The thick and thin of it
This is an unusually well-preserved ammonite. It came from the Junction Bed which contains at least four ammonite zones defined by different species of ammonite, yet the rock layer is less than one metre thick. On the Yorkshire coast, the same amount of time is represented by over 100 metres of black shale with the same range of ammonites.
In Yorkshire, during this time period known as the Upper Lias, sediments were continually raining down onto the sea bed and building up into thick layers of rock complete with the evolving or changing ammonites. On the Dorset coast, deposition was slow and at times there was even erosion, leading to a thin layer of rock representing the same period of time – this is called a ‘condensed sequence’. Because deposition was slow most of the fossils are poorly-preserved and some are even partially eroded away, which makes this specimen quite unusual.
Find out more about ammonites on the molluscs pages.