Ammonite

Ammonite is a common name for extinct ammonoids, cephalopods of the subclass Ammonoidea that lived in oceans from the Devonian to the Cretaceous. They are not the same as the living nautilus; nautiloids belong to a separate lineage (Nautiloidea).

Ammonites possessed typically planispirally coiled shells, though some forms were more open or irregularly coiled, and reached sizes from a few centimeters to over a meter. The shell is chambered, separated by septa, with a central or subcentral siphuncle used to regulate buoyancy. The outer shell material is usually aragonite. The suture lines where the septa meet the outer wall are intricately folded, becoming more complex through time and used as a key taxonomic character.

Ammonites were highly diverse and occupied a range of ecological roles as predators or scavengers in marine ecosystems. They existed from the Devonian period through the end of the Cretaceous, becoming extinct at the Cretaceous-Paleogene boundary about 66 million years ago. Their rapid speciation and wide geographic distribution make them valuable as index fossils for dating rocks and correlating marine strata.

Fossils of ammonites are abundant in marine sedimentary rocks and are commonly used in biostratigraphy. They are preserved in fine detail, including sutures and shell ornamentation, providing insight into paleoenvironments and evolutionary history.