The moderately rich ammonite assemblage of the distractum Biohorizon is described from the Lambertiknollen Bed, a set of phosphatic concretions layers in the upper part of the Callovian Ornatenton Formation of SW Germany. The distractum Biohorizon is dominated by the highly variable Subboreal species Quenstedtoceras lamberti (Sowerby, 1819), the index ammonite of the Middle Jurassic Lamberti Zone. In addition, there are several species of Submediterranean oppeliids besides Submediterranean aspidoceratids, peltoceratids, perisphinctids, and Subboreal kosmoceratids. Tropical faunal elements such as lytoceratids and phylloceratids did not reach the area during this time interval. After a short hiatus indicated by an intercalated belemnite breccia, another layer with phosphatic concretions follows. It contains mostly reworked and fragmented ammonites of the newly introduced aff. zudacharicum Biohorizon, which is the youngest biohorizon of the Lamberti Zone in the Swabian Jurassic. Despite the presence of Submediterranean taxa, the ammonitic assemblages have a rather Subboreal character. This Subboreal character of the ammonite assemblages as well as the great abundance of phosphatic concretions, clay sedimentation and glauconite formation points to rather cold seawater temperatures at least at the seafloor probably caused by upwelling. A comparison of the two succeeding ammonite assemblages points to an increased cooling trend towards the Callovian-Oxfordian boundary.
1. Introduction
The Swabian Jurassic became a classical area of biostratigraphy due to the outstanding monographs on Jurassic ammonites by Zieten (1830–1833), Quenstedt (1845–1849, 1885–1888) and Oppel (1862–1863) and the rock succession containing these time-diagnostic fossils (Quenstedt 1843, 1856–1858; OPPEL 1856–1858). Despite this long tradition and plenty of subsequent studies dedicated to the refinement of the stratigraphic knowledge, a high-resolution biostratigraphy, as it is needed i.e. for exact dating of strata and correlation of beds and deciphering short-term climate changes or faunal migrations, is still missing for larger parts of the Jurassic section in this area. The reasons for this deficiency are manifold, but one main reason is that some parts of the section are rarely exposed and permanently accessible natural outcrops of these intervals do not exist at all. This is especially true for the clayey upper part of the Braunjura Group of Swabia, which is often covered with thick hillside debris of the overlying Upper Jurassic limestones and marls of the Weißjura Group.
The focus of this paper is to fill the gap of stratigraphic information for the Lambertiknollen Bed of latest Middle Jurassic age. This glauconitic bed belongs to the clayey Ornatenton Formation, which continues into the Oxfordian and is characterized by a mass occurrence of phosphatic concretions (Herde 1887; Käss 1954). The abundance of phosphatic concretions in the terminal Callovian deposits of Southern Germany and other areas might reflect a global signal for a phosphorous excursion (Ramkumar et al. 2012), possibly triggered by climatic conditions.
2. History of research
2.1. Lithostratigraphy
Quenstedt (1843) was the first who reported on the Lambertiknollen Bed in the Middle Jurassic of Swabia, then called “schwarze Mergelkugeln der obersten Lagen des Ornatentons” (“black marl nodules in the uppermost layers of the Ornatenton Formation”). Subsequently, Quenstedt's student Zakrzewski (1886) studied the transitional beds between the Middle and the Upper Jurassic of Swabia. He introduced the term “Lambertiknollenschicht” for the concretions-bearing beds. In the same year, Quenstedt (1886) used the terms “Lambertischicht” and “Schwarze Knollen” (= black nodules) as synonyms. Subsequently, a variety of names and spellings was used for the same bed, such as “Lambertiknollen” (Herde 1887; Scholz 1966), “Knollenschichte” (Engel 1908, 1911), “Lamberti-Knollen” (Engel 1908; Stahlecker 1926; Model 1935), “Lambertiknollenschicht” (Stahlecker 1934), “Lamberti-Zone” (TERZIDIDS 1966), “lambertiKnollen” (ETZOLD et al. 1975; Geyer & Gwinner 1979) or “Lamberti-Knollen-Horizont” (Lippold 1983). Here we use the term Lambertiknollen Bed for this thin but significant rock interval located in the upper part of the Ornatenton Formation of the Braunjura Group.
2.2. Previous studies of ammonites of the Lambertiknollen Bed
In 1832, Zieten (1830–1833: 38, pl. 28, fig. 1) was the first who reported on the occurrence of Ammonites Lamberti Sowerby at Reichenbach im Täle and in the vicinity of Stuifen Mountain, however, without providing stratigraphic details. Subsequently, Quenstedt (1843: 384) identified the horizon containing this ammonite as a concretion layer (“schwarze Mergelkugeln”) in the uppermost part of the Braunjura Group. Later, he already realized that Ammonites Lamberti is a highly variable species; for some of these varieties he introduced third names (Lamberti macer, Lamberti pinguis, Lamberti inflatus; Quenstedt 1845–1849). He also published other ammonites – as we know today – that originate from the same bed, however, without referring to their precise horizon (Quenstedt 1856–1858). An early overview on the transitional beds between the Middle and Upper Jurassic (“Lamberti-Cordatus-Schichten”) in Central Europe was provided by Bertschinger (1883). However, his study is not very detailed and ammonites were not illustrated at all; therefore, it was not relevant for our research. Finally, Quenstedt (1887: pls. 82, 89) illustrated numerous ammonites assigned to the Lambertiknollen Bed (then termed as “Schwarze Knollen”) from various localities of the Swabian Jurassic. One year earlier, Quenstedt's student Zakrzewski (1886) had reported on the lithostratigraphic position of the Lambertiknollen Bed and listed a few ammonite taxa. However, Zakrzewski (1886) illustrated only two specimens, “Amm. Lamberti inflatus QU.” and “Amm. convolutus gigas QU.”, from this bed. The absence of illustrations of sections and ammonites in subsequent literature cited above hampers comparisons with the herein studied material. Some ammonites of the Lambertiknollen Bed or from adjacent strata were also discussed in systematic palaeontology papers (Pietzcker 1911; Prieser 1937; Zeiss 1959, 1962); these studies, however, are not based on modern species concepts and tended to an oversplitting of taxa despite lacking precise biostratigraphic control of the samples. Schlegelmilch (1985) and Riegraf (1991) illustrated photographically for the first time ammonites from the Lambertiknollen Bed of Swabia. The first detailed description of an ammonite-bearing section containing the Lambertiknollen Bed in the Jurassic of Swabia was provided by Scholz (1966) in his unpublished diploma thesis. This only temporarily accessible section was located in the construction pits of the “Pumpspeicherwerk Glems” near Metzingen, not far away from the often-cited classical locality “Linsengraben”, a small creek located NE of the village Glems where the Ornatenton Formation was exposed. In the same year, Terzidis (1966, fig. 3) published another short section from Glems that covers the Lambertiknollen Bed and reported the presence of Quenstedtoceras lamberti and a few other ammonites, again without illustrations. Later, an artificially excavated section at the foot of Achalm Mountain was studied by Lippold (1983). Unfortunately, however, her samples were not deposited in a public collection and the fate of the material is unknown. However, it should be noted that Lippold (1983), Geyer & Karapantelakis (1980) and Riegraf (1987a, 1987b) assigned the depositional age of the Lambertiknollen Bed in some sections of Swabia and Franconia to the lowermost Oxfordian Marie Zone. Similarly, Riegraf interpreted this bed as highly condensed and the phosphatic concretions as reworked from older clayey beds spanning in age from the Jason to the Athleta zones, despite the undoubted presence of the zonal index of the late Callovian Lamberti Zone itself, Quenstedtoceras lamberti. Possibly, his view was triggered by descriptions of a condensed layer of phosphatic concretion occurring at the Middle-Upper Jurassic boundary of northern Franconia (Reuter 1908; Model 1935; Model & Model 1938; Dietl & Mönnig 2016). Indeed, many ammonite taxa listed from this thin layer point to some reworking from older beds, since these taxa normally do not co-occur in the same bed. However, it is not clear how precise the sampling for those studies was. In the Callovian of Swabia, numerous ammonite biohorizons of the Coronatum, Athleta and Lamberti zones could be identified within the clayey Ornatenton Formation without any evidence of re-elaboration and mixing (Dietl 1993, 2011, 2019, 2022). Therefore, the radiolarians and protoglobigerine foraminifera etched out from phosphatic concretions of Swabia are not reworked from a huge stratigraphic interval as suggested by Riegraf (1987a, 1987b) but must be Jason and Lamberti Zone in age, respectively.
3. Material and methods
The Lambertiknollen Bed is present all over the Swabian Alb and adjacent areas (Wutach area, Franconia), but due to the absence of permanent natural outcrops there exists only some randomly sampled ammonite material both in public and private fossil collections. In 1989, during a renewed construction of the motorway A8 from Stuttgart to Munich, a scientific excavation team of the Stuttgart Natural History Museum (G. Dietl, M. Rieter, M. Kapitzke) recovered over 1.000 fossils – mostly ammonites – from the Lambertiknollen Bed at Gruibingen (Fig. 1). The bulk of these ammonites are three-dimensionally preserved macroconchs which are partly or completely embedded in phosphatic nodules. Lappeted peristomes of microconchs are never preserved. Sometimes only the body-chambers are preserved in this mode, but also their nuclei can be phosphatic. This clearly indicates an early diagenetic phosphatization at place. Cracks within the concretions are often filled with barite or sphalerite. At the occasion of the 3rd International Jurassic Symposium held in Poitier, France, the first scientific results of these excavations were presented. In this context, the herein studied distractum Biohorizon was informally introduced (Dietl 1991). Subsequently, Thierry et al. (1997) cited this piece of information and included it in their summary on the late Callovian Lamberti Zone in Europe. More recently, a provisory faunal list and a few illustrations of ammonites from the motorway construction outcrop near Gruibingen were provided by Dietl (2013, figs. 66–68). That preliminary report, however, gives only a short glimpse on the potential biostratigraphic, autecological and biogeographic information connected with the vast material. Therefore, we here document both the field observations and discuss the ammonite assemblages recovered from the Lambertiknollen Bed, with a focus on the distractum Biohorizon.
A significant portion of the studied ammonites was prepared mechanically with needles and pneumatic chisels; additional specimens, which are partly enclosed within the phosphatic nodules, were identified by direct comparison of their ventral aspects with the prepared specimens. Most of the illustrated specimens were photographed with a Leica V-Lux digital camera. The individual photos were arranged to figures or plates with Photoshop version CS5.1. All studied material is stored in the collection of the Natural History Museum in Stuttgart, Germany (acronym: SMNS).
Abbreviations: [m] = microconchiate specimen/taxon, [M] = macroconchiate specimen/taxon.
Fig. 1.
A – Simplified geological map of Germany with focus on the Jurassic of Southern Germany and showing the location of the study area (black arrow) (map modified from https://www.bgr.bund.de/). B – Detailed geological map showing the location of the studied section (red arrow). Blueish colours: Upper Jurassic; greenish colours: Middle Jurassic; dark red: Miocene volcanic diatreme; light red: landslide areas; dark beige: Quaternary valley deposits; light beige: areas with hillside rubble; grey: anthropogenic deposits (map modified from https://maps.lgrb-bw.de/).
4. Lithological section of the Lambertiknollen Bed at Gruibingen
Aside other partial sections of the Middle to lowermost Upper Jurassic Braunjura Group which were temporarily exposed along the motorway construction between the villages Aichelberg and Gruibingen (Fig. 1), a generalized profile of the Ornatenton Formation (uppermost Bathonian – lower Oxfordian) was preliminarily published in Dietl (2013, fig. 44), however, lacking details for the herein studied rock interval containing the Lambertiknollen Bed. Hence, a more detailed section of this interval is provided in Fig. 2.
Fig. 2.
Stratigraphic section of the transition upper Middle Jurassic (Braunjura Group) – lowermost Upper Jurassic (Weißjura Group). The detailed section shows the studied part with two layers of phosphorite concretions (L1, L2) in the bottom, followed by a belemnite accumulation (B) in the middle part and reworked concretions (L3) in the upper part. D = Dentalienton Formation; Imp. F. = Impressamergel Formation; Bath. = Bathonian; Herv. = Herveyi Zone; Koen. = Koenigi Zone; Call. = Calloviense Zone; Coron. = Coronatum Zone; Lamb. = Lamberti Zone; Transv. Z. = Transversarium Zone.
In the Gruibingen section, the Lambertiknollen Bed is underlain by ca. 120 cm thick dark clays of the Ornatenton Formation. Apart from very rare ammonites no other macrofossils have been recorded. Below, sporadically follow large septaria-type concretions with diameters exceeding 20 cm. These limestone concretions are late diagenetic in origin and lack any macrofossils. A closely comparable section was reported from the foot of Irrenberg Mountain near Bisingen-Thanheim (Dietl 2011) in the area of the western Swabian Alb. In the latter section occurs a similar layer of large septaria concretions in an almost identical stratigraphical position; however, shortly above this layer another ammonite-bearing level was detected. The ammonite assemblage of this level was informally termed as “compressum”-Horizont (Dietl 2011), named after the preoccupied and thus invalid Kosmoceras “compressum” (Quenstedt, 1846), the holotype of which is lost due to pyrite decay. This biohorizon could not be traced in the Gruibingen section.
At Gruibingen, the rock interval of the Lambertiknollen Bed consists of three distinct phosphatic concretion layers within a dark-grey clayey glauconitic matrix. The lowest one was termed as layer L1. This layer contains scattered larger, mostly well-rounded phosphatic concretions with rare ammonite. Immediately below this layer a single specimen of Quenstedtoceras praelamberti Douvillé, 1912 (SMNS 70710, Pl. 2, Fig. 8) was found. This taxon is age-diagnostic for the lower part of the Lamberti Subzone (Thierry et al. 1997) and gives a maximum age for the Lambertiknollen Bed. Immediately above layer L1 follows L2. Here, the phosphatic concretions are smaller than in L1, but extremely abundant and relatively rich in ammonites of the Lamberti Subzone. The bulk of the studied ammonite material originates from L2. A few examples of ammonite-bearing concretions of this layer were illustrated by Dietl (2013, figs. 67, 68). Layer L2 terminates with a belemnite breccia indicating a hiatus of unknown duration. Similar belemnite breccias also occur in other levels of the Callovian Ornatenton Formation (e.g., Dietl & Mönnig 2026; Dietl 2019; Schweigert et al. 2021). Immediately above this belemnite breccia follows the highest concretion layer, here named L3. The phosphatic concretions of layer L3 significantly differ from those of the layers L1 and L2 below in their characteristic state of preservation. They are mostly fragmented and exhibit a blackish to dark-greenish coloured polished surface. This peculiar preservation style clearly points to longer exposure at the seafloor. However, we could not find any faunistic overgrowth on these hiatus concretions nor any borings.
Finally, the Lambertiknollen Bed is overlain by grey glauconitic sandy marls of the Glaukonitsandmergel Member, which represents the uppermost member of the Ornatenton Formation and of the entire Braunjura Group. Ammonite assemblages sampled from this member in the same outcrop at Gruibingen indicate the presence of the lower Oxfordian Cordatum Zone and the middle Oxfordian Plicatilis Zone. The ammonite assemblages of this interval have not been studied in detail yet. Little higher up in the section, crushed cardioceratids and peltoceratids of the Cordatum Zone appear. Then follow grey calcareous marls and thin light-grey limestone beds of the Upper Jurassic Impressamergel Formation (Transversarium Zone, middle Oxfordian).
The lowest zone of the Oxfordian, the Mariae Zone, is either extremely thin or completely missing in the Gruibingen section, as in most parts of the Swabian Jurassic. Only in the vicinity of Reutlingen, at Gönningen, the Mariae Zone could be safely identified by the record of the age-diagnostic ammonite Creniceras renggeri (Oppel, 1862) (SMNS 70714, former collection of Stuttgart University). Lippold (1983) also reported this species from the Achalm Mountain near Reutlingen. If sediments of the latest Lamberti or Mariae zones are present in the Gruibingen section, this concerns only the lowermost centimetres of the Glaukonitsandmergel Member. From immediately above Bed L3, a single small Quenstedtoceras sp. (Fig. 3; SMNS 70713) in phosphatic preservation (but not within a concretion) was recovered. This specimen is indistinguishable from a small specimen illustrated as Cardioceras (Pavloviceras) redcliffense (Page et al. 2009b, pl. 1, fig. E), a species that was suggested to characterize the earliest biohorizon of the Oxfordian at Redcliff Point, one of the suggested but still unratified GSSP sections for the base of the Oxfordian Stage. However, it is impossible to determine the age of this sole specimen, because a high-resolution biostratigraphy with cardioceratids is only possible with a statistically significant amount of specimens. This is why even the definition of C. redcliffense by Page et al. (2009) was criticized as being vague and this taxon was better included in Quenstedtoceras paucicostatum Lange, 1973 (see Fortwengler et al. 2012; Kiselev et al. 2013).
5. Ammonite assemblages of the Lambertiknollen Bed
All three concretion layers (L1, L2, L3) yielded ammonite assemblages. In layer L1 only a few ammonites were found, represented mainly by species of Peltoceras und Kosmoceras aside some Quenstedtoceras lamberti (Sowerby, 1819) and hecticoceratids. Taxonomically, these ammonites are fully identical with those of layer L2. The few specimens do not allow a statistical analysis about the distribution of taxa in this layer compared with the vast material of layer L2. From the latter, 988 ammonite specimens were recovered. This number is statistically significant; thus, the assemblage from layer L2 is used here as a basis for the formal description of the distractum Biohorizon. In the entire area of Swabia, no further locality is known with a similarly rich and diverse ammonite assemblage of this age. Of course, this richness must not reflect favorite local preservation conditions but can be interpreted as the result of scientific bulk sampling from a uniquely large outcrop, whereas material from anywhere else became widely dispersed by plenty of amateur collectors and fossil traders. Random sampling indicates the presence of the distractum Biohorizon over large distances across most parts of the Swabian Alb (e.g., Stuifen Mountain, Gairen near Reichenbach im Täle, Achalm Mountain and Pfullingen (Ursulaberg) near Reutlingen, Lochen Mountain near Balingen, Egesheim).
The following ammonite taxa characterize the distractum Biohorizon; examples are illustrated on Plates 1–9 and Figs. 3–5:
Quenstedtoceras lamberti (Sowerby, 1819) [M + m] (Fig. 4; Pl. 1, Figs. 1–7; Pl. 2, Figs. 1–7; Pl. 3, Figs. 1–5)
Paraspidoceras distractum (Quenstedt, 1857) [M] (Pl. 6, Figs. 1, 4)
Euaspidoceras hirsutum (Bayle, 1878) [M] (Pl. 5, Figs. 1–3)) Euaspidoceras cf. subbabeanum (Sintzow, 1888) [M] (Pl. 6, Figs. 2, 3)
Peltoceras subtense (Bean in Leckenby, 1859) [M + m] (Pl. 3, Figs. 6–9; Pl. 4, Figs. 1–6)
Kosmoceras n. sp. aff. kuklicum (Buckman, 1926) [M] (Fig. 6; Pl. 7, Fig. 10; Pl. 8, Figs. 1–6)
Choffatia poculum (Leckenby, 1859) [M + m] (Fig. 6A, B; Pl. 7, Figs. 1–5)
Alligaticeras alligatum (Leckenby, 1859) [M + m] (Fig. 7A, B; Pl. 7, Figs. 6, 7)
Distichoceras nodulosum (Quenstedt, 1887) [M] (Pl. 7, Figs. 8, 9; Pl. 9, Figs. 3, 4)
Lunuloceras paulowi (DE Tsytovitch, 1911) [M] (Pl. 9, Fig. 7) Orbignyiceras pseudopunctatum (Lahusen, 1883) [M + m] (Pl. 9, Figs. 1, 2, 5, 6)
Putealiceras puteale (Leckenby, 1859) [M + m] (Dietl 2013, fig. 67 right)
Richeiceras sp. [M] (Fig. 8C, D)
Rollieria cf. kormosi (Lóczy, 1915) [M] (Fig. 8A, B)
Despite the very large number of specimens we could not find any phylloceratids or lytoceratids; at least the former are still present in some beds of the older Athleta Zone (e.g., Dietl 1993). Also in other samples from the same rock interval in Swabia, these Tethyan ammonite groups seem to be completely missing. Nautilids have not been recorded in our sample from Gruibingen, either. However, extremely rare specimens of Pseudaganides sp. in phosphatic preservation from Swabia and Franconia most likely originate from equivalents of layer L2.
Within the rock interval of the distractum Biohorizon we noticed a few small oysters (Liostrea sp.) directly attached to ammonites and an incomplete article of an erymid lobster; however, no further macrofauna was recorded. In addition, a single piece of driftwood was recovered. In the body-chamber of a very large specimen of Quenstedtoceras lamberti (Sowerby, 1819) we noticed Chondrites targionii (BRONGNIART, 1818) burrows (Fig. 4) indicative for a dysoxic environment (Bromley & Ekdale 1984).
Layer L2 terminates with the abovementioned belemnite accumulation consisting of large-sized but mostly fragmented rostra of Hibolithes semihastatus (Blainville, 1827). This peculiar belemnite breccia was already reported by Engel (1908). Above this belemnite accumulation follows another concretion layer named L3. This layer is much less rich in ammonites compared to L2. Only a total of 50 ammonite specimens was recovered from L3. These ammonites are often still enclosed within the phosphatic concretions, but most of the concretions are fragmented so that also the ammonites are less complete than those of layer L2. At first glance, the ammonite assemblage of layer L3 looks close to that of the distractum Biohorizon because it is also predominated by cardioceratids. However, the accompanying ammonite taxa are quite different; peltoceratids are missing and even the variation within Quenstedtoceras has significantly shifted and new morphologies appear, which are characterized by a strong and distant prorsiradiate ribbing and a complete loss of sculpture in the relatively small-sized adult stage. These advanced macroconchiate species is identified here as Quenstedtoceras pseudolamberti (Sintzow, 1888). Specimens with a broad whorl section occur as well but seem to be rarer than in layer L2. Some evolute, possibly microconchiate forms with very coarse and distant ribbing are already close to Qu. paucicostatum Lange, 1973, a species based on a few strongly crushed specimens from the Ornatenton Formation of northwestern Germany. The latter species is used as an index for a biohorizon of the topmost Lamberti Zone (Fortwengler et al. 1997, 2012; Page 2004; Kiselev et al. 2013; Kiselev 2022), but its exact position at the German type locality remains unknown. As the most diagnostic taxon of this assemblage we identified Kosmoceras “zudacharicum KAZANSKII, 1909”, which was reported from European Russia from a probably time-equivalent stratigraphic level in the upper Lamberti Zone (Kiselev et al. 2013). More recently, the same material was determined as Kosmoceras cf. mrazeci Simionescu, 1899 (Kiselev 2022). K. mrazeci, a species originally described from the upper Callovian of Bulgaria is considered a synonym of K. duncani(Sowerby, 1819) (Howarth & Stephanov 1965). This view is confirmed by the typical broad bundles of ribs meeting in ventrolateral nodes, a character not seen in the specimens from the paucicostatum Biohorizon of European Russia, which is we do not follow this determination. The stratigraphically latest Kosmoceras species from the Gruibingen section is much smaller-sized than Kosmoceras n. sp. aff. kuklicum of layer L2. The ribbing of the inner whorls shows numerous irregularities and looped ribs. On the body-chamber the ribbing becomes coarse and distant, with few furcations, and the venter becomes well-rounded. However, the holotype of Kosmoceras zudacharicum illustrated by KAZANSKII (1909) is a very fragmentary specimen and does hardly allow to recognize species-diagnostic characters; hence, we should refrain from using the name zudacharicum in the original sense for the poorly known latest Kosmoceras species. Similar morphologies occur in the French lamberti Biohorizon of the upper but not topmost Lamberti Zone, were this taxon was informally termed “Kosmoceras duncani in BADALUTA 1976” (Fortwengler et al. 1997, 2012). From the French paucicostatum Biohorizon no kosmoceratids have been reported yet. This could be interpreted in two ways. Either kosmoceratids persisted longer in the epicontinental sea of present-day Russia, or the French and Russian paucicostatum biohorizons are not exactly coeval.
Fig. 4.
Largest specimen of Quenstedtoceras lamberti (Sowerby, 1819) [M], SMNS 70711/39, with Chondrites targionii (BRONGNIART, 1828) in its body-chamber; distractum Biohorizon, Ornatenton Formation, Gruibingen, SW Germany. Scale bar equals 50 mm.
Our limited and incomplete material does not allow claiming an index for the peculiar biohorizon represented in layer L3; hence, we here tentatively named it “aff. zudacharicum” Biohorizon. The ammonite taxa recorded from this biohorizon are:
Quenstedtoceras pseudolamberti (Sintzow, 1888) [M] (Pl. 10, Figs. 1–5)
Quenstedtoceras cf. paucicostatum Lange, 1973 [m] (Pl. 10, Figs. 6–9)
Kosmoceras “zudacharicum KAZANSKII, 1909” sensu Kiselev et al. 2013 [M] (Pl. 10, Figs. 11–18)
Choffatia poculum (Leckenby, 1859) [M + m] (Pl. 10, Fig. 19)
Grossouvria sp. [m]
Euaspidoceras sp. [M]
Hecticoceras sp. [m] (Pl. 10, Fig. 10)
6. Remarks on some ammonites of the distractum Biohorizon
6.1. Cardioceratidae
The co-occurrence of Quenstedtoceras lamberti (Sowerby) and its microconch counterpart Quenstedtoceras leachi (Sowerby) – the latter here included in the former – allows an assignment of the distractum Biohorizon to the Late Callovian Lamberti Subzone (Thierry et al. 1997). The huge number of specimens Quenstedtoceras lamberti (Sowerby, 1819) recovered from the distractum Biohorizon allows us to demonstrate an enormous variation among cardioceratids, which is typical of ammonites of this lineage (Callomon 1985) and has led to the description of a vast number of morphospecies (e.g., Maire 1938). There is a continuous range from oxycone (“Lamberticeras”) to cadicone (“Goliathiceras”) morphologies and thus we interpreted this observation as intraspecific variation of a single longer-ranging palaeobiospecies, despite some condensation which might have led to further mixing up of morphologies. For the macroconchs we used the oldest available name, Quenstedtoceras lamberti (Sowerby, 1818) (see Fig. 4 and Pls. 1–3). Disregarding this variation, some authors grouped co-occurring variants not only into various morphologically defined species but used several individual but co-occurring cardioceratid genera (e.g., Buckman 1920; Maire 1938; Kiselev 2022). In the microconchiate specimens of Qu. lamberti (= Quenstedtoceras leachi) the intraspecific variation is less extreme than in the macroconchs but still significant. In our material, however, macroconchiate Quenstedtoceras specimens dominate by far the ammonite assemblage of the distractum Biohorizon.
6.2. Aspidoceratidae
The macroconchiate aspidoceratid Paraspidoceras distractum (Quenstedt, 1857) was chosen by Dietl (1991) as the nominal species of this biohorizon. In our material, this species is restricted to layer L2, which hence must be the type horizon of this taxon. Quenstedt (1857) had established this species based on two illustrated specimens (Quenstedt 1849, pl. 16, figs. 7–8 and Quenstedt 1857, pl. 71, fig. 4). Both specimens come from the foot of the Stuifen Mountain near the town of Schwäbisch Gmünd in eastern Swabia. Zeiss (1962) designated the specimen illustrated in 1849 and re-illustrated in Quenstedt (1887, pl. 89, fig. 6) as the lectotype of P. distractum. Our material led us to the conclusion that further specimens illustrated by Quenstedt (1887, pl. 89, figs. 5, 7, 10) represent Paraspidoceras distractum as well, contrary to Zeiss (1962), who renamed some varieties as P. interninodatum and P. raricostatum. Probably due to this systematic confusion and the small size and incomplete illustrations of the material on which P. distractum is based, this taxon has been often disregarded.
Fig. 6.
Perisphinctidae of the distractum Biohorizon, Ornatenton Formation, at Gruibingen, SW Germany. A, B – Choffatia poculum (Leckenby, 1859). A: SMNS 70711/40, [M], an adult body-chamber resembling Choffatia trina sensu Cox (1988). B: SMNS 70711/41, [m]. Scale bar equals 50 mm.
Besides P. distractum, but less abundant, we recorded aspidoceratids of the genus Euaspidoceras Spath, 1931 and assigned most of them to E. cf. subbabeanum (SINTZOV, 1888). These forms differ from Paraspidoceras by a fine-ribbed juvenile stage lacking spines and by the absence of spatulate or shovel-shaped ventromarginal spines in later ontogenetic stages. Furthermore, Euaspidoceras has microconchiate counterparts formerly included in a separate genus Mirosphinctes Schindewolf, 1926 (e.g., Bonnot et al. 1994; Bonnot 1995), whereas in Paraspidoceras microconchs are unknown (Parent et al. 2020). However, in our sample of the distractum Biohorizon we could not trace any microconchiate specimens of this lineage. A single adult specimen (Pl. 5, Figs 1–3) shows an almost ventral position of spines and fits well with Euaspidoceras hirsutum (Bayle, 1878). (compare Courville 2013; Courville et al. 2013). The nucleus of this specimen, however, does not show any fine-ribbed stage typical of Euaspidoceras. For the time being we concur with previous systematic placements of this taxon in Euaspidoceras.
6.3. Peltoceratidae
Quenstedt (1887: pl. 89, figs. 11–14) illustrated several peltoceratids with densely ribbed inner whorls from the Lambertiknollen Bed (“Schwarze Knollen”) of various Swabian localities. Schlegelmilch (1985) and Riegraf (1984) re-illustrated photographically Quenstedt's specimen (1887, pl. 89, fig. 12) from the foot of Stuifen Mountain as Peltoceras unispinosum (Quenstedt, 1847) thus enhancing Quenstedt's incomplete illustration. This macroconchiate specimen is very close to the newly sampled peltoceratids from the distractum Biohorizon of Gruibingen. Prieser (1937, pl. 6, fig. 7) illustrated a microconchiate specimen under the same name. Indeed, also the latter specimen corresponds well to the macroconchiate examples making this case of dimorphism reliable. In contrast, however, the holotype of Ammonites athleta unispinosus itself is a very fragmentary specimen earlier illustrated by Quenstedt (1847, pl., fig. 4). It was said to come from the “Braunjura ς” (= Ornatenton Formation) in the vicinity of Reichenbach near Göppingen. Since there are only Upper Triassic and Lower Jurassic beds present in the area around Reichenbach an der Fils and the town of Göppingen, Quenstedt's cited locality obviously refers to the village of Reichenbach im Täle near Geislingen an der Steige, where Middle and Upper Jurassic strata crop out. The monotypic holotype is clearly distinct from all other specimens later assigned to Peltoceras unispinosum by a broader whorl section and a denser ribbing at comparable size. In consequence, our material of the distractum Biohorizon cannot be assigned to this taxon, and due to the very fragmentary state of the holotype we consider this taxon as a nomen dubium. When comparing speci- mens from the distractum Biohorizon with other late Callovian peltoceratids it became evident that our material represents Peltoceras subtense (BEAN in Leckenby, 1859) originally described from Southern England. We use this name for both dimorphs.
Fig. 7.
Perisphinctidae of the distractum Biohorizon, Ornatenton Formation, at Gruibingen, SW Germany. A, B – Alligaticeras alligatum (Leckenby, 1859) [M], SMNS 70711/1, an adult body-chamber in ventral (A) and lateral (B) views. Scale bar equals 100 mm.
6.4. Kosmoceratidae
Layer L2 yielded only macroconchiate specimens of Kosmoceras, both juveniles and nearly adults. The species is relatively large-sized and shows some continuous variation in respect of its ribbing density and strength, the presence or absence of tubercles at the diverging point of the ribs and the onset of the adult stage where the previously tabulate venter becomes fully rounded. Interestingly, in Quenstedt's monographs (1857, 1887) not a single example of this significant species is illustrated. In subsequent faunal lists, this Kosmoceras chronospecies was identified either with Ammonites ornatus compressus Quenstedt, 1887 or with Ammonites duncani Sowerby, 1816, even in the course of this study. The former was based on a specimen that is lost now due to pyrite decay (Schlegelmilch 1985). The pyritic preservation excludes its origination from the rock interval of the Lambertiknollen Bed and it must have its type horizon somewhat below. Recently, Dietl (2022) reported it from the late Callovian fraasi Biohorizon. Kosmoceras duncani, on the other hand, is often used as a waste-basket taxon although Arkell (1939, pl. 11, fig. 6) defined a neotype originating from the Athleta Zone which shows diagnostic characters (see Callomon 1985) not seen in any of our specimens. The finer-ribbed variants of our sample somewhat resemble Kosmoceras kuklicum (Buckman, 1926), but all studied specimens from the distractum Biohorizon differ constantly from that species by showing a characteristic retroradiate ribbing style of the secondary ribs (Fig. 5), which are instead prorisradiate in true K. kuklicum, a species coming from a stratigraphically much older level (cf. Kiselev 2022). In previous literature, this unnamed species is constantly hidden within Kosmoceras “duncani” (Bayle 1878; Douvillé 1916; Fortwengler et al. 1997).
6.5. Perisphinctidae
Choffatia poculum (Leckenby, 1859) [m + M]
A fragmentary ammonite body-chamber illustrated by Quenstedt (1845–1849: pl. 13, fig. 6) and named as “Amm. convolutus gigas” was renamed by Oppel (1857) as Ammonites Orion, thus representing the monotypic holotype of this taxon. Its origination from the Ornatenton Formation is undoubted, but its preservation as a three-dimensional pyritic mould excludes the Lambertiknollen Bed as its type horizon. Two more complete and superficially similar perisphinctids from the Lambertiknollen Bed of Gammelshausen, a locality not far away from the Gruibingen section, were illustrated by Zakrzewski (1886, pl. 2, figs. 2, 3) under the name originally introduced by Quenstedt. In his discussion of this species he also referred to Oppel'S taxon but continued to use Quenstedt's name. The two illustrated specimens seem to differ significantly in their inner whorls, but this results from preparation artefacts and somewhat idealized illustrations. The specimen of Zakrzewski's pl. 2, fig. 3 is refigured here (Pl. 7, Figs. 2, 3) and is well comparable to our new material from Gruibingen. Quenstedt himself (1887, pl. 81, fig. 25) illustrated another specimen from the Lambertiknollen Bed under the name Ammonites convolutus plicomphalus and a well-preserved corresponding macroconch under the name Ammonites polygyratus (Quenstedt 1887, pl. 89, fig.22). There is no doubt at all that the latter specimens as well as the newly collected material from Gruibingen are conspecific with Ammonites poculum Leckenby, 1859, whereas Ammonites orion Oppel, 1858 shows a more evolute coiling, a very regular ornamentation lacking prominent constrictions and another whorl section; it is clearly different from poculum and not even belonging to the same genus. Recently, Dietl (2022) listed it from the fraasi Biohorizon. Choffatia poculum has a characteristic ribbing style with wide-spaced thickened primaries giving rise to bundles of secondaries. Later in ontogeny, the ribbing becomes simplified, with less prominent primaries and a gradual reduction of the number of secondaries. In addition, deep oblique constrictions are present in the microconchs of Ch. poculum. Cox (1988) illustrated macroconchs and microconchs of Choffatia (Poculisphinctes) poculum under the same name. The holotype of this species was illustrated photographically by Buckman (1020, pl. 185) and by Cox (1988, pl. 12, fig. 9). From its very involute coiling we assume it is a juvenile macroconch that matches our incomplete specimen illustrated on Pl. 7, Fig. 1. Adult specimens of Ch. poculum from the distractum Biohorizon are often preserved as phosphatic body-chambers (Fig. 6A) while the inner whorls are missing. There is also great resemblance to a macroconchiate specimen assigned by Cox (1988, pl. 14, fig. 2) to Choffatia trina (Buckman, 1922), the holotype of which is based on a fully septate specimen lacking precise stratigraphic information. The maximum diameter of the macroconchs is less than 15 centimetres; this is remarkably small for perisphinctids. We concur with the species concept of Cox and follow Énay & Howarth (2019) who considered Poculisphinctes Buckman, 1920 a junior subjective synonym of Choffatia Siemiradzki, 1898. In the Gruibingen section, Ch. poculum is not restricted to the distractum Biohorizon but also recorded from layer L3 (Pl. 10, Fig. 19).
Alligaticeras alligatum (Leckenby, 1859)
A single small-sized perisphinctid from Gruibingen fits perfectly with the species established by Leckenby (1859). Cox (1988) provided a photograph of the microconchiate holotype. Our specimen is also well comparable with a specimen from the Lamberti Subzone of the Oxford Clay of Woodham illustrated by Page (1991, pl. 20, fig. 15). Among the perisphinctids of the distractum Biohorizon of Gruibingen there is only a single macroconchiate specimen assignable to Alligaticeras alligatum (Leckenby). This incomplete but threedimensionally preserved specimen (Fig. 7) consists only of the body-chamber but it fits well with a coarsely ribbed specimen with preserved inner whorls illustrated by Quenstedt (1887, pl. 89, fig. 21) under the name “Amm. cf. colubrinus”. Quenstedt's historical determination refers to Reinecke's (1818) Nautilus colubrinus, a perisphinctid ammonite taxon from the Kimmeridgian of Franconia that is nowadays assigned to Orthosphinctes Schindewolf, 1925 (e.g., Geyer 1961; Heller & Zeiss 1972). In the 19th century, many perisphinctids with a bipartite ribbing style were lumped under this name and thus appear in faunal lists ranging from the Callovian up to the Tithonian. Of course, there is no closer relationship with the late Callovian species. Our macroconchiate specimen represents the ultimate adult stage of Alligaticeras alligatum; subadult specimens were illustrated both from the late Callovian of SE France (Fortwengler et al. 1997, 2012), Normandy (Courville et al. 2013) and from S England (Cox 1988).
6.6. Oppeliidae
Hecticoceratinae are the most frequent oppeliids within the assemblage of the distractum Biohorizon. The entire subfamily is still in a chaotic systematic state due to an exorbitant oversplitting of taxa based on poorly defined and often monotypic type material (e.g., Spath 1927–1933; Gérard & Contaut 1936; Jeannet 1951; ZEISS 1955, 1959; Heller & Zeiss1972). In the course of erecting new genera without knowledge of dimorphism and other sexual phenomena even the generic assignment is often controversy. In our sample from Gruibingen macroconchs predominate, but in many cases it is hardly possible to differentiate between microconchs and juveniles due to their incomplete preservation notoriously lacking apertures. Within our material, three forms are easily distinguishable which do not intergrade. The rarest form was determined as Lunuloceras paulowi (DE Tsytovitch, 1911); however, the generic assignment of this involute species remains tentative. More common is the coarse-ribbed Putealiceras puteale (Leckenby, 1859), the type species of this genus. A nice example from the distractum Biohorizon of Gruibingen was illustrated by Dietl (2013, fig. 67 right). The most common form is represented by Orbignyiceras pseudopunctatum (Lahusen, 1883), which is the type species of Orbignyiceras Gérard & Contaut, 1936. In literature, it is often assigned to Lunuloceras or Hecticoceras and reported from various biohorizons of the Lamberti Zone (e.g., Fortwengler et al. 1997; Thierry et al. 1997; Kiselev 2022).
Fig. 8.
Rare Oppeliidae of the distractum Biohorizon, Ornatenton Formation, at Gruibingen, SW Germany. A, B – Rollieria cf. kormosi (Lóczy, 1915) [M]; A: SMNS 70711/4, B: SMNS 70711/5. C, D – Richeiceras sp. [M], SMNS 70711/6. Scale bar equals 20 mm.
Distichoceratinae are a minor component within the ammonite assemblage of the distractum Biohorizon. In contrast to Hecticoceratinae, all studied specimens represent macroconchs. Quenstedt (1846, 1887) described morphologically similar specimens from upper Callovian beds of the southwestern Swabian Alb as two new species. It is very likely that the type horizons of Distichoceras bidentatum (Quenstedt, 1846) and D. nodulosum (Quenstedt, 1887) as well as that of D. bicostatum (Stahl, 1824) are located in beds assignable to the Athleta Zone, but the Lamberti Zone cannot be excluded, either. This is why Riegraf (1991) cited the “athleta-/lamberti-Zone” for the supposed chronostratigraphic ages of Quenstedt's specimens. The stratigraphic ranges of species of Distichoceratinae are possibly longer than those of most other late Callovian ammonite genera. This diminishes their value for a precise biostratigraphic resolution. On the other hand, Distichoceratinae has a remarkably wide palaeobiogeographic distribution reaching even South America (Gröschke & Zeiss 1990).
Further rare oppeliids (Fig. 8) are represented by the Mediterranean genera Rollieria Jeannet, 1951 and Richeiceras Jeannet, 1951, Callovian representatives of which are still rather incompletely known due to the widespread hiatuses or unfavourable preservation conditions in the Tethyan Realm and their complete absence in the Boreal seas.
7. Correlation with other areas
A comparison of the distractum Biohorizon and the following “aff. zudacharicum Biohorizon“with other biohorizons of the Lamberti Zone described from elsewhere is limited by the condensed and incomplete nature of the sections in the Jurassic of Swabia. The Lamberti Subzone was defined by Callomon & Sykes in Cope et al. (1980). The ammonite taxa listed therein indicate that the distractum Biohorizon belongs to this subzone, despite some outdated determinations. At the type locality of this subzone, Brora in Northern Scotland, besides the ammonite assemblage of this subzone a biohorizon (̔clynelishense Horizon') of the latest Lamberti Zone was reported in which kosmoceratids are absent. The same horizon was said to be present in Normandy, and there is no doubt that this horizon coincides with the assemblage later termed as paucicostatum Biohorizon in southeastern France by French authors (see below).
In the Wutach area, which is adjacent to the Jurassic of Swabia and links this area with Northern Switzerland, Callovian deposits are mainly represented by various iron oolites nowadays summarized in the Wutach Formation (Dietl 2010). The Lambertiknollen Bed is present in the top of this formation and contains small phosphatic concretions with ammonites (Zeiss 1955). This ammonite assemblage is almost exclusively represented by Quenstedtoceras spp. of the Lamberti Zone; the sole record of a Reineckeia sp. and the reported occurrence of phosphatic concretions with brownish iron ooids instead of dark-green glauconite peloids points to some reworking and mixing up of beds of the underlying Athleta Zone even if the Reineckeiinae reach the Lamberti Zone.
In northern Switzerland, the Callovian-Oxfordian boundary beds were mainly studied from the former iron mine at Herznach. Parts of the iron-oolitic beds of the Herznach Member of the Ifenthal Formation (Bitterli-Dreher 2012) including the interval of the Lamberti Zone were originally assigned to the Oxfordian (Jeannet 1951). A precise restudy of the full succession and its faunas is still missing. More recently, Hostettler (2014) studied the ammonite assemblages of the clayey Callovian and Oxfordian deposits in northwestern Switzerland. From his lamberti Biohorizon, which is mostly represented by relatively small specimens in phosphatic preservation, he reported Quenstedtoceras lamberti (Sowerby, 1819), Alligaticeras alligatum (Leckenby, 1859), Kosmoceras spino-sum (Sowerby, 1826) s.l., Euaspidoceras hirsutum (Bayle, 1878), Distichoceras bicostatum (Stahl, 1824) and Alcidia canaliculata (Quenstedt, 1846). Alcidia canaliculata is herein identified as Rollieria cf. kormosi (Lóczy, 1915). Despite the smaller number of recorded taxa and specimens there is no doubt that this assemblage represents the distractum Biohorizon as well.
In the famous Sengenthal section of Franconia, Callomon et al. (1987) reported the presence of the Lamberti Zone. Two ammonite-bearing levels could be distinguished there. The lower one yields varieties of Quenstedtoceras lamberti (Sowerby, 1819), Putealiceras puteale (Leckenby, 1859), Alligaticeras alligatum (Leckenby, 1859) and the index species of the distractum Biohorizon, Paraspidoceras distractum (Quenstedt, 1846), the latter termed as “Aspidoceras (Euaspidoceras) ferrugineum (= hirsutum Bayle)” (Callomon et al. 1987: 29). However, it should be noted that ferrugineum Jeannet, 1951 is a valid species of the genus Euaspidoceras and has nothing to do with E. hirsutum or with the genus Paraspidoceras. This species characterizes the base of the Lamberti Zone, Henrici Subzone (see Bonnot 1998; Courville & Bonnot 1998). From the upper ammonite-bearing level, the illustrated “Quenstedtoceras aff. lamberti” (Callomon et al. 1987, pl. 5, fig. 1) corresponds to Qu. pseudolamberti (SINTZOV, 1888) as reported here from the aff. zudacharicum Biohorizon of the Gruibingen section. This provides us a hint that the upper level at Sengenthal may correlate with this biohorizon. As noticed above, sections in Northern Franconia are often characterized by a strongly condensed layer of phosphatic concretions with very poorly preserved ammonites (Dietl & Mönnig 2015) or by a long hiatus with reworking of older strata and the formation of dysphotic stromatolite layers (V. DIETZE, pers. comm.).
In the Jurassic of Northern Germany detailed information on the Lamberti Zone is missing. Only Lange (1973) reported the presence of this zone in several sections, however, without any further subdivision into subzones or biohorizons. His reports of ammonites of the Lamberti Zone are scarce and do not allow closer comparisons with other areas. Despite these deficits, one thereby introduced ammonite taxon, Quenstedtoceras paucicostatum Lange, 1973, was used as the index of a biohorizon in the uppermost Callovian of France (Vidier et al. 1993; Fortwengler & MARCHAND 1994; Fortwengler et al. 1997, 2012; Courville & Bonnot 1998), Southern England (Page et al. 2004, 2009a, 2009b) and European Russia (Kiselev et al. 2013; Kiselev & Rogov 2018; Kiselev 2022). Sections in SE France and S England have been proposed as GSSP candidates for the base of the Oxfordian. This search has led to an increased knowledge on the Callovian-Oxfordian boundary beds and their ammonite contents. The presence of kosmoceratids in the aff. zudacharium Biohorizon seems to indicate that this biohorizon must be somewhat older than the biohorizons with Qu. paucicostatum in France and England, but it may overlap with the equally named biohorizon in Russia. Conversely, the upper bed (5B) of the French lamberti Biohorizon preceeding the French paucicostatum Biohorizon yields an ammonite assemblage with very rare kosmoceratids different from those of the distractum biohorizon (Fortwengler et al. 1997, fig. 6.2). Identical kosmoceratids as in the distractum Biohorizon, termed as Kosmoceras duncani (Sowerby) sensu Bayle (1878) and Douvillé (1912) are reported from the lower bed (5A) of the lamberti Biohorizon in SE France (Fortwengler et al. 1997, fig. 5.5). Consequently, the distractum Biohorizon corresponds approximately to the lower bed of the French lamberti Biohorizon and the aff. zudacharium Biohorizon to the upper bed of this biohorizon. In the Boulonnais area (Vidier et al. 1993), a bed containing the ‘Lamberi Horizon’ approximately corresponds as well with the distractum Biohorizon and not with the marly bed following above the “Schwarze Knollen” as suggested by these authors.
In Russia, Kiselev & Rogov (2004) distinguished a mojarowskii Biohorizon that separates the lamberti Biohorizon from the paucicostatum Biohorizon. Its index species is a dwarfish microconchiate kosmoceratid that is neither recorded from Swabia nor from Franconia.
Fig. 9.
Diagrams showing the distribution of ammonite families in the distractum (A) and “aff. zudacharicum” (B) biohorizons of the Ornatenton Formation at Gruibingen, SW Germany, both based on the samples excavated at the occasion of the motorway construction in 1989.
In Northern Iran, the late Callovian Lamberti Zone is solely proven by the occurrence of Quenstedtoceras ex gr. lamberti (Sowerby) (Seyed-Emami et al. 2020).
In the Alpine, Carpathian and Apennine regions, late Callovian to early Oxfordian ammonite-bearing sections are extremely rare because at that time large areas dropped below calcite compensation depth and at least radiolarites without macrofossils were deposited. From the Villany site in Hungary, Lóczy (1915) reported ammonites of Callovian age; among those, the rare occurrence of Rollieria cf. kormosi (Lóczy, 1915) from the Lambertiknollen Bed at Gruibingen is the only hint for a time-equivalent of the late Lamberti Zone and the possibility of a faunal exchange from the eastern Tethyan Realm. From the Balkan Mountains of Bulgaria, Metodiev (2019, fig. 13) reported several perisphinctids of supposed late Callovian age. Among these, a specimen of Choffatia poculum (Leckenby, 1859) from there was misidentified as “Binati-sphinctes aff. binatus (Leckenby)”.
In Spain – and many other areas of the Circum-Mediterranean – the Callovian-Oxfordian transition is mostly characterized by a marked hiatus, and ammonites of the Lamberti Zone are only occasionally found in a reelaborated state (see Page et al. 2004 and references therein).
Aspidoceratidae and Peltoceratidae have an almost pandemic distribution except for the Boreal realms and both are well-represented and diverse in the Indo-Madagascan Province (e.g., Waagen 1873–1875; Spath 1927–1933; Collignon 1958; Alberti et al. 2011; Bonnot & THOMAS 2023). Their lineages are well-studied (Bonnot 1995; Bonnot et al. 1997, 2002) and thus would allow long-distance correlations with Southern India (Kachchh) or Madagascar; however, this possibility is again strongly hampered by large and widespread hiatuses or unfavourable preservation conditions for ammonites around the Callovian-Oxfordian boundary (Singh 1989).
Correlations with North and South America are hampered by strong endemism and possibly also by widespread hiatuses around the Callovian-Oxfordian boundary (see Parent 2006). From the late Callovian of Argentina, Parent & Garrido (2015) reported Choffatia cf. poculum (sensu Cox 1988) as a possible link to the western Tethys; the otherwise co-occurring Euaspidoceras subbabeanum (Sintzow, 1888) was reported from a slightly younger bed of the same section thus pointing to a series of immigration events rather than to a single one. In Northern Chile and Peru, Distichoceras species including its dimorphic partner Horioceras allowed the identification of upper Callovian beds (Gröschke & Zeiss 1990); however, a precise correlation at the level of subzones or even biohorizons is impossible due to the supposedly endemic nature of these species.
8. Discussion and conclusions
Within the succession of the Lambertiknollen Bed, which is a thin bed of glauconitic marls with abundant phosphatic concretions in the upper part of the Ornatenton Formation of the Swabian Jurassic, two biohorizons are distinguishable. Both biohorizons are separated by a hiatus expressed by a belemnite breccia. The ammonite assemblage of the lower one is described as distractum Biohorizon and assigned to the Lamberti Subzone of the Lamberti Zone. It is moderately diverse and dominated in number by cardioceratids and oppeliids, but kosmoceratids, perisphinctids, aspidoceratids and peltoceratids are also well-represented. In contrast, the upper one is much less diverse and by far dominated by cardioceratids; oppeliids are very rare and peltoceratids have not been recorded at all (Fig. 9). The upper horizon is described as aff. zudacharicum Biohorizon. It is the latest recorded biohorizon of the Lamberti Zone in the Jurassic of Swabia. In the Gruibingen section follows another prominent hiatus most likely comprising the latest Lamberti Zone with equivalents of the British and French paucicostatum Biohorizon and the entire Mariae Zone of the Oxfordian. Both the reported hiatuses and the low sedimentation rates that stimulated the formation of glauconite and phosphatic concretions do not allow to reconstruct a complete chronological succession of biohorizons across the Callovian-Oxfordian boundary in Southern Germany.
The faunal shift from the moderately diverse distractum Biohorizon to the low-diverse aff. zudacharicum Biohorizon can be explained by cooler seawater temperatures at least at the seafloor for which the Subboreal cardioceratids were better adapted. Cool seawater temperatures and very low sedimentation rates are also indicated by abundant glauconite (Odin & Matter 1991). Tethyan and Submediterranean ammonite taxa are still relatively abundant in the distractum Biohorizon except for the pelagic phylloceratids but drastically declined in the aff. zudacharicum Biohorizon. An increased climatic cooling could explain why Subboreal cardioceratids widely expanded towards the South (starting already with the beginning of the Lamberti Zone). A further possible mechanism for regional cooling is by cold upwelling waters from the Penninic Ocean in the South that flooded temporarily the European epicontinental shelves. This upwelling is documented by the occurrences of protoglobigerine planktic foraminifers and radiolarians in some of the levels containing phosphatic concretions (Riegraf 1987a, 1987b). Besides the herein presented palaeontological evidence, further interdisciplinary studies are necessary to decipher these environmental effects on regional sedimentation and diagenesis during this crucial time interval.
Plate 1
Variation of Quenstedtoceras in the distractum Biohorizon, Ornatenton Formation, at Gruibingen, SW Germany.
(1–5) Quenstedtoceras lamberti (Sowerby, 1819) [M]; 1, 2: SMNS 70711/7 in lateral (1) and ventral (2) views); 3–5: SMNS 70711/8 in lateral (3) and ventral (4, 5) views.
(6, 7) Quenstedtoceras lamberti (Sowerby, 1819) [m] (= Ammonites leachi Sowerby, 1819), SMNS 70711/9 in ventral (6) and lateral (7) views.
Scale bar equals 50 mm.
Plate 2
(1–7) Morphological variation of Quenstedtoceras lamberti (Sowerby, 1819) in the distractum Biohorizon, Ornatenton Formation, at Gruibingen, SW Germany. 1, 2: SMNS 70711/10 in lateral (1) and ventral (2) views; 3, 4: SMNS 70711/11 in lateral (3) and ventral (4) views; 5, 6: SMNS 70711/12 in lateral (5) and ventral (6) views; 7: SMNS 70711/13, lateral view of Pl. 3, Fig. 1.
(8) Quenstedtoceras praelamberti Douvillé, 1912, SMNS 70710, sampled immediately below concretion layer L1, Gruibingen, SW Germany.
Scale bar equals 50 mm.
Plate 3
Ammonites of the distractum Biohorizon, Ornatenton Formation, at Gruibingen, SW Germany.
(1–5) Quenstedtoceras lamberti (Sowerby, 1819) [M]; 1: SMNS 70711/13, ventral view of Pl. 2, Fig. 7; 2, 3: SMNS 70711/14 in ventral (2) and lateral (3) views; 4, 5: SMNS 70711/15 in lateral (4) and ventral (5) views.
(6–9) Peltoceras subtense (BEAN in Leckenby, 1859) [m]; 6, 7: SMNS 70711/16 in lateral (6) and ventral (7) views; 8, 9: SMNS 70711/17 in lateral (8) and ventral (9) views.
Scale bar equals 50 mm.
Plate 4
Peltoceratinae in the distractum Biohorizon, Ornatenton Formation, at Gruibingen, SW Germany.
(1–6) Peltoceras subtense (BEAN in Leckenby, 1859) [M]; 1, 2: SMNS 70711/18 in lateral (1) and ventral (2) views; 3: SMNS 70711/19, a nucleus in lateral view; 4–6: SMNS 70711/20 in ventral (4, 5) and lateral (6) views.
Scale bar equals 50 mm.
Plate 5
Aspidoceratidae in the distractum Biohorizon, Ornatenton Formation, at Gruibingen, SW Germany.
(1–3) Euaspidoceras hirsutum (Bayle, 1878) [M] SMNS 70711/21 in lateral (1) and ventral (2) views; (3) shows the three-dimensionally preserved phosphatic nucleus of the same specimen; distractum Biohorizon, Gruibingen, SW Germany.
Scale bar equals 100 mm for 1 and 2 and 20 mm for 3, respectively.
Plate 6
Aspidoceratidae in the distractum Biohorizon, Ornatenton Formation, at Gruibingen, SW Germany.
(1, 4) Paraspidoceras distractum (Quenstedt, 1857) [M]; 1: SMNS 70711/22, an incompletely preserved adult, 4: SMNS 70711/23, showing an intermediate stage.
(2, 3) Euaspidoceras cf. subbabeanum (SINTZOV, 1888) [M], SMNS 70711/24 in ventral (2) and lateral (3) views.
Scale bar equals 100 mm.
Plate 7
Ammonites of the distractum Biohorizon, Ornatenton Formation, at Gruibingen, SW Germany.
(1–5) Choffatia poculum (Leckenby, 1859) [M & m]; 1: incomplete [M], SMNS 70711/25; 2, 3: SMNS 60015, a more complete juvenile macroconchiate specimen for comparison from the distractum Biohorizon of Gammelshausen, SW Germany (= Orig. Zakrzewski 1886, pl. 2, fig. 3). 4, 5: [m], SMNS 70711/26, in lateral (4) and ventral (5) views.
(6, 7) Alligaticeras alligatum (Leckenby, 1859) [m], SMNS 70711/27 in ventral (6) and lateral views (7).
(8, 9) Distichoceras nodulosum (Quenstedt, 1887) [M]; 8: SMNS 70711/28, 9: SMNS 70711/29.
(10) Kosmoceras n. sp. aff. kuklicum (Buckman, 1926), [M], SMNS 70711/30, a coarse-ribbed variety in ventral view.
Scale bar equals 50 mm.
Plate 8
Kosmoceratinae in the distractum Biohorizon, Ornatenton Formation, at Gruibingen, SW Germany.
(1–6) Kosmoceras n. sp. aff. kuklicum Buckman, 1926) [M]. 1, 2: lateral and ventral views of a fine-ribbed juvenile variety, SMNS 70711/31; 4, 5: fine-ribbed variety in lateral and ventral views, SMNS 70711/32; 3: coarse-ribbed juvenile specimen, SMNS 70711/33; 6: coarse-ribbed variety, SMNS 70711/34.
Scale bar equals 50 mm.
Plate 9
Oppeliidae of the distractum Biohorizon, Ornatenton Formation, at Gruibingen, SW Germany.
(1, 2, 5, 6) Orbignyiceras pseudopunctatum (Lahusen, 1883) [M], 1, 2: SMNS 70711/35 in lateral (1) and ventral (2) views; 5, 6: SMNS 70711/36 in lateral (5) and ventral (6) views).
(3, 4) Distichoceras nodulosum (Quenstedt, 1887) [M], SMNS 70711/37 in lateral (3) and ventral (4) views.
(7) Lunuloceras paulowi (DE Tsytovitch, 1911) [M], SMNS 70711/38.
Scale bar equals 50 mm.
Plate 10
Reworked ammonites of the aff. zudacharicum Biohorizon, Ornatenton Formation, at Gruibingen, SW Germany.
(1–5) Quenstedtoceras pseudolamberti (SINTZOV, 1888) [M]; 1, 2: SMNS 70712/1. 3: SMNS 70712/2. 4, 5: SMNS 70712/3.
(6–9) Quenstedtoceras cf. paucicostatum Lange, 1973 [M]; 6, 7: SMNS 70712/4; 8, 9: SMNS 70712/5.
(10) Hecticoceras sp. [m], SMNS 70712/6.
(11–18) Kosmoceras aff. zudacharicum KAZANSKII, 1909 (sensu Kiselev et al. 2013) [M]; 11, 12: SMNS 70712/7. 13, 14: SMNS 70712/8. 15, 16: SMNS 70712/9. 17, 18: SMNS 70712/10.
(19) Choffatia poculum (Leckenby, 1859) [M], SMNS 70712/11.
Scale bar equals 20 mm.
Acknowledgements
This study would not have been possible without the enthusiastic fieldwork of our former excavation team Martin Kapitzke and Markus Rieter (both Stuttgart) who both also did the skillful preparation of the material. Special thanks go to Dipl.-Geol. Olga Dietl (Stuttgart) for her technical support to the first author. We greatly acknowledge the fruitful discussions with Prof. Dr. John H. Callomon (†, London), Dr. Reinhart Gygi (†, Basel) and Dr. Eckhard Mönnig (Coburg). Bernhard Hostettler (Bern) and an anonymous referee are heartily thanked for their invaluable and constructive reviews.
