Ammonites are found in great diversity in the Jurassic rocks of the Posidonia Shale. All ammonites of the grey shale of Holzmaden are preserved flattened to a millimetre. The largest forms have a diameter of up to 1.5 metres.
Ammonites belong to the molluscs like the mussels and snails. However, they are classified as cephalopods and not as snails.
The shell of the ammonites was rolled up and divided into individual chambers. These could be filled with gas and the animal could climb upwards. If the chambers were flooded, it sank into the depths like a submarine. Only the foremost chamber was the living chamber; it housed the soft body.
Ammonites fed on plankton and carrion.
It is assumed that the nautilus is a distant relative of the ammonites still living today.

Common species in Holzmaden:

Harpoceras belongs to the family Hildoceratidae and is an extinct genus of ammonites. These cephalopods lived in the Jurassic period, which began about 201.3 million years ago and ended about 145 million years ago. We are talking about fast-moving nectonic carnivores. The shells of the Harpoceras species show a strong dimorphism in their size, which means that we are talking about two distinctly different appearances of the same species. While the microconchs reach a diameter of 24-51 mm, the shells of the macroconchs are 115-430 mm wide. The whorl sides are flat and have a rather strong trunk. The ribs are in the shape of a wave but initially straight, spoon-shaped or sickle-shaped and thus concave on both sides, stronger and protruding. Sometimes the ribs are broad and shallowly pointed on the outside of the whorl, in some species they are striate on the inside of the whorl. Some species have wavy depressions on the inner half of the whorl and, for example, a groove running down the middle.

Dactylioceras was a widespread genus of ammonites from the Lower Jurassic, about 180 million years ago this species occurred. Dactylioceras are often rather smaller and have an average diameter of 65 millimetres. Their shell is strong and ribbed, the ribs are slightly tilted forward, also run outside the ammonite and are either simple or split at the outer end in the shape of a fork. The type of ribbing was still copied by numerous subsequent ammonite genera - until the complete group died out with the dinosaurs 66 million years ago.

Hildoceras is a genus of ammonites from the Jurassic period - from the family Hildoceratidae. The shells are recognisable by a narrow disc-shaped form, a truncated abdomen, concave ribs along the outer sides and a shallow spiral groove. The whorls overlap slightly, the cross-sections are condensed and the abdominal rump is restricted on both sides by a shallow groove.