Ammonites were sea creatures that resembled the modern-day nautilus, although they are more closely related to the octopus.
The term "ammonite" comes from the Egyptian god Amun, who had a ram's head (and they look like tiny ram horns).
Our ammonite fossil specimens come from the Mahajanga Province in Madagascar, and formed in the Middle Cretaceous period. They are about 110 million years old!
They first appeared in the Devonian period, branching off from similar creatures like the Goniatite (I also sell fossil pendants of them!).
They were cephalopods, like the modern day squid and nautilus, and propelled themselves with jets of water. They lived amongst the plankton that they ate, and while their shell provided some protection, they were preyed upon by large predators like the Mosasaurus.
However, they had a supremely successful evolutionary design and they survived FOUR major extinction periods!! Throughout the MILLIONS of years that they flourished in Earth's oceans, they diversified greatly. Some species were small, but some grew QUITE large (and it is speculated by some that these larger fossils were used by the ancient Greeks as the first throwing disks in the Olympics!).
Various species also adapted some spectacular ornamentation, like this red iridescent specimen here! These iridescent ammonites are only found in Canada and Madagascar (mine are from Madagascar, which fun fact, I have visited to study indigenous and rare flora and fauna!).
The shiny layer is called nacre. Basically, it's a thin layer of argonite leftover from the shell of the creature which still reflects/diffuses light in such a way that you get these rainbow or dazzling red displays. Think of an abalone shell for a modern-day example of nacre.
This iridescence, or nacre, would NOT have been visible in a living ammonite, as this is a layer of argonite that would have been under the outermost protective layer of shell.
Unfortunately for the ammonites, they were unable to survive a fifth extinction period, and died out with the dinosaurs 65 million years ago
As a STEAM Educator, I also LOVE how mathematical these shells are. They are the perfect physical manifestation of the Fibonacci Sequence, and the derivative Golden Ratio (hence the second photo on graph paper!). The Fibonacci Sequence is a sequence of numbers where the subsequent number equals the sum of the two numbers before it (1,1,2,3,5,8...). The golden ratio is derived from the dividing a subsequent number from the number before it (e.g. 3/2, 5/3, 8/5...). As you get into the higher numbers of the sequence, these quotients gets closer and closer to 1.61803..., which is the golden ratio! Like Pi, this number (named "Phi") is irrational, and is usually written as the Greek letter in mathematical equations since it's an ongoing number.
In any case, the shape of the shell follows the curves of the golden ratio!