Ammonites were marine molluscs that lived for 360 million years and went extinct 65.5 million years ago. They had a distinctive spiral shell, large eyes, and tentacles. Ammonite fossils are common and some have an iridescent coating, making them valuable gemstones called ammolite.
Ammonites were a major group of marine molluscs that were numerous in Earth’s oceans for 360 million years, from about 425 million years ago during the Silurian period to 65.5 million years ago, when they went extinct along with dinosaurs and many other clades.
Ammonites were cephalopods, most closely related to living octopuses, squids, and cuttlefish, although all of their closest relatives are extinct. They lived in a distinctive spiral shell, which they could fill with gas and use to float to a desired level in the water. Some ammonites had non-spiral shells, including complex vaulting patterns. The Ammonites received their name from Pliny the Elder, who called them “Ammon’s horns” after the Egyptian god who was often depicted with the horns of a ram.
At the time they evolved, ammonites would have been one of the smartest species on the planet, along with other sophisticated molluscs. They had large eyes used to locate prey and tentacles with which to grasp it. Some ammonites were good swimmers, with hydrodynamic shells, others probably just floated around and were bottom feeders. Reconstructing the ammonite’s way of life is difficult because they have no living close relatives, but much has been learned by making models of their shells and experimenting with them in water tanks.
Ammonite fossils are very common because their free-floating mode of life meant they often lived on very oxygen-poor seabeds, so when they died and sank, there were few organisms to degrade the remains. The cement built up around the shell in a structure known as a concretion, which further preserved it and prevented it from deteriorating. Today these concretions are found on the ground as round nodules. Breaking them with a pickaxe may reveal a fossil inside.
Some ammonite fossils have been preserved so well that an internal iridescent coating is preserved, which covered the inner layer of the animal’s shell during life. When polished, some of these fossils become gemstones, a mineral that has been dubbed ammolite. Ammolite is commercially mined in small quantities in Canada and used for jewelry. It is one of the few biogenic precious gems, along with amber and pearl.
Protect your devices with Threat Protection by NordVPN
