The Aalenian stage of the Middle Jurassic is a notoriously tricky interval for correlation. Its rocks in southwest Germany were historically lumped together as the Braunjura beta of Quenstedt, but modern lithostratigraphy splits them into the Achdorf Formation in the west, the Eisensandstein Formation in the east, and the Murchisonaeoolith-Formation in the Upper Rhine Valley. The base of the Achdorf Formation is defined by the base of the Untere Wilflingen-Bank Bed, and it is precisely at the Wilflinger Steige, northwest of Gosheim, that this boundary bed was formally designated its type locality. The new paper now provides the full lithological and biostratigraphic description that such a type section demands.

The fieldwork was opportunistic in the best sense. During embankment stabilisation works on the road in 2009, a complete profile from the Untere Wilflingen-Bank Bed up to the Obere Sehndensis-Bank Bed was measured and sampled. Between 2020 and 2025, landslides and natural outcrops in the immediate vicinity yielded additional partial sections. Almost all of the recovered ammonites were prepared mechanically with pneumatic chisels and air abrasion using abrasive iron powder, and nearly all are now housed in the collection of the State Museum of Natural History Stuttgart. Only two specimens are elsewhere, and the ammonites from the highest bed, exposed solely during the roadworks, are no longer available for study.

The section itself reads like a layered archive of a shallow, periodically oxygen-poor sea. The Untere Wilflingen-Bank Bed, about 2.1 metres thick, is a chamosite-oolitic, grey-green to dark brown marly limestone in which fossils occur sporadically but densely in particular levels. Its ammonite fauna is dominated by macroconchs of Leioceras crassicostatum and Leioceras goetzendorfense, accompanied by the microconchs Leioceras striatum and Leioceras paucicostatum, and it defines the crassicostatum biohorizon of the Bifidatum Subzone in the Opalinum Zone. Bivalves such as Pseudisocardia cordata, Clavitrigonia brodiei and Pholadomya fidicula round out the assemblage, and rare Tethyan immigrants like the genus Tmetoceras, with its T. scissum, signal connections to warmer southern waters.

Above a monotonous interval of clayey marl, the Obere Wilflingen-Bank Bed presents one of the most taxonomically challenging ammonite assemblages in the whole succession. The fauna sits morphologically between formally named taxa of older biohorizons of the Bifidatum Subzone and younger ones of the Haugi Subzone, so the authors classify all graphoceratid ammonites from this bed either as the morphogenus Leioceras or as the morphogenus Ancolioceras, treating the assemblage as a single evolving chronospecies in which all morphologies intergrade. Macroconchs range from coarsely ribbed Ancolioceras citaae through A. viallii, A. subfalcatum, A. krymholzi and A. noszkyi to the densely ribbed A. substriatum, while microconchs are assigned with reservations to Leioceras cf. striatum and L. cf. paucicostatum. This transitional fauna defines the viallii biohorizon and has so far been recorded only between Gosheim and Wochenberg hill.

The real headline of the study, however, lies higher in the section. The Untere Sehndensis-Bank Bed, historically known as the Sinon-Bank, yields an ammonite fauna dominated by the genus Staufenia, with rare Ludwigia. The authors place all discoidal, sharp-keeled ammonites from this bed in the chronospecies Staufenia sehndensis, whose variation spans smooth shells through weakly ribbed inner whorls to rare nodose specimens. Together with early Ludwigia forms such as Lu. pustulifera, Lu. wilsoni and Lu. obtusa, this assemblage defines a newly introduced pustulifera biohorizon, and the entire interval is assigned to a new subzone, the Obtusiformis Subzone of the Murchisonae Zone in the Upper Aalenian. It is the first formal recognition of this subzone in southwest Germany, anchored on the English succession where the Obtusiformis Subzone was established.

The evolutionary story embedded in these beds is as compelling as the stratigraphy. Staufenia evolved from the genus Ancolioceras in or before the early Haugi Subzone, and its phyletic lineage can be traced through a sequence of chronospecies: from S. sinon in the sinon biohorizon, through S. sehndensis and S. latiumbilicus, to S. discoidea and finally S. staufensis in the Bradfordensis Subzone. The new material shows that S. sehndensis is an intermediate chronospecies linking S. sinon and S. latiumbilicus, distinguishable mainly by statistics of ribbing frequency, adult size and whorl shape rather than by any single diagnostic feature. Ludwigia, by contrast, is remarkably conservative through the Haugi and Obtusiformis subzones, only giving way in the Murchisonae Subzone to the increasingly common Lu. murchisonae group with its falcate ribbing and smooth ventral band, and later to the slender-whorled genus Brasilia.

Above the Untere Sehndensis-Bank, the authors propose a provisional sehndensis/murchisonae biohorizon for the Obere Sehndensis-Bank Bed, characterised by slightly larger Staufenia together with the first abundant Ludwigia of the murchisonae group. Notably, the two Staufenia assemblages of the pustulifera and sehndensis/murchisonae biohorizons are practically indistinguishable from one another, underscoring how gradual the transition is. Strata of the Haugi Subzone itself, curiously, could not be detected at Gosheim or its surroundings, implying a subtle gap or non-sequence at exactly the level where one might expect it. A separate latiumbilicus biohorizon, revealed in 2020 during pipeline construction for a pumping station in Gosheim village, completes the local Upper Aalenian record.

The correlation implications reach far beyond the Swabian Alb. The ammonite assemblage of lens 1 in the Geisingen clay pit, previously assigned to the Bifidatum Subzone, is now revised and placed in the Obtusiformis Subzone, roughly corresponding to the pustulifera biohorizon, a decade of new research having shifted the Lower to Upper Aalenian boundary. In Dorset, England, recent work shows that the pustulifera biohorizon of Gosheim correlates with the obtusiformis biohorizons of Chideock and other southwest English localities, where Staufenia remains known from only two British sites. Intriguingly, both Staufenia and Ludwigia tend to be coarser-ribbed and larger in southwest Germany than in England, a difference the authors attribute to contrasting palaeoecological conditions rather than to different ages.

What emerges from this roadside section is a template for high-resolution ammonite biostratigraphy in a classical area of Jurassic research, one that ties the type locality of a formation-defining bed to biohorizons recognisable from the French Jura to the Dorset coast. By formally naming the Obtusiformis Subzone and its pustulifera biohorizon in the Achdorf Formation, and by proposing the provisional sehndensis/murchisonae biohorizon at the base of the Murchisonae Subzone, the study gives geologists a finer ruler for Aalenian time. Every landslide and road cut along the Wilflinger Steige has now been converted into a calibrated archive, and the ammonites of Gosheim, once scattered curiosities, have become the yardsticks against which Middle Jurassic seas across Europe can be measured.

Subject of Research: Ammonite biostratigraphy and evolution in the Aalenian Achdorf Formation at the Wilflingen-Bank type locality in the Swabian Alb, Germany

Article Title: Ammonites and stratigraphy of the Achdorf Formation (Braunjura Group; Aalenian) at the type locality of the Wilflingen-Bank near Gosheim (W Swabian Alb)

Article References: Dietze, V., Hofbauer, A., Chandler, R. B., Auer, W., & Schweigert, G. (2025). Ammonites and stratigraphy of the Achdorf Formation (Braunjura Group; Aalenian) at the type locality of the Wilflingen-Bank near Gosheim (W Swabian Alb). Zitteliana, 99, 33-61. https://doi.org/10.3897/zitteliana.99.161763

Image Credits: AI Generated

DOI: 10.3897/zitteliana.99.161763

Keywords: ammonites, Aalenian, Middle Jurassic, Achdorf Formation, Swabian Alb, biostratigraphy, Staufenia, Ludwigia, Leioceras, Ancolioceras, Murchisonae Zone, Opalinum Zone