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Revue de micropaléontologie 49 (2006) 199–214

Original article

Maastrichtian calcareous nannofossil biostratigraphy and paleoecologyin the Equatorial Atlantic (Demerara Rise, ODP Leg 207 Hole 1258A)

Biostratigraphie et Paléoécologie des nannofossilescalcaires du Maastrichtien dans l’Atlantique équatorial(Plateau de Demerara, Leg ODP 207, Site 1258A)

Nicolas Thibault*, Silvia Gardin

Paléobiodiversité et paléoenvironnements, UMR 5143 du CNRS, département de géologie sédimentaire,université Pierre et Marie-Curie Paris-VI, case 104, 4, place Jussieu, 75252 Paris cedex 05, France

Abstract

Species richness, absolute and relative abundances of Maastrichtian calcareous nannofossils were analyzed in Hole 1258A. Absolute abun-dances of taxa were used for the biozonation of this hole and the record of successive bio-events. Distribution patterns of cool water taxa such asAhmuellerella octoradiata, Gartnerago segmentatum and Kamptnerius magnificus underscore two major cooling events. These events occurredduring Chron C31r and during undifferentiated Chrons C31n to C30n. The latter event appears to be accompanied by lowered productivity, asexpressed by the lower abundances of fertility index taxa Biscutum constans and Zeugrhabdotus spp. An end-Maastrichtian warming event isrecorded in Chron C29r by the sudden drop in abundance of cool water taxa and acme of tropical taxon Micula murus. The disappearance ofB. constans just before this event and the significant increase of Micula decussata, respectively, suggest lowered productivity and high-stressenvironmental conditions prior to the K/P boundary (Cretaceous–Paleogene). These results correlate very well with the climatic scenario pro-posed by Li and Keller (1998a) in South Atlantic Hole 525A as suggested by stable isotopes and distribution patterns of planktic foraminifera.© 2006 Elsevier Masson SAS. All rights reserved.

Résumé

La richesse spécifique, les abondances absolues et relatives des nannofossiles calcaires du Maastrichtien ont été analysées sur le site 1258A.L’abondance absolue de certains taxons nous a permis d’établir la biozonation de ce site et d’enregistrer plusieurs bioévénements supplémentai-res. La distribution des taxons d’« eaux froides » Ahmuellerella octoradiata, Gartnerago segmentatum et Kamptnerius magnificus soulignentdeux événements de refroidissement accru au cours du Maastrichtien. Le premier est situé au sein du Chron C31r. Le second est situé au sein del’intervalle indifférencié C31n à C30n. La plus faible abondance des taxons indices de fertilité Biscutum constans et Zeugrhabdotus spp. au coursde ce second événement froid suggère une plus faible productivité. Un événement de réchauffement accru est enregistré dans le Chron C29r parla soudaine chute d’abondance des taxons d’« eaux froides » et par l’acmé du taxon tropical Micula murus. La disparition de B. constans justeavant cet événement et la forte augmentation de Micula decussata suggèrent respectivement une basse productivité et des conditions environne-mentales stressantes précédant la limite K/P (Crétacé-Paléogène). Ces résultats concordent avec l’évolution climatique proposée par Li et Keller(1998a) sur le Site Sud-Atlantique 525A à partir des isotopes stables et des caractéristiques des foraminifères planctoniques.© 2006 Elsevier Masson SAS. All rights reserved.

Keywords: Calcareous nannofossils; Maastrichtian; Equatorial Atlantic; Biostratigraphy; Paleoecology; Paleoclimate

Mots clés : Nannofossiles calcaires ; Maastrichtien ; Atlantique équatorial ; Biostratigraphie ; Paléoécologie ; Paléoclimat

* Corresponding author.E-mail address: nithibau@ccr.jussieu.fr (N. Thibault).

0035-1598/$ - see front matter © 2006 Elsevier Masson SAS. All rights reserved.doi:10.1016/j.revmic.2006.08.002

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Fig. 1. Bathymetric position of Hole 1258 drilled on Demerara Rise duringODP Leg 207. The boxed area (down right) allows to situate Demerara Rise on

1. Introduction

Until recently, Maastrichtian time was mostly recognized asa period of progressive global cooling (Douglas and Savin,1973; Arthur et al., 1985; Barrera et al., 1997). The focus onthe K/P boundary had overshadowed investigations of the evo-lution of climate in this last stage of the Cretaceous.

Recent global stable isotope studies show that significantclimatic and temperature fluctuations occurred in the Maas-trichtian (Barrera and Savin, 1999; Li and Keller, 1999).Superimposed on the long-term trend, these authors evidencedtwo major episodes of global cooling. Between 73 and 70 Ma(1st episode), intermediate water (IW) temperature decreasedby 5–6 °C globally and sea-surface temperature (SST) by4–5 °C in the middle and high-latitudes. Between 70 and68.5 Ma (2nd episode), IW warmed by 2 °C in low and mid-latitudes. Global cooling resumed between 68.5 and 66 Ma1

with a decrease of IW by 3–4 °C and of SST by 3 °C in middleand high-latitudes.

About 450 ky before the Cretaceous–Paleogene boundary,intermediate and surface waters warmed rapidly and globallyby 3–4 °C and then cooled slightly by 2–3 °C during the last100–200 ky of the Maastrichtian. This rapid global warming ofthe Uppermost Maastrichtian seems to be correlated with thetiming of the main eruptive episode of Deccan volcanismwhich could have caused greenhouse conditions (Barrera andSavin, 1999; Li and Keller, 1999; Ravizza and Peucker-Ehrenbrink, 2003).

Important biotic events occurred in this period of climatechange:

● the extinction of Inoceramid bivalves (MacLeod et al.,1996);

● the extinction of tropical rudist bivalves (Johnson et al.,1996);

● a diversification of planktic foraminifera in the South Atlan-tic, coincident with the two episodes of cooling (Li and Kel-ler, 1998a);

● a decrease of planktic foraminifera species richness at about66 Ma in the South Atlantic and Tethys basins, coincidentwith the end-Maastrichtian global warming (Li and Keller,1998a, 1998b).

Few events corresponding to these Maastrichtian climaticchanges have yet been recorded in the calcareous nannoplank-ton assemblage. Huber and Watkins (1992) observed an expan-sion of southern high-latitudes assemblages of both plankticforaminifera and calcareous nannofossils to mid-latitudes inthe Early Maastrichtian, coincident with the first episode ofcooling. Friedrich et al. (2005) underscored an interval ofenhanced surface water productivity by means of calcareousnannofossils and benthic foraminifera on the Kronsmoor sec-tion. This interval coincides with a major cooling phase

1 Li and Keller (1998a) used absolute ages of Gradstein et al. (1995) todescribe climatic trends. It has been corrected in this paper using Gradsteinet al. (2004).

between 71.1 and 70.8 Ma in the Boreal Realm that could becorrelated to the first global episode of cooling.

Three reasons can be advanced for the general lack of nan-nofossil data: (1) no gradual decline of nannofossil speciesrichness has ever been recorded before the K/P boundary(Perch-Nielsen et al., 1982; Ehrendorfer, 1993; Pospichal,1994; Gartner, 1996; Gardin and Monechi, 1998; Gardin,2002); (2) most of Maastrichtian nannofossil studies essentiallyfocused on biostratigraphy and (3) yet, the understanding of thepaleoecological features of Upper Cretaceous nannofossil spe-cies is still limited.

In modern and ancient oceans, it has been demonstrated thatcalcareous nannoplankton species are particularly sensitive totemperature and nutrient supply variations. So the abundanceand composition of nannofossil assemblages should help todelineate these climatic events in the Maastrichtian.

This paper presents new calcareous nannofossil data fromODP Leg 207 Hole 1258A drilled on Demerara rise in thewestern Equatorial Atlantic (Fig. 1). It discusses the resultsrelative to the Maastrichtian climate evolution.

2. Demerara Rise: setting and objectives

The principal objective of Leg 207 was to recover anexpanded section of pelagic Cretaceous and Paleogene sedi-ments, from shallow buried depths on Demerara Rise off Sur-iname, South America (Fig. 1). Sediments recovered shouldallow a paleoceanographic study of critical stratigraphic inter-vals (Cretaceous–Paleogene and Paleocene–Eocene bound-aries), biotic and climate change in the tropical Atlantic inthese periods. This sequence should also elucidate the historyof opening of the equatorial gateway between the North andSouth Atlantic in the mid-Cretaceous.

a bathymetric map of the western tropical Atlantic. After Danelian et al. (2005).Fig. 1. Localisation sur le fond bathymétrique du Site 1258 foré sur le plateaude Demerara lors de la campagne ODP 207. La cartouche en bas à droite permetde localiser le plateau de Demerara sur une carte bathymétrique de l’Atlantiquetropical occidental. D’après Danelian et al. (2005).

Table 1Composition of the main groups of taxa cited in the textTableau 1Composition des groupes de taxons cités dans le texte

Group SpeciesEiffellithus spp. E. gorkae, E. parallelus, E. turriseiffeliiMicrorhabdulus spp. M. belgicus, M. decoratus, M. undosusMicula spp. M. decussata (~90%), M. cubiformis, M. swastica,

M. concava, M. praemurus, M. murus, M. prinsiiP. cretacea P. cretacea (~80%), P. majungae,

P. microrhabdulinaRetecapsa spp. S. crenulata (~80%), R. ficula, R. schizobrachiata,

R. surirellaZeugrhabdotus spp. Z. spiralis (50–80%), Z. bicrescenticus (40% in

core 34R), Z. embergeri, Z. erectus, Z. sigmoides,small Zeugrhabdotus

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The studied interval belongs to Hole 1258A, located in awater depth of 3192.2 meters below sea-level (mbsl) on thegently dipping western slope of Demerara Rise, 380 km northof Suriname. These sediments were deposited in an upper bath-yal setting (Erbacher et al., 2004). During the Maastrichtian,the estimated paleolatitude of Leg 207 Holes was 4 °N (Suga-numa and Ogg, 2006).

3. Materials and methods

The uppermost part of the studied interval belongs to sub-unit IIB which consists of calcareous chalk with foraminifersand is pervasively bioturbated (Erbacher et al., 2004). Lowersamples belong to subunit IIC which consists of a greenishgray nannofossil chalk with foraminifers and clay and is mod-erately bioturbated. The lowest sample lies in the upper part ofsub-unit III which consists of calcareous nannofossil clay(Fig. 2).

A detailed investigation of both absolute and relative abun-dances of calcareous nannofossil taxa was performed on 72samples spanning the whole Maastrichtian. Samples were col-lected at about 1-m intervals with one additional sample takenabout 6.5 m lower in sub-unit IIIA. Samples were processed asfollows: sediments were gently disaggregated in a mortar and50 mg of dried sediment were weighed and dispersed inexactly 50 ml of distilled water. The suspension was ultrasoni-cated for 15 s and agitated with a magnetic stirrer. Then 1 mlof this suspension was extracted with a micropipette anddropped so that it completely covered the smear-slide. Wepaid attention to the homogeneity of the deposition so that cal-careous nannofossils are evenly distributed on the slide.

Relative species abundances were calculated as the percen-tage of at least 400 specimens randomly counted at a magnifi-cation of × 1575. Absolute abundances were calculated for onespecies as the ratio between the number of specimens encoun-tered and the number of fields of view examined to attain it.Absolute abundance of total calcareous nannofossils was cal-culated as the ratio between total specimens of calcareous nan-nofossils encountered and the total number of fields of viewexamined for each sample. Then, after measuring the area ofone field of view (0.0172 mm2), we were able to transformthese abundances into number of specimens per mm2. As thegrain density is similar for all smear-slides (1 g/l), we assumethat these abundances per unit area are approximately propor-tional to the selected species per gram of sediment. Thismethod is comparable to that used in Backman and Shackleton(1983) or Henriksson (1993a).

In addition, three traverses of the slide, corresponding toabout 570 fields of view, were examined to document speciesrichness encountered during the analysis of one slide.

Species which were too rare for accurate quantitative analy-sis were grouped into higher taxonomic levels such as Eiffe-lithus spp., Microrhabdulus spp., Micula spp., Retecapsa spp.and Zeugrhabdotus spp. as presented in Table 1.

We also analyzed the CaCO3 content of all samples andchecked the preservation state of the nannofossil assemblage,following the method proposed by Roth (1978). Three classes

of preservation are considered here: Poor (P), Moderate (M)and Good (G).

For biostratigraphy, we used absolute abundance patterns ofnannofossil markers to determine the “base” and “top” of stra-tigraphic ranges which denote the first and last occurrence ofan index species (Fig. 2). This biochronologic method has beenproved to be much more efficient to distinguish levels of lastoccurrences from reworked patterns (Backman and Shackleton,1983; Backman, 1986; Rio et al., 1990; Raffi, 1999, 2002).The zonations of Perch-Nielsen (1985) and Burnett (1998)were used for the biostratigraphic analysis of this hole(Fig. 2).

Calcareous nannofossil species considered in this article arelisted in the Appendix A, following taxonomic concepts ofPerch-Nielsen (1985) and Young and Bown (1997).

4. Results

4.1. Biostratigraphy

Since a magnetostratigraphic study has been conducted inHole 1258A (Suganuma and Ogg, 2006), it is possible to com-pare our results with the global biomagnetostratigraphicscheme proposed by Bralower et al. (1995) (abbreviated as B95 in the following) implemented by additional events such asthe base of Micula prinsii (base of Chron C29r according toHenriksson, 1993b), and the base of C. kamptneri (C31n/C30r boundary according to Self-Trail, 2001) (Fig. 3).

Our study shows some similarities as well as differenceswith this global biomagnetostratigraphic scheme (Fig. 2).

The top of Reinhardtites levis lies within C31r in our study(Fig. 2) in agreement with the scheme of B 95.

The base of Lithraphidites quadratus lies within the undif-ferentiated Chrons C31n–C30n, well above the top of C31r(Fig. 2). B 95 place it near the base of C31n. Thus, the base ofL. quadratus may be younger than in the previously proposedscheme. Moreover, this event lies above the base of Miculamurus, in an opposite order of that given by zonation schemes.

The base of M. murus lies at the top of C31r (Fig. 2)whereas B 95 record it within C30n. Self-Trail (2001) showedthat the base of M. murus varies according to the paleodepth ofthe section studied (shallow vs. deep-water setting). Thus, itscorrelation with the polarity time scale is dependent on envir-

Fig. 2. Main nannofossil bioevents in Hole 1258A using absolute abundances of taxa and inferred biozonations. (*) Lithostratigraphy and Planktic foraminifera zones from Erbacher et al. (2004). Magnetostratigraphyfrom Suganuma and Ogg (2006).Fig. 2. Principaux bioévénements des nannofossiles calcaires sur le Site 1258A mis en évidence par les abondances absolues des taxons et biozonations associées. (*) Lithostratigraphie et zones de foraminifèresplanctoniques d’après Erbacher et al. (2004). Magnétostratigraphie d’après Suganuma et Ogg (2006).

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Fig. 3. Global bio-magnetostratigraphic scheme of calcareous nannofossils for the Maastrichtian. Based on a compilation of data from Bralower et al. (1995) withadditional bio-events from Henriksson (1993b) and Self-Trail (2001).Fig. 3. Schéma biomagnétostratigraphique global des nannofossiles calcaires pour le Maastrichtien. Basé sur une compilation des données de Bralower et al. (1995)avec les bioévénements supplémentaires de Henriksson (1993b) et Self-Trail (2001).

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onmental factors at the time of deposition. In this deep-oceanhole, our findings are close to the results of Self-Trail (2001,2002) in Hole 1052E on the Western North Atlantic margin.

The base of C. kamptneri lies within the undifferentiatedChrons C31n–C30n (Fig. 2). Perch-Nielsen (1979, 1985)used the base of C. kamptneri as a proxy for the base ofCC26 in low latitudes, instead of the base of Nephrolithus fre-quens which originally defined the base of CC26. But this lat-ter event had been proved to be strongly diachronous, depend-ing on paleolatitudes (Worsley and Martini, 1970; Pospichaland Wise, 1990; Huber and Watkins, 1992). Burnett (1998)used the base of C. kamptneri for the base of UC20c in Teth-yan and Intermediate Provinces. However, in Hole 1258A, thisevent lies above the base of M. murus, in an opposite order ofthat given by zonation schemes (Fig. 2).

The base of M. prinsii lies within Chron C29r in Hole1258A (Fig. 2), in agreement with Henriksson (1993b).

In Hole 1258A, the calcareous nannofossil zonations ofPerch-Nielsen (1985) and Burnett (1998) are hardly applicablebecause several events appear in an opposite order of thatgiven by zonation schemes. As a consequence, we only used:

● the top of R. levis, which defines the top of CC24 andUC18, for the lower Maastrichtian (Fig. 2);

● the base of M. prinsii, which defines the base of CC26b andUC20d, for the upper Maastrichtian (Fig. 2).

In addition, the base of C. kamptneri, which lies within theundifferentiated Chrons C31n–C30n (Fig. 2), could help inter-preting the magnetostratigraphical signal and differentiate thesechrons. Indeed, this event is supposed to be reliable and syn-chronous in both nearshore and deep-ocean environments andoccurs almost concurrently with the C31n/C30r boundary(Self-Trail, 2001, 2002).

Some additional bio-events could be useful to further refinethe biostratigraphy in this region. Three species (Ahmuellerellaoctoradiata, Zeugrhabdotus bicrescenticus and Gartneragosegmentatum) show some spotty occurrences before theirextinction, well after the very end of a clear decreasing trendof their absolute abundances (Fig. 2). These spotty occurrencesmay not be stratigraphically significant because of possiblereworking. For these three taxa, we chose to present both levelsof top and levels of Last Consistent Occurrences (LCO) whichcorrespond to these marked changes in absolute abundances(Fig. 2). In addition, the top of Biscutum constans occursabout 4 m below the C30n/C29r boundary and the base ofPseudomicula quadrata is recorded in the lowermost part ofundifferentiated Chrons C31n–C30n (Fig. 2).

All nannofossil events are plotted against stratigraphic infor-mation on Fig. 4 and their stratigraphic levels are given inTable 2.

4.2. Species richness and nannofossil abundance

Species richness slightly decreases from 53 at the base ofthe section to 40 species in core 32R (Fig. 5). The base ofcore 31R is marked by a slightly lowered species richness(32 species). Then species richness increases to 47 species inthe upper part of core 31R. The following upper part of theMaastrichtian is characterized by strong oscillations of speciesrichness between 35 and 47. It drops abruptly above the K/Pboundary to attain nine species in the last Danian sample.

Total nannofossil abundance is very low in the lower part ofthe section (decreasing from 316 specimens/mm2 in the firstsample of the core 34R to 67 in core 32R; Fig. 5). Then, nan-nofossil abundance increases to attain a maximum of 1039 incore 29R. The interval between the top of core 29R and thelower part of core 28R is characterized by lower abundances(between 350 and 530 specimens/mm2) while the end of the

Fig. 4. Synthesis of nannofossil biostratigraphic events found in Hole 1258A.Main events in bold character.Fig. 4. Synthèse des événements biostratigraphiques des nannofossiles calcairessur le Site 1258A. Principaux événements en caractères gras.

Table 2Sub-bottom depths (mbsf) of nannofossil events. LCO: Last ConsistentOccurrenceTableau 2Profondeur des bio-événements de nannofossiles calcaires (mbsf). LCO :Niveau de Dernière Occurrence Consistante

Nannofossil events Depth (mbsf)T. operculata bloom 252.50Base of M. prinsii 259.79Top of B. constans 266.60Top of G. segmentatum 268.00Top of Z. bicrescenticus 277.40Base of P. quadrata 284.10Base of L. quadratus 284.50Base of C. kamptneri 285.50LCO of G. segmentatum 294.50Base of L. praequadratus 295.30Base of M. murus 296.30Top of A. octoradiata 304.00LCO of Z. bicrescenticus 306.60Top of R. levis 309.50LCO of A. octoradiata 310.20

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Maastrichtian interval is characterized by variable abundancesbetween 400 and 870 specimens/mm2. Nannofossil abundanceshows an abrupt drop to less than 10 specimens/mm2 in theDanian interval.

4.3. Composition of the Maastrichtian assemblage

A total of 104 species were encountered 26 of which showconsistent occurrences throughout the studied interval. Thedominant taxa of the Maastrichtian assemblage are Miculadecussata (5–49%), Watznaueria barnesiae (16–45%), Cribro-sphaerella ehrenbergii (2–13%), Prediscosphaera cretacea (4–16%) and Retecapsa spp. (2–20%). These taxa represent 55–90% of the total nannofossil assemblage. Other common nan-nofossil taxa are Arkhangelskiella cymbiformis, Ahmuellerellaregularis, Chiastozygus spp., Eiffelithus spp., Microrhabdulusspp., Prediscosphaera stoveri, Tetrapodorhabdus decorus,Thoracosphaera operculata and Zeugrhabdotus spiralis. Theoverall assemblage is very similar to those reported from lowto mid-latitudinal sites. However, three species (A. octoradiata,G. segmentatum and Kamptnerius magnificus) which are con-sidered as high-latitude taxa (Thierstein, 1976, 1981; Wind,1979; Pospichal and Wise, 1990; Lees, 2002) show unusuallyhigh relative abundances in some intervals of this low-latitudinal hole (Figs. 5 and 6).

A. octoradiata is about 3% at the beginning of the sectionand then disappears in core 33R. G. segmentatum shows highrelative abundances (3–8%) from core 34R to 33R, abovewhich, it decreases until it disappears at the base of core 31R.K. magnificus is less than 1% abundant at the base of the sec-tion, then it reaches 3–4% in the upper part of core 33R andremains consistent until the middle part of core 31R where itsuddenly increases up to 13%. The abundance of this taxonremains high till the upper part of core 29R where it suddenlydecreases. Then, its signal becomes sporadic and inconsistentuntil the K/P boundary.

A. cymbiformis, A. regularis, Eiffelithus spp., Microrhabdu-lus spp. and T. operculata are all less than 4% in abundance.None of them shows any peculiar trend throughout the Maas-trichtian interval (Fig. 5).

Besides four successive peaks in core 32R, the relativeabundance of M. decussata does not exceed 15% in the lowerpart of the section (Fig. 5). A sudden increase occurs justabove the unrecovered interval in core 30R. Then it graduallyincreases and becomes the dominant species at the end of theMaastrichtian (core 28R) with a maximum of 49%.

W. barnesiae is the dominant species of this assemblagefrom the base of the section to the middle part of core 31Rand accounts for a mean of 35% in this interval. In the upperpart of core 31R, W. barnesiae suddenly decreases to 29%.Then it shows a slight overall decreasing trend in the upperpart of the section to attain a mean of 20% at the end of theMaastrichtian (core 28R) (Fig. 5).

C. ehrenbergii and P. cretacea show quite stable abundancesand do not generally exceed 10% all along the section.

Retecapsa spp. shows quite stable abundances (~7%),except between cores 33 and 32R where it shows higher abun-dances around 16% (Fig. 5).

P. stoveri accounts for about 6% in cores 34 and 33R. Itbecomes less abundant in cores 32 and 31R (~1%). In theupper part of the section, P. stoveri is only sporadically present(Fig. 5).

Fig. 5. CaCO3 content, species richness, total nannofossil abundance and relative abundances of selected calcareous nannofossil taxa in the Maastrichtian in Hole 1258A. (*) Lithostratigraphy and Plankticforaminifera zones from Erbacher et al. (2004). Magnetostratigraphy from Suganuma and Ogg (2006). Nannofossil zones from this study.Fig. 5. Contenu en CaCO3, richesse spécifique, abondance totale des nannofossiles calcaires et abondances relatives de taxons choisis au Maastrichtien sur le Site 1258A. (*) Lithostratigraphie et zones deforaminifères planctoniques d’après Erbacher et al. (2004). Magnétostratigraphie d’après Suganuma et Ogg (2006). Zones de nannofossiles d’après cette étude.

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B. constans is the only common species of Biscutum in thishole. At the beginning of the section, the relative abundance ofB. constans is only 1.5%. Between cores 33 and 32R, this spe-cies shows higher abundances with several peaks up to 5%(Figs. 5 and 6), followed by a decrease down to 1% fromcore 31R to 29R. In the upper part of core 29R, B. constansdisappears.

Zeugrhabdotus spp. are the most abundant in the first sam-ple of our study (~14%) and decrease to attain less than 1% incore 33R (Fig. 5). This trend is largely due to the decrease andthen the disappearance of Z. bicrescenticus. Zeugrhabdotusspp. shows relatively higher abundances (~4%) in cores 31and 30R. In the uppermost part of the section, Zeugrhabdotusspp. is less than 1% abundant.

M. murus is a rare to frequent component of Maastrichtiancalcareous nannofossil assemblages. Its relative abundance isgenerally low and rarely considered in paleoecological studiesof calcareous nannofossils. In Hole 1258A, both the relativeand absolute abundances of this species are very low until themiddle part of core 29R (Fig. 6). Then they slightly increaseuntil the middle part of core 28R while K. magnificus abruptlydrops and B. constans disappears. They strongly increase in theupper part of core 28R and delineate an acme zone in theuppermost Maastrichtian.

5. Paleoecology of selected nannofossil speciesin the Maastrichtian

5.1. M. decussata: preservation index, temperature indexor high-stress marker?

M. decussata is a major component of Late Cretaceousassemblages. Its paleoecological significance is not yet wellunderstood. Since the studies of Hill (1975) and Thierstein(1980), its distribution has largely been interpreted as drivenby preservation. Few authors reported very high abundancesof this species in well-preserved Uppermost Maastrichtian sam-ples showing no evidence of dissolution or overgrowth (Eshet etal., 1992; Tantawy, 2002). Different paleoecological signifi-cances of this species exist in the literature. Several authors sug-gested that this taxon might have preferred cooler temperatures(Wind, 1979; Doeven, 1983; Watkins and Self-Trail, 2005).However, biogeographic studies of Wind (1979); Thierstein(1981); Shafik (1990), Henriksson and Malmgren (1997) andLees (2002) showed that this taxon is clearly cosmopolitanand can reach as far as 80% in both tropical and sub-tropicalassemblages. Eshet et al. (1992) and Tantawy (2002) interpretedthe high abundances of M. decussata as indicative of very lowsurface productivity and high-stress environmental conditions.

5.2. Temperature indices

Several nannofossil species are recognized as cool waterindicators. These are mainly A. cymbiformis, P. stoveri,A. octoradiata, G. segmentatum, K. magnificus andN. frequens. Wind (1979) and Thierstein (1981) referred

A. cymbiformis as a high-latitude taxon. However, Lees(2002) shows that this species is common down into tropicalpaleolatitudes even if it prefers high-latitudes. P. stoveri is alsoa common species of tropical paleolatitudes but it stronglyincreases in abundance in both southern and northern high-latitudes in the Late Maastrichtian (Pospichal and Wise, 1990,1992; Lottaroli and Catrullo, 2000). This event is interpreted asa cooling in high-latitudes and the increase of temperature gra-dient between low and high-latitudes. A. octoradiata,G. segmentatum, K. magnificus and N. frequens are predomi-nantly high-latitude taxa and sporadically occur in low-latitudinal sites (Thierstein, 1976, 1981; Wind, 1979; Pospichaland Wise, 1990; Lees, 2002). These species, which are muchmore frequent at high-latitudes, are certainly the best indicatorsof cool surface waters.

M. murus is clearly restricted to warm tropical waters and istotally absent from the high-latitude areas all along its biostra-tigraphical range (Worsley and Martini, 1970; Thierstein,1981; Watkins et al., 1996; Lees, 2002). Thus, it could be con-sidered as a good warm-water indicator.

5.3. Fertility indices

Some nannofossil species are good indicators of surface-water fertility. Numerous mid-Cretaceous studies demonstratedthat B. constans and Zeugrhabdotus erectus thrive at differentlevels of fertility, B. constans being favored in conditions ofenhanced surface water fertility (Erba, 1990; Erba et al.,1992; Williams and Bralower, 1995; Fisher and Hay, 1999).Eshet and Almogi-Labin (1996) distinguished two distinctgroups of fertility using statistical correlations. The low-fertility group comprises Eiffelithus spp., Prediscosphaeraspp. (excluding P. stoveri), Lithraphidites spp. and Stauro-lithites spp. whereas the high-fertility group includes Zeugr-habdotus spp., Biscutum spp. and Thoracosphaera saxea.

W. barnesiae is a cosmopolitan species which is generallydominant in tropical latitudes (about 30%) and only commonin high-latitude sites (less than 10%). Thus, several authorsused it as a warm-water indicator (Doeven, 1983; Watkins etal., 1996; Watkins and Self-Trail, 2005). However, several stu-dies show that W. barnesiae is mainly a low-nutrient indicator(Roth and Krumbach, 1986; Erba et al., 1992; Lamolda et al.,1992; Williams and Bralower, 1995; Fischer and Hay, 1999).

Other common taxa of Maastrichtian assemblages such asC. ehrenbergii and Retecapsa spp. do not show any latitudinalpreferences nor seem to be related to surface water fertility.Thus, significance of their distribution patterns remainsunknown. A summary of nannofossil Temperature and Fertilityindices is presented on Table 3.

6. Discussion

6.1. Preservation of calcareous nannofossil assemblages

Since the early studies of Hill (1975) and Thierstein (1980),W. barnesiae and M. decussata (= Micula staurophora for

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Fig. 6. Tentative of correlation of patterns of selected calcareous nannofossil taxa at site 1258A with stable isotope records of surface ( ) and deep-water ( ) foraminifera in DSDP South Atlantic Hole 525A from Li and Keller (1998a).Fig. 6. Tentative de corrélation des caractéristiques de taxons choisis de nannofossiles calcaires sur le site 1258A avec les résultats des isotopes stables des foraminifères des eaux de surface

R. rugosa A. acuta

( ) et profondes( ) de Li et Keller (1998a) sur le Site DSDP 525A (Atlantique Sud).

R. rugosaA. acuta

N. Thibault, S. G

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icropaléontologie 49 (2006) 199-214207

Fig. 7. Scatter diagrams of species richness vs relative abundance of Miculaspp. (a), total nannofossil abundance vs relative abundance of Micula spp. (b)and absolute abundance of Micula spp. vs its relative abundance (c).Fig. 7. Diagrammes x–y de la richesse spécifique vs l’abondance relative deMicula spp. (a), de l’abondance totale des nannofossiles calcaires vsl’abondance relative de Micula spp. (b) et de l’abondance absolue de Miculaspp. vs son abondance relative (c).

Table 3Calcareous nannofossil paleoecological indices considered in this study. SeeSection 5 for discussionTableau 3Espèces de nannofossiles calcaires considérées comme indices paléoécologi-ques dans cette étude. Voir le chapitre 5 pour la discussion

Fertility indices Temperature indicesHigh-fertility taxa Cool water taxaB. constans A. octoradiata,

G. segmentatum,K. magnificus, N. frequens,

Mid-fertility taxa P. stoveri?Zeugrhabdotus spp.

Low-fertility taxa Warm-water taxaEiffellithus spp., Lithraphidites spp.,Prediscosphaera spp. (excluding P. stoveri),Staurolithites spp., W. barnesiae

(tropical)M. murus

N. Thibault, S. Gardin / Revue de micropaléontologie 49 (2006) 199–214208

some authors) are considered as two high solution-resistantspecies and are generally used to test the preservation degreeof the assemblage.

However, in the uppermost Cretaceous, Micula spp. (mostlyM. decussata) reaches high abundances and its dissolution ratiois more than ten times that of W. barnesiae (Thierstein, 1980).Consequently, if preservation modifies the original assemblage,the relative abundance of Micula spp. will be favored as com-pared to all other taxa (including W. barnesiae). The relativeabundance of Micula spp. should be negatively correlated tospecies richness and total nannofossil abundance, and poorlycorrelated to its absolute abundance which is a good proxy ofits original flux in the water column.

In Hole 1258A, there is no clear correlation between speciesrichness and the relative abundance of Micula spp. (Fig. 7a).Total nannofossil abundance shows a weak positive correlationwith Micula spp and not a negative one (Fig. 7b); relative andabsolute abundances of this taxa have a very good correlation(Fig. 7c). Moreover, solution-susceptible taxa such as Biscutumspp. and P. stoveri are present when the total nannofossil abun-dance is very low in the lower part of the section and othersolution-susceptible taxa C. ehrenbergii and P. cretacea donot decrease when Micula spp. strongly increases in theupper part of the section (Fig. 5). These results suggest thatdiagenetic processes have not modified the original nannofossilassemblage. The relative abundances as well as absolute abun-dances of calcareous nannofossils can be interpreted in termsof their ecological patterns.

6.2. Interpretation of the nannofossil assemblage patternsin Hole 1258A

6.2.1. Paleoclimatic evolution of Hole 1258A as suggestedby calcareous nannofossil assemblages

The most important findings of this study are the trends ofthe cool water taxa A. octoradiata, G. segmentatum andK. magnificus and that of the tropical taxon M. murus.

The high abundance of cool water taxa in this equatorialhole is unusual for tropical paleolatitudes and suggest two suc-cessive cool events in the Maastrichtian. Moreover, the succes-

sive flourishments of the three species suggest different cooloceanic conditions: at the base of the section, a first coolingevent is suggested by the higher abundances ofA. octoradiata and G. segmentatum in cores 34 and 33R(Fig. 6). During this event, A. octoradiata disappears in core33R and seems to be replaced by higher abundances ofK. magnificus, G. segmentatum being the dominant speciesamong cool water taxa.

In core 32R, G. segmentatum abruptly drops in abundanceand disappears, while the abundance of K. magnificus remains

N. Thibault, S. Gardin / Revue de micropaléontologie 49 (2006) 199–214 209

stable until the upper part of core 31R. Considering the wholeabundance of cool water taxa, the interval between the upperpart of core 32R to the upper part of core 31R could testify ofwarmer conditions.

The sudden increase of K. magnificus in core 31R and thehigher abundances of this species until core 29R suggest a sec-ond cooling event.

The dramatic drop of the abundance of K. magnificus at thebase of core 28R and its sporadic presence until the K/Pboundary suggest much warmer conditions at the end of theMaastrichtian (Fig. 6).

The first cooling event seems to show a gradual evolution inthe lower part of the section, as supported by the replacementof A. octoradiata by K. magnificus and the overall decrease ofP. stoveri, the fertility index Zeugrhabdotus spp. and the spe-cies richness (Fig. 5). Two peaks of abundance of the highfertility taxon B. constans also take place in this interval andanother one takes place just after the disappearance ofG. segmentatum (Fig. 6). Then B. constans decreases duringthe warmer event in cores 32 and 31R and becomes a minorcomponent of the assemblage. This sequence of events couldsuggest an overall warming trend and slight decrease of pro-ductivity during this first cooling event which terminatesabruptly by a sudden warming. Likely, when surface waterswarmed, ocean was less stratified which disallowed deepnutrient-rich waters to shallow.

The second cooling event, lying in cores 31R to 29R, ismarked by the higher abundances of K. magnificus and is notaccompanied by higher abundances of P. stoveri or B. constans(Fig. 5). Only Zeugrhabdotus spp. seem to be slightly moreabundant during this event. This cooling event could be lesspronounced and fertile than the first one.

The drop of K. magnificus, followed in turn by the disap-pearance of B. constans and the increase of M. murus in core28R (Fig. 6) suggest that warming and lowered productivity ofsurface waters took place in the Uppermost Maastrichtian. Thiswarming reached a maximum with the acme of M. murus at theend of this stage.

6.2.2. Paleoenvironmental significance of total nannofossilabundance patterns

Total nannofossil abundance is much higher in the upperpart of the section (Fig. 5). This may be partly explained bythe fact that CaCO3 content is higher (about 72%) in the upperpart of the section than in the lower part (about 60%). Butthese differences do not seem to be sufficient to explain suchdiscrepancies of absolute abundance. Calcareous nannofossilscould have been diluted by another source of carbonates whichstill needs to be characterized during the deposition of sedi-ments in the lower part of the section. This trend could beexplained at least in part by a change in surface water fertility.Indeed, fertility index taxa are less abundant in the upper partof the section when nannofossil abundance is the highest(Fig. 5). This could be explained by the fact that a majorityof Upper Cretaceous taxa preferred well-stratified and oligo-trophic surface waters as extant coccolithophores do in the

present-day ocean. Hence, when surface water fertilitydecreased, these k-selected taxa, which account for the mainpart of the assemblage, tended to increase their populationgrowth rate and subsequently increased the total nannofossilabundance.

6.2.3. Paleoecological significance of W. barnesiaeand M. decussata

Variations of W. barnesiae are somewhat difficult toexplain. This species is more abundant in the lower part ofthe section than in the upper part and none of its supposedpaleoecological significances work with our previous assump-tions. If W. barnesiae indicates warmer temperatures or low-fertility conditions, its abundance should be higher in theupper part of the section where cool water taxa are only spora-dic, where high fertility taxon B. constans is absent and mid-high fertility taxon Zeugrhabdotus spp. is low (Fig. 5). This isnot the case. However, a gradual increase of its abundanceoccurs in the presumably warming and less fertile interval ofcores 32R to 31R where the abundance of cool water taxa islow. There, W. barnesiae reaches a maximum and suddenlydrops while cool water taxon K. magnificus abruptly increases.This abundance pattern of W. barnesiae could be in a relativeagreement with its supposed preferences (warm or low-fertilityindex). In the upper part of the section, this species seems toshow a slight overall decrease which may be compensated bythe increase and predominance of M. decussata. Though, wehardly explain this trend.

We do not believe that the increase of M. decussata iscaused only by poorer preservation. In Hole 1258A, samplesof the uppermost Maastrichtian with high abundances ofM. decussata are not particularly affected by poor preservation.Highest abundances of M. decussata occur in the warmer con-ditions of the Uppermost Maastrichtian, where cool water taxaare only sporadically present and where B. constans is absent(Fig. 5). Therefore, in agreement with Eshet et al. (1992) andTantawy (2002), we interpret the strong increase ofM. decussata at the end of the Maastrichtian as due to theonset of unfavorable, stressful environmental conditions beforethe K/P boundary, probably caused by a strong warming event.

6.3. Comparison with past climatic studiesin the Maastrichtian Atlantic Ocean

No stable isotopic data are yet available for Hole 1258A.However, our results seem to fit very well to previous findingson Maastrichtian climate, productivity and faunal turnovers inplanktic foraminifera in the South Atlantic.

Based on isotopic studies on South Atlantic Hole 525A(Fig. 6), Li and Keller (1998a) delineated a major cooling insurface and bottom waters and increased surface productivitybetween 71.2 and 69.2 Ma. This climate change corresponds toa diversification event and increased species richness in plank-tic foraminifera and ends inside magnetochron C31r, above theboundary between CF7 and CF6 planktic foraminifera zoneswhich corresponds to the boundary between KS30b and

N. Thibault, S. Gardin / Revue de micropaléontologie 49 (2006) 199–214210

KS30a in our study (First Appearance of Rosita contusa). Thisevent might correspond to our first cooling and more fertileinterval characterized by higher abundances of cool watertaxa and fertility index (Fig. 6).

According to Li and Keller (1998a), a rapid warming of2–3 °C occurred at 69.2 Ma and warmer but fluctuating tem-peratures continued from the upper part of Chron C31r untilthe base of C30n (about 67.3 Ma). In the same time, accordingto Li and Keller (1998a), productivity ceases to increaseapproximately at the boundary between Chrons C31r andC31n. This event corresponds well to our warming intervalwith lower abundances of cool water taxa and the decrease ofB. constans (Fig. 6).

At the base of Chron C30n, temperatures cooled again andmaximum cool temperatures prevailed from 67.2 to 66 Ma nearthe boundary between Chrons C30n and C29r (Li and Keller,1998a). This event marks the beginning of the decline in spe-cies richness of planktic foraminifera and might correspond toour second cooling event characterized by higher abundancesof K. magnificus. In the same time, according to Li and Keller(1998a), deep-water productivity remained relatively stablewhereas surface water productivity started to decline at66.2 Ma in the uppermost part of C30n. That may correspondin our study to the disappearance of B. constans (Fig. 6).

An abrupt and major warming by 3–4 °C occurs in thelower part of Chron C29r at about 65.5 Ma and lasts untilabout 100–200 ky before the K/P boundary (Fig. 6). Thisshort-term rapid warming seems to be a global event (Barreraand Savin, 1999; Li and Keller, 1999) and is supposed to berelated to a greenhouse effect linked to major Deccan Trapvolcanic degassing (Courtillot et al., 1988, 1996; Baksi,1994; Ravizza and Peucker-Ehrenbrink, 2003). It correspondsto a strong decline in planktic foraminifer species richness (Liand Keller, 1998a). In our study, the warming seems to startmore gradually before the boundary between Chrons C30n andC29r with the decrease and then sporadic presence ofK. magnificus but the major greenhouse warming eventappears to be well marked by the M. murus acme zone(Fig. 6). Rapid cooling by 2–3 °C occurs within the last 100–200 ky before the K/P boundary in Hole 525A but neitherdecrease of M. murus nor a return of cool water taxa is obser-vable in our study. However, the presence of an unrecoveredinterval in the basal part of core 27R claims for an incompleteK–P interval and may explain this missing event.

The beginning of the decline of Maastrichtian planktic for-aminifera species richness in Hole 525A starts at 67.5 Ma, nearthe base of Chron C30n (Li and Keller, 1998a) which maycorrespond to the unrecovered interval of core 30R (Fig. 6).The beginning of the increase of M. decussata is just abovethis unrecovered interval. This concordance of events is anadditional argument to interpret the increase of this species asthe onset of high-stress environmental conditions.

Even if the distribution patterns of cool water taxa in Hole1258A are concordant to climatic events delineated in Hole525A, it does not explain why their abundances are so highin this tropical hole. Thierstein (1981) indicated higher abun-

dance of cool water taxon K. magnificus (>2%) in other sites ofthe western equatorial region of the Atlantic (DSDP Holes 354and 146). What kind of paleoceanographic organization ledthese cooling events to be so much pronounced here? The pre-sence of an upwelling system seems unlikely. Calcareous nan-nofossil fertility index are not very abundant in this hole andthe Total Organic Carbon (TOC) content is very low alongsideof this coast off Suriname (Handoh et al., 2003). Upwellingsystems appear to take place as today along the western coastof Africa and the western equatorial region of the Atlanticexhibits low primary productivity in the Maastrichtian (Handohet al., 2003). So, this question needs to be further investigated.

This work also brings concerns about the evolution of Northand South Atlantic climates. Equatorial Hole 1258A belongs tothe North Atlantic basin whereas Hole 525A is situated at apaleolatitude of about 36 °S in the South Atlantic basin(Zachos and Arthur, 1986). In the Maastrichtian, both basinswere connected by an equatorial gateway (Erbacher et al.,2004). However, it appears that the climate evolution is differ-ent in the North and South Atlantic basins: stable isotopesexpress a long-term Maastrichtian cooling throughout theSouthern Hemisphere (Barrera and Savin, 1999; Li and Keller,1999) and a brief and intense warming pulse between 500 and100–200 ky before the K/P boundary. Whereas in the NorthAtlantic, stable isotope studies on material from Blake NosePlateau and numerous other data hint at long-term warmingand increased water stratification in the Maastrichtian(MacLeod et al., 2005). These authors explain these regionaldifferences by an import of heat from the South Atlantic and anintensification of the North Atlantic polar front. Our data sug-gest that the western equatorial region of the North Atlanticbasin was tightly connected to the South Atlantic and followedthe same evolution.

7. Conclusions

Absolute abundances of Maastrichtian calcareous nannofos-sils allowed us to infer the biozonation and record local bio-events in Hole 1258A. The biozonation schemes of Perch-Niel-sen (1985) and Burnett (1998) are hardly applicable in thisregion. We could not differentiate subzones CC25a to CC26a,nor zones UC19 to UC20c.

The base of C. kamptneri which is supposed to be a reliableand synchronous event could help interpreting the magnetos-tratigraphic signal of this hole and differentiate the C31n toC30n interval because this event occurs almost concurrentlywith the C31n/C30r boundary.

Distribution patterns of calcareous nannofossils in Hole1258A indicate extreme climatic variability in the Maastrich-tian (Fig. 8). Two cool events are respectively expressed by thehigh abundances of (1) A. octoradiata + G. segmentatum and(2) K. magnificus. Only the first one is accompanied by higherabundances of fertility index taxa (B. constans and Zeugrhab-dotus spp.) which suggests that, among cool water taxa,K. magnificus preferred less fertile surface waters. An end-Maastrichtian warming is highlighted by the drop in abundance

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66

N. Thibault, S. G

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icropaléontologie 49 (2006) 199-214211

Fig. 8. Summary of the main paleoecological features of calcareous nannofossils in Hole 1258A and correlation with stable isotopes record of surface and deep-water foraminifera of Li and Keller (1998a) in SouthAtlantic DSDP Hole 525A.Fig. 8. Résumé des principaux événements paléoécologiques des nannofossiles calcaires sur le Site 1258A et tentative de corrélation avec les enregistrements des isotopes stables des foraminifères planctoniques etbenthiques de Li et Keller (1998a) sur le Site DSDP 525A (Atlantique Sud).

N. Thibault, S. Gardin / Revue de micropaléontologie 49 (2006) 199–214212

of cool water taxa and the acme of the tropical taxon M. murus.The disappearance of fertility index B. constans in this intervalsuggests lowered productivity at the end of the Maastrichtian.In addition, the increase of M. decussata in the upper Maas-trichtian suggests the onset of high-stress environmental condi-tions. This climatic evolution, evidenced by means of calcar-eous nannofossils, is in agreement with that of South AtlanticHole 525A, inferred by both stable isotopes and patterns ofplanktic foraminifera (Li and Keller, 1998a). However, it isnot concordant with the climatic evolution proposed for theNorth Atlantic sites (MacLeod et al., 2005). Therefore, we sup-pose that the western equatorial region of the North Atlanticbasin was tightly connected to the South Atlantic basin andfollowed the same evolution.

Our study confirms that the end-Maastrichtian was a time ofhigh-stress conditions. Some authors recently claimed forchanges in calcareous nannoplankton prior to the K/P bound-ary, in concordance with paleoclimate change (Tantawy, 2002;Lamolda et al., 2005). Even if species richness doesn’tdecrease significantly at this time, the strong increase ofM. decussata which is recorded in many places testifies ofstressful conditions.

Acknowledgements

This research used samples provided by the Ocean DrillingProgram (ODP). ODP is sponsored by the US National ScienceFoundation (NSF) and participating countries under the man-agement of Joint Oceanographic Institutions (JOI), Inc. Theauthors gratefully acknowledge Laurence Le Callonnec forbringing back all our samples on her own from Bremen andEclipse II Program for funding. We thank Tim Bralower andJörg Mutterlose for a detailed review that greatly improved themanuscript.

Appendix A

Alphabetical list of calcareous nannofossil species consid-ered in this study.

References not cited in this paper can be found in Perch-Nielsen (1985) and Bown (1998).

A. octoradiata (Gorka, 1957) Reinhardt (1966).A. regularis (Gorka, 1957) Reinhardt and Gorka (1967).A. cymbiformis Vekshina (1959).B. constans (Gorka, 1957) Black in Black and Barnes

(1959).Chiastozygus amphipons (Bramlette and Martini, 1964)

Gartner (1968).Chiastozygus antiquus (Perch-Nielsen, 1973) Burnett

(1998).C. ehrenbergii (Arkhangelsky, 1912) Deflandre in Piveteau

(1952).Cruciplacolithus primus (Perch-Nielsen, 1977).Eiffelithus gorkae Reinhardt (1965).Eiffelithus parallelus Perch-Nielsen (1973).

Eiffelithus turriseiffelii (Deflandre in Deflandre and Fert,1954) Reinhardt (1965).

G. segmentatum (Stover, 1966) Thierstein (1974).K. magnificus Deflandre (1959).Lithraphidites carniolensis Deflandre (1963).Lithraphidites praequadratus Roth (1978).L. quadratus Bramlette and Martini (1964) emend. Roth

(1978).Microrhabdulus belgicus Hay and Towe (1962).Microrhabdulus decoratus Deflandre (1959).Microrhabdulus undosus Perch-Nielsen (1973).M. decussata Vekshina (1959).Micula cubiformis Forchheimer (1972).Micula concava (Stradner in Martini and Stradner, 1960)

Verbeek (1976).Micula swastica Stradner and Steinmetz (1984).Micula praemurus (Bukry, 1973) Stradner and Steinmetz

(1984).M. murus (Martini, 1961) Bukry (1973).M. prinsii Perch-Nielsen (1979).Neobiscutum parvulum (Romein, 1979) Varol (1989).Neobiscutum romeinii (Perch-Nielsen, 1981) Varol (1989).N. frequens Gorka (1957).P. cretacea (Arkhangelsky, 1912) Gartner (1968).P. stoveri (Perch-Nielsen, 1968) Shafik and Stradner (1971).Prediscosphaera grandis Perch-Nielsen (1979).Prediscosphaera microrhabdulina Perch-Nielsen (1973).Prediscosphaera spinosa Bramlette and Martini (1964)

Gartner (1968).Prediscosphaera majungae Perch-Nielsen (1973).P. quadrata Perch-Nielsen in Perch-Nielsen et al. (1978).Retecapsa ficula Stover (1966) Burnett (1998).Retecapsa surirella (Deflandre and Fert, 1954) Grün in

Grün and Allemann (1975).Retecapsa schizobrachiata (Gartner, 1968) Grün in Grün

and Allemann (1975).Stradneria crenulata (Bramlette and Martini, 1964) Noël

(1970).T. decorus (Deflandre in Deflandre and Fert, 1954) Wind

and Wise in Wise and Wind (1977).T. operculata Bramlette and Martini (1964).W. barnesiae (Black, 1959) Perch-Nielsen (1968).Z. spiralis (Bramlette and Martini, 1961) Burnett (1998).Z. bicrescenticus (Stover, 1966) Burnett in Gale et al.

(1996).Zeugrhabdotus embergeri (Noël, 1958) Perch-Nielsen

(1984).Z. erectus (Deflandre in Deflandre and Fert, 1954) Rein-

hardt (1965).Zeugrhabdotus sigmoides (Bramlette and Sullivan, 1961)

Bown and Young (1997).

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