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

Ekbert Seibertz
email: e.seibertz@web.de

Christian Spaeth
email: chspaeth@web.de

Luc G Bulot
email: bulot@cerege.fr

Ammonite stratigraphy and the belemnite genus Hibolithes in the higher Serdj Formation (Aptian-Albian boundary interval) in north central Tunisia

Keywords: Die Bestimmung einiger Ammonitenfunde aus dem Grenzbereich Aptium/Albium der Typlokalität für die Serdj-Formation am Djebel Serdj in nord-zentral Tunesien werden revidiert und ihre biostratigraphische Aussage diskutiert. Aus diesem stratigraphischen Abschnitt wird ein angefertigter Dünnschliff zeigt den internen Aufbau von Rostrum und Phragmokon, inklusive einiger Septen, und ermöglicht die Bestimmung als Hibolithes cf. obtusirostris biostratigraphy, Lower Cretaceous, Aptian-Albian boundary interval, ammonites, belemnites, Hibolithes, Tunisia Biostratigraphie, Unterkreide, Aptium/Albium-Grenzbereich, Ammoniten, Belemniten, Hibolithes, Tune-

The identifications of some ammonite findings from the Aptian-Albian boundary interval of the type locality of the Serdj Formation at Djebel Serdj in north central Tunisia are revised and their biostratigraphic value is discussed. From this stratigraphic section, a single belemnite finding is described that has been previously identified as Neohibolites. A newly prepared thin section of the guard shows the internal structure of rostrum and phramoconus, including some septa, and allows an assignment as Hibolithes cf. obtusirostris (Pavlow in Pavlow & Lamplugh 1891). It represents the youngest record of this genus from the Tethys where the youngest record of Hibolithes was hitherto early Barremian. The new evidence is also of palaeobiogeographic interest, since Hibolithes is originally a genus of the Tethys, which migrated early into the Boreal and from there later on again into the Tethys.

Introduction

During the last fifteen years, Aptian (Lower Cretaceous) carbonate deposits of the Serdj Formation at its type locality Djebel Serdj, north central Tunisia (Figs. 1 and2) were studied by Tunisian, French and German geologists in detail with regard to lithology, lithostratigraphy, biostratigraphy (mainly ammonites), chemostratigraphy (including isotopic stratigraphy), and microfacies [START_REF] Heldt | Microfacies, biostratigraphy, and geochemistry of the hemipelagic Barremian-Aptian in north-central Tunisia: Influence of the OAE 1a on the southern Tethys margin. -Palaeogeogr[END_REF][START_REF] Heldt | Increased terrigenous influx but no drowning: Palaeoenvironmental evolution of the Tunisian carbonate plat-form margin during the Late Aptian[END_REF][START_REF] Lehmann | Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia[END_REF][START_REF] Ben Chaabane | Aptian-Lower Albian Serdj carbonate platform of the Tunisian Atlas: Development, demise and petroleum implication[END_REF]. These data were used for international correlation of these Tunisian successions [START_REF] Weiß | Small planktonic foraminiferal assemblages from the Boreal Aptian Hoheneggelsen KB 9 core (Hannover, northern Germany) -Distribution, biostratigraphy and palaeoceanography[END_REF][START_REF] Núnez-Useche | A negative carbon isotope excursion within the Dufrenoyia furcata Zone: Proposal for a new episode for chemostratigraphic correlation in the Aptian. -Carnets Géol[END_REF]) and allow to reconstruct the palaeoenvironmental evolution of a south-Tethyan carbonate platform margin and to investigate its response to global climate change. The unusually 600 metres thick deposits consist of limestones, marlstones and siltstones, suggesting an accumulation in mid-and inner-ramp positions. The hermatypic platform biota in these deposits induce a eschweizerbart_xxx u n c o r r e c t e d _ p r o o f this stratigraphic section, a single belemnite finding is described that has been previously identified as u n c o r r e c t e d _ p r o o f this stratigraphic section, a single belemnite finding is described that has been previously identified as newly prepared thin section of the guard shows the internal structure of rostrum and phramoconus, including some septa, (Hamada, Serdj, Hameima and Fahdene formations;[START_REF] Castany | Stratigraphy of the Hameima and lower Fahden formations in the Tadjerouine area (northern Tunisia)[END_REF][START_REF] Latil | Early Albian ammonites from Central Tunisia and adjacent areas of Algeria[END_REF][START_REF] Fadhel | Radiolarian and planktic foraminifera biostratigraphy of the Early Albian organic rich beds of Fahdene Formation, northern Tunisia[END_REF], and lagoonal facies with coral-rudist reefal buildings dominantly in the salt dome zone (Serdj Formation; [START_REF] Lehmann | Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia[END_REF][START_REF] Heldt | Increased terrigenous influx but no drowning: Palaeoenvironmental evolution of the Tunisian carbonate plat-form margin during the Late Aptian[END_REF][START_REF] Ben Chaabane | Aptian-Lower Albian Serdj carbonate platform of the Tunisian Atlas: Development, demise and petroleum implication[END_REF] of northern Atlas Tunisia are recorded from 36° N (including the Serdj-Guitoun-Bargou ranges; The Barremian Hamada Formation, as well as the Aptian-lower Albian Serdj Formation (informally subdivided as Lower Serdj and Upper Serdj by Ben Chaabane et al. 2019) crop out as a well exposed succession south of the town of Sidi Hamada (GPS Post Office: 35.96065°N, 9.54625°E) in the Serdj Mountain, which is the type locality for both formations (Fig. 2).

Ammonites and biostratigraphy

The Aptian ammonite fauna in north central Tunisia contains deshayesitids, pseudohaploceratids and hypacanthoplitids, and is, therefore, typical for the shelf environmental regions of the Tethys. This hemipelagic fauna shows affinities to faunas from the western and central Tethys as well as the Boreal region that was indistinctly defined compared to other stages of the Cretaceous [START_REF] Lehmann | Ammonoid paleobiology: From macroevolution to paleogeography. -Topics in Geobiol[END_REF]. Ammonites with a supposed pelagic mode of life that occur in more distal Tethyan environments, such as lytoceratids, phylloceratids and heteromorphs, are a minor component, if present at all. In the late Aptian, large acanthohoplitids are found in inner ramp and lagoonal facies. Ammonites of this group are rare in the uppermost strata of Djebel Serdj (Heldt et al. 2010: 710); their poor preservation and importance in respect to the Aptian-Albian boundary interval are reasons why some identification were quite often debated. [START_REF] Latil | Early Albian ammonites from Central Tunisia and adjacent areas of Algeria[END_REF] and later partly Ben Chaabane et al. (2019, with Latil as a co-author) re-assigned some of the ammonites described originally by [START_REF] Lehmann | Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia[END_REF] with partly contradictory results (Table 1). Furthermore, ammonite taxonomy is also difficult since parahoplitid ammonites (sensu lato) generally tend to be homoeomorphic and morphological differences are slight compared to other Cretaceous stages. Consequently, the identifications are briefly discussed in the following for the biostratigraphical dating of the belemnite specimen. The abbreviation GSUB refers to Geosciences Collection of the University of Bremen, Germany, and indicates the repository of specimens.

Discussion of Mellegueiceras

Mellegueiceras [START_REF] Latil | Early Albian ammonites from Central Tunisia and adjacent areas of Algeria[END_REF] is a hypacanthohoplitid ammonite that is an allegedly endemic genus from Tunisia that was recently also reported from Morocco [START_REF] Luber | A revised ammonoid biostratigraphy for the Aptian of NW Africa: Essaouira-Agadir Basin, Morocco[END_REF][START_REF] Luber | A revised chronostratigraphic framework for the Aptian of the Essaouira-Agadir Basin, a candidate type section for the NW African Atlantic Margin[END_REF]. Since this genus is critical for the stratigraphical in- stable oligotrophic water regime. Stable isotopes show a mid-late Aptian warming phase corresponding to siliciclastic limestones and siltstones, resulting rather from higher precipitation rates and intensified weathering, presumably of the southeasterly situated Kairouan Island and the southerly Kasserine Island, than from a drowning episode [START_REF] Heldt | Increased terrigenous influx but no drowning: Palaeoenvironmental evolution of the Tunisian carbonate plat-form margin during the Late Aptian[END_REF].

Geologic setting

During Cretaceous times, Tunisia formed part of the North-African epicontinental border, and the southern part of the Tethys, respectively. Trans-and regression cycles can be combined in the Lower Cretaceous into four megasequences (Burollet et al. 1983: 263), the third of them comprises the time interval from the late early Aptian ("Gargasian") to the middle Albian. Dominated by the Kairouan and Kasserine islands in central Tunisia, this time interval is characterised by the following facies distribution (comp. Castany 1953: Pl. III, VI and[START_REF] Burollet | Le Crétacé inférieur de Tunisie. Aperçu stratigraphique et sédimentologique[END_REF]

Ammonites and biostratigraphy

u n c o r r e c t e d _ p r o o f

Ammonites and biostratigraphy

The Aptian ammonite fauna in north central Tunisia contains terpretation of the Aptian-Albian boundary interval in Tunisia, it is discussed here. Mellegueiceras was separated from Hypacanthoplites Spath, 1923 by "its irregular ribbing and the lack of tuberculation" according to Latil (2011: 371). Klein & Bogdanova (2013: 208) question if Mellegueiceras should be regarded as a separate genus without discussing this issue any further.

The type species of Mellegueiceras, M. ouenzaensis, shows a somewhat irregular ribbing on the inner whorls only and this feature is not more irregularly developed compared to some Hypacanthoplites species referred to as Hypacanthoplites. This is true for Hypacanthoplites elegans for example, with its neotype that is showing an irregularity between the bullae and a high variability in the number and height of onset of secondary ribs in between primary ribs. However, H. elegans is much more finely ornamented compared to the type species of Hypacanthoplites, Hypacanthoplites milletianus (d'Orbigny, 1841), and it remains unclear if species showing stronger ribbing or even strongly tuberculated inner whorls like H. plesiotypicus [START_REF] Fritel | Sur les variations morphologiques d'Acanthoceras Milletianum, d'Orb. sp[END_REF]) can be reasonably placed in the same genus. Therefore, it possibly makes sense to accommodate H. elegans in Mellegueiceras.

The thickened ribs at the umbilical margin of the inner whorls in Mellegueiceras are very close to be called bullae and this is not clearly separating it from Hypacanthoplites. M. ouenzaensis can be separated from Hypacanthoplites though, by the inner whorls that are well-rounded and thus contrast the angular cross section seen in at least many wellknown European species of Hypacanthoplites. Other species of Mellegueiceras, M. hameimaense and M. minutum (both first described by Latil 2011), also show angular inner whorls and thus these features do not serve to distinguish both genera properly. "Melleguieiceras" chihaouiae appears to be quite different in cross section as well as ornament and rather resembles Hypacanthoplites than the type species of Mellegueiceras and the other species attributed to this genus. As an indication of uncertainty, we added quotation marks to the genus name herein.

A separation of Mellegueiceras from Parahoplites Anthula, 1900 is based on "ribs that are never bent forward on the venter, persistent irregular ribbing and wider umbilicus" following Latil (2011: 371). The first feature mentioned is not a criterion for exclusion and thus less advantageous for a distinction. It is believed to be a key feature to distinguish Parahoplites from acanthohoplids, but maybe this feature that is based on Parahoplites melchioris Anthula, 1900, is not strong enough to define Parahoplites in the current sense. Regarding to the second feature several Parahoplites species show an irregular ribbing (e.g. Parahoplites daveyi [START_REF] Casey | A monograph of the Ammonoidea of the Lower Greensand, part VI. -Paleontograph[END_REF] and Parahoplites irregularis [START_REF] Casey | A monograph of the Ammonoidea of the Lower Greensand, part VI. -Paleontograph[END_REF] and thus the irregularity of ribbing is not critical either. At last, a tendency to a more evolute coiling is developing within the Parahoplites, Acanthohoplites, Hypacanthoplites lineages during the late Aptian-early Albian and thus it does not constitute a unique feature as well.

This discussion shows that the definition of Mellegueiceras, Hypacanthoplites and Parahoplites is currently unsatisfying and that a new genus concept is needed. Furthermore, the affiliation of some species currently attributed to Hypacanthoplites and Parahoplites needs to be reviewed.

In the following the revised identifications of ammonites from the uppermost section at Djebel Serdj (section metres 810 to 835) given in Table 1 are

Section metre 812: GSUB C4048

Originally: Parahoplites cf. nutfieldiensis, revised: "Melleguieiceras"? cf. chihaouiae (Fig. 3H) [START_REF] Latil | Early Albian ammonites from Central Tunisia and adjacent areas of Algeria[END_REF], [START_REF] Klein | Lower Cretaceous ammonites VI. Douvilleiceratoidea and Deshayesitoidea[END_REF] and Ben Chaabane et al. ( 2019) suggested to attribute some of the specimens figured as P. cf. nutfieldiensis by [START_REF] Lehmann | Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia[END_REF] to "M." chihaouiae. After one of us (JL) was studying the type specimens of P. nutfieldiensis at the Natural History Museum [START_REF] Lehmann | Parahoplitid ammonites and narrowing down the Aptian/Albian boundary interval in northern Iran[END_REF]: Supplementary Data Appendix B) we agree that the Tunisian specimen are not conspecific with P. nutfieldiensis. Based on the fairly open coiling and according to the ornament, it appears likely that they represent "Mellegueiceras". GSUB C4048 [START_REF] Lehmann | Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia[END_REF]: Fig. 8D) as well as C4053 (Figs. 3D,E; [START_REF] Lehmann | Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia[END_REF]: Fig. 8I,J) are very poorly preserved and thus it is not convenient to determine them on the species level. The same is true for C4043 (Figs. 3I, J; [START_REF] Lehmann | Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia[END_REF]: Fig. 7I,J) from a lower level in the section.

The specimen C4048 shows simple, strong ribs and secondary ribs that do not arise before mid-flank and that are as strong as the primary ribs on the inner half of the flank. There is usually one secondary rib in between two primary ribs. This rib pattern contrasts the type material of P. nutfieldiensis from the Lower Greensand Group, but some new material at hand (Mark Bayliss collection, to be transferred to the Natural History Museum, London) from the same strata demonstrates a considerable variability. The coiling of C4048 appears to be more evolute than in P. nutfieldiensis and generally in Parahoplites and agrees with the re-identification given above.

A stratigraphic position as lowermost Albian and not upper Aptian [START_REF] Latil | Early Albian ammonites from Central Tunisia and adjacent areas of Algeria[END_REF]) is confirmed by data from Morocco where "Melleguieiceras" chihaouiae is associated with Douvilleiceras [START_REF] Luber | A revised ammonoid biostratigraphy for the Aptian of NW Africa: Essaouira-Agadir Basin, Morocco[END_REF][START_REF] Luber | A revised chronostratigraphic framework for the Aptian of the Essaouira-Agadir Basin, a candidate type section for the NW African Atlantic Margin[END_REF].

Section metre 815: GSUB C4042

Originally: Parahoplites cf. maximus, revised: "Mellegueiceras"? sp. (Figs. 3F,G) This specimen is less distorted compared to the other specimens revised here. We orientate this fragment different from [START_REF] Lehmann | Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia[END_REF] and the whorl section appears to be broadest around mid-flank and not near the umbilical rim. However, in our re-identification we disagree with the interpretation of [START_REF] Latil | Early Albian ammonites from Central Tunisia and adjacent areas of Algeria[END_REF], Ben Chabaane et al. ( 2019) and [START_REF] Klein | Lower Cretaceous ammonites VI. Douvilleiceratoidea and Deshayesitoidea[END_REF] that GSUB C4042 represents "Mellegueiceras" chihaouiae. It is different from the "M." chihaouiae macroconch of a similar size (Latil (2011: Pl. X and XI, 4), by a ribbing on the inner flank with ribs that are more distant and that are forming bullae on the very broad ribs. Near the umbilical edge the ribbing is generally reflecting backwards in GSUB C4042, leading to a rib style that is flexuous, as in the type of P. maximus, but even more distinct than in some of the specimens figured by [START_REF] Sinzow | Untersuchungen einiger Ammonitiden aus dem Unteren Gault Mangyschlaks und des Kaukasus[END_REF] Pl. I, Fig. 3). In cross section, GSUB C4042 is much broader and less evenly rounded than "M." chihaouiae. The particularly strong umbilical bullae that are bending forward on the lowermost flank, and then very strongly forwards again, is also present in acanthohoplitids. A similar ribbing pattern can be seen in some latest Aptian acanthohoplitids, e.g. in mid-size specimen figured by Sinzow (1908: Pl. VIII, Figs. 8 and 11). They are referred to as Nolaniceras spp. by [START_REF] Klein | Lower Cretaceous ammonites VI. Douvilleiceratoidea and Deshayesitoidea[END_REF], a view that is not followed by [START_REF] Bulot | New insight on the genus Nolaniceras Casey, 1961 (Ammonoidea, Cretaceous) and its consequences on the biostratigraphy of the Aptian Stage[END_REF]: according to these authors, Nolaniceras is strictly defined to the holotype of the type species. It is also true for the paratype of Acanthohoplites aschiltaensis Anthula (1900: Pl. XI, Fig. 1), but also in ?Parahoplites tenuicostatus [START_REF] Sinzow | Untersuchungen einiger Ammonitiden aus dem Unteren Gault Mangyschlaks und des Kaukasus[END_REF]: Pl. VII, Fig. 2). The exact stratigraphic assignment of the latter species is unclear and thus it is unknown if this feature is of stratigraphic relevance (compare Szives et al. 2007: 72).

Section metre 825: GSUB C4053

Originally: Parahoplites cf. nutfieldiensis, revised: "Melleguieiceras"? cf. chihaouiae (Figs. 3D,E)

The ribs of specimen C4053 are all of equal strength, more delicate than in C4042 (Figs. 3F,G) described above and also finer than in C4043 (all referred to as P. cf. nutfieldiensis by [START_REF] Lehmann | Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia[END_REF], but with C4053 from a much lower stratigraphic level). C4053 agrees with the other specimens identified earlier as P. cf. nutfieldiensis in showing one secondary rib in between two primary ribs only. The poor preservation permits only to assume that the secondary ribs arise probably as low as at mid-flank and that the cross segment is very well-rounded probably shows the greatest whorl breadth around mid-flank.

Except for the finer ribbing, the features of C4053 also distinguish it from the type material of P. nutfieldiensis (compare 3.2). We disagree with [START_REF] Klein | Lower Cretaceous ammonites VI. Douvilleiceratoidea and Deshayesitoidea[END_REF] who assigned this specimen to P. nutfieldiensis (omitting the "cf." by [START_REF] Lehmann | Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia[END_REF]), but agree with the re-identification of [START_REF] Latil | Early Albian ammonites from Central Tunisia and adjacent areas of Algeria[END_REF].

Section metre 834: GSUB C4051

Originally: Riedelites sp., revised: Acanthohoplites? sp. (Figs. 3A-C)

The worn and distorted fragment was originally assigned to Riedelites because it recalls the type of Parahoplites inconstans (Riedel 1938: Pl. 6, Figs. 8-10; now Riedelites inconstans; see Klein & Bogdanova 2013: 151), by its prominent and distant ribs and particularly "the large smooth areas on the inner flank between primary ribs", and the secondary ribs that do not arise before a high position on the flank (Lehmann et al. 2009: 907). Despite these similarities with the type of [START_REF] Riedel | Amonitas del cretácico inferior de la Cordillera Oriental. -Estud[END_REF], as well as the other material referred to Riedelites (e.g. Etayo-Serna 1979; [START_REF] Sharikadze | Aptian and Early Albian Douvilleiceratidae, Acanthohoplitidae and Parahoplitidae of Colombia[END_REF], we revise the initial identification here. The Tunisian specimen shows extremely strong umbilical bullae and even a more robust ribbing.

A similar morphology compared to GSUB C 4051 shows also Parahoplites robustus [START_REF] Sinzow | Untersuchungen einiger Ammonitiden aus dem Unteren Gault Mangyschlaks und des Kaukasus[END_REF] unknown detailed stratigraphic occurrence, but the bullae are even more distant in the Tunisian specimen. In this aspect, it resembles Mellegueiceras hameimaense of [START_REF] Latil | Early Albian ammonites from Central Tunisia and adjacent areas of Algeria[END_REF] in which the bullae appear to be closer to each other. GSUB C 4051 is not conspecific with the material attributed to "M." chihaouiae [START_REF] Latil | Early Albian ammonites from Central Tunisia and adjacent areas of Algeria[END_REF] as suggested by Latil (2011: 371, 372) as well as by [START_REF] Klein | Lower Cretaceous ammonites VI. Douvilleiceratoidea and Deshayesitoidea[END_REF], because of its pronounced and particularly very distant umbilical bullae without any ribs in between -an ornament unlike that of juvenile representatives of "M." chihaouiae. We place GSUB C 4051 into acanthohoplitids because it is similar in showing distant umbilical bullae although representatives of these usually contrasts by secondary ribs that are also present in between the umbilical bullae and are mostly not smooth in between as seen the GSUB C4051.

Discussion of the geological age

Based on the revised ammonite record from section metres 812 to 834 we interpret that the dating of this part of the Djebel Serdj section by [START_REF] Lehmann | Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia[END_REF] is probably slightly too old.

As already stated by [START_REF] Szives | Aptian-Campanian ammonites of Hungary[END_REF] the "stratigraphical distribution of Parahoplites, and also the co-occurrence with Acanthohoplites and Hypacanthoplites is problematic" and [START_REF] Owen | Uppermost Wealden facies and Lower Greensand Group (Lower Cretaceous) in Dorset, southern England: correlation and palaeoenvironment" by Ruffell & Batten (1994) and "The Sandgate Formation of the M20 Motorway near Ashford, Kent and its correlation[END_REF] states that the upper limit of Parahoplites falls together with the occurrence of Hypacanthoplites in the Late Aptian Nolaniceras nolani Zone that is debated though [START_REF] Casey | Lower Greensand ammonites and ammonite zonation[END_REF]. [START_REF] Szives | Aptian-Campanian ammonites of Hungary[END_REF] suggested that Acanthohoplites, Hypacanthoplites and Parahoplites are partly cooccurring in northern Iran interpreting [START_REF] Raisossadat | The ammonite family Parahoplitidae in the Sanganeh Formation of the Kopet Dagh Basin, north-eastern Iran[END_REF]. This interpretation might be partly based on general statements of ammonite occurrence but no detailed ranges are given by [START_REF] Raisossadat | The ammonite family Parahoplitidae in the Sanganeh Formation of the Kopet Dagh Basin, north-eastern Iran[END_REF]. Probably only Acanthohoplites and Parahoplites co-occur (cf. Raisossadat 2006: Fig. 2). The results of the most recent paper on this topic also did not find evidence for a co-occurrence of Acanthohoplites, Hypacanthoplites and Parahoplites [START_REF] Lehmann | Parahoplitid ammonites and narrowing down the Aptian/Albian boundary interval in northern Iran[END_REF]. At least it is clear that acanthohoplitids overlap with Hypacanthoplites and Parahoplites based on the record in Iran [START_REF] Lehmann | Parahoplitid ammonites and narrowing down the Aptian/Albian boundary interval in northern Iran[END_REF].

Out of this discussion the taxonomic re-interpretations of the individual ammonite record from section metres 812 to 834 by [START_REF] Klein | Lower Cretaceous ammonites VI. Douvilleiceratoidea and Deshayesitoidea[END_REF] is problematic from a stratigraphic point of view. If GSUB C4048 (Fig. 3H) is interpreted an unequivocal representative of P. nutfieldiensis, as done by these authors, the index of the P. nutfieldiensis Zone of the middle upper Aptian would co-occur with latest Aptian or earliest Albian ammonites ("M." chihaouiae).

The critical interval belongs to the T4 and Cu5 units of Ben [START_REF] Ben Chaabane | Aptian-Lower Albian Serdj carbonate platform of the Tunisian Atlas: Development, demise and petroleum implication[END_REF], attributed to the "M." chihaouiae Zone sensu [START_REF] Latil | Early Albian ammonites from Central Tunisia and adjacent areas of Algeria[END_REF]. This zone is not well-dated in Tunisia, further index ammonites are missing. An alleged condensation by "reworked phosphatised Epicheloniceras" (Latil 2011: 372), respectively "Reworked, phosphatised, juvenile Epicheloniceras sp." (Ben Chaabane et al. 2019: 588) is actually based on a misidentification. These reported che-loniceratids are actually douvilleiceratids, possibly of a hitherto new genus (pers. obs. by one of us: LGB). However, although it remains uncertain if it represents latest Aptian or earliest Albian sediments from the Tunisian record (compare Ben Chaabane et al. 2019: Fig. 17) the association of "M." chihaouiae with Douvilleiceras in Morocco places it into the Albian [START_REF] Luber | A revised ammonoid biostratigraphy for the Aptian of NW Africa: Essaouira-Agadir Basin, Morocco[END_REF]). Nevertheless, our revised dating is in accordance with the interpretation of the T4 and Cu5 units as "Aptian-Albian transition" by Ben Chaabane et al. ( 2019) and places the uppermost faunal horizon of [START_REF] Lehmann | Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia[END_REF] probably into the earliest Albian (comp. Figs. 4A andB).

The belemnite findings

Belemnites in the southwestern Tethys

Belemnites are generally rare findings in the northwest African Lower Cretaceous, except in some few layers. In the north-Tethyan regions from the Vocontian Basin in southeast France to the Betic Cordillera in southeast Spain, they are known to be common (see [START_REF] Janssen | Mediterranean Neocomian belemnites, part 4: Belemnites of the Barremian stratotype section. -Carnets Géol[END_REF] with references herein). Contemporaneous beds in the western High Atlas of Morocco contain lots of belemnites, so that these beds can be characterised by belemnite assemblages similar to those of France and Spain [START_REF] Mutterlose | Early Cretaceous (Valanginian -Hauterivian) belemnites from western Morocco: Stratigraphy and palaeoecology[END_REF]. One of the first who investigated northwest African Lower Cretaceous belemnites and used them for biostratigraphy was Coquand (1862: 27, 28), characterising in the Algerian region of Constantine an "Étage néocomien caractérisé par les Belemnites dilatatus Blainv." (corresponding to Duvalia dilatata of the Hauterivian), and an "Étage aptien caractérisé par le Belemnites semi-canaliculatus Blainv." (corresponding to Mesohibolites semicanaliculatus of the Aptian). South of Constantine, Peron (1889-90: 1) found "les Belemnites latus, B. bipartitus, etc. dans les marnes néocomien" in the Batna region (corresponding to Duvalia lata Blainville, and Pseudobelus bipartitus Blainville of the Valanginian-Hauterivian interval). Gentil (1902: 192, 193) found in the Tafna Basin of central Algeria "Pseudobelus bipartitus Blainv." and "Duvalia dilatata d'Orb." from the Hauterivian at Arlal, and "Hibolites sp." and "Duvalia Grasi Duv. sp." from the Barremian of the same location.

The Djebel Serdj belemnites

Pervinquière (1903: 41) described the section of the Djebel Serdj in detail giving lists of fossils for every lithological unit but without belemnites. In fossil-lists of the neighbouring Djebel Bargou and Djebel Guitoun (comp. Fig. 1), Pervinquière (1903: 45, 46) cited "Belemnites (Hibolites) semicanaliculatus Blainv." which is now Mesohibolites semicanaliculatus (de Blainville 1827). Four years later, Pervinquière (1907: 403 ff.) described a lot of belemnites from the Tunisian Lower Cretaceous that belong to the genus Duvalia (5 species), Pseudobelus (1 species) and Hibolithes (9 species). From the latter genus, only one species remains today in the genus concept of Hibolithes, the other species are representatives of the genus Mesohibolites and Neohibolites. Castany (1951: 31) cited from the Djebel Serdj "Belemnopsis semicanaliculatus Blainv.", which is nowadays Mesohibolites semicanaliculatus (de Blainville 1827), a form with great affinities to our finding. Since then, belemnites are recorded only as "belemnites" or "rostra" (e.g. [START_REF] Fadhel | Albian oceanic anoxic events in northern Tunesia: Biostrati-graphic and geochemical insights[END_REF] or even only as symbols aside a lithostratigraphic column (e.g. Latil 2011).

The GSUB C4049 belemnite

The belemnite GSUB C4049 from section metre 828 of the composite section of [START_REF] Lehmann | Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia[END_REF]Fig. 3; GPS North 35.96674°, East 009.59452°) was neither figured nor described, but is the specimen referred to as Neohibolites sp. in their range chart, and also mentioned on their p. 901-903 and their Table 1. It was also mentioned by [START_REF] Heldt | Increased terrigenous influx but no drowning: Palaeoenvironmental evolution of the Tunisian carbonate plat-form margin during the Late Aptian[END_REF] Table 1) and comes from the uppermost part of the outcrop of the Serdj Formation at its type locality at Djebel Serdj (Fig. 4C) in north central Tunisia. Originally this stratigraphic interval was placed into the middle upper Aptian based on the ammonite records of Parahoplites cf. nutfieldiensis, Parahoplites maximus and Riedelites in between section metres 810 to 835 [START_REF] Lehmann | Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia[END_REF]: Fig. 2), are herein revised (Table 1). This stratigraphic interval (Figs. 4A andB) is now placed into the highest upper Aptian or lowermost Albian. 7) as well as the detailed new description of Swinnerton (1952: 61-62, Pl. XVI, Figs. 3-7) allowed an unreserved definition -the use of open nomenclature is due to the fact that no external features are to be seen. A preparation of the rostrum was not possible because of the hardness of the limestone around it. So, most of the features for identifying are from a thin section. In the following, the rostral nomenclature is adopted from [START_REF] Spaeth | Untersuchungen an Belemniten des Formenkreises um Neohibolites minimus (Miller, 1826) aus dem Mittelund Ober-Alb Nordwestdeutschlands[END_REF] (comp. Fig. 5A).

The belemnite finding is a slightly eroded rostrum with an elongated outline (Fig. 6A). The apical line is straight and central (Figs. 5B and 6B,C). A short peak is preserved (Fig. 6A), which lies centrally without forming a distinct mucro. The alveolar angle is 17 degrees, which coincides with data from Pavlow (1891: 262) and Spaeth (1989: 79). The protoconch is overgrown by calcite. This coarse-grained overgrowth is induced by a ventral furrow at the alveolar end (comp. Fig. 6G) which is a third of the orthorostrum. Therefore, the view has to be perpendicular to the dorsoventral plain, which means it is a lateral view. The minimal thickness Dmin (blue line in Fig. 5B) shifts ontogenetically to the alveolus, whereas the maximal thickness Dmax (red line in Fig. 5B) remains more or less at one point of the rostrum, the ratio Dmax/Dmin has a constant value of 1.5, comparable to some specimens from the Isle of Heligoland (according to unpublished data by one of us: CS). The reason for this Dmax position is a reduced growth of the length in relation to thickness and breadth during ontogeny. Therefore, the apical part becomes more obtuse, shown in Fig. 5C by a striking bend in the graph. The Tunisian form is smaller compared to those from the Isle of Heligoland [START_REF] Spaeth | Hibolites obtusirostris (Pavlow, 1892 [sic]) und sein Vorkommen mit anderen Belemniten der Unterkreide von Helgoland (Nordsee, NW-Deutschland)[END_REF] or the Speeton Clay [START_REF] Swinnerton | A monograph of British Cretaceous belemnites; part IV. -Palaeontogr[END_REF]), but the congruence of the bluecircle graph and the green-square graph shows that the Tunisian form is an adult specimen. Fig. 6B shows the thin section in incident light with dark background, so that mineral overgrowth is visible very well: besides the calcite of the ventral furrow, bright points of calcification are visible in the rostrum. Fig. 6C is the thin section seen in transmitted light. The calcitic rostrum of the guard shows only little diagenetic overgrowth because the accretion lines are visible excellently. In Fig. 6D, the area of the peak is enlarged to show an offspring, presumably the first chambers of a bryozoan colony. This means that the rostrum was exposed at the sea bottom for only short time before it was covered by carbonate mud. Fig. 6E shows the section of the rostrum after the sawing of the thin section. The plane of intersection cuts here the siphuncle, which was filled by carbonate mud. In spite of the coarse-grained calcitic filling of the phragmoconus, segmentation into eight chambers is still visible. Fig. 6F is a magnification of part of the phragmoconus with the siphuncle and the alveolar wall, which shows a slight undulation because of the roundness of the chambers (comp. Doguzhaeva & Melendez 2017: Fig. 9). For comparison with older Early Cretaceous Hibolithes species, two sketches of the typical Hauterivian-Barremian H. jaculoides Swinnerton, 1937 are given in Figs. 6G and H. Though the outer diagnostic features of the belemnite cannot be seen, various measurements and internal features discussed in the previous chapter lead to the determination as Hibolithes cf. obtusirostris (Pavlow in Pavlow & Lamplugh, 1891). The co-occurring Mesohibolites semicanaliculatus (de Blainville, 1827) (see chapter 4.2) is less elongate/more stout and less obtuse with a distinct mucro and a greater alveolar angle. Some neohibolitids with obtuse ends similar to the Tunisian specimen, e.g. Neohibolites minimus obtusus [START_REF] Stolley | Studien an den Belemniten der unteren Kreide Norddeutschlands. -Jber[END_REF](comp. Spaeth 1971: 62), are smaller, show a different apical angle graph and are of middle Albian age.

The stratigraphic position of latest Aptian-earliest Albian (see chapter 3.6) is very young and hitherto unknown for this genus for the Tethys; the youngest specimens of Hibolithes are known from Tethyan occurrences of Early Barremian age (e.g. [START_REF] Janssen | Mediterranean Neocomian belemnites; part 1: Río Argos sequence (province of Murcia, Spain): The Berrias-Valanginian and the Hauterivian-Barremian boundaries[END_REF][START_REF] Janssen | Neocomian belemnites from the Bersek Hill (Gerecse Mountains, Hungary)[END_REF]. In contrast to this, from Boreal localities (e.g. northern Germany) Hibolithes is known from Aptian sediments, too [START_REF] Stolley | Studien an den Belemniten der unteren Kreide Norddeutschlands. -Jber[END_REF][START_REF] Mutterlose | Zur Paläobiogeographie von Belemniten der Unter-Kreide in NW-Europa[END_REF]Spaeth 1989 and references therein). Under palaeobiogeographic aspects, the originally Tethyan belemnite genus Hibolithes was able to migrate early into the Boreal regions as well as re-migrated later on into Tethyan areas again [START_REF] Stevens | Cretaceous belemnites[END_REF][START_REF] Mutterlose | Zur Paläobiogeographie von Belemniten der Unter-Kreide in NW-Europa[END_REF]. This scenario was repeated by Upper Cretaceous belemnitellids, as they also immigrated occasionally into northern Tethyan margins [START_REF] Christensen | Palaeobiogeography and migration in the Late Cretaceous belemnite family Belemnitellidae[END_REF]).

  u n c o r r e c t e d _ p r o o f newly prepared thin section of the guard shows the internal structure of rostrum and phramoconus, including some septa, (Pavlow in Pavlow & Lamplugh 1891). It represents the youngest u n c o r r e c t e d _ p r o o f (Pavlow in Pavlow & Lamplugh 1891). It represents the youngest was hitherto early Barremian. The new eviu n c o r r e c t e d _ p r o o f was hitherto early Barremian. The new eviis originally a genus of the Tethys, which migrated early into u n c o r r e c t e d _ p r o o f is originally a genus of the Tethys, which migrated early into Die Bestimmung einiger Ammonitenfunde aus dem Grenzbereich Aptium/Albium der Typlokalität für die u n c o r r e c t e d _ p r o o f Die Bestimmung einiger Ammonitenfunde aus dem Grenzbereich Aptium/Albium der Typlokalität für die Serdj-Formation am Djebel Serdj in nord-zentral Tunesien werden revidiert und ihre biostratigraphische Aussage diskutiert. u n c o r r e c t e d _ p r o o f Serdj-Formation am Djebel Serdj in nord-zentral Tunesien werden revidiert und ihre biostratigraphische Aussage diskutiert. Neohibolites u n c o r r e c t e d _ p r o o f Neohibolites bestimmter einzelner Belemnitenfund beschrieu n c o r r e c t e d _ p r o o f bestimmter einzelner Belemnitenfund beschrieben. Ein neu angefertigter Dünnschliff zeigt den internen Aufbau von Rostrum und Phragmokon, inklusive einiger Septen, u n c o r r e c t e d _ p r o o f ben. Ein neu angefertigter Dünnschliff zeigt den internen Aufbau von Rostrum und Phragmokon, inklusive einiger Septen, obtusirostris u n c o r r e c t e d _ p r o o f obtusirostris (Pavlow in Pavlow & Lamplugh 1891). Der Fund ist der u n c o r r e c t e d _ p r o o f (Pavlow in Pavlow & Lamplugh 1891). Der Fund ist der Hibolithes u n c o r r e c t e d _ p r o o f Hibolithes bislang nur bis in das frühe Barremium bekannt war. Der neue u n c o r r e c t e d _ p r o o f bislang nur bis in das frühe Barremium bekannt war. Der neue Nachweis ist auch paläobiogeographisch interessant, da u n c o r r e c t e d _ p r o o f Nachweis ist auch paläobiogeographisch interessant, da Hibolithes u n c o r r e c t e d _ p r o o f Hibolithes u n c o r r e c t e d _ p r o o f ursprünglich eine Gattung der Tethys ist, die früh in das u n c o r r e c t e d _ p r o o f ursprünglich eine Gattung der Tethys ist, die früh in das Boreal migrierte und später zurück in die Tethys einwanderte. u n c o r r e c t e d _ p r o o f Boreal migrierte und später zurück in die Tethys einwanderte. biostratigraphy, Lower Cretaceous, Aptian-Albian boundary interval, ammonites, belemnites, u n c o r r e c t e d _ p r o o f biostratigraphy, Lower Cretaceous, Aptian-Albian boundary interval, ammonites, belemnites, Biostratigraphie, Unterkreide, Aptium/Albium-Grenzbereich, Ammoniten, Belemniten, u n c o r r e c t e d _ p r o o f Biostratigraphie, Unterkreide, Aptium/Albium-Grenzbereich, Ammoniten, Belemniten, During the last fifteen years, Aptian (Lower Cretaceous) caru n c o r r e c t e d _ p r o o f During the last fifteen years, Aptian (Lower Cretaceous) carbonate deposits of the Serdj Formation at its type locality u n c o r r e c t e d _ p r o o f bonate deposits of the Serdj Formation at its type locality Djebel Serdj, north central Tunisia (Figs. 1 and 2) were stud-u n c o r r e c t e d _ p r o o f Djebel Serdj, north central Tunisia (Figs. 1 and 2) were studied by Tunisian, French and German geologists in detail with u n c o r r e c t e d _ p r o o f ied by Tunisian, French and German geologists in detail with ing with shales and sandstones

Fig. 1 )

 1 to about 34.15° N (Zaghbib-Turki 2003); (3) open marine/pelagic organic-rich shales intercalated with thin limestones containing planktic faunas (Fahdene and Bahloul formations; Soua 2016) are represented in northern Tunisia.

Fig. 1 :

 1 Fig. 1: Main study area at Djebel Serdj as part of the Serdj-Bargou anticline in north central Tunisia, with villages and the sampling locality of the described ammonites and belemnites.

  Fig. 1): (1) sandstones and shallow marine carbonates as well as dolomites alternating with evaporites are found in south central and southern Atlas Tunisia (Orbata and Kebar formations; Trabelsi 2012) and the Chott regions (Berrani Formation; Hfaiedh et al. 2013); (2) extensive shallow marine carbonate deposits interfinger-eschweizerbart_xxx u n c o r r e c t e d _ p r o o f 9.54625°E) in the Serdj Mountain, which is the type locality u n c o r r e c t e d _ p r o o f 9.54625°E) in the Serdj Mountain, which is the type locality for both formations (Fig. 2). u n c o r r e c t e d _ p r o o ffor both formations (Fig.2).

  u n c o r r e c t e d _ p r o o f The Aptian ammonite fauna in north central Tunisia contains deshayesitids, pseudohaploceratids and hypacanthoplitids, u n c o r r e c t e d _ p r o o f deshayesitids, pseudohaploceratids and hypacanthoplitids, and is, therefore, typical for the shelf environmental regions u n c o r r e c t e d _ p r o o f and is, therefore, typical for the shelf environmental regions of the Tethys. This hemipelagic fauna shows affinities to fauu n c o r r e c t e d _ p r o o f of the Tethys. This hemipelagic fauna shows affinities to faunas from the western and central Tethys as well as the Boreal u n c o r r e c t e d _ p r o o f nas from the western and central Tethys as well as the Boreal region that was indistinctly defined compared to other stages u n c o r r e c t e d _ p r o o f region that was indistinctly defined compared to other stages of the Cretaceous (Lehmann et al. 2015). Ammonites with a u n c o r r e c t e d _ p r o o f of the Cretaceous (Lehmann et al. 2015). Ammonites with a supposed pelagic mode of life that occur in more distal Tethu n c o r r e c t e d _ p r o o f supposed pelagic mode of life that occur in more distal Tethyan environments, such as lytoceratids, phylloceratids and u n c o r r e c t e d _ p r o o f yan environments, such as lytoceratids, phylloceratids and heteromorphs, are a minor component, if present at all. In the u n c o r r e c t e d _ p r o o f heteromorphs, are a minor component, if present at all. In the anticline in north central Tunisia, with villages and the sampling u n c o r r e c t e d _ p r o o f anticline in north central Tunisia, with villages and the sampling stable oligotrophic water regime. Stable isotopes show a u n c o r r e c t e d _ p r o o f stable oligotrophic water regime. Stable isotopes show a u n c o r r e c t e d _ p r o o f mid-late Aptian warming phase corresponding to siliciclastic u n c o r r e c t e d _ p r o o f mid-late Aptian warming phase corresponding to siliciclastic limestones and siltstones, resulting rather from higher preu n c o r r e c t e d _ p r o o f limestones and siltstones, resulting rather from higher precipitation rates and intensified weathering, presumably of u n c o r r e c t e d _ p r o o f cipitation rates and intensified weathering, presumably of the southeasterly situated Kairouan Island and the southerly u n c o r r e c t e d _ p r o o f the southeasterly situated Kairouan Island and the southerly u n c o r r e c t e d _ p r o o f Kasserine Island, than from a drowning episode (Heldt et al. u n c o r r e c t e d _ p r o o fKasserine Island, than from a drowning episode(Heldt et al.

Fig. 2 :

 2 Fig. 2: Central part of Djebel Serdj, with the section trail from the town of Sidi Hamada to the top of the mountain range with the finding location of the belemnite (red point). The red line marks the Aptian-Albian boundary interval.

  discussed. eschweizerbart_xxx u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f Central part of Djebel Serdj, with the section trail from the town of Sidi Hamada to the top of the mountain range with the fi u n c o r r e c t e d _ p r o o f Central part of Djebel Serdj, with the section trail from the town of Sidi Hamada to the top of the mountain range with the fi location of the belemnite (red point). The red line marks the Aptian-Albian boundary interval. u n c o r r e c t e d _ p r o o f location of the belemnite (red point). The red line marks the Aptian-Albian boundary interval. was separated from u n c o r r e c t e d _ p r o o f was separated from Spath, 1923 by "its irregular ribbing and u n c o r r e c t e d _ p r o o fSpath, 1923 by "its irregular ribbing and the lack of tuberculation" according toLatil (2011: 371).

  u n c o r r e c t e d _ p r o o f the lack of tuberculation" according toLatil (2011: 371). Mellegueiceras u n c o r r e c t e d _ p r o o f Mellegueiceras should be regarded as a separate genus without discussing u n c o r r e c t e d _ p r o o f should be regarded as a separate genus without discussing Mellegueiceras u n c o r r e c t e d _ p r o o f Mellegueiceras, u n c o r r e c t e d _ p r o o f , M. ouenzaensis u n c o r r e c t e d _ p r o o f M. ouenzaensis shows a somewhat irregular ribbing on the inner whorls u n c o r r e c t e d _ p r o o f shows a somewhat irregular ribbing on the inner whorls only and this feature is not more irregularly developed comu n c o r r e c t e d _ p r o o f only and this feature is not more irregularly developed com-Hypacanthoplites u n c o r r e c t e d _ p r o o f Hypacanthoplites species referred to as u n c o r r e c t e d _ p r o o f species referred to as . This is true for u n c o r r e c t e d _ p r o o f . This is true for Hypacanthoplites elegans u n c o r r e c t e d _ p r o o f Hypacanthoplites elegans u n c o r r e c t e d _ p r o o f for example, with its neotype that is u n c o r r e c t e d _ p r o o f for example, with its neotype that is showing an irregularity u n c o r r e c t e d _ p r o o f showing an irregularity between the bullae and a high variability in the number and u n c o r r e c t e d _ p r o o fbetween the bullae and a high variability in the number and height of onset of secondary ribs in between primary ribs.

  u n c o r r e c t e d _ p r o o f height of onset of secondary ribs in between primary ribs. is much more finely ornamented com-u n c o r r e c t e d _ p r o o f is much more finely ornamented com-Hypacanthoplites u n c o r r e c t e d _ p r o o f Hypacanthoplites and thus these features do not serve to distinguish both genu n c o r r e c t e d _ p r o o f and thus these features do not serve to distinguish both genera properly. " u n c o r r e c t e d _ p r o o f era properly. " quite different in cross section as well as ornament and rather u n c o r r e c t e d _ p r o o f quite different in cross section as well as ornament and rather resembles u n c o r r e c t e d _ p r o o f resembles legueiceras u n c o r r e c t e d _ p r o o f legueiceras an indication of uncertainty, we added quotation marks to the u n c o r r e c t e d _ p r o o f an indication of uncertainty, we added quotation marks to the

Fig

  371 and 372: only C4043 is "tentatively referred to" Mellegueiceras chihaouiae among the ammonites that were originally identified as Parahoplites cf. nutfieldiensis; C4053 is from the lower Lower Serdj Formation: 420 m] Parahoplites nutfieldiensis [p. 219, erroneously not referred to as Fig. 8I, J, but 6I, J] C4053, see above] Parahoplites nutfieldiensis [p. 219, erroneously not referred to as Fig. 8D, but 6D] o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f Ammonite identifications from the uppermost interval bearing ammonites at the Djebel Serdj section, above section meter 812 of the composite section by Lehmann et al. (2009: u n c o r r e c t e d _ p r o o f Ammonite identifications from the uppermost interval bearing ammonites at the Djebel Serdj section, above section meter 812 of the composite section by Lehmann et al. (2009: Neodeshayesites nicholisoni u n c o r r e c t e d _ p r o o f Neodeshayesites nicholisoni (based on GSUB C4042, following Lehmann et al. 2009) and u n c o r r e c t e d _ p r o o f (based on GSUB C4042, following Lehmann et al. 2009) and u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f u n c o r r e c t e d _ p r o o f by u n c o r r e c t e d _ p r o o f by Identification u n c o r r e c t e d _ p r o o f Identification tary] u n c o r r e c t e d _ p r o o f tary] Possibly u n c o r r e c t e d _ p r o o f Possibly Mellegueiceras chihaouiae u n c o r r e c t e d _ p r o o f Mellegueiceras chihaouiae [p. 371, 372] u n c o r r e c t e d _ p r o o f [p. 371, 372] [p. 371 and 372: only C4043 is u n c o r r e c t e d _ p r o o f [p. 371 and 372: only C4043 is " u n c o r r e c t e d _ p r o o f "tentau n c o r r e c t e d _ p r o o f tenta-Mellegueiceras u n c o r r e c t e d _ p r o o f Mellegueiceras among the ammonites that u n c o r r e c t e d _ p r o o f among the ammonites that Parahoplites u n c o r r e c t e d _ p r o o f Parahoplites C4053 is from the u n c o r r e c t e d _ p r o o f C4053 is from the lower Lower Serdj Formation: 420 m] u n c o r r e c t e d _ p r o o f lower Lower Serdj Formation: 420 m] but 6I, J] u n c o r r e c t e d _ p r o o f but 6I, J] Mellegueiceras chihaouiae u n c o r r e c t e d _ p r o o f Mellegueiceras chihaouiae [p. 209] u n c o r r e c t e d _ p r o o f [p. 209] Parahoplites u n c o r r e c t e d _ p r o o f Parahoplites nutfieldiensis u n c o r r e c t e d _ p r o o f nutfieldiensis [p. 219, erroneously not u n c o r r e c t e d _ p r o o f [p. 219, erroneously not referred to as Fig. 8D, but 6D] u n c o r r e c t e d _ p r o o f referred to as Fig. 8D, but 6D]

  species that is of eschweizerbart_xxx u n c o r r e c t e d _ p r o o f demonstrates a considerable variability. The coiling of u n c o r r e c t e d _ p r o o f demonstrates a considerable variability. The coiling of nutfieldiensis u n c o r r e c t e d _ p r o o f nutfieldiensis agrees with the re-identiu n c o r r e c t e d _ p r o o f agrees with the re-identi-A stratigraphic position as lowermost Albian and not upu n c o r r e c t e d _ p r o o f A stratigraphic position as lowermost Albian and not upper Aptian (Latil 2011) is confirmed by data from Morocco u n c o r r e c t e d _ p r o o f per Aptian (Latil 2011) is confirmed by data from Morocco is associated with u n c o r r e c t e d _ p r o o fis associated with[START_REF] Luber | A revised ammonoid biostratigraphy for the Aptian of NW Africa: Essaouira-Agadir Basin, Morocco[END_REF][START_REF] Luber | A revised chronostratigraphic framework for the Aptian of the Essaouira-Agadir Basin, a candidate type section for the NW African Atlantic Margin[END_REF].

  u n c o r r e c t e d _ p r o o f[START_REF] Luber | A revised ammonoid biostratigraphy for the Aptian of NW Africa: Essaouira-Agadir Basin, Morocco[END_REF][START_REF] Luber | A revised chronostratigraphic framework for the Aptian of the Essaouira-Agadir Basin, a candidate type section for the NW African Atlantic Margin[END_REF].

3. 3

 3 Section metre 815: GSUB C4042 u n c o r r e c t e d _ p r o o f 3.3 Section metre 815: GSUB C4042 Parahoplites u n c o r r e c t e d _ p r o o f Parahoplites cf. u n c o r r e c t e d _ p r o o f cf. maximus u n c o r r e c t e d _ p r o o f maximus, revised: " u n c o r r e c t e d _ p r o o f , revised: " This specimen is less distorted compared to the other u n c o r r e c t e d _ p r o o f This specimen is less distorted compared to the other specimens revised here. We orientate this fragment different u n c o r r e c t e d _ p r o o f specimens revised here. We orientate this fragment different 3.4 Section metre 825: GSUB C4053 u n c o r r e c t e d _ p r o o f 3.4 Section metre 825: GSUB C4053 Parahoplites u n c o r r e c t e d _ p r o o f Parahoplites cf. u n c o r r e c t e d _ p r o o f cf. nutfieldiensis u n c o r r e c t e d _ p r o o f nutfieldiensis chihaouiae u n c o r r e c t e d _ p r o o f chihaouiae (Figs. 3D, E) u n c o r r e c t e d _ p r o o f (Figs. 3D, E) The ribs of specimen C4053 are all of equal strength, u n c o r r e c t e d _ p r o o f The ribs of specimen C4053 are all of equal strength, more delicate than in C4042 (Figs. 3F, G) described above u n c o r r e c t e d _ p r o o f more delicate than in C4042 (Figs. 3F, G) described above and also finer than in C4043 (all referred to as u n c o r r e c t e d _ p r o o f and also finer than in C4043 (all referred to as diensis u n c o r r e c t e d _ p r o o f diensis by Lehmann et al. 2009, but with C4053 from a much u n c o r r e c t e d _ p r o o f by Lehmann et al. 2009, but with C4053 from a much lower stratigraphic level). C4053 agrees with the other speciu n c o r r e c t e d _ p r o o f lower stratigraphic level). C4053 agrees with the other specimens identified earlier as u n c o r r e c t e d _ p r o o f mens identified earlier as secondary rib in between two primary ribs only. The poor u n c o r r e c t e d _ p r o o f secondary rib in between two primary ribs only. The poor preservation permits only to assume that the secondary ribs u n c o r r e c t e d _ p r o o f preservation permits only to assume that the secondary ribs arise probably as low as at mid-flank and that the cross segu n c o r r e c t e d _ p r o o f arise probably as low as at mid-flank and that the cross segment is very well-rounded probably shows the greatest whorl u n c o r r e c t e d _ p r o o f ment is very well-rounded probably shows the greatest whorl breadth around mid-flank. u n c o r r e c t e d _ p r o o f breadth around mid-flank.

Fig. 3 :

 3 Fig. 3: Ammonite records from Djebel Serdj, northern Tunisia. (A-C) Acanthohoplites? sp., GSUB C4051. (D, E) "Mellegueiceras"? sp., GSUB C4053. (F, G) "Mellegueiceras"? sp., GSUB C4042. (H) "Mellegueiceras"? cf. chihaouiae, GSUB C4048. (I, J) "Mellegueiceras"?, GSUB C4043. Scale bar 10 mm.

  eschweizerbart_xxx u n c o r r e c t e d _ p r o o f in the u n c o r r e c t e d _ p r o o f in the Zone that is debated though u n c o r r e c t e d _ p r o o f Zone that is debated though Acanthou n c o r r e c t e d _ p r o o f Acanthoare partly cou n c o r r e c t e d _ p r o o f are partly cooccurring in northern Iran interpreting Raisossadat (2006).

  u n c o r r e c t e d _ p r o o f occurring in northern Iran interpreting[START_REF] Raisossadat | The ammonite family Parahoplitidae in the Sanganeh Formation of the Kopet Dagh Basin, north-eastern Iran[END_REF]. This interpretation might be partly based on general stateu n c o r r e c t e d _ p r o o f This interpretation might be partly based on general statements of ammonite occurrence but no detailed ranges are u n c o r r e c t e d _ p r o o f ments of ammonite occurrence but no detailed ranges are given by Raisossadat (2006). Probably only u n c o r r e c t e d _ p r o o f given by Raisossadat (2006). Probably only Acanthohoplites u n c o r r e c t e d _ p r o o f Acanthohoplites co-occur (cf. Raisossadat 2006: Fig. 2). u n c o r r e c t e d _ p r o o f co-occur (cf. Raisossadat 2006: Fig. 2). The results of the most recent paper on this topic also did not u n c o r r e c t e d _ p r o o f The results of the most recent paper on this topic also did not find evidence for a co-occurrence of u n c o r r e c t e d _ p r o o f find evidence for a co-occurrence of Acanthohoplites u n c o r r e c t e d _ p r o o f Acanthohoplites Parahoplites u n c o r r e c t e d _ p r o o f Parahoplites (Lehmann et al. 2019). At u n c o r r e c t e d _ p r o o f (Lehmann et al. 2019). At least it is clear that acanthohoplitids overlap with u n c o r r e c t e d _ p r o o f least it is clear that acanthohoplitids overlap with Parahoplites u n c o r r e c t e d _ p r o o f Parahoplites based on the record in Iran u n c o r r e c t e d _ p r o o f based on the record in Iran Out of this discussion the taxonomic re-interpretations of u n c o r r e c t e d _ p r o o f Out of this discussion the taxonomic re-interpretations of 4.1 Belemnites in the southwestern Tethys u n c o r r e c t e d _ p r o o f 4.1 Belemnites in the southwestern Tethys Belemnites are generally rare findings in the northwest Afriu n c o r r e c t e d _ p r o o f Belemnites are generally rare findings in the northwest African Lower Cretaceous, except in some few layers. In the u n c o r r e c t e d _ p r o o f can Lower Cretaceous, except in some few layers. In the north-Tethyan regions from the Vocontian Basin in southeast u n c o r r e c t e d _ p r o o f north-Tethyan regions from the Vocontian Basin in southeast France to the Betic Cordillera in southeast Spain, they are u n c o r r e c t e d _ p r o o f France to the Betic Cordillera in southeast Spain, they are known to be common (see Janssen et al. 2012 with referu n c o r r e c t e d _ p r o o f known to be common (see Janssen et al. 2012 with references herein). Contemporaneous beds in the western High u n c o r r e c t e d _ p r o o f ences herein). Contemporaneous beds in the western High Atlas of Morocco contain lots of belemnites, so that these u n c o r r e c t e d _ p r o o f Atlas of Morocco contain lots of belemnites, so that these beds can be characterised by belemnite assemblages similar u n c o r r e c t e d _ p r o o f beds can be characterised by belemnite assemblages similar to those of France and Spain (Mutterlose & Wiedenroth u n c o r r e c t e d _ p r o o f to those of France and Spain (Mutterlose & Wiedenroth 2008). One of the first who investigated northwest African u n c o r r e c t e d _ p r o o f 2008). One of the first who investigated northwest African Lower Cretaceous belemnites and used them for biostratiu n c o r r e c t e d _ p r o o f Lower Cretaceous belemnites and used them for biostratigraphy was Coquand (1862: 27, 28), characterising in the u n c o r r e c t e d _ p r o o f graphy was Coquand (1862: 27, 28), characterising in the Algerian region of Constantine an "Étage néocomien caracu n c o r r e c t e d _ p r o o f Algerian region of Constantine an "Étage néocomien caractérisé par les u n c o r r e c t e d _ p r o o f térisé par les

Fig. 4 :

 4 Fig. 4: Correlation of the composite sections of (A) Ben Chaabane et al. (2019: Fig. 5), (B) Lehmann et al. (2009: Fig. 3), and (C) Google Earth map with the position of the belemnite finding. The red line between T4 and Cu5 marks the Aptian-Albian boundary interval, the red point the belemnite. Scale bar aside section A is equal to that in section B.

  Hibolithes Denys de Montfort, 1808 Hibolithes cf. obtusirostris (Pavlow in Pavlow & Lamplugh, 1891) Figs. 5B, 6A-F Comments to the species authorship: The Imperial Society of Naturalists of Moscow published their Bulletin quarterly. The article of the two authors about the Speeton Clay and their Russian equivalents appeared in four parts: introduction by both authors, geology of the Speeton Clay by Lamplugh, belemnites by Pavlow and ammonites by Pavlow. The first three parts appeared in the double-number 2 & 3 delivered in September 1891, the fourth part in December 1891. The cover for the whole volume was published in the first quarter of the next year, which is 1892. Therefore, the authorship has to be cited as "Pavlow 1891" or "Pavlow in Pavlow & Lamplugh, 1891".Description: The short but effective diagnosis of the species made byPavlow (1891: 262-263, Pl. VII, Fig.

  Fig. 6G is a ventral side with typical slender alveolar end and a ventral furrow, Fig. 6H is a lateral sight with a typical peeling of the alveolar end. eschweizerbart_xxx u n c o r r e c t e d _ p r o o f ness Dmin (blue line in Fig. 5B) shifts ontogenetically to the u n c o r r e c t e d _ p r o o f ness Dmin (blue line in Fig. 5B) shifts ontogenetically to the alveolus, whereas the maximal thickness Dmax (red line in u n c o r r e c t e d _ p r o o f alveolus, whereas the maximal thickness Dmax (red line in Fig. 5B) remains more or less at one point of the rostrum, the

Fig. 5 :

 5 Fig. 5: (A) Sketch of a belemnite guard with the main features important for identification (redrawn from Spaeth 1971). (B) Thin section of the described belemnite with position of features and dimensions mentioned in the text (lateral section, comp. Fig. 6C). (C) Graphic description of the ontogenetic changes of the apical angle in relation to the apical length. Blue-circle graph comprises data from the Tunisian belemnite (specimen GSUB C4049), green-square graph from a specimen from the Isle of Heligoland (redrawn and completed from original material of Spaeth 1989).

Fig. 6 :

 6 Fig. 6: Hibolithes cf. obtusirostris (Pavlow in Pavlow & Lamplugh 1891) (specimen GSUB C4049 from Djebel Serdj, northern Tunisia (A-F) and sketches of Hibolithes jaculoides (G, H) for comparison. (A) Original sight of the belemnite finding, lateral section. (B) The thin section in incident light with dark background, lateral section. (C) Thin section seen in transmitted light, lateral section. (D) Area of the peak, enlarged to show an offspring (bryozoan?). (E) Section of the rostrum after the sawing of the thin section, lateral section. (F) Magnification of part of the phragmoconus with the siphuncle. (G) Sketch of Hibolithes jaculoides, showing the ventral side with typical slender alveolar end and a ventral furrow. (H) Sketch of Hibolithes jaculoides in lateral sight with a typical peeling of the alveolar end. The scale bar represents 10 mm and is valid for all rostra.

Table 1 :

 1 Ammonite identifications from the uppermost interval bearing ammonites at the Djebel Serdj section, above section meter 812 of the composite section byLehmann et al. (2009: Fig. 3). Note that[START_REF] Heldt | Increased terrigenous influx but no drowning: Palaeoenvironmental evolution of the Tunisian carbonate plat-form margin during the Late Aptian[END_REF] mentioned Neodeshayesites nicholisoni (this was erroneously published since it is based on an outdated field identification of GSUB C4051), Parahoplites maximus (based on GSUB C4042, following[START_REF] Lehmann | Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia[END_REF]) and P. cf. nutfieldiensis (general citation, not referring to any inventory numbers).

	Section meter [m]	(Lehmann et al.	2009:

Acknowledgements

We thank Ralf Bätzel (Bremen) for making the thin section of the belemnite specimen. Markus Wilmsen (Dresden) and Hugh G. Owen (London) kindly discussed some ammonite identifications and Matthias Heldt (Emden) the stratigraphy of the Djebel Serdj section. Jean-Louis Latil (Lazer) kindly provided some original figures of his publications. This research is based on material collected in the year 2006 with support of the DFG (German Research Foundation, project no Ba-1571-1-1). We thank Jörg Mutterlose (Bochum) as well as Martin Košťák (Prague), for valuable hints and their critical reviews and Markus Wilmsen for his careful editorial work.

 1). This stratigraphic interval (Figs. 4A u n c o r r e c t e d _ p r o o f herein revised (Table 1). This stratigraphic interval (Figs. 4A andB) is now placed into the highest upper Aptian or lower-