The Ultimate Guide to Ammonites: Coiled-Shell Cephalopods

Leave a comment / / Updated on: 13th November 2023

NameAmmonite DietPredators, Filter-feeders and Scavengers 
Name MeaningAmmon’s hornHeightN/A
Pronunciationa-muh-niteLength0.01–2 meters (0.033–6.6 feet)
EraPaleozoic to MesozoicWeightUp to 100 kilograms (220 pounds) 
ClassificationMollusca, Cephalopoda, AmmonoideaLocationWorldwide

Ammonite Pictures

Ammonite | estt via iStock

The Ammonites

Gage Beasley Prehistoric's Ammonite Concept
Gage Beasley Prehistoric’s Ammonite Concept

Ammonites were once the most successful and diverse group of animals in Earth’s marine ecosystem. 

More than 10,000 species of these shelled cephalopods have been identified so far from virtually all continents. 

The name “ammonite” is commonly used to refer to all animals in the Ammonoidea subclass. 

The earliest members of this group evolved during the Devonian Period, about 408 million years ago, and they thrived throughout the Paleozoic and Mesozoic eras. 

Ammonites are closely related to living cephalopods such as nautilus, squids, cuttlefish, and octopuses. 

Despite their superficial similarities to the coil-shelled nautilus species, ammonites are more closely related to the coleoids (octopuses and squids). 

Antique animal illustration of Duncan Ammonoid | ilbusca via iStock

Their name was inspired by the spiral-shaped shells, which resemble the coiled horns of a ram.

The name translates as “horns of Ammon,” referring to the Egyptian god Ammon, who is often represented with ram horns.

Ammonites are famous in the scientific world due to their use as index fossils and their significance in unraveling ancient ecosystems and changes in Earth’s geological past. 

In this post, we will explore some of the most fascinating details about ammonites, including their physical characteristics, ecological role, and cultural and scientific significance. 

Gage Beasley's Prehistoric Shirt Collection
Gage Beasley’s Prehistoric Shirt Collection
Gage Beasley's Prehistoric Plush Collection
Gage Beasley’s Prehistoric Plush Collection

Physical Characteristics

Close-up of an ammonite fossil on a grey slate background | PaulaConnelly via iStock

Ammonites were marine mollusks famous for their coiled shells. 

Their shell consisted of multiple chambers divided by walls known as septa. 

These chambers were filled with gas or fluid, which helped control the animal’s buoyancy. 

The last chamber, called the living chamber, housed the soft body of the ammonite.

Ammonites were quite diverse in terms of their sizes and general body structure. 

They ranged from small, thumbnail-sized species to giants that measured several feet in diameter.

The largest known species found so far is Parapuzosia seppenradensis

Gage Beasley Prehistoric's Ammonite Size Comparison Charts
Gage Beasley Prehistoric’s Ammonite Size Comparison Charts

This Late Cretaceous ammonite had a diameter of about 2.5 to 3.5 meters.

The smallest ammonoids were the Maximites

They lived during the Carboniferous Period, and adults grew to a maximum shell diameter of 10 millimeters (0.39 inches).

Little is known about the soft body of this prehistoric cephalopod since it is rarely preserved in the fossil record. 

Only in one instance has the fossilized imprint of the ammonite’s body been preserved as fossils. 

It had tentacled arms, but it isn’t clear if the arms were strong and muscular like those of modern coleoids or thin and delicate like nautiluses. 

Ammonites of the species Dactylioceras athleticum from the Lower Jurassic of Southern Germany | weisschr via iStock

One of the most distinctive features of ammonites was the intricate suture patterns on their shells. 

These suture lines represented where the septa connected with the outer shell wall. 

The wavy or frilly patterns varied from one species to the other. 

Experts think these sutures strengthened the ammonite shells. 

Ammonite shells also featured various forms of ornamentation, such as ribs, nodes, and spines. 

These structures also varied from one species to the other and provided added structural support for the shells. 

The chambered part of the ammonite’s shell is also referred to as the phragmocone. 

An ammonite that exhibits gorgeous mineralization within the chambers, each separated by white calcite scepta (divisions) | Dan Olsen via iStock

It consists of a series of large chambers known as the camerae. 

Each camera is divided from the next one by a thin wall (septa).

The last chamber is the largest and is also referred to as the body chamber because that’s where the animal itself lives. 

As the ammonite grew, newer and larger chambers were added to the open end of the coil. 

A thin tube (siphuncle) extended from the ammonite’s body through the empty shell chambers. 

The ammonite used this tube to vary the fluid pressure within the shell and control its buoyancy. 

Habitat and Distribution

Ammonites have a global distribution | PeterPencil via iStock

Ammonites were prolific marine creatures that inhabited Earth’s oceans during the Paleozoic and Mesozoic eras. 

They had a global distribution, with fossils found on every continent, including Antarctica. 

Mesozoic ammonites are generally more popular. 

They’re known from regions that were once part of the supercontinent Pangaea, which includes North America, Europe, Asia, and South America.

Ammonites occupy various marine habitats, depending on the species and their ecological niche.

They mostly inhabited shallow coastal waters and continental shelves, but a few species lived in deep environments. 

The maximum depth inhabited by these mollusks didn’t exceed 400 meters.

A few preferred soft, muddy substrates, but most ammonite species were free-swimming, meaning they lived in open ocean waters and coral reefs. 

Behavior and Diet

Antique illustration of Ammonite | ilbusca via iStock

Ammonites were free-swimming marine cephalopods. 

They moved around by changing the amount of fluids present in their chambered shells through a hyperosmotic active transport process

All ammonites had a thin, tube-like structure known as a siphuncle that ran along the outer rims of their shells. 

This little siphon helped to pump fluid through the shell chambers, and this provided the buoyancy needed to move the ammonites through the water. 

It isn’t clear whether the ammonites were fast and efficient swimmers and whether their tentacled arms helped them move through the water. 

Living nautiloids have siphuncles, too, but they run through the center of their shell chamber and not along the outer rims like that of the ammonites. 

Ammonites were solitary animals, with no evidence to suggest that they exhibited any form of advanced social behavior. 

Individuals likely interacted occasionally, especially during mating and spawning events. 

Fossilized groups of ammonites have been found in some locations, but these are probably aggregations for breeding or other activities.

Reconstruction of an Ammonites Dactylioceras commune | CorbalanStudio via iStock

Ammonites were primarily carnivorous predators. 

They had a beak-like structure similar to that of modern squids and octopuses, which they used to crush and consume prey. 

Their diet likely consisted of small marine organisms, including small fish, crustaceans, and other mollusks.

They had tentacles which may have been lined with tiny hooks. 

These appendages were likely used to capture and immobilize prey. 

Once a prey was ensnared, the ammonite would then use its beak to crush and consume it.

As free-swimming animals, ammonites were capable of swimming around in the water column in search of food. 

They probably relied on their swimming abilities to pursue and catch prey. 

Some ammonite species may have been opportunistic scavengers, too. 

This means they fed on the carcasses of larger marine organisms that had sunk to the ocean floor.

Life Cycle

Computer-generated 3D illustration with an ammonoid | MR1805 via iStock

Since soft body parts are hardly preserved in the fossil record, the exact mechanism for reproduction in ammonites isn’t known. 

Experts can only make deductions based on the lifecycle of living cephalopods.

Ammonites reproduced sexually since they had distinct male and female individuals. 

Males of some species have been known to possess long prongs (lappets) that stuck out of their shell openings.

They used these prongs to hold onto the females while mating, similar to how shark claspers work. 

Fertilization was external, with males and females releasing games into the water. 

Males held on to the body of females to increase the chances of successful fertilization. 

It’s also possible that some ammonite species exhibited hermaphroditic behavior, with individuals having both male and female reproductive organs.

Early ammonites produced a small number of offspring, but later forms were explosive breeders. 

Oso Ammonoids Ammonite | MR1805 via iStock

Producing a large batch of eggs was probably the key to the rapid proliferation of ammonoids and their ability to survive through the extinction events that occurred during the Paleozoic and Mesozoic eras. 

Once fertilized, ammonite eggs were probably attached to the substrate or floated freely in the water column. 

Ammonite juveniles had very small shells. 

They grew by forming new chambers in their coiled shells. 

The new chambers were typically added to the open section of their shells. 

As the ammonite grew, a new, larger chamber would be created while the old one would be sealed off with septa. 

This created their characteristic spiral shape and the sutures on their shell.

The size and shape of the shells changed as they aged, but the final, largest chamber always contained the living animal.

Evolution and History

Ammonite fossil on a dark background | Helmut Feil via iStock

Ammonites had one of the longest evolutionary histories of all of Earth’s prehistoric animal groups. 

They were around for about 350 million years, with an existence that spanned across two eras and several geological periods. 

The earliest ammonites appeared during the Devonian Period, around 400 million years ago. 

The ammonoids reached the peak of their diversity during the Jurassic Period, with the emergence of the true ammonites (suborder Ammonitina) about 200 million years ago. 

The group eventually went extinct about 65 million years ago during the Cretaceous-Paleogene extinction. 

The mold of an ammonite | Andyworks via iStock

The earliest ammonites were straight-shelled cephalopods that gradually developed a characteristic coiled shell as they evolved.

Their shell structures became more complex and tightly coiled as they evolved during the Paleozoic and Mesozoic eras. 

The early ammonites of the Paleozoic era had simple suture lines across their shell wall. 

Later forms that emerged during the Mesozoic had sutures that formed intricate patterns. 

Experts think the increasing complexity of the ammonite shells may have given them greater buoyancy control compared to their predecessors. 

Ammonites were the ultimate survivors of the Paleozoic and Mesozoic eras. 

Mass extinction in the past | Denis-Art via iStock

They survived three mass extinctions, diversifying further after each one. 

Although only a few species survived each event, this small number often diversified into various forms within a short period. 

Ammonites became less abundant in the second half of the Mesozoic Era. 

They came to the end of their long reign about 66 million years ago, during one of Earth’s biggest and most recent extinction events. 

The same event wiped out the dinosaurs and several other groups of prehistoric organisms. 

The extinction of the ammonites has been attributed to the sudden disappearance of marine plankton, which was their main food source.

Interactions With Other Species

Ammonite found in an ancient seabed | GoodLifeStudio via iStock

Ammonites played diverse roles in ancient marine ecosystems. 

While some species were active predators, others may have been scavengers or filter feeders.

The size, body form, and specific roles of the different species of ammonites determined how they interacted with other species within their ecosystem. 

The scavenger species probably ate small plankton, vegetation growing on the seafloor, and other forms of organic matter that were available to them. 

Many ammonite species, especially the larger ones, preyed on smaller, slow-moving species in their habitat. 

Their typical prey may have included foraminifera, ostracods, small crustaceans, brachiopods, corals, and bryozoa. 

They used their tentacles to capture and immobilize prey, while their beaks helped to crush and consume it.

Ammonites in the permanent collection of The Children’s Museum of Indianapolis | The Children’s Museum of Indianapolis via Wikipedia CC BY-SA 3.0

As prey, larger marine organisms prey on ammonites. 

The marine reptiles like the ichthyosaurs and plesiosaurs were the major predators of ammonites. 

These reptiles had adaptations for capturing and consuming these coiled mollusks, which formed the bulk of their diet.

Several ammonite shells have been discovered, with visible triangular holes on both sides of the shell. 

Initially, this was interpreted as limpets that attached themselves to the surface of the shells. 

However, more recent studies suggest that these holes correspond to the upper and lower jaws of a medium-sized mosasaur that preyed on these ammonites. 

Ammonites may have competed against members of their species and other marine organisms sharing similar ecological niches for resources such as food, space, and breeding territories.

Cultural Significance

A collection of pyritised ammonite fossils | Blackbeck via iStock

Ammonites are among the most recognizable fossil animals due to their distinctive spiral shape. 

Even before they were studied scientifically, rocks with ammonite remains were recognized and considered very special in various ancient cultures. 

In medieval Europe, for instance, ammonite fossils were thought to be petrified coiled snakes and were referred to as “snakestones” or “serpent stones.”  

These stones were considered sacred evidence of the acts of saints. 

Consequently, healing and oracular powers were ascribed to them. 

To enhance their appearance, artists and traders sometimes carved or painted the wide end of the ammonite fossils to make them look more like petrified snakes. 

A trader examining an ammonite | GoodLifeStudio via iStock

In some other cultures, ammonites were believed to be the fossilized dung of worms. 

These also had spiritual powers and were used in religious rituals to ward off witches. 

In various Asian cultures and religions, ammonites were believed to be concrete manifestations of gods like the Hindu god, Vishnu.

Ammonite fossils were also regarded as symbols of time and continuity due to their spiral shape. 

In some cultures, they represented the cyclical nature of life, death, and rebirth.

These rocks were also used as decorative elements in architectural designs, jewelry, and other ornamental purposes, particularly in regions where they were abundant.

Since the 1850s, ammonites have become very important to geologists because of their stratigraphic significance. 

A group of fossil ammonites in a slab of rock | Gannet77 via iStock

Geologists use them as index fossils to establish the relative ages of different sedimentary formations. 

Their usefulness as index fossils is due to how rapidly they evolve, their widespread geographical distribution, and their abundance in the fossil record.

Ammonite fossils are also considered important environmental Indicators. 

The abundance of ammonite samples in a particular area indicates the presence of a prehistoric sea in that region.

Their presence or absence in ancient marine sediments can also provide information about past environmental conditions, including water depth, temperature, and salinity.

For instance, the diversity of abundance of ammonite fossils tends to change after major extinction events. 

This makes the species very important to scientists trying to understand the broader impact of such events on Earth’s ecosystems.


Ammonites are a class of marine invertebrates that thrived in Earth’s prehistoric seas during the Paleozoic and Mesozoic eras. 

They first emerged about 408 million years ago and survived up to three major extinction events before going extinct about 65 million years ago. 

Ammonites are closely related to modern cephalopods like the cuttlefish, octopus, squid, and nautilus. 

Their unique, chambered shell shapes make them one of the most recognizable fossil species. 

This distinct appearance also makes them attractive to collectors.

Scientists use ammonites as index fossils to determine the relative age of rocks and reconstruct the geography and nature of the earth’s ecosystem millions of years ago. 


Where are ammonite fossils usually found? 

Ammonite fossils are found on virtually all continents on the planet, including Antarctica. 

Some of the most notable ammonite fossil sites include the Jurassic Coast of England, the Jura Mountains of France and Switzerland, the Alps of Europe, and the Rocky Mountains of North America.

How much is an ammonite fossil worth?

Collectors value ammonites based on their size, rarity, and uniqueness of their sutures. 

The largest ammonites can cost as much as $1,000, but most ammonite fossils are cheap, only costing a few dollars. 


About The Author

Leave a Reply

Discover more from Gage Beasley Prehistoric | Recapping Timeless Creatures

Subscribe now to keep reading and get access to the full archive.

Continue reading

Scroll to Top