Many different species of ammonites are found in the Jurassic. They are the classic guide fossils for defining the individual strata. All the ammonites found in the Posidonia shale of Holzmaden have been preserved pressed flat, and are only a few millimetres thick. The largest forms can reach a diameter of up to 1.5 metres. Like mussels and snails, ammonites are molluscs. They are assigned to the cephalopods, however, not to the snails.
The ammonites’ shell was coiled and divided into chambers. These chambers could be filled with gas, and the animal could then rise towards the surface. If the chambers were flooded, then it sank like a submarine. Only the outermost chamber was inhabited, and it was here that the soft body tissue was to be found. The ammonites probably ate plankton and carrion.
It is assumed that the nautilus, which is still in existence today, is a distant relative of the ammonites.