The ammonites were first subdivided at generic level on the basis of shell coiling, heteromorphs or ‘uncoiled’ shells being separated after 1799 from the normally coiled shells. The latter were referred to a single genus, Ammonites, until the late nineteenth century. After the publication of Charles Darwin's Origin of species in 1859, the more forward-looking workers adopted the ideal of a classification based on phylogenetic relationships. This necessitated the use of a wider range of characters. Increasing realization of the complexity of phylogenetic relationships brought a proliferation of names at lower taxonomic levels—genus and family. For a while, the belief that the growth stages of the shell provided a reliable key to ancestry dominated the work of several specialists.
In the early twentieth century, Schindewolf favoured reliance on a single character, sutural ontogeny, as the decisive criterion in determining relationships. Other workers favoured a balanced appreciation of several characters. In the mid-twentieth century some felt that the detailed lineages, of which glimpses are seen through the fossil record, might never be disentangled, and that classification at generic and specific level must often be arbitrary. Recently, more abundant material and better stratigraphical information have led to the recognition of lineages based on closely-spaced assemblages which can be used as a basis for classification. The usefulness of the present classification is briefly considered.