What is an Ammonite?
Our general knowledge of today’s ocean is very limited, so information about what lived there millions of years ago is hard to come by. An exception to that rule is the ammonites. These extinct shelled cephalopods have been found in fossil form worldwide, often in large concentrations.
Ammonite fossils give us a fascinating glimpse into past ocean life. At the same time, the history of their discovery tells us a great deal about how fossils were perceived and classified in bygone eras.
At Fossils UK, we have a huge passion and admiration for everything prehistoric. Each discovery teaches us more about the world before we inhabited it and how conditions and creatures have evolved over time. Read on to learn all you need to know about the life, death and properties of ammonites.
Table of contents
- What were ammonites?
- What is inside an ammonite?
- How did ammonites live?
- The evolution of ammonites
- Why did ammonites go extinct?
- Ammonites at Fossils UK
What were ammonites?
Before accurate descriptions and understandings of ammonites were developed, many believed they were coiled-up snakes that had turned to stone after death. This assumption led to their being referred to as ‘snakestones’. Despite their reptile-like appearance, ammonites were actually cephalopods that lived in the ocean.
The closest living relatives to ammonites are coleoids, including squids, octopuses and cuttlefish. With over 10,000 species found from almost every corner of the globe, as posited in the Swiss Journal of Palaentology (Klug, Schweigert, Tischlinger, Pochmann, 2021), ammonites were once among Earth’s most diverse and populous animals.
As they developed, ammonites grew their initially tiny shells by building chambers on top of them and moving their whole bodies into entirely new chambers. Their old shells were sealed off with walls known as sepia. It is not known for certain how often ammonites moved their shells.
What is inside an ammonite?
The coiled nature of ammonite fossils means the appearance of the animals beyond the shell is initially hard to decipher. This is made more allusive due to the previously mentioned layers of chambers that the ammonites grew into through their lifetime. An ammonite’s body was contained within the largest and final shell, the head or living chamber.
Tentacles would extend from this head chamber to catch prey, as ammonites were natural ocean predators. While we aren’t certain, many have speculated that the opening of the shell that the tentacles would emerge from was covered by a protective shield that could open and close for opportunistic preying and protection from other predators.
The chambered interior of an ammonite shell is known as the phragmocone. While ammonites were alive, the phragmocone contained gasses which controlled the ammonite’s buoyancy in the water. The chambers were connected by a small tube called the siphuncle.
How big were ammonites?
One of the most fascinating facts about ammonites is the fluctuation in size. Ammonites could range from just a few millimetres to several metres in diameter. The larger ammonites were more prevalent from the Late Jurassic period onwards.
Parapuzosia seppenradensis was the largest ammonite species in the Late Cretaceous period. A tongue twister to say and a jaw dropper to see, the largest ever found was 1.8 meters, but it was incomplete. Were this fossil complete it could have reached 2.5-3.5 metres in diameter.
How did ammonites live?
Various species of ammonites lived long and fascinating lives throughout several prehistoric eras, making them a scientific marvel. While stating that ammonites lived in the ocean sounds broad and non-specific, this is because they were so widespread worldwide that it’s difficult to pin them to exact locations.
However, we are closer to determining the type of water ammonites would reside in. They were much more likely to live in shallow seas with a maximum depth of roughly 400 metres. Ammonites didn’t venture deeper because their shell structures likely couldn’t withstand the water pressure beyond a certain depth.
What did ammonites eat?
A huge part of any being’s life cycle is its diet, especially extinct animals. A creature’s feeding habits can tell us a great deal about their environment, what other animals surrounded them, their nutritious needs and where they sat on the food chain. Ammonites were predators, but due to the vast spectrum of their size, their diet is unlikely to have been consistent.
Larger ammonites would likely have preyed upon similar creatures as living cephalopods, like crustaceans, fish and bivalves. Plankton would likely have been on the menu for smaller ammonites. While there was plenty for ammonites to feed on, other marine animals would consider them prey. Evidence has shown that extinct marine reptiles, including mosasaurs and ichthyosaurs, would have eaten them.
The evolution of ammonites
Ammonites first appeared around 450 million years ago as the last of an ancient group of creatures called the ammonoids. Most ammonoids were extinct by the Jurassic period, leaving ammonites as the only living representative of the group.
The ammonite’s signature shell structure was a significant evolutionary change from the small planktonic organisms they began as. These shells and chambers protected ammonites from predators and the continued development saw ammonites get larger in a short space of time. Some evidence suggests that the females were up to 400% larger than males at one period.
During their long life, ammonites were no strangers to a close shave. The shelled creatures became survival specialists as they avoided catastrophes that wiped out other species, like oxygen-starved oceans and the disastrous impacts of climate change. Ammonites were extinct around 66 million years ago, at the end of the Cretaceous period.
Why did ammonites go extinct?
The asteroid that wiped out virtually all life on Earth hit at the end of the Cretaceous period. Ammonites felt the impact more in the aftermath than at the point of impact, as the harsh winter lacked photosynthesis. This lack of light and energy eliminated all food sources on land and in the ocean, leaving ammonites and other creatures to starve.
Research had suggested that this was somewhat of a merciful end to a species in decline with no likely survival in sight. However, recent research published in Nature Communications (2024) disputes this. It indicates that ammonites were cut short of a long and fruitful existence.
The article suggests that specific fossil records may have biased previous findings, and the researchers wanted a broader understanding. After analysing a vast new database of ammonite fossils from the period alongside previous findings, the scientists found that speciation and extinction rates varied in different regions.
Due to the sheer amount of ammonites worldwide, these creatures’ climates differed. Ammonites in different areas had to evolve in many ways to adapt to their specific environment, filling several ecological roles.
This constant evolution, alongside the survival skills shown during rising sea levels and dropping temperatures, suggests that the ammonites still had a long life before the devastating end of the Cretaceous period.
Ammonites at Fossils UK
You can learn even more about these fascinating prehistoric creatures by viewing them with your own eyes. Our collection of ammonite fossils at Fossils UK celebrates the life of one of the Earth’s most interesting and historically significant animals.
Choose ethically sourced fossils hand-selected directly from finders, collectors, dig sites and mines in Britain and worldwide. Place a stunning cut and polished ammonite fossil proudly on your desk, bookshelf or mantelpiece for everyone to admire.
Calcite Ammonite 
Jurassic Ammonite
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