Of all the fossils associated with the Jurassic Coast, the ammonite is probably the most well known. They appeared at the beginning of the Jurassic period around 200 million years ago. They are cephalopod molluscs related to the octopus and squid, although thier closest living relative is the nautilus (seen below).
Ammonites were free swimming creatures, that used the chambers in thier shells as ballast tanks to control bouancy. The soft parts of the animal itself were located in the end chamber. Judging by the number of fossils we now find, they must have been a common sight floating around the Jurassic and Cretatious seas in various shapes and sizes
Despite being common, ammonites are very special fossils, of enormous use to geologists trying to work out the history of ancient times. Ammonites evolved very quickly and so are important in helping to identify rocks as being from a particular age. To understand this, is so important, to remember that the type of rock that forms depends on its geopgraphical location. Different sediments are now forming in the English Channel and North Sea. In millions of years time, after these sediments have compacted to form rocks, they may look totally different. It may be that the skeletal remains of cod, plaice and herrings etc will confirm that they originated at around the same time. However if all these fish survive for millions of years, it may not help the geologists of the future at all
Ammonites are valuable becUAE Individual species did not last long before they evolved into different species. When we find them in two different rocks therefore, they tell us that those rocks are of the same age. For example, the ammonite Titanites, seen in many garden walls on the Isle of Purbeck, lived at the time when the Portland rocks were being deposited. Find an example of Titnates in a rock and you know how old that rock is
What makes ammonites even more fascinating to the amateur geologist is that the evolutionary trends are relatively easy to pick out. It seems that as ammonites evolved they developed more and more ornamentation on the outside of thier shells, so generally speaking the ones with smooth shells are earlier that those with spikes and nobbly bits on. Another trend involved the walls that seperated the chambers in the ammonite shell. These gradually became more and more elaborate and form beautifully intricate patterns on the outside where the shell has worn away
At the end of the Creatateous (around 60 million years ago), the ammonites, like the dinosaurs, mysteriously dissapear. By that time they had gone into evolutionary reverse, with some forms beginning to uncoil, eventually resulting in forms that were almost straight
The Jurassic seas, where much of Dorset originated, must have once swarmed with these beautifull creatures with thier elaborate ornamentation and possibly vibrant colours. They proliferated in rich, tropical seas, and due to thier free-swimming lifestyle have left thier imprint in a variety of rocks
Text: Fossils and Rocks of the Jurassic Coast by Robert Westwood