Ammonite Animal Facts: Discover An Amazing Prehistoric Ocean Animal

Ammonite facts, pictures and information.

Ammonites are prehistoric marine mollusks with distinctive curled shells. Once abundant in Earth’s oceans, ammonites – along with the dinosaurs and many other species – became extinct during the Cretaceous-Paleogene Extinction Event that occurred around 66 million years ago.

Ammonites were cephalopods, and are related to living cephalopods such as octopuses and nautiluses.

Read on to find out more about these once-abundant prehistoric animals…

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What Is An Ammonite?

An ammonite is a prehistoric marine animal belonging a now-extinct group of mollusks known as Ammonoidea. Specifically, ammonites are ammonoids of order Ammonitida, but the term ammonite is also used to refer to all ammonoids.

(On this page we’ll use the more specific definition unless specified.)

A typical ammonite fossil.

Ammonites belonged to the mollusk group Cephalopoda, and are related to living cephalopods such as octopuses, squid and nautiluses.

Ammonites had flat, coiled external shells. They ranged in size from 3mm / 0.12 in. to over 3m / 10 ft. in diameter.

When we think of ammonites, we usually picture just those tightly-coiled shells, because in most cases these are the only body parts to have become fossilized.

When living, however, the ammonite’s head and tentacles would have projected from the end of the curled shell.

The shells of some ammonite species were heavily ridged, while others were smooth. Some ammonite shells had bumps and spines.

Ammonites were named after the Greek god Ammon, because their shells resemble the ram’s horns with which the god is often depicted.

Ammonites were abundant in Earth’s prehistoric oceans from the early Jurassic Period up to the latter half of the Cretaceous Period.

Ammonites were abundant in many parts of the world. Ammonites are among the most common types of fossil.

Ammonites were free-swimming animals that probably fed on plankton, marine vegetation and slow-moving marine animals.

They swam by forcibly expelling water from their bodies – pushing themselves through the water. This means of jet propulsion is still used by modern cephalopods such as octopuses and nautiluses.

When Did Ammonites Live?

Ammonoids (the group of animals that includes ammonites and other, related animals) first appeared around 409 mya (million years ago), during the Devonian Period of the Paleozoic Era.

Ammonites appeared in the Jurassic Period, which began around 201.3 mya.

Ammonite Fossil
Ammonites appeared in the Early Jurassic Period.

When Did Ammonites Become Extinct?

Ammonites became extinct, along with the dinosaurs and 75% of all life on Earth, during the Cretaceous-Paleogene Extinction Event, which occurred around 66 mya.

This global extinction event marked the end not just of the Cretaceous Period, but also of the entire Mesozoic Era.

The number and diversity of ammonites had already begun to decline in the second half of the Cretaceous Period.

The ammonites left no direct descendants. Their extinction marked the end of the subclass Ammonoidea.

Paleocene Ammonites

There is some evidence to suggest that certain ammonites (most notably those of genus Scaphites) may have survived the Cretaceous-Paleogene Extinction Event and lived several million years into the Paleocene epoch.

Ammonites have been found in rock formations dating to the Paleocene in Turkmenistan in Central Asia, and in the Tinton Formation in New Jersey.

The presence of ammonites in the Paleocene remains controversial due to the small amount of fossil evidence.

An Ammonite’s Body

Fossil Ammonites

An ammonite’s shell is divided into numerous chambers. This both strengthens the shell and provides a means for the ammonite to swim at the correct depth.

The ammonite’s soft body inhabited only the last chamber. As the ammonite grew, new, chambers were continually added to the front (open) end of the shell. The ammonite would then move into the new chamber.

The ammonite’s head, beak-like jaws and tentacles projected from the open end of the last chamber.

A small tube – known as a siphuncle – ran through the chambers of an ammonite’s shell. The ammonite could alter the amount of gas in the chambers via this tube, thereby controlling its buoyancy and the depth at which it swam.

The wall between each chamber is known as a septa. Lines known as sutures mark where each septa joins the ammonite’s outer shell.

An ammonite can be distinguished from other ammonoids by the ornate patterning of its sutures.

Female ammonites were larger than males, probably to allow the production of large quantities of eggs.

This size difference led paleontologists to erroneously classifying males and females of the same species as being two different species!

Ammonites Used As Index Fossils

Paleontologists study ammonites not only to find out about the animals themselves, but also to find out more about other species.

Because ammonites were so abundant, the numbers and types of ammonites present in a particular layer of rock gives a good indication of when that rock was formed – and, therefore, when other fossils found in that rock were likely to have been formed.

Fossils that can be used to date other fossils in this way are known as index fossils.

One of the pioneers of this method of dating rock layers was English geologist William Smith (1769 –1839). The self-educated Smith’s work led to his recognition as the “Father of English Geology”. (Source)

Common Octopus
Ammonites are the extinct relations of living cephalopods such as the common octopus, above.

Ammonites are mollusks (molluscs in British English) in the subclass Ammonoidea, which itself is part of the class Cephalopoda – a group of mollusks whose living members include octopuses, squid, cuttlefish and nautiluses.

Today, two subclasses of Cephalopoda remain: Coleoidea (octopuses, squid, and cuttlefish); and Nautiloidea (nautiluses and related animals).

Although ammonites resemble nautiluses, which today are found in the coral reefs of the Indo-Pacific, they are more closely related to the other branch of living mollusks: Coleoidea.

Types Of Ammonite

Over 10,000 species of ammonite have been found. Below are examples of ammonites from the Jurassic and Cretaceous Periods.


Dactylioceras commune Ammonite
Dactylioceras commune. Photo: James St. John (jsj1771), (cropped / resized by CC BY 2.0

Dactylioceras is a genus of ammonite that lived in the Early Jurassic Period. It was widespread and abundant, with specimens having been found on most continents. It was likely a slow-moving scavenger.

At least 50 Dactylioceras species have been identified. Most were around 4 in. / 10 cm in diameter


Stephanoceras Ammonite
Ammonite of genus Stephanoceras

Stephanoceras was a genus of ammonite from the Middle Jurassic. It can be identified by its relatively large size (specimens with diameters of up to 10 in. / 27 cm have been found), and by its ridges and bumps.


Mortoniceras inflatum Ammonite
Mortoniceras inflatum. Photo: H. Zell, (cropped / resized / background enlarged by, CC BY-SA 3.0

Mortoniceras is a genus of ammonite from the Early Cretaceous Period. Specimens, which can be up to 12 in. / 30 cm in diameter, have been found in Europe, Asia and both North and South America.


Ammonite of genus Scaphites
Ammonite of genus Scaphites. Photo by: Hectonichus, (cropped/resized by CC BY-SA 3.0

Scaphites is a genus of ammonite that lived in the Late Cretaceous Period. It can be recognized by its unusual, hook-like shape. In its early stages it grew in the usual curled shape, but later in its life its shell straightened out, before curling once more to form a hook.


Mantelliceras tuberculatum ammonite species
Mantelliceras tuberculatum. Photo by: Hectonichus, (cropped / resized / rotated by, CC BY-SA 3.0

Mantelliceras is a relatively large (diameter up to 5 in. / 13 cm) ammonite genus that lived during the Late Cretaceous. Specimens have been found in Europe, Asia (Japan), and North America.  


Ammonite of genus Parapuzosia
Ammonite of genus Parapuzosia. Photo by: Ghedoghedo, (cropped/resized by CC BY-SA 3.0

Parapuzosia is a genus of ammonite that lived during the Late Cretaceous. It contains Parapuzosia seppenradensis, the largest-known ammonite, with specimens of diameter 11 ft. / 3.5 m having been discovered.

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