Dinoflagellate cyst biostratigraphy of the Opalinuston Formation (Middle Jurassic) in the Aalenian type area in southwest Germany and north Switzerland SUSANNE FEIST-BURKHARDT AND JO ¨ RG PROSS Feist-Burkhardt, S. & Pross, J. 2010: Dinoflagellate cyst biostratigraphy of the Opalinus- ton Formation (Middle Jurassic) in the Aalenian type area in southwest Germany and north Switzerland. Lethaia, Vol. 43, pp. 10–31. In order to provide a detailed dinoflagellate cyst stratigraphy of the Lower Aalenian Opalinuston Formation from the Aalenian type area, 68 samples from four boreholes and one outcrop section were analysed. The sample localities are Hausen an der Fils and Wittnau in southwest Germany, Weiach in north Switzerland and Mont Russelin in the Swiss Jura Mountains. Dinoflagellate cyst assemblages were recovered from the Late Toarcian Aalensis Zone to the Late Aalenian Murchisonae Zone. The samples yielded rich, well-preserved and diverse assemblages with 51 dinoflagellate cyst taxa identified in total. The dinoflagellate cyst distribution data obtained from this study allow a high- resolution biostratigraphical subdivision of the lowermost Middle Jurassic Opalinuston Formation into four palynostratigraphical units. First and last occurrences, acmes and consistent presence of the species Batiacasphaera sp. A, Evansia cf. granochagrinata, Kallosphaeridium praussii, Nannoceratopsis triangulata, Phallocysta? frommernensis and Wallodinium laganum were selected as the criteria for defining these units. The obtained high-resolution palynostratigraphical scheme provides a basis for establishing and further refining early Middle Jurassic biostratigraphy in the Boreal and Tethyan realms. h Aale- nian, biostratigraphy, dinoflagellate cysts, Germany, Jurassic, Switzerland, Toarcian. Susanne Feist-Burkhardt [s.feist-burkhardt@nhm.ac.uk], Palaeontology Department, Natural History Museum, Cromwell Road, London SW7 5BD, UK; Jo ¨rg Pross [joerg.pross@ em.uni-frankfurt.de], Palaeoenvironmental Dynamics Group, Institute of Geosciences, Goethe University Frankfurt, Altenho ¨ferallee 1, D-60438 Frankfurt am Main, Germany; manuscript received on 31 ⁄ 12 ⁄ 2007; manuscript accepted on 06 ⁄ 01 ⁄ 2009. The Lower and Middle Jurassic strata of southwest Germany and north Switzerland have long been in the focus of palaeontological research, because of their general richness in well-preserved invertebrate and vertebrate fossils. This is not only documented by the fact that many stratigraphically significant ammonite taxa have been described from these strata (e.g. Zieten 1830; Quenstedt 1845, 1856, 1882, 1886; Oppel 1862; see Schlegelmilch 1976, 1985 for an extensive review), but also by the selection of two type localities for Jurassic stages from the region: the Pliensbachian (Lower Jurassic, 189.6 ± 1.5–183.0 ± 1.5 Ma; Ogg 2004) and the Aalenian (Middle Jurassic, 175.6 ± 2.0– 171.6 ± 3.0 Ma; Ogg 2004). The Aalenian stage was first proposed by Mayer (1864), with reference to the town of Aalen in south- west Germany (Fig. 1). Although some agreement was found at the Colloque du Jurassique a ` Luxem- bourg 1962, the definition of precise boundaries for the Aalenian stage and its subdivision have long been controversial (e.g. Rieber 1963; Maubeuge 1964; see Rieber 1977 for a detailed discussion). In the type area, the Upper Aalenian can be well subdivided because of the presence of lithological marker beds, highly diverse and rich ammonite faunas and abun- dant outcrops resulting from iron ore mining (e.g. Dietl 1977; Dietl & Etzold 1977; Rieber 1977). Strati- graphical control for the Lower Aalenian, by con- trast, is less well established. The Lower Aalenian is represented by a monotonous, up to 150-m-thick succession of partly sandy marls and clays with single sandstone beds, the so-called Opalinuston Formation (Bloos et al. 2005). Its low-diversity ammonite assemblages (e.g. Ohmert et al. 1991) render limited biostratigraphical control, and a subdivision based on lithological characteristics (Engel 1908) has local applicability only (Kobler 1972; Geyer & Gwinner 1984, 1986). Stratigraphical studies are further impeded by the plasticity of the Opalinuston Forma- tion sediments, which causes the tendency to land- slides and hinders the development of continuous, larger scale outcrops (Dietl & Etzold 1977). Hence, a micropalaeontological approach based on core mate- rial seems to be best suited in order to improve the stratigraphical control of the Lower Aalenian succession. DOI 10.1111/j.1502-3931.2009.00170.x Ó 2009 The Natural History Museum, Journal compilation Ó 2009 The Lethaia Foundation