JURASSIC AMMONITE IMPRESSION WITH APTYCHI - 150 million years old - Solnhofen, Germany

A faint impression of an ammonite with the aptychi present. The ammonite shell (consisting of aragonite) has dissolved but leaving its impression in the mud which has now hardened to limestone. The aptychi, however, consists of calcite and so it as fossilised. It is rare for an ammonite fossil to be preserved with the aptychi in place.

The aptychus (plural aptychi) is composed of two parts, each a mirror image of the other, which are located in the body chamber of the ammonite. Their precise function is still debated. It was previously thought that aptychi served the function of closing the shell aperture against intruders but it may be that they were part of the jaw apparatus.

The fossil consists of a thin slab of limestone. Please note that the aptychi are extremely small and are best viewed with a hand lens. Also, the photos are taken with oblique lighting making the ammonite impression easier to see. A curious fossil!