Ammonites, from the Jurassic seas to the beach

Ammonites: sea animals in spiral shells, gone for 66 million years.

What ammonites were, how they lived and died out, how they became fossils, where to find them on the Jurassic Coast and how to collect responsibly, the story of Mary Anning, and the snakestone legends. Every fact is linked to its source, and there is a quiz at the end.

Meet the animal Take the quiz

A dark grey fossil ammonite with thick ribs coiled in a tight spiral, on a museum stand — Asteroceras obtusum, an Early Jurassic ammonite from Charmouth in Dorset.Picture: Notafly, CC BY-SA 3.0, via Wikimedia Commons (cropped and resized)

66 millionyears since most ammonites died out NHM

10,000+species discovered, possibly over 20,000 NHM

1.8 macross: the largest specimen found NHM

155 kmof Jurassic Coast World Heritage Site UNESCO

The short version

Ammonites were sea animals with shells: extinct cephalopods, the group that includes today’s squid, octopuses and nautiluses. NHM

Every ammonite had a shell divided into chambers, which it used to float. NHM

The animal lived only in the last chamber, the body chamber. The earlier chambers held gas or fluid, which it could adjust to rise, sink and move, much like a submarine. BGS

Because ammonites were common and evolved fast, much of Europe’s Mesozoic rock has been divided into ‘ammonite zones’. NHM

In England, ammonite fossils were called snakestones, because they look like coiled snakes turned to stone. NHM, snakestones

Sources don’t always agree, for example on how deep ammonites could swim or how big the biggest one was. Where they differ, we show each version side by side.

A flat grey limestone shore at low tide, under a pale sky, with low cliffs to the left and the sea beyond — The ammonite pavement on Monmouth Beach, Lyme Regis, at low tide.Picture: RealGatba, CC0, via Wikimedia Commons (cropped and resized)

Inside an ammonite (our drawing, simplified). The soft parts are a reconstruction: no fossil arms have been found.Drawing: our own, simplified and not to scale

Explore

The animalChambers, sutures, arms and jets.

Life and deathFrom 20 mm to 1.8 m, and the asteroid.

Fossils and timeFool’s gold and ammonite zones.

The Jurassic Coast185 million years in 155 km.

Fossil huntingWhat the codes say, and what to leave.

Mary AnningThe fossil hunter of Lyme Regis.

SnakestonesSaints, serpents and carved heads.

GamesQuiz, myth or fact and the shell sizer.

Fact-check logEvery fact checked against its source.

Tile pictures are credited on their pages and on the credits page.

Try the games

A 12-question quiz, a myth-or-fact challenge, and a shell sizer that measures you against the biggest and smallest ammonites.

Play now