Ammonite Fossils have long captured people’s imaginations, but these distinctive spiral shells’ true nature and origin were once a mystery. One of the earliest explanations for ammonites was that they were fossilised snakes, dubbed ‘snakestones’. But as it turns out, cephalopod molluscs were the trustworthy source of these fossils.

In the late 1800s, Karl Alfred von Zittel helped classify these creatures, discovering that their shells were once inhabited by sea-dwelling molluscs. Ammonites are a fascinating group of extinct marine molluscs that first appeared during the Devonian period and continued until the end of the Cretaceous–Paleogene extinction event.

What’s most surprising about Ammonites is that they are more closely related to living coleoids, such as octopuses, squid, and cuttlefish than to shelled nautiloids like the living Nautilus species. Ammonites belong to the Ammonoidea subclass of Cephalopoda, a larger group of animals that also includes squid and octopuses.


Ammonites were incredibly diverse creatures with a wide range of sizes and shapes. Some of these prehistoric molluscs were tiny, measuring less than an inch in diameter, while others were truly massive, with shells that could grow up to eight feet wide. While most ammonites had coiled shells, there were also species with long, straight shells, making them look quite different from their coiled brethren.

Inside the spiral shells, there were a series of increasingly larger compartments separated by delicate walls known as septa. To move through the water, ammonites used a siphuncle, a thin tube that allowed air to flow through the inner chambers of their shells. 

Diet and habitat 

During the Mesozoic epoch, Ammonites were abundant in the seas. Ammonites lived in warm, shallow waters most of their lives and are believed to have had a carnivorous diet. Ammonites inhabited the areas just above the seafloor, where oxygen levels were too low for other animals to live. This was due to their floating and free-swimming behaviour.

Ammonites were stealthy hunters who stalked their prey before grabbing it with their tentacles and consuming it with their powerful jaws. They had sharp, beak-like mouths that helped them trap prey such as plankton, crabs, molluscs, fish, and cephalopods.

Myths and legends

Gods and deities 

Ammonites owe their name to the ancient Egyptian god Amun, also known as Ammon in Greek mythology. Amun was often depicted with ram's horns, which may have led to the association with the spiral-shaped shells of ammonites. However, it is interesting that Ammonite Fossils are scarce in Egypt. It is more likely that the shells of giant snails found in the Eocene Limestone of Mokattam, near Cairo, inspired the name.

The Greek word 'keras', meaning horn, also features in the names of many Ammonite genera, further emphasising the connection with Amun's distinctive horns.

Protection from snakes and reptiles

Ammonites, with their spiral form, have often been compared to snakes throughout history. Early works of natural history dubbed them as "snakestones" due to their resemblance to serpents. This connection between Ammonites and snakes dates back to medieval times in Europe, where ammonites were believed to be fossilised snakes.

According to lore, Saxon Abbess Saint Hilda was tasked with removing snakes from her village in the early 1600s. She cast a spell that turned the snakes to stone and threw them off cliffs. This myth became so widely known that local collectors and dealers in fossils began carving snakeheads into ammonites to perpetuate the legend.

St Hilda from Whitby, England

In medieval England, Ammonites were considered a protective charm against snakes and were often worn by those who feared the reptiles. The snakestone legend is mainly associated with the town of Whitby, which was home to the Anglo-Saxon abbess St Hilda. Three 'snakestones' are included in the town's coat of arms, symbolising the town's connection to the legend.

Frequently Asked Questions

Ammonites lived during the Jurassic and Cretaceous periods, spanning an extensive period of approximately 140 million years. The Jurassic period commenced roughly 201 million years ago, and the Cretaceous period ended about 66 million years ago. Ammonites did not survive the end of the Cretaceous, and their extinction coincided with the disappearance of dinosaurs.

Ammonites varied greatly, some less than an inch long and others over nine feet in diameter. Their shells were typically coiled, with chambers increasing and separated by thin walls. However, some Ammonites had long, straight shells. What’s incredible is that the organism living inside the shell grew more shell material throughout its life and always lived in the outside chamber. Experts believe that Ammonites used a thin tube, known as a siphuncle, to allow air to flow through the inner chambers of the shell, which helped them move through the water.  In terms of appearance, it is thought that ammonites resembled modern cephalopods, with soft body tissue, tentacles, and possibly sharp, beak-shaped jaws used for hunting prey.

Cephalopoda is a class of bilaterally symmetrical and exclusively marine animals. Among the cephalopods, ammonites and nautiluses are distinct groups of molluscs belonging to sub-classes Ammonoidea and Nautiloidea, respectively.

Ammonites were early cephalopods that emerged during the Devonian period and went extinct during the Cretaceous–Paleogene extinction event. Due to their characteristic spiral shape, they are visibly distinguishable from nautiloids, and palaeontologists highly prize their fossils.

On the other hand, Nautiluses are still extant today and are easily recognisable due to their distinctive spiral shells. They first appeared during the Cambrian period; approximately 30 species are known today. Although Ammonites and Nautiluses may seem similar at first glance, they differ significantly, particularly in their evolutionary history.

Ammonites were fierce predators during their time, known for their voracious appetite for various marine creatures such as molluscs, fish, and even fellow cephalopods. They were believed to silently stalk their prey before rapidly extending their tentacles to grasp them, similar to how modern cephalopods hunt.

While the image of these beautiful creatures gliding effortlessly through the ancient seas is enticing, fossil evidence reveals that they were also prey themselves. The bite marks on ammonite shells left by mosasaurs indicate that they, too, fell victim to the violence of their environment.

The shell of an ammonite can reveal a lot about these creatures that roamed the sea in ancient times. The spiral shell, composed of linked chambers, housed the Ammonite’s body within the final open section called the head chamber. Tentacles would extend from this chamber to catch prey.

As the Ammonite grew, new chambers were added behind the head chamber, forming the phragmocone. This chambered interior carried gasses allowing for buoyancy regulation within the water column. The Siphuncle, a small tube, links the chambers. Intricate patterns, called sutures, can also be found on some Ammonite shells beneath the external shell wall. 

In Medieval Europe, Ammonites were known as snakestones because they were believed to resemble petrified curled-up snakes. Interestingly enough, this belief was perpetuated by a legend about St Hilda, a 7th-century Saxon abbess of Whitby, who was said to have rid the area of snakes by turning them into stone.

It wasn’t until the Victorian era that people began carving heads onto these fossils to fetch a higher price. Today, the ammonite variety commonly found around Whitby is named Hildoceras after St Hilda.

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