The Aalenian Stage was proposed by Mayer-Eymar (1864) for the lowest part of the “Brauner Jura” in the vicinity of Aalen at the northeastern margin of the Swabian Alps (Baden-Württemberg, southwestern Germany) where iron ore was mined from the associated ferruginous oolite sandstones (Dietl and Etzold, 1977). His Aalenian stratotype truncated the Bajocian Stage of d’Orbigny (1842–1851, 1852) at the base of the Sonninia sowerbyi ammonite zone.

The biostratigraphic placement of the base of the Middle Jurassic was the evolution of the ammonite subfamilies Grammoceratinae and Leioceratinae, especially the first occurrence of species of the genus Leioceras, which evolved from Pleydellia. The Aalenian GSSP in the Fuentelsaz section in the Iberian Chain of Spain corresponds to this ammonite marker (Goy et al., 1994, 1996; Cresta et al., 2001). The base of marl Bed FZ107 in the Fuentelsaz section of alternating marl and limestone coincides with the first occurrence of Leioceras opalinum (base of L. opalinum Zone) (Fig. 26.6). The magnetostratigraphy of Fuentelsaz correlates to a composite magnetic pattern derived from other sections in Europe. A secondary reference section for basal Aalenian is at Wittnau near Freiburg in south Germany (Ohmert, 1996).