Ammonites are an extinct group of cephalopods that entered the fossil record 400 million years ago. They survived several mass extinction events, including the Permian–Triassic "Great Dying" which wiped out 96% of all marine species. They finally succumbed during the Cretaceous-Paleogene extinction event 66 million years ago, which also wiped out the dinosaurs.

This specimen is a fossilized ammonite shell from Madagascar. There are two styles: split and whole. Split specimens are precision cut to reveal the interior spiral (pictured right).

Whole specimens (pictured below left) are suitable for tabletop display. They range in size from 3cm to 5cm in diameter. The outer surface is polished to reveal the sutures in the shell.

All specimens come inside our classic, glass-topped riker display cases. The cases measure 4 1/2" x 3 1/2". A small information card is also enclosed that also serves as the certificate of authenticity.

Ammonites are an extinct group of cephalopods which entered the fossil record 400 million years ago. They survived several mass extinction events, including the Permian–Triassic "Great Dying" which wiped out 96% of all marine species. They finally succumbed during the Cretaceous-Paleogene extinction event 66 million years ago, which also wiped out the dinosaurs.

The size of ammonite shells range from sub-centimeter dwarf species to giants nearly three meters in diameter. Most iconic shells exhibit a nearly perfect logarithmic spiral.

This ammonite is from the genus Cleoniceras, a subset of the ammonite class that lived during the Cretaceous period. Given its abundance in the fossil record, Cleoniceras (along with other ammonites) are often used to estimate the age of the geological layer in which the fossils are found. Cleoniceras have been found across the world, from the European mainland to along the Caspian Sea. The fossils in our collection come from Madagascar and lived in the seaway growing between the island and the paleocontinent Gondwana.

The shells of Cleoniceras are separated into chambers called septa, which show highly complex sutured patterning. These beautiful patterns evolved over time to solve a difficult problem: shell strength. In order to survive at deeper pressures, ammonites needed tougher shells, but increasing the size and thickness of the shell could slow them down.

Sutured shells could spread the water pressure across the surface, increasing the strength of each chamber without making it much heavier. This made the shells safer at high depths while staying efficient to grow and kept the animal swimming quickly.

How these creatures lived is of intense interest to science, as ammonites likely played a vital role in the food chain in the ancient seas. Evidence exists to suggest that ammonites were a prime food source for Mosasaurs and fishes, while other studies suggest the bite marks on their remains were created after death by limpets or even by other cephalopods.

Many thousands of distinct species make up the long-lived ammonoid subclass. Though most ammonite shells are the classic spiral, there are also straight and gastropod-like shells and even some shells that are partially uncoiled. The surface of the shells also vary quite widely, from smooth to wildly thorny.

Ammonites were an incredibly diverse and plentiful group of animals that survived for hundreds of millions of years and lived all across the planet. Their rapid diversification and tough, rocky shells means there are many different and easily identifiable species in the fossil record. Because of this, scientists can use them to easily identify the age of other fossils and geologic deposits found in the same layer of the ammonites. They're a welcome sight to the eyes of any inquisitive geologist!

Aside from their complex shells, there is little direct evidence regarding the appearance of ammonites due to the absence of soft tissue fossils. However, many scientists believe ammonites had bodies similar to that of the present-day Nautilus.

Ammonite shells grew in a natural spiral and made a consistent, mathematically significant pattern. This special shape is known as a logarithmic spiral.

The main property of a logarithmic spiral is that the shape of the spiral is unaltered as it increases in size. Each turn is a pure geometrical progression of the last with a common ratio. This form is found in many natural phenomena, from the shape of galaxies to patterns on sunflower heads.