2023 | 76/2 | 57–72 | 11 Figs. | 2 Supplements. | www.geologia-croatica.hr Journal of the Croatian Geological Survey and the Croatian Geological Society 1. INTRODUCTION The Central Paratethys disintegrated towards the end of the mid- dle Miocene, leaving in its wake the Late Miocene Lake Pannon (MAGYAR et al., 1999; PILLER et al., 2007). The brackish Lake Pannon lost its marine life, and the largely endemic ostracod and molluscan fauna of the lake is well-known from its fossil record. Upper Miocene ostracods from the Krško Basin, SE Slovenia Miha Marinšek1, Valentina Hajek-Tadesse2, Marijan Poljak3, Tea Kolar-Jurkovšek1 and Luka Gale1,4 1 Geological Survey of Slovenia, Dimičeva ul. 14, 1000 Ljubljana, Slovenia; (miha.marinsek@geo-zs.si, tea.kolar-jurkovsek@geo-zs.si, luka.gale@geo-zs.si) 2 Croatian Geological Survey, Sachsova 2, 10 000 Zagreb, Croatia; (tadesse@hgi-cgs.hr) 3 Zgornje Gorenjce 27, 8333 Semič; (marijanpoljak7@gmail.com) 4 University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Geology, Aškerčeva cesta 12, 1000 Ljubljana, Slovenia; (luka.gale@ntf.uni-lj.si) doi: 10.4154/gc.2023.07 Abstract The purpose of this study was to identify the ostracod assemblage from a 43 m thick section of the Bizeljsko Formation, which constitutes the middle part of the Upper Miocene Posavje Group. The succession comprises sandstone, siltstone, and marlstone, deposited in a delta front envi- ronment. The determined ostracods belong to the families Cyprididae, Cytheridae, Darwinulidae, and Loxoconchidae. In total, 30 species were identified. Additionally, 8 morphotypes were de- termined at the genus level. The most common genera are Candona, Hemicytheria, and Cypri­ deis. The assemblage belongs to the Caspiocypris labiata subzone from the upper Pannonian. The ostracod assemblage from the Krško Basin is similar in species composition to Pannonian ostracod assemblages from Croatia and Serbia. Finally, the determined ostracod assemblage was compared to assemblages of the same age from the Paratethys area. Data pre- sented here serve as an important contribution to the understand- ing of the distribution of ostracods in Lake Pannon as the last known scientific paper on Miocene ostracods from Slovenia was published in 1989 (STEVANOVIĆ & ŠKERLJ, 1989). A short Article history: Manuscript received January 10, 2023 Revised manuscript accepted June 29, 2023 Available online July 11, 2023 Keywords: Lake Pannon, Bizeljsko Formation, Pannonian, biostratigraphy, palaeoenvironment Figure 1. Palaeogeographic map of the Central Paratethys (Pannonian age) with the approximate location of Slovenia (SB: Styrian Basin; MZB: Mura-Zala Basin) modified after KOVAC et al., 2017). Research on the ostracod fauna is thus extremely important in de- termining the relative age, the palaeo ecological conditions, and the palaeogeographic connections bet ween individual sub-basins of Lake Pannon (KRSTIĆ, 1972, 1973; SOKAČ, 1981, 1989; JIŘI­ ČEK & RIHA, 1991; RUNDIĆ, 1997; TER BORGH et al., 2013). This paper presents the ostra- cod assemblage recovered from a 43 m thick succession of the Up- per Miocene Bizeljsko Formation near the village of Bizeljsko, situ- ated on the NE hilly rim of the Krško Basin in SE Slovenia (Figs. 1, 2). The succession is well ex- posed along a forest road, starting at coordinates Lat: 45°59’49.10’’, Lon: 15°41’01.23’’ (Figs. 3, 4). The relative age of the studi ed section and palaeosalinity were determined, based on ostracods. mailto:miha.marinsek@geo-zs.si mailto:tea.kolar-jurkovsek@geo-zs.si mailto:luka.gale@geo-zs.si mailto:marijanpoljak7@gmail.com mailto:luka.gale@ntf.uni-lj.si G eo lo gi a C ro at ic a Geologia Croatica 76/258 description of ostracods from the Krško Basin and the Bizeljsko section are given by VESEL­LUKIĆ (2012) and VESEL­LUKIĆ et al., (2013, 2014). There was also a Masters thesis done on the studied section by MARINŠEK (2020). 2. GEOLOGICAL AND STRATIGRAPHIC SETTING Slovenia is located in the western part of Central Paratethys, where the Transtethyan corridor was theorised to be located in the Badenian (BARTOL, 2009). Sediments of the Central Para- tethys in Slovenia are predominantly found in the eastern part of the country and extend westwards towards central Slovenia in a series of synclines that once formed interconnected basins (Fig. 2). The NE part of Slovenia was connected to the “Styrian” and Mura­Zala basins (JELEN & RIFELJ, 2002) and most of the SE part was connected to the North Croatian Basin (JELEN et al., 1992; PAVELIĆ, 2005). The structural development of eastern Slovenia resulted in four tectono­stratigraphic units (TTU) (JELEN et al., 1992). From north to south, these are: TTU A1, TTU A2, TTU B1, and TTU B2 (Fig. 2). Unit A1 covers the area north of the Periadriatic fault zone, namely the Northern Karavanke, Pohorje, and the Pannonian Plain of northeastern Slovenia. Unit A2 is a narrow area between Figure 2. The present-day distribution of sediments and sedimentary rocks of the tectono-stratigraphic units of the Central Paratethys in eastern Slovenia. BL Ba- laton Line, CNL Celje-Nadžkaniža Line, CTZ Celje Tectonic Zone, DL Donat Line, MHZ Middle Hungarian Tectonic Zone, PAL Periadriatic Line, SF Sava Fault, ŠF, Šoštanj Fault, SB Senovo Basin, KB Krško Basin. For an explanation of the tectonostratigraphic units (A1, A2, B1, B2) see text. The star symbol marks the position of the sam- pled section. Modified after JELEN & RIFELJ (2002). Figure 3. Position of the studied area. The exact position of the section is shown by a dashed rectangle in Figure 2B. The section starts at coordinates Lat: 45°59’49.10’’, Lon: 15°41’01.23’’. G eologia C roatica Marinšek et al.: Upper Miocene ostracods from the Krško Basin, SE Slovenia 59 the Periadriatic Fault Zone and the Don Tectonic Zone, which in- cludes the Southern Karavanke and Haloze areas. Unit B1 stretches from the Julian Alps to the Sava Tectonic Zone and con- tinues to the area of Varaždin in Croatia. Unit B2 covers the en- tire south­eastern part of Slovenia, including the Krško basin and thus also the area of Bizeljsko. After deposition of the Neogene sediments, the area underwent compression, resulting in the for- mation of the Sava Folds, a W­E extending series of antiforms and synforms (PLACER, 1999a). The general facies succession of the Miocene sediments in Slovenia is composed of alternating marl/marlstone, sand/sandstone with cobblestone, and in some places, there are beds of lignite (PLENIČAR, 1954; CIGIT, 1958; VONČINA, 1966; PLENIČAR, 1968; PREMRU & MARINČIĆ, 1971; KUŠČER, 1973; ARSOVSKI et al., 1975; MIOČ & ŽNIDARČIČ, 1978; KALOPER 1984; ANIČIĆ & JURIŠA 1985a, b; ŽNIDARČIČ & MIOČ, 1987; MARKOVIĆ & MIOČ, 1988; MIOČ & ŽNIDARČIČ, 1989; DJURASEK, 1995; PLACER, 1999b; ŠRAM et al., 2015). In Slovenia, the evolution of Lake Pannon is best recorded in the Krško and Senovo basins (Fig. 2). Panonnian calcareous marls conformably overlie Sarma- tian beds; which are overlain by “ostracod marls”. In some parts of the Krško Basin the Pannonian lies unconformably on the Sar- matian beds (POLJAK et al., 2016) The Pannonian succession ends with quartz sand and marly-clayey intercalations with lig- nite on top. The entire Neogene succession can be interpreted as a transition from marine to brackish and finally freshwater con- ditions (PAVŠIČ & HORVAT, 2009; IVANČIČ, 2021). Important fossil groups that have been investigated in the Upper Miocene sediments of eastern Slovenia are ostracods (PIERAU, 1958; SOKAČ, 1981; STEVANOVIĆ & ŠKERLJ, 1985, 1989; HAJEK­ TADESSE, 2007; VESEL­LUKIĆ, 2012; VESEL­LUKIĆ et al., 2013, 2014; MARINŠEK, 2020), and molluscs (MIKUŽ, 2005, 2014; MIKUŽ et al., 2015). The Bizeljsko area corresponds to the northern limb of the Krško synclinorium and is a part of the B2 TTU (PLACER, 1999a, b; POLJAK, 2017). The geological maps of the study area were produced by ANIČIĆ & JURIŠA (1985a, b), and more re- cently by POLJAK et al. (2017). A simplified geological map is shown in Figure 5. The stratigraphic succession in the study area is summarized after POLJAK (2017) (Fig. 6). The oldest deposits of the Central Paratethys in the research area belong to the Bade- nian–Sarmatian Laško Formation. It comprises shallow marine coralline algal­rich limestone, the “Lithothamnium limestone”, and the deeper marine “Laško marl”. The Laško Formation is, after another prominent unconformity, followed by marls and sands of the deltaic Pannonian Drnovo Formation, which in its upper part passes into the Bizeljsko Formation. The latter com- prises quartz sand, marl, and subordinate sandstone and gravel, deposited within a delta front environment. The uppermost Pan- nonian Raka Formation almost entirely consists of quartz sand, which was also deposited in a deltaic environment. The Drnovo, Bizeljsko, and Raka Formations together comprise the Posavje Group, which represents the final remnants of the Central Para- tethys. The youngest sediments in the area are represented by Quaternary gravel (POLJAK, 2017). Figure 4. Field pictures of the studied section. A – bottom of the marl in the middle of the section (view towards the south).; B – a detailed picture of the marl sec- tion; C – transition from the marl to sand; D – sand section in the upper part of the outcrop (Author: Marina Vesel-Lukić, 2012) G eo lo gi a C ro at ic a Geologia Croatica 76/260 The Bizeljsko Formation is best exposed in the eastern Krško Basin, especially in the Bizeljsko area. The formation was defined by POLJAK (2017). It was previously also researched by LAPA- JNE (1976), ŠIKIĆ et al. (1979), SKABERNE (1990), ŠKERLJ (1990), TRAJANOVA (2006), HAJEK­TADESSE (2007), VESEL­LUKIĆ et al. (2013, 2014); MARINŠEK (2020) and POLJAK (2017). The formation consists mainly of marls and sands, which alternate over short distances. Beds of sand locally show traces of slumping and/or contain rip-up clasts of marl. It reaches its thickest point (up to 800 m) in the wider Bizeljsko area. The age was first determined based on bivalves (Congeria cro- atica, C. rhomboidea, C. zagrabiensis, Lymnocardium majeri, L. riegeli, Paradacna abichi and Paradacna okrugici, as well as ostracods (ŠIKIĆ et al., 1979; HAJEK­TADESSE, 2007; POL- JAK, 2017). POLJAK (2017) interpreted the succession as a delta front deposit based on individual layers of sand showing signs of synsedimentary slumping with frequent layers containing rip-up marl clasts. It represents the part of the Lake Pannon evolution where it began to shrink and transitioned locally from brackish to freshwater conditions. The Bizeljsko Formation is completely missing in the western part of the Krško Basin. Instead, the up- per Pannonian deposits are represented there by a much thinner succession of marls and sandy marls of the Drnovo Formation (POLJAK, 2017). 3. MATERIAL AND METHODS The section of the Bizeljsko Formation was logged and sampled with a total stratigraphic length of 43 m (Fig. 7). In total, 38 sam- ples were collected and subsequently processed in the laboratory. Individual samples weighed between 1 and 2 kg. Each sample was quartered to a weight of 250g – 500g. Samples were dissolved in a 5% hydrogen peroxide solution; samples that contained a large number of ostracod valves filled with sediment were put in an ultrasonic bath for 30 seconds. The dissolved material was then wet sieved using mesh sizes of 0.500 mm, 0.250 mm, 0.125 mm, and 0.063 mm. The specimens were hand­picked un- der a ZEISS Stemi 2000­C binocular magnifier with a magnifi- cation of × 50. Approximately 100 adult ostracod valves were collected from each sample. Picking was nonselective to preserve the original proportions of the species within the assemblage. This was done by collecting valves either until we reached 100 valves or until the sample was looked through fully. Selected specimens were photographed on the JEOL JSM 6490LV scan- ning electron microscope at the Geological Survey of Slovenia. For determining the families of ostracods, the suprageneric clas- sification scheme proposed by MOORE (1961) and HORNE et al. (2002) was used, as they are widely used classification schemes for ostracods. Samples are stored at the Geological Survey of Slo- venia under field numbers GeoZS B­019 – B­057. The diversity was calculated using the Shannon­Wiener index (H) for each sample (SHANNON & WEAVER, 1949). 4. RESULTS The total stratigraphic thickness of the section is 43 m (Fig. 7). A thin gravel layer at the base of the section is overlain by alternat- ing sands and marls in 3 m thickness, with each sand and marl layer being approximately 20 to 30 cm thick (with a 1-meter-thick unlogged interval). The following 8-metre-thick sand has a sharp erosional base and clay intraclasts at the bottom part. The sand grades upward into marl, again overlain by a 10-metre-thick, dominantly sandy unit with an erosional base at 13 m. This unit contains thin clay and marl layers and even pebbly intervals at 17 m and between 20 and 21 m (Fig. 7). The sand is overlain by a 7-metre-thick clay with sand intercalations. The top of the clay is eroded by a channel sandstone at 30 metres, with pebbles above the erosion surface. This sandstone is 7 metres thick and contains a few thin marl layers. Marl predominates in the uppermost 6 m of the outcrop (Fig. 7). A total of 30 ostracod species were deter- mined (Figs. 8­10 + Supplement 1). In addition, 8 morphotypes were determined to the genus level. Approximately 70 % of the collected valves were intact, and the rest were fragmented. Only adult specimens were included in the palaeontological determi- nation. Diversity is strongly controlled by lithology: higher in marlstone, and lower in more sandy facies and claystone. The maximum value of diversity is 2.29 in sample B-024 at the 4 me- tre point of the section (taken from clay lithoclasts), but on aver- Figure 5. A simplified geological map by POLJAK et al. 2017, slightly modified. G eologia C roatica Marinšek et al.: Upper Miocene ostracods from the Krško Basin, SE Slovenia 61 Figure 6. A general stratigraphic column of the Miocene deposits within the Krško Basin (Modified after POLJAK et al., 2017). G eo lo gi a C ro at ic a Geologia Croatica 76/262 Figure 7. Sedimentary log and stratigraphic distribution of ostracod species in the section. The “diversity” column shows values of the Shannon-Wiener index as a measure of species richness and evenness. G eologia C roatica Marinšek et al.: Upper Miocene ostracods from the Krško Basin, SE Slovenia 63 age the Shannon­Wiener diversity index values are between 1.0 and 1.5 (Fig. 7). The most abundant specimens are from the genera Cyprideis and Hemicytheria. Loxoconcha is present in most of the section but represents only a small percentage of the total assemblage. Of the species observed here, the most important ones for determining the age were: Amplocypris dorsobrevis SOKAČ, Caspiocypris alta (ZALÁNYI) and Camptocypria aff. acronas- uta (LIVENTAL). 5. DISCUSSION 5.1. Depositional environment The studied section begins with sandy facies which suggests a higher energy environment. Indicators for this are mud chips and interbedded gravel. The environment changes at 22 metres from the beginning of the succession where we see a thick sequence of clay with thin layers of sand. At the 30 metre mark, the facies change again, with a rise in energy with an erosional channel in the clay, in which sand was deposited. The section ends with a lower energy environment indicated by marl deposition. These shifts in diversity and lithological changes correspond to rapidly changing conditions and lateral migration of the delta ( DALRYMPLE et al., 1992). The ostracod genera Cyprideis, Hemicytheria, Leptocythere and Loxoconcha in the lowermost sand layer (0-2 m) indicate brackish conditions. Ostracod diversity is low in this sandy part. Moving upwards through the succession the ostracod diversity fluctuates significantly. Diversity is highest in the marl­dominated part of the section, which can be explained by low-energy sedi- mentation and the reduced influence of fresh water. The diversity of the ostracod community reaches its highest value in sample B­024, which was taken from clay lithoclasts (Fig. 6). The species Cyprideis ex. gr. pannonica from that sample is considered by the authors to be reworked from older beds. Further along the section, the diversity of the ostracod assemblage decreases drastically. The low values of diversity between samples B-035 and B-043 are due to the sudden intake of sand. The presence of Cyprideis, Hemicy- theria, and Loxoconcha indicates that beds from the upper part of the sequence were deposited under brackish conditions. Previous research of the Bizeljsko Formation suggested de- position in a delta­front environment (POLJAK, 2017), and this interpretation is accepted here. It is to be noted that POLJAK (2017) does not describe any progradation and/or cyclic deposi- tion, which would be typical for deltaic deposition as described and studied in SZTANÓ et al. (2016), SEBE et al. (2020) and ŠPELIĆ et al. (2023). This studied outcrop is too short to log any cyclic deposition apart from alternating marl and sand. Accord- ing to HAJEK­TADESSE (2007), the composition of the ostracod community of the Bizeljsko profile, the preservation and abun- dance of ostracod shells depend on the change in the salinity of the lake water and the sedimentation. In general, the same author (HAJEK­TADESSE, 2007) concluded that several changes in lake water salinity and ostracod fauna can be distinguished on the Bizeljsko profile. At the beginning of the late Pannonian, wa- ter salinity increases to mesohaline values, while in the upper- most samples of the late Pannonian, the ostracod fauna of the Bizeljsko profile takes on a more freshwater character and indi- cates deposition near fresh-water input and a near-shore environ- ment as stated in POLJAK (2017). We agree with the paleoenvi- ronmental interpretation of both HAJEK­TADESSE (2007) and POLJAK (2017). 5.2. Biostratigraphy There are quite a few ostracod biostratigraphic zonal schemes available for the Pannonian stage (e.g., POKORNÝ, 1944; BRESTENSKÁ, 1961; KRSTIĆ, 1973, 1985; JIŘIČEK, 1975, 1985; SOKAČ 1972, 1989; JIŘIČEK & RIHA 1991; and RUNDIĆ, 1998, 2006). According to OLTEANU (2011), the ostracod as- semblages are well­differentiated only between two units, the Lower Pannonian and the »Portaferrian« (with Congeria rhom- boidea), and most of the known species cannot be considered re- liable index fossils. The same author concludes that the biostra- tigraphy of the brackish-water facies, generally and especially of the Pannonian Basin, is also difficult to dtermine. A similar view was expressed by KOVÁCS et al. (2016). The same authors con- clude that different ostracod biostratigraphic schemes have been erected in different parts of the Pannonian Basin System, but they usually reflect local characteristics and differ significantly from each other. The four most commonly used schemes for biozonation are presented in Fig. 8. Here, the biozonation by SOKAČ (1989) was used. The great majority of the investigated section can be placed within the Caspiocypris labiata subzone (Fig. 8), which is the oldest subzone in the Upper Pannonian (previously known as up- per Pontian) Bakunella dorsoarcuta zone (SOKAČ, 1972; KRSTIĆ 1973; SOKAČ, 1989). The subzone is defined by Cas- piocypris labiata (Zalányi), Caspiocypris alta (Zalányi) and C. Caspiocypris pontica Sokač. Although the species Caspiocypris labiata itself is absent, the defined subzone contains the typical Caspiocypris form, such as Caspiocypris alta. In addition, Am- plocypris aff. dorsobrevis SOKAČ, Camptocypria aff. acronas- uta, Typhlocypris cf. centropunctata, Loxoconcha schweyeri are relatively abundant in this zone which is characteristic for this subzone. The Camptocypria flectimarginata, which reaches its acme in the Camptocypria flectimarginata subzone, rarely occurs in the studied section. However, the younger Pontoniella truncata subzone is excluded due to the absence of the index species, as well as the lack of the usual accompanying species e.g., Ponton- iella paracuminata and Hastacandona loczyi (JIŘIČEK, 1983; SOKAČ, 1989). The subzone was chosen out of all the others in the literature, due to the proximity of the localities where the sub- zone was defined, in agreement with the results of previous re- search undertaken around the studied area (SOKAČ, 1981; HAJEK­TADESSE, 2007). Ostracods determined here only cor- respond to the described ostracod assemblages of the Caspiocy- pris labiata subzone. The lowermost 3.5 m of the section does not contain species diagnostic of the Caspiocypris labiata subzone, nor are there any species that would indicate the presence of the preceding, lower level of upper Pannonian Hemicytheria prisca SOKAČ biozone. The latter ostracod zone was defined by KRSTIĆ (1973) based on the nominate species, and the presence of the species Ponton- iella ilica KRSTIĆ, Zalanyiella longissima KRSTIĆ, Campto- cypria acuta SOKAČ, Bakunella anae SPADI (SPADI et al., 2019), Cyprideis salopeki SOKAČ, and C. parallela KRSTIĆ. In the investigated section, the species Camptocypria acuta SOKAČ and B. anae SPADI occur in the lower part. The stratigraphic ranges of these species, however, extend to the upper Pannonian. The lowermost 3.5 m of the section is therefore not defined in age. The species Bakunella anae, characteristic of the upper Pan- nonian (previously known as the lower and upper Pontian) (SOKAČ, 1972), was also observed in the sample B­024. The sample was collected from a marl lens within a sandstone bed, which is, as mentioned before, was interpreted as a clast eroded G eo lo gi a C ro at ic a Geologia Croatica 76/264 from the underlying beds. Resedimentation of the fossils is also possible for samples B-035, B-037, and B-048 (see Fig. 7). According to JIŘIČEK & RIHA (1991), the equivalent os- tracod zone for the Caspiocypris labiata subzone is the lower Pontian Ostracod Zone 20 “Candona (C.) balcanica- Candona (C.) lobata” in which there is the first occurrence of Bakunella dorsoarcuata. However, in the biozonation set by JIŘIČEK (1983), the species Caspiocypris labiata is placed in the upper Pannonian Zone E 1-2. The correlation of biozones is further complicated by the different compositions of ostracod assem- blages in various parts of the Central Paratethys (see POKORNÝ, 1944; BRESTENSKÁ, 1961; KRSTIĆ, 1973, 1985; JIŘIČEK, 1975, 1985; RUNDIĆ, 1998, 2006). Also, there is much confusion related to the ostracod taxonomy, which needs a serious revision (see OLTEANU, 2011; SPADI et al., 2019). 5.3. Palaeosalinity levels The Pannonian microfauna is dominated by brackish to freshwa- ter ostracods which tolerate an increase in salinity (Figs. 8 and 10). This suggests that the composition of the Lake Pannon ostra- cod assemblages is strongly facies dependent, as was previously demonstrated for the Pannonian mollusc fauna (MAGYAR et al., 1999). Many of the detected ostracod species may tolerate a con- siderable range of salinity. The number of freshwater species di- minishes when salinity surpasses 3‰, and the number of true brackish water species in the range from 3-10‰ is not numerous but the number of specimens is large (MORKHOVEN, 1962). The important genera for determining palaeosalinity levels in the logged part of the Bizeljsko Formation are Darwinula, Candonopsis, Pontoniella, Lineocypris, Camptocypria, Caspio- cypris, Typhlocypris, Bakunella, Hemicytheria, Leptocythere, Amplocypris, and Loxoconcha (TUNOĞLU, 2003). Candonopsis and Darwinula are specific to freshwater environments. According to Meisch (2000), recent Darwinula tolerates an increase in salinity up to a maximum of 15‰, and Candonopsis from a maximum salt content of 5 ‰. Considering the poor preservation of their valves in sample B-024 and their probable re-deposition they were excluded from the environmental interpretation. Paleosalinity was determined to be in the range of brackish conditions. The interpretation is based on the presence of the genus Hemicytheria, which was abundant in the section ( MORKHOVEN,1962; MEISCH, 2000; FRENZEL & BOOMER, 2005). Figure 8. Four selected biozonation schemes proposed by different authors (KRSTIĆ, 1973; SOKAČ, 1989; JIŘIČEK & RIHA, 1991; RUNDIĆ, 2006) with the stratigrap- hic position of the study site. G eologia C roatica Marinšek et al.: Upper Miocene ostracods from the Krško Basin, SE Slovenia 65 Figure 9. SEM pictures of Pannonian ostracods from the Bizeljsko section. 1. Amplocypris subacuta ZALÁNYI (LV, external view, sample B-024, fragmented valve); 2. Amplocypris cf. dorsobrevis SOKAČ (LV, external view, sample B-051, fragmented valve); 3. Herpetocyprella cf. pannonica ZALANYI (LV, external view, sample B-048, fragmented valve); 4. Bakunella anae (Vekua) SPADI (RV, external view, sample B-024), alternatively, possible juvenile of B. dorsoarcuata; 5a., 5b. Bakunella dorsoar- cuta ZALÁNYI (a. LV, external view, sample B-024; b. RV, internal view, sample B-024); 6. Camptocypria acuta SOKAČ (LV, internal view, sample B-027); 7. Camptocypria cf. acronasuta LIVENTAL (RV, internal view, sample B-027); 8. Camptocypria flectimarginata SOKAČ (LV, external view, sample B-031); 9. Camptocypria? sp. (LV, exter- nal view, sample B-030); 10. Candonopsis cf. arcana KRSTIĆ (LV, external view, sample B-024); 11. Caspiocypris alta ZALÁNYI (LV, internal view, sample B-031); 12. Li- neocypris reticulata (MÉHES) (LV, external view, sample B-057); 13. Lineocypris hodonensis POKORNÝ (RV, external view, sample B-027);14. Lineocypris zagrabiensis SOKAČ (RV, external view, sample B-041). G eo lo gi a C ro at ic a Geologia Croatica 76/266 5.4. Comparison between the ostracod fauna from the Pannonian deposits of the Krško Basin and other parts of the Paratethys The ostracod fauna from the Upper Pannonian deposits of the Krško Basin, which includes taxa that were determined both here and in previously published papers (SOKAČ, 1981; STEVANOVIĆ & ŠKERLJ, 1989; HAJEK­TADESSE, 2007), was compared to the ostracod faunas of the same age reported from similar locali- ties in the Central Paratethys. The results are presented in Sup- plement 2. Figure 10. SEM pictures of Pannonian ostracods from the Bizeljsko section. 1. Lineocypris cf. granulosa ZALÁNYI (LV, external view, sample B-024); 2. Pontoniella sag- ittosa KRSTIĆ (RV, external view, sample B-024); 3. Pontoniella sp. 1 MANDELSTAM (LV, external view, sample B-024); 4. Pontoniella sp. 2 (LV, sample B-036); 5. Typhlo- cypris cf. centropunctata SUZIN (LV, external view, sample B-038); 6. Typhlocypris sp. (HÉJJAS) (LV, external view, sample B-040); 7. Cyprideis macrostigma KOLLMANN (a. RV, external view, b. LV, internal view, sample B-036); 8. Cyprideis macrostigma spinosa SOKAČ (a. LV, external view, b. LV, internal view, sample B-032); 9. Cyprid- eis ex gr. pannonica MÉHES (RV, external view, sample B-024); 10. Cyprideis ex gr. pannonica MÉHES (RV, external view, sample B-027); 11. Cyprideis ex gr. pannonica MÉHES (RV, external view, sample B-020); 12. Cyprideis sp. 1 (RV, external view, sample B-023); 13. Cyprideis sp. 2 (LV, external view, sample B-023). G eologia C roatica Marinšek et al.: Upper Miocene ostracods from the Krško Basin, SE Slovenia 67 In the comparison, from the literature, we only included Up- per Pannonian ostracod species. The ostracod assemblage from the Krško Basin is most similar to that from Mt. Medvednica as the locality is very close by. Out of 73 species found in the Krško Basin, in total, Croatia has 43 of the same species from similar deposits. The resemblance with Croatia was expected as it be- longs to the same system of supposed basins and the location of the outcrop and samples are only a few kilometres away from the comparison locations studied on Mt. Medvednica and reported in the literature. Ostracod assemblages from Romania and Serbia have, respectively, 29 and 33 species in common with Krško Ba- sin. The Vienna Basin only has 13 species similar to those re- corded at Bizeljsko. Regarding Serbia and Romania, the number of similar species indicates a good connection with the Central Paratethys. Papers involving the older Pannonian ostracods (e.g., VRSALJKO, 1999; KOVÁCS et al., 2016; CSOMA et al., 2021), as well as Pannonian ostracods from Austria /Vienna Basin (GROSS, 2004; GROSS et al., 2008; STAREK et al., 2010) are not included in Supplement 2. Apart from these localities, some ostracods are the same species as the ones that migrated from Paratethys to the Palaeo-Mediterranean. These are Caspiocypris alta and Euxinocythere (M.) cf. praebaquana (GLIOZZI et al., 2012). Figure 11. SEM pictures of Pannonian ostracods from the Bizeljsko section. 1. Cyprideis sp. 3 (LV, external view, sample B-023); 2. Hemicytheria croatica SOKAČ (LV, external view, sample B-024); 3. Hemicytheria cf. marginata SOKAČ (LV, external view, sample B-024); 4. Hemicytheria cf. prisca SOKAČ (RV, external view, sample B-030); 5. Hemicytheria dubokensis KRSTIĆ (LV, juvenile, external view, sample B-026); 6. Amnicythere naca (MÉHES) (LV, external view, sample B-052); 7. Amnicythere sp. (LV, external view, sample B-020); 8. Leptocythere? sp. (LV, external view, sample B-051); 9. Euxinocythere (Maeotocythere) cf. praebaquana (LIVENTAL) (RV, exter- nal view, sample B-033); 10. Darwinula cf. stevensoni BRADY & ROBERTSON (LV, external view, sample B-024) fragmented due to handling of the specimen; 11. Loxo- concha schweyeri SUZIN (LV, external view, sample B-024); 12. Loxoconcha cf. spinosa SOKAČ (LV, external view, sample B-024). G eo lo gi a C ro at ic a Geologia Croatica 76/268 6. CONCLUSIONS The Pannonian ostracods studied from the 43 m thick lithological succession (Bizeljsko Fm., Krško basin) are relatively well pre- served and diverse. The determined species delineate the late Pan- nonian Caspiocypris labiata subzone. Palaeosalinity levels, as in- dicated by the ostracods, show brackish water conditions. Comparison with the ostracod assemblages from the region shows the highest similarity in species composition to the fauna from Croatia, less so with faunas from Serbia and Romania. Within the Bizeljsko ostracod assemblage, two species are recognised that migrated from Paratethys to the Palaeo-Mediterranean during the Lago­Mare event of the Messinian Salinity Crisis. ACKNOWLEDGMENT This work is part of the ongoing PhD research financially sup- ported by the Slovenian Research Agency (ARRS), within the program group Regional Geology (P1­0011). We thank Marija PETROVIĆ for the preparation of the samples. REFERENCES ANIČIĆ, B. & JURIŠA, M. (1985a): Osnovna geološka karta SFRJ, 1:100.000, list Ro- gatec [Basic Geological Map of the SFRY, 1:100000, sheet Rogatec – in Slove- nian].– Zvezni geološki zavod, Beograd. ANIČIĆ, B. & JURIŠA, M. 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Class Ostracoda LATREILLE, 1802 Subclass Podocopa SARS, 1866 Order Podocopida SARS, 1866 Suborder Cypridocopina BAIRD, 1845 Superfamily Cypridoidea BAIRD, 1845 Family Cyprididae BAIRD, 1845 Amplocypris ZALÁNYI, 1944 Amplocypris subacuta ZALÁNYI, 1944 Amplocypris cf. dorsobrevis SOKAČ, 1972 Subfamily Herpetocyprellinae BRONSTEIN, 1947 Herpetocyprella DADAY, 1909 Herpetocyprella cf. pannonica ZALÁNYI, 1959 Family Candonidae KAUFMANN, 1900 Subfamily Candoninae KAUFMANN, 1900 Bakunella SCHNEIDER, 1958 Bakunella anae (VEKUA) SPADI, 2019 Bakunella dorsoarcuta ZALÁNYI, 1929 Camptocypria ZALÁNYI, 1959 Camptocypria acuta SOKAČ, 1972 Camptocypria cf. acronasuta (LIVENTAL, 1929) Camptocypria flectimarginata SOKAČ, 1967 Camptocypria? sp. Candonopsis VAVRA, 1891 Candonopsis cf. arcana KRSTIĆ, 1968 Caspiocypris MANDELSTAM in SCHNEIDER et al., 1956 Caspiocypris alta (ZALÁNYI, 1929) Lineocypris ZALÁNYI, 1929 Lineocypris reticulata (MÉHES, 1907) Lineocypris hodonensis POKORNÝ, 1952 Lineocypris zagrabiensis SOKAČ, 1972 Lineocypris cf. granulosa ZALÁNYI, 1959 Pontoniella MANDELSTAM in LUEBIMOVA et al., 1960 Pontoniella sagittosa KRSTIĆ, 1968 Pontoniella spp. Typhlocypris VEJDOVSKÝ, 1882 (sensu NAMIOTKO et al. 2014) Typhlocypris cf. centropunctata (SUZIN, 1956) Typhlocypris sp. Superfamily Cytheroidea BAIRD, 1850 Family Cytherideidae SARS, 1925 Subfamily Cytherideinae SARS, 1925 Cyprideis JONES, 1857 Cyprideis macrostigma KOLLMANN, 1958 Cyprideis macrostigma spinosa SOKAČ, 1972 Cyprideis ex. gr. pannonica (MÉHES, 1908) Cyprideis spp. Family Hemicytheridae PURI, 1953 Subfamily Hemicytherinae PURI, 1953 Hemicytheria POKORNÝ, 1955 Hemicytheria croatica SOKAČ, 1963 Hemicytheria cf. marginata SOKAČ, 1972 Hemicytheria cf. prisca SOKAČ, 1972 Hemicytheria dubokensis KRSTIĆ, 1963 Family Leptocytheridae SARS, 1925 Amnicythere DEVOTO, 1965 Amnicythere naca (MÉHES, 1909) Amnicythere sp. Leptocythere SARS, 1925 Leptocythere sp. Euxinocythere (Maeotocythere) STANCHEVA, 1968 Euxinocythere (M.) cf. praebaquana (LIVENTAL, 1956) Family Loxoconchidae SARS, 1925 Loxoconcha SARS, 1866 Loxoconcha schweyeri SUZIN, 1956 Loxoconcha cf. spinosa SOKAČ, 1972 Superfamily Darwinuloidea BRADY & ROBERTSON, 1885 Family Darwinulidae BRADY & ROBERTSON, 1885 Darwinula BRADY & ROBERTSON, 1885 Darwinula cf. stevensoni BRADY & ROBERTSON, 1870 G eo lo gi a C ro at ic a Geologia Croatica 76/272 Su pp le m en t 2 . T ab le s ho w in g th e re gi on al d is tr ib ut io n of U pp er P an no ni an o st ra co d sp ec ie s fo un d in th e Kr šk o Ba si n (S O KA Č, 1 97 2; K RS TI Ć, 1 97 3; S O KA Č, 1 98 1, 1 98 9; S TE VA N O VI Ć & Š KE RL J, 19 89 ; S ZU RO M I-K O RE Cz e t a l., 2 00 4; H A JE K- TA D ES SE , 2 00 7; C ZI CZ ER e t a l., 2 00 9; F LO RO IU e t a l., 2 01 1; O LT EA N U , 2 01 1; R U N D IĆ e t a l., 2 01 1; T ER B O RG H e t a l., 2 01 3; M AG YA R et a l., 2 01 9; R U N D IĆ e t a l., 2 01 9; L A ZA RE V et a l., 2 02 0; B A RT H A e t a l., 2 02 1; A VA N IĆ e t a l., 2 02 2b ; BA SC H e t a l., 2 02 2) .