Three principal intervals can clearly be distinguished, each beginning with

1. random coiling (early cretaceous to lower palaeocene and the oligoceneto early lower miocene)
2. followed by period of preferred coiling that ends abruptly at cretaceous/Plaleocene
3. Eocene/oligocene boundary

The oligocene-recent interval shows distinctly preferred coiling directions for species of several genera.
The reason for the sudden change from preferred back to random coiling at the end of cretaceous, and at the end of Eocene, lies in the sudden extinction at these levels of the planktonic genrea and species that distinctly preferred a coiling direction. Surviving or newly appearing, more primitive, species at the base of the Palaeocene and the oligocene respectively, coil at random.