Paläontologische Zeitschrift

, Volume 85, Issue 2, pp 169–184

Phenotypic plasticity and taxonomy of Schloenbachia varians (J. Sowerby, 1817) (Cretaceous Ammonoidea)

Authors

    • Museum für Mineralogie und Geologie, Sektion PaläozoologieSenckenberg Naturhistorische Sammlungen Dresden
  • Abdolmajid Mosavinia
    • Science DepartmentPayame Noor University
Research Paper

DOI: 10.1007/s12542-010-0086-5

Cite this article as:
Wilmsen, M. & Mosavinia, A. Paläontol Z (2011) 85: 169. doi:10.1007/s12542-010-0086-5

Abstract

Taxonomic concepts of Early–Middle Cenomanian representatives of Schloenbachia Neumayr, 1875 are evaluated based on well-preserved, abundant faunas from NE Iran (Atamir Formation, Koppeh Dagh) and Germany (Essen Greensand and Baddeckenstedt formations at several localities in northern Germany; Meißen Formation of Saxony). From a single Lower Cenomanian horizon in the Atamir Formation, a complete suite of specimens of Schloenbachia varians (J. Sowerby, 1817), ranging from moderately involute, compressed and finely ornamented (“forma subplana”) to more evolute, depressed and strongly spinose forms (“forma ventriosa”) with all transitional morphologies, is documented. Also in the shallow-water faunas of the “Klippenfazies” of the Essen Greensand Formation at Mülheim-Broich and of the Meißen Formation, all morphotypes co-occur. Thus, these forms cannot be regarded as geographic subspecies or successive chrono-subspecies, but reflect a high degree of variability in shell form and ornament (phenotypic plasticity) in S. varians. Interestingly, strongly tuberculate forms are rare to absent in contemporaneous offshore settings as documented by the Baddeckenstedt Formation. The phenotypic plasticity in Early Cenomanian S. varians populations is explained by ecophenotypic variation along a depth (proximal–distal) gradient: strongly tuberculate, depressed forms reflect comparatively shallow, nearshore environments with higher water energy and predation pressure, whereas compressed, weakly ornamented morphs are forms of open (and deeper) marine waters. The complete range of different forms is also observed in micro- and macroconchs, suggesting that the morphological variability is not controlled by sexual dimorphism. Based on its similarity to S. varians, also the Middle Cenomanian Ammonites coupei Brongniart, 1822 may be placed in the synonymy of the former.

Keywords

CenomanianSchloenbachiidaeSystematic palaeontologyPhenotypic plasticity

Kurzfassung

Taxonomische Konzepte früh- bis mittelcenomaner Vertreter von Schloenbachia Neumayr, 1875 werden auf der Basis gut erhaltener, reicher Faunen aus dem NE Iran (Atamir-Formation im Koppeh Dagh) und Deutschland (Essen Grünsand- und Baddeckenstedt Formationen verschiedener Lokalitäten in Norddeutschland, Meißen-Formation in Sachsen) bewertet. Aus einem einzigen Horizont im Untercenoman der Atamir-Formation kann eine vollständige Sequenz von involuten, hochmündigen und schwach ornamentierten Formen (“forma subplana”) hin zu breitmündigen und stark skulpturierten Formen (“forma ventriosa”) von Schloenbachia varians (J. Sowerby, 1817) mit allen morphologischen Übergängen dokumentiert werden. Auch in den Flachwasserabfolgen (“Klippenfazies”) der Essen Grünsand-Formation in Mülheim-Broich und der Meißen Formation kommen alle Morphotypen gemeinsam vor. Diese können daher nicht als geographische oder chronologische Unterarten gedeutet werden, sondern reflektieren eine ausgeprägte Variabilität in Schalenform und Ornament (phänotypische Plastizität) bei S. varians. Interessanterweise fehlen die stark skulpturierten Formen der Art in zeitgenössischen Abfolgen des offenen Schelfs (Baddeckenstedt-Formation). Die Variabilität in früh-cenomanen S. varians-Populationen wird als ökophenotypische Variation infolge von sich ändernden Umweltbedingungen entlang eines Tiefengradienten von proximal nach distal gedeutet: die stark skulpturierten, breitmündigen Vertreter sind Flachwasserformen und reflektieren höhere Wasserenergie und Räuberdrücke, wohingegen die hochmündigen, schwach ornamentierten Vertreter Formen des tieferen Wassers sind. Das Vorhandensein aller möglichen Übergangsformen zwischen den morphologischen Endgliedern, die sowohl in Micro- als auch in Macroconchen beobachtet wird, zeigt weiterhin, dass die Variabilität in Schalenform und Ornament nicht durch Geschlechtsdimorphismus gesteuert wird. Aufgrund der weitgehend übereinstimmenden Morphologie mit S. varians sollte auch Ammonites coupei Brongniart, 1822 aus dem Mittelcenoman in die Synonymie von Sowerby’s Art gestellt werden.

Schlüsselwörter

CenomanSchloenbachiidaeSystematische PaläontologiePhänotypische Plastizität

Introduction

In recent decades, Cretaceous, and especially Cenomanian, Ammonoidea have been well described, stratigraphically documented, systematically treated and comprehensively monographed (e.g., Kennedy and Juignet 1983, 1984, 1993, 1994a, b; Wright and Kennedy 1984, 1987, 1990, 1995, 1996; Kennedy et al. 1986; Kaplan et al. 1998). However, the genus Schloenbachia Neumayr, 1875, which is very common and widely distributed in the Cenomanian of the Boreal Realm, forms an exception and is in urgent need of revision. Surprisingly, it was excluded from previous monographic revisions (see Kennedy 1971, 44–45). As pointed out by many authors, e.g., Kennedy et al. (1979), Marcinowski (1980), Hiss (1982), Thomel (1992) and Kaplan et al. (1998), the morphological variation within the genus is enormous and species concepts are not always clearly defined, resulting in the establishment of several morphotypes (varieties, forms, sometimes regarded as subspecies; see discussion in Hiss 1982) and stratigraphically defined species [i.e., Schloenbachia varians (J. Sowerby, 1817) for Early Cenomanian and S. coupei (Brongniart, 1822) for Middle Cenomanian representatives of the genus].

Based on well-dated, superbly preserved and, in part, new Lower Cenomanian material collected from the Atamir Formation of the Koppeh Dagh Mountains (NE Iran) and from Germany (Essen Greensand Formation of Mülheim-Broich, Baddeckenstedt Formation of the Münsterland, Lower Saxony and Saxony-Anhalt as well as Meißen Formation of Saxony), we discuss the wide morphological variability of the commonest and important species of the genus, Schloenbachia varians, in terms of phenotypic variation due to environmental changes along a proximal–distal gradient.

Taxonomy of Schloenbachia Neumayr, 1875: the status quo

The Boreal Cenomanian genus Schloenbachia comprises a group of usually medium-sized, keeled ammonites with markedly wide morphological variability. Morphotypes range from strongly compressed, relatively involute forms with faint ribbing and weak tuberculation to strongly spinose (hypernodose), depressed and relatively evolute forms (e.g., Juignet and Kennedy 1976; Kennedy et al. 1979; Marcinowski 1980; Hiss 1982; Kaplan et al. 1998). The genus was probably derived from the latest Albian (Stoliczkaia dispar Zone) Arrhaphoceras (Praeschloenbachia) Scholz, 1973 and had its maximum distribution in the Early Cenomanian (Kaplan et al. 1998).

Taxonomic concepts for Schloenbachia have varied widely because of the morphological variability of the genus (see account by Spath 1938). According to Kennedy et al. (1979, p. 29), Kaplan et al. (1998, p. 104; referring to unpublished work of the late Prof. J.M. Hancock) and Kennedy et al. (2008), Early Cenomanian forms are assigned to a single species, S. varians, with a number of morphological variants (Fig. 1). Between inflated and hypernodose to compressed and almost smooth end-members (i.e., ventriosa Stieler, 1922 versus subplana Mantell, 1822), a series of intergrading variants exists, termed varians s.str. J. Sowerby, 1817, subtuberculata Sharpe, 1853, and tollotiana Pictet, 1847 (in Pictet and Roux 1847–1854) (from strongly to less tuberculate and inflated to compressed, respectively; see Spath 1938 and Kennedy et al. 1979, 2008 for taxonomic details of the different forms in S. varians). However, these forms are only artificial categories that cannot be sharply separated, because all possible transitional morphologies exist; this results in the characterization of specimens as “passage forms” between two forms (e.g., Thomel 1992; Kennedy et al. 2008, p. 130). Middle Cenomanian representatives are assigned to S. coupei, which has virtually the same morphological variation, similarly expressed in a number of intergrading variants from hypernodose forms via trituberculata, quadrata, some unnamed forms to costata (Fig. 1). Formerly, many of these Early and Middle Cenomanian variants were regarded as species or subspecies (for the full taxonomic history see Spath 1938). Late Cenomanian taxa include S. lymense Spath, 1926 and S. baylissi (Kennedy and Wright, 1981), as well as some additional, hitherto undescribed species (Kaplan et al. 1998).
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Fig. 1

Taxonomy of Early and Middle Cenomanian representatives of the genus Schloenbachia Neumayr, 1875 (according to Kaplan et al. 1998 and Kennedy et al. 2008). See text for further explanation

Sowerby (1817, p. 169, pl. 176) based Ammonites varians on a series of predominantly tuberculate forms from southern England which included also one compressed, less tuberculate variant (pl. 176, lower left figure) without designating a holotype. Mantell (1822) was the first to subdivide Sowerby’s species by including all forms of pl. 176 except the one in the lower left in his var. tuberculata. At the same time, he considered the less tuberculate variants as the “usual form of the species” (Mantell’s var. intermedia and var. subplana). However, from Sowerby’s description it is clear that the tuberculate forms are the “true” A. varians and, consequently, Spath (1938) designated the lectotype from this group (pl. 176, upper figure; “forma varians s.str.”). Ammonites coupei was erected by Brongniart (1822, p. 83, pl. 6, fig. 3A–C) on the basis of a small, tuberculate, relatively inflated form from the Middle Cenomanian of Rouen, Seine-Maritime, France, while figuring as A. varians a more compressed form (pl. 6, fig. 5A–C). However, as Stieler (1922, p. 27) later stated, it is impossible to separate S. varians from S. coupei, especially when comparable variants and large populations are studied. Kaplan et al. (1998) also noted that it is much easier to differentiate the extreme variants of S. varians (i.e., S. varianssubplana from S. variansventriosa) than the hypernodose variants of both species. More data on taxonomic problems related to high intraspecific variability in (Cretaceous) ammonoids can be found in Kennedy and Cobban (1976, pp. 38–44).

Geological settings

The Schloenbachia faunas investigated herein originate from very different palaeogeographical and environmental settings (Fig. 2). The depositional settings comprise siliciclastic shelves of a passive margin (Atamir Formation of the Koppeh Dagh, Iran) to epicontinental shelves in (hemi-)pelagic offshore facies (Baddeckenstedt Formation, northern Germany) and high-energy nearshore facies associated with elevated cliffs (“Klippenfazies” of the Essen Greensand Formation of Mülheim-Broich and the Meißen Formation of Saxony). The German settings are well known and are thus only briefly described below. The Koppeh Dagh site, however, has so far been less thoroughly investigated and is therefore outlined in more detail below.
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Fig. 2

Palaeogeographic map of the Cenomanian (modified after Philip and Floquet 2000) with position of the four studied Schloenbachia faunas and palaeobiogeographic distribution of S. varians in general (obliquely hatched). Note the southernmost occurrence on the Central-East Iranian Microcontinent (CEIM) at 15–20° northern palaeolatitude. 1, Atamir Formation in the Koppeh Dagh, NE Iran; 2, Meißen Formation in Saxony. 3, Essen Greensand Formation at the Kassenberg, Mülheim-Broich; 4, Baddeckenstedt Formation in northern Germany

Atamir Formation (Koppeh Dagh, NE Iran)

Mid-Cretaceous ammonoids are well known from Iran (Seyed-Emami 1988), having been recorded from several localities in the Koppeh Dagh Mountains, NE Iran (e.g., Seyed-Emami and Aryai 1981; Seyed-Emami et al. 1984; Immel et al. 1997; Raisossadat 2006; Mosavinia et al. 2007). The Koppeh Dagh exposes an expanded Cretaceous succession of sedimentary rocks, ranging from the lower Lower Cretaceous (“Neocomian”) Shurijeh Formation, up to the Maastrichtian Kalat Formation. The strata were laid down on the northern shelf of a marine basin separating the Central-East Iranian Microcontinent (CEIM) from the Turan Plate (part of Eurasia) during the Cretaceous (Fig. 2). Fault-related subsidence resulted in the deposition of several kilometres of Cretaceous sediments in the Koppeh Dagh Basin (e.g., Berberian and King 1981).

Following non-marine deposition of the Shurijeh Formation, the transgressive phase started with shallow-water limestones of the Barremian–lowermost Aptian Tirgan Formation, followed by increasingly deeper marine limestones, marls and shales of the Aptian–Lower Albian Sarcheshmeh and Sanganeh formations. The Albian–Cenomanian Atamir Formation (Fig. 3) represents an up to 1,000 m thick succession of fairly fossiliferous, fine- to medium-grained, often glauconitic and/or bioclastic sandstones, with sandstone–siltstone intercalations and dark-grey to green, silty to fine sandy shales of inner, mid- and outer shelf origin (Mosavinia 2009).
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Fig. 3

Stratigraphical log of part of the Atamir Formation at Ghorghoreh, Koppeh Dagh, NE Iran. The level with the Schloenbachia fauna is indicated

From the locality Ghorghoreh (ca. 80 km east of Mashad, co-ordinates N 36°13′34″, E 60°29′18″), fairly abundant (35% of total ammonite fauna) Schloenbachia varians were collected from the transition of a sandstone bed into a thick sequence of shales (Fig. 3). Based on the co-occurrence of a rich and diverse ammonoid fauna comprising Mantelliceras mantelli (J. Sowerby), M. saxbii (Sharpe), M. cantianum Spath and several turrilitid heteromorphs [Mariella (Mariella) lewesiensis (Spath), M. (M.) cenomanensis (Schlüter), M. (M.) bicarinata (Kner), M. (M.) cf. dorsetensis Spath and Hypoturrilites wiedmanni Collignon], the bed can be dated as Early Cenomanian, upper Mantelliceras mantelli Zone of the standard ammonite zonation. A few specimens have been collected from the Middle Cenomanian (Acanthoceras rhotomagense Zone) of Gharehsu and Salehabad.

The Ghorghoreh fauna comes from the transitional interval overlying a 2–3 m thick fine- to medium-grained, glauconitic sandstone bed which is, at its base, abruptly intercalated into a thick Lower Cenomanian shale sequence (Fig. 3). In a sequence-stratigraphic interpretation, the base of the sandstone bed represents a significant sea-level fall (sequence boundary). From a biostratigraphic viewpoint, this sequence boundary is equivalent to the sequence boundary recognized in upper Mantelliceras mantelli zonal successions elsewhere (sequence boundary SB Ce 2; Robaszynski et al. 1998; Wilmsen 2000, 2003). The transition into the overlying thick unit of distal shelf shales suggests a rapid deepening of the depositional environment, and, thus, the early transgressive systems tract of Cenomanian depositional sequence DS Ce 3.

Meißen Formation (Saxony, Germany)

The Meißen Formation (Tröger and Voigt in Niebuhr et al. 2007) records the first marine ingression of the Late Cretaceous transgression in Saxony (southeastern Germany). It is confined to the northwest of the Saxonian capital Dresden in the vicinity of Meißen and consists of reddish-brown monzogranite conglomerates with a fossiliferous matrix of bioclastic calcareous sandstone. The “Meißen beds” were for long time regarded as Upper Cenomanian (Dietze 1960), but Prescher (1981) and Prescher and Tröger (1989) demonstrated the Early Cenomanian (Mantelliceras dixoni Zone) age on the basis of ammonoid and inoceramid occurrences. The Meißen Formation is preserved only in depressions of the Variscan basement and is unconformably overlain by the Upper Cenomanian Mobschatz Formation. It records nearshore, high-energy environmental conditions in the vicinity of elevated cliffs (Klippenfazies) and yields a diverse fauna of corals, bryozoans, brachiopods, bivalves, echinoids and cephalopods (belemnites and ammonoids). Schloenbachia varians has been recorded from Meißen by Dietze (1960) and Prescher and Tröger (1989).

Essen Greensand Formation (Kassenberg, Mülheim-Broich, Germany)

At the southern margin of the Münsterland Basin, the Late Cretaceous transgression is exemplified by the southward-directed onlap of Lower Cenomanian–Turonian strata onto Palaeozoic rocks of the Rhenish Massif [see Wilmsen et al. (2005) for a synopsis and additional references]. A famous locality representing a nearshore cliff setting is the Klippenfazies of the Kassenberg at Mülheim-Broich, where, in the Rauen quarry, fossiliferous sediments were preserved in wave-cut pockets within Carboniferous sandstones (Kahrs 1927). The oldest sediments of the Klippenfazies are the so-called Rotkalk mit Brauneisenschwarte (Kahrs 1927), which yielded a rich and well-preserved ammonoid fauna of the lower Lower Cenomanian (Mantelliceras mantelli Zone; Hancock et al. 1972; Wiedmann and Schneider 1979; Kaplan et al. 1998). Schloenbachia varians has also been recorded in all morphological varieties from the Rotkalk which lithostratigraphically is included within the Essen Greensand Formation (Hiss and Wilmsen in Niebuhr et al. 2007).

Baddeckenstedt Formation (northern Germany)

Cenomanian sediments of northern Germany were deposited on a northward-deepening graded shelf with a mixed siliciclastic-calcareous inner shelf facies belt (Essen Greensand and Herbram Formations; see Wilmsen et al. 2005 for a synopsis). The mid-shelf facies belt is documented by fossiliferous, cyclic marl-limestone alternations of the Baddeckenstedt Formation (Wilmsen and Hiss in Niebuhr et al. 2007), while the outer shelf facies is represented by poorly fossiliferous calcareous nannofossil limestones (Brochterbeck Formation). Due to the onlapping character of this biosedimentary system, the formations mentioned show a pronounced N–S diachronism, i.e., they become younger southwards. The Baddeckenstedt Formation ranges from the upper Mantelliceras mantelli Zone of the Lower Cenomanian into the Middle Cenomanian and is characterized by fine-grained, hemi-pelagic calcisphere limestones and marls deposited below storm wave base (Wilmsen 2003; Wilmsen et al. 2005). Macrofossils are common and include inoceramid and non-inoceramid bivalves, brachiopods, irregular echinoids, siliceous sponges and serpulids as well as heteromorph and planispiral ammonoids. Among the latter, Schloenbachia varians is by far the most abundant species, being especially common in the M. dixoni Zone of the upper Lower Cenomanian of the Baddeckenstedt Formation (see Wilmsen 2008). Compressed and weakly tuberculate morphotypes predominate.

Systematic palaeontology

The ammonites are stored in the collections of the Museum für Mineralogie und Geologie (MMG) of the Senckenberg Naturhistorische Sammlungen Dresden (material from the Atamir Formation of the Koppeh Dagh as well as specimens from the Meißen Formation of Saxony and most of the specimens from the Baddeckenstedt Formation of northern Germany) and of the Ruhr-Museum in Essen (RM, formerly Ruhrland-Museum; material from Mülheim-Broich). A part of the fauna was coated with magnesium oxide prior to photography (the Klippenfazies specimens were left uncoated in order to show the characteristic reddish colours).
  • Order Ammonoidea von Zittel, 1884

  • Suborder Ammonitina Hyatt, 1889

  • Superfamily Hoplitoidea H. Douvillé, 1890

  • Family Schloenbachiidae Parona and Bonarelli, 1897

  • Genus Schloenbachia Neumayr, 1875

Type species:

Ammonites varians J. Sowerby, 1817, by subsequent designation of H. Douvillé (1890).

Schloenbachia varians (J. Sowerby, 1817) Figs. 4, 5, 6, 7
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Fig. 4

Schloenbachia varians (J. Sowerby, 1817) from the Lower Cenomanian Atamir Formation at Ghorghoreh, Koppeh Dagh, NE Iran (all figures natural size): a specimen At.go.F40-2; b specimen At.go.F26-1; c specimen At.go.F16-3; d specimen At.go.F11-1; e specimen At.go.F11-2; f specimen At.go.F61-2; g specimen At.go.F39-1; h specimen At.go.F51-1; i specimen At.go.F11-3; j specimen At.go.F8; k specimen At.go.F31-2; l specimen At.go.F10-3; m specimen At.go.F10-1; n specimen At.go.F50-2; o specimen At.go.F10-2

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Fig. 5

Schloenbachia varians (J. Sowerby, 1817) from the Lower Cenomanian Klippenfazies of the Meißen Formation at the type locality at Meißen-Zscheila (MMG collection): a specimen SaK 74 (original of Dietze 1960, pl. 16, fig. 3a); b specimen SaK 75 (original of Dietze 1960, pl. 16, fig. 3a); c specimen SaK 14476a; d specimen SaK 14476b; e specimen SaK 14476c. All figures natural size

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Fig. 6

Schloenbachia varians (J. Sowerby, 1817) from the Lower Cenomanian Klippenfazies of the Essen Greensand Formation at the Kassenberg, Mülheim-Broich (RM collection): a strongly tuberculate specimen A2030; b intermediate specimen A5214; c weakly ornamented, more involute specimen A 1248/7. All figures natural size

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Fig. 7

Schloenbachia varians (J. Sowerby, 1817) from the Lower Cenomanian “Plänerfazies” of the Baddeckenstedt Formation of northern Germany (different collections; only A is from southern England and G is from the lower Middle Cenomanian): A large intermediate form from bed B24 of Southerham, Lewes, Sussex, southern England; B strongly tuberculate specimen from the Orbirhynchia/Schloenbachia Event of Wunstorf; C compressed specimen from Halle-Ascheloh (note tectonic deformation parallel to coiling plane); D compressed specimen from the Schloenbachia/virgatus Event of Wunstorf (see Wilmsen 2008); E large compressed specimen from the Schloenbachia/virgatus Event of Baddeckenstedt; F compressed specimen from the Schloenbachia/virgatus Event of Baddeckenstedt; G large, compressed macroconch from the lower Middle Cenomanian of Wunstorf (specimen DZ-W-C3; original of Wilmsen et al. 2007, Fig. 4h). H compressed specimen from the Schloenbachia/virgatus Event of Baddeckenstedt

1817

Ammonites varians J. Sowerby: 169 (pars), pl. 176 (top figure, middle figure in row below, figures of lower row)

1960

Schloenbachia varians (J. Sowerby 1817). Dietze: 54, pl. 16, fig. 3, 3a. [specimens from Meißen-Zscheila]

1972

Schloenbachia varians (J. Sowerby). Hancock et al.: 447, pl. 81, figs. 3–7. [specimens from Mülheim-Broich, variably assigned to different morphological variants]

1979

Schloenbachia varians (J. Sowerby, 1817). Kennedy et al.: 31, pl. 3, figs. 2–7; pl. 4, figs. 1–5. [specimens from the Esfahan area, Central Iran]

1979

Schloenbachia varians (J. Sowerby). Wiedmann and Schneider: 664, pl. 1, figs. 1–3; pl. 6; pl. 7; pl. 8, figs. 1, 2; pl. 9, figs. 1–4; text-fig. 8. [specimens from Mülheim-Broich]

1989

Schloenbachia varians (Sowerby) ssp. Prescher and Tröger: 162, pl. 28, figs. 1, 3. [specimens from Meißen-Zscheila]

1998

Schloenbachia varians (J. Sowerby, 1817). Kaplan et al.: 107, pl. 10, fig. 12; pl. 11, fig. 5; pl. 12, figs. 1–4, 6, 9–12; pl. 13, figs. 3–5, 13, 14; pl. 14, figs. 1–21; pl. 15, figs. 1–13; pl. 16, figs. 1–14. [see for comprehensive synonymy]

2007

Schloenbachia varians (J. Sowerby, 1817). Wilmsen et al.: 433, fig. 5H, I

2008

Schloenbachia varians (J. Sowerby, 1817). Wilmsen: fig. 5Q

2008

Schloenbachia varians (J. Sowerby, 1817). Kennedy et al.: 129, pl. 5, figs. 10–12, 14, 15; pl. 6, figs. 3, 6–13

Type:

Lectotype, designated by Spath (1938, p. 544), is specimen BMNH 43962b, hosted in the British Museum of Natural History, London, the original of Sowerby (1817, pl. 176, top figure, refigured by Kennedy and Hancock 1978, pl. 3, fig. 1).

Material:

37 internal moulds from the Atamir Formation (Koppeh Dagh, Iran), 6 specimens from the Meißen Formation, 95 from the Essen Greensand Formation and 74 from the Baddeckenstedt Formation (all Germany).

Description:

In the Iranian material (Fig. 4) and the populations from the Essen Greensand Formation of Mülheim-Broich (and also in the small faunule from the Meißen Formation of Saxony; Figs. 5, 6) there is a complete morphological suite of specimens, from moderately involute, compressed and finely ribbed forms with fastigate venter and a delicate, elevated keel, to depressed and strongly tuberculate (spinose) forms, whose strong keels do not arise above the ventrolateral tubercles. In compressed forms, the umbilicus is shallow, with an inclined (ca. 45°) umbilical wall; at narrowly rounded umbilical shoulders, flexuous primaries arise at distinct tubercles. The number of ribs is roughly doubled by intercalatories starting near mid-flank. In some specimens, they also appear to bifurcate (e.g., Fig. 4m). Each rib carries a prominent clavus at ventrolateral shoulders, forming two clavate spiral bands joining an elevated keel on a narrow venter. Primaries may carry indistinct lower lateral tubercles on inner flanks or near the point of bifurcation. The depressed forms lose their ribs at the cost of very large, spinose tubercles on the lower flanks and ventrolateral shoulders. The whorl section is polygonal, with concave portions between tubercles. The keel is strong but does not rise above the prominent ventrolateral tubercles. The umbilicus is deep and relatively wider than in compressed forms.

The contemporaneous populations from the Baddeckenstedt Formation (mainly from the localities Halle-Ascheloh, Wunstorf and Baddeckenstedt; Fig. 7) are clearly predominated by compressed, more involute and feebly ornamented forms. Tuberculate, depressed forms occur very rarely (Fig. 7b), yet intermediate morphs are present (Fig. 7a). However, their percentage remains below 10–15% of the total population.

Local occurrences:

Lower Cenomanian (upper Mantelliceras mantelli Zone) of the Atamir Formation at Ghorghoreh, E of Mashad, Koppeh Dagh Mountains (Iran); Lower Cenomanian (lower Mantelliceras mantelli Zone) of the Essen Greensand Formation of the Kassenberg (Mülheim-Broich, Germany); and Lower Cenomanian (upper Mantelliceras mantelli and M. dixoni zones) of the Baddeckenstedt Formation across northern Germany.

Palaeobiogeographical and temporal distribution:

Schloenbachia varians is widely distributed in the Lower Cenomanian of the Boreal Realm. The species is known from the Transcaspian area and Iran in the east to eastern Greenland in the west, being the commonest ammonite species of the Lower Cenomanian, sometimes accounting for up to 90% of complete ammonite assemblages (for further details, see Kennedy 1971; Kennedy et al. 1979; Kaplan et al. 1998).

Discussion

The genus Schloenbachia is in urgent need of taxonomic revision. As pointed out by Kennedy (1971), Kennedy et al. (1979), Marcinowski (1980), Hiss (1982), Thomel (1992) and Kaplan et al. (1998), the morphological variability within the genus is conspicuous and species concepts are not always clearly defined. This has resulted in the establishment of several morphotypes, varieties, subspecies or stratigraphic species (Fig. 1). However, we do not see any sense in drawing arbitrary boundaries between morphological entities and follow a different approach (see below).

Based on material from the Lower Cenomanian of the Koppeh Dagh and northern Germany, variants of Schloenbachia varians are not separated significantly either in space or in time (in terms of geographic subspecies or successive chrono-subspecies). Furthermore, they are all connected by transitional morphologies. Thus, they represent different morphs of a single, highly variable species, and there is no real justification for separation of the different variants or subspecies as displayed in Fig. 1 (see also Kennedy 1971, p. 45; Kennedy and Cobban 1976). Also the very similar sutures of the different morphotypes (Wiedmann and Schneider 1979, p. 666, fig. 8) support the uniformity of the group.

A plausible explanation for the apparent variability in Early Cenomanian Schloenbachia varians is ecophenotypic variation related to environmental change along a proximal–distal (i.e., depth) gradient. When the different morphs are reduced to three basic types, i.e., compressed and weakly ornamented (encompassing ±“forma subplana” and “tollotiana”), intermediate (±“forma subtuberculata”), and inflated and strongly ornamented forms (±“forma varians s.str.” and “ventriosa”), and their proportions for the different settings are plotted (Fig. 8), an interesting picture arises: strongly tuberculate, depressed forms are much commoner in comparatively shallower, nearshore environments, whereas compressed, weakly ornamented morphs completely predominate in open marine settings (Fig. 9). The mid-shelf assemblage of the Koppeh Dagh is intermediate in character. In a broader context, this observation fits the more general scenario of (Jurassic–)Cretaceous ammonoid morpho-group distribution, where strongly ornamented forms are usually placed in the neritic domains (compare Westermann 1996; Tsujita and Westermann 1998).
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Fig. 8

Distribution of compressed/weakly ornamented, intermediate and inflated/strongly tuberculate morphotypes of Schloenbachia varians (J. Sowerby, 1817) in the nearshore and offshore populations studied

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Fig. 9

Scenario of different habitats of Schloenbachia varians (J. Sowerby, 1817) on a distal–proximal transect with different morphotypes (“forma subplana” through “ventriosa”) shown in apertural views. Trends in shell morphology and principal palaeoenvironmental positions of the populations investigated are also shown

Phenotypic plasticity (Gause 1947) in relation to various environmental factors such as water energy and specialized predators is common in Recent molluscs (e.g., Trussell and Smith 2000), including cephalopods (Boyle and von Boletzky 1996). It has also been recorded for Cretaceous inoceramids and ammonoids (Reyment 1988; Reyment and Kennedy 1991; Diedrich 2001; Crampton and Gale 2005; Kin 2010) and may also account for the morphological variation in S. varians. The great variability in shell inflation and ornament of Albian Knemiceras persicum (Collignon) from southwest Iran was interpreted by Reyment and Kennedy (1991) as ecophenotypic in origin. They related this phenotypic plasticity more generally to the highly variable environmental conditions in shallow epicontinental seas and claimed the Tethyan vascoceratids of the Trans-Saharan Seaway as another potential example (cf. Reyment 1988).

The development of coarse ornament is often interpreted as an adaptive response to life in shallow environments, because those shells may be more resistant to mechanical damage or obtain a more stable position in turbulent waters (Fig. 9). Even more importantly, the coarse shell ornament (especially the spines on strong tubercles) was of a defensive nature: in the course of the “Mesozoic marine revolution” (Vermeij 1977), which intensified during the Jurassic and Cretaceous, shell-breaking (durophagous) predators such as carnivorous gastropods, decapod crustaceans, teleost fishes, marine reptiles (e.g., mosasaurid lizards), rays and ptychodontoid sharks became increasingly important (e.g., Kelley and Hansen 2001). Since attacks of durophagous predators are more common in shallow than in deeper waters (Ward 1996; Kelley and Hansen 2001), predatory pressure may have triggered the development of defensive morphologies, i.e., ornamented, predation-resistant shells, in Cretaceous shallow-water cephalopods. Ward (1981, 1996) demonstrated that coarse ornament in ammonoids, apparently defensive in nature, became increasingly important during the Mesozoic, especially during the Cretaceous. The development of predator-induced defences can in fact be very rapid (within several months) in species with high phenotypic plasticity, as has been shown by Trussell and Smith (2000) for marine gastropods. Furthermore, the authors suggest that predator-induced plasticity may have evolved as a general defence against spatially and temporally variable predators and that the impressive magnitude of those changes suggests that phenotypic plasticity plays an important role in shaping ecological communities. For Schloenbachia varians this means that, in shallow waters, defensive devices (spines; furthermore, in well-preserved depressed forms, the strong keel projects forwards at the aperture as a long rostrum; Kennedy 1971, p. 44) and stable shells were more important compared with the inferred better swimming skills (velocity, manoeuvrability) of compressed, weakly ornamented deep-water forms. This interpretation is clearly supported by the data from the Cenomanian of northern Germany, where compressed and weakly ornamented forms of S. varians characterized open marine deposits (the marl-limestone alternations of the Baddeckenstedt Formation), whereas the strongly tuberculate forms are common in proximal deposits (e.g., in the Klippenfazies of the Essen Greensand Formation at Mülheim-Broich and the Meißen Formation in Saxony; Fig. 8). The shallow- versus deeper-water interpretation for the different morphs is furthermore supported by the general trend in the population structure of NW European Schloenbachia faunas during the Cenomanian (Kennedy et al. 1979; Kaplan et al. 1998): inflated, tuberculate forms constantly decreased in importance throughout the stage, following the pronounced Cenomanian sea-level rise, associated with widespread loss of shallow-water habitats (Wilmsen 2003; Wilmsen et al. 2005). Also Diedrich (2001) demonstrated environmental variability of shell ornament in the large ammonite Puzosia dibleyi (Spath) from the Upper Cenomanian of Germany. Interestingly, he likewise recognized shallow-water morphotypes with strong ornament and spines, and deep-water forms with more delicate ornament patterns. The facies dependence of many ammonoids suggests a nekto-benthic (demersal) mode of life (also suggested by oxygen isotopic studies; Moriya et al. 2003), and a demersal mode of life is assumed for S. varians as well (see below).

Of course, there was no sharp boundary between the shallow- and deep-water morphotypes in Schloenbachia varians populations (Fig. 9). However, the nearshore forms are apparently more clearly restricted from offshore settings than vice versa (compressed forms also occur in Klippenfazies in significant numbers). This may be explained by (1) onshore transport of (dead) shells due to post mortem drift and/or storms, or (2) adaptation of the compressed forms to a wider range of environments (they almost certainly were better swimmers than their inflated, spiny counterparts). The co-occurrence of the different morphs in roughly equal proportions at one horizon in the Atamir Formation of NE Iran (Figs. 4, 8) is explained by the mid-shelf position of the section, recording influences from all principal environments (i.e., distal, medial, proximal). Furthermore, it may be related to the superposition of faunas representing different, originally laterally separated depth environments, because the level studied represents a transgressive bed deposited during the early transgressive systems tract of Cenomanian depositional sequence 3 (see Robaszynski et al. 1998 and Wilmsen 2003 for details on Cenomanian sequence stratigraphy). In support of this interpretation, Mosavinia (2009) showed that the tuberculate morphs were commoner at the base of the bed whereas the compressed forms increased in number in the upper part, thus reflecting the deepening-upwards trend. It should be noted that Thomel (1992) investigated Schloenbachia populations from the Vocontian Basin (southern France; see Fig. 2) and found Early Cenomanian S. varians in full morphological variability to be commonest in silty-clayey marl facies (which he referred to as “talus” but may better be regarded as middle to outer shelf), whereas the species is rare in basinal and nearly absent in shallow-water facies. Even if no quantitative data on the distribution of the morphotypes are available, the compressed and weakly ornamented morphs predominate with >55% (cf. Thomel 1992, pls. 43–46), in agreement with their offshore provenance. The nearly complete absence of the species in the proximal parts of the Vocontian Basin is somewhat surprising but may be related to the palaeogeographic situation at the northern margin of the Tethys, near the southern limit of the palaeobiogeographic distribution of the Boreal genus Schloenbachia (see also below). Shallow settings were certainly warm-water environments as shown by their biofacies (orbitolinid-exogyrine oyster facies; Thomel 1992) and may have confined S. varians to deeper (and thus cooler) settings. This also implies that S. varians did not inhabit the upper parts of the water column but more likely had a demersal mode of life (Fig. 9). This is straightforward for the shallow-water morphs anyway but was also suggested for the distal populations by Wilmsen (2008).

Schloenbachia is regarded as a dimorphic genus (e.g., Kennedy 1971), and one may argue that the morphological variability in S. varians is controlled by sexual (size) dimorphism. However, this can be discarded by the present Schloenbachia faunas: the Iranian material comprises specimens of more or less comparable diameter (~20–50 mm), with no really large specimens (i.e., macroconchs) being present. In this small size class (microconchs?), all morphs are present. The macroconchs figured by Kaplan et al. (1998, pl. 12, fig. 11) and Wilmsen et al. (2007, fig. 5H) from the Baddeckenstedt Formation reach up to 120 mm in diameter and are relatively compressed and smooth. However, in the collections of the Ruhr-Museum there are also strongly tuberculate macroconchs of Schloenbachia varians from the Essen Greensand Formation of Wattenscheid (specimens A 0489 and A 0567) of up to 145 mm in diameter, and also Thomel (1992, pl. 43, fig. 6) illustrated tuberculate macroconchs up to 120 mm in diameter from the Lower Cenomanian (M. mantelli Zone) of the Vocontian Basin (France). These faunas show that the complete range of morphological variation is present in both small (i.e., microconchs) and large S. varians (i.e., macroconchs) and that the morphological variability of shell form and ornament as discussed herein is not controlled by sexual size dimorphism. Furthermore, the presence of all intermediate stages between the morphological end-members argues against sexual dimorphism of ornamental features (compare Reyment 1988).

Schloenbachia varians from Iran, especially those from Central Iran (Kennedy et al. 1979), occur considerably further south (i.e., at subtropical 15–20°N) than the contemporaneous faunas from Europe (Fig. 2). This is surprising, as the species is regarded as a typical Boreal element which is missing from the Tethyan Realm (Juignet and Kennedy 1976; Kennedy et al. 1979). However, also the co-occurring ammonoid faunas from Central Iran are typically Boreal in character (Kennedy et al. 1979; Seyed-Emami 1982, 1988), supporting the rather temperate palaeobiogeographic affinity of the species but casting some doubt on the existing palaeogeographic reconstructions for the Central-East Iranian Microcontinent during the Cenomanian.

It is suggested here also to include Ammonites coupei Brongniart (1822, p. 83, pl. 6, fig. 3A–C) in the synonymy of Schloenbachia varians (compare Spath 1938; Thomel 1992). The species was erected by Brongniart on the basis of a tuberculate Middle Cenomanian variant which is very difficult to distinguish from an Early Cenomanian tuberculate form of S. varians. We only have limited Middle Cenomanian material from Iran (four specimens from the Acanthoceras rhotomagense Zone of Gharehsu and Salehabad; Fig. 10 shows a specimen which is close to, but larger than, the specimen illustrated by Brongniart 1922 on pl. 6) to support this statement, but the specimens figured as S. coupei by Kaplan et al. (1998, pl. 12, figs. 5, 7, 8, 12, 14; pl. 18, figs. 4, 8) from the Middle Cenomanian of the Münsterland Cretaceous Basin are virtually identical to Early Cenomanian representatives of S. varians illustrated in the same monograph and only separated based on their younger age. However, we question the concept of naming species on the basis of stratigraphical occurrence alone (i.e., S. varians for Early Cenomanian and S. coupei for Middle Cenomanian representatives of the genus), but further studies and larger Middle Cenomanian populations such as those from the Vocontian Basin (Thomel 1992) are needed to clarify this taxonomic dilemma.
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Fig. 10

A Middle Cenomanian (Acanthoceras rhotomagense Zone) representative of the genus Schloenbachia from the Koppeh Dagh (specimen At.sa.1F1-51 from Salehabad; natural size)

Conclusions

The taxonomic concepts of Early and Middle Cenomanian representatives of the genus Schloenbachia Neumayr, 1875 are evaluated based on new, well-preserved and abundant material from the Atamir Formation of the Koppeh Dagh (NE Iran) as well as both new and historical material from Germany (Essen Greensand Formation of Mülheim-Broich, Baddeckenstedt Formation of several sites in northern Germany, and the Meißen Formation of Saxony). From a single Lower Cenomanian horizon in the Atamir Formation, a complete suite of specimens of Schloenbachia varians (J. Sowerby, 1817), ranging from moderately involute, compressed and finely ribbed forms with elevated keel (“forma subplana”) to depressed and strongly tuberculate forms (“forma ventriosa”) with all transitional morphologies, is documented, indicating that those varieties were not separated either in space or significantly in time. Also in the shallow-water faunas of the nearshore Klippenfazies of Mülheim-Broich and Meißen, all morphotypes co-occur, with inflated and tuberculate forms being common. Interestingly, strongly tuberculate forms are rare to absent in contemporaneous offshore settings characterized by the Baddeckenstedt Formation. Thus, these varieties cannot be regarded as geographic subspecies or successive chrono-subspecies, but reflect a high degree of phenotypic plasticity of the species.

The morphological variability in Early Cenomanian S. varians is related to ecophenotypic variation along a proximal–distal (i.e., depth) gradient. Strongly tuberculate, depressed forms reflect comparably shallower, nearshore environments with higher water energy and predation pressure, whereas compressed, weakly ornamented morphs are forms of the open (and deeper) marine realm. This interpretation is clearly supported by the distribution pattern of the different forms in nearshore and offshore settings. The presence of the complete range of different forms in both micro- and macroconchs suggests that the morphological variability of shell form and ornament as discussed herein is not controlled by sexual dimorphism.

Schloenbachia varians is a Boreal species that occurs in Iran considerably further south (inferred 15–20°N) than the contemporaneous faunas from Europe, suggesting a warm-temperate setting of (Central) Iran during the (Early) Cenomanian and casting some doubt on the existing palaeogeographic reconstructions.

It is suggested here also to include the morphologically similar and likewise morphologically variable Middle Cenomanian Schloenbachia coupei (Brongniart, 1822) in the synonymy of S. varians. However, further studies and larger Middle Cenomanian populations are needed to clarify this taxonomic problem.

Acknowledgments

We are thankful for constructive reviews by J.W.M. Jagt (Maastricht) and A. Lukeneder (Vienna). Discussions with C.J. Wood (Minehead) and I. Walaszczyk (Warszawa) improved the paper considerably. Financial support for fieldwork in northern Germany to M.W. by the Deutsche Forschungsgemeinschaft (DFG, code WI 1743/3-1) is gratefully acknowledged. A.M. acknowledges a grant of the Iranian government and logistic help by the Payame Noor and Shahid Beheshti Universities. Furthermore, he thanks his thesis supervisors M.R. Chahida (Tehran) and K. Seyed-Emami (Tehran) for support. We also thank U. Scheer (Ruhr-Museum Essen) for access to the material from Mülheim-Broich, H. Schönig (Würzburg) and R. Winkler (Dresden) for their excellent photographic work and N. Richardt (Dresden) for the donation of S. varians from Halle-Ascheloh.

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