Ammonites are probably one of the most recognisable and commonly collected fossils, with the majority of collections having some form of ammonites in their collection. But have you ever thought about what an ammonite was?
Get ready to learn all about what we know of ammonites, how they lived, and when they became extinct.
Ammonites were marine predators and they were both successful and diverse. In fact, there were more than 10,000 species and they can be found all over the world. They are ammonoids, which arose during the Devonian period around 416 million years ago. Ammonites are distant relatives to other cephalopods, including the squid, octopus, and cuttlefish.
True ammonites are a suborder of these ammonoids, and they appeared in the Jurassic period around 200 million years ago. They survived three different mass extinctions, including the Permian extinction that resulted from global warming. This extinction killed 96% of the marine species on the planet.
The surviving ammonites diversified after this event, and they became dominant in the seas until they became extinct at the same time as the dinosaurs, 66 million years ago.
There are a number of fossilised ammonites that show they varied significantly in size, from less than an inch to the biggest ammonite at over nine feet in diameter. Most of them had coiled shells with chambers that increased in size and were separated by septa, or thin walls. Others had long, straight shells.
The organism living inside the shell grew more shell material throughout its life, and it always lived in the outside chamber. It is believed that a thin tube called a siphuncle allowed air to flow through the inner chambers of the shell, which helped ammonites move through water. They also believe that ammonites resembled modern cephalopods and had soft body tissue and tentacles that they used to catch prey. They may have had jaws that were sharp and shaped like a beak.
Throughout the Mesozoic era, ammonites evolved. They started out as very small planktonic organisms that were probably less than 1 mm in diameter. At this time, they were likely food for other predators, so they evolved to add the protective shell. Evidence has been discovered that indicates that they grew quickly, and the females were as much as 400% larger than the males.
Their shell structures developed tighter coils and became smaller and more complex. The earlier ammonites had simple lines across their shells, and in the Cretaceous period, the sutures started to show detailed patterns. However, they died out in the same extinction that killed the non-avian dinosaurs because they had a sudden decline in marine plankton, which was their main source of food.
Ammonites are thought to have spent most of their time in shallow waters, which is supported by looking at their diets. The food they ate was mostly found in warm, shallow waters. In addition, it is not likely that their shells would have handled the pressure at depths of more than 100 meters.
A living ammonite was a predator, and plankton was a large part of their diet. However, they likely ate other marine critters, including mollusks, fish, and other cephalopods. It is likely that they silently stalked their prey, and then they would grab the prey with their tentacles. Next, they ate it with their powerful jaws which were located between their eyes at the base of their tentacles.
An ammonite shell fossil is coiled, and it is made up of chambers that are linked together. The body usually rested in the outside coil, which is called the living chamber. The tentacles came out of the end of this chamber, and they could shoot out to catch prey. It is also possible that the opening was covered with a protective shield.
When ammonites grew, new chambers developed behind the head chamber. They call the interior the phragmocone, and it contained gases that allowed the ammonite to have buoyancy in water. Ammonite fossils reveal details about these chambers.
An ammonite fossil has patterns on the outside of the shell, and they are called sutures. These sutures are found below the shell wall, and they are visible if the shell has been polished or weathered. If you study the patterns, you can see where the Septum of the walls of the chambers join the outer wall of the shell. These patterns are also helpful when you want to distinguish different species of ammonites.
Ammonites went extinct at the same time as the non-flying dinosaurs. This happened at the end of the Cretaceous Period when an asteroid collided with Earth and set off a global mass extinction. It is thought that dust clouds and ash rose up followed by a winter that prevented photosynthesis from taking place. This led to a shortage of food, such as plankton, which was ammonites’ primary diet. Nautilus are a relative of ammonites that did survive this event, but they live much deeper in the ocean.
Today, ammonite shells are used as index fossils. They are very helpful for dating other fossils that are found in marine rock. They work well because they were found all over the planet, and they have specific patterns that mark different time periods.
Ammonite animals evolved rapidly, and each species has a short enough lifespan that it is easy to date fossils. They are found in a lot of different types of rock all over the world. They are one of the best guide fossils for stratigraphy because the ammonite age can separate intervals for less than 200,000 years at a time. This makes it very precise.
Ammonites are fossils that can teach a lot about the past of Earth and other species that lived here. There are many species, and they evolved quickly relative to other organisms. As a result, they are very useful in dating other fossils that are found alongside them. They have wonderful patterns and are amazing to look at. If you are interested in collecting ammoniates we have a huge range for you to look at.
