Males and females are known to differ in their sexual organs. However, because such soft parts decompose quickly, males and females can only be recognized by differences in the shell.

Scientists have repeatedly observed that two completely different-looking ammonite species are found in exactly the same layer – and only there! This means that the two must have lived exclusively at the same time.

They appeared together and died out together. It was also striking that one specimen of this pair was always small and its shell had a beak-like extension at the mouth. The other specimen, on the other hand, was often many times larger and did not have this beak.

At some point, the scientists suddenly realized: These had to be males and females of the same species. Previously, these two different shells had been regarded as two different species of ammonite, which had also been given two different names. This was now just as absurd as referring to a woman and a man as two different species of human!