3.1.1. Teufelsloch Member

The Teufelsloch Member, formally introduced by Franz & Nitsch (2009), has not been further subdivided. Here we use traditional terms taken from older literature for some distinguishable beds within this member.

Fig. 2.

Combined section of the Opalinuston Formation (Lower Aalenian) in the Teufelsklingenbach creek at Bad Boll-Eckwälden (section above the Dilucidum-Schichten) and in the tributaries of the Pliensbach creek (section up to the Dilucidum-Schichten). The grey parts of the section were not studied in detail due to the outcrop situation or the rarity of ammonites. Ju. F. = Jurensismergel Formation; Op = Opalinuston Formation; L. ‘Pentacrinitenplatten’ = Level of the ‘Pentacrinitenplatten'; Opaliniforme-Sch. = Opaliniforme-Schichten; WfS = Wasserfallschichten; ZoP = Level of the ‘Zopfplatten'; Achd. F. = Achdorf Formation; H. Sbz. = Haugi Subzone, Mu. Z. = Murchisonae Zone, Up. Aal. = Upper Aalenian, Tor. S. = Torulosum Subzone, Aal. Z. = Aalensis Zone, Toarc. = Toarcian.

The exact position of the base of the Teufelsloch Member in the Teufelsklingenbach creek is unknown due to poor exposure (Gropper 1925). Therefore, we included information from the western and eastern tributaries of the nearby Pliensbach creek and from the former clay pit Mohring in Heiningen. Gottlieb (1955) described the underlying Jurensismergel Formation (“Lias zeta”) of the Teufelsklingenbach creek.

We focused on parts of the section presently accessible for investigations and which are relatively rich in ammonites and thus, biostratigraphically interpretable. A comparison with previous studies (Kobler 1967, 1972; Ohmert 1993) revealed that all biostratigraphically relevant parts of the Teufelsloch Member could be restudied. Only the lower part of this member, the claystone unit above the “Dilucidum-Schichten”, contains very rare fossils.

3.1.1.1. Range area of the ‘Torulosum-Knollenlage’ and ‘Dilucidum-Schichten'

A. Teufelsklingenbach creek

Gottlieb (1955) briefly described the underlying Jurensismergel Formation (“Lias zeta”) of the Teufelsklingenbach creek. Due to lack of exposure in the Teufelsklingenbach creek no new material from the basal Opalinuston Formation was collected. The ammonites studied are from museum collections. All these specimens were preserved inside limestone concretions or in the body chambers of large lytoceratids.

Ammonites: L. opalinum α (Pl. 1, Fig. 11), L. opalinum var. comatum (Pl. 1, Fig. 4), L. opalinum α var. buckmani (Pl. 1, Figs. 7, 9, 10, 13), L. opalinum α var. tomum (Pl. 1, Fig. 14), L. opalinum α var. costulatum (Pl. 1, Fig. 15), L. opalinum α var. subcostosum (Pl. 1, Fig. 12), L. opalinum α var. leurum (Pl. 1, Fig. 8; Pl. 2, Fig. 6), L. opalinum α var. subglabrum (Pl. 1, Figs. 2, 5, 6), Pleurolytoceras torulosum (Pl. 1, Figs. 1, 3), P. penicillatum.

B. Tributaries of the Pliensbach creek

The two tributaries of the Pliensbach creek are located west of the village of Bad Boll-Eckwälden (Fig. 1). They meet just before the road L 1214. In the outcrop area of the basal Opalinuston Formation, the gradient of the creeks is so low that the thicknesses of the incompletely exposed section can only be roughly estimated.

In the western tributary of the Pliensbach creek, we recorded the following incomplete section at the base of the Opalinuston Formation:

Transition Jurensismergel Formation/Opalinuston Formation:

• (0.3 m): Brownish marl at the top of the Jurensismergel Formation.

• (∼ 1–1.5 m): Within this interval the boundary between the Jurensismergel and Opalinuston formations and the basal-most part of the Opalinuston Formation with the Torulosum-Knollen is located. These beds are now obscured. The thickness of the beds following up to c. 1.0 m above the “Dilucidum-Schichten” (Opalinuston-Formation) can only be roughly estimated due to the poor exposure, weathering and almost horizontal bedding (after Kobler 1967, the dip of the beds in the Teufelsklingenbach creek is 2–3° towards the Southeast).

Gottlieb (1955: 39) described a 0.08 m thick bed of calcareous marl overlain by 0.97 m steel-grey marl containing fragments of Cotteswoldia aalensis. Above, 0.4 m of ochre-coloured marl with Leioceras opalinum and P. torulosum is overlain by a 0.05 m thick layer with white aragonitic shells, 0.3 m grey-green marl and finally 0.25 m grey-olive marly mudstones full of aragonitic P. torulosum, bivalve debris and belemnites. We have doubts regarding the correct determination of Leioceras opalinum in the abovementioned marls. They may represent morphologically similar Pleydellia spp. of the latest Toarcian. Gottlieb (1955) placed the boundary between the Jurensismergel and Opalinuston formations at the base of the 0.25 m thick claystone with P. torulosum.

Dilucidum-Schichten:

• (∼ 0.7–1.0 m [beds below not exposed]): Strongly weathered claystone with numerous compressed white aragonitic shells of Leioceras opalinum β and Pleurolytoceras dilucidum (“beer-mat shells” sensu Seilacher et al. 1976).

• (∼ 0.35 m): Claystone with a layer of grey, oval limestone concretions, rarely with fragments of white-shelled L. opalinum β and Pleurolytoceras dilucidum on their upper surface. Claystone with mass occurrence of compressed white aragonitic shells of L. opalinum β and abundant P. dilucidum.

• (∼ 1.3 m; in the middle there is a small waterfall with a height of c. 0.7 m): mass-occurrence of compressed white-shelled L. opalinum β and abundant P. dilucidum. Two specimens of Bredyia subinsignis have been recovered from the claystones at the top of the small waterfall, which yield a mass occurrence of the gastropod Toarctocera subpunctata. Nuclei of ammonites are sometimes three-dimensionally preserved. A large limestone concretion from just above the small waterfall has yielded the fish Dapedium ballei (Maxwell & Lópezarbarello 2018).

Ammonites: Leioceras opalinum β (Pl. 3, Figs. 2, 3, 6, 10, 12), L. opalinum β var. buckmani (Pl. 3, Figs. 4, 5, 7), L. opalinum β var. undatum (Pl. 3, Figs. 8, 11), L. opalinum β var. subglabrum (Pl. 3, Fig. 9), Bredyia subinsignis (Pl. 3, Fig. 13), Pleurolytoceras dilucidum (Pl. 3, Figs. 14–17).

• Above, there follow several metres of mudrock that has not been studied.

In the eastern tributary of the Pliensbach the outcrop is more degraded than in the western tributary. In the bank of a small pool, 1.4 m claystones of the Dilucidum-Schichten are exposed. Further up in the creek, these beds crop out again, but are strongly weathered there.

Ammonit e s: Leioceras opalinum β, Pleurolytoceras dilucidum.

During a recent flooding event, several slabs of claystone were detached from the floor of the c. 0.5–0.8 m deep water pool. These yielded compressed white-shelled L. opalinum α (very common) and P. torulosum (= range area of the ‘Torulosum-Knollen').

Ammonites: Leioceras opalinum α var. buckmani (Pl. 2, Fig. 11), L. opalinum α var. subglabrum (Pl. 2, Fig. 10), Pleurolytoceras torulosum (Pl. 2, Fig. 9).

In the field notebook of the late W. Ohmert, there is a section from the eastern tributary of the Pliensbach creek dated from June 27th, 1986, in which he noted from the stream bed c. 0.1 m unconsolidated bright yellowish detrital marl with “fluitans + torulosum” following above c. 0.35 m light grey calcareous marl with an ammonite mass occurrence predominated by Dumor-tieria and “Pleydellia aalensis” at the top. From a c. 2 m-thick “lateral slump”, c. 2.5–2.8 m above the bright yellowish marl (later crossed out and replaced by “1.4 m Gottlieb (1955)”) he mentioned ammonite-bearing concretions at the base. Above, a 0.2 m thick ammonite accumulation follows comprising compressed, white-shelled “∼ Pl. loth. – buckmani” and a little further up Lytoceras cf. dilucidum. This means that Ohmert (1993, fig. 2), when compiling his schematic section of the Teufelsloch gorge, in fact illustrated the situation of the nearby Pliensbach creek.

C. Creek bed of the Pliensbach

Ex-situ specimens from the ‘Torulosum-Knollen’ layer (collected from scree; coll. G. Gassmann) are housed in the collection of the SMNS:

Ammonites: Leioceras opalinum α (Pl. 2, Figs. 7, 8), L. opalinum α var. comatum (Pl. 2, Figs. 2, 3), L. opalinum α var. subglabrum (Pl. 2, Figs. 4, 5), Pleurolytoceras torulosum (Pl. 2, Fig. 1), P. penicillatum.

D. Heiningen

Our data on stratigraphy of the former clay pit Mohring in Heiningen (Fig. 3) are based on sections by Al-Sayari (1970), Ohmert (fieldbook data) and one of us (S.G.). The latter took partial sections in a temporary sewer trench in the southern part of the former clay pit (in 1996) and at the construction of a new building of the company Mayer (in 1998) up to the ‘Torulosum-Knollenlage'. There, Ohmert studied the uppermost parts of the Jurensismergel Formation. Al-Sayari (1970) described a section of more than 7 m of claystones following above the Torulosum-Knollenlage.

Fig. 3.

Section in the transition Jurensismergel Formation/Opalinuston Formation in the former clay pit Mohring in Heiningen. Jur. F. = Jurensismergel Formation; Tor.-Kn = Torulosum concretion layer.

Jurensismergel Formation

• (at least 0.1 m): Clay-marlstone with jet, belemnites (long, thin) and undetermined ammonites.

• (0.07–0.08 m): Nodular bed of marl-limestone.

• (0.55–0.62 m): Clay-marlstone, partly rich in shell detritus.

• (0.05–0.1 m): Grey bed of marl-limestone, partly nodular, poor in fossils, with two compressed Cotteswoldia aalensis.

• (0.05–0.15 m): Detrital, grey clay-marlstone. The transition to the above following claystone is gradational.

Opalinuston Formation (Teufelsloch Member), range area of the ‘Torulosum-Knollenlage'

• (1.9–2.0 m): Dark grey to blackish grey claystone; from 0.7 m above the base onwards with rare dark limestone concretions, mostly lacking fossils, rarely with small nuclei of leioceratids and shell detritus. Two body chambers of lytoceratids have been recorded. In two of the concretions and in the two body chambers, ammonites up to 5.5 cm in diameter occurred. All fossils from this part of the section are preserved with grey shells.

Ammonites: L. opalinum α, Pleurolytoceras penicillatum.

• (0–0.1 m): Only locally developed lenticular bed developed within the claystones below, greyish-black claystones with white-shelled, compressed fossils. The boundary to the underlying claystone lacking white-shelled fossils is very sharp. The next bed above follows with a very sharp boundary as well. Numerous limestone concretions and lytoceratid body chambers contain three-dimensionally preserved white-shelled fossils.

• (0.12–0.13 m): Greyish-black claystone with accumulation of white-shelled fossils, which are less deformed and compressed than in the lower, locally occurring claystone lenses. Calcareous concretions with white-shelled, three-dimensionally preserved fossils are abundant, but usually smaller than in the claystone lenses below. The limestone concretions have smoother surfaces than those in the claystone lenses below. Lytoceratid body chambers with white-shelled fossils are abundant as well. The concretions often show cracks filled with gypsum which may cut the ammonites inside. Some ammonites are partly dissolved by acidic pore water that later fostered to gypsum precipitation or partial deformation. The fossils in the body chambers of lytoceratids are better preserved, since gypsum occurs only marginally. The fossil content of this continuous bed is the same as in the underlying beds of the Opalinuston Formation.

Ammonites (from both concretion layers and the body chambers): L. opalinum α (Figs. 5.15–5.18), L. opalinum α var. arkelli (Fig. 5.12), L. opalinum α var. arcuatum (Fig. 5.9), L. opalinum α var. gracile (Fig. 5.4), L. opalinum α var. tomum (Figs. 5.5–5.8), L. opalinum α var. angulatum (Fig. 5.2), L. opalinum α var. buckmani (Fig. 5.13), L. opalinum α var. undatum (Fig. 5.1), L. opalinum α var. subcostosum (Figs. 5.10, 5.11), L. opalinum α var. comatum (Fig. 5.14), L. opalinum α var. leurum (Fig. 5.3), L. opalinum α var. subglabrum (Figs. 4.1, 4.2, 4.4), L. opalinum α var. pseudoaalensis (Fig. 4.3), Bredyia subinsignis (Figs. 5.19, 5.20), Canavarella fasciata (Figs. 13a–c), Pleurolytoceras torulosum, P. taeniatum, P. penicillatum, P. dilucidum.

Fig. 4.

(1, 2, 4): Leioceras opalinum α var. subglabrum [M], (1) SMNS 70608/1 (limestone concretion from the upper concretion layer); (2) SMNS 70608/2; (4) SMNS 70608/3. (3): L. opalinum α var. pseudoaalensis [M], SMNS 70608/4. (1–4) former clay pit Mohring in Heiningen; Opalinuston Formation, Teufelsloch Member, range area of the Torulosum concretion layer; Lower Aalenian, Opalinum Zone, Opalinum Subzone, opalinum Biohorizon. Ammonites slightly reduced in size. Scale bar equals 30 mm. The beginning of the body chamber is marked by an asterisk.

Fig. 5.

(1): Leioceras opalinum α var. undatum [m], SMNS 70608/5. (2) L. opalinum α var. angulatum [m], SMNS 70608/6. (3) L. opalinum α var. leurum [m], SMNS 70608/7. (4) L. opalinum α var. gracilis [m], SMNS 70608/8. (5–8) L. opalinum α var. tomum [m], (5) SMNS 70608/9, (6) SMNS 70608/10, (7) SMNS 70608/11 (8) SMNS 70608/12. (9) L. opalinum α var. arcuatum [m], SMNS 70608/13. (10–11) L. opalinum α var. subcostosum [m], (10) SMNS 70608/14, (11) SMNS 70608/15. (12) L. opalinum α var. arkelli [m], SMNS 70608/16. (13) L. opalinum α var. buckmani [m], SMNS 70608/17. (14) L. opalinum α var. comatum [m], SMNS 70608/18. (15–18) L. opalinum α [m], (15) SMNS 70608/19, (16) SMNS 70608/20, (17) SMNS 70608/21 (18) SMNS 70608/22. (19) Bredyia subinsignis [M], SMNS 70608/23. (20) Bredyia subinsignis [m], SMNS 70608/24. (1–20) former clay pit Mohring in Heiningen; Opalinuston Formation, Teufelsloch Member, range area of the Torulosum concretion layer; Lower Aalenian, Opalinum Zone, Opalinum Subzone, opalinum Biohorizon. Ammonites slightly reduced in size. Scale bar equals 30 mm. The beginning of the body chamber is marked by an asterisk.

Beds above the ‘Torulosum-Knollen-lage'

Our description of the c. 7 m of beds following above the ‘Torulosum-Knollenlage’ as well as that of Hegele (1995: 99, profile at right) was compiled from the diploma thesis of Al-Sayari (1970). Above the ‘Torulosum-Knollenlage’, the limestone concretions are rich in fossils and white-shelled fossils become generally scarce. Al-Sayari (1970) was unable to make a lithological subdivision of the uniform blackish-grey claystones and subdivided the section artificially into 11 horizons, which we refer to with numbers in square brackets. The only recognizable differences are the occurrences of Bositra shell fragments in the 3rd and 5th horizon, whereas these are lacking in all other horizons. The determination of the ammonites is after Al-Sayari (1970). All ammonites from this part of the section are compressed.

Fig. 6.

Lithologic variability of the Lucinenbank (LB; Layer 3) encountered in the Teufelsloch gorge. (A) Schematic drawing of the outcrop in September 2020 and location of the samples (within frame) shown in (B) and (C). (B) Grading of components within the LB having crinoid debris (c) enriched (frame marking the area shown enlarged within the inset, enriched crinoid debris) at the base (b), upright to inclined (fragmented) shells and small nodules (n) in the middle part (m), and densely packed mollusk shells with umbo up (arrow) indicating current-stable position (u). (C) Micritic limestone impregnated by pyrite (pi) with slightly nodular appearance exhibiting crinoid-rich (c), nodular (n), and shell-rich (s) domains (for details see Chapter 8.2.).

• (0.1 m [2]): Blackish-grey claystone, poor in fossils. Rare ammonites (L. opalinum, Pleurolytoceras “lineatum”).

• (0.75 m [3]): Alternation of blackish-grey and medium-grey claystones with shell detritus (L. opalinum, P. torulosum, P. “lineatum”).

• (0.91 m [4]): Blackish-grey, micaceous claystone rich in fossils, with small disc-shaped limestone concretions. In the upper part a level with burrows. Fossils partly accumulated (L. opalinum. P. torulosum, P. “lineatum”).

• (0.6 m [5]): Grey claystone with shell detritus (L. opalinum).

• (0.8 m [6]): Black-grey, micaceous, thinly bedded, claystone, rich in fossils (L. opalinum, P. torulosum).

• (0.72 m [7]): Dark grey, finely bedded claystone (L. opalinum).

• (0.5 m [8]): Light? grey claystone, rich in fossils (L. opalinum, P. torulosum, P. “lineatum”).

• (0.7 m [9]): Brownish weathered claystone, rich in ammonites (L. opalinum, P. torulosum).

• (0.9 m [10]): Light grey, partly silty claystone. In the lower part shell fragments are abundant. Accumulations of ammonites up to 0.4 m in diameter (L. opalinum, L. “costosum”, P. torulosum, “Hammatoceras subinsigne”).

• (1.16 m [11]): Bright grey claystone with four levels of siderite concretions, which are mostly, embedded within up to 0.05 m thick calcareous lenses of siltstone. Fossils are rare (L. opalinum).

3.1.1.2. Lucinenbank

• (c. 0.1–0.15 m) This dark grey bed forms a small barrier in the stream bed (Fig. 6A), c. 14.5 m below the Pentacrinitenplatte. Some meters below the Lucinenbank, there are several levels with siderite concretions.

The upper and lower extent of the Lucinenbank is bounded by a limestone layer with cone-in-cone structures (Wenk 1927; for lithologic-sedimentological details see section 8.2). This bed yields abundant Mesomiltha (“Lucina”) plana (Zieten) and other bivalves (Quenstedt 1857; Gropper 1925; Hegele 1995: 97, fig. 3). The only ammonites from this bed are scarce compressed and deformed leioceratids, mostly preserved with their opalescent shell. At several places, the Lucinenbank is represented by lithified clays (Gropper 1925). Immediately above the Lucinenbank, there are thin (c. 0.1 m), stronger lithified claystones with numerous white-shelled fossils.

Ammonites: Leioceras opaliniforme var. opalinum (Fig. 7.1), L. ex gr. opaliniforme (Fig. 7.2).

Fig. 7.

(1): Leioceras opaliniforme var. opalinum [m], SMNS 70608/25. (2): L. ex gr. opaliniforme (Buckman) [M], SMNS 70608/26. (1, 2) Opalinuston Formation, Teufelsloch Member, Lucinenbank; Teufelsklingenbach gorge at Bad Boll-Eckwälden; Lower Aalenian, Opalinum Zone, Opalinum Subzone, ?opaliniforme Biohorizon. Ammonites at natural size. Scale bar equals 30 mm.

3.1.1.5. Zillhausen Member

Concerning the lithostratigraphical subdivision of the Zillhausen Member, we follow the suggestions by Franz & Nitsch (2009). In our description of the section, we additionally refer to data by Söll (1954), Weber (1964), Kobler (1967, 1972) and Ohmert (1993, and unpublished notes in his field book). The lower boundary of the Zillhausen Member is identified by the onset of fine-sandy silty-claystones or sandy limestone and sandstone beds above the claystones of the Teufelsloch Member (Kobler 1967, 1972; Franz & Nitsch 2009). In the Teufelsloch gorge the base of the Zillhausen Member coincides with the base of the ‘Wasserfallschichten’ (= “Abschnitt C” of Kobler 1967, 1972; Franz & Nitsch 2009: 131). Due to an increase of the silt fraction (mostly quartz) the claystones are more indurated. A marked increase of the sand content (Kobler 1967, 1972) occurs slightly higher in the section, approximately coeval with the change from white-shelled aragonitic fossils to brownish calcitic replacement of the shells.

Fig. 8.

Section of the Wasserfallschichten (Zillhausen Member) in the Teufelsklingenbach gorge. TL M. = Teufelsloch Member, Op = Opalinuston Formation, WB = Wasserfallbank.

3.2.1. Wilflingen-Bank (0–0.1 m)

The very compact, grey, fine-sandy siltstones yield irregular lenses of brown lime-marlstone (0.05–0.08 m) with body chambers or, more rarely, complete leioceratids. Occasionally, such body chambers occur as well in the siltstones. Every 1–2 m there is a hard calcareous marl nodule with blackish cortex and numerous mostly small-sized leioceratids and abundant bivalve shell detritus. The lenses or nodules containing ammonites occur at c. 0.9 below the top of the above following platy limestone bed. The lithology and position in the section fits well with the Wilflingen-Bank exposed at the Aichelberg slope of the motorway A 8. Ohmert (1993) estimated that the bed with limestone concretions containing leioceratids is encountered c. 4 m above the ‘Zopfplatten'. This value must be corrected; in fact, this bed occurs in a vertical distance of c. 6.6 m. Weber (1964) mentioned the Comptumbank, now Wilflingen-Bank, as being rich in ammonites nearby at Gammelshausen, where it is located c. 6 m above the ‘Zopfplatten'.

Ammonites : Leioceras evolutum var. striate sensu Contini (Pl. 24, Figs. 6–8, Pl. 25, Figs. 2, 3b–5), L. evolutum var. subcostate sensu Contini (Pl. 24, Fig. 5), L. evolutum var. costate sensu Contini (Pl. 24, Fig. 4; Pl. 25, Figs. 1, 3a), L. evolutum var. crassicostatum (Pl. 24, Figs. 2, 3), L. evolutum var. comptocostosum (Pl. 24, Fig. 1), L. evolutum var. striatum (Pl. 25, Figs. 6b, 9–12), L. evolutum var. paucicostatum (Pl. 25, Figs. 6a, 7, 8).

3.2.2. Beds above the Wilflingen-Bank (c. 2 m studied)

Ammonites: Ancolioceras opalinoides (Figs. 12.1–12.3, 12.5–7, 12.9, 12.11, 12.12, 12.14–12.16), ?Leioceras sp. (Figs. 12.4, 12.8, 12.13).

4.1.1. Comments on the species concept

All leioceratids of the genus Leioceras in the study area are treated as a succession of chronospecies. The same concept has been successfully applied in our revisions of the genera Phlycticeras (Schweigert et al. 1998), Garantiana (Dietze et al. 2002), Sonninia (Dietze et al. 2005), Strigoceras (Schweigert et al. 2007) as well as in the description of the genus Brasilia in the Upper Aalenian of the Geisingen clay pit (Dietze et al. 2014). A similar concept was already used by Hoffmann (1913) for the description of the Late Aalenian leioceratids of Sehnde near Hannover. An indispensable basis for this species concept is a sufficient number of bed-by-bed collected adult specimens (Chandler & Callomon 2009). This allows studying the variation in succeeding populations. Indeed, in every studied time slice, the genus Leioceras shows a great intraspecific variation, with all possible combinations of characters. Traditionally, these varieties have been often treated as morphospecies having a vertical range or even as different morphogenera (e.g., Leioceras [m] – Costiceras [m] – Cypholioceras [M] – Cylicoceras [M]). Studying ammonite populations in their stratigraphical succession indicates that the evolution of the genus Leioceras from the Torulosum Subzone (Aalensis Zone, Upper Toarcian) until the end of the Opalinum Zone (Lower Aalenian) is a chronocline (Chandler & Callomon 2009; Chandler et al. 2012; Chandler 2019). The range of variation of the ammonite populations differs in each time slice. Some conservative morphologies can have vertical ranges across several time slices. Each chronospecies represents a sole population or generations of populations within the chronocline of Leioceras. Within the c. 130 m thick Opalinuston Formation leioceratids are not continuously present, but concentrated in a few, mostly thin intervals, and they are very rare or missing in the remaining parts of the section. Therefore, the individual chronospecies represent only short, unequal segments of a continuous evolution. The unrecorded time makes them fairly well distinguishable from each other. Hence, a chronospecies is defined – analogous to a biological species – by the characters of a reproductive population and their isolation from other contemporaneous populations (Callomon 1985; Mayr 2000; Dietze et al. 2005).

How can this species concept fit with the rules of nomenclature? In the optimal case, the name of a leioceratid ammonite should imply (1) the chronocline, (2) the chronospecies and (3) the morphological characters. Waagen (1869) was the first to suggest a solution, which, however, was rejected because of its complexity (Willmann 1985; Dietze et al. 2005). Waagen (1869) concurrently named (1) the chronocline (Waagen: “Formenreihe” or “Collectivart”), (2) the evolutionary position within this chronocline (Waagen: “Mutation”), the chronospecies, and (3) the morphological characters of the individual ammonite (Waagen: “Varietät”). We here follow this concept, with some practical modifications:

In practice, we slightly modified the procedure suggested by Dietze et al. (2005). As an example, Leioceras opalinum (Reinecke, 1818) var. subglabrum ex Buckman, 1902 sp. (1) is a specimen belonging to the chronocline of Leioceras, (2) it comes either from the opalinum Biohorizon or from the dilucidum Biohorizon, (3) and morphologically, it corresponds to L. subglabrum. In short: Leioceras opalinum var. subglabrum. In the very special case of the chronospecies Leioceras opalinum, we further distinguished an α-form and a β-form. The reason is that the populations of the opalinum and dilucidum biohorizons slightly differ with respect to a higher proportion of involute forms in the dilucidum Biohorizon, especially in microconchs. However, since an increase of the adult size of macroconchs, which is an important evolutionary character, is not recorded, we separated only an α-form and a β-form, L. opalinum α und L. opalinum β, instead of two chronospecies, followed by the morphological variety.

A higher number of varieties within a chronospecies does not necessarily imply a greater variation. The higher number of described varieties mainly results from a historical focus of studies on special beds or intervals. This is the case in the transition Toarcian/Aalenian or in the Bifidatum Subzone. In contrast, only few morphospecies exist for ammonites from the top of the Opalinum Subzone. To avoid an inflation of purely artificial taxa, we did not erect additional morphospecies, but we gave the existing varieties a broader definition. In the case of ammonites from intervals with more described morphospecies, we used these names for a better characterization of the morphological diversity.

4.1.2. Relationships between Leioceras Hyatt, 1867 and Pleydellia Buckman, 1899

The phylogenus Leioceras evolved from the phylogenus Pleydellia of the latest Toarcian. The respective type species of both genera, Nautilus opalinus Reinecke, 1818 and Pleydellia comata Buckman, 1899, differ only little from each other. The types of both taxa mainly differ by a slightly wider umbilicus and a broader whorl section in P. comata. In the basal Opalinum Subzone, there is an overlap of both genera (Ohmert 1993; Contini et al. 1997; Cresta et al. 2001; Schulbert 2001). Following our chronospecies concept, we assign ammonites of the Opalinum Subzone with a Pleydellia-like morphology to varieties of L. opalinum. Although the apportionment of genera within a chronocline is arbitrary, we favour a division at the Toarcian/Aalenian stage boundary. Taking Pleydellia as a junior synonym of Leioceras, the genus Cotteswoldia Buckman, 1902 should also be included (Howarth 2013).

4.1.3. The chronospecies of Leioceras and its varieties in the succession of ammonite biohorizons

4.1.3.9. evolutum Biohorizon (Wilflingen-Bank [formerly Comptumbank])

Leioceras evolutum Contini, 1969 [HT: Leioceras (Cypholioceras) comptum (Rein.) evolutum morphotype subcostate, p. 19, pl. 9, figs. 1a, b].

From the “Comptum” Subzone [= Bifidatum Subzone] of Franche-Comté Contini (1969) reported very evolute leioceratids up to 8 cm in diameter. The broad, fastigate venter of the phragmocone becomes rounded on the body chamber. With respect to their variable ribbing, he distinguished between the morphological variants striate, subcostate and costate. Here we use them in the same sense. Specimens showing a coarse ribbing in all stages have been assigned to L. (Cylicoceras) crassicostatum Rieber (Contini 1969: 4). This species is remarkably common at Amaurandes du Bas. Leioceras evertens (Buckman, 1899) is not an appropriate index species, since after Buckman (1899: suppl. pl. 1, figs. 10–12) its type horizon at Mapperton (Dorset) is unclear. Either it comes from the Murchisonae Zone or from the Scissum Zone of southern England (= Bifidatum Subzone). The chronospecies of this time slice should not be named after a species with an unknown type horizon, possibly coming from the Upper Aalenian.

A. Types of the name-bearing morphospecies of the described varieties

L. evolutum var. striate sensu Contini, 1969 [M] [pp. 19, 20; pl. 8, fig. 9; Maubeuge 1955, pl. 3, fig. 3; pl. 5, fig. 2]

This variety is characterized by its fine, relatively dense ribbing (Pl. 24, Figs. 6–8; Pl. 25, Figs. 2, 3b, c, 4–5; Pl. 26, Figs. 7–12).

L. evolutum var. subcostate sensu Contini, 1969 [M] [pp. 19, 20; pl. 8, fig. 10; pl. 9, figs. 1a, b]

Typical are wider spaced, stronger primaries; the body chamber can become smooth (Pl. 24, Fig. 5; Pl. 26, Figs. 4–6).

L. evolutum var. costate sensu Contini, 1969 [M] [pp. 19, 20, pl. 9, figs. 2, 3; Buckman 1899, Pl. 7, figs. 10–12; pl. 11, figs. 10–12]

Continuously sculptured, moderately strongly ribbed specimens represent the variant costate sensu Contini (Pl. 24, Fig. 4; Pl. 25, Figs. 1, 3a; Pl. 26, Figs. 1–3). The ammonite illustrated on Pl. 25, Fig. 1 still shows a slightly fastigate, less rounded venter.

L. evolutum var. crassicostatum ex Rieber, 1963 sp. [M]

We partly follow Contini (1969), who has determined ammonites with a strong ribbing up to the aperture as L. crassicostatum. The holotype and the specimens illustrated by Rieber (1963) as topotypes and chorotypes are sligthly more involute than our material. Therefore, here we use this taxon name only to describe a variant of the chronospecies L. evolutum (Pl. 24, Figs. 2, 3).

L. evolutum var. comptocostosum ex Chandler & Callomon, 2009 sp. [M] [HT: p. 120, pl. 1, figs. 1a, b]

A single specimen (Pl. 24, Fig. 1) exhibits a remarkably broad whorl section and is ornamented with thick, widely spaced ribs. It compares favourably with specimens from Southern England that are described as “biospecies” L. comptocostosum (Chandler & Callomon 2009, pl. 1, fig. 1 [HT], pl. 2, fig. 1).

L. evolutum var. paucicostatum ex Rieber, 1963 sp. [m]

Following Rieber (1963), who identified the microconchiate leioceratids from the slightly older crassicostatum Biohorizon of the western Swabian Alb as L. paucicostatum and L. striatum, the coarser-ribbed specimens are here termed as variant paucicostatum, and finely ribbed to almost smooth-shelled ones are assigned to the variant striatum. The whorl sections of microconchiate leioceratids of the evolutum Biohorizon are broader than those of the crassicostatum Biohorizon. Microconchs are almost restricted to concretions. Some of them are full of microconchiate leioceratids and bivalves and shell fragments. The ammonites illustrated on Pl. 25, Figs. 6a, 7, 8, 13–18, 23a are determined as L. evolutum var. paucicostatum, whereas those of Pl. 25, Figs. 16, 18 are transitional to L. evolutum var. striatum.

L. evolutum var. striatum ex Buckman, 1899 sp. [m] [syntypes see above]

The microconchs illustrated on Pl. 25, Figs. 9–12, 19–22, 23b–25 represent L. evolutum var. striatum. The specimens on Pl. 25, Figs. 19–20 and 22 are transitional to L. evolutum var. paucicostatum.

B. Variation within the chronospecies Leioceras evolutum

Adult macroconchiate specimens of the chronospecies L. evolutum are relatively uniform, with a diameter of c. 8 cm, a rounded venter on the body chamber, a depressed whorl section and an evolute coiling. Most specimens exhibit a weak ornamentation; only a few ones are stronger or coarse-ribbed. In the microconchs, the section of the venter on the body chamber is usually broad fastigate, occasionally rounded. The ribbing varies from almost smooth to coarse. Typical are, as in the macroconchs, a relatively broad whorl section and a large umbilicus.

Fig. 13.

(1, 2): Canavarella fasciata (Buckman) [M], (1) Heiningen, Teufelsloch Member, range area of the Torulosum concretion layer, opalinum Biohorizon, SMNS 70608/43, (2) Aichelberg slope of the motorway A8, Zillhausen Member, base of the Wasserfallschichten, hansrieberi Biohorizon, SMNS 70608/44. (3): Canavarella fasciata (Buckman) [m], Teufelsklingenbach gorge, Teufelsloch Member, Opaliniforme-Schichten, opaliniforme Biohorizon, SMNS 70608/45. (1–3) Opalinuston Formation; Lower Aalenian, Opalinum Zone, Opalinum Subzone. Ammonites slightly reduced in size. Scale bar equals 30 mm.

5.1.1. Opalinum Subzone

A. opalinum Biohorizon

The opalinum Biohorizon is recorded in the basal part of the Opalinuston Formation all over the foothills of the Swabian Alb. Since the relative proportions of Pleydellialike ammonites and leioceratids of the morphospecies L. opalinum – “Pleydellia” buckmani is variable in different localities, the beds assigned to the opalinum Biohorizon might have been deposited at different times, however, not further biostratigraphically distinguishable.

The misera Biohorizon (sensu Ohmert & Rolf 1991; Ohmert 1993; Ohmert et al. 1993; Ohmert 1996a, b) of Wittnau (Upper Rhinegraben valley) and the opalinum Biohorizon of Mistelgau (Schulbert 2001) correlate with the opalinum Biohorizon of the study area.

The lower part of the Opalinum Zone of Gretenberg (near Sehnde at Hannover) (Hoffmann 1913) corresponds approximately to the opalinum Biohorizon.

B. dilucidum Biohorizon

In the Swabian Alb, the dilucidum Biohorizon is additionally recorded from the clay pit at Weilen unter den Rinnen near Schömberg (ex coll. N. Wannenmacher †, now coll. V.D.; LGRB, coll. Ohmert) and at Mössingen near Tübingen (Seilacher et al. 1976: 330, 333).

Schulbert (2001) described the dilucidum Biohorizon from the clay pit at Mistelgau near Bayreuth (Upper Franconia). However, it remains an open question, whether it is fully identical with the dilucidum Biohorizon in the study area because Schulbert (2001) mentioned a common occurrence of P. dilucidum already in the opalinum Biohorizon, whereas this taxon is extremely rare in the opalinum Biohorizon in vicinity of Bad Boll. At Mistelgau, the dilucidum Biohorizon is restricted to the level “0 m” yielding abundant, partly extremely large-sized P. dilucidum. We assume that the dilucidum Biohorizon is widespread in the area of the Swabian and Franconian Alb despite there are no further records in the literature.

C. opaliniforme Biohorizon

The opaliniforme Biohorizon is recorded in the area between Donzdorf (Dietze & Schweigert 2016) to Weilheim an der Teck. Most likely, it is also present in the western Swabian Alb, a few tens of metres below the ‘Wasserfallschichten'.

D. hansrieberi Biohorizon

This biohorizon is well recorded in the study area. Furthermore, it occurs in the Breitenbach creek at Reutlingen and in the western Swabian Alb, for instance at the Zillhausen waterfall. In northern Franconia, the hansrieberi Biohorizon is also present (Dorn 1935).

E. bifidatum/rieberi Biohorizon

Up to now, the bifidatum/rieberi Biohorizon is only recorded in the area Gammelshausen – Boll – Weilheim an der Teck (Weber 1964; Ohmert 1993 [Ohmert named this horizon simultaneously lineatum horizon and lineatum-costosum horizon]). In the private collection of the late N. Wannenmacher (†, now coll. V.D.), there are a few ammonites possibly representing the bifidatum/rieberi Biohorizon coming from a level a few metres above the ‘Zopfplatten’ in the Steinlach creek near Mössingen. Further studies are necessary for a confirmation.

The ammonites illustrated by Hoffmann (1913, pl. 2) from the “Costosum Subzone” of Sehnde near Hannover (northern Germany) are a convolute of more or less stronger ribbed ammonites probably coming from various stratigraphic levels. Most of them are museum's collection material that has been subsequently assigned to the “Costosum Subzone”. Besides “Ludwigia costosa”, Hoffmann (1913) mentioned numerous body chambers of large-sized “Ludwigia opalina” from the “Costosum Subzone”. Consequently, a correlation of the bifidatum/rieberi Biohorizon and that “Costosum Subzone” is not possible. The large-sized body chambers of leioceratids more likely point to a correlation of the “Costosum Subzone” with the opaliniforme or hansrieberi Biohorizons.

Fig. 16.

Correlation of the biohorizons in SW Germany, S England and France. Remarks: ¹In France, this subzone is called Bifidatum Subzone, in S England Scissum Subzone. Both correspond to the Comptum Subzone in the former German usage, now re-named as Bifidatum Subzone. ²A re-naming of the ‘horizon à Crassicostatum' into the ‘horizon à Crassicostatum/Evolutum’ was necessary to avoid any confusion with the crassicostatum Biohorizon of the western Swabian Alb. The latter is the type horizon of Leioceras crassicostatum. ³The crassicostatum/evolutum Biohorizon does not exactly correlate with the evolutum Biohorizon (see Chapter 6.2). ⁴The bifidatum Biohorizon in France and S England is not exactly correlatable with the uncinatum or bifidatum/rieberi biohorizons (see Chapter 6.2). ⁵The French subglabrum Biohorizon and the opalinum Biohorizon are not exactly correlatable (see Chapter 6.2.). ⁶For the moment we keep the Swabian hansrieberi and the French and Southern English lineatum biohorizons separated and put the hansrieberi biohorizon in the top of the Opalinum Subzone (see Chapter 6.2).

F. uncinatum Biohorizon

In the entire Swabian Alb, this biohorizon is only known from the creek Teufelsklingenbach. The “Grenzbank” bed and the “Hauptmuschelbank” of northern Franconia (Schmidtill 1925–1926) yields leioceratids of the Bifidatum Subzone, possibly of the uncinatum Biohorizon. However, those Franconian sections and their ammonite faunas have to be re-evaluated.

G. crassicostatum Biohorizon

The crassicostatum Biohorizon is missing in the entire study area. However, it is widespread in the western Swabian Alb and adjacent Wutach area (Rieber 1963, 1966) and also occurs in the northern Franconian Alb (Schmidtill 1925–1926; Dorn 1935). For the moment, we switch the crassicostatum biohorizon between the uncinatum and evolutum biohorizons for several reasons: The average size of the ammonites of the crassicostatum horizon is between the average size of the ammonites of the uncinatum and evolutum biohorizons. The tendency of possessing a rounded venter on the body chamber also in smaller-sized macroconchs increases from the uncinatum via the crassicostatum to the evolutum biohorizon.

H. evolutum Biohorizon

This biohorizon is recorded with certainty only from the study area. Remarkably small-sized and evolute ammonites, which could represent this or a little older biohorizon, have been sampled from the Steinlach creek near Mössingen (coll. N. Wannenmacher †, now coll. V.D.), from the Hohenzollern area (coll. N. Wannenmacher †, now coll. V.D.), the Raißenbühl near Balingen-Frommern (GPIT and SMNS) and the Plettenberg near Schömberg (GPIT, SMNS) as well as from a concretion layer in the top of the Wilflingen-Bank near Gosheim (Rieber 1963, fig. 1; SMNS). Further studies of these localities are necessary.

A. France

Mostly based on previous studies by Contini (1969), Contini et al. (1997) subdivided the Opalinum Zone of France into five ammonite biohorizons (Fig. 16):

B. Southern England

A correlation with Southern England (Callomon & Chandler 1994; Callomon & Chandler 2009) is difficult for some intervals.

The British Leioceras opalinum horizon (Opalinum Subzone) corresponds to the opalinum Biohorizon. In Burton Bradstock (Southern England, however, the number of Pleydellia-like morphologies is lower than in the study area.

The British L. opaliniforme horizon (Opalinum Subzone) corresponds to the opaliniforme Biohorizon of the study area.

In contrast, the correlation of the British L. lineatum horizon and the L. bifidatum horizon with the herein newly introduced biohorizons is difficult. It is likely that the Scissum Bed of Burton Bradstock comprises time equivalents of the bifidatum/rieberi and uncinatum biohorizons. Based on observations by one of us (V.D.) we assume that the Scissum Bed of Burton Bradstock contains ammonite faunas of different ages over short distances making it almost impossible to distinguish between the beds of the lineatum and the bifidatum horizons in the field.

The British L. comptocostosum horizon must be younger than the evolutum Biohorizon, because of the high abundance of Ancolioceras morphologies within the chronospecies L. comptocostosum. Moreover, in the L. comptocostosum horizon the first ammonites with a Ludwigia-morphology appear, which are still missing in the evolutum Biohorizon. Because of their classification of the ammonites belonging to the genus Leioceras, Chandler & Callomon (2009) included the L. comptocostosum horizon in the British Scissum Subzone that corresponds to the Bifidatum Subzone.

C. Switzerland

Maubeuge (1955: pl. 2, figs. 3, 6; pl. 3, fig. 3; pl. 5, fig. 2) illustrated ammonites of the evolutum Biohorizon from Northern Switzerland.

Etter (1990) studied the lower Opalinuston Formation in various outcrops of Northern Switzerland. The illustrated, strongly compressed ammonites (Etter 1990: Pl. 7, figs. 6–8; pl. 8, figs. 1–5; pl. 9, figs. 1–6; Pl. 7, fig. 2) come from the opalinum and/or dilucidum biohorizons of the Opalinum Subzone.

Based on ammonite collections of Bodmer and Lieb in the Natural History Museum Basel, Christ (1999) subdivided the Lower Aalenian of the central Swiss Jura Mountains into four biohorizons, of which his opalinumsubglabrum Horizont corresponds to our opalinum and dilucidum biohorizons. A further correlation of Christ's (1999) biohorizons with those in our study area is impossible having only the listed leioceratid taxa and the few illustrated specimens available.

A rich Leioceras fauna of the lineatum Biohorizon is known from Schinberg (canton Aargau) (coll. Jutzi, Schwerzenbach/CH, coll. V.D.). Further samples from various amateur collectors led us to the conclusion that the Opalinum Zone of Northern Switzerland must contain a similar number of biohorizons as in SW Germany.

D. Spain

The GSSP of the Toarcian/Aalenian boundary is located at Fuentelsaz in the Keltiberian Ranges of Eastern Spain. It is defined by the first appearance of L. opalinum and the onset of the Opalinum Zone in the base of bed FZ 107 of that section (Cresta et al. 2001). The Lower Aalenian section of Fuentelsaz has been divided into the Opalinum and “Comptum” subzones. Ammonites from the c. 1.5 m-thick Opalinum Subzone and the over 20 m-thick “Comptum” Subzone of Fuentelsaz were illustrated by Goy & Ureta (1987) and Goy et al. (1991, 1994, 1996). At the base of the Opalinum Subzone, L. opalinum and L. lineatum occur – both ranging up into the basal Comptum Subzone – accompanied by P. buckmani and P. leura. L. costosum ranges a little higher in the Opalinum Subzone. Hence, bed FZ 107 correlates with the opalinum Biohorizon. A further subdivision of the Opalinum Subzone is impossible. The basal part of the “Comptum” Subzone with the last occurrences of L. lineatum and L. opalinum most likely corresponds to the lineatum Biohorizon of Southern England and France. A correlation of the higher up-section with the bifidatum/rieberi, uncinatum or crassicostatum biohorizons is not possible and any ammonites indicative of the evolutum Biohorizon are unknown from Fuentelsaz.

Ureta Gil (1983: pl. 3, figs. 2–11; pl. 4, figs. 1–12) described an ammonite fauna of the “Sz Comptum” from the Keltiberian Ranges of Eastern Spain, which corresponds very well with the bifidatum/rieberi Biohorizon of Southwestern Germany.

E. Further important records in Europe

In the section of Bakonycsernye (Hungary), Géczy (1967) distinguished a “Zone à Leioceras opalinum” and a “Zone à Leioceras comptum”. He illustrated numerous leioceratids, especially of the “Zone à L. comptum”. In Italy, sections of the Opalinum Zone are mostly incomplete or strongly condensed. Leioceratids of the Opalinum Zone have been reported, for instance from the Southern Alps (Vacek 1886) and from Sicily (Gemmellaro 1886; Pallini et al. 2004).

Sapunov (1970) described leioceratids from a condensed level comprising the Opalinum and “Comptum” subzones in Bulgaria.

A. Range area of the ‘Torulosum-Knollenlage’ and ‘Dilucidum-Schichten'

The concretions (‘Torulosum-Knollen') were formed at the storm wave base (Olivero 2007). The gypsum in the ammonites in the upper ‘Torulosum-Knollenlage’ has formed during recent weathering processes.

B. Lucinenbank

The levels with siderite concretions some meters below the Lucinenbank exhibit in cross-section small-scale ripples and lamination and hence, represent distal tempestites (e.g., Wetzel & Allia 2003).

The typical Lucinenbank represents a grain- to rudstone composed of variable amounts of crinoid debris, (fragmented) mollusc shells including disarticulated shells of the lucinid bivalve Mesomiltha plana, and dark grey, irregular-shaped to spherical, micritic limestone nodules a few millimetres to centimetres in size within a fine-grained marl or pyrite-bearing limestone matrix (Fig. 6 B, C). These nodules contain considerable amounts of pyrite, disseminated within the concretion interior or concentrated along the margin. The components constituting the Lucinenbank exhibit a grading as crinoid debris is enriched at the base and exhumed nodules and (fragmented) shells constitute the upper part of the bed (Fig. 6B). The lithology of the Lucinenbank varies laterally from an about 5 cm-thick limestone bed rich in mollusc shells associated with cone-in-cone structures (Fig. 6B) to a 1–2 cm-thick bioclast-rich interval covering a nodular limestone (Fig. 6C). The Lucinenbank (= Layer 3) needs to be interpreted in context with the interval underneath, comprising two horizons (layers 1, 2) with ellipsoidal concretions flattened parallel to bedding (Fig. 6A). The concretions in the upper layer (= Layer 2) exceed 10 cm in lateral extent whereas the deeper ones (= Layer 1) are somewhat smaller. The Lucinenbank interval documents a considerable episode of omission and repeated sediment reworking by at least 3 successive sediment winnowing to reworking episodes.

When growing by diffusion, stratabound concretions of the encountered size need to reside within the same geochemical zone for several hundreds to more than thousands of years (e.g., Raiswell & Fisher 2004; Blouet 2020 pers. comm.) and hence, mark a sedimentary hiatus (Wetzel & Allia 2000). Given that the concretion size corresponds to the duration of the sedimentary hiatus, the Layer 1 concretions formed during a shorter time span than those in Layer 2. However, currents causing the hiatus were too weak to exhume the nodules. In contrast, during formation of Layer 3, first, currents removed crinoids living in a supposedly somewhat elevated area from their substrate (e.g., Wetzel & Meyer 2006). Later, currents reworked an at least 10 cm-thick sediment layer as indicated by the enrichment of the lucinid shells and some small-sized concretions that were bored when exposed on the seafloor. In modern, organic-rich, shallow-marine sediments, similar-sized lucinids live closely above the sulfate reduction zone in about 3–10 cm sediment depth or more (e.g., Stanley 1970). Without exception, lucinids are chemosymbiotic, ‘sulfide-mining’ organisms (Taylor & Glover 2006). The sulphide is taken via tubes produced by the bivalve downward and processed in its gills by symbionts that provide nutrition (Taylor & Glover 2006). Consequently, sulfide must have been available within the reach of the lucinids probably in the range of a few centimeters or more (e.g., Stanley 1970). The exhumed nodules, therefore, must have formed below the lucinids or somewhat deeper within the sulfate reduction zone, maybe 10–15 cm. Currents reworked this interval. With time, the number of exhumed nodules increased and paved the seafloor together with exhumed shells and hence, prevented further erosion. The lateral lithological variability of the Lucinenbank implies that the seafloor was structured by a relief. Consequently, the layers 1–3 document an increasing intensity of bottom current reworking. However, after formation and burial of Layer 3, deposition was still retarded for some time during that cementation within the sulfate reduction zone took place as indicated by pyrite impregnation of the matrix. The development of cone-in-cone limestone layers remains to be resolved.

C. Levels of the ‘Pentacrinitenplatten'

The crinoid accumulations are caused by storm events and form small, laterally thinning lenses (Wetzel & Meyer 2006).

D. Opaliniforme-Schichten

The sampled concretions of layer K 9 exhibit on all sides, up to 2 cm deep and 0.8 cm wide Gastrochaenolites borings being truncated to a varying degree. They are filled with marl and some contain the still articulated shells of the producers. Locally, only preserved within depressions, oysters are attached to the nodules. Thus, this interval documents an episode of sediment reworking and omission. Prior to exhumation of the nodules, sediment deposition was retarded to allow their formation (e.g., Wetzel & Allia 2000; see above). Later, the concretions became exhumed by currents and bored by the Gastrochaenolites producers. The currents were strong enough to turn the nodules and to cause their partial abrasion. When the amount of suspension increased, oysters adapted to live in muddy water settled on the nodules. Before final burial, currents accelerated and partly removed the oysters.

Further up, about 30 cm below the fossil-rich bed K10, another horizon with stratabound concretions marks a sedimentary discontinuity. These nodules may reach a size of several centimeters and rarely contain fossils. The concretions appear to be genetically related to the fossil-rich layer at the base of bed K10, as winnowing currents enriched the shells and retarded deposition for a period of time sufficient for concretion formation (e.g., Raiswell & Fisher 2004; Blouet 2020, pers. comm.).

Within bed K10, the different layers with concretions may represent different stages of repeated sediment reworking, as elsewhere in this section. The Opalinusknollen-type concretions probably formed after storms when combined flow enriched fossils in pot-casts (e.g., Cheel & Leckie 1993; Olivero 2007). The stratabound limestone concretions containing no or only few fossils were not exhumed, but nonetheless represent simple sedimentary hiatus (see above).

E. Belemnite Breccia

The Belemnite Breccia is intercalated in typical, dark-grey, platy Opalinuston mudrock and marks another sedimentary discontinuity. The Belemnite Breccia represents a rudstone composed of (fragmented) mollusc shells, locally high concentrations of belemnite rostra, and two types of exhumed concretions embedded in marly limestone; (1) large, ellipsoidal, light brownish-beige nodules, 5 cm high and >15 cm wide, and (2) flat, dark-grey to black, pyrite-bearing limestone concretions up to 6 cm in size that occasionally exhibit Gastrochaenolites borings, up to 1.5 cm deep and 0.5 cm wide.

The brownish concretions exhibit locally small-scale current ripples, but mainly bioturbational structures that show a succession indicative of progressive substrate induration; (1) indistinct bioturbational/biodeformational structures that disturb the original bedding, (2) Phycosiphon, (3) Planolites-like burrows, (4) Chondrites, and (5) probably Rhizocorallium (e.g., Wetzel 2015). The first two burrow types (Suite-1) contain material similar to the concretion body whereas the latter traces (Suite-2) are actively filled with the dark marl housing the concretions (= matrix of the Belemnite Breccia). Both Suite-1 and Suite-2 burrows are truncated at the surface of the nodules. Obviously, the locally preserved small-scale cross-bedding records the depositional currents. The Suite-1 burrows document the colonization of this muddy event bed (e.g., Wetzel & Uchman 2001; Celis et al. 2018). The dark marl filling the Suite-2 traces implies burial of the horizon wherein these concretions grew but they still resided within the bioturbated zone and became increasingly cemented. Below, in the sulfate reduction zone, the dark limestone concretions probably formed at the same time.

The onset of bottom current activity and hence, the slow-down of deposition appears to be recorded by ripple formation, followed by omission documented by the bioturbation structures indicating progressive induration of the brownish nodules, while the dark concretions grew below. Then both types of concretions became exhumed, transported, and embedded within the lag deposits. Obviously, the limestone concretions were more indurated than the brownish nodules, as indicated by Gastrochaenolites borings compared to the not-bored brownish nodules exhibiting truncated burrows. The bottom currents were locally and temporarily so fast that only belemnite rostra accumulated.

F. Level of the ‘Zopfplatten'

The upper part of the Zopfplatten interval (ZoP 5, 6) exhibit typical hummocky cross-stratification characterized by a wave-length of 1–1.5 m and an amplitude of 10–15 cm that is typical of tempestites (e.g., Cheel & Leckie 1993). These beds formed slightly above storm-wave base in a water depth of c. 30 m or less (Wetzel & Allia 2003). They consist of fine-sand laminae separated by thin dark mud veneers and contain locally greyish ellipsoidal limestone concretions up to 3–4 cm wide and 2–3 high having their long axis inclined to bedding. The sand laminae are bent around and do not crosscut the exhumed concretions and hence, indicate transport of the latter during the storm. The dark muddy interlayers were preferentially reworked by producers of Chondrites. These traces continue into the concretions. Therefore, the concretions were only slightly lithified when they became exhumed during a storm, otherwise the Chondrites burrows would not continue from the surrounding sediment into the nodules. This observation provides striking evidence that concretion formation was a slow process (see above) and nodules forming near the sediment surface were only partially, but sufficiently cemented to be resistant against compaction but still in a plastic state (Wetzel 1992).

G. Transformation of the (white) aragonitic shell into (brown) cacitic shells of the fossils

Within the Opalinuston ammonites with white, aragonitic shells occur from the base up to -4.9 m below the top of the Wasserfallschichten, whereas further up, the ammonite shells consist of brownish calcite (e.g., Andalib 1973). Within a thin transitional extent of strata, ammonite shells may show partial replacement of aragonite by calcite starting from the outer surface (Andalib 1973). Both aragonite as well as calcite crystals are 3–5 µm in size. The preservation of the original shell mineralogy, in particular of metastable aragonite, appears to depend on several factors; most important are (1) reducing conditions within the host sediment (Hall & Kennedy 1967), (2) low permeability of the host sediment (Hall & Kennedy 1967; Andalib 1973; Janiszewska et al. 2018), and (3) seawater composition at the time when the aragonitic fossils became embedded.

Reducing conditions within the sediment maintain the protective effect of the organic skeletal matrix (Hall & Kennedy 1967). Within the claystones of the Opalinuston, reducing conditions developed at shallow levels within the sediment as evidenced by the occurrence of (i) dark color (e.g., Myrow 1990), (ii) a relatively high organic carbon content >0.2% (e.g., Nagra 2008), (iii) the recurrent presence of the trace fossil Chondrites (e.g., Bromley & Ekdale 1984), (iv) framboidal pyrite (e.g., Lerouge et al. 2014; Lauper et al. 2018; Taylor & Macquaker 2000), and (v) early diagenetic concretions formed within the sulfate-reduction zone (e.g., Wetzel & Allia 2000).

Low permeability prevents movement of fluids and retards diffusion such that both can cause mineral transformation if pore water composition changes. In fact, the Opalinuston Formation exhibits a very low permeability in the range of 10^-13 m s^-1 in the compacted state (e.g., Nagra 2008). This is mainly caused by the high clay content that may exceed 60% in the lower part of the formation whereas towards the top the clay content may decrease to 35–40% and less as determined about 130 km to the SSW of the studied site in the well Benken (Switzerland; Nagra 2008). Palaeogeographically relative to Benken, the studied site is located more proximally to the basin (e.g. Ziegler 1990). The above given lithologic data are confirmed by our analyses (see section 8.1).

During the Aalenian, inorganic precipitation of aragonite was facilitated by the Mg:Ca ratio of seawater >2, whereas during the Bajocian–Bathonian it shifted to <2 fostering inorganic calcite precipitation (e.g., Lowenstein et al. 2001). The original Mg²⁺:Ca²⁺ ratio of seawater was very likely kept during early diagenesis and transformation of aragonite to calcite was hindered (e.g., Fernándezdíaz et al. 1996) due to the low permeability of the host sediment.

In the studied section a few metres below the Wasserfallschichten, the sand content increases on the expense of the clay content as sediment reworking by waves and currents becomes more frequent as indicated by coarse-grained layers, chamositic oolites, and horizons containing exhumed early diagenetic concretions and, finally, by the Eisensandstein Formation (e.g., Franz & Nitsch 2009). The Wasserfallschichten already contain up to 25% sand in the decalcified sediment fraction (Andalib 1973). The increased sand content caused an increasing permeability facilitating downward diffusion of seawater having a Mg²⁺:Ca²⁺ ratio <2 since the Bajocian, favoring calcite precipitation (e.g., Lowenstein et al. 2001). This interpretation is supported by experimental evaluation of diffusion in compacted Opalinuston mudrock showing differences between the basal and top interval of the Opalinuston, being somewhat higher and lower, respectively (e.g., Degueldre et al. 2003). The boundary between preserved aragonitic and replaced calcitic ammonite shells evidently depends on mudrock properties and thus, may vary locally in depth below the top of the Wasserfallschichten, as already mentioned by Andalib (1973).