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geological geological geological geological periodperiodperiodperiod The Cretaceous is a geologic period which began145 million years ago. In the geologic timescale the Cretaceous follows on from the Jurassic Period. The Cretaceous was a period with a relatively warm climate and high sea levels. The oceans and seas were populated with now extinct marine reptiles and ammonites with the land being populated by dinosaurs. At the same time, new groups of mammals and birds as well as flowering plants appeared. The Cretaceous ended with one of the largest mass extinctions in Earth History, the K-T Extinction, when many species, including the dinosaurs and large marine reptiles, disappeared.

Image by Robert Nicholls

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Many ammonites lived in the open water of ancient seas. Ammonites preyed on fish, crustaceans and other small creatures, while they themselves were preyed upon by marine reptiles. Ammonites began life very tiny, less then 1mm in diameter, and were vulnerable to attack from predators. Most ammonites only lived for two years, however, some lived longer becoming very large. The largest ever found was in Germany which was 2 metres in diameter. The chambered part of the ammonite shell is called a phragmocone. The phragmocone contains a series of progressively larger chambers that are divided by thin walls called septa. Only the last chamber was occupied by the living animal. As it grew, it added newer and larger chambers to the open end of the coil. A thin living tube called a siphuncle passed through the septa. Using this tube the ammonite could empty water out of these shell chambers. This enabled it to control the buoyancy of the shell and thereby rise or descend in the water.

Ammonite—155 million years old

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Which direction would the ammonite swim? Looking at the artist reconstruction (above right) of an ammonite, how do you think the animal would have moved, eaten and protected itself? Are there any animals, on land or sea, that have similar features to the ammonite? The Nautilus is often referred to as a living fossil? Can you see why? What features are similar to that of the ammonites? Ammonites are an example of rapid evolution. What is evolution? What ‘pressures would make a species evolve

(above) Ammonites by Robert Nicholls (below) Nautilus