Jens Lehmann
email: jens.lehmann@uni-bremen.de

Georges Bulot
email: bulot@cerege.fr

A Dufrenoyia (Ammonoidea, Deshayesitidae) fauna from Germany: Improving the correlation of the lower/upper Aptian boundary

Keywords: Ammonites, lower Aptian/upper Albian, historical material, northern Germany, Correlations

Dufrenoyia is a relevant ammonite genus for discussing the lower/upper Aptian boundary on a supraregional scale. We present the first comprehensive account on the genus from Germany, recording the two index species Dufrenoyia furcata and Dufrenoyia transitoria from the Ahaus area. This is based on a yet undescribed historical collection by the German nineteenth century palaeontologist Clemens Schlüter. Our stratigraphic conclusions improves the correlation of the poorly-known highest lower Aptian in Germany, particularly with the well-known successions in England and France. An alleged dimorphism in Dufrenoyia furcata is discussed.

Introduction

Almost 60 years ago Raymond Casey recommended to place the lower/upper Aptian boundary at the last occurrence of Dufrenoyia, at the top of the Tropaeum bowerbanki Zone, based on the well-exposed Lower Greensand succession in southern England [START_REF] Casey | The stratigraphical palaeontology of the Lower Greensand[END_REF]. In fact, in the Boreal-Atlantic subrealm Dufrenoyia furcata can be used as an alternative to the uppermost lower Aptian zonal index Tropaeum bowerbanki [START_REF] Lehmann | Ammonite biostratigraphy of the Cretaceous -an overview[END_REF]. Although this has been debated over the years (e.g. Kemper, 1971) it is now widely accepted [START_REF] Reboulet | Report on the 4th International Meeting of the IUGS Lower Cretaceous Ammonite Working Group, the "Kilian Group[END_REF][START_REF] Reboulet | Report on the 6th International Meeting of the IUGS Lower Cretaceous Ammonite Working Group, the Kilian Group[END_REF]2014). Our current knowledge indicates that the ammonite genus Dufrenoyia is a well-established marker for recognizing the lower/upper Aptian boundary in the Boreal as well as in the Tethys [START_REF] Raisossadat | The Early Aptian ammonite biozonation in Mediterranean Province[END_REF], whereas the distribution of other important markers like Tropaeum and Australiceras is comparatively restricted [START_REF] Lehmann | Ammonoid Paleobiology: From macroevolution to paleogeography[END_REF].

The Aptian ammonites of the genus Dufrenoyia from North Germany described here are a part of a collection assembled in the nineteenth century. At that time, many small clay pits where existing in that area from which a large number of fossils were collected. Nowadays, there is hardly any outcrops left. This is the main reason why only 15 papers illustrating ammonites from in situ occurrences from the Aptian in North Germany were published in the past 50 years (Kemper, 1971a(Kemper, , 1973a;;[START_REF] Kemper | Der Grenz-Tuff Apt/Alb von Vöhrum[END_REF][START_REF] Gaida | Das Oberapt von Sarstedt und seine Tuffe[END_REF][START_REF] Kemper | Geologischer Führer durch die Grafschaft Bentheim und die angrenzenden Gebiete mit einem Abriß der emsländischen Unterkreide[END_REF]Kemper, , , 1982a[START_REF] Kemper | Die Entfaltung der Ammoniten und die Meeresverbindungen im borealen Unter-und Mittel-Apt[END_REF][START_REF] Weber | Die Tongrube 4 der Ziegelei Schnermann in Rothenberge: Lithound Biostratigraphie eines Ober-Aptprofils in NW-Deutschland[END_REF][START_REF] Mutterlose | Die Litho-und Biostratigraphie des Apt der Tongruben Schnermann am Rothenberg[END_REF] Owen in [START_REF] Mutterlose | The Vöhrum section (northwest Germany) and the Aptian/Albian boundary[END_REF][START_REF] Hoffmann | Stratigraphie und Cephalopodenfauna des Unter-Apt von Alstätte (NRW)[END_REF][START_REF] Vespermann | Die neue Tongrube Vöhrum[END_REF][START_REF] Mutterlose | Neue Tagesaufschlüsse der Unter-Kreide (Hauterive -Unter-Apt) im Großraum Hannover -Braunschweig: Stratigraphie und Faunenführung[END_REF][START_REF] Lehmann | Early Aptian bay deposits at the southern margin of the Lower Saxony Basin: Integrated stratigraphy, palaeoenvironment and OAE 1a[END_REF][START_REF] Lehmann | Morphological variability in response to palaeoenvironmental change -a case study on Cretaceous ammonites[END_REF]. Among those, ammonite systematic is only addressed in a couple of contributions (Owen in [START_REF] Mutterlose | The Vöhrum section (northwest Germany) and the Aptian/Albian boundary[END_REF][START_REF] Hoffmann | Stratigraphie und Cephalopodenfauna des Unter-Apt von Alstätte (NRW)[END_REF] and thus each further account is reasonable.

Geological setting

All specimens referred to here have been obtained from the vicinity of the city of Ahaus in northern Germany. During the Early Cretaceous this area was at the western rim of the Lower Saxony Basin (LSB in the following; Fig. 1). The LSB contains a several thousand meters thick sedimentary succession, spanning a time interval from the latest Jurassic to the Late Cretaceous. During Aptian times, the LSB was subdivided by small swells and islands into subbasins, but generally it constituted a large epicontinental sea extending to the Proto-North Sea area, and connected to the (proto-)Atlantic Ocean in the southwest and northwest [START_REF] Kemper | Die Unterkreide Nordwestdeutschlands. Ein Überblick[END_REF][START_REF] Ziegler | Geological Atlas of Western and Central Europe[END_REF]. In the distal areas predominately shales were deposited, including the local expression of OAE 1a (e.g. [START_REF] Lehmann | Early Aptian bay deposits at the southern margin of the Lower Saxony Basin: Integrated stratigraphy, palaeoenvironment and OAE 1a[END_REF], whereas towards the basin margin, silty to sandy dark-grey claystone prevailed [START_REF] Mutterlose | The Barremian-Aptian interval in NW Germany: a review[END_REF]. The material described here comes from a restricted southernmost extension of the LSB called the Alstätte Bay (e.g. [START_REF] Lehmann | Early Aptian bay deposits at the southern margin of the Lower Saxony Basin: Integrated stratigraphy, palaeoenvironment and OAE 1a[END_REF][START_REF] Von Bargen | Benthic ecosystem response to the deposition of lower Aptian black shales in an epicontinental sea[END_REF][START_REF] Schlak | Paleoecological and paleoenvironmental implications of planktonic foraminifera from a restricted basin deposit of the Early Aptian of North Germany[END_REF] =Alstätte Embayment of Kemper, 1973b and[START_REF] Herngreen | Age and depositional environment of the Kuhfeld Beds (Lower Cretaceous) in the Alstätte Embayment (West Germany, East Netherlands)[END_REF]. It represents one of several small bays at the southwestern margin of the LSB [START_REF] Wolburg | Schwellen und Becken im Emsland-Tektogen mit einem paläogeographischen Abriß von Wealden und Unterkreide[END_REF][START_REF] Schott | Paläogeographischer Atlas der Unterkreide von Nordwestdeutschland mit einer Übersichtsdarstellung des nördlichen Mitteleuropa[END_REF]. Sediments of the Alstätte Bay were poorly exposed for many decades and thus very little research was done for many years. This changed fairly recently and a whole series of papers on the temporary EGW section in Ahaus-Alstätte that was exposed from 2009 to 2019 was published [START_REF] Hoffmann | Stratigraphie und Cephalopodenfauna des Unter-Apt von Alstätte (NRW)[END_REF][START_REF] Lehmann | Early Aptian bay deposits at the southern margin of the Lower Saxony Basin: Integrated stratigraphy, palaeoenvironment and OAE 1a[END_REF][START_REF] Bottini | Integrated stratigraphy of Early Aptian black shales in the Boreal Realm: calcareous nannofossil and stable isotope evidence for global and regional processes[END_REF][START_REF] Mutterlose | Stratigraphie und kalkige Nannofossilien des Unter-Apt von Alstätte (NRW)[END_REF][START_REF] Von Bargen | Benthic ecosystem response to the deposition of lower Aptian black shales in an epicontinental sea[END_REF][START_REF] Von Bargen | Ostracod response to habitat change in an early Aptian shelf sea[END_REF][START_REF] Mutterlose | High seasurface temperatures during the early Aptian Oceanic Anoxic Event 1a in the Boreal Realm[END_REF][START_REF] Schlak | Paleoecological and paleoenvironmental implications of planktonic foraminifera from a restricted basin deposit of the Early Aptian of North Germany[END_REF][START_REF] Lübke | The impact of OAE 1a on marine biota deciphered by size variations of coccoliths[END_REF][START_REF] Lehmann | Morphological variability in response to palaeoenvironmental change -a case study on Cretaceous ammonites[END_REF]. However, the Ahaus area became a classic area for Aptian palaeontology in Germany already in the 19 th century due to a large number of fossiliferous, but now abandon clay pits (e.g. [START_REF] Ewald | Über die fossile Fauna des unteren Gault bei Ahaus in Westphalen[END_REF][START_REF] Hilbck | Geognostische Darstellung des Eisenstein -Vorkommens in der älteren Kreideformation von Ahaus[END_REF][START_REF] Ziegler | Petrefakten aus dem Gault von Ahaus[END_REF]. It is quite surprising that the material focussed on herein can add quite a few new data to this classical and seemingly well-explored area of palaeontological research in Central Europe.

Figure 1 somewhere near here

Material and conventions

Previously known material

Dufrenoyia is a very rarely reported ammonite from Early Cretaceous deposits of Germany. Already von Strombeck (1858: 444) reported an ammonite, erroneously referred to as Ammonites Deshayesi, that is a Dufrenoyia since the cross section shows a distinct ventrolateral edge ("Die Kante zwischen Rücken und Seite pflegt dagegen bis ins höchste Alter hervorzustechen").

First finds attributed to a species that is now included in Dufrenoyia were mentioned by Ewald (1861: 338; not 1860 as cited by von Koenen 1902) from the Ahaus area (Barler Berg) as Ammonites furcatus. The same specimens were later briefly described by von Koenen (1902: 202-204), referring to them as Hoplites furcatus Sowerby. Besides 10 specimens from the Ahaus area von [START_REF] Von Koenen | Die Ammonitiden des norddeutschen Neocom (Valanginien, Hauterivien, Barrêmien und Aptien). Abhandlungen der königlich preussischen geologischen Landesanstalt und Bergakademie[END_REF] mentions two doubtfully conspecific specimens from Weenzen, Duingen area, in the Hildesheim district. More recently, Kemper (1964: 42) referred to one of these specimens in the University of Münster collection as Dufrenoya dufrenoyi (d'Orbigny, 1840) [sic] -apparently the only specimen that survived until then. Kemper (1971: pl. 26, fig. 2) figured Dufrenoyia furcata and since his figure captions refers to being a part of the University of Münster collection it can be assumed that he refers to the same specimen and actually revised his earlier identification. In fact, this historical specimen remained the only material depicted in the literature from Germany and consequently [START_REF] Kemper | Geologischer Führer durch die Grafschaft Bentheim und die angrenzenden Gebiete mit einem Abriß der emsländischen Unterkreide[END_REF]Kemper ( : pl. 24, fig. 1 and, 1995: pl. 5, fig. : pl. 5, fig. 2) refigured this important find.

New material and conventions

One of us (JL) was visiting the historical collection of Clemens August Schlüter (Fig. 2A) as a part of the University of Bonn collection in 2016, recognizing that the Aptian/Albian part of this collection was scientifically not explored yet. All of the material is housed in the Goldfuß-Museum für Paläontologie, University of Bonn, Germany. Its acronym is IGPB (for "Institut für Geowissenschaften, Paläontologie, Bonn"). Since these specimens are of historical importance additional to their scientific value, no preparation works have been done to reveal the suture lines.

However, IGPB KrU16-5 is a slightly worn internal mould that shows the suture. We used the suture terminology of [START_REF] Wedekind | Über Lobus, Suturallobus und Inzision[END_REF], as applied by [START_REF] Kullmann | Significance of sutures in phylogeny of Ammonoidea[END_REF] and modified by [START_REF] Korn | The A-mode sutural ontogeny in prolecanitid ammonoids[END_REF]: E -external lobe, Aadventive lobe (that is L of traditional nomenclature), U -umbilical lobe (I -internal lobe is not applicable for the present material). The E/A (that is E/L of traditional nomenclature) is the saddle between E and A and A/U (L/U of traditional nomenclature) and corresponds to the saddle between A and U. The differentiation into U2 and U3 is used here according to the conclusions on suture ontogeny of Dufrenoyia by [START_REF] Schindewolf | Studien zur Stammesgeschichte der Ammoniten[END_REF]. For most specimens it was not possible to give conch dimensions, D represents the diameter, U the width of the umbilicus (further details and measurements see Supplementary data).

The nineteenth century palaeontologist Schlüter (Fig. 2A) became famous by publishing 164 papers on Late Cretaceous and Devonian fossils from North Germany in between 1859 and 1902 [START_REF] Steinmann | Sitzungsberichte der Niederrheinischen Gesellschaft für Natur-und Heilkunde zu Bonn[END_REF]Riegraf and Scheer, 1991b, b). Many of Schlüter's papers are based on fossils collected by himself, he was a successful collector from an early age [START_REF] Steinmann | Sitzungsberichte der Niederrheinischen Gesellschaft für Natur-und Heilkunde zu Bonn[END_REF]. During the first part of his career he almost exclusively published on Late Cretaceous fossils from Germany and in 1878 this changed and he also concentrated on the description of Devonian fossils (Riegraf and Scheer, 1991a). In fact, Early Cretaceous fossils never drawn his attention, except for a paper on glacial erratics [START_REF] Schlüter | Über Geschiebe des unteren Jura und der unteren Kreide im baltischen Diluvium[END_REF]. This is one reason why he never worked on the fossils at hand, but also because he borrowed it to J. Ewald who used it together with a collection of von der Marck from the same locality to give an overview, however, without figuring them [START_REF] Ewald | Über die fossile Fauna des unteren Gault bei Ahaus in Westphalen[END_REF]. Nevertheless, the Ewald (1861) paper presents identifications that are referred to in the recent literature as doubtful (Klein and Bogdanova, 2013: 125) and thus it is relevant to figure all of the original material that survived here.

Figure 2 somewhere near here In this respect it is astonishing that Edwin Kemper never used the collection of the University of Bonn. Kemper was a geologist and palaeontologist widely known for his comprehensive work on the Early Cretaceous of the LSB in Germany (e.g. Kemper, 1982b;[START_REF] Kemper | Das Klima der Kreidezeit, Geologisches Jahrbuch[END_REF]. His work comprised Aptian ammonites quite intensively, but although he used historical material from various sources, he probably overlooked Schlüter's collection, because the latter was widelyrecognized for his collection of Late Cretaceous and Devonian fossils only. This assumption is underlined by a statement in his famous "Bentheimführer" [START_REF] Kemper | Geologischer Führer durch die Grafschaft Bentheim und die angrenzenden Gebiete mit einem Abriß der emsländischen Unterkreide[END_REF]) that there is nice material is kept in the collection of Münster University and the Natural History Museum in Enschede (The Netherlands).

Locality details

In 1853 quarrying first exposed Aptian sediments in the Ahaus area [START_REF] Ewald | Über die fossile Fauna des unteren Gault bei Ahaus in Westphalen[END_REF]. This exposure is now long covered again, but together with succeeding clay pit activities it is known today as the classical Barler Berg near Ahaus locality for fossils in Germany [START_REF] Kemper | Geologischer Führer durch die Grafschaft Bentheim und die angrenzenden Gebiete mit einem Abriß der emsländischen Unterkreide[END_REF][START_REF] Lehmann | Early Aptian bay deposits at the southern margin of the Lower Saxony Basin: Integrated stratigraphy, palaeoenvironment and OAE 1a[END_REF]. Most of the specimens described here are, however, lettered in red "Wüllen" (Fig. 2C). Besides, some of these have an additional label associated in the specimen box that states "Wüllen bei Ahaus, Unterer Gault, C. Schlüter leg." Some labels show that Schlüter identified the Dufrenoyia specimens correctly based on the state of the art of his time. Several identifications are given on the labels associated, and in one case (old inventory number 223, IGPB KrU16-5) a label is fixed on the specimen with that information and the identification "Ammonites furcatus Sow. = A. Dufrenoyi d'Orb." (Fig. 2B). Apparently, Schlüter was aware of his finds, but simply did not do the scientific work by himself (compare 3.2). Some fossils in the collection are labelled with the locality name "Barler Berg", as also indicated by red letters on the specimens (e.g. IGPB KrU15-13, a phylloceratid; IGPB KrU15-10, a pleurotomariid gastropod). In this case the year of collecting is given (1899, respectively 1864). A couple of labels associated with the Dufrenoyia material states that they were collected from "Wüllen" in 1863 (IGPB KrU18-10) respectively 1865 (IGPB KrU-14-25). However, Wüllen is the name of the next larger settlement and is used synonymous with "Barler Berg", that is a more precise name for indicating where the former pits were located (Fig. 3). We assume both locality names are used synonymous by Schlüter who, however, apparently did not changed the name used for this collecting locality at one point in time. We call the locality Barler Berg here, since "Wüllen" is widely used in the literature to refer to a former limestone quarry [START_REF] Riegraf | Nicht mehr zugänglich[END_REF] of Late Cretaceous (Turonian) age (e.g. [START_REF] Wittler | Zur systematischen Position der Isis vertebralis Hennig (Oktokorallia) zugeordneten Wurzelgeflechte in der Oberkreide[END_REF][START_REF] Wiese | Additional Prionocyclus germari (Reuss, 1845) (Cretaceous Ammonoidea, Upper Turonian) from NW Germany[END_REF][START_REF] Karl | The first shell remains of Rhinochelys pulchriceps (Owen, 1851) from the Upper Cretaceous of NW Germany (Testudines: Protostegidae)[END_REF] and even a formation in the Turonian of Westphalia is named after Wüllen [START_REF] Niebuhr | Lithostratigraphie der norddeutschen Oberkreide[END_REF].

Figure 3 somewhere near here

The Barler Berg locality is also called "an der Frankenmühle" [START_REF] Von Strombeck | Ueber den Gault bei der Frankenmühle unweit Ahaus[END_REF][START_REF] Ewald | Über die fossile Fauna des unteren Gault bei Ahaus in Westphalen[END_REF], referring to the name of a farmyard about 1 km WSW of the Frankenmühle, as clarified by Kemper (1976: figure caption to pl. 24, fig. 1;Fig. 3). This locality name was alternatively spelled "Frankemölle" (Bentz 1930) or "Frankenmölle", see [START_REF] Kemper | Geologischer Führer durch die Grafschaft Bentheim und die angrenzenden Gebiete mit einem Abriß der emsländischen Unterkreide[END_REF]) in historical times. Hosius (1860: 68) reports an alternation of glauconitic marlstones and ironcontaining sandstones, sometimes silicified, of varying hardness from the Frankenmühle locality. The matrix of the specimens at hand are in general accordance with this observation. All specimens show a carbonaceous matrix and mostly this contains many coarse quartz grains (Fig. 2B), almost all are reddish and this indicates that they are rich in iron, but there is are also a few specimens preserved in a light grey claystone. In fact, there are is an examples where a specimen preserved in grey claystone is associated with a specimen in carbonaceous matrix in the same box and the same label in the collection (IGPB KrU14-25 and IGPB KrU14-25b). Although these two preservations indicate an origin from beds of contrary rock types, this is in accordance with the alternation of different lithology at one and the same spot as described by [START_REF] Hosius | Beiträge zur Geognosie Westphalens[END_REF].

Systematic palaeontology

Suborder Ammonitina Hyatt, 1889 Superfamily Deshayesitoidea [START_REF] Stoyanow | Lower Cretaceous stratigraphy in southeastern Arizona[END_REF] Family Deshayesitidae [START_REF] Stoyanow | Lower Cretaceous stratigraphy in southeastern Arizona[END_REF] Dufrenoyia [START_REF] Kilian | Contribution à l`étude des faunes paléocrétacés du sud-est de la France. I. La faune de l`Aptien inférieur des environs de Montélimar. II. Sur quelques ammonites des environs de Montélimar (Dròme)[END_REF] Preliminary remarks. Among Cretaceous ammonoids, the Deshayesitidae show a particularly large variability. As already outlined by [START_REF] Bersac | Two ammonite species under the same name: Revision of Deshayesites deshayesi (d'Orbigny, 1841) based on topotype material (Lower Aptian, Lower Cretaceous, Northeast of France)[END_REF] and [START_REF] Lehmann | Morphological variability in response to palaeoenvironmental change -a case study on Cretaceous ammonites[END_REF], many of the species and even genera described are in need of a critical evaluation.

Several attempts have been made in the last two decades to evaluate the intraspecific variability of the western European populations of Dufrenoyia. Large number of specimens show that there is a considerable variability in the course of time, but also in the individual assemblages (fossil "populations") that are from a short stratigraphic interval. Such "populations" were investigated from France by [START_REF] Martin | Intraspecific variation of Dufrenoya furcata Sowerby (Ammonoidea, Ancylocerataceae, Deshayesitidae) from the Gargasian in the Vaucluse (Southern France)[END_REF] and Dutour (2005). Interestingly these two authors outlined the variability of the French "populations", but reached different systematic conclusions regarding the number of taxa represented. According to [START_REF] Martin | Intraspecific variation of Dufrenoya furcata Sowerby (Ammonoidea, Ancylocerataceae, Deshayesitidae) from the Gargasian in the Vaucluse (Southern France)[END_REF] all specimens from SE France, including Dufrenoyia dufrenoyi (d'Orbigny, 1841), fall within the range of variation of Dufrenoyia furcata (J. de C. Sowerby, 1836). To the difference, Dutour (2005) retains three successive palaeospecies (? forming a chronospecies): Dufrenoyia furcata, Dufrenoyia praedufrenoyi [START_REF] Casey | A monograph of the Ammonoidea of the Lower Greensand[END_REF] Dufrenoyia dufrenoyi. Since, a quantitative study of more than 1200 specimens from Spain was conducted by [START_REF] García | Dufrenoyia furcata (Sowerby, 1836) ammonite aptiense del Aptiense inferior (Cretácico inferior) del Perelló (Tarragona)[END_REF], who concluded that sexual dimorphism occurs in D. furcata. These later authors did not directly address the distinction between D. furcata and D. dufrenoyi, even so they kept the two species separate.

The Mediterranean faunas on which those population studies were carried out are based on juvenile pyritized internal moulds (diameter < 30 mm) that cannot be compared with our material. With two exceptions, our material represents later growth stages with a diameter ranging from 50 to 75 mm. Preserved suture lines indicate a reconstructed diameter of at least 100 mm for some specimens.

The collection herein studied compares best with the Dufrenoyia from the Lower Greensand of England monographed by [START_REF] Casey | A monograph of the Ammonoidea of the Lower Greensand[END_REF]Casey ( , 1980)). The variability of these forms was recently addressed since by [START_REF] Bersac | Ontogenesis, variability and evolution of the Lower Greensand Deshayesitidae (Ammonoidea, Lower Cretaceous, Southern England): reinterpretation of literature data; taxonomic and biostratigraphic implications[END_REF], who concluded that the many taxa described by Casey fall within the range of variation of two palaeospecies: Dufrenoyia furcata (J. de C. Sowerby, 1836) and Dufrenoyia dufrenoyi (d'Orbigny, 1841). Bersac and Bert (2012: 219 and 223) were aware of the limited quality of their database that was compiled from literature data and photographic material. They already pointed out that "the specimens studied from the Monograph were initially selected by Casey to illustrate an essentially typological taxonomic division, and thus they were not sampled in order to conduct a population study" and that "the number of specimens per sample is low to very low".

Regarding Dufrenoyia, the quantitative study performed by those authors is based on 31 specimens originating from three different geographic and geological settings (Hythe Beds, Kent; Carstone, Norfolk and Ferruginous Sands, Isle of Wight). The precise stratigraphic position in the T. bowerbanki Zone of at least a third of the material studied is either vague, inferred or unknown [START_REF] Casey | The stratigraphical palaeontology of the Lower Greensand[END_REF][START_REF] Casey | A monograph of the Ammonoidea of the Lower Greensand[END_REF]. In our opinion, the conclusions reached by [START_REF] Bersac | Ontogenesis, variability and evolution of the Lower Greensand Deshayesitidae (Ammonoidea, Lower Cretaceous, Southern England): reinterpretation of literature data; taxonomic and biostratigraphic implications[END_REF] 1968 Dufrenoya furcata (J. de C. Sow.); Kemper, pl. 24, fig. 1 (= Kemper 1971a and1976, both pl. 24, fig. 1).

1971b Dufrenoyia furcata (J. de C. Sow.); Kemper, pl. 26, fig. 2 (= Kemper, 1968, 1971a, as well as Kemper 1976: each pl. 24, fig. 1).

1995 Dufrenoyia furcata (J. de C. Sow.); Kemper, pl. 5, fig. 2 (= Kemper, 1968, pl. 24, fig. 1).

2015 Dufrenoyia furcata; Lehmann,Fig. 15.2 (holotype refigured) Remarks. Full revision of D. furcata is beyond the scope of the present contribution. A full list of citations is given by [START_REF] Klein | Lower Cretaceous Ammonites VI. Douvilleiceratoidea & Deshayesitoidea[END_REF], who did not revise the species. Additionally, specimens attributed to this species were illustrated by [START_REF] Delanoy | Xerticeras gen. nov., a new genus of micromorphic heteromorph ammonite (Ancyloceratina, Ancyloceratidae) from the lower Aptian of Spain[END_REF], [START_REF] Pictet | The Perte-du-Rhône Formation, a new Cretaceous (Aptian-Cenomanian) lithostratigraphic unit in the Jura mountains (France and Switzerland)[END_REF] and [START_REF] Bersac | The lower Aptian ammonites of the Les Ferres Aptian Basin(Lower Cretaceous, Southeast of France)Part I: Introduction and biostratigraphy[END_REF].

Material. 15 specimens, IGPB KrU14-13, IGPB KrU14-14, IGPB KrU14-25a, IGPB KrU17-5a, IGPB KrU17-5b, IGPB KrU17-5c, IGPB KrU17-8a, IGPB KrU17-8d, IGPB KrU17-11, IGPB KrU17-12, IGPB KrU17-13, IGPB KrU18-10, KrI18-8b, IGPB KrU18-11, IGPB KrU18-22; probably conspecific is KrU14-25b, short and distorted fragments referred to as Dufrenoyia sp. because of their poor preservation.

Figures 4, 5 and 6 somewhere near here Description. Specimens IGPB KrU17-5a and IGPB KrU17-5b are juveniles that compare with the early whorls of IGPB KrU.17-8b. At a diameter of 25 to 30 mm, the coiling is moderately involute (U/D around 0.29, see Supplementary data) with a C whorl section. Both specimens are densely ribbed with usually one secondary rib in between two primary ribs (10 primary and 11 secondary ribs per half whorl in IGPB KrU17-5a: Fig. 4.E). Height of the onset of secondary ribs varies strongly, either on the lower third of the flank high or at about mid-flank. All ribs weaken on the ventral area. They bear small clavate tubercles on the ventral shoulder. A smooth siphonal band can be observed. This feature is less expressed on IGPB KrU17-5a (Fig. 4.E) than in IGPB KrU17-5b (Fig. 4.F). IGPB KrU17-5c (Fig. 4.D) is a poorly preserved fragment of a juvenile marked by coarser ornamentation and well expressed ventrolateral clavi and smooth siphonal band.

Most specimens at hand are mid-size with a maximum reconstructed diameter ranging from 50 to 100 mm. The coiling is moderately involute (U/D around 0.26, see Supplementary data) with a subrectangular whorl section. The flanks are ornamented by very strong, radiate and flexuous ribs, with primary ribs very prominent and high on the lower and mid flank of most specimens even. This is less marked on IGPB KrU14-14 (Fig. 4.I) that is distorted, but nevertheless differs from all other specimens by a shallower umbilical wall and a more evolute coiling; additionally, the flexuous ribbing is generally prorsiradiate and not radiate. IGPB KrU17-11 (Fig. 5) is distorted and thus the ribbing is less distinct on the right side of the shell, but the left side better retains the original inflation of the shell and here the strength of ribbing is as prominent as usually at this growth stage. However, ribbing in IGPB KrU17-11 is less flexuous and denser compared to other specimens of the same size.

The last rib preserved in IGPB KrU17-13 (Fig. 6; estimated diameter of 70 mm, maximum whorl height of 30 mm) is standing closer compared to previous ribs and the last three ribs are clearly thinner than previous ones; additional a broadening of the body chamber is noted at the aperture. Furthermore, the umbilical wall is shallower for the last quarter of the whorl compared to the other specimens and the coiling appears more evolute.

There are in between 9-10 primary and secondary ribs per half whorl (Table 1).

Secondary ribs are regularly alternating with primary ribs to the exception of IGPB KrU17-8d (Fig. 4.B) where a pair of secondary ribs can be observed between to primary ribs. All specimens show ventrolateral clavi united by transversal ribs across the siphonal area (i.e. without smooth siphonal band), giving the venter a tabulate appearance in cross section with clear ventrolateral edges and a flat venter in between.

Table 1 somewhere near here Discussion. The Barler Berg material illustrates well the ontogenetic stage 2 (ventrolateral clavi bordering the smooth siphonal band) and 3 (ventrolateral clavi united by transversal ribs) of the genus Dufrenoyia as defined by [START_REF] Casey | A monograph of the Ammonoidea of the Lower Greensand[END_REF].

Specimens IGPB KrU17-5a (Fig. 4.E) and IGPB KrU17-5b (Fig. 4.F) clearly shows that the beginning of ontogenetic stage 3 is at about 25 to 30 mm. This is typical of the Dufrenoyia from the lower part of the Tropaeum bowerbanki Zone (D.

transitoria Subzone) of the Lower Greensand.

Among those, Dufrenoyia formosa and Dufrenoyia mackesoni corresponds best to most of the specimens at hand. Even so their morphology is more slender than that of the holotype of Dufrenoyia furcata (refigured here, Fig. 7) with a slightly more involute coiling, a slightly narrower whorl section and a somewhat finer ribbing, we consider them as conspecific. This is in agreement with [START_REF] Bersac | Ontogenesis, variability and evolution of the Lower Greensand Deshayesitidae (Ammonoidea, Lower Cretaceous, Southern England): reinterpretation of literature data; taxonomic and biostratigraphic implications[END_REF] treatment of those taxa.

Figure 7 somewhere near here Among the juvenile specimens from Barler Berg IGPB KrU17-5b (maximum diameter 25mm, maximum diameter around 12 mm; Fig. 4.F) corresponds well to alleged macroconchs of same size figured by [START_REF] García | Dufrenoyia furcata (Sowerby, 1836) ammonite aptiense del Aptiense inferior (Cretácico inferior) del Perelló (Tarragona)[END_REF] in its comparatively to microconchs fine and less dense ribbing of primary and secondary ribs, and a siphonal zone that gets smooth by a discontinuous ribbing across the venter. In contrast IGPB KrU17-5a (Fig. 4.E, reconstructed maximum diameter 30mm, maximum whorl height 14mm) shows continuous ribs across the venter, and pretty distinct ventrolateral bullae, as well as a dense ribbing and thus could be regarded as a microconch. We cannot see a fairly evolute coiling in our juvenile microconch though, however, it needs to be emphasized that the conclusions by [START_REF] García | Dufrenoyia furcata (Sowerby, 1836) ammonite aptiense del Aptiense inferior (Cretácico inferior) del Perelló (Tarragona)[END_REF] are based on a sample size that is statistically significant. The advantage of our material compared to that of [START_REF] Martin | Intraspecific variation of Dufrenoya furcata Sowerby (Ammonoidea, Ancylocerataceae, Deshayesitidae) from the Gargasian in the Vaucluse (Southern France)[END_REF] and [START_REF] García | Dufrenoyia furcata (Sowerby, 1836) ammonite aptiense del Aptiense inferior (Cretácico inferior) del Perelló (Tarragona)[END_REF] is that it represents adult or at least subadult specimens that are supposed to show indications for dimorphism more clear than juveniles, particularly modifications of the mature body chamber [START_REF] Klug | Mature modifications and sexual dimorphism[END_REF]. Another advantage is that some also allow -at least to some extend -an assessment of the juvenile stage morphology. The features of IGPB KrU17-13 (Fig. 6) described above (broadening of section and modified ribbing at the aperture, umbilical wall shallower, coiling more evolute) probably indicate that it reached maturity at its estimated maximum diameter of 70 mm (maximum whorl height 30 mm). Contrary specimen IGPB KrU17-22 does not show signs of maturity although it is larger. This fragment (maximum whorl height of 35 mm) is still septate at its initial part preserved, and the estimated diameter is larger than 120 mm.

The irregularity of ribbing in this group of ammonites is remarkable. It is wellknown from systematic descriptions that primary and secondary ribbing is not perfectly alternating in Dufrenoyia. Additionally, in several cases an individual primary rib on one side of the shell corresponds to a secondary rib on the other in our material (IGPB KrU14-14:

4.B).

A pretty extensive occurrence of pathologies could be related to a partially demersal life style (e.g. Keupp andIlg, 1992, 1994), but the general abundance of irregularities is difficult to explain.

To summarize the Dufrenoyia material at hand is varying much. A sexual dimorphism is not apparent in our material and the morphological disparity might be also explained by a wide range of phenotypic plasticity.

Distribution. In the eponymous zone widely distributed in Europe (England, Germany, France, Spain), Turkmenistan, northern Caucasus in Russia, Mangyshlak peninsula in Kazakhstan [START_REF] Casey | A monograph of the Ammonoidea of the Lower Greensand[END_REF][START_REF] Kemper | Geologischer Führer durch die Grafschaft Bentheim und die angrenzenden Gebiete mit einem Abriß der emsländischen Unterkreide[END_REF][START_REF] Ropolo | The Deshayesitidae Stoyanov, 1949 (Ammonoidea) of the Aptian historical stratotype region at Cassis-La Bédoule (SE France)[END_REF][START_REF] García | Dufrenoyia furcata (Sowerby, 1836) ammonite aptiense del Aptiense inferior (Cretácico inferior) del Perelló (Tarragona)[END_REF]. Not all of the non-European occurrences are certain, as well as the records from Egypt [START_REF] Aly | Aptian cephalopods from Gabal Abu Ruqum, north Sinai, Egypt[END_REF] and Japan [START_REF] Obata | Integrated mega-and microfossil biostratigraphy of the Lower Cretaceous Choshi Group, Japan[END_REF]. Casey, 1961 Figs. 8, 9 1961 Dufrenoyia transitoria sp. nov., Casey, p. 594;pl. 83, figs. 3a-b. 1964 Dufrenoyia transitoria Casey, Casey, p. 388, pl. LXV, fig. 6, pl. LXVI, fig. 2a-b (= CASEY, 1961, pl. 83, fig. 3a-b), fig. 3a-b, text-fig. 140b, 141, 142. A specimen from Spain that is doubtfully representing the species from our point of view was figured by Moreno-Bedmar et al. (2012: appendix fig. 6i) Material. 2 specimens, IGPB KrU16-5 (former inventory number 223) and IGPB KrU18-23.

Dufrenoyia transitoria

Description. The best preserved specimen (IGPB KrU16-5) is a half whorl, representing a part of a phragmocone and a partial body chamber with a maximum whorl height of 47 mm and a reconstructed maximum diameter of around 95 mm (Fig. 8).

The ribbing is prominent on the flanks, but not very strong, and generally irregular.

The strength of the primary ribs is variable. Some are well individualised while others tend to fade on the lower third of the flank. Some secondary ribs are intercalated and arise on the lower third of the flank or at mid-flank; others are branching from the primary ribs at an umbilical bullae; and thus have almost the same appearance as the primary ribs. There are 30 primary and secondary ribs per half a whorl. The ribs are very strong across the venter, more prominent compared to the flanks. In cross section the venter is tabulate, with a rounded ventrolateral edge and a very slightly arched venter between the edges. The flanks are broadest on the lower third of the flank. The umbilical seam is well-rounded.

A second fragment with a whorl height of about 90 mm is slightly worn, with dense, weak but fairly broad and slightly flexuous ribs that are most prominent near the umbilicus and at the ventrolateral edge. Occasionally secondary ribs are intercalated that arise on the upper flank; the ribs are crossing the venter less prominently.

Figure 8 and 9 somewhere near here Although the surface of IGPB KrU16-5 is slightly worn the characteristics of the suture line is fairly well-visible on the penultimate suture of the right side of the shell (Fig. 9). The E lobe is slender and high, the A lobe is the most prominent element of the suture line, trifid and barely symmetrical. Lobe U2 is trifid and asymmetrical. The U3 is very small, located just above the umbilical edge, in comparison with the U2, and little frayed. The E/A saddle is broader than the A lobe and shows a deep and moderately broad incision in the middle. The A/U2 saddle is prominent, symmetrical, with a deep and moderately broad incision in the middle.

Discussion. An early tendency to weak ornament (called "rapid degeneration of sculpture" by [START_REF] Casey | A monograph of the Ammonoidea of the Lower Greensand[END_REF] is characteristic for this species and the cross-section matches well the cross section given by Casey (1964: Fig. 140b). The suture line was described by Casey (1964: 388) as "with subquadrate ventral lobe and symmetrically trifid first lateral lobe". His ventral lobe corresponds to the external lobe and the first lateral lobe is the A lobe (see conventions in chapter 3.2). A subquadrate external lobe is also reported for the earliest ontogeny by [START_REF] Schindewolf | Studien zur Stammesgeschichte der Ammoniten[END_REF], in a juvenile suture referred to as representing Dufrenoyia dufrenoyi, but the E is well-visible in IGPB KrU16-5 and is in fact higher and slender. IGPB KrU16-5 agrees in having a trifid A, however, it is not symmetrical.

Lobes and saddles are less regularly frilled with stouter folioles and lobules (sensu [START_REF] Yacobucci | Ammonoid septal formation and suture asymmetry explored with a geographic information systems approach[END_REF] compared to the suture lines figured by Casey (1964: pl. 66, fig. 3a, b;text-fig. 141), and thus the general appearance of the suture line is less symmetrical in IGPB KrU16-5. The surface of IGPB KrU16-5 is slightly worn and it is most likely that these differences are due to a less favourable preservation of this feature compared to the Lower Greensand specimens. Bersac and Bert (2016) regards Dufrenoyia transitoria as one of the slender morphologies of D. furcata. In our opinion, D. transitoria has a highly distinctive dense and irregular ribbing style that do not compare with any other Dufrenoyia from the Lower Greensand. As such, we believe that the species should be kept separate until more material comes available to discuss Bersac and Bert's view.

Distribution. Outside England and Germany, the species was identified the Paris basin [START_REF] Amédro | Les ammonites aptiennes de la bordure orientale du bassin de Paris: un aperçu[END_REF]. Occurrences from Romania [START_REF] Avram | Les fossiles du flysch éocrétacé et des calcaires tithonique des hautes vallées de la Doftana et du Tirlung (Carpates Orientales)[END_REF] and southeast France [START_REF] Ropolo | The Deshayesitidae Stoyanov, 1949 (Ammonoidea) of the Aptian historical stratotype region at Cassis-La Bédoule (SE France)[END_REF] Kemper (1971) introduced a new biostratigraphical scheme for the Aptian of NW-Germany that was derived from the stratigraphical scheme for the Lower Greensand in southern England established by Casey (1961 and succeeding publications). Kemper (1971[START_REF] Kemper | Die Entfaltung der Ammoniten und die Meeresverbindungen im borealen Unter-und Mittel-Apt[END_REF] used both, Dufrenoyia furcata and Tropaeum bowerbanki, as a zonal index to characterize the highest lower Aptian (Fig. 10).

Biostratigraphy and correlations

The occurrence of both species was later used to correlate the lower/upper Aptian boundary between both areas [START_REF] Casey | Observations on the lithostratigraphy and ammonite succession of the Aptian (Lower Cretaceous) Lower Greensand of Chale Bay, Isle of Wight, UK[END_REF], although the British scheme is formally slightly different. [START_REF] Casey | The stratigraphical palaeontology of the Lower Greensand[END_REF] defined a Tropaeum bowerbanki Zone, subdivided into a Dufrenoyia transitoria Subzone at the base and a Cheloniceras (Cheloniceras) meyendorfi Subzone at the top. This interval characterises the uppermost lower Aptian and the genus Dufrenoyia ranges from its base to close to its upper limit (Casey et al 1998: Fig. 6). [START_REF] Casey | Observations on the lithostratigraphy and ammonite succession of the Aptian (Lower Cretaceous) Lower Greensand of Chale Bay, Isle of Wight, UK[END_REF] retained this zonation, and unequivocally stated that they regard the Tropaeum bowerbanki Zone as a synonym for the Dufrenoyia furcata Zone (p. 530). A correlation with the West Mediterranean province is thus clear, in fact a Dufrenoyia furcata Zone is widely used in this part of the Tethys [START_REF] Reboulet | Report on the 3rd International Meeting of the IUGS Lower Cretaceous Ammonite Working Group, the "Kilian Group[END_REF]2011;[START_REF] Reboulet | Report on the 6th International Meeting of the IUGS Lower Cretaceous Ammonite Working Group, the Kilian Group[END_REF]2014), but obscured by different zonal names. This is the reason why we prefer to use a Dufrenoyia furcata Zone. It makes sense to homogenize the biostratigraphical zonation since D. furcata can be recognized in a much wider area than Tropaeum bowerbanki.

Morphochange and its palaeoenvironmental background

Representatives of the ammonite family Deshayesitidae are widely known as abundant in shallow habitat, less dominant in hemipelagic settings (e.g. the Vocontian basin and Carpathian belt), since we know that Deshayesites itself is missing in pelagic deep-water [START_REF] Mikhailova | Ammonites of the genus Deshayesites from Aptian (Lower Cretaceous) sediments in the Crimea mountains[END_REF]. Despite the fact that it is a challenging field, some studies try to link the distribution of morphotypes to facies respectively environmental change [START_REF] Martin | Intraspecific variation of Dufrenoya furcata Sowerby (Ammonoidea, Ancylocerataceae, Deshayesitidae) from the Gargasian in the Vaucluse (Southern France)[END_REF][START_REF] Lehmann | Morphological variability in response to palaeoenvironmental change -a case study on Cretaceous ammonites[END_REF]. In the case of Deshayesites it has been demonstrated that variability among alleged chronospecies of this family is steered by palaeoenvironmental change and thus might eventually lead to divergent selection [START_REF] Lehmann | Morphological variability in response to palaeoenvironmental change -a case study on Cretaceous ammonites[END_REF].

In southern France, [START_REF] Martin | Intraspecific variation of Dufrenoya furcata Sowerby (Ammonoidea, Ancylocerataceae, Deshayesitidae) from the Gargasian in the Vaucluse (Southern France)[END_REF] reports a change to coarser ornamentation through time in Dufrenoyia. The average number of ribs per whorl changes from 37 to 28 in the three succeeding "populations" (for each "population" material from different localities is involved) individualised by this author. This trend is interpreted as an adaptive reaction to prograding shallow water facies. This appears to be opposite to the changes interpreted for Deshayesites [START_REF] Lehmann | Morphological variability in response to palaeoenvironmental change -a case study on Cretaceous ammonites[END_REF], but agrees with the idea of a principal tendency of strongly tuberculated ammonites, with depressed cross-sections, prefer nearshore deposits [START_REF] Wilmsen | Phenotypic plasticity and taxonomy of Schloenbachia varians (J. Sowerby, 1817) (Cretaceous Ammonoidea)[END_REF]. The Dufrenoyia material at hand shows primarily a coarse ornament and the high number of quartz grains shows that they were embedded in a turbulent water setting. However, a straightforward transfer of the morphochange pattern observed for Dufrenoyia by [START_REF] Martin | Intraspecific variation of Dufrenoya furcata Sowerby (Ammonoidea, Ancylocerataceae, Deshayesitidae) from the Gargasian in the Vaucluse (Southern France)[END_REF] to the Barler Berg fauna is impossible. Our coarsely ribbed specimens could be interpreted with corresponding to Marin's (2003) upper population sample, but the density of ribbing is different and our sample is not statistically relevant. As long as we need to deal with historical collections of Dufrenoyia from North Germany this issue cannot be solved any further. Berg and Frankenmühle, 2: Alstätte EGW section. Assembled after various sources, mainly [START_REF] Schott | Paläogeographischer Atlas der Unterkreide von Nordwestdeutschland mit einer Übersichtsdarstellung des nördlichen Mitteleuropa[END_REF], [START_REF] Ziegler | Geological Atlas of Western and Central Europe[END_REF] and [START_REF] Mutterlose | The Barremian-Aptian interval in NW Germany: a review[END_REF]. England [START_REF] Casey | The stratigraphical palaeontology of the Lower Greensand[END_REF][START_REF] Casey | Observations on the lithostratigraphy and ammonite succession of the Aptian (Lower Cretaceous) Lower Greensand of Chale Bay, Isle of Wight, UK[END_REF]. Note that [START_REF] Kemper | Die Entfaltung der Ammoniten und die Meeresverbindungen im borealen Unter-und Mittel-Apt[END_REF] is not formally calling his scheme a zonation, but a succession of index species and that the scheme shown is only applied to the southern part of the Lower Saxony Basin in NW-Germany.

  Figs. 4.A-J, 5, 6

  Fig. 4.G) and occasionally pathologies occur (IGPB KrU14-13: Fig. 4.C; IGPB KrU17-12: Fig. 4.J) or other irregularities in ornament occur (IGPB KrU17-8d: Fig.

  are poorly documented (ventral view not illustrated) and cannot be ascertained. Moreno-Bedmar et al. (2010) illustrated a single specimen from Spain that superficially match D. transitoria. The ribbing style and ventral area are closer to those of D. dufrenoyi from the same locality than to typical D. transitoria.

Figure

  Figure 10 somewhere near here

Fig. 1 .

 1 Figure captions, Table caption, Plate caption

Fig. 2 .

 2 Fig. 2. A Portrait of Clemens Schlüter (1835-1906) who collected the fossils described here (courtesy of the late J. Wiedmann). B Presumably Schlüter's original handwriting on a label sticking on specimen IGPB KrU16-5: "Wüllen bei Ahaus, Unterer Gault, Amm. furcatus Sow.: A. Dufrénoyi d'Orb., Cl. Schlüter leg.", referring to the Barler Berg locality. C Red label (IGPB KrU17-8d) indicating the locality name "Wüllen" on almost all of the specimens in the collection, showing many quartz grains in a carbonaceous matrix.

Fig. 3 .

 3 Fig.3. Locality map of modern landmarks and assumed sampling localities of the material described in here. Aptian fossils from this area are either labelled "Wüllen", Frankenmühle" (and variations in spelling) or "Barler Berg", but this all refers to the couple of potential localities marked in the geological map in the right lower corner. Geological map afterBentz (1930). The latter author divided the Cretaceous deposits in several mapping units that are not any longer in use, the modern age assignment is given in brackets: 1 Kuhfeldschichten, = Kuhfeld Beds (Valanginian), 2 Gildehauser Sandstein, = Gildehaus sandstone (Hauterivian), 3 "Hoplites weissi" beds (lower Aptian), 4 Rothenberg Sandstein, = Rothenberg

Fig. 4 .

 4 Fig. 4. Dufrenoyia furcata from the Aptian of Barler Berg near Ahaus, Germany.A1-A2, IGPB KrU17-8a. B1-B3, IGPB KrU17-8d. C1-C2, IGPB KrU14-13, note the pathology near the aperture in C1. D1-D3, IGPB KrU17-5c. E1-E3, IGPB KrU17-5a. F1-F3, IGPB KrU17-5b. G1-G3, IGPB KrU14-25a. H1-H2, GPB KrU18-11. I1-2, IGPB KrU14-14. J1-J2, IGPB KrU17-12.

Fig. 5 .

 5 Fig. 5. Dufrenoyia furcata from the Aptian of Barler Berg near Ahaus, Germany. Note the distortion of the specimen. IGPB KrU17-11.

Fig. 6 .

 6 Fig. 6. Dufrenoyia furcata from the Aptian of Barler Berg near Ahaus, Germany.Note the denser ribbing close to the aperture and the finer ultimate and penultimate rib compared to all specimens depicted in Fig.4this specimen is furthermore distinct by a shallow umbilical wall on the last quarter of the fragment. IGPB KrU17-13.

Fig. 7 .

 7 Fig. 7. Holotype of Dufrenoyia furcata (J. de C. Sowerby, 1836) from the Lower Greensand Group of Hythe, Kent, England. Coll. Fitton, 1911. Original in the British Geological Survey, Geol. Soc. Coll. 2290. Photo of plaster cast made by J. O. Barlow, mould no. 1013 in the Natural History Museum London (photo JL).

Fig. 8 .

 8 Fig. 8. Dufrenoyia transitoria from the Aptian of Barler Berg near Ahaus, Germany. IGPB KrU16-5.

Fig. 9 .

 9 Fig. 9. Suture of Dufrenoyia transitoria from the Aptian of Barler Berg near Ahaus, Germany. IGPB KrU16-5. Left: Continuous line indicates the umbilical seam, dashed line indicates the position of the umbilical edge.

Fig. 10 .

 10 Fig. 10. Position of the Dufrenoyia furcata Zone in NW-Germany within a biostratigraphical scheme for the Boreal Aptian and correlation with southern

Acknowledgements

We are am indebted to Georg Heumann (Bonn) for giving access to the Schlüter collection and his comprising support of this project. Markus Bertling (Münster) is thanked for giving access to the collection at the University of Münster. Martin Krogmann (Bremen) took the photos.