Cleoniceras Ammonites: Ancient Spirals of the Sea

Polished half of a Cleoniceras Ammonite

Cleoniceras ammonites are some of the most striking and beautifully preserved fossils in the world. These ancient marine creatures lived over 100 million years ago, during the Cretaceous period, when much of the Earth was covered in warm, shallow seas. The Cleoniceras genus is known for its elegant spiral shape and beautiful mineral inclusions, making it a favourite among fossil collectors.

Artist impression of an Ammonite

Found primarily in Madagascar, Cleoniceras ammonites are carefully extracted from layers of limestone and shale that were once the floor of a prehistoric ocean. Madagascar’s hot, dry climate and stable geology have helped preserve these fossils in exceptional detail. Once excavated, they are often cut and polished to reveal the beautiful inner chambers of the shell, sometimes filled with sparkling minerals like calcite. These polished specimens often display stunning colours, from creamy whites and browns to hints of red, orange, and gold.

Outer shell of a Cleoniceras Ammonite

One of the most easily recognisable features of Cleoniceras ammonites is their symmetrical spiral shell and visible sutures that run across the surface. Sutures are wavy lines where the inner walls of the shell met the outer shell, which form beautiful natural patterns. Most Cleoniceras fossils are between 5 to 15 cm in diameter, but some can be much larger. When polished the internal structure of these fossils reveals a chambered design, which was used by the living animal to control buoyancy, much like modern-day nautiluses.

Why are some Cleoniceras Ammonites iridescent?

The beautiful rainbow-like shimmer found on some Cleoniceras ammonites comes from the preservation of a thin outer shell layer made of aragonite, a form of calcium carbonate. When this aragonite layer remains intact and is preserved under just the right conditions, it reflects light in a way that creates iridescence (not to be confused with Ammolite)

Fire Opal Cleoniceras Ammonite

Mother of Pearl Cleoniceras Ammonite

Alternatively, some ammonite fossils are replaced with minerals like pyrite or hematite during fossilization because of the unique chemical conditions in the sediments where they were buried.

Ammonite replaced with Hematite

When an ammonite dies and sinks into the seafloor, it may end up in an environment low in oxygen but rich in iron and sulfur. In these conditions, bacteria break down organic material and produce hydrogen sulfide, which reacts with iron to form pyrite. Over time, this pyrite can replace the original shell material, preserving the fossil in stunning metallic detail.

In other cases, if the environment becomes more oxygen-rich, iron can oxidize and form hematite instead, giving the fossil a reddish, black or metallic appearance. These replacements are relatively rare and depend entirely on the chemistry of the sediment and water at the time of fossilization.

Ammonites as a whole were an incredibly successful group of marine animals that existed for hundreds of millions of years before going extinct at the same time as the dinosaurs, around 66 million years ago. They were cephalopods, related to squid, octopuses and cuttlefish, they also used jet propulsion to move through the water. Their widespread presence in the fossil record makes them useful as index fossils, helping scientists date the rock layers in which they’re found.

Whether you’re drawn to their scientific value, natural artistry or prehistoric mystery, these Madagascan fossils offer a unique window into life from millions of years ago, frozen in time and full of life’s past energy.