EGEOL Estudios Geológicos Estud. geol. 0367-0449 1988-3250 Consejo Superior de Investigaciones Científicas egeol.44880.624 10.3989/egeol.44880.624 Artículos Ostracods of the Cenomanian-Turonian transition in the Ksour and Amour Mountains (Saharan Atlas, Algeria): systematic and palaeobiogeographic implications Ostrácodos del tránsito Cenomaniense-Turoniense en los Montes Ksour y Amour (Atlas Sahariano): sistemática e implicaciones paleobiogeográficas https://orcid.org/0009-0006-3047-1390 Benadla Mustapha Department of Earth and Universe Sciences, University of Tlemcen, P.O. Box 119 Tlemcen, Algeria. https://orcid.org/0000-0001-9119-9198 Marok Abbas Department of Earth and Universe Sciences, University of Tlemcen, P.O. Box 119 Tlemcen, Algeria. https://orcid.org/0000-0001-9084-6285 Soulimane Choukri Department of Earth and Universe Sciences, University of Tlemcen, P.O. Box 119 Tlemcen, Algeria. https://orcid.org/0000-0003-4211-3946 Reolid Matías mreolid@ujaen.es Departamento de Geología, Universidad de Jaén, Campus Las Lagunillas sn, 23071 Jaén, Spain. 19 04 2023 06 2023 79 1 e152 14 11 2022 07 03 2023 10 05 2023 © 2023 CSIC 2023 This is an open-access article distributed under the terms of the Creative Commons Attribution- Non Commercial (by-nc) Spain 4.0 License. Abstract

The study of ostracods from the Cenomanian-Turonian transition in the Ksour and Amour Mountains (Saharan Atlas, Algeria) has allowed the identification of fossil assemblages characterising this relevant time interval characterised by global environmental changes. The ostracod assemblages consist of fifteen species and seven genera, and are dominated by the Family Cytherellidae (mainly genus Cytherella), and secondarily by the families Paracyprididae (exclusively Paracypris) and Trachyleberididae (mainly Cythereis). Less common are components of families Bairdiidae, Bythocypridae and Macrocyprididae. The studied ostracod assemblages were compared with those assemblages from basins belonging to palaeobiogeographic provinces of North Africa-Middle East (Gondwana Palaeomargin) to search for possible similarities among basins. Thus, the results obtained show the proximity of the ostracod fauna of the Moroccan and Egyptian basins, to which the two basins belonging to the Middle East (Jordan and Oman) are related, the strong similarity between the basins of the Saharan Atlas (Algeria and Tunisia) and finally, the isolation of the ostracod fauna of the Lebanese Basin. This palaeobiogeographical topology shows the probable existence of communication routes during the Cenomanian-Turonian transition or equivalent palaeoenvironmental conditions in different basins.

 Resumen

El estudio de los ostrácodos de la transición Cenomaniense-Turoniense (Cretácico superior) en los Montes Ksour y Monte Amour (Atlas Sahariano, Argelia) ha permitido la identificación de asociaciones fósiles típicas de este periodo caracterizado por cambios ambientales a escala global. La asociacion de ostrácodos consiste en 15 especies y 7 géneros, y se encuentra dominada por la familia Cytherellidae (principalmente el género Cytherella), y en menor medida por las familias Paracyprididae (exclusivamente Paracypris) y Trachyleberididae (principalmente Cythereis). Las formas menos comunes corresponden a las familias Bairdiidae, Bythocypridae y Macrocyprididae. Las asociaciones de ostrácodos del Atlas Sahariano fueron comparadas con las asociaciones de cuencas vecinas pertenecientes a la provincia paleobiogeográfica del Norte de África y Oriente Medio (margen septentrional de Gondwana) con el fin de encontrar similitudes entre cuencas. Así, el resultado obtenido muestra una gran similitud entre la fauna de ostrácodos de las cuencas del Atlas Sahariano en Argelia y Túnez. Por otro lado, existe similaridad entre las asociaciones de las cuencas de Marruecos y Egipto, y de ambas a su vez con las cuencas de Oriente Medio (Jordán y Omán). Finalmente, la fauna de la Cuenca Libanesa aparece relativamente aislada. Estas similaridades entre distintas cuencas desde el punto de vista palaeobiogeográfico pueden evidenciar cierta comunicación entre las mismas o condiciones ambientales equivalentes durante el tránsito Cenomaniense-Turoniense.

 Ostracoda Upper Cretaceous North Gondwana Palaeomargin Palaeobiogeography Similarity Ostrácodos Cretácico superior Paleomargen septentrional de Gondwana Paleobiogeografía Similaridad Spanish Ministry of Economy and Competitiveness PID2019-105537RB-100 We thank the constructive comments of three reviewers (Julio Rodríguez Lázaro and two anonymous) which have improved this work. This study had the support of funding from Spanish Ministry of Economy and Competitiveness, project PID2019-105537RB-100. Introduction

The Cenomanian-Turonian transition was marked by palaeoceanographic and palaeoclimatic perturbations related to long-lasting carbon isotope anomalies and to an oceanic anoxic event (OAE2; Jenkyns, 1980, 1997; Schlanger et al., 1987; Jarvis et al., 1988; Robaszynski, 1989; Kaiho & Hasegawa, 1994; Erbacher & Thurow, 1997; Huber et al., 2002; Friedrich et al., 2006; Turgeon & Creaser, 2008; Voigt et al., 2008; Gebhardt et al., 2010; Monteiro et al., 2012; Erba et al., 2013; Elderbak et al., 2014; Reolid et al., 2016).

In North Africa-Middle East, these both bioevent and isotopic event have widely been studied in Morocco (e.g. Gebhardt et al., 2004, 2010; Ettachfini & Andreu, 2004; Ettachfini et al., 2005; Ettachfini, 2006; Jati et al., 2010; Lézin et al., 2012; Prauss, 2012; Andreu et al., 2013; Aquit et al., 2013; Wang et al., 2021), in Algeria (e.g. Naili et al., 1995; Harket & Delfaud, 2000, Grosheny et al., 2008, 2013; Ruault-Djerrab et al., 2012, 2014, Benadla et al., 2018), in Tunisia (e.g. Accarie et al., 2000; Amédro et al., 2005; Caron et al., 2006; Zagrarni et al., 2008; Robaszynski et al., 2010; Negra et al., 2011, Grosheny et al., 2013; Zaghbib-Turki & Soua, 2013; Reolid et al., 2015; Aguado et al., 2016; Touir et al., 2017), in Egypt (e.g. Lüning et al., 1998; Aly et al., 2001; Bauer et al., 2002; Zakhera & Kassab, 2002; Ismail et al., 2009; Nagm, 2009; Gertsch et al., 2010; Nagm et al., 2010; El-Sabbagh et al., 2011; Ayoub-Hannaa et al., 2013; Shahin & Elbaz, 2013a; Wilmsen & Nagm, 2013; Nagm et al., 2021), in Jordan (e.g. Schulze et al., 2004; Aly et al., 2008; Morsi & Wendler, 2010; Wendler et al., 2010; Bergue et al., 2016; Nagm et al., 2017; Momani, 2021) and in Oman (Athersuch, 1988).

The OAE2 has been associated to climatic and palaeoceanographic changes including a sea-level transgression (Hallam, 1992), a perturbation of the carbon cycle (e.g. Kuypers et al., 2002; Erba, 2004; Pogge von Strandmann et al., 2013), a greenhouse warming (e.g. Huber et al., 2002; Norris et al., 2002; Pogge von Strandmann et al., 2013), and a probable massive magmatic episode (e.g. Kuroda et al., 2007; Turgeon & Creaser, 2008; Erba et al., 2013). The impact of this event on fossil assemblages have been focused on different groups of organisms such as cephalopods (e.g. Monnet, 2009; Nagm et al., 2017; Kostak et al., 2018), bivalves (e.g. Takahashi, 2005; Negra et al., 2011; Posenato et al., 2020), foraminifera (e.g. Gebhardt et al., 2004; Caron et al., 2006; Friedrich et al., 2006; Ismail et al., 2009; Elderbak et al., 2014; Reolid et al., 2015, 2016; Bryant & Belanger, 2023) and nannoplankton (e.g. Wan et al., 2003; Hardas & Mutterlose, 2007; Erba et al., 2013; Aguado et al., 2016; Farouk et al., 2022). However, recent studies on ostracod assemblages from Cenomanian-Turonian transition are comparatively scarcer (Horne et al., 2011; Andreu et al., 2013; Benadla et al., 2018; Khalil, 2020; Shahin & Elbaz, 2013b; Mebarki et al., 2016; Tchenar et al., 2020).

The purpose of this present work is to study the ostracods of the Cenomanian-Turonian transition in the Ksour and Amour Mountains (Saharan Atlas, Algeria). It is a systematic and palaeobiogeographic study that allowed us to highlight a global biological event corresponding to the explosion of smooth-shaped ostracods, represented mainly by the Family Cytherellidae (Barroso-Barcenilla et al., 2011; Shahin & Elbaz, 2013b; Benadla et al., 2018). The biogeographic analysis is carried out to test for the presence of potential similarities between the ostracod assemblages of the Atlasic Basin and other basins of the northern palaeomargin of Gondwana outcropping in the North Africa and the Middle East (Fig. 1).

Geographic location of the analysed regions.

1. Morocco, 2. Algeria, 3. Tunisia, 4. Egypt, 5. Jordan, 6. Lebanon, 7. Oman.

(After Andreu et al., 2013) Geological Setting

The Ksour Mountains (Western Saharan Atlas) and Amour Mountains (Central Saharan Atlas) are part of a vast mountainous area, the Atlas Cordillera, stretching for almost 2000 km from Agadir in Morocco to Gabes in Tunisia. It includes from west to east: the Moroccan High Atlas, the Saharan Atlas, the Aurès and finally the Tunisian Atlas (Fig. 2A). This cordillera presents a general NE-SW orientation. In Algeria, the Saharan Atlas is represented by a structural alignment extending over more than 1000 km, from the Algerian-Moroccan borders in the west to the western limit of the Aurès Mountains in the east. It is composed of the Ksour Mountains, Amour Mountains and the Ouled Naïl Mountains (Fig. 2B). The Zibane and the Aurès follow these clusters. A total of three sections have been studied from Ksour Mountains (Rhoundjaïa, M’Daouer and Chellala Dahrania) and other from Amour Mountains (El Kohol).

Geographic and geologic sketch.

A) Situation map of the Saharan Atlas, B) Geological map of the area studied.

 Ksour Mountains (Western Saharan Atlas)

Part of the great orographic barrier of the Saharan Atlas, the Ksour Mountains are located approximately 360 km south of Oran. They are limited to the north by the Oran High Plains, to the south by the Saharan platform, to the east by Amour Mountains and finally to the west by the Moroccan High Atlas (Fig. 2B). The Ksour Mountains formed at the location of more or less subsident intra-plate basins (Aït Ouali, 1991; Aït Ouali & Delfaud, 1995) and this sub-basin shows a tectonic style that is more brittle in the west and more flexible in the east. The so-called soft tectonics is represented by narrow anticlines with straightened sides and more or less horizontal vaults separating large synclines with generally flat bottoms, wider and more elongated where the Cretaceous terrain is preserved (Yelles-Chaouche et al., 2001). It should be noted that the major phase that structured the Ksour Sub-basin in question generated isopaque folds with a SW-NE direction and is dated to the Lutetian-Priabonian (Coiffait et al., 1984). Geologically, the filling of the Ksour Sub-basin consists of Triassic and Jurassic rocks, predominantly carbonates (Bassoullet, 1973; Delfaud, 1975; Elmi et al., 1998; Mekahli, 1998; Reolid et al., 2012). The Lower Cretaceous is constituted by siliciclastic sedimentary rocks (fluvial and deltaic) overlaid by the first marine carbonate deposits attributed to the Upper Cretaceous. Thus, during the Cenomanian a major transgression flooded the Sahara and the Ksour Sub-basin (Busson et al., 1999; Grosheny et al., 2008, 2013). There are two major sequences in the Upper Cretaceous correlatable at the scale of the Saharan Atlas (Delfaud, 1986; Harket & Delfaud, 2000): CI (Cenomanian-Turonian) and CII (Coniacian to Maastrichtian). According to the geological map of Galmier (1972) and the work of Bassoullet (1973), the Cenomanian and Turonian deposits form the upper level of the Mesozoic folded series. The Cenozoic deposits are essentially continental siliciclastics (sandstones and conglomerates).

 Amour Mountains (Central Saharan Atlas)

The Amour Mountains are bounded to the north by the Oran High Plains, to the south by the Saharan platform, to the east by the Ouled Naïl Mountains and to the west by the eastern end of the Ksour Mountains (Fig. 2B). Unlike the Ksour Mountains, this part of the Saharan Atlas is characterised by large synclinal and anticlinal folds (Kazi-Tani, 1986), elongated NE-SW in the western part and E-W in the eastern part (Bettathar, 2009). Furthermore, the so-called brittle tectonics is expressed in this sub-basin by three major N-S, E-W and NW-SE trending faults (Guiraud, 1990). According to Bettahar et al. (2007), the evolution of the Amour Sub-basin records several compressive tectonic phases from the Early Cretaceous to the Mio-Pliocene. Geologically, the stratigraphic series of Amour Mountains is formed by a thick sedimentary series covering the Mesozoic-Cenozoic stratigraphic interval. It consists of Triassic gypsum and salt-rich clays, with local doleritic volcanic rocks, overlaid by Jurassic carbonates, marlstones and sandstone-siltstone alternations. The Lower Cretaceous is characterised by limestones and sandstones-silstones rich in gypsum and claystones, whereas the Upper Cretaceous is constituted by dolomitic limestones, gypsum-rich marls and marly-limestone alternations (Guillemot & Estorges, 1981; Abed, 1982; Kazi-Tani, 1986; Bracene, 2001; Bettahar, 2009; Zazoun et al., 2015). The Cenozoic is represented mainly by continental siliciclastic deposits.

 Material and Methods

The chronological interval studied in the different sections, the Whiteinella archaeocretacea Zone that contains the Cenomanian-Turonian boundary (see Caron et al., 2006; Reolid et al., 2015), consists mainly of limestone beds and some marlstone levels. A total of 107 samples have been analyzed (35 from Rhoundjaïa, 18 from M’Daouer, 20 from Chellala Dahrania, and 34 from El Kohol). Most of the samples were from limestones and prepared for thin sections and analysis of microfacies, where as a total of 30 marl samples (500 g/sample) were washed under a gentle jet of water over a set of standard stainless-steel sieves (250 µm, 125 µm and 63 µm) and sorted for examining ostracods and foraminifera. The identification and systematic classification of ostracods is based mainly on the work of Bassoullet & Damotte (1969), Andreu et al. (2013) and Benadla (2019). The most systematically and biostratigraphically representative species were selected and gold-coated for analyzing under a scanning electron microscope Merlin Carl Zeiss SEM at the University of Jaén (Centro de Instrumentación Científico-Técnica).

In this work, the application of quantitative biogeography is used to compare the studied ostracod assemblages with those from different basins belonging to different palaeobiogeographic provinces. Therefore, the basins included in the palaeobiogeographic analysis are located in Morocco (Agadir Basin, Central High Atlas, Middle Atlas and Preafrican Basin) (Andreu, 2002; Ettachfini & Andreu, 2004; Ettachfini et al., 2005; Jati et al., 2010), Algeria (Ksour, Amour and Tébessa sub-basins) (Benadla, 2019; Ruault-Djerrab et al, 2012), Tunisia (Central Tunisia) (Salmouna et al., 2014), Egypt (East and central Sinai) (El-Nady et al., 2008; Shahin & Elbaz, 2013a), Jordan (Central Jordan) (Morsi & Wendler, 2010), Lebanon (Damotte & Saint-Marc, 1972) and finally western Oman (Athersuch, 1988). This biogeographical quantification is based on two types of data processing:

For the analysis of the quantitative data (abundance), the PAST-PAlaeontological STatistics software, ver. 1.89 (Hammer et al., 2009) was used. In this software, the matrix obtained in terms of number of genera per family for each region (Table 1) is processed using the Principal Coordinates Analysis. The latter is the result of the distance measure based here on the Bray-Curtis coefficient. It should be noted that the distance calculation algorithm depends on the type of matrix constructed.

For the processing of qualitative (binary) data, we chose the BG-Index ver. 1.1 β software (Escarguel, 2001). This is done with the aim of comparing the degree of similarity or dissimilarity between each pair of lists generated by the database. In this analysis, a degree is calculated by the similarity (Jaccard and Dice coefficients) or distance (Bray-Curtis coefficient) indices. The results of these calculations are represented in the form of a phenogram which will be transformed later into a “Hierarchical Association Diagram”.

 Lithostratigraphy

As indicated before, the studied ostracod assemblages come from four sections (Fig. 3): Rhoundjaïa and M’Daouer (western Ksour Mountains), Chellala Dahrania (eastern Ksour Mountains) and El Kohol (Amour Mountains).

Geological setting of the studied sections in the Ksour and Amour Mountains (Saharan Atlas).

A) Location map of the study sections (1. Rhoundjaïa, 2. M’Daouer, 3. Chellala Dahrania, and 4. El Kohol). B and C), Geological map of the Ksour Mountains. D) Geological map of the Amour Mountains.

 Rhoundjaïa section

Located at 60 km west of locality of Aïn Séfra, the Rhoundjaïa section (32º44’45.00’’N, 0º14’24.58’’W) was studied on the south-western end of a SSW-NNE syncline (Fig. 3B). This section is of particular interest for the study of the Cenomanian-Turonian transition (Bassoullet & Damotte, 1969; Galmier, 1972; Bassoullet, 1973; Marok et al., 2009; Mebarki et al., 2016; Benadla et al., 2018). Lithostratigraphically, the two geomorphologically detectable bars (Fig. 4A) correspond to the Rhoundjaïa Formation (58.15 m) which can be subdivided into three members (Fig. 5A):

Lower Member (24.25 m) overlyes the M’Daouer Formation composed by gypsum-rich claystones, limestones and dolostones, and this lower member is mainly composed of bioclastic bioturbated limestones (wackestones to packstones of ostracods and planktic foraminifera) with common Thalassinoides and Planolites. Locally it is recorded mudstone with planktic foraminifera (Benadla et al., 2018). In the Lower Member are recorded Dicarinella sp., Rotalipora sp., Muricohedbergella delrioensis, M. planispira, Planoheterohelix moremani, Helvetoglobotruncana praehelvetica and Guembelitria cretacea. The genus Rotalipora is restricted to the lowermost part of the section (samples Rh-4 and Rh-5) and Guembelitria is recorded in the top of the Lower Member (from sample Rh-14) (Benadla et al., 2018).

Middle Member (30 m) is an alternation of whitish bioclastic limestones (wackestones to packstones with foraminifera, filaments and echinoderms) and marls with some lumpy levels rich in sea urchins (Holaster subglobosus, Mecaster pseudofournelli, Hemiaster syriacus and Prionocidaris granulostriata), ammonites (Vascoceras gamai and Vascoceras sp.) and gastropods (Tylostoma sp.). Locally, there are dense accumulations of serpulids. The bioturbated carbonate levels correspond to biomicrites. Towards the top, this alternation is followed by limestones with flint nodules and bioturbated limestones with Thalassinoides. Planktic foraminiferal assemblage is dominated by Planoheterohelix moremani, P. reussi, Guembelitria cretacea and G. cenomana. The species of the genus Muricohedbergella are recorded in the lower part of this member.

Upper Member (10.60 m) is constituted by massive beds of slightly bioclastic (wackestones to packstones of foraminifera, ostracods and filaments) and highly bioturbated limestone (Thalassinoides). Planktic foraminifera are scarce and only Planoheterohelix moremani is recorded.

 Outcrop view of studied sections.

A. Rhoundjaïa section, B. M’Daouer section, C. Chellala Dahrania section, and D. El Kohol section.

 Stratigraphic columns of the Rhoundjaïa, M’Daouer, Chellala Dahrania and El Kohol (A to D).

In this section, the age of the Rhoundjaïa Formation is based on the palaeontological record. Thus, the few ammonites (Vascoceras gamai and Vascoceras sp.) collected in levels Rh-15’, Rh-18’, Rh-26’ and Rh-29 indicate the upper Cenomanian. On the other hand, the ammonites collected in the bed Rh-31 and correlated with the M’Daouer sections give a lower Turonian age.

The last occurrence of Rotalipora at the top of bed Rh-5 would indicate the upper boundary of the Rotalipora cushmani Zone (Benadla et al., 2018). The beginning of the Whiteinella archaeocretacea Zone is signalled in the Rhoundjaïa section by the absence of R. cushmani and the record of Helvetoglobotrunana praehelvetica in bed Rh-6. According to the Robaszynski & Caron (1995) the base of the W. archaeocretacea Zone is defined by the last occurrence of R. cushmani. The record of Helvetoglobotruncana praehelvetica has been reported from the base of the W. archaeocretacea Zone (Wan et al., 2003).

 M’Daouer section

This section was studied on the south-eastern flank of M’Daouer Mountain (Fig. 3B) (32º39’26.04’’N, 0º04’46.15’’W). With a thickness of 64.40 m, it is formed by two large limestone bars (Fig. 4B). This is the Rhoundjaïa Formation which shows the succession of three members (Fig. 5B):

Lower Member (31.40 m) corresponds to massive beds of micritic and bioclastic limestone (wackestones to packstones of foraminifera, ostracods and filaments), affected by bioturbation (mainly Thalassinoides). At the top, this lower member ends in two lumpy beds (bioclastic wackestone rich in echinoderms and planktic foraminifera) very rich in echinoids and ammonites (Vascoceras sp., V. cf. cauvini, V. gamai, and Neolobites vibrayeanus) (Benadla, 2019). Planktic foraminifera are dominated by Muricohedbergella planispira and M. delrioensis, and Planoheterohelix moremani is exclusively recorded in the top of the member.

Middle Member (20.20 m) is constituted mainly by marls with discontinuous beds of marly limestone (mudstones with filaments and planktic foraminifera) locally rich in irregular sea urchins (Mecaster pseudofournelli). Planoheterohelix moremani, P. reussi, Guembelitria cenomana and G. cretacea are very abundant whereas trochospiral forms such as Muricohedbergella are not recorded.

Upper Member (12.80 m) comprises massive beds of bioturbated limestones followed by limestones with flint nodules and micritic limestone. Some beds are affected by dolomitization. The planktic foraminifera are scarce, dominated by small forms of Planoheterohelix, with secondary Muricohedbergella.

In the M’Daouer section, the ammonites collected (Vascoceras cf. cauvini, V. gamai, Vascoceras sp., and Neolobites vibrayeanus) and the foraminiferal record give to the Rhoundjaïa Formation an upper Cenomanian-lower Turonian age.

Chellala Dahrania section

Located in the eastern part of the Ksour Mountains, the Chellala Dahrania section was studied in the eastern end of the Milok El Guelbi Mountain (Fig. 3C) (33º05’32.27’’N, 0º15’55.33’’E). In this 58.40 m thick section (Fig. 4C), the Rhoundjaïa Formation consists of three members (Fig. 5C).

Lower Member (19.60 m) overlies the gypsum-rich clays and silts of the M’Daouer Formation. It is formed mainly of micritic limestones (wackestones of planktic foraminifera), followed by channeled beds of slightly bioclastic limestones (wackestones to packstones with planktic and benthic foraminifera, and fragments of bivalves, gastropods and echinoderms) and micritic chalky limestones (mudstones to wackestones with planktic foraminifera). At the top, the member ends with a succession of decimetric beds of lumpy limestones with a nodular appearance, very rich in ammonites (Vascoceras cf. gamai and Metoicoceras aff. geslinianum). Planktic foraminifera in the lower part are mainly Muricohedbergella planispira, M. delrioensis, Planispira moremani and P. reussi, whereas to the top biserial (Planoheterohelix) and triserial forms (Guembelitria) are dominant.

Middle Member (3.60 m) corresponds to an alternation of marly limestones and marls very rich in ammonites (Vascoceras gamai and Vascoceras sp.). Microfacies are wackestones rich in biserial and triserial planktic foraminifera.

Upper Member (35.20 m) begins with a marly limestone bed with ammonites (Choffaticeras sp.), followed by a succession of micritic, slightly bioclastic limestones and massive dolomitic beds (mudstones to wackestones). Among the planktic foraminifera reappear the trochospiral form Muricohedbergella, but species of Planoheterohelix and Guembelitria keep dominant.

In this section the Rhoundjaïa Formation has provided an ammonite fauna (Vascoceras gamai, Vascoceras sp., Choffaticeras sp.) that indicate the upper Cenomanian-lower Turonian transition.

 El Kohol section

The section was studied on the northern part of El Kohol Mountain (Fig. 3D) (33º03’17.03’’N, 1º28’18.52’’E). It is distinguished by the presence of a single bar marking the boundary between the M’Daouer and Rhoundjaïa formations (Fig. 4D). With a thickness of 38.65 m, in the Rhoundjaïa Formation can be recognised the upper two members (Fig. 5D):

Middle Member (5.40 m) is resting concordantly on the gypsum-rich clays and silts of the M’Daouer Formation. It consists of alternating decimetric beds of bioclastic limestones (wackestones to packstones rich in fragments of echinoderms and bivalves) and marl, with discontinuous beds of micritic limestone (wackestones of bioclasts) at the top. Thalassinoides are locally common. Planktic foraminifera are not recorded.

Upper Member (33.25 m) is constituted by micritic bioturbated limestones, sometimes in chanalised banks. The bioclastic limestones in pseudo-nodular beds are very rich in ammonites (Vascoceras gamai and Vascoceras sp.). This ensemble is followed by centimetric to decimetric beds of well-stratified limestones. These are essentially slightly bioclastic limestones (wackestones to packstones with echinoderms and foraminifera and locally rich in filaments) with ammonites (Fikaites sp.), platy limestones and flint nodules. Planktic foraminifera in the lower part (samples Kh-11 and Kh-12) are scarce and only represented by Muricohedbergella planispira. Biserial and triserial forms (Planoheterohelix and Guembelitria) appear and are abundant in the top of the member (from sample Kh-29 to Kh-44).

In the El Kohol section, the ammonites have enabled the two members of the Rhoundjaïa Formation to be dated with precision. Thus, the ammonites Vascoceras gamai with the foraminifera identified at the base give a upper Cenomanian age (Muricohedbergella planispira). Furthermore, the upper member of this formation has been dated to the lower Turonian thanks to the collection of a few ammonites of the genus Fikaites. It should be noted that Rerbal (2008) recorded in this member the ammonites Pseudotissotia sp. which confirms the lower Turonian.

 Systematic palaeontology of ostracoda

The ostracod assemblages of the studied sections are characterised by the dominance of Family Cytherellidae (mainly genus Cytherella), followed by components of the families Paracyprididae (exclusively genus Paracypris) and Trachyleberididae (mainly genus Cythereis). Less common are components of families Bythocypridae and Macrocyprididae. The microfauna studied yielded 15 species, seven of which are left to open nomenclature. These taxonomic categories of ostracods belong to seven genera. In this systematics work, we adopted the classification of the European Register of Marine Species http://erms.biol.soton.ac.uk, and Integrated Taxonomic Information System http://www.itis.usda.gov. The systematic established by Horne et al. (2002) was also used to bring more precision to our work. Note that L, H, and W, represent measurements of length, height, and width respectively.

Class Ostracoda Latreille, 1802

Subclass Podocopa Sars, 1866

Order Platycopida Sars, 1866

Suborder Platycopina Sars, 1866

Superfamily Cytherelloidea Sars, 1866

Family Cytherellidae Sars, 1866

Genus Cytherella Jones, 1849

Cytherella gr. ovata Roemer, 1841 (Fig. 6a)

 Ostracods from the upper Cenomanian-lower Turonian (<italic>Whiteinella archaeocretacea</italic> Zone) of the Rhoundajaïa Formation.

All are carapaces. Samples are indicated. a) Cytherella gr. ovata, left lateral view (Md-81); b) Cytherella gigantosulcata, right lateral view (Kh-3’); c) Cytherella sp. 1, left lateral view (Kh-3’); d) Cytherella ? sp. 2, left lateral view (Kh-3’); e) Cytherelloidea sp., left lateral view (Rh-17’); f) Paracypris dubertreti, right lateral view (Md-83); g) Paracypris mdaouerensis,right lateral view (Md-83); h) Bythocypris sp., right lateral view (Kh-4’); i) Macrocypris sp., right lateral view (Rh-17’); j and k) Bairdia sp. 1, left lateral view (j), right lateral view (k) (Kh-4’); l) Bairdia sp. 2, left lateral view (Rh-17’); (m-o) Cythereis mdaouerensis, right lateral view (m), dorsal view (n), left lateral view (o) (Md-81); p) Cythereis ziregensis right lateral view (Rh-17’); q) Cythereis sp. 1, left lateral view (Rh-31’); r) Cythereis sp. 2, right lateral view (Rh-31’). Scale bar = 50 µm.

1841 Cytherina ovata Roemer, p. 104, pl. 16, fig. 21.

1845 Cytherina ovata Roemer; Reuss, p. 16, pl. 5, fig. 35.

1849 Cythere (Cytherella) ovata (Roemer); Jones, p. 28, pl. 7, figs. 24b-g.

1851 Cytherina ovata Roemer; Reuss, p. 48, pl. l 7, figs. 2b-d.

1899 Cytherella obovata (Roemer); Egger, p. 186, p l. 2 7, figs. 54-56.

1940 Cytherella obovata (Roemer); Bonnema, p. 93, pl. l , figs. 1- 1 6.

1952 Cytherella obovata (Roemer); Dupper, p. 106, pl. 5 , fig. 3.

1956 Cytherella obovata (Roemer); Deroo, p. 1508, pl. l, fig s. 4-6.

1966 Cytherella obovata (Roemer); Gründel, p. 12, pl. 1, fig. 2.

1969 Cytherella gr. ovata Roemer; Bassoullet & Damotte, pl. 2, fig. 13.

1974 Cytherella ovata Roemer; Damotte & Freytet, p. 207, pl. I, fig. 1.

1976 Cytherella “ovata” (Roemer); Bremen, p. 82, pl. l , fig . la-b.

1980 Cytherella gr. ovata Roemer; Babinot, pl. 1, figs. 12, 13; pl. 2, figs. 1-3.

1991 Cytherella gr. ovata Roemer; Shahin, p. 133, pl. 1, fig. 5.

2006 Cytherella aff. ovata Roemer; Andreu & Bilotte, p. 59, pl. 1, figs. 1-5.

2008 Cytherella ovata Roemer; El-Nady et al., p. 561, pl. I, fig. 6.

2018 Cytherella gr. ovata Roemer; Benadla et al., p. 420, figs. 8A-C.

Material: More than 200 specimens.

Dimensions: L: 0.08-0.62 mm; H: 0.06-0.40 mm; W: 0.02-0.28 mm.

Locality: Rhoundjaïa, M’Daouer, Chellala Dahrania and El Kohol.

Description: Form of genus Cytherella, ovoidal to subquadrangular in lateral view. The species is charaterized by an oval outline. Right valve larger, overlapping left valve along entire periphery. Posterior and anterior margin are rounded. Valve surface is smooth.

Age: Upper Cenomanian-lower Turonian.

Stratigraphic and geographic distribution: Cytherella gr. ovata is known from the lower Cenomanian of Egypt (El-Nady et al., 2008), upper Cenomanian-Turonian of the Western Saharan Atlas (Bassoullet & Damotte, 1969) and Tinrhert Basin of eastern Algeria (Tchenar et al., 2020), France (Babinot, 1980; Jolet et al., 2001; Andreu & Bilotte, 2006) and Egypt (Shahin, 1991; Shahin et al., 1994). It has also been identified in the Turonian-Coniacian of the Tunisian Atlas (Salmouna et al., 2014) and in general, the Upper Cretaceous of Europe and America (Damotte & Freytet, 1974).

Cytherella gigantosulcata Rosenfeld, 1974 (Fig. 6b)

1932 Cytherella sulcata Van Veen, p. 336, pl. 4, figs. 1-18

1959 Ostracode U. 10 Glintzboeckel & Magne, p. 64, pl. 3, fig. 31

1969 Cytherella ? U. 10 Glintzboeckel & Magne; GREKOFF, pl. 1, fig. 6

1977 Cytherella sulcata Rosenfeld; Al Abdul Razzaq, p. 50, pl. 4, figs. 1-5

1980 Cytherella sulcata Rosenfeld; Ben Youssef, p. 92, pl. 5, figs. 6-8

1981 Cytherella sulcata Rosenfeld; Bismuth et al., p. 223, pl. 6, figs. 3-4

1983 Cytherella sulcata Rosenfeld; Gargouri & Razgallah, p. 148, pl. 33. fig. 1

1988 Cytherella posterosulcata Rosenfeld; Athersuch, p. 202, pl. 5, fig. 1.

1991 Cytherella gigantosulcata Rosenfeld; Szczechura et al., pl. 1, figs. 7-12.

2008 Cytherella sulcata Rosenfeld; El-Nady et al., p. 561, pl. I, fig. 13.

2013 Cytherella gigantosulcata Rosenfeld; Shahin & Elbaz, p. 107, pl. 1, figs. 9-10.

2016 Cytherella gigantosulcata Rosenfeld; Bergue et al., p. 199, fig. 2 f-g.

2022 Cytherella gigantosulcata Rosenfeld; Slami et al., p. 9, figs. 4.1-4.5.

Material: 11 specimens.

Dimensions: L: 0.88 mm; H: 0.55 mm; W: 0.44 mm.

Locality: El Kohol.

Description: Carapace of medium size, oval in lateral view. Anterior margin is rounded and slightly compressed in dorsal view. Posterior margin is strongly rounded and larger. Dorsal and ventral margins are curved. Maximum height at mid-length. Valve surface is smooth.

Age: Upper Cenomanian

Stratigraphic and geographic distribution: This species has been collected in the Cenomanian of Tunisia (Glintzboeckel & Magné, 1959; Grekoff, 1969; Bismuth et al., 1981), Tinrhert Basin of eastern Algeria (Tchenar et al., 2020), Morocco (Andreu, 1989), Iran (Grosdidier, 1973), Kuwait (Al-Abdul-Razzaq, 1979) and Egypt (Hataba & Ammar, 1990; Shahin, 1991; El-Nady, 2008). In Jordan, it has been collected from the upper Cenomanian (Babinot & Basha, 1985) and the upper Albian-lower Cenomanian (Bergue et al., 2016).

Cytherella sp.1 Ruault-Djerrab, 2012 (Fig. 6c)

2012 Cytherella sp.1 Ruault-Djerrab, p. 195, pl. 3, fig. F.

2013 Cytherella sp.1 Andreu et al., p. 237, pl. 1, figs. 1-4.

2018 Cytherella sp.1 Benadla et al., p. 420, fig. 8D-E.

Material: More than 200 specimens.

Dimensions: L: 0.13-0.73 mm; H: 0.06-0.42 mm; W: 0.04-0.26 mm.

Locality: Rhoundjaïa, M’Daouer, Chellala Dahrania and El Kohol.

Description: Medium-size carapace, valves are elongated and subequal. The right valve is slightly larger than the left valve with an almost identical outline. Dorsal margin straight. Ventral margin becoming concave postero-dorsally. Posterior and anterior margins simmetrically rounded.

Age: Upper Cenomanian-lower Turonian.

Stratigraphic and geographic distribution: Middle to Upper Cretaceous of southeast Constantine, Algeria (Ruault-Djerrab, 2012) and the upper Cenomanian-lower Turonian of Morocco (Andreu et al., 2013).

Cytherella ? sp. 2 Ruault-Djerrab, 2012 (Fig. 6d)

Material: More than 200 specimens.

Dimensions: L: 0.08-0.55 mm; H: 0.11-0.35 mm; W: 0.06-0.22 mm.

Description: Form potentially assigned to genus Cytherella. Oval in lateral view. Dorsal and ventral margins have very strong ray of curvature. Maximum height at mid-length. The right valve overlaps the left valve along the entire periphery. In dorsal or ventral view, the outline lozenge, subrombic is very characteristic, regular and rounded.

Locality: Rhoundjaïa and M’Daouer.

Age: Upper Cenomanian-lower Turonian.

Stratigraphic and geographic distribution: The Cenomanian-Coniacian of the South-East Constantine, Algeria (Ruault-Djerrab, 2012) and the Maastrichtian of the El Koubbat syncline of the Middle Atlas of Morocco (Andreu & Tronchetti, 1996).

Genus Cytherelloidea Alexander, 1929

Cytherelloidea sp. (Fig. 6e)

Material: Seven specimens.

Dimensions: L: 0.42-0.57 mm; H: 0.28 mm; W: 0.11-0.15 mm.

Locality: Rhoundjaïa, M’Daouer and El Kohol

Description: Subrectangular in lateral view, sculpted (nets ornamentation, muri and reticules). Anterior and posterior margins are well rounded. Dorsal and ventral margins concave at mid-length. Ribs are longitudinal, short and sinuous.

Age: Upper Cenomanian-lower Turonian.

Suborder Podocopina Sars, 1866

Superfamily Cypridoidea Baird, 1845

Family Paracyprididae Sars, 1923

Genus Paracypris Sars, 1866

Paracypris dubertreti Damotte and Saint-Marc, 1972 (Fig. 6f)

1972 Paracypris dubertreti Damotte & Saint-Marc, Pl. I, fig. 6.

1972 Paracypris dubertreti n. sp. Damotte & Saint-Marc, p. 276, pl. 1, fig. 1.

1974 Paracypris acutocaudata n. sp. Rosenfeld, p. 8, pl.1, figs. 22-24.

1977 Paracypris sp. 1 Al Abdul Razzaq, p. 87, pl. 15, figs. 1-3.

1985 Paracypris dubertreti Damotte & Saint-Marc; Viviere, p.149, pl. 3, figs. 6-7

1994 Paracypris acutocaudata Rosenfeld; Shahin et al., p. 41, pl. 1, fig. 23.

1999 Paracypris acutocaudata Rosenfeld; Ismail, p. 310, pl. 3, figs. 16-17.

2001 Paracypris dubertreti Damotte & Saint-Marc; Hewaidy & Morsi, p. 239, pl. 2, fig. 6.

2008 Paracypris acutocaudata Rosenfeld; El-Nady et al., p. 563, pl. II, figs. 11-12.

2013 Paracypris dubertreti Damotte & Saint-Marc; Shahin & Elbaz, p. 107, pl. 1, fig. 30.

2016 Paracypris dubertreti Damotte & Saint-Marc; Bergue et al., p. 201, figs. 3K-L.

2018 Paracypris dubertreti Damotte & Saint-Marc; Benadla et al., p. 201, p. 420, fig. 8F.

Material: More than 200 specimens.

Dimensions: L: 0.33-0.68, H: 0.15-0.33 mm, W: 0.06-0.22 mm.

Locality: Rhoundjaïa, M’Daouer, Chellala Dahrania and El Kohol.

Description: This species is characterised by posterior margin very tapering and pointed. Anterior margin well rounded. Valve surface smooth and becoming strongly arched ventrally. Dorsal margin straight at mid-length.

Age: Upper Cenomanian-lower Turonian.

Stratigraphic and geographic distribution: This species has been described in the Aptian-Cenomanian of Egypt (Boukhary et al., 1977; Shahin et al., 1994; Ismail, 1999; Morsi & Bauer, 2001; Hewaidy & Morsi, 2001; Bassiouni, 2002), middle and upper Cenomanian of Lebanon (Damotte & Saint-Marc, 1972) and Jordan (Morsi & Wendler, 2010), Cenomanian-Turonian of Algeria (Vivière, 1985; Majoran, 1989; Slami et al., 2022), middle Cenomanian of Morocco (Andreu, 1991), middle Cenomanian-lower Turonian of central Egypt (Boukhary et al., 2009; Shahin & Elbaz, 2013a, b), lower Turonian of South-East Constantine Algeria (Ruault-Djerrab, 2012), Turonian of the Potiguar Basin, North-East Brazil (Poivesan et al., 2014) and finally the Turonian-Coniacian of the Tunisian Atlas (Salmouna et al., 2014).

Paracypris mdaouerensis Bassoullet & Damotte, 1969 (Fig. 6g)

1969 Paracypris mdaouerensis n. sp. Bassoullet & Damotte, p. 143, pl. 2, fig. 10.

1996 Paracypris cf. mdaouerensis Bassoullet & Damotte; Andreu & Tronchetti, p.57, pl. 5, figs. 18-19

2000 Paracypris aff. mdaouerensis Bassoullet & Damotte; Viviers et al., p. 418, fig. 10, n°12, 13, 16.

2001 Paracypris mdaouerensis Bassoullet & Damotte; Morsi & Bauer, p. 386, pl. 2, fig. 6.

2008 Paracypris mdaouerensis Bassoullet & Damotte; El-Nady et al., p. 563, pl. II, fig. 13.

2012 Paracypris mdaouerensis Bassoullet & Damotte; Ruault-Djerrab, p. 195, pl. 3, fig. A., pl. 10, fig. A.

2013 Paracypris mdaouerensis Bassoullet & Damotte; Shahin & Elbaz, p. 107, pl. 1, figs. 31-32.

2018 Paracypris mdaouerensis Bassoullet & Damotte; Benadla et al., p. 420, figs. 8G-H.

2022 Paracypris mdaouerensis Bassoullet & Damotte; Slami et al., p. 12, figs. 7.1-7.2.

Material: More than 200 specimens.

Dimensions: L: 0.17-0.8 mm; H: 0.06-0.33 mm; W: 0.04-0.22 mm.

Locality: Rhoundjaïa, M’Daouer and Chellala Dahrania

Description: Similar to Paracypris dubertreti, but it differs in its ventral margin which is not arched.

Age: Upper Cenomanian-lower Turonian.

Stratigraphic and geographic distribution: The species Paracypris mdaouerensis collected for the first time in the Monts des Ksour (Bassoullet & Damotte, 1969) has been cited in the lower Cenomanian of Jordan (Babinot & Basha, 1985), the Cenomanian of Gabon (Neufville, 1973), the Albian-Cenomanian of the Brazilian Basin (Viviers et al., 2000), the Cenomanian to Coniacian-Santonian of the Eastern Saharan Atlas of Algeria (Ruault-Djerrab, 2012; Mebarki et al., 2016; Slami et al., 2022), and Tinrhert Basin of eastern Algeria (Tchenar et al., 2020), the lower Cenomanian-Turonian of Egypt (El-Nady et al., 2008; Shahin & Elbaz, 2013a, b) and Morocco (Andreu, 1989; Ettachfini et al., 2005), in the Turonian-Coniacian of the Tunisian Atlas (Salmouna et al., 2014) and the Albian-Turonian of Morocco (Andreu, 1991; Andreu & Tronchetti, 1996; Andreu et al., 2013).

Family Bythocyprididae Maddocks, 1969

Genus Bythocypris Brady, 1880

Bythocypris sp. (Fig. 6h)

Material: 50 specimens.

Dimensions: L: 0.26-0.77 mm; H: 0.28 mm; W: 0.06-0.15 mm.

Locality: Rhoundjaïa, M’Daouer, Chellala Dahrania and El Kohol.

Description: Carapace of medium size. Subrectangular to suboval in lateral view, tighten and convex in dorsal view. Maximum height at mid-length. The left valve overlaps the right valve along the entire periphery, except the dorsal view. Ventral margin straight. Valve surface smooth.

Age: Upper Cenomanian-lower Turonian.

Family Macrocyprididae Müller, 1912

Genus Macrocypris Brady, 1867

Macrocypris sp. (Fig. 6i)

Material: 20 specimens.

Dimensions: L: 0.48-0.73 mm; H: 0.40-0.42 mm; W: 0.11-0.22 mm.

Locality: Rhoundjaïa, M’Daouer, Chellala Dahrania and El Kohol.

Description: Studied carapace are usually deformed. Anterior margin truncated and extends downwards. The right valve overlaps the left valve along the entire periphery. Dorsal margin sinuous along anterior margin.

Age: Upper Cenomanian-lower Turonian.

Superfamily Bairdioidea Sars, 1865

Family Bairdiidae Sars, 1885

Genus Bairdia McCoy, 1844

Bairdia sp.1 (Fig. 6j, k)

Material: 50 specimens.

Dimensions: L: 0.80-1.11 mm; H: 0.46-0.73 mm; W: 0.31-0.37 mm.

Locality: Rhoundjaïa and El Kohol.

Description: Carapace of large size, smooth and finely porous. Anterior margin short, pointed and slightly turned up. Posterior margin tapered and rounded. Dorsal margin strongly convex with inflection well marked in the two extremity. Ventral margin convex to subrectlinear in mid-length. The left valve overlaps the right valve along the entire periphery.

Age: Upper Cenomanian.

Bairdia sp. 2 (Fig. 6l)

Material: 22 specimens.

Dimensions: L: 0.84-1.11 mm; H: 0,53-0,64 mm; W: 0.31-0.48 mm.

Locality: Rhoundjaïa, M’Daouer and El Kohol.

Description: Carapace elongated and smooth. Dorsal margin convex with sharp inflection at extremity. Ventral margin convex. Anterior margin rounded. Posterior margin short and pointed. The left valve overlaps right valve along the entire periphery.

Age: Upper Cenomanian.

Superfamily Cytheroidea Baird, 1850

Family Trachyleberididae Sylvester-Bradley, 1948

Genus Cythereis Jones, 1849

Cythereis mdaouerensis Bassoullet & Damotte, 1969 (Fig. 6m, n, o)

1969 Cythereis mdaouerensis n. sp. Bassoullet & Damotte, p. 141, pl. 1, fig. 5.

2018 Cythereis mdaouerensis Bassoullet & Damotte; Benadla et al., p. 420, fig. 8M-O

Material: More than 100 specimens.

Dimensions: L: 0.11-0.64 mm; H: 0.06-0.33 mm; W: 0.03-0.26 mm.

Locality: Rhoundjaïa, M’Daouer and Chellala Dahrania.

Description: Rectangular shape in lateral view, ornament with a network reticulations. Anterior margin rounded in a semicircle. Posterior margin pointed. We note the presence of three fine longitudinal ribs. Median ribs prolong to subcentral tubercle in continuity. Ventral ribs are located clearly above the ventral margin that it does not cover.

Age: Upper Cenomanian-lower Turonian.

Stratigraphic and geographic distribution: The upper Cenomanian-lower Turonian of the Western Saharan Atlas (Bassoullet & Damotte, 1969; Bassoullet, 1973) and the lower Turonian of Tunisia (Bismuth et al., 1981).

Cythereis ziregensis Bassoullet & Damotte, 1969 (Fig. 6p).

?1959 Ostracode E8 Glintzboeckel & Magné, pl. 3, fig. 32.

Material: 20 specimens.

Dimensions: L: 0.46-0.66 mm; H: 0.28-0.31 mm; W: 0.26 mm.

Locality: Rhoundjaïa and M’Daouer.

Description: Subrectangular in lateral view, flattened laterally in dorsal view. Dorsal margin straight, very long, underlined by a denticulate ridge on its outer edge. Ventral margin short, slightly inclined and up towards the posterior margin. Anterior margin rounded almost in a semicircle. Posterior margin triangular, denticulate. Median ribs is non-existent. Low convexity forms the subcentral tubercle.

Age: Upper Cenomanian.

Stratigraphic and geographic distribution: Upper Cenomanian of the Western Saharan Atlas (Bassoullet & Damotte, 1969).

Cythereis sp. 1 (Fig. 6q)

Material: 11 specimens.

Dimensions: L: 0.69 mm; H: 0.38 mm; W: 0.22 mm.

Locality: Rhoundjaïa, El Kohol and Chellala Dahrania.

Description: The specimens differ from other individuals of Cythereis by the presence of cross-linking along the entire surface and the lateral costulation fading slightly in the mid-lenght of the valve.

Age: Upper Cenomanian.

Cythereis sp. 2 (Fig. 6r)

Material: 5 specimens.

Dimensions: L: 0.65 mm; H: 0.35 mm; W: 0.23 mm.

Locality: Rhoundjaïa.

Description: Specimens shows deterioration of ornamentation which results in the appearance of irregular tubers at the posterior endings of ventral and dorsal ribs, as well as in the mid-length of the carapace.

Age: Upper Cenomanian.

 Interpretation and comparison with other regions

The ostracod fauna of the upper Cenomanian-lower Turonian transition in the Ksour and Amour mountains has been previously studied and figured by Bassoullet & Damotte (1969) and Benadla (2019) in which several species determined for the first time, remain endemic. From a biostratigraphic point of view, two assemblages of ostracods could be found in the Cenomanian-Turonian transition (Figs 7-10).

Stratigraphic distribution of species of ostracods recorded in Rhoundjaïa section. Stratigraphic distribution of species of ostracods recorded in M’Daouer section. Stratigraphic distribution of species of ostracods recorded in Chellala Dahrania section. Stratigraphic distribution of species of ostracods recorded in El Kohol section.

The first rich and diverse assemblage consists of Cytherella gr. ovata, Cytherella gigantosulcata, Cytherella sp. 1, Cytherelloidea sp., Paracypris dubertreti, Paracypris mdaouerensis, Bythocypris sp., Macrocypris sp., Bairdia sp. 1, Bairdia sp. 2, Cythereis ziregensis, Cythereis mdaouerensis, Cythereis sp. 1, and Cythereis sp. 2. This species assemblage indicates an upper Cenomanian age. At the North Africa scale, this recognised association in the Western Saharan Atlas corresponds to Cythereis algeriana Zone defined in Tunisia (Bismuth et al., 1981) and in Egypt (Ismail, 2001).

The second assemblage, which is very rich but not very diverse, consists mainly of Cytherella sp. 2, and Cytherelloidea sp., and secondarily by Cytherella gr. ovata, Cythereis mdaouerensis, Paracypris dubertreti, Paracypris mdaouerensis, Cytherella sp. 1, This association indicates a lower Turonian age and corresponds to Cythereis mdaouerensis Zone (Bismuth et al., 1981).

The ostracod assemblages determined for the stratigraphic interval of the Cenomanian-Turonian transition indicate the presence of a biological event corresponding to the explosion of smooth ostracods called the Cytherellid Event, also described in Spain (Barroso-Barcenilla et al., 2011), Egypt (Shahin & Elbaz, 2013a) and Algeria (Benadla et al., 2018). Cytherellids are relatively resistant to oxygen depleted conditions (Whatley, 1991, 1995) and Cytherella has been interpreted as an ostracod indicative of warm waters adapted to survive during low oxygen episodes (Depêche, 1984; Whatley, 1995; Bonnet et al., 1999; N’Zaba-Makaya et al., 2003; Reolid, 2020; Reolid & Ainsworth, 2022). In the Rhoundjala section the Cytherellid Event coincides with the negative carbon isotopic excursion of the OAE2 (Benadla et al., 2018).

Intra-family comparisons with other regions

This analysis compare the ostracod assemblages from Moroccan Basin (MB; Agadir Basin, Central High Atlas Basin, Middle Atlas Basin), Algerian Basin (AB; Ksour Sub-basin, Amour Sub-basin, Tébessa Sub-basin), Central Tunisian Basin (TB), Egyptian Basin (EB, East and Central Sinai Basin), Lebanese Basin (LB), Central Jordanian Basin (JB), and Western Oman Basin (OB).

The results of the processing of the generic matrix by the program PAST-Palaeontological STatistics, ver.1.89 (Hammer et al., 2009; Table 1) are presented in the form of planar graphs (principal coordinate analysis, PCA) (Fig. 11A) and trees whose branch lengths are proportional to the distance between the taxonomic composition of the different regions (Fig. 11B). Thus, the intra-family generic diversity shows the following structure:

A first group made up of assemblages from basins belonging to four regions: Morocco (MB), Egypt (EB), Jordan (JB) and Oman (OB), which are more or less isolated on figures 11A, B.

A second group formed by the Algerian Saharan Atlas basins (AB) and Central Tunisian Basin (TB). It should be noted that the generic intra-family composition of the study region (Ksour and Amour mountains) is relatively close to Jordanian Basin.

Finally, the Central Lebanese Basin (LB) is isolated.

 A. Principal coordinates analysis (PCA) of the distance matrix of Bray-Curtis. B. Phenogram (tree of Neighbor-Joining) allowing the visualization of the proximity between basins as recorded in the distance matrix of Bray-Curtis.

These different basins on the northern edge of Gondwana show an abundance of smooth forms represented by families Cytherellidae, Cytherideidae, Cytheruridae and Paracyprididae, and usually ornamented individuals of Trachyleberidae. In the Saharan Atlas Basin Cytherellidae and Paracypridae dominated during the Cenomanian-Turonian transition.

Previous works by Khalil (2020) and Shahin & Elbaz (2021) have stablished two different bioprovinces for Ostracoda: the North African Province (or South Tethysian Province; Shahin & Elbaz, 2021) including Morocco, Algeria, Tunisia and Egypt, and the Middle East Province including Lebanon, Oman, Saudi Arabia, Kuwait and Iran. Mebarki et al. (2016) found that ostracod species from Guir Basin (southwestern part of Saharan Atlas) are closer in affinity to those from Atlasic Basin of Morocco, and secondarily with assemblages fropm Tunisia and Egypt. Our results differ from these other proposals but have in common the dominance of smooth ostracods, mainly cytherellids, during the Cenomanian-Turonian transition.

Quantitative comparison of the taxonomic composition with other regions

In this biogeographical quantification analysis, we dealt with 48 genera of which 33 (68%) are present in the Egyptian basins. The results given in the form of a phenogram (Fig. 12A) and a hierarchical association diagram (Fig. 12B), allowed us to reconstitute the following topology:

Proximity of the ostracod fauna of the Moroccan and Egyptian basins. These two regions share 12 genera (25%) (Bairdia, Brachycythere, Cythereis, Cytherella, Limburgina, Metacytheropteron, Nigeroloxoconcha, Ovocytheridea, Parakrithe, Paracypris, Reticulocosta, and Spinoleberis). To the Moroccan and Egyptian basins, the two basins belonging to the Middle East, Jordanian and Oman basins, are related, the Central Jordanian Basin with 4 genera in common (Brachycythere, Cythereis, Cytherella, and Parakrithe) and the Oman Basin with 4 genera in common (Brachycythere, Cythereis, Cytherella, and Metacytheropteron).

The ostracod fauna of the Algerian and Tunisian basins are very similar. This resemblance is reflected in the presence of 5 shared genera to both regions (Cythereis, Cytherella, Cytheropteron, Dolocytheridea, and Paracypris).

The remoteness of the ostracod fauna of Lebanese Basin compared to the regions analysed.

 A. Reconstituted phenogramm for Cenomanian-Turonian transition. B. Hierarchical association diagram between basins.

The similarity between the ostracod faunas from different basins shows the probable existence of communication routes during the Cenomanian-Turonian transition or the existence of equivalent palaeoenvironmental conditions.

 Pielou criteria Test

The obtained values of Q/Qmax (Table 2) show that the matrix, MB (Morocco), AB (Algeria), TB (Tunisia), EB (Egypt), LB (Lebanon), JB (Jordan), OB (Oman) is completely disordered (ungraded matrix). The order of the list of regions does not follow any geographical sequence.

 Conclusions

The study of ostracods from the Cenomanian-Turonian transition (Whiteinella archaeocretacea Zone) through four sections surveyed in the Ksour Mountains (Western Saharan Atlas) and the Amour Mountains (Central Saharan Atlas) allowed the identification of fifteen species, seven genera and five families. The average ostracod assemblage is dominated by the Family Cytherellidae (mainly genus Cytherella), and secondarily by the families Paracyprididae (exclusively Paracypris) and Trachyleberididae (mainly Cythereis). Less common are components of families Bairdiidae, Bythocypridae and Macrocyprididae.

Two ostracod biozones have been identified within the Whiteinella archaeocretacea foraminiferal Zone, the Cythereis algeriana Zone of the upper Cenomanian, and the Cythereis mdaouerensis Zone of the lower Turonian.

From palaeoecological point of view, the studied assemblages highlight a global biological event corresponding to the explosion of smooth-shaped ostracods, represented by the Family Cytherellidae. The Cytherellid Event is related to the biotic crisis of the Cenomanian-Turonian transition (OAE2) and related to the increased temperature of sea water and oxygen depleted conditions in the bottom.

Furthermore, the calculation of ostracod similarity and distance indices by the BG-Index allowed the comparison of seven regions belonging to palaeobiogeographic provinces of North Africa-Middle East (Gondwana palaeomargin). The results thus obtained show a general topology in the Cenomanian-Turonian transition, marked by the binary similarity between the Moroccan and Egyptian basins on the one hand and the basins of the Saharan Atlas (Algeria, Tunisia) on the other. This palaeobiogeographical topology indicates the probable existence of communication routes between some basins and the isolation of the ostracod fauna of the Lebanese Basin.

 Acknowledgements

We thank the constructive comments of three reviewers (Julio Rodríguez Lázaro and two anonymous) which have improved this work. This study had the support of funding from Spanish Ministry of Economy and Competitiveness, project PID2019-105537RB-100.

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