Ammonite Fossils: Age, Species and Whitby Snakestones

Ammonites are the coiled, chambered shells of an extinct group of marine animals that lived in the world's oceans for roughly 340 million years. They are among the most collected fossils in Britain, and on the Yorkshire coast they carry a local name and a local legend of their own. This guide covers what they actually were, how old they are, how the shell was built, and why geologists still rely on them.

What were ammonites?

Ammonites were cephalopods, the same class of molluscs that today includes squid, octopuses, cuttlefish and the nautilus. Despite the obvious resemblance of the shell, ammonites are thought to sit closer to squid and octopuses than to the modern nautilus.

In life an ammonite probably looked like a squid tucked into a coiled shell, with a ring of tentacles emerging from the opening. Only the shell survives. The soft parts decayed, which is why no ammonite fossil shows the animal itself, and why almost everything we say about how they lived is inferred from their living relatives.

Most were probably active hunters in open water. They were also prey: ammonite shells are sometimes found with bite marks left by mosasaurs, large marine reptiles of the same period.

How old are ammonites?

Ammonoids first appear in the fossil record in the Early Devonian, about 409 million years ago. They died out at the end of the Cretaceous, about 66 million years ago, in the same extinction event that ended the dinosaurs.

That matters when you buy one, because age depends entirely on where it was collected. The ammonites found in the shales of the Yorkshire coast are Jurassic, roughly 180 million years old. Specimens from other deposits can be far older or much younger. A seller who cannot say where a piece came from cannot really say how old it is either.

The Whitby snakestones

On the Yorkshire coast around Whitby, ammonites wash out of the Jurassic shales in large numbers, and for centuries local tradition called them snakestones. The legend held that they were snakes turned to stone in the time of Saint Hilda, the seventh-century abbess of Whitby. The coiled shell with no visible head made the story easy to tell.

Victorian carvers took it further. They would carve a snake's head onto the open end of the shell and sell the result to visitors, which is why genuine antique Whitby snakestones sometimes carry a small carved head that the fossil never had. Three ammonites still appear on the town's coat of arms.

The link runs into the science as well. Hildoceras, a genus of Jurassic ammonite common in the Whitby shales, is named after Saint Hilda.

How many species were there?

Over 10,000 ammonite species have been described, and some estimates run higher. The real figure is uncertain, and part of the reason is interesting.

Ammonites showed marked differences between the sexes, with small and large forms of the same species known as microconchs and macroconchs. Before that was understood, the two forms were often catalogued as separate species, so the historic species count is inflated by an unknown amount.

Species are told apart by shell shape and size, the number and spacing of the ribs, and features such as spines or thickened ornament, together with the age and location of the rock they came from.

How the shell was built

An ammonite started life with a tiny shell and added chambers to it as it grew, moving its body forward into each new chamber and sealing the one behind with a dividing wall called a septum.

Where each septum meets the outer shell wall it leaves a line called a suture, and in many ammonites these sutures are extraordinarily intricate, folded and refolded like a coastline. The complexity was not decorative. Folding increased the surface area where the septum joined the shell, which made the join far stronger while keeping the wall itself thin and light. The result was a shell that was both strong and buoyant.

Those sutures are often the most striking thing about a polished or sectioned specimen, because cutting the shell exposes the chambers and the pattern at once.

Why geologists use ammonites

Ammonites were abundant, they spread across the world's oceans, and individual species existed for relatively short spans of time. That combination makes them unusually good index fossils.

If the same ammonite species turns up in two beds of rock hundreds of miles apart, those beds can be treated as the same age. Much of the detailed timescale for the Jurassic and Cretaceous rests on ammonite zones, which is why they matter to geologists far more than their looks alone would suggest.

Caring for a fossil ammonite

A fossil shell is a mineral replacement, not the original material, and what it has been replaced by varies. Some are calcite, some aragonite, some pyrite. That changes how you should treat them.

Keep ammonites dry and dust them with a soft dry brush rather than washing them. Pyrite specimens in particular react badly to damp air and can break down over time, so they are better kept in a dry room than a bathroom or a conservatory windowsill. Avoid oils and polishes, which trap dust in the suture lines. Store specimens so they are not resting against each other, because the exposed chamber edges chip easily.

Collecting natural stone alongside fossils

If you collect ammonites for the structure and the pattern rather than any single stone, polished tumblestones scratch much the same itch: small natural pieces, each one different, chosen for colour and banding. They also survive handling far better than a fossil does, which makes them easier to keep somewhere you will actually look at them.

Polished tumblestones at Crystals Healing UK

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Frequently asked questions

How old is an ammonite fossil?

Ammonoids lived from about 409 million years ago to about 66 million years ago, so the age of any given fossil depends on the deposit it came from. Ammonites from the Yorkshire coast are Jurassic, around 180 million years old.

What is an ammonite, an animal or a shell?

Both. The animal was a cephalopod related to squid and octopuses. What survives as a fossil is only its coiled, chambered shell.

Why are they called ammonites?

Because the coiled shell resembles a ram's horn. Roman writers called them the horns of Ammon, after an Egyptian deity depicted with ram's horns, and the name stuck.

What are snakestones?

Snakestones is the old Yorkshire name for ammonites, from a Whitby legend that they were snakes turned to stone in the time of Saint Hilda. Victorian carvers sometimes added a carved snake's head to the shell before selling it to visitors.

Are ammonites rare?

Common ammonites are not rare at all, which is why they are affordable and widely collected. Value comes from size, how complete the shell is, how clearly the sutures show, and the quality of preparation.