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Ancient Cephalopods
More than 17,000 species of fossil cephalopods have been named. There are three main groups of fossil cephalopods. The oldest group, nautiloidea, lived about 505 to 408 million years ago. They resembled modern nautiluses with coiled, chambered shells. The ammonoidea group first appeared when the nautiloid group declined and survived throughout the age of dinosaurs, until 65 million years ago. These animals also had hard, round shells. All of the ammonoid cephalopods are extinct. The coleoidea group had long, pointed shells and lived during the age of dinosaurs.
Modern Cephalopods
Eight hundred living species of cephalopods include octopuses, squid, cuttlefish and chambered nautiluses. All cephalopods are predators with at least eight arms and a beak. Octopuses have no shells and generally cannot create fossils from their soft bodies. Squids and cuttlefish have flexible, internal shells that may form fossils under the right conditions. Nautiluses have hard, external shells that can become fossilized. After 300 million years, a few species of nautilus are the only surviving descendents of the nautiloid cephalopods. Squids and octopuses are believed to descend from coleoid ancestors.
The Fossil Record
The fossil record shows the presence of cephalopods more than 500 million years ago. Cephalopods without shells leave behind few fossils because their hard parts, such as a beak, are not preserved because they are not calcified. Nautiloid and ammonoid cephalopods left behind fossils of flat, coiled shells with internal chambers that likely aided in buoyancy. Coleoids, such as belemnites left behind long, thin, conical fossils remains of their shells. Four-feet long belemnite shell fossils found near the Kentucky River were created from cephalopods that were probably 8 feet long in life.
Fossil Formation
Cephalopod shells were made of hard minerals. Some types of fossils were formed when the minerals in the shell were replaced by other minerals, which preserved the form of the shell. Other types of fossils were formed from the impression the shell left in a soft substrate that later hardened. In some ammonoid fossils, the shell itself has been worn away and the impression left by the internal chambers remains. Belemnites, a type of coleoid cephalopod, left fossilized molds of their shells. Once the soft tissue of the animal's body is gone, the buried shell fills with sediment. The sediment hardens to form a mold of the shell. Over time, the shell decays or is worn away but the mold remains.