The Sirhan (also known as the Azraq-Sirhan) Basin is a northwest–southeast-oriented, regional antisynclinal structure, located in the northern Arabian Peninsula (Fig. 1). It is part of the Syrian Arc System (Guiraud et al., 2001) where subsidence in the early Paleogene Tethyan seafloor resulted in deposition of carbonate and mixed sediments (Al-Rawi, 2014). The Rashrashiyah Formation crops out in the eastern flank of the Sirhan Basin (31.46∘ N, 37.36∘ E) and consists of upper Eocene chalks, claystone and limestone (Meissner et al., 1990; Halawani, 2001; Aljahdali et al., 2020) (Figs. 2 and 3). We investigated the uppermost 10 m of the Rashrashiyah Formation. Five samples were obtained, with a resolution of one sample per meter, and assigned the prefix A–E (Figs. 2 and 3, Table 1). To provide stratigraphic constraints, an additional sample (Sample F) was taken close to the upper contact boundary between Rashrashiyah and Sirhan formations. The sampling is the same as in Aljahdali et al. (2020) for calcareous nannofossils.

For planktonic foraminiferal analysis, samples were washed over a 63 µm sieve and oven-dried at <40 ∘C. The dried sample was separated into > 425, 355–425, 250–355, 150–250 and 63–150 µm size fractions, and each size fraction was examined under the light microscope. Following the taxonomy of Pearson et al. (2006), Pearson and Wade (2015), and Wade et al. (2018a), the abundance of each species was semi-qualitatively assessed as follows: A – abundant; C – common; F – few; R – rare (Table 1). Preservation was assessed as VG (very good), where specimens are glassy under the light microscope with no infilling; G (good), where specimens are semi-translucent and with no infilling; M (moderate), where specimens are recrystallized and the test walls are opaque, and VP (very poor), where specimens are fragmented, opaque and infilled.

Selected specimens were picked for z-stacked light microscope and SEM imaging. A z-stacking light microscope was used to take images in three views (umbilical, edge and spiral view). For SEM imaging specimens were placed on stubs using double-sided tape. Each stub was coated in an argon and gold atmosphere using a sputter coater. The illustrated specimens (Figs. 4–9) are deposited in the Natural History Museum, London, UK (NHMUK PM PF 75192–75251).

Four criteria were presented by Pearson and Burgess (2008) for distinguishing foraminifera tests that are not significantly recrystallized: (1) tests should be glassy or translucent in reflected light; (2) ultrafine features such as spines (if initially possessed) should survive; (3) smooth parts of the test such as the apertural lips, sutures, outer surface (in some species) and inner surface (in most species) should appear smooth at the submicron scale in high-resolution SEM images; and (4) in cross section the submicron microgranular texture of the wall (if originally possessed) should be clear when the test is broken.

Test preservation is very good in Sample A, specimens are glassy beneath the light microscope and spines are preserved on Globigerinatheka, satisfying the criteria for excellent preservation of Pearson and Burgess (2008). In Samples B and D, the preservation is good, with specimens translucent under the light microscope. Moderate preservation is recorded for Samples C and E, with some infilling, encrustation and fragmentation (Figs. 4–9). Preservation is superior to other sections in this region (e.g., Ramadan et al., 2021).

The planktonic foraminiferal assemblage indicates that all samples are of Priabonian age. No Morozovelloides or large Acarinina were found (except a singular dwarfed A. rohri in Sample A), indicating that all samples are above the middle–late Eocene turnover (MLET; Kamikuri and Wade, 2012; Wade et al., 2012). The extinction of M. crassatus marks the base of the Globigerinatheka semiinvoluta HOZ Zone (Zone E14) (Berggren and Pearson, 2005; Wade et al., 2011). The top (T) M. crassatus occurs within Chron C17n.3n and is calibrated to 38.073 ± 0.005 Ma at the Varignano section in Italy (Luciani et al., 2020) based on the Pälike et al. (2006) magnetochronology. This converts to 38.0 Ma on the Westerhold et al. (2014) magnetochronology, which is the current standard used in the most recent edition of the Geologic Time Scale (GTS2020; Speijer et al., 2020). The entire section studied is therefore younger than 38 Ma.

We found a singular dwarfed specimen of Acarinina rohri in Sample A (Table 1). In the western North Atlantic (ODP Site 1052) a reduction in specimen size of Morozovelloides crassatus from 500 to 350 µm was recorded during the MLET, due to photosymbiont bleaching and environmental stress (Wade et al., 2008; Wade and Olsson, 2009). Small <250 µm specimens of Acarinina consisting of A. collactea, A. echinata and A. medizzai are found in all samples (Fig. 4). Small acarininids have been shown to range beyond the MLET and into the Oligocene (Berggren et al., 2006; Wade and Hernitz Kucenjak, 2018; Luciani et al., 2020), though they are not consistently present in late Eocene samples from Jordan (Farouk et al., 2015).

Globigerinatheka is common, with specimens of G. barri, G. index, G. korotkovi, G. kugleri, G. mexicana, G. semiinvoluta, and G. tropicalis (Premoli Silva et al., 2006). Globigerinatheka semiinvoluta is present in samples C, D and E but absent from samples A and B. The Base (B) G. semiinvoluta is a secondary bioevent within Zone E14 but was previously used as a primary bioevent to mark the base of Zone P15 (Berggren et al., 1995). It has been calibrated to Chron C17n in several sections including ODP Site 1052 (Wade, 2004; Wade et al., 2012), Varignano (Luciani et al., 2020) and Alano (Agnini et al., 2021). At the Varignano section, B G. semiinvoluta is calibrated to Chron C17n.2n with an age of 37.665 ± 0.006 Ma (timescale of Pälike et al., 2006) (Luciani et al., 2020), which converts to 37.54 Ma on Westerhold et al. (2014) magnetochronology (Fig. 11). The lack of G. semiinvoluta in Samples A and B raises the possibility that these samples could be from the short stratigraphic interval between the T M. crassatus and the B G. semiinvoluta. This interval was designated the Turborotalia pseudoampliapertura Zone by Haggag (1990) and has been recognized in several sections worldwide (Canudo and Molina, 1992; Wade, 2004; Agnini et al., 2011; Strougo et al., 2013). The B G. semiinvoluta may be diachronous, as the duration of the interval between T M. crassatus and B G. semiinvoluta is only 20 kyr at ODP Site 1052 (Wade, 2004; Wade et al., 2012), 330 kyr at Alano and 408 kyr at the Varignano section (Luciani et al., 2020). Note, however, the discrepancy could be due to the criteria used to separate G. semiinvoluta from its ancestor G. mexicana (Premoli Silva et al., 2006). Thus the bioevent may be isochronous, but independent workers have not been unified in the discrimination of the first true G. semiinvoluta morphospecies.

T Planorotalites capdevilensis is slightly older than B G. semiinvoluta in the Italian sections of Alano (Agnini et al., 2011) and Varignano (Luciani et al., 2020). Wade (2004) found a longer range of P. capdevilensis from ODP Site 1052, western North Atlantic. P. capdevilensis was not found in this study of the Rashrashiyah Formation.

The characteristics of the assemblage are very consistent with stratigraphic equivalent sections studied in the Adriatic, Egypt and Armenia (Wade et al., 2012; Strougo et al., 2013; Cotton et al., 2017; Ramadan et al., 2021; Salama et al., 2021). However, despite the high diversity, no Catapsydrax are present. This contrasts Adriatic cores where Catapsydrax increase in abundance at the base Priabonian (Wade et al., 2012). We record a high diversity of species belonging to Globoturborotalita (Figs. 8, 9 and 10), and G. pseudopraebulloides is unusually abundant (Fig. 9).

The stratigraphic distribution of the recorded species is shown in Fig. 10. The stratigraphic ranges of many of the non-marker species are different to published schemes (e.g., Pearson et al., 2006; Aze et al., 2011; Wade et al., 2018a). Many species thought to have evolved in Zone E16 or Zone O1 are found at this section in Zone E14, suggesting their stratigraphic range needs revision. For example, Globoturborotalita barbula, Globoturborotalita pseudopraebulloides, Globoturborotalita paracancellata, Globoturborotalita cancellata, Globoturborotalita gnaucki and Subbotina projecta (Figs. 5, 8 and 9) were all thought to evolve between upper Eocene Zone E15 and lower Oligocene Zone O2 (Spezzaferri et al., 2018; Wade et al., 2018b, c). However, they are all present in Zone E14, with some species, for instance, Globoturborotalita pseudopraebulloides in high abundance (Fig. 9). The discrepancy between first occurrence in the published literature and this study is most likely due to the excellent preservation of this section coupled with the lack of recent taxonomic investigations on the late Eocene. Within the combined nannofossil and planktonic foraminifera assemblages, we do not see any evidence for reworking in these sediments as an explanation for the lower occurrences of certain species compared to their previously published stratigraphic ranges.

Dentoglobigerina is a diverse genus that evolved in the middle Eocene. We find and illustrate (Fig. 5.6 and 5.7) two specimens that we could not confidently place in any of the previously described species: we refer to these as Dentoglobigerina sp. 1 and Dentoglobigerina sp. 2, pending further investigations (see Appendix A for taxonomic notes). Many of the stratigraphic ranges presented in Pearson et al. (2006) have already been extended to older levels by Wade et al. (2018a) and Fayolle and Wade (2021). Dentoglobigerina eotripartita is present from the base sample (Sample A), confirming the stratigraphic range suggested in Wade et al. (2018a). Dentoglobigerina taci was thought to be confined to the Eocene–Oligocene boundary interval (Zone E16 to Zone O1) (Pearson and Wade, 2015; Wade et al., 2018a). Here we find Dentoglobigerina taci in Zone E14 (Figs. 5 and 10), extending its evolution to several million years earlier. We find a singular rare occurrence of D. tripartita in Sample C (Fig. 5.5).

Globigerinathekids are abundant in the samples. Premoli Silva et al. (2006) state that the extinction of G. barri occurs towards the end of Zone E14. Here we find G. barri in Samples A, B and C (Fig. 6), but not in the younger part of the section, potentially constraining the extinction of G. barri to early Zone E14. Our extinction horizon for G. barri is consistent with results from coeval Egyptian sections (Strougo et al., 2013). In Premoli Silva et al. (2006) the extinction of Globigerinatheka kugleri is given as Zone E13, but we find this species ranging higher (Zone E14).

Globoturborotalita is a long-ranging genus, from the Eocene to the present. Many species have been described, though their ranges, in general, are poorly constrained. Globoturborotalita barbula was previously only known from the Eocene–Oligocene boundary interval (Pearson and Wade, 2015; Spezzaferri et al., 2018). We find specimens in Samples A and B, suggesting this species evolved earlier than previously thought (Pearson and Wade, 2015; Spezzaferri et al., 2018). The stratigraphic range of G. cancellata is not well constrained, and until Spezzaferri et al. (2018) it had not been recorded outside of the Oligocene Globorotalia opima opima Zone, from which it was described. Spezzaferri et al. (2018) found and illustrated specimens from Zone O1 and suggested a questionable range from Zone E16 to Zone O5. We find and illustrate specimens from Zone E14 (Fig. 8), indicating that this species has a much longer stratigraphic range than previously suggested. G. paracancellata was described from the upper Oligocene, with recorded specimens occurring from the upper Eocene Zone E16 (Spezzaferri et al., 2018). Here we extend the stratigraphic range of G. paracancellata with specimens illustrated from Zone E14 (Fig. 9). G. gnaucki is abundant in Samples B, C and D (Table 1, Fig. 8). This species was thought to evolve in Zone E15 (Spezzaferri et al., 2018), but this study suggests it evolved in Zone E14 or older (Fig. 10). We find specimens that we refer to as Globoturborotalita cf. G. labiacrassata (Fig. 9). These have a high-arched umbilical aperture and lobate profile but lack the thick rim boarding the aperture that is characteristic of this species. Our specimens are in Zone E14 and thus much older than the first appearance of G. labiacrassata (Zone O2) suggested by Spezzaferri et al. (2018). Further investigations are required to determine if these forms are a new species.

In Pearson et al. (2006) the evolution of Pseudohastigerina naguewichiensis from P. micra occurs at the base of Zone E15. We find and illustrate P. naguewichiensis in Zone E14 (Fig. 4). Our earlier evolution of this species is in agreement with Cotton et al. (2017), though an older range (Zone E13) is suggested by Strougo et al. (2013). The oldest previously recorded specimens of Subbotina projecta are from upper Eocene Zone E16. Here we document and illustrate specimens from Zone E14 (Table 1, Figs. 5 and 10). The occurrence of T. quinqueloba in Sample D (Figs. 8 and 10) confirms the stratigraphic range recorded in Pearson and Kučera (2018).

Despite the relatively good preservation and the high diversity of species recorded from the Rashrashiyah Formation (Table 1, Figs. 4–10), there are some species that were expected to be present, based on previous range chart compilations (e.g., Pearson et al., 2006; Aze et al., 2011), that were not found. These include Paragloborotalia griffinoides and P. nana, Chiloguembelina ototara, C. cubensis, and species of the genus Catapsydrax. However, we note that these species are recorded in upper Eocene sections from Egypt (Ramadan et al., 2021). We suspect that the absence of these taxa in the Rashrashiyah Formation is due to the environment, which is indicative of warm and oligotrophic conditions.

The amendment of planktonic foraminifera stratigraphic ranges has ramifications for the phylogenies and tropical–subtropical diversity charts. The range charts presented in Pearson et al. (2006), Wade et al. (2018a), incorporated into Time-Scale Creator (Fordham et al., 2018), and the Mikrotax online portal (Huber et al., 2016) will require revision. Many species evolved earlier than previously thought, particularly within the Globoturborotalita genus. The extensions of the range of Pseudohastigerina naguewichiensis and Turborotalita quinqueloba were already suggested in Cotton et al. (2017) and confirmed here. Our study implies that the late Eocene tropical–subtropical diversity is higher than previously suggested in compilations (Ezard et al., 2011; Fraass et al., 2015; Lowery et al., 2020). Many of the species found in this study of the late Eocene extend through the Eocene–Oligocene transition and into the Oligocene, suggesting that the rate of turnover at the Eocene–Oligocene transition is not as large as previously thought and requires re-investigation.

The calcareous nannofossil assemblages were studied by Aljahdali et al. (2020) indicating that the section corresponds to Zone CNE17 of Agnini et al. (2014) and Zone NP18 of Martini (1971). The section is Priabonian age (upper Eocene), as indicated by the presence of Chiasmolithus oamaruensis. The T C. grandis between Samples B and C allows identification of the base Zone CP15 of Okada and Bukry (1980). The base common (Bc) of Cribrocentrum erbae also occurs between Samples B and C. Throughout the section C. erbae increases in abundance consist with Zone CNE17 of Agnini et al. (2014). A single specimen of Isthmolithus recurvus was documented in Sample E (Aljahdali et al., 2020); however, this is not used in the biostratigraphic interpretation. The planktonic foraminifera biostratigraphy is consistent with the calcareous nannofossil biostratigraphy and can also be compared to other integrated calcareous biostratigraphic studies (e.g., Strougo et al., 2013; Farouk et al., 2015; Cotton et al., 2017; Luciani et al., 2020; Agnini et al., 2021).

The base of the Priabonian Global Stratotype Section and Point (GSSP) was recently defined by Agnini et al. (2021) at the Alano di Piave section (north-eastern Italy). The Bartonian–Priabonian boundary is placed at the prominent 14–16 cm crystal tuff layer known as the “Tiziano bed” at 63.57 m. With our current 1 m sampling resolution and the three bioevents between Samples B and C, we are unable to confidently calculate sedimentation rates, but it would appear that the base of the section is either at, or very close to, the Bartonian–Priabonian boundary.