JURASSIC AMMONITE APTYCHUS - 180 million years old - Whitby, Yorkshire

Although resembling a bivalve shell, this is, in fact, part of a fossil ammonite, although the precise function within the ammonite's shell is still debated.

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. It is very rare for an ammonite fossil to be preserved with the aptychus in place. 

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.

​​​​​​​Presented in a lidded gift box.