and octopus but possessing a large internal shell, that first appeared about 345 million years ago, during the Early Carboniferous Period, and became extinct during the Eocene Epoch, which ended about 36.6 million years ago. The internal shell was straight in most species but was loosely coiled in some. The shell was complex in structure and served for support and muscle attachment and as a hydrostatic device enabling the animal to compensate for depth and its own body weight. During the Jurassic and following Cretaceous periods, the belemnites reached their greatest diversity and abundance. It is probable that the belemnoids originated from the more primitive simple-shelled nautiloid cephalopods, animals related to the modern pearly nautilus. Many distinctive and easily recognized belemnoids are known. Some are of widespread occurrence and allow correlation of widely separated rocks.