Learn about Ammonites
Ammonites
The architecture of vanished seas.
Ammonites were extinct, shell-bearing cephalopods whose fossils occur in rocks around the world. Their shells unite biological function, geological record and visual order—one reason they remain among the most immediately recognisable forms in natural history.
The Animal
Relatives, not ancestors
Ammonites were not the ancestors of modern squid and octopuses. They formed an extinct branch of the cephalopod family tree and are thought to have been more closely related to coleoids—the group containing squid, octopuses and cuttlefish—than their resemblance to a living nautilus might suggest.
Ammonoids first appeared in the Palaeozoic, while the animals usually called true ammonites became especially diverse during the Jurassic and Cretaceous. Most disappeared in the mass extinction at the end of the Cretaceous, approximately 66 million years ago.
Their scale varied enormously. Some shells measured only millimetres across; the largest confirmed specimen is incomplete but approximately 1.8 metres in diameter. Between those extremes evolved a remarkable range of ribs, keels, spines, apertures and coiling patterns.
A shell built through growth
The familiar spiral was not simply decoration. It was a growing chambered structure that supported and recorded the life of the animal.
Where the animal lived
The soft body occupied the open final chamber. As the ammonite grew, it extended the shell forward and left its earlier chambers behind.
A structural record
Internal walls called septa divided the chambers. Where they met the shell, they produced suture patterns that help identify and classify ammonites.
Hydrostatic balance
A narrow tube connected the chambers and helped regulate their liquid content as the shell grew. How this supported fine buoyancy control remains an active subject of research.
The spiral is compelling because growth remains visible: every whorl is both structure and history.
Form and Meaning
One outline, extraordinary variation
Many ammonites formed a flat, coiled shell known as a planispiral. Yet the group was far less uniform than the familiar silhouette suggests. Heteromorph ammonites evolved open coils, hooks, towers and irregular forms that can appear almost experimental.
People noticed these forms long before modern palaeontology. The name ammonite derives from the coiled ram’s horns associated with the ancient Egyptian deity Amun. In the Northern Plains, ammonite fragments known as iniskim have longstanding ceremonial importance within Blackfoot traditions, where their shapes are associated with bison.
These histories are distinct, but both reflect the same underlying fact: ammonites possess a form that repeatedly invites recognition and interpretation.
Cultural context: Alberta Historic Places — Rainbow Fossils and Bison Calling.
Southern Alberta
The iridescent ammonites of the Bearpaw Formation
Some Late Cretaceous ammonites from the Bearpaw Formation retain layers of their original aragonite shell. When light interacts with this finely layered structure, it produces shifting colour—from red and green through rarer blue and violet. The effect is structural rather than painted or caused by an ordinary pigment.
Gem-quality material derived from these shells is known as ammolite. Burial, compression and later geological alteration influenced how the shell survived. In some specimens, compression and natural healing created the fractured, mosaic-like appearance for which Alberta ammolite is especially known.
An intact iridescent ammonite is more than a source of gemstone material. It retains the relationship between shell, animal and matrix—the fossil as a coherent natural-history object.
Gemological context: Gemological Institute of America — Ammolite from Southern Alberta.
The Collector’s Eye
The category name is only the beginning.
Ammonites range from abundant decorative fossils to materially and scientifically exceptional specimens. Quality depends on how the particular object brings together form, preservation and record.
Shape and legibility
Completeness, symmetry, ornament and the relationship between shell and matrix determine how clearly the specimen reads.
What has survived
Original shell, mineralisation, surface texture and natural colour should be distinguished from polishing, repair and reconstruction.
What can be supported
Taxonomy, locality, geological age, preparation, restoration and provenance give the object its necessary scientific and collecting context.
Explore
The ammonite collection
Our changing selection includes ammonites chosen for preservation, locality, shell material, sculptural form or exceptional presence.