Revised middle Eocene-upper Oligocene calcareous nannofossil biozonation for the Southern Ocean

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Original article

Revised middle Eocene-upper Oligocene calcareous nannofossil biozonation for the Southern Ocean Zonation révisée à nannofossiles calcaires pour l’Éocène moyen-Oligocène supérieur de l’Océan Austral Chiara Fioroni a,∗ , Giuliana Villa b , Davide Persico b , Sherwood W. Wise c , Laura Pea b a

Università degli Studi di Modena e Reggio Emilia, Dipartimento di Scienze della Terra, L.go S. Eufemia, 19, 41100 Modena, Italy b Università degli Studi di Parma, Dipartamento di Scienze della Terra, V.le delle Scienze, 78, 43100 Parma, Italy c Florida State University, Department of Geological Sciences, 108, Carraway Building, Tallahassee, FL 32306-4100, USA

Abstract Calcareous Nannofossils are widely utilized in biostratigraphic studies of Cenozoic sediments. In recent years large datasets have been acquired, and several bioevents showed to be unreliable when correlated over distant areas. New biostratigraphic investigations of middle Eocene-upper Oligocene deep-sea cores have highlighted problematic areas in the currently used calcareous nannofossil zonal schemes for the Southern Ocean. Quantitative analysis on sediments from seven ODP (Ocean Drilling Program) sites from this area, characterized by abundant, diverse, and moderately to well-preserved nannofloral assemblages, have enabled a revision of the existing zonation for the Southern Ocean, and the development of a high-resolution zonal scheme. Eleven zones and six subzones are introduced or emended to replace the ten zones and two subzones of the Wei and Wise (1990a) and Wei and Thierstein (1991), with the identification of several new biohorizons. New age calibrations are provided improving regional correlations and evaluating the reliability of the identified events for supraregional correlations. Comparisons are made between the proposed zonation and the existing schemes, both on nannofossils from different geographic areas, and on foraminifers from southern high latitudes. © 2012 Elsevier Masson SAS. All rights reserved. Keywords: Eocene; Oligocene; Calcareous Nannofossils; Biostratigraphy; Southern Ocean

Résumé Les nannofossiles calcaires sont largement utilisés dans les études biostratigraphiques des sédiments cénozoïques. Au cours des dernières années, de nombreuses données ont été obtenues mais plusieurs bioévénements n’ont pu être utilisés pour des corrélations à grandes distances. Notre étude a permis d’obtenir une nouvelle biozonation à haute résolution pour l’Eocène moyen-Oligocène supérieur pour l’océan austral, grâce à une analyse quantitative des assemblages des nannofossiles calcaires menée sur sept forages de l’Ocean Drilling Program (ODP). Les nouvelles données ont permis une révision de la zonation existante. Onze zones et six sous-zones sont présentées ou modifiées pour remplacer les dix zones et deux sous-zones de Wei et Wise (1990a) et Wei et Thierstein (1991). Plusieurs bioévénements supplémentaires ont été reconnus. Cette étude comprend une nouvelle calibration d’âge pour les bioévénements permettant ainsi d’améliorer les corrélations régionales. La zonation proposée ici est comparée aux autres biozones existantes, soit à nannofossiles (de différents secteurs géographiques) soit à foraminifères (pour les hautes latitudes méridionales). © 2012 Elsevier Masson SAS. Tous droits réservés. Mots clés : Éocène ; Oligocène ; Nannofossiles calcaires ; Biostratigraphie ; Océan Austral

1. Introduction

∗

Corresponding author. E-mail address: [email protected] (C. Fioroni).

0035-1598/$ – see front matter © 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.revmic.2012.03.001

The Southern Ocean (SO) is oceanographically quite distinct from the southern part of the three main oceans (Atlantic, Pacific, and Indian). Its northern boundary is conventionally traced at

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∼52◦ S, between the Antarctic Divergence (AD) (∼65◦ S) and the Sub Tropical Front (∼40◦ S), the natural boundaries of the Antarctic Circumpolar Current. Today, sea-surface temperature of the SO decreases by about 3 ◦ C from the Polar front (PF) (∼55◦ S) to the Antarctic continent. The vertical water-mass distribution around the continent results from a pronounced gradient in temperature and salinity between the Antarctic surface water (with winter temperatures close to 3 ◦ C) and the Antarctic Circumpolar Water, slightly warmer and more saline. The physical and chemical conditions of the SO compared to the mid and low latitudes are therefore well delineated. Calcareous nannoplankton, which live mainly in the upper photic zone, are directly influenced by sea-surface temperature and nutrient content (e.g., McIntyre and Bé, 1967; Haq and Lohmann, 1976). The current oceanographic setting of the SO did not develop until the opening of the oceanic seaways between Antarctica and South America, and between Antarctica and Tasmania. The timing of these tectonic events has not been clearly determined yet (Lawver and Gahagan, 2003; Livermore et al., 2005; Lagabrielle et al., 2009). Following the Paleocene-Eocene Thermal Maximum (PETM), a weak latitudinal climatic gradient throughout the Eocene greenhouse world existed during a long and gradual cooling (Miller et al., 1991; Zachos et al., 2001). Therefore, it is not surprising that, even in pre-ice sheet conditions, calcareous nannofossils were sensitive to water temperature changes and their biogeographical distribution was well delineated. Since the pioneer coring in the Antarctic sector of the South Atlantic, it has been evident that the widely used Eocene-Oligocene “standard” zonation of Martini (1971) and the low- to middle-latitude zonation of Okada and Bukry (1980) were not applicable in the high southern latitudes where some warm- and temperate-water markers were rare or absent. Consequently, Edwards (1971) first established a calcareous nannofossil zonation for the subantarctic Paleogene of New Zealand, and later Wise (1983) proposed a Cenozoic zonation for the Falkland Plateau. The first detailed calcareous nannofossil zonal scheme for the Eocene and the Oligocene for the Southern Ocean was developed by Wei and Wise (1990a) using well-preserved nannofossil assemblages from ODP Leg 113 (Weddell Sea). Biostratigraphic correlations between low and high latitudes were presented by Wei and Wise (1990b), who showed latitudinal diachrony in the distribution of some taxa when correlated with magnetic reversal stratigraphy. On this basis, Wei and Thierstein (1991) emended the previous zonation (Wei and Wise, 1990a) by adding a biozone (the Reticulofenestra bisecta Zone) in the late Oligocene. These studies provided an expanded database on species stratigraphic distribution and emphasized the importance of nannofossil assemblage studies in the Southern Ocean for high-latitude biostratigraphic correlations. Recent updates on the EoceneOligocene biostratigraphy at SO sites (e.g., Persico and Villa, 2004; Villa and Persico, 2006; Villa et al., 2008; Persico et al., in press; Pea, 2011) have pointed out the need for revision of the existing zonations. This revision is also based on the refinement of calcareous nannofossil taxonomy (Persico and Villa, 2008) and the recognition of several stratigraphic discrepancies in taxa distribution at southern latitudes (Crux, 1991; Aubry, 1992a; Arney and Wise, 2003). The new zonation proposed here

represents a base for paleoceanographic studies, and facilitates middle Eocene-upper Oligocene correlation between the SO and the lower latitudes. Problems in applying standard zonations were also encountered in mid-latitude sections (Mediterranean Sea and Atlantic Ocean) by Fornaciari et al. (2010), who proposed a new biostratigraphic framework for the middle-late Eocene by adding several new biohorizons. However, most of these bioevents were not found or reproducible in the SO. Biostratigraphic data discussed in this work are derived from a reanalysis of species distribution and from their correlation between ODP Sites 738, 748, and 744 (Kerguelen Plateau), ODP Sites 689 and 690 (Maud Rise), ODP Site 1172 (Tasman Plateau), and ODP Site 702 (Islas Orcadas Rise) (Fig. 1). This approach resulted in the delineation of a detailed nannofossil biostratigraphy framework (Fig. 2) that also highlights major hiatuses not detected previously in the middle Eocene through the upper Oligocene (∼43 to ∼25 Ma) of the region (Persico et al., in press). A comparison with previous high-latitude zonations (Wei and Wise, 1990a; Wei and Thierstein, 1991) (Fig. 3) reveals in greater detail the stratigraphic framework obtained in this work. The new biostratigraphic scheme will allow more reliable paleoecological interpretations of nannofossil Eocene and Oligocene assemblages, which in turn are used for paleoceanographic and paleoclimatic reconstructions of this crucial time interval. The new zonal scheme is intended to better constrain in time the paleogeographic changes and to add contributions to the debate about the timing of the opening of ocean gateways surrounding Antarctica. 2. Material and methods Several ODP sites were drilled in the SO, but only a few have a near complete record of the middle Eocene through the upper Oligocene because of incomplete recovery and/or the presence of hiatuses. The most complete sections were drilled at ODP Sites 738, 748, and 744 (Kerguelen Plateau), ODP Sites 689 and 690 (Maud Rise), ODP Site 1172 (Tasman Plateau), and ODP Site 702 (Georgia Basin) (Figs. 1 and 4). At most of these sections, the paleomagnetic signal is well preserved and reliable, and was interpreted by Roberts et al. (2003) at Sites 744 and 748, by Florindo and Roberts (2005) at Sites 689 and 690, by Stickley et al. (2004a) and Bijl et al. (2010) at Site 1172, by Ciesielski and Kristoffersen (1988), and Clement and Hailwood (1991) at Site 702. At Site 748 the magnetostratigraphic interpretation is available only from the upper Eocene Chron C13n upwards. A reliable magnetostratigraphy is not available for Site 738. The sites studied were also selected based on the availability of oxygen stable-isotope records (Salamy and Zachos, 1999; Stickley et al., 2004b; Villa et al., 2008; Bohaty et al., 2009), as some prominent oxygen-isotope shifts were used as tie points and chronostratigraphic horizons. The nannofossil dataset used in this work was acquired by quantitative analyses of samples from ODP Holes 744A, 689D and 690C (Persico and Villa, 2004), ODP Hole 748B (Villa et al., 2008), ODP Holes 738B, 1172A, 689B, and 690B (Persico et al., in press), and ODP Hole 702B (Pea, 2011). The simplified

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Fig. 1. Geographic location of ODP Sites included in this study.

lithology of the studied sections is shown next to the recovery in Fig. 4. Most of the middle Eocene-upper Oligocene intervals were sampled every 10 cm. The completed database consists of 2629 samples (for supplementary data see Persico and Villa, 2004; Villa et al., 2008; Persico et al., in press; Pea, 2011). Samples were prepared using the settling technique described by De Kaenel and Villa (1996) and examined by light microscope at 1250× magnification. Samples from ODP Site 702 were prepared as smear slides. In each slide, at least 300 to 500 specimens were counted to estimate species abundance and two additional long traverses of each slide were scanned to find rare taxa. Calcareous nannofossil species recognised in this work are listed in Appendix A. 3. Age model and biochronology The age of the bioevents was defined using available magnetostratigraphic records by applying a linear interpolation between nearest reversal boundaries. Clearly, the assumption of a constant sedimentation rate between Chron boundaries can lead to a slightly different age assignment for the bioevents at the sites studied. The middle-late Eocene magnetostratigraphic signal at Site 748 is considered unreliable (Roberts et al., 2003). As a consequence, an age model was constructed for this part of the section by calibrating the stable-isotope record of Site 748 with the magnetostratigraphic and isotopic record available at Site 689 (Villa et al., 2008). The shipboard magnetic polarity of

Site 738 has been considered ambiguous (Barron et al., 1991; Zachos et al., 1992). The age model presented here for Site 738 was constructed using as tie points the high resolution stable-isotope records for the Middle Eocene Climatic Optimum (MECO) event (Bohaty et al., 2009) and for the Eocene Oligocene Transition (EOT) (Scher et al., 2011), a spherule layer dated at 35.48 Ma (Liu et al., 2009), three nannofossil bioevents (Villa et al., 2008), and three foraminifera bioevents (Huber and Quillévéré, 2005) (Fig. 5). The complete age model proposed by Huber and Quillévéré (2005) for Site 738 was not used here as some nannofossil bioevents are considered unreliable (i.e., the lowest occurrence (LO) of Chiasmolithus oamaruensis and the highest occurrence (HO) of Chiasmolithus solitus). Nevertheless, in our age model, we also considered as tie points the HO of Subbotina linaperta. This event was considered unreliable by Huber and Quillévéré (2005) due to a misinterpreted hiatus based on the absence of the Discoaster saipanensis Zone (Wei and Thierstein, 1991) (see the next paragraph for discussion). Using our model, the calculated age of this foraminifer bioevent is in agreement with the age assigned by Berggren et al. (1995), confirming both the reliability of our age model and the absence of the hiatus (Persico et al., in press). At all studied sites, the ages of chron boundaries were assigned using the astronomically-tuned calibration of Pälike et al. (2006). Ages of nannofossil bioevents are presented in Table 1; calculated ages were also converted to the time scale

56 Table 1 Summary of the key nannofossil bioevents in the study sites with mean depths and estimated ages calibrated according to the time scale of Pälike et al. (2006). Only datums calibrated with magnetostratigraphy are dated also according to Berggren et al. (1995). Bioevent

702A Mean depth

Age (Pälike et al., 2006)

Age Mean (Berggren depth et al., 1995)

689D Age (Pälike et al., 2006)

Age Mean (Berggren depth et al., 1995) 72.35 108.83 113.17 117.52 120.36 121.65 129.18 130.47 131.68

73.15

71.15 74.65 74.20 98.05

40.209

40.019 40.359 40.313 42.481

40.241

40.063 40.373 40.333 42.482

689B Age (Pälike et al., 2006)

Age Mean (Berggren depth et al., 1995)

25.508 31.769 32.579 33.346 33 34.086 35.72 35 36.01

26.15 31.4 32.34 33.18 33.57 33.91 35.82 36.1 36.35

364.08 368.85 415.47

36.167 37.05 40.123

36.167 37 40.165

131.9 132.35 132.85 137.35 144.2

384.85

38.564

38.546

148.55

395.9 395.9

39.029 39.029

39.03 39.03

153.51 153.51

406.11

39.458

39.48

414.44

39.808

39.84

690B Age (Pälike et al., 2006)

36.082 36.135 36.19 36.845 37.5

Age Mean (Berggren depth et al., 1995)

36.09 36.14 36.19 36.80 37.453

90.04 93.05 93.05 93.05 93.05 93.05 93.05 93.05

Age (Pälike et al., 2006)

Age (Berggren et al., 1995)

32.81 Hiatus Hiatus Hiatus Hiatus Hiatus Hiatus Hiatus

32.657

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HO C. altus HCO R. umbilicus HO I. recurvus LO C. altus IB R. daviesii HO R. oamaruensis LO R. oamaruensis LCO I. recurvus HO C. reticulatum HO C. eoaltus HO N. dubius LO D. bisectus HO S. linapertaa LO I. recurvus HO C. solitus LCO C. oamaruensis HCO C. solitus LO C. oamaruensis LO C. eoaltus HO A. primitivaa HCO E. formosa HCO Discoaster spp. HO R. clatrata LO C. reticulatum LO R. umbilicus LO G. indexa

1172A

Table 1 (Continued ) Bioevent

a b

738A

Mean depth

Age (Pälike et al., 2006)

Age (Berggren et al., 1995)

57.75

25.80

26.35

Mean depth

748B Age (Pälike et al., 2006)

19.48 24.31 38.99 38.99 38.99 41.05 44.92 57.31 60.45

33.695 33.97 Hiatus Hiatus Hiatus 36.093 36.422 37473 37.74

61.89

37.884

70.99 70.99 73.65 82.47 94.49 100.05 101.47 145.15

38.794 38.794 39.06 39.491 40.127 40.561 40.672

Age (Berggren et al., 1995)

744A

Mean depth

Age (Pälike et al., 2006)

Age (Berggren et al., 1995)

Mean depth

Age (Pälike et al., 2006)

Age (Berggren et al., 1995)

71.6 105.94 109.01 112.95 115.76 115.86 125.20 127.48 128.35 130.35 134.34 143.90

25.58 31.51 32.49 33.31 33.76 33.97 35.54 35.77 35.92 36.23 36.70 37.59

26.2 31.52 32.46 33.30

128.11 135.45 141.05 146.98 147.85 165.35

31.523 32.68 33.357 33.75 33.901 35.744

31.4 32.55 33.19 33.605 33.71 35.78

148.25 149.45 149.25 151.35 155.01 155.81

37.98 38.08 38.06 38.34 38.80 38.88

157.11 165.99 170.01 171.15

39.02 40.08 40.56 40.69 hiatus

37.70

39.06b

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HO C. altus HCO R. umbilicus HO I. recurvus LO C. altus IB R. daviesii HO R. oamaruensis LO R. oamaruensis LCO I. recurvus HO C reticulatum HO C. eoaltus HO N. dubius LO D. bisectus HO S. linapertaa LO I. recurvus HO C. solitus LCO C. oamaruensis HCO C. solitus LO C. oamaruensis LO C. eoaltus HO A. primitivaa HCO E. formosa HCO Discoaster spp. HO R. clatrata LO C. reticulatum LO R. umbilicus LO G. indexa

690C

44.08b

Datums from Huber and Quillévéré (2005). Recalibrated from Huber and Quillévéré (2005).

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Fig. 2. Revised Antarctic Eocene/Oligocene Zonation proposed in this study. Nannofossil datums are plotted against the GPTS with magnetic reversal calibrated according to Pälike et al. (2006).

of Berggren et al. (1995) to facilitate the comparison with the literature (Table 1). 4. Southern Ocean middle Eocene-upper Oligocene Biozonation The reliability of a bioevent is defined by its degree of synchroneity over a wide paleogeographic area. As long noted, several nannofossil markers used in the zonations of Martini (1971) and Okada and Bukry (1980) are rare or absent at high latitudes. Using a compendium of all the biostratigraphic data from our previous studies at SO ODP sites (see references above) and comparing our records with datasets from other latitudes, we found that several bioevents can be recognized and easily correlated at a global scale, while other bioevents are only reproducible at a regional scale. Our database led to the compilation of a new middle Eocene-upper Oligocene biozonation of the Southern Ocean based mainly on the species distributions and biostratigraphic events at ODP sites located between 65◦ and 50◦ S paleolatitude (Fig. 1) (Persico and Villa, 2004; Villa and Persico, 2006; Villa et al., 2008; Persico et al., in press; Pea, 2011) taking in account the fact that Site 1172, presently located

at 44◦ S latitude, was between ∼65◦ and ∼60◦ S paleolatitude during the time in question (Exon et al., 2001). The used bioevents are labelled as Lowest Occurrence (LO), Lowest Common Occurrence (LCO), Highest Occurrence (HO), Highest Common Occurrence (HCO) and Increase Beginning (IB). Eleven zones and six subzones are described (Figs. 2 and 4), together with a number of additional biohorizons, intended to provide a high-resolution subdivision for the Eocene-Oligocene of the Southern Ocean. Utilized index species are documented in Plates 1 and 2. Our approach was to follow as much as possible the Wei and Wise (1990a) Zonation. Nevertheless, ten zone and subzone boundaries are redefined, and six additional bioevents are introduced. From the middle Eocene to the late Oligocene the following biozones are described: Reticulofenestra umbilicus Zone, Wei and Wise (1990a). Base: LO of R. umbilicus. Top: LO of Cribrocentrum reticulatum. Range: middle Eocene. Remarks: At Site 702, the base of this Zone (LO of R. umbilicus) was detected at 42.481 Ma (98.05 mbsf). This

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Fig. 3. Middle Eocene-upper Oligocene biozonation proposed in this study correlated with the high-latitude biozonations of Wei and Wise (1990a) and Wei and Thierstein (1991), the low-to middle-latitude biostratigraphies of Martini (1971) and Okada and Bukry (1980), and the high latitude foraminiferal biozonation of Huber and Quillévéré (2005). Zonal boundary correlations indicate a relative position between the zonal schemes and have not a absolute age connotation.

event falls within the lower part of Chron C19r, in general agreement with previous records from mid and high latitudes (Poore et al., 1983; Backman, 1987; Pospichal and Wise, 1990; Berggren et al., 1995; Lupi and Wise, 2006) (Fig. 4). At Site 1172, R. umbilicus was discontinuous near the base of Chron C18r but this site is strongly affected by dissolution below the middle part of Chron C18n.2n (Wei, 2004; Stickley et al., 2004a). As a consequence, the LO occurrence of R. umbilicus cannot be pinpointed at this site. Cribrocentrum reticulatum-Ericsonia formosa Concurrentrange Zone, Wei and Wise (1990a), emended this paper. Base: LO of C. reticulatum. Top: HCO of E. formosa. Range: middle Eocene.

Remarks: The C. reticulatum Zone of Wei and Wise (1990a) is here emended: we use as the upper boundary the HCO of E. formosa instead of the HO of Chiasmolithus solitus. This zone can be subdivided into three subzones. Cribrocentrum reticulatum-Reticulofenestra clatrata Concurrent-range Subzone (defined here). Base: LO of C. reticulatum. Top: HO of R. clatrata. Range: middle Eocene. Remarks: At ODP Site 738, the LO of C. reticulatum was found at 101.47 mbsf, with an estimated age of 40.672 Ma on the basis of our proposed age model (Fig. 5; Table 1). At ODP Site 748, the age of this bioevent is uncertain because of the presence of a hiatus (Fig. 6). At Site 1172, it was found in the middle

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Fig. 4. Biostratigraphic correlations among the sites studied. For every site, simplified lithologic and recovery logs are shown. The GPTS magnetic reversal ages are calibrated according to Pälike et al. (2006).

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Plate 1. Fig. 1. Chiasmolithus altus; crossed nicols. Sample 748B 10H 4 84-85. Fig. 2. Chiasmolithus altus; crossed nicols. Sample 748B 9H-4 53-54. Fig. 3. Chiasmolithus solitus; phase contrast. Sample 738A 10H 2 110-111. Figs. 4, 5. Chiasmolithus solitus; crossed nicols. Sample 738A 10H 2 110-111. Figs. 6, 7. Chiasmolithus oamaruensis; crossed nicols. Sample 738A 8H 7 36-38. Fig. 8. Chiasmolithus oamaruensis; crossed nicols. Sample 748B 16H 4 55-56. Fig. 9. Chiasmolithus eoaltus; crossed nicols. Sample 738A 5H 6 132-134. Figs. 10–13. Chiasmolithus eoaltus; crossed nicols. Sample 738A 8H 7 36-38. Figs. 14, 15. Isthmolithus recurvus; crossed nicols. Sample 738A 7H 4 86-88. Fig. 16. Neococcolithes dubius; crossed nicols. Sample 738A 8H 7 36-38. Fig. 17. Reticulofenestra oamaruensis; crossed nicols. Sample 738A 4H 3 116-118. Figs. 18, 19. Dictyococcites bisectus; crossed nicols. Sample 738A 8H 7 36-38. 20. Cribrocentrum reticulatum; crossed nicols. Sample 7H 3 82-83. All specimens at 1250×.

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Plate 2. Figs. 1–5. Cribrocentrum reticulatum; crossed nicols. Sample 738A 7H 3 82-83. Figs. 6–8. Reticulofenestra clatrata; crossed nicols. Sample 748B 20H 2 66- 68. Fig. 9. Ericsonia formosa; crossed nicols. Sample 738A 12H 3 101.5-103.5. Fig. 10. Ericsonia formosa; phase contrast. Sample 738A 12H 3 101.5-103.5. Figs. 11–13. Reticulofenestra daviesii; crossed nicols. Sample 748B 20H 1 26- 28. Figs. 14–16. Reticulofenestra umbilicus; crossed nicols. Sample 738A 12H 5 58-60. Fig. 17. Cyclicargolithus floridanus; crossed nicols. Sample 738A 12H 5 58-60. Fig. 18. Dictyococcites scrippsae; crossed nicols. Sample 738A 5H 6 132-134. Fig. 19. Zygrhablithus bijugatus; crossed nicols. Sample 738A 11H 3 81-83. Fig. 20. Coccolithus pelagicus; crossed nicols. Sample 738A 11H 3 81-83. All specimens at 1250×.

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discussed in Villa et al. (2008), who pointed out that this species probably corresponds to Reticulofenestra onusta of Wei and Wise (1990a). We consider the characteristics of the observed specimens as corresponding to the R. clatrata of Muller (1970), who reported this species from early Oligocene sediments from Germany. However, our data suggest that the stratigraphic distribution of R. clatrata described by Muller (1970) may have been determined by reworked specimens, or alternatively its distribution may be latitudinally controlled.

Fig. 5. Age-depth curve constructed for ODP Site 738 based on reliable tiepoints (bold): Foraminifera datums taken from Huber and Quillévéré (2005) (1); MECO event from Bohaty et al. (2009) (2); Nannofossils datums from Villa et al. (2008) (3); spherule layer from Liu et al. (2009) (4); Oi-1 event from Scher et al. (2011) (5). See Table 1 for the ages assigned to the bioevents.

part of Chron C18n.2n (39.808 Ma), in agreement with Wei (2004). However, this datum should be considered with caution at Site 1172, due to the strong carbonate dissolution affecting the section. Some uncertainties arise from the biostratigraphic analysis at Site 702, where the LO of C. reticulatum is found at 40.313 Ma (74.20 mbsf). In fact, at this site many specimens showing the same size and outline of C. reticulatum, but lacking the central cross, were found, preventing a unambiguous assignment. Therefore, the precise stratigraphic position of the LO of C. reticulatum could not be determined. R. clatrata was identified at Sites 748, 702, and 738. Its HO is estimated to have occurred at 40.56 Ma, 40.36 Ma, and 40.56 Ma, respectively. Even though this species is constantly present in low abundances, its distribution is considered useful here for regional correlations. At Site 1172, this event was not found, probably because of the strong dissolution affecting the assemblages. The definition of R. clatrata was

Chiasmolithus expansus Partial-range Subzone (defined here). Base: HO of R. clatrata. Top: HCO of Discoaster spp. Range: middle Eocene. Remarks: The lower boundary of this subzone is discussed above. The upper boundary is defined by the HCO of rosette-shaped Discoaster spp. Discoasters are considered to be oligotrophic/warm-water (Kelly et al., 1996; Bralower, 2002; Kahn and Aubry, 2004; Tremolada and Bralower, 2004; Dunkley Jones et al., 2008; Villa et al., 2008) and dissolutionresistant taxa (Perch-Nielsen, 1985; Raffi and De Bernardi, 2008; Raffi et al., 2009). Their early regional disappearance at high southern latitudes has been interpreted as a paleoecological bioevent plausibly related to the combined effect of sea-surface temperature and eutrophication across the MECO (Villa et al., 2008; Luciani et al., 2010; Pea, 2011). At ODP Sites 738, 748 and 702, the HCO of Discoaster spp. occurs in the same stratigraphic position, just below the negative peak that in oxygen isotopes (warming peak) marks the MECO event (Bohaty et al., 2009). Following this event at these sites Discoaster abundance drops below 0.5% of the total assemblage. Conversely, at Site 1172 the strong carbonate dissolution affecting the assemblages below 414 mbsf makes Discoaster distribution difficult to interpret. Within this subzone, C. expansus is constantly present. Sphenolithus moriformis Partial-range Subzone (defined here). Base: HCO of Discoaster spp. Top: HCO of E. formosa. Range: middle Eocene. Remarks: Similarly to the HCO of Discoster spp. discussed above, the HCO of E. formosa is only significant on a regional scale. Being that E. formosa is considered to be a warm-water taxon (Wei and Wise, 1990a, 1990b; Aubry, 1992b; Kelly et al., 1996; Bralower, 2002; Tremolada and Bralower, 2004; Agnini et al., 2006; Villa et al., 2008), we suggest that its HCO may have been influenced by the cooling that followed the MECO event. At high southern latitudes, the HCOs of Discoaster spp. and E. formosa are consistently found at a lower level (middle Eocene), thus they are considered to be reliable regional bioevents. Their early disappearance here relative to lower latitudes could be related to the reorganization of the water masses, which involved an increase in nutrient availability and the onset of unfavorable conditions for the oligotrophic discoasters (Pea, 2011).

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Fig. 6. Time intervals for the investigated sites. For every site dark grey intervals are magnetostratigraphically calibrated. Identified hiatuses are marked by wavy lines.

The HCO of E. formosa was dated at 39.458 Ma at Site 1172, 39.02 Ma at Site 748, and 39.49 Ma at Site 738. This event is, therefore, significantly older at southern high latitudes than at low latitudes where the HO of E. formosa was dated at 32.919 Ma by Blaij et al. (2009). In the sites mentioned above, after its HCO, the relative abundance of E. formosa drops below 0.5% and shows a scattered distribution. In the interval following the HCO of E. formosa, S. moriformis shows several abundance peaks, reaching at Site 748 values as high as 7% of the total assemblage. Ericsonia formosa Interval Zone Wei and Wise (1990a), emended this paper. Base: HCO of E. formosa. Top: LO of C. oamaruensis. Range: middle Eocene. Remarks: The top of this zone was defined by Wei and Wise (1990a) as the LO of C. oamaruensis. However, at all of the sites studied (see also Villa et al., 2008; Persico et al., in press), this LO occurred at a lower level than previously reported, resulting

in a longer stratigraphic range of the zonal marker. In fact, the LO of C. oamaruensis is dated at 38.80 Ma at Site 748, 38.79 Ma at Site 738, and 39.029 Ma at Site 1172, while this event was found in Chron C17 (from 38.159 to 36.668) in previous works (Poore et al., 1983; Wei and Wise, 1989; Berggren et al., 1995). The LO’s of C. oamaruensis and C. eoaltus, are present at the same stratigraphic level at Sites 738, 748, 689, and 1172 (Villa et al., 2008; Persico et al., in press). The LO of C. eoaltus is considered to be a good additional biohorizon in the SO. Chiasmolithus oamaruensis Interval Zone Wei and Wise (1990a), emended this paper. Base: LO of C. oamaruensis. Top: LCO of I. recurvus. Range: middle-late Eocene. Remarks: The lower boundary of this zone is discussed above. As upper boundary we use the LCO of I. recurvus instead of its LO. In fact, at Hole 748B, we found the LO of this marker in a lower position compared to Wei and Wise (1990a), as discussed in detail below.

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This zone can be subdivided into three subzones: Chiasmolithus oamaruensis/Chiasmolithus solitus Concurrent-range Subzone (defined here). Base: LO of C. oamaruensis. Top: HO of C. solitus. Range: middle Eocene. Remarks: At mid and low latitudes, the Discoaster saipanensis Zone is defined at the base by the HO of C. solitus and at the top by the LO of C. oamaruensis. However, at high southern latitudes these two species show an inverse stratigraphic order probably due to an extended range of both species. Previous studies noted that the LO of C. oamaruensis is significantly older in the SO than at low latitudes (Wei and Wise, 1990b), and that the HO of C. solitus is significantly younger (Wise and Mostajo, 1983; Wei and Wise, 1989). Arney and Wise (2003) suggested that “C. solitus might have persisted in the high latitudes after its disappearance at low latitudes”, and as a consequence, in the Southern Ocean, the D. saipanensis Zone would have a reduced range when compared with low latitudes (Wei et al., 1992). Crux (1991) did not identify the D. saipanensis Zone at Sites 699, 702 and 703 and, to explain the juxtaposition of the HO of C. solitus with the LO of C. oamaruensis, he suggested either the presence of a regional hiatus, or the possibility that specimens of C. solitus found at a stratigraphic level younger than the “traditional” HO, had been reworked. Our data clearly show that the ranges of C. solitus and C. oamaruensis overlap in all of our study sections, and therefore the D. saipanensis Zone is not represented at high southern latitudes (Figs. 3 and 5). At Site 748, C. solitus and C. oamaruensis ranges are in line with the other sites. In addition, both the HCO of C. solitus and the LCO of C. oamaruensis are easily detectable in a position close to their relative HO and LO (Villa et al., 2008). At the other studied sites, the identification of the HCO of C. solitus and the LCO of C. oamaruensis are not so clear, probably because of the lower sampling resolution and/or the dissolution effect. Nevertheless, we consider the HCO of C. solitus and the LCO of C. oamaruensis as possible useful events. We believe that in the zonation of Wei and Wise (1990a), the HO of C. solitus and the LO of C. oamaruensis were positioned in coincidence of their HCO and LCO, respectively. Ages assigned to the biohorizons discussed above are shown in Table 1. Neococcolithes dubius Interval Subzone (defined here). Base: HO of C. solitus. Top: HO of N. dubius. Range: middle-late Eocene. Remarks: Villa et al. (2008) proposed the HO of N. dubius as an additional upper Eocene bioevent in the SO, as previously suggested by Wei and Wise (1990a), Madile and Monechi (1991), and Marino and Flores (2002). This event was also used at high northern latitudes by Sheldon (2003). Varol (1998) used the HO of Neococcolithes spp. to mark the base of Zone NNTe11B. As a consequence, this event can be considered useful for supraregional correlations. N. dubius was detected in all of our study sites, and its HO is easily recognisable even if this

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species is constantly present in low abundances or if overgrowth occurs. At the Maud Rise and Kerguelen Plateau sites, Dictyococcites bisectus has its LO within this subzone occurring at Site 748 at 37.59 Ma (143.90 mbsf), at Site 689B at 37.45 Ma (144.20 mbsf), and at Site 738 at 37.47 Ma (57.31mbsf). However, at Sites 1172, 702, and at Site 699 (Crux, 1991), the LO of this species is recorded within the upper part of Chron C18r, and is dated 40.12 Ma at Site 1172 and 40.20 Ma at Site 702. Wei and Wise (1990b) noted this time-transgressive behavior over different latitudes, which was interpreted as driven by different environmental conditions (Wei and Wise, 1990a; Wei et al., 1992). These inconsistencies have prevented the use of this LO as a zonal marker at the southern high latitudes. Fornaciari et al. (2010) found similar results in the Central Tethys (40.59 Ma) and Western Atlantic (40.35 Ma) successions. However, it must be noted that Fornaciari et al. (2010) consider as D. bisectus specimens larger than 10 ␮m, while the holotype of Hay et al. (1966) is smaller than 10 ␮m. Consequently, we follow this latter criterion and classify forms bigger that 10 ␮m as Dictyococcites stavensis. Within this subzone, at Holes 689B and 748B, C. oamaruensis increases in abundance. At Site 1172, this increase was recorded in a slightly lower position, at the top of the C. solitus subzone. At Site 748, probably because of high-resolution sampling, rare but unmistakable specimens of I. recurvus were found within this subzone at 37.98 Ma (Villa et al., 2008), highlighting the early presence of this species. The LO of I. recurvus has been only recorded at Site 748, and not at the other sites studied here, probably because of the lower resolution sampling. This event was found in Chron C17n at South Atlantic Site 523 (Backman, 1987), in Chron C17n.1n in some Italian Alpine sections and in Chron C17n.1r-Chron C17n.1n (base) on the Blake Nose at Site 1052 (Fornaciari et al., 2010; Agnini et al., 2011). These data emphasize a small diachrony over different latitudes. We consider the LO of I. recurvus as an useful additional event recognizable at high-resolution sampling. Nevertheless, it has to be used with caution in supraregional correlations. Zygrhablithus bijugatus Partial-range Subzone (defined here). Base: HO of N. dubius. Top: LCO of I. recurvus. Range: late Eocene. Remarks: The HO of Chiasmolithus eoaltus (Persico and Villa, 2008) was recorded within this zone at Holes 1172A, 689B, 738B, and 748B, while at Hole 690B it coincides with a major hiatus separating the lower part of Chron C16n.2n from the lower part of Chron C13n (Fig. 6). C. eoaltus was previously assigned to C. altus (Wei and Wise, 1990a; Wei and Thierstein, 1991; Firth and Wise, 1992; Wei, 2004) from which it is distinguished by the larger central opening, the thinner rim, and the cross-bar structure not perfectly aligned with the major and minor axes (one cross-bar is slightly sigmoidal). The HO of C. eoaltus is considered to be a reliable regional additional

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event even if sometimes it cannot be easily detected due to low abundance. The HO of C. reticulatum was recognised within this subzone at Holes 689B, 689D, and 748B, while at Holes 690B and 738B it is not reliable because of the presence of several unconformities (Fig. 6). This event is diachronous with respect to the middle latitudes (Fornaciari et al., 2010). Isthmolithus recurvus Interval Zone, Martini (1971), emended Wei and Wise (1990a), and this paper. Base: LCO of I. recurvus. Top: LO of Reticulofenestra oamaruensis. Range: late Eocene. Remarks: We emend this zone using as base the LCO of I. recurvus. In fact the LO marks the base of a very short interval of presence followed by a ∼2 My interval of absence until the re-entrance with continuous occurrence (with peaks of 4-5%) used here. We report this event at Sites 748 and 689, since at Sites 690 and 738 it falls within a hiatus (Persico et al., in press). The estimated age of this event at Site 689 is 35.87 Ma (Table 1), comparable with the age obtained at Site 748 (35.77 Ma) (Villa et al., 2008). These data are in agreement with Fornaciari et al. (2010) who reported the LCO of I. recurvus within Chron C16n.2n with an age of 36.032 Ma (following Cande and Kent, 1995) in the Massignano section; however at ODP Site 1052 (Blake Nose, North Atlantic) the same event is observed in the upper part of Chron C17n.1n (∼36.7 Ma). We can conclude that the LCO of I. recurvus is reliable within the high latitudes, but supraregional correlations are problematic. Fornaciari et al. (2010) proposed to use the LO of Cribrocentrum isabellae as a proxy for the LCO of I. recurvus. C. isabellae is distinguished from C. reticulatum primarily by its larger size (12 ␮m). A gradual size increase within the genus Cribrocentrum was noted in our Southern Ocean sites, and at South Atlantic sites by Wei and Wise (1990b). However, a specific biohorizon, marked by a size change, was not identified here. The top of this zone is defined by the LO of R. oamaruensis, which is quite rare, especially at the beginning of its range. Moreover, the central area of this species is sometimes missing, and broken specimens can be confused with Chiasmolithus rims. For these reasons, the LO of R. oamaruensis should always be defined using a high number of specimens/sample. At Site 738, it was noted that, in the lower part of its range, R. oamaruensis is significantly smaller (∼8 ␮m) than the holotype (14–17 ␮m). Biometric studies are in progress at other SO sites to confirm if the evolutionary trend of increasing size is synchronous, and to verify if the size change can be used as a reliable bioevent. Reticulofenestra oamaruensis Taxon-range Zone, Wei and Wise (1990a). Base: LO of R. oamaruensis. Top: HO of R. oamaruensis. Range: late Eocene. Remarks: The biostratigraphic value of R. oamaruensis is widely accepted. Its HO is used to approximate the

Eocene/Oligocene boundary at high latitudes (Wei and Thierstein, 1991) as a substitute for the HO of the large-bodied rosette-shaped discoaster (e.g., D. saipanensis). The majority of studies on Eocene-Oligocene nannofossils from the Northern Hemisphere did not record R. oamaruensis (Wei, 1991), being basically a species that is distributed in the southern high latitudes (Berggren et al., 1995). The HO of R. oamaruensis is isochronous between SO sites (Wei, 1991). We found this event near the top of Chron C13r at all sites studied, just below the Eocene/Oligocene boundary (33.79 Ma), which was placed by calculating the sedimentation rate between Chron boundaries. The R. oamaruensis Zone is not represented at Hole 690B due to the presence of a major hiatus (Fig. 6). An impact event is recorded at Sites 689 and 738 (Liu et al., 2009) within this biozone. The effects of such events on living organisms are still unclear, but our data show that nannoplankton assemblages were not significantly affected by the late Eocene impact. Reticulofenestra samodurovii Partial-range Zone (defined here). Base: HO of R. oamaruensis. Top: LO of Chiasmolithus altus. Range: early Oligocene. Remarks: The upper boundary of this zone is placed on the basis of the recently re-defined biostratigraphic distribution of C. altus (Persico and Villa, 2008). This zone correlates with the lower part of the Blackites spinosus Zone of Wei and Wise (1990a) (Fig. 3), within the zone R. samodurovii can reach values of 6%. Near the base of this zone a sharp increase of Reticulofenestra daviesii begins perduring for most of the Oligocene and, being easily recognizable at all the studied sites, it can be used as an additional marker at high latitudes. This event coincides with the top of Chron C13r at Sites 689, 744 and 748, where a reliable magnetostratigraphy is available (Figs. 4 and 6). The sharp increase (IB) in abundance of the cool-water taxon R. daviesii has been related to the global deterioration in climate that occurred at the EOT (Pearson et al., 2008). At all sites, the IB of this species occurs synchronously with Oi-1 (Step 2) (Pearson et al., 2008) of the oxygen isotope record, recently recognized also at Site 738 (Scher et al., 2011). Blackites spinosus Partial-range Zone Wise (1983), emended Wei and Wise (1990a), emended this paper. Base: LO of C. altus. Top: HO of I. recurvus. Range: early Oligocene. Remarks: We herein emend the definition of the B. spinosus Zone of Wei and Wise (1990a) to define its base, as the LO of C. altus instead of the HO of R. oamaruensis. The age of the LO of C. altus has long been matter of discussion, varying from the middle Eocene to the early Oligocene (Wei and Wise, 1990a; Wei, 2004). A recent biometric and biostratigraphic study (Persico and Villa, 2008) has better defined the stratigraphic

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range of C. altus, and clarified the differences between this species and C. eoaltus, which has a short stratigraphic range in the middle-late Eocene. The LO of C. altus is well defined both at Maud Rise (Hole 689D) and at Kerguelen Plateau (Sites 748 and 744) (Fig. 4; Table 1). At Hole 690B, this event is not detectable as it falls within a hiatus (Figs. 4 and 6). The top of this zone is defined by the HO of I. recurvus. This event was found at Holes 748B, 744A, 689D and 690B (Fig. 4). The HO of I. recurvus has been previously considered to be a good biostratigraphic event for middle (Premoli Silva et al., 1988; De Kaenel and Villa, 1996) and high latitudes (Wei and Wise, 1990a, 1990b). Reticulofenestra daviesii Interval-range Zone, Wise (1983) emended Wei and Wise (1990a), emended this paper. Base: HO of I. recurvus. Top: HCO of R. umbilicus. Range: early Oligocene. Remarks: The upper boundary of this zone is defined here by the HCO of R. umbilicus instead of its HO. The HO of R. umbilicus is the primary criterion used to define the top of the R. daviesii Zone of Wei and Wise (1990a) (Fig. 3). The identification of the HCO is easier compared to the HO, suggesting its use as a more reliable zonal boundary. The HCO of R. umbilicus is picked where its abundance drops below ∼0.5% of the total assemblage. Previous assignments at sites 744 and 689 (Persico and Villa, 2004) would be better labelled as HCO as scattered rare presence, which is not considered reworked, continue after this level as at Site 748. Chiasmolithus altus Interval Zone, Wise (1983), emended this paper. Base: HCO of R. umbilicus. Top: HO of C. altus. Range: early-late Oligocene. Remarks: The top of this zone is the HO of C. altus (following Wei and Wise [1990a]) and its base corresponds to the HCO of R. umbilicus, discussed above. At Holes 748B, 689D, and 690C, the HO of C. altus is a clearly detectable event (Fig. 4), herein considered to be a reliable marker for the southern high latitudes and more easily traceable than the HCO as proposed by Wise (1983). Dictyococcites bisectus interval Zone, Wise (1983), emended Wei and Thierstein, 1991. Base: HO of C. altus. Top: HO of D. bisectus. Range: late Oligocene. Remarks: The base of this zone is discussed above. Its top was not investigated since our study sections do not extend to the top of the Oligocene. However, at Sites 744 and 748, this bioevent was located by Roberts et al. (2003) within Chron C6Cn. The HO of D. bisectus is considered latitudinally controlled (Raffi et al., 2006), but it is nonetheless useful at high latitudes for regional correlations (Perch-Nielsen, 1985).

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5. Conclusions Abundant and well-to-moderately-preserved nannofossil assemblages from seven Southern Ocean ODP sites have enabled the detection of traditional and new datums, which has led to a revision of the existing zonal schemes for the Southern Ocean. The revised zonal framework and the age assignment of the biostratigraphic events will facilitate correlations of Paleogene carbonate sequences within and outside the Southern Ocean. We propose 15 bioevents for the time interval from ∼43 to 25 Ma (Fig. 2). Most of these events are reliable and easily traceable within the high southern latitudes; six others, reported as additional bioevents, are considered less utilitarian, mainly because of the scarcity of the taxa or because of taxonomic controversies. The proposed zonal scheme offers an average biostratigraphic resolution of ∼800 ky over a ∼18 Ma time interval, which significantly improves previous subdivisions. The nannofossil bioevents were calibrated by using available magnetostratigraphic records, providing an improved biochronologic framework for the studied time interval. Some of the Paleogene biostratigraphic datums used here show more restricted or expanded stratigraphic ranges at high latitudes compared to low and middle latitudes. In particular, the ranges of C. solitus and C. oamaruensis are more extended at high latitudes, and those of Discoaster spp. and E. formosa appear reduced, most probably in response to different paleoclimatic conditions. Thus, the long cooling trend following the MECO event (∼40 Ma) is considered responsible for the early exclusion of warm-water taxa (Discoaster and E. formosa) from the southern high latitudes as well as for their time-transgressive behaviour between low and high latitudes. The different latitudinal distributions of these taxa are shown to be regionally reproducible, and therefore of biostratigraphic value. Furthermore, some of the bioevents used in the low-middle latitude zonations or in the recent paper of Fornaciari et al. (2010) are not present. For example, the late middle Eocene re-entry of large Dictyococcites or the middle-late Eocene Cribrocentrum erbae acme and the late Eocene LCO of C. isabellae were not noted at our Southern Ocean sites. We sought to correlate the high-latitude foraminifers (Huber and Quillévéré, 2005) with our new calcareous nannoplankton biozonation. A more detailed correlation of the calcareous plankton groups is the object of future research to further refine the biostratigraphic framework for the SO and to obtain an integrated biochronology for this region. The proposed scheme, besides increasing biostratigraphic resolution, clarifies some long discussed enigmas (e.g., the D. saipanensis Zone), and improves the correlation between high and low-mid latitude nannofossil biostratigraphy. Acknowledgments We are grateful to Taniel Danelian, Joerg Mutterlose, Jean Self-Trail and an anonymous referee for their constructive reviews of this manuscript. Samples utilized in this study were provided by the Ocean Drilling Program (ODP). Funding for this

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research was provided by MIUR/PRIN COFIN 2007 (I. Premoli Silva). Appendix A. Taxonomic list Blackites spinosus (Deflandre and Fert, 1954) Hay and Towe (1962) Chiasmolithus Hay, Mohler and Wade (1966) Chiasmolithus altus Bukry and Percival (1971) Chiasmolithus eoaltus Persico and Villa (2008) Chiasmolithus expansus (Bramlette and Sullivan, 1961) Gartner (1970) Chiasmolithus grandis (Bramlette and Riedel, 1954) Radomski (1968) Chiasmolithus oamaruensis (Deflandre, 1954) Hay, Mohler and Wade (1966) Chiasmolithus solitus (Bramlette and Sullivan, 1961) Locker (1968) Cribrocentrum reticulatum (Gartner and Smith, 1967) Perch-Nielsen (1971) Dictyococcites bisectus (Hay, Mohler and Wade, 1966) Roth, 1970 Discoaster Tan Sin Hok (1927) Discoaster saipanensis Bramlette and Riedel (1954) Ericsonia formosa (Kamptner, 1963) Haq (1971) Isthmolithus recurvus Deflandre (1954) Neococcolithes dubius (Deflandre, 1954) Black (1967) Reticulofenestra clatrata Müller, 1970 Reticulofenestra daviesii (Haq, 1968) Haq, 1971 Reticulofenestra oamaruensis (Deflandre in Deflandre and Fert, 1954) Stradner in Haq (1968) Reticulofenestra onusta (Perch-Nielsen) Wise, 1983 Reticulofenestra samodurovii (Hay, Mohler and Wade, 1966) Roth (1970) Reticulofenestra umbilicus (Levin, 1965) Martini and Ritzkowski (1968) Sphenolithus moriformis (Brönnimann and Stradner, 1960) Bramlette and Wilcoxon (1967) Zygrhablithus bijugatus Deflandre in Deflandre and Fert, 1954

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