For a period of 300 million years ammonites dominated the marine enviroment, until around 65 million years ago. Ammonites came in all different sizes and shapes although typically as a partitioned spiral cone.
The design of a chambered spiral shell to give buoyancy and propulsion was so successful that it has continued and evolved to this day, some 500 million years later! Today they are represented by some species of squid such as the second largest known living invertebrate Architeuthis, and Cuttlefish.
The ammonites we find on the Yorkshire Coast line belong to the classification Ammonoidea, which begins in the early Devonian and becomes extinct towards the end of the Cretaceous period. To narrow this down even further our ammonites are from the Jurassic period, some 200 million years ago.
It surprises many people to learn that Ammonites were not sea bottom dwellers but instead swam throughout the Jurassic Seas. They did this through a form of pulse jet propulsion. Water would be drawn into the mantle cavity which was then sealed off, pressure was then increased due to the mantle muscles reducing the chamber space, the cavity was then opened and water was ejected at a high velocities propelling the Ammonite along.
We most commonly find Dactylioceras Ammonites on our trips to North Yorkshire(pictured above), however there are many many different types of Ammonites from the Jurassic period alone! One of my favourite Ammonite species I like to find is the Eleganticeras Ammonite, I am yet to find a really nice example of one of these. Ian was lucky enough to find a really huge beautiful one on his last trip, however he has not dared to crack it open yet….. 😮