Ammonite
geological
period
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
the object
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
things to
ask . .
(above) Ammonites by Robert Nicholls (below) Nautilus
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