The discrete cosine transform (DCT) is a Fourier-related transform widely used in signal processing and well suited to analyzing open outlines such as ammonite ribs. The method is applied here to depict and decipher the ribbing morphospace of a large group of Lower Jurassic ammonites composed of the Oxynoticeratidae and their close ancestors. Because they are clearly associated with buoyancy and/or swimming ability, the usually clearly involute, comparatively smooth and compressed shells of these ammonites may well be misleading taxonomic markers. In this context, quantitative analysis of the ribbing pattern using the DCT may significantly improve our perception of the ornamental patterns expressed within the group. A set of 251 specimens illustrating the worldwide fauna and selected from more than 80 publications is analyzed. Big differences are found in the evolutionary patterns of the two main lineages of Oxynoticeratidae currently accepted in the literature. A previously unsuspected Mediterranean group comprising principally the genus Parasteroceras is identified from its distinctive ornamentation. The northwest European and Mediterranean genera Eparietites, Oxynoticeras, and Parasteroceras do not feature among the American (East Pacific) faunas. This finding calls into question some generally accepted correlations between European and American stratigraphic frameworks. The study shows that the DCT is a valuable tool for discriminating between species within the huge and often puzzling range of ornamental variation of the main genera (e.g., Gleviceras and Radstockiceras).