Ammonites and stratigraphy of the Achdorf Formation (Braunjura Group; Aalenian) at the Wochenberg hill near Schömberg-Schörzingen (W Swabian Alb, SW Germany)

In sections of the Achdorf Formation at the Wochenberg hill (western Swabian Alb, SW Germany), a succession of five ammonite biohorizons is distinguished (from bottom to top): the crassicostatum and viallii biohorizons of the Lower Aalenian (Bifidatum Subzone, Opalinum Zone) and the latiumbilicus , discoidea α and discoidea β biohorizons of the Upper Aalenian (Murchisonae Subzone, Murchisonae Zone). The herein newly introduced viallii biohorizon is the youngest hitherto identified biohorizon of the Opalinum Zone (Bifidatum Subzone). A lectotype is designated for Staufenia latiumbilicus (Quenstedt, 1886), the index ammonite of the latiumbilicus biohorizon. The succession of biohorizons of the Murchisonae Subzone reflects the evolution of the graphoceratid late Aalenian ammonite genera Staufenia and Ludwigia .


Introduction
The Upper Aalenian Achdorf Formation is an up to 60 metres thick series of claystones, arenaceous claystones and occasional limestone beds that crops out in the Wutach area as well as in the adjacent western and middle Swabian Alb (Franz and Nitsch 2009). Historically, this formation was lumped with a coeval succession of ferruginous sandstones of the Eisensandstein Formation in eastern Swabia and Franconia in the more biostratigraphically defined "Braunjura β". The area of eastern Swabia had been well-known for its rich and diverse fossil content, namely ammonites, recovered from the long-abandoned iron mines in the vicinity of Aalen (Quenstedt 1856(Quenstedt -1857Oppel 1858) and was subsequently designated as name-bearing type area of the Aalenian Stage. In these early days of stratigraphical investigations, only few Upper Aalenian ammonites from other areas in Swabia were reported. An exception is the area of the Wochenberg hill near Schömberg-Schörzingen. The Wochenberg is a hill located a few kilometres in front of the Upper Jurassic escarpment of the Swabian Alb. It is characterized by steep forested hillsides and an almost flat plateau formerly used for agriculture. Quenstedt (1886Quenstedt ( -1887 was the first to describe ammonites from there. Lörcher (1939) studied the litho-and biostratigraphy of several sections in the vicinity of Schörzingen and provided descriptions of the strata as well as their fossil content. The lithostratigraphic section and ammonites from this area were studied in greater detail by Rieber (1963) in his monograph on the ammonites of the "Braunjura β". Apart from ammonites, only few other groups of fossils were studied from the area of the Wochenberg hill. Recently, Weis in Weis et al. (2021) described a new species of the belemnite genus Acrocoelites based on specimens from the Untere Wilflingen-Bank (formerly: Comptum-Bank) of the Wochenberg hill.
Since natural outcrops along the steep hillsides of the Wochenberg only expose short intervals of the lithological succession, the complete section could only be roughly estimated in former times and the succession of ammonite faunas was still incompletely known. Moreover, extensive ammonite material housed in institutional and private collections mostly lacked detailed information about the exact locality and bed from where it was collected. These deficiencies hampered the recognition of evolutionary trends in ammonite lineages, sexual dimorphism, and changes of faunal composition through time as well as faunal migrations. Thus, the only possibility to add these missing data are excavations, where the complete succession is sampled bed-by-bed. This precise sampling of sections allows deciphering the ammonite evolution and results in a high-resolution biostratigraphy, although intraspecific variation within large samples complicates determinations.
The aim of this study, which focusses on the ammonites, is to present the scientific results of our excavations, which were executed between 2014 and 2022.

Material and methods
Our scientific excavations (2014)(2015)(2016)(2017)(2018) of the Staufensis-Bank took place at the southwestern edge of the Wochenberg hill, southeast of a model airfield. Several hundred ammonites of the genera Staufenia and Ludwigia/Brasilia and a sole Planammatoceras were collected precisely bed-by-bed. Subsequently (2021)(2022), the rock interval between the Untere Wilflingen-Bank and the Staufensis-Knollenlage was excavated at another section c. 20 metres further to the west of the previous one, with special focus on the ammonite-bearing Obere Wilflingen-Bank. In addition, two smaller excavations (2017,2021) at a distance of c. 50 m and c. 170 m further southeast (in the direction to road L 435 connecting the town Schömberg and the village of Deilingen) focussed on the Untere Wilflingen-Bank. Additional sections of the Untere Wilflingen-Bank were sampled at the headwaters of the Schörzinger Starzel River and 350 m in southeastern direction, as well as along the steep escarpment of the Wochenberg hill north of road L 435.
The herein studied ammonites have been prepared mechanically using pneumatic chisels and airbrasive iron powder. All illustrated specimens are stored in the collection of the Staatliches Museum-für Naturkunde in Stuttgart (acronym: SMNS). The specimens are illustrated in natural size, if not otherwise stated.

Description of the section
Our lithostratigraphic description of the uppermost Opalinuston and Achdorf formations starts from the bottom of the section. It is a combination of our recent observations and measurements and those of Rieber (1963). We studied in detail the interval from the Untere Wilflingen-Bank up to the Staufensis-Bank (sensu Rieber 1963).

Opalinuston Formation
Above the 'Wasserfallschichten' [Waterfall Beds] (c. 8.8 m;Rieber 1963, text-fig. 2) follows the 'sandige Tonmergel' [Sandy Clay-Marls] (8.6 m) with the 'Zopfplatten' [Gyrochorte Beds] (4-4.5 m); the latter are positioned in the upper half of the 'sandige Tonmergel' (Fig. 2). The Untere Wilflingen-Bank (formerly: Comptum-Bank, see Dietze et al. 2021a) consists of a chamosite-oolitic, gray-green to dark brown marly limestone, partly rich in fossils. In our excavation sites at the southern hillside of the Wochenberg, most ammonites occurred in the well-lithified beds uWB-4 and uWB-7. The thicknesses and lithologies of individual beds are almost identical in both sections. In other outcrops at the headwaters of the Schörzinger Starzel River, the Untere Wilflingen-Bank has an overall similar thickness, but at each section it shows some variation in respect of the thickness and lithology of individual beds. Like in our two excavation sites at the Wochenberg hill, the ammonites occur in two layers located in more or less the same position within the Untere Wilflingen-Bank. The frequency of ammonites in both fossiliferous horizons varies at the various localities.  Rieber (1963, text-fig. 2, Profil c Wochenberg). Rieber (1963) interpolated the distance between the "Comptum"-Bank up to the Staufensis-Bank across the Wochenberg hill from the northern to the southern hillside in several steps; this procedure may have resulted in significant measurement errors.

Obere Wilflingen-Bank [Upper Wilflingen Bed]
(1.6-1.7 m; Fig. 3) In one of his sections, Rieber (1963, text- fig. 2, Profil c) showed a separate bed with a maximum thickness of 1 m that follows at a vertical distance of 14 m above the Untere Wilflingen-Bank. It seems that this obviously discontinuously developed bed may reach even greater thicknesses at some places of the Wochenberg hill. In the Katzensteige section near Gosheim, Rieber (1963 text- fig. 2, Profil b) termed a c. 1.45 m thick bed following 3.3 m above the "classical" ammonite-bearing "Comptum"-Bank ["Comptum"-Bed] (= Untere Wilflingen-Bank) as well as "Comptum"-Bank. This is why we refer to the higher bed as 'Obere Wilflingen-Bankʼ.  The beds of the Concava-Bank and the Sowerbyi-Oolith (Rieber 1963) should be expected in a distance of c. 2 m above the top of the Staufensis-Bank; however, these beds were not present at our excavation sites. We were even unable to locate these beds anywhere else at the Wochenberg hill, except of its northern hillside, beyond the road L 435.

Preliminary remarks
Within Graphoceratidae, the Staufenia lineage is easily divisable into a succession of only moderately variable chronospecies. In contrast, the taxonomy of Leioceras/ Ancolioceras and Ludwigia/Brasilia is much more complex. There are no focussed evolutionary trends observable, and some morphologies can reappear independently multiple times. Moreover, ammonites of these groups vary considerably within a single biohorizon. Plenty of the recorded morphologies are not restricted to a single biohorizon, but range within several succeeding ones. Finally, from some well-sampled Aalenian beds such as the Scissum Bed of southern England (Buckman 1887(Buckman -1907 or the Lower/Upper Aalenian transition of Bakonycsernye in Hungary (Géczy 1967) an extremely large number of potentially valid taxa were formally described. However, few or no nominal taxa of this time interval were reported from other regions. The distinction of these nominal taxa is difficult. Considering the great variation within this group of ammonites, both in coeval beds as well as in a succession of beds, it is disputable whether to assign a specimen to a taxon described from a stratigraphically older or younger bed. Adding new taxa would even complicate this taxonomic maze. Therefore, both in Leioceras/Ancolioceras and Ludwigia/Brasilia new species should be only introduced based on very strong arguments in favour. Fortunately, this was not necessary in the course of this study.
Despite the partly large samples recovered by bed-by-bed sampling it was often impossible to identify corresponding macroconchs (M) and microconchs (m) -females and males -with accuracy. Specimens of the Tethyan genera Planammatoceras and Tmetoceras are very rare.

The ammonite fauna of the Untere Wilflingen-Bank [= Comptum-Bank auct.]
The ammonite fauna of the Untere Wilflingen-Bank (formerly: "Comptum-Bank") in the western Swabian Alb was described by Rieber (1963) in great detail. He determined coarsely ribbed and broad macroconchiate specimens as L. crassicostatum Rieber; all other macroconchs were assigned to L. comptum (Reinecke). In the microconchs, the coarser ribbed forms were distinguished as L. paucicostatum Rieber (the holotype of the latter species originates from the Wochenberg hill), and the weakly sculptured ones as L. striatum (Buckman). Concerning the microconchs, we follow the classification and determinations by Rieber (1963). For the sake of completeness, we here illustrate one example of L. striatum ( Fig. 4.5) and one of L. paucicostatum (Fig. 4.9). It is remarkable that at the Wochenberg hill and in the headwaters of the Schörzinger Starzel River microconchs are significanthy more common than in the sections at Gosheim located only a few kilometres farther to the southwest. A possible reason for this variation might be that both living and preservational conditions for microconchs had been more favorable in the present day Wochenberg area due to a calmer environment (Rieber 1963). In Gosheim, macroconchiate specimens of Leioceras with a better rounded bodychamber are more common than at the Wochenberg hill. Thus, we cannot exclude a slight difference in age of the Untere Wilflingenbank at both places; however, this difference is below the biostratigraphical resolution. The former determination of the macroconchs as 'L. comptumʼ must be rejected since the rediscovered holotype of Reinecke´s Nautilus comptus was found to be a latest Toarcian Pleydellia (Chandler and Callomon 2010). The Leioceras specimens illustrated in Fig. 4 provide an overview on the great variation within adult macroconchs from the Wilflingen-Bank at the Wochenberg hill. We assigned them to the two morphospecies L. crassicostatum Rieber (Fig. 4.1a,b) and -following Chandler and Callomon (2010) In contrast to the illustrated material, most macroconchiate graphoceratids from the Wilflingen-Bank are juveniles. They still lack the typical rounded venter, which is only well developed on the bodychamber of adults. However, the bulk of our specimens is close to the morphology of the specimen illustrated in Fig. 4.4 (compare the specimens in Rieber 1963, pl. 1, figs 2-4, 8, 9, 12, 14).
Tmetoceras scissum (Figs 4.6a, b, 7a, b;Rieber 1963) is a rare Tethyan immigrant in the Jurassic of Swabia (Dietze and Schweigert 2020). Unfortunately, a perfectly preserved specimen of c. 45 mm diameter was lost during preparation.

The ammonite fauna of the Obere Wilflingen-Bank
The taxonomy and determination of the ammonites from the Obere Wilflingen-Bank is extremely complicated, since the stratigraphical position of this fauna is transitional between formally named taxa from older horizons of the Bifidatum Subzone (Opalinum Zone) and younger ones of the Haugi Subzone (Murchisonae Zone), similarly to the recently described case of the slightly younger subfalcatum biohorizon (Dietze et al. 2021b). Usually, the ammonites of the (Untere) Wilflingen-Bank both, in the eastern and western Swabian Alb, are classified in a few macroconchiate and microconchiate taxa (L. "comptum", L. evolutum, L. crassicostatum and L. striatum, and L. paucicostatum, respectively) (Rieber 1963;Dietze et al. 2021a). By contrast,   in Ancolioceras from the Haugi Subzone of the Wutach area and the eastern Swabian Alb plenty of nominal species and varieties are distinguished (Horn 1909;Dietze et al. 2021b). There is a gradual transition between the (morpho-)genera Leioceras and Ancolioceras (see Dietze et al. 2021b). The ammonites from the Obere Wilflingen-Bank exhibit numerous combinations of characters of nominal species, which hampers the determination of individual specimens. We classified all graphoceratid ammonites from the Obere Wilflingen-Bank either as Leioceras Hyatt, 1867or Ancolioceras Buckman, 1899[in Buckman 1887-1907 and considered further taxa mostly introduced by Buckman (1887-1907) (e.g., Cypholioceras, Cylicoceras, Geyerina, Hyattina, Mansellia, etc.) as subjective younger synonyms. Their continuous usage would make ammonite taxonomy from the Lower/Upper Aalenian transitional beds extremely complicated.
The maximum size of Staufenia latiumbilicus is represented by a specimen of nearly 30 cm diameter (Fig. 8.5a, b). The unusually large size and the rounded umbilical edge of this specimen not developed in any other representatives of this species points to a pathology. Specimens with preserved bodychamber (Fig. 7.1a, b) are very rare; most specimens are phragmocones with a rather uniform diameter of c. 16-17 cm (Fig. 10.2a, b). Inner whorls can be smooth (Fig. 7.5), weakly ribbed (Fig. 7.4) or coarser ribbed ( Fig. 9.1, 3). The ammonite illustrated in Fig. 9.3 and the holotype of St. sehndensis ( Fig. 9.4a, b) are almost identical. The specimen of Staufenia Fig. 9.7 is strikingly similar to the paralectotype of St. latiumbilicus ( Fig. 9.6a, b), except for the slightly earlier beginning of the egression of the bodychamber as indicated by the preserved spur line. The specimen of Fig. 9.5 is still reminiscent to Ancolioceras.

discoidea α biohorizon
• Genus Staufenia Pompeckj, 1906 The ammonites of the genus Staufenia from the interval St-B 6 to St-B 8b represent Staufenia discoidea (Quenstedt, 1886). Hoffmann (1913) designated the specimen from Schörzingen (= W or S hillside of the Wochenberg) illustrated by Quenstedt (1886, pl. 58, fig. 3) (Fig. 11.6a, b) as lectotype of St. discoidea. Accoring to its morphology, it originates from the discoidea α biohorizon. For an exhaustive description of St. discoidea we refer to Rieber (1963: 44). At the Wochenberg hill, the succession of an older St. discoidea α and a younger St. discoidea β form can be recognized (Fig. 21). In St. discoidea α, the umbilicus in juvenile and median stages is wider and the spur line of the outer whorl egregates less rapidly than in St. discoidea β (e.g., St. discoidea α in Figs 11.1a, b, 5 and St. discoidea β in Figs 18.1, 18.4, Fig. 21, respectively). In complete adults (Fig. 13.1), the egregation of the bodychamber exceeds the mid-flank of the previous whorl. The maximum adult size of St. discoidea α reaches 25 cm, but most specimens are much smaller. The conchs become smooth at diameters of at least 50-60 mm so that larger specimens are remarkably similar to one another, except for the presence and strength of primary ribs in the umbilicus (Figs 11.1a, b, 11.5, 13.1, 14.3a, b). The sculpture of inner whorls can be almost smooth as in the lectotype of St. discoidea (cf. Figs 11.6a, b, 12.1, 13.3), weakly ribbed ( Fig. 12.2; Fig. 13.4) or even coarse-ribbed ( Fig. 12.3a, b). Some of the smooth-shelled inner whorls are still reminescent to the ancestral genus Ancolioceras. Microconchs, here termed as St. discoidea α [m], are either involute and weakly ribbed (Figs 11.2a, b, 13.2, 5), slightly stronger ribbed ( Fig. 11.3a, b) or coarse-ribbed and evolute (Figs 11.4a, b, 13.6). Among macroconchs, specimens with weakly sculptured inner whorls predominate, whereas among the microconchs the coarse-ribbed evolute forms are more common. The sculpture continues up to the aperture in the microconchs; sometimes it weakens a little. When preserved, the microconchs show a spatulate apophysis (Fig. 13.5).
St. latiumbilicus is more evolute than St. discoidea α and exhibits a more gradational egregation of the outer whorl as indicated by the preserved spur line (Fig. 21).

• Genera Ludwigia Bayle, 1878 and Brasilia Buckman, 1899 [in Buckman 1887-1907]
Concerning macroconchs, in the discoidea α biohorizon Ludwigia spp. with a subquadratic whorl section predominate; however, in several specimens, the ventral shoulder is rounded. In contrast, ammonites of the morphogenus Brasilia are comparatively rare. Most specimens of Ludwigia spp. correspond to the "Artengruppe der L. murchisonae (Sowerby)" ("species-group of L. murchisonae (Sowerby)") of Rieber (1963: 53) which he defined as follows: The primary ribs divide into two or sometimes three secondaries in the area between the umbilical edge and mid-flank. The primaries are prorsiradiate, whereas the secondaries are strongly rursiradiate just from the diverging point onwards or immediately distal from this position. Towards the ventromarginal shoulder, the ribs become slightly elevated.            After forming a very gentle forwardly bended curve, the coarse ribs end abruptly near the ventromarginal edge, so that a relatively broad, smooth band is developed in the area between the ventromarginal edge and the keel.

• Genera Ludwigia Bayle, 1878 and Brasilia Buckman, 1899 [in Buckman 1887-1907]
Specimens of Ludwigia spp. from the discoidea β biohorizon generally exhibit a more slender whorl section than those from the discoidea α biohorizon. Specimens assignable to the genus Brasilia are still rare.
Ammonites of the genus Brasilia from the discoidea β biohorizon are represented by B. bradfordensis (Buckman, 1887) [in Buckman 1887Buckman -1907 (Fig. 20.2a, b) and B. elmii (Géczy, 1967) (SMNS 70640/96). Buckman, 1922[in Buckman 1909-1930 A single, strongly weathered specimen ( Fig. 20.1) is determined as Planammatoceras aff. planinsigne (Vacek, 1886). It differs from the lectotype of P. planinsigne by having weakly spinose diverging points of the ribs positioned higher on the flanks. In consequence, these nodes are arranged directly along the umbilical seam. (1886) ammonites from the Achdorf Formation of the Wochenberg hill or from Schörzingen Quenstedt (1886) not only mentioned the Wochenberg hill itself but also the nearby village Schörzingen as finding localities of some ammonites listed below. Since the Achdorf Formation is not present within the village of Schörzingen, the specimens labeled with "Schörzingen" must either originate from the western or from the southern hillside of the   Wochenberg. All specimens originate from the Murchisonae Subzone of the Murchisonae Zone (Upper Aalenian).

Bio-/chronostratigraphy
In our stratigraphical analyses, we focus on the beds of the Achdorf Formation and the directly overlying Sowerbyi-Oolith of the Wedelsandstein Formation.

• crassicostatum biohorizon
The Untere Wilflingen-Bank contains the crassicostatum biohorizon. Typical ammonites of this horizon are the coarseribbed L. crassicostatum along with L. goetzendorfense [formerly misidentified as L. "comptum"] and numerous morphological varieties. For a distinction from the older uncinatum biohorizon we refer to Dietze et al. (2021a). In the evolutum biohorizon, the graphoceratids are markedly smaller and mostly weaker ribbed than those of the crassicostatum biohorizon (Dietze et al. 2021b).

• viallii biohorizon
The herein newly introduced viallii biohorizon of the Wochenberg hill was detected in the c. 1.7 m thick Obere Wilflingen-Bank. Its ammonite fauna is transitional between the ammonite fauna of the crassicostatum biohorizon (Rieber 1963) and that of the subfalcatum biohorizon (Dietze et al. 2021b). It yields ammonites of the genera Leioceras and Ancolioceras as well as transitional forms linking both morphogenera. L. goetzendorfense and L. striatum which first appeared in the crassicostatum biohorizon are now accompanied by L. evertens and L. capillare. The name-bearing species of the viallii biohorizon, A. viallii, is closer to Ancolioceras and hence assigned to this morphogenus. The morphospecies A. citaae, A. aff. subacutum, A. krymholzi and A. noszkyi are already present in the viallii biohorizon, but become predominant in the younger subfalcatum biohorizon of the Haugi Subzone.
The ammonite fauna of the crassicostatum biohorizon is more uniform than that of the viallii biohorizon. It is dominated by L. goetzendorfense (= L. comptum sensu Rieber 1963) with several extreme morphological varieties (see above and Rieber 1963). In comparison, the ammonite fauna of the viallii biohorizon looks much less uniform; approximately half of the ammonite fauna is represented by Ancolioceras, the other half by Leioceras (Figs 4, 5). The Tethyan genus Tmetoceras is only recorded from the crassicostatum biohorizon, but not yet from the viallii biohorizon. In the evolutum biohorizon described from the middle Swabian Alb relatively small-sized specimens of L. evolutum with some intraspecific variation predominate. L. evolutum itself is not present in the viallii biohorizon, but A. citaae exhibits some resemblance to L. evolutum var. comptocostosum (Dietze et al. 2021a, pl. 24, fig. 1). Apart from its coarser ribbing, L. evertens resembles L. evolutum var. costate (Dietze et al. 2021a, pl. 24, fig. 4). Dietze et al. (2021a) suspected that the evolutum biohorizon was younger than the crassicostatum biohorizon (Fig. 20). However, this can neither be verified nor falsified in the Wochenberg section, since no ammonites were recorded from the 3.6 m thick interval between the crassicostatum and the viallii biohorizon. In the viallii biohorizon, Leioceras spp. is still abundant; such morphologies become extremely rare in the subfalcatum biohorizon, where they are mostly replaced by Ancolioceras krymholzi, A. subfalcatum and A. subacutum. Additionally, in the subfalcatum biohorizon the first representatives of Ludwigia and questionable Staufenia spp. appear (Dietze et al. 2021b).

Haugi Subzone (Murchisonae Zone, Upper
Aalenian) The Haugi Subzone was not recorded at the Wochenberg hill. However, it is quite possible that sediments of this age locally occur, such as the calcareous marls below the "Sehndensis-Knollenlage" ("Sehndensis nodular layer") which were not studied in detail by us or a 0.8 m thick alternation of limestones and marly limestones which correspond to the questionable Sinon-Bank sensu Rieber. Rieber (1963) reported "Staufenia" sinon from this bed at the Wochenberg hill.

• latiumbilicus biohorizon
This biohorizon (beds St-B 4-5) is exclusively characterized by its index ammonite Staufenia latiumbilicus. Ammonites of the genus Ludwigia are extremely rare and do not contribute to the recognition of this biohorizon. A differentiation towards the next older described opalinoides biohorizon is rather simple, since in the latter the genus Ancolioceras predominates (see Horn 1909;Rieber 1963), Wochenberg Staufenia discoidea (Quenstedt) discoidea α Ammonites discus Zieten,p. 461,pl. 57,fig. 6 Wochenberg Staufenia discoidea (Quenstedt) discoidea α whereas large-sized specimens of the lanceolate Staufenia latiumbilicus dominate in the latiumbilicus biohorizon. However, we want to point out that at least one or two further still undescribed biohorizons are developed between the opalinoides and the latiumbilicus biohorizon in the Aalenian of SW Germany (Aalen, Gosheim, Geisingen) ( Table 2).

• discoidea β biohorizon
The discoidea β biohorizon (Schicht St-8b) is characterized by the index species Staufenia discoidea β and an accompanying fauna containing Ludwigia spp., which Rieber (1963) summarized as the "Artengruppe der Ludwigia (L.) murchisonae". The discoidea β biohorizon mainly differs from the next older discoidea α biohorizon by the evolutionary state of the genus Staufenia; St. discoidea β is morphologically transitional between St. discoidea α and the younger, extremely involute St. staufensis. Ludwigia specimens of the discoidea β biohorizon have more slender whorl sections and a better rounded venter; a trend which continues up to Brasilia spp. of the Bradfordensis Zone. In the next younger staufensis biohorizon (Rieber 1963;Dietze et al. 2017), the ammonites of the genus Staufenia with its index St. staufensis are still more involute than those of the discoidea β biohorizon. The "Artengruppe der L. murchisonae sensu Rieber" is later replaced by the "Artengruppe der L. bradfordensis" sensu Rieber (1963); however, numerous transitional forms exist.

Bradfordensis Zone (Upper Aalenian)
Beds of the staufensis biohorizon (Dietze et al. 2017) have not been recorded at the Wochenberg hill. Rieber (1963) interpreted nuclei of Staufenia from the discoidea β biohorizon as belonging to St. staufensis. This erroneously implied the presence of the Bradfordensis Zone and is corrected here. No beds of the Bradfordensis Zone have been recorded from the Wochenberg hill.
Concavum Zone (Upper Aalenian) Rieber (1963) reported the "Concava-Bank" near the Wochenberg hill, although he could not find any ammonites in this bed. The "Concava-Bank" most likely represents the Concavum Zone, as it is the case in other parts of the western Swabian Alb (Rieber 1963;Wannenmacher et al. 2021).
A fragmentary Sonninia sp. labelled as originating from the Sowerbyi-Oolith of the Wochenberg hill is stored in the palaeontological collection of Tübingen University. These two finds clearly indicate that the Sowerbyi-Oolith in the vicinity of the Wochenberg hill belongs to the Discites Zone, as it is the case in all other studied places of the western Swabian Alb (Rieber 1963;Dietze et al. 2019).

Correlation within Germany
North Germany: Hoffmann (1913) described a rich ammonite fauna from the Aalenian of Sehnde near Hannover. The Sehndensis-Subzone of North Germany includes time-equivalent strata of the latiumbilicus biohorizon, and the Discoidea-Subzone strata of the discoidea α and β biohorizons.

SW Germany:
The crassicostatum biohorizon is widespread in the western Swabian Alb (Rieber 1963). A set of beds occurring a few metres above the (Untere) Wilflin gen-Bank in a section at the Wilflinger Steige near Gosheim contain ammonites which could represent the viallii biohorizon; their description is still in progress. In 2020, the discoidea α biohorizon was recorded by the first author for the first time in Gosheim within a c. 1 m thick, probably only locally developed set of arenaceous limestone beds which were temporarily exposed along the connectiong road to the industrial area Sturmbühl. The somewhat deeper occurring "Schichten mit Staufenia sehndensis" ["beds with Staufenia sehndensis"] at Gosheim (Rieber 1963;Dietze 1989a, b), which are very rich in ammonites and shell detritus, are slightly older than the latiumbilicus biohorizon. Since bed-by-bed collections from further localities of the Swabian and Franconian Alb as well as from the Wutach area and the Upper Rhinegraben valley are still lacking, further correlations at the level of biohorizons are impossible.

Correlation outside Germany
France (Franche-Comté): Contini (1969, Tableau I) correlated the ammonite successsion of the French Jura Mountains with the successions of North Germany (Hoffmann 1913) and SW-Germany (Rieber 1963). This correlation was later adopted and revised by Contini et al. (1997, Tableau Va) in a rather confusing way. Different zonal schemes were used for the so-called "biome franco-germanique" [partly inluding the French Jura Mountains] on the one hand and the Submediterranean and Subboreal provinces on the other. Most strangely, the discoidea-Horizont sensu Rieber was totally ignored within these "horizons" of the "biome franco-germanique". The stratigraphic position of the "Horizon à Sehndensis" [biome franco-germanique] was placed between the "Horizon à Obtusiformis" and the "Horizon à Murchisonae" [of the Submediterranean/ Subboreal provinces] in Tableau Va; however, in the corresponding text it was explained that the faunas of the "Horizon à Sehndensis" in the French Jura Mountains and in SW Germany [biome franco-germanique] are identical with the fauna of the "Horizon à Obtusiformis" [Submediterranen/Subboreal provinces]. This is why we prefer the more consistent and reproducable subdivision of Contini (1969) for our correlation between SW Germany and the French Jura Mountains. Continiʼs "Horizon à Staufenia sehndensis" with St. sehndensis und Ludwigia obtusiformis is slightly older than the latiumbilicus biohorizon at the Wochenberg hill, where L. obtusiformis has not been recorded yet. At Gosheim, only a few kilometres south of the Wochenberg hill, L. obtusiformis is recorded and co-occurs with smaller-sized St. "sehndensis" (Dietze 1989a(Dietze , p. 125 figure bottom right, 1989b; hence, we suspect these beds correlate with the French Horizon à Sehndensis. The discoidea α und discoidea β biohorizons correspond to the Horizon à Murchisonae of the French Jura Mountains. South England (Dorset): Despite of numerous previous studies (Buckman 1887(Buckman -1907Callomon and Chandler 1990;Chandler 1982Chandler , 1997, a correlation between SW Germany and S England remains problematic. The main reasons are that Staufenia discoidea and St. staufensis do not occur in S England and the corresponding faunas with Ludwigia spp. from England are generally more weakly sculptured and more rounded in ventral aspect than those of SW Germany (Chandler et al. 2012). The faunal horizon of Ludwigia obtusiformis with St. sehndensis, L. obtusiformis and Ancolioceras spp. (Chandler 1982(Chandler , 1997Chandler 1990, 1997) must be slightly older than the latiumbilicus biohorizon based on the same arguments as explained above for our correlation with the French Jura Mountains. For a correlation of the faunal horizon of L. patellaria only its index L. patellaria can be used (Callomon and Chandler 1990). Since specimens of Ludwigia are extremely rare in the latiumbilicus biohorizon of SW Germany, it is impossible to decide whether both horizons could be time-equivalent or which one is the older. Possibly, the latiumbilicus biohorizon is the younger one of the two, since the few Ludwigia specimens already show a smooth band along the keel; this is not the case in L. obtusiformis. The discoidea α und β biohorizons correlate with the faunal horizon of L. murchisonae. The newly introduced viallii biohorizon is either slightly older or coeval with the comptocostosum horizon of Chandler and Callomon (2010).
Scotland (Isle of Skye): Morton and Hudson (1995) divided the rock succession at the type locality of Ammonites murchisonae Sowerby, 1827 -the type species of Ludwigia Bayle, 1878 -into the same faunal horizons as in South England (Callomon and Chandler 1990). The beds O 7-O 11 of the Ollach Sandstone Member were assigned to the Murchisonae Subzone, with the two faunal horizons of L. patellaria and L. murchisonae. These beds correlate with the latiumbilicus, discoidea α und discoidea β biohorizons in the Wochenberg section.
Switzerland (Central Swiss Jura): Christ (1999Christ ( , 2000 divided the rock in the central Swiss Jura Mountains into a succession of several ammonite faunas; however, his study is merely hypothetical since it is mostly based on imprecisely collected museum material (collection Lieb/ Bodmer). His fauna 6 "murchisonae und ssp." approximately correlates with the discoidea α und β biohorizons.
In the Aalenian of Luxembourg, Sadki et al. (2020) divided the Murchisonae Subzone into biohorizons; however, the few specimens of Ludwigia spp. presented in their study do not allow a precise correlation. A high-resolution correlation with further European regions is impossible due to the lack of published data.

Discussion and conclusions
The evolution of the genus Staufenia is well-recorded in sections of the Wochenberg and nearby Plettenberg hills (Rieber 1963;Dietze et al. 2017, this study). Although time-equivalent populations show some morphological variation, this variation shifts continuously during evolution. Four evolutionary trends are observed through time (Fig.  21): (1) The umbilical width decreases. (2) The narrowing of the umbilicus is reinforced by a gradual change from a steep to an overhanging umbilical wall during ontogeny. (3) The strength of ribbing decreases and ribbing becomes restricted to earlier and earlier ontogenetic stages. (4) The suture line becomes simplified. At least the first three evolutionary trends are closely related with each other and occur in many other highly variable ammonite genera. This phenomenon is well-known as "Buckmanʼs rules of covariation" (see Monnet et al. 2015 for more details). Concerning the simplification of the suture line, there is no consistant correlation between complexity of the suture lines and water depth (Lemanis 2020). Therefore, this simplification must be an adaptation to other environmental conditions hardly decipherable without any autecological data.
But why are evolutionary trends in Staufenia appear to be continuous and targeted, whereas they are discontinuous and random in Leioceras/Ancolioceras? A possible reason could lie in the different distribution ranges of these genera. Staufenia is an endemic genus restricted to present-day Germany, E France and S England [only recorded there up to the Obtusiformis Subzone], whereas Leioceras/Ancolioceras had a much wider geographic distribution including the entire Tethyan realm. In the latter taxa, different environmental conditions (e.g. water temperature, food supply) in various parts of the distribution area might have led to different adaptations in the various populations. When such temporarily spatially isolated populations of Leioceras/Ancolioceras showing an onset  (Dietze et al. 2017, pl. 4, fig. 7), all other ammonites from the Wochenberg hill. The staufensis biohorizon is not present at the Wochenberg hill due to Quaternary erosion. Scale bar: 3 cm. of speciation later became reunified, e.g. in the course of a sealevel rise reconnecting neighbouring basins and thus allowing an interbreeding of their populations, this may have resulted in seemingly random developments.