3.1. Historical review

Zieten (1830: pl. 6) was the first to figure several ammonites from the iron ore seams of Aalen. Most of these specimens from the Unteres Flöz are no longer traceable. Subsequently, Quenstedt (1846/1849, 1886) figured additional ammonites from Aalen and introduced several new taxa. Later, Horn (1909) introduced another new ammonite species based on a specimen from the Unteres Flöz of Aalen.

3.2. Ammonite taxa based on ammonites from the Unteres Flöz/Unterer-Donzdorf-Sandstein of Aalen

Fig. 2.

Section of the Eisensandstein Formation (with the exception of the top of the Oberer-Donzdorf-Sandstein Member) at Aalen-Wasseralfingen (modified from Schuler 1865), to show the wedging out of the smaller iron ore seams over short distances. The “Sohlstein” bed is shaded in grey, and the Unteres Flöz in red. Abbreviations: Mu. = Murchisonae; Bradf. = Bradfordensis; Sbz. = Subzone.

3.3. Remarks on the taxonomy of the ammonites from the Unteres Flöz

In the lower/upper Aalenian transition of SW Germany, the family Graphoceratidae Buckman, 1905 splits into two separate lineages (chronoclines), which range up to the Discites Zone (Lower Bajocian) and Bradfordensis Zone (Upper Aalenian), respectively:

(1) The first of these chronoclines is that of the genus Ludwigia Bayle, 1878. In its juvenile stage, the whorl section is broad; in the adult stage it becomes sub-quadrate to high-rectangular or high-triangular. The keeled venter is flattened or roof-shaped and laterally separated by a marked edge from the flanks. In the juvenile stage the flanks are ribbed, whereas they mostly become smooth in the adult stage. In stratigraphically older forms, the ribs may pass on the venter and are prosiphonate. The lobes and saddles of the suture line are rather deep and complex (Rieber 1963).

All ammonites of this chronocline from the Unteres Flöz at Aalen are assigned to Ludwigia. Buckman (1887–1907) introduced numerous other genera, e.g., Welschia, Hyattia, Kiliania, and Apedogyria. We concur with Howarth (2013) and consider them as younger subjective synonyms of Ludwigia. Only for traditional reasons and also for providing a temporal structure for this chronocline, we advocate the purely artificial naming of successive morphologies by the succession of the (morpho-) genera Ludwigia → Brasilia → Graphoceras → Hyper-lioceras (Chandler et al. 2012). When only a few specimens are studied or transitional forms are neglected these morpho-genera seem to be well-defined. In reality these genera represent a morphological and temporal continuum.

(2) The second chronocline is represented by the genus Staufenia Pompeckj, 1906. It is characterized by a discoidal whorl section and a sharp keel that is hardly separated from the slightly convex flanks and a steep or overhanging umbilical wall. The inner whorls can display weak or even coarse ribs; the sculpture disappears at around a diameter of c. 10 cm. The suture line is much simpler than that in Ludwigia, with shallower and less incised saddles and lobes but generally variable (Rieber 1963).

A separation of Staufenia from the ancestral genus Leioceras Hyatt, 1867 is difficult due to a long transitional interval containing numerous intermediate morphologies. Various authors (e.g., Rieber 1963; Contini 1969; Géczy 1967; Ureta Gil 1983; Sadki et al. 2020; Dietze et al. 2021b) assigned the ammonites occurring up to the Opalinum/Murchisonae zonal transition to the genus Leioceras (or to synonymous genera, e.g., Cypholioceras, Cylico-ceras). Younger forms originating from the Haugi Subzone were termed as Staufenia (Costileiocras) (Rieber 1963), Costileioceras (Géczy 1967), or Ancolioceras and Staufenia, respectively (Contini 1969; Ureta Gil 1983; Dietze et al. 2021a). In the Murchisonae Subzone, ammonites of this chronocline are predominantly assigned to Staufenia (Rieber 1963; Contini 1969; chandler 1982; Dietze et al. 2022a).

Here we assign the discoidal graphoceratid ammonites occurring in the Unteres Flöz at Aalen to the genus Staufenia. The ammonites of the genus Staufenia form a phyletic lineage: S. sinon → S. latiumbilicus → S. discoidea α → S. discoidea β → S. staufensis (see Dietze et al. 2022b). At present, it is still unclear whether S. sehndensis is another distinguishable chronospecies of this lineage or if it is a synonym of S. latiumbilicus. Since S. sinon is the type species of the genus Costileioceras Maubeuge, 1950, we consider Costileioceras a younger subjective synonym of Staufenia.

Fig. 3.

Section from the Wasserfallbank Member (Opalinuston Formation) and the Eisensandstein Formation up to the Sowerbyi-Oolith Member (Wedelsandstein Formation) of Aalen-Wasseralfingen (SW Germany) (based on Weber 1964, fig. 3, right column). The iron ore beds are indicated by thick dots; the Unteres Flöz is shaded in red. Left of the section: lithostratigraphy; right of the section: biostratigraphy (biohorizons, subzones, zones, stages). Abbreviations: WF = Wedelsandstein Formation; W = Wasserfallbank Member; Z = Zopfplatten Member; U. D.-S. = Unterer-Donzdorf-Sandstein Member; UF = Unteres Flöz; Personatensandstein = Personatensandstein Member; Ob. Flözh. = Oberer Flözhorizont Member; Ob. Donzdorf-Sds. = Oberer-Donzdorf-Sandstein Member; AF = Aalener Flöz; GF = Geislinger Flöz; SO = Sowerbyi-Oolith Member; Murch. = Murchisonae; Bradf. = Bradfordensis; Ov. = Ovale; Laev. = Laeviuscula.

In contrast to SW Germany, ammonites of the “leioceratid” genera Leioceras and Ancolioceras Buckman, 1904 occur frequently in beds up to the Bradfordensis Zone of SW England (Buckman 1887–1907; Chandler 1997), whereas ammonites of the genus Staufenia range only up to the basal Murchisonae Subzone there (Chandler 1982).

3.4. The genus Ludwigia Bayle, 1878 in the Unteres Flöz at Aalen

The bulk of the specimens of Ludwigia from the Unteres Flöz at Aalen (Pl. 6, Figs. 1a–2b; Pl. 7, Figs. 1a–5b; Pl. 8, Figs. 1a–4b, Pl. 9, Fig. 1a, b) are morphologically grouped around L. obtusa (QUenstedt, 1846) [= L. haugi auct.] and L. obtusiformis (Buckman, 1899). The same observation was made by Horn (1909) and Rieber (1963) for Ludwigia spp. from the lower part of the Oberbeta-Oolith (Haugi Subzone, Murchisonae Zone) at Blumberg-Achdorf in the Wutach area. Rieber (1963) distinguished between L. haugi haugi and L. haugi obtusiformis, whereas Horn (1909) formally treated both as different species. On the other hand, Horn (1909) pointed to an extremely large variation within the genus Ludwigia which led to numerous transitional forms between the two nominal morphospecies so that almost every specimen shows an individual morphology.

In addition to the common morphologies around L. obtusa and L. obtusiformis, both at Blumberg-Achdorf and in the Unteres Flöz at Aalen, some extreme morphologies, connected by transitional forms with L. obtusa and L. obtusiformis, also occur. All these forms together show the range of variation within the genus Ludwigia in the Haugi Subzone. We include all specimens from the Unteres Flöz as representatives of a chronospecies with highly variable individual morphologies. In a purely biological understanding, this chronospecies represents a (palaeo-) biospecies (cf. Chandler & Callomon 2009).

At one extreme of the variability occur coarsely ribbed specimens with a broad whorl section assigned to L. crassa (Horn, 1909) (Pl. 5, Figs. 1a–3b). There also exist transitional forms between L. crassa and L. obtusa (Pl. 6, Figs. 1a, b).

At the opposite margin of variation both in the Unteres Flöz and in the Wutach area specimens with a high-trapezoidal section and a very weak ventrolateral edge occur (Pl. 9, Figs. 2a–4b). Specimens which are transitional between L. obtusiformis and L. obtusa occur as well (Pl. 8, Fig. 4a, b; Pl. 9, Fig. 1a, b). It is worthy to note that neither Horn (1909) nor Rieber (1963) figured any of such examples despite their occasional occurrence in the Haugi Subzone of the Wutach area (coll. SMNS, coll. Auer).

Within the genus Ludwigia, L. crassa (Pl. 5, Figs. 1a–3b) is characterized by relatively wide-spaced ribs and nodes and a broad whorl section. All specimens illustrated on Pl. 5 are syntypes of Horn's (1909) L. crassa.

The specimen figured on Pl. 6, Fig. 1a, b as L. cf. obtusa is morphologically intermediate between L. crassa and L. obtusa. A very similar specimen was described by Buckman (1904) under the name Kiliania armipotens.

The ammonites figured on Pl. 6, Fig. 2a, b and on Pl. 7, Figs. 2a–3b are both assigned to L. obtusa because of their broad whorl section, thickened primaries and generally coarse ribbing. The lost lectotype of this species (Pl. 7, Fig. 4a, b) fits well with the topotypes illustrated on Pl. 7, Figs. 2a–3b.

L. obtusiformis (Pl. 7, Figs. 1a, b; 5a, b, Pl. 8, Figs. 1a, b) has a narrower whorl section than L. obtusa and exhibits a somewhat denser, less marked ribbing.

Specimens morphologically transitional between L. obtusa and L. obtusiformis are determined as L. cf. obtusa (Pl. 8, Figs. 2a–3b).

We refrain from introducing additional nominal taxa within the genus Ludwigia in respect of the already numerous synonyms introduced by S.S. Buckman. Some of these Buckman-taxa can be used for the morphological characterisation of specimens with a high-trapezoidal to high-triangular whorl section, such as L. aff. subtabulata (Buckman, 1898) (Pl. 9, Fig. 3a, b), L. aff. wilsoni (Buckman, 1899) (Pl. 9, Fig. 2a, b) and L. aff. pustulifera (Buckman, 1899) (Pl. 9, Fig. 4a, b). These purely morphological determinations should imply that these are extreme varieties opposite to the morphology of L. crassa.

L. cf. obtusiformis (Pl. 9, Fig. 1a, b) is transitional between L. obtusiformis and these extreme varieties. L. cf. obtusa (Pl. 8, Fig. 4a, b) is transitional to these extreme variants in respect of the narrowing of the whorl section, to L. obtusa in respect of the thickened bases of the primaries) and to L. obtusiformis in respect of the narrow whorl section and ribbing density.

3.5. The genus Staufenia Pompeckj, 1906 in the Unteres Flöz at Aalen

Specimens of the genus Staufenia occurring in a stratigraphically non-condensed horizon are treated as belonging to the same chronospecies and are named the same unless some morphological variation is noted (Rieber 1963; Dietze et al. 2017; Dietze et al. 2022b). Since there is a rather broad morphological variation concerning the strength of the ribbing and correlating whorl section in the specimens of the sinon biohorizon, we differentiate the smooth-shelled forms below sub-specific level as “variety opalinoides”. Varieties are not affected by the strict requirements of the ICZN, especially the Principle of Priority.

The type horizon of Ancolioceras opalinoides (Mayer, 1864) must occur below the iron-oolitic part of the Unteres Flöz as is proven by the preservation of the type. There is no doubt that the lectotype of S. sinon (Bayle, 1878) originates from the iron-oolitic part of the Unteres Flöz at Aalen-Wasseralfingen. Consequently, the oldest available name for Staufenia spp. from the iron-oolitic part of the Unteres Flöz is Staufenia sinon. Apart from the lectotype of A. opalinoides no other ammonites were reported from the “Sohlstein” or from the Unterer-Donzdorf-Sandstein at Aalen. Hence, it is unclear whether the lectotype of A. opalinoides is a representative of the ammonite assemblage termed as sinon biohorizon or of an older biohorizon. To solve this problem, we formally assigned all Staufenia specimens from the iron-oolitic part of the Unteres Flöz to the chronospecies S. sinon. Among these specimens the smooth-shelled examples are informally distinguished as S. sinon var. opalinoides.

S. sinon var. opalinoides (Pl. 1, Figs. 1a–2b, 4a–5b) exhibits weak, densely spaced ribs on the inner whorls (Pl. 1, Figs. 2a, b, 4a, b). The adult whorl section is high and narrow, with an arched venter. A ventro-marginal edge is absent or very weakly developed. On the label of the ammonite illustrated on Pl. 1, Fig. 2a, b it is noticed: “Ludwigia Sehndensis … (det. Horn)”. This implies that Horn was already aware that the Staufenia specimens from Aalen are biostratigraphical younger than the Ancolioceras specimens from the Wutach area termed by him as “Lioceras acutum“.

All specimens of Staufenia which are stronger sculptured than S. sinon var. opalinoides are assigned to S. sinon due to their close similarity with Bayle's syntypes (1878, pl. 83, figs. 1–4). The specimen illustrated on Pl. 2, Fig. 2a, b is almost identical to the syntype of Bayle (1878, pl. 83, fig. 1 here refigured on Pl. 2, Fig. 3a, b), which was erroneously taken as the lectotype by some authors. The Staufenia specimen illustrated on Pl. 4, Fig. 2a, b is close to Bayle's syntype also, but differs by its more regular and longer persisting ribbing. Staufenia sinon specimens which show a coarse ribbing in the inner whorls and a thickened basis of the primaries (Pl. 3, Figs. 1a–2b, 4a–6b; Pl. 4, Fig. 3a, b) are grouped around the lectotype (Pl. 3, Fig. 3) or the third syntype illustrated by Bayle (1878, pl. 83, figs. 2, 3). In the latter the ribs are slightly wider spaced than in the lectotype. The Staufenia sinon specimen illustrated on Pl. 2, Fig. 1a, b is somewhat exceptional regarding its very large size, long-persisting ribbing and low aperture. Occasionally, unusually large specimens also occur in S. latiumbilicus (Dietze et al. 2022b, pl. 8, fig. 5) or in S. staufensis (coll. W. Auer). This anomalous size can be explained as pathologic gigantism either caused by parasitic castration or by a genetic defect (Keupp 2012: 225).

Fig. 4.

Biohorizons of the Aalenian in SW Germany. The here newly introduced sinon biohorizon is highlighted in gray.

Finally, we noticed remarkable differences in the suture lines of S. sinon. Already Horn (1909) found that in the slightly older specimens from the Haugi Subzone of Blumberg-Achdorf (termed by him as “Lioceras” acutum and “L.” sinon) the suture line was highly variable, a view about which Rieber (1963) concurred. In some of the specimens studied of the genus Staufenia from the Unteres Flöz the suture line is extremely reduced (Pl. 3, Fig. 4a, b), like in the lectotype (Pl. 3, Fig. 3a, b). In the majority of specimens, however, the suture line is more variably differentiated (compare Pl. 2, Fig. 2a, b with Pl. 2, Fig. 2a, b or Pl. 3, Fig. 5a, b with Pl. 4, Fig. 2a, b [suture line on the opposite flank is illustrated in Rieber 1963, text-fig. 13f]). Consequently, as a very flexible character the suture line is not diagnostic.

3.6. Occurrence of “Leioceras” comptocostosum Chandler & Callomon, 2009 in the Unteres Flöz at Aalen

A single specimen determined here as “Leioceras” comptocostosum (Pl. 4, Fig. 1a, b), which originates from the collection of the famous F. A. V. Alberti – who introduced the Triassic System –, is markedly different from all other ammonites of the genera Ludwigia or Staufenia from the Unteres Flöz. It shows a rounded venter of the terminal body-chamber but a keeled phragmocone. In this respect, it falls in the variation of Leioceras comptocostosum, a taxon described from the uppermost Lower Aalenian of Dorset (SW England) (cf. Chandler & Callomon 2009, pl. 3, figs. 1a–2b; coll. V.D., coll. R.B. Chandler). Alternatively, this sole specimen could be assigned to Ancolioceras; it represents a descendant of “Leioceras” – a rare occurrence for SW Germany. The various specimens of Ancolioceras described by Horn (1909; pl. 9, figs. 1a–7b, pl. 10, figs. 1a–4b, pl. 11, figs. 1a–8a) from Blumberg-Achdorf differ by a fastigate venter up to the aperture. The lateral lobe as well as the umbilical lobes of the specimen illustrated on Pl. 4, Figs. 1a, b are significantly narrower and much deeper incised than in Staufenia. In this respect, there is a closer resemblance to the suture line of Ludwigia (cf. Pl. 8, Figs. 1a, b, 3a–4b). The latter examples are distinguished from “Leioceras” comptocostosum by their whorl section and a stronger ribbing on the outer flanks.

4.1. Bio-/chronostratigraphy

The age of the Unterer-Donzdorf-Sandstein Member at Aalen corresponds to the Haugi Subzone (Murchisonae Zone, Upper Aalenian), as indicated by records of Ancolioceras opalinoides (Weber 1964; Dietze et al. 2022a). A more detailed age at the level of biohorizons cannot be provided since any further ammonites originating from the Unterer-Donzdorf-Sandstein of Aalen have not been recorded. Weber (1964, section no. 38) noticed Staufenia (Costileioceras) sinon, Staufenia opalinoides and Ludwigia haugi obtusiformis from the Unterer-Donzdorf-Sandstein of Essingen, only few kilometres west of Aalen. Unfortunately, only few specimens of S. opalinoides from his collection have survived and are now in the SMNS collection. Their preservation is identical to that of the lectotype of A. opalinoides. The surviving specimens from Essingen are smaller than the S. sinon var. opalinoides material from Aalen. This provides a valuable argument that the Unterer-Donzdorf-Sandstein of Aalen cannot be assigned to the sinon biohorizon.

Except the only locally developed “Sohlstein” bed, the Unteres Flöz dates completely into the herein newly introduced sinon biohorizon within the Haugi Subzone (Murchisonae Zone, Upper Aalenian).

4.2. Correlations

SW Germany: We consider that the opalinoides biohorizon of the Wutach area (Horn 1909; Rieber 1963) is older than the sinon biohorizon of the Unteres Flöz at Aalen. The Ludwigia assemblages of the opalinoides and sinon biohorizons are not distinguishable on the basis of the studied specimens, however, there are differences in respect of Ancolioceras/Staufenia. The Staufenia specimens of the sinon biohorizon are larger than their ancestral forms of the genus Ancolioceras from the opalinoides biohorizon of the Wutach area (cf. Horn 1909, pls. 9–11). Moreover, in the latter the whorl section is on average more inflated and with a slightly stronger expressed ventrolateral edge (cf. Horn 1909, pls. 9–11) compared with S. sinon of the Unteres Flöz. In respect of the general evolutionary trends reported from the Staufenia chronocline (Dietze et al. 2017, 2022b) we regard the specimens from the Unteres Flöz to be more advanced than those still assigned to Ancolioceras of the Oberbeta-Oolith in the Wutach area and therefore must be younger. Consequently, the sinon biohorizon must be younger than the opalinoides biohorizon as defined in the Wutach area. A further consideration is if the opalinoides biohorizon of the Wutach area corresponds to the type horizon of A. opalinoides at Aalen. The inner whorls of A. opalinoides from the opalinoides biohorizon in the Wutach area (Horn 1909, pl. 9, figs. 4a, b, 10a, b, 9a, b), of the lectotype of A. opalinoides (Pl. 1, Figs. 3a, b) and inner whorls of Staufenia sinon var. opalinoides (Pl. 1, Fig. 4a, b) are practically indistinguishable. Although the type horizon of A. opalinoides, as explained above, is slightly older than the type horizon of S. sinon, the difference in age lies below biostratigraphical resolution. Therefore, the opalinoides biohorizon of the Wutach area must not be renamed.

The latiumbilicus biohorizon (Murchisonae Subzone) defined at the Wochenberg hill near Schömberg is the next younger defined biohorizon following above the sinon biohorizon (Rieber 1963; Dietze et al. 2022b). This ammonite assemblage differs from that of the sinon biohorizon by larger diameters of adult Staufenia specimens showing a weaker and shorter ribbing stage. The specimens of Ludwigia from here are grouped around L. murchisonae (Sowerby, 1829), contrary to those from the sinon biohorizon below (Rieber 1963; Dietze et al. 2022b). In these Ludwigia specimens the ribs terminate along the ventrolateral edge, whereas they continue towards the venter in specimens of the sinon biohorizon. Moreover, in Ludwigia specimens of the Haugi Subzone, the ribs are projected forward between the ventrolateral edge and the keel – a feature reminiscent to the ancestral genus Leioceras.

Further studies are necessary on the age of the rock interval “Schichten mit Staufenia sehndensis” (Rieber 1963). At the Wochenberg hill near Schörzingen, these beds can be assigned to the latiumbilicus biohorizon (Dietze et al. 2022b). At Gosheim, many ammonites determined as Staufenia “sehndensis” in various collections originate from several individual shell beds (lumachelles) occurring at different stratigraphic levels (Fischer 1924). It is planned to sample this interval carefully bed-by-bed in the near future. Ludwigia spp. from this interval at Gosheim in museums' and private collections – which are much rarer than coeval Staufenia – mostly show morphologies typical of the Haugi Subzone; however, younger forms group around L. murchisonae are present as well.

France: After the ammonite taxa listed in Contini (1969) and Contini et al. (1997), only a rough correlation of the sinon biohorizon with their Haugi Subzone is possible. From their obtusiformis horizon of the Murchisonae Subzone, these authors have neither cited Ludwigia crassa nor Staufenia sinon.

SW England: A correlation is extremely difficult, since ammonites of the genus Staufenia are almost absent in SW England (Chandler 1982, 1997) and the potentially age-diagnostic ammonite taxa of the genus Ludwigia listed by Chandler (1997, table 1) have long ranges covering several biohorizons. Chandler (1997, pl. 2, figs. 4a, b, 6a, b) illustrated Staufenia sehndensis und S. aff. sehndensis from the faunal horizon of Ludwigia obtusiformis, these specimens are slightly more discoidal and more involute than S. sinon var. opalinoides of the sinon biohorizon. Based on that material, the sinon biohorizon has to be placed between the English faunal horizons of Ancolioceras opalinoides and of Ludwigia obtusiformis.