Special environmental conditions developed in the Tarfayan Basin of southwestern Morocco during the Cenomanian to Coniacian, presumably in response to eustatic changes of sea-level. The passage of <i>V. rotunda</i> to <i>V. rahhalii</i> took place over approximately 2 × 10<sup>5</sup> years, the appearance of the new species being heralded by the development of a new morph displaying some of the properties of the descendant species. This morph coexisted with the normal morph of <i>V. rotunda.</i> Using methods of quantitative genetics it is shown that speciation could have occurred either by weak selection or by random genetic drift in a deme of moderate size. The phenotype is regionally stable, hence the observed morphological changes in the lineage are of genetic origin. The morphological trend is regional.