What does ammonite mean?
An ammonite is an extinct marine mollusc belonging to the order Ammonoidea, famous for its distinctive coiled, chambered spiral shell. These creatures thrived in the world's oceans for hundreds of millions of years, from the Devonian period until they perished alongside the dinosaurs at the end of the Cretaceous, roughly 66 million years ago. Today ammonites survive only as fossils, and because they evolved rapidly and spread across the globe, geologists use them to date rock layers with remarkable precision. Their shells, ranging from under an inch to more than a metre across, are among the most recognizable and widely collected fossils in the world, turning up along eroding coastlines such as England's Jurassic Coast. The name derives from Pliny the Elder's phrase 'horns of Ammon,' likening the coils to the ram-horned Egyptian deity. Closely related is 'ammolite,' the iridescent gemstone cut from fossilized ammonite shells.
nounAn extinct marine mollusc of the order Ammonoidea, characterized by a coiled, chambered, spiral shell, which flourished from the Devonian to the end of the Cretaceous period and is now known only as a fossil.
- An extinct marine cephalopod mollusc with a coiled, chambered spiral shell, known today only through its abundant fossils.
"The cliff face at Lyme Regis crumbles each winter, revealing fresh ammonites in the blue lias shale."
"The palaeontologist carefully split the nodule to reveal a perfectly preserved ammonite inside."
"Victorian collectors prized fine ammonites from the Dorset coast as natural curiosities."
The standard plural; used both for multiple specimens and collectively for the group as a whole.
"Ammonites are among the most abundant fossils found in the Jurassic rocks of southern England."
Pliny the Elder called these coiled fossils 'horns of Ammon' after an Egyptian god with a ram's head — and their name literally gave us the word 'ammonia.'
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of ammonite
The word derives from Latin 'ammonites' (lapis), meaning 'Ammon stone,' so named by Pliny the Elder because the fossils' coiled shape resembled the ram's horns of the Egyptian god Ammon, whom the Greeks identified with Zeus as Zeus-Ammon. English adopted the term in the early modern period, around the late sixteenth or early seventeenth century. The same divine name underlies 'ammonia' and 'ammonium,' since sal-ammoniac was historically associated with the temple of Jupiter Ammon in Libya.
How ammonite is actually used
Used almost exclusively in scientific, educational, and collector contexts; the plural 'ammonites' is standard when referring to multiple specimens. The related biblical sense — a member of the ancient Semitic people of Ammon — is rare in modern usage and usually written 'the Ammonites' in historical or scriptural contexts.
Easily confused with ammonite
Ammolite is the iridescent gemstone cut from the fossilized shells of ammonites, not the creature or fossil itself.
An amniote is a vertebrate whose embryos develop within an amniotic membrane — a living animal group, entirely unrelated to the fossil molluscs called ammonites.