High-resolution nannofossil biochronology of middle Paleocene to early Eocene at ODP Site 1262: Implications for calcareous nannoplankton evolution

Marine Micropaleontology 64 (2007) 215 – 248 www.elsevier.com/locate/marmicro

High-resolution nannofossil biochronology of middle Paleocene to early Eocene at ODP Site 1262: Implications for calcareous nannoplankton evolution Claudia Agnini a,⁎, Eliana Fornaciari a , Isabella Raffi b , Domenico Rio a,c , Ursula Röhl d , Thomas Westerhold d a

Dipartimento di Geoscienze, Università di Padova, Via Giotto, 1, I-35137 Padova, Italy Dipartimento di Geotecnologie per l'Ambiente e il Territorio, Università “G. d'Annunzio” di Chieti-Pescara, Campus Universitario, Via dei Vestini 31, I-66013 Chieti Scalo, Italy Istituto di Geoscienze e Georisorse, CNR-Padova c/o Dipartimento di Geoscienze, Università di Padova, I-35137 Padova, Italy d Center for Marine Environmental Sciences (MARUM), Bremen University, Leobener 8 Strasse, Bremen, 28359, Germany b

c

Received 19 January 2007; received in revised form 8 May 2007; accepted 9 May 2007

Abstract Over the last several decades debates on the ‘tempo and mode’ of evolution have centered on the question whether morphological evolution preferentially occurs gradually or punctuated, i.e., with long periods of stasis alternating with short periods of rapid morphological change and generation of new species. Another major debate is focused on the question whether long-term evolution is driven by, or at least strongly influenced by changes in the environment, or by interaction with other life forms. Microfossils offer a unique opportunity to obtain the large datasets as well as the precision in dating of subsequent samples to study both these questions. We present high-resolution analyses of selected calcareous nannofossils from the deep-sea section recovered at ODP Site 1262 (Leg 208) in the South-eastern Atlantic. The studied section encompasses nannofossil Zones NP4–NP12 (equivalent to CP3–CP10) and Chrons C27r–C24n.We document more than 70 biohorizons occurring over an about 10 Myr time interval, (∼62.5 Ma to ∼52.5 Ma), and discuss their reliability and reproducibility with respect to previous data, thus providing an improved biostratigraphic framework, which we relate to magnetostratigraphic information, and present for two possible options of a new Paleocene stratigraphic framework based on cyclostratigraphy. This new framework enabled us to tentatively reconstruct steps in the evolution of early Paleogene calcareous nannoplankton through documentation of transitional morphotypes between genera and/or species and of the phylogenetic relations between the genera Fasciculithus, Heliolithus, Discoasteroides and Discoaster, as well as between Rhomboaster and Tribrachiatus. The exceptional record provided by the continuous, composite sequence recovered at Walvis Ridge allows us to describe the mode of evolution among calcareous nannoplankton: new genera and/or new species usually originated through branching of lineages via gradual, but relatively rapid, morphological transitions, as documented by the presence of intermediate forms between the end-member ancestral and descendant forms. Significant modifications in the calcareous nannofossil assemblages are often “related” to significant changes in environmental conditions, but the appearance of structural innovations and radiations within a single genus also occurred during “stable”

⁎ Corresponding author. Fax: +39 049 8272070. E-mail address: [email protected] (C. Agnini). 0377-8398/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.marmicro.2007.05.003

216

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

environmental conditions. These lines of evidence suggest that nannoplankton evolution is not always directly triggered by stressed environmental conditions but could be also driven by endogenous biotic control. © 2007 Elsevier B.V. All rights reserved. Keywords: Calcareous nannofossils; Biochronology; Evolution; ODP; Early Paleogene

1. Introduction Within the multifaceted evolution of Earth's climate over the past 65 Ma, the Paleogene time interval represents a transition from the Cretaceous “greenhouse world” to the Oligocene “icehouse world” (Hambrey et al., 1991; Zachos et al., 1992, 2001). The Paleogene is a crucial interval, both for biotic evolution and environmental changes, and during this period the global warming that persisted through the early Paleocene finally ended during the early Eocene (EECO — Early Eocene Climatic Optimum) (Zachos et al., 2001, 2003; Wing et al., 2003; Billups et al., 2004). During this interval the global environment did not change in a simple, unidirectional way; on the contrary several shortterm, rapid changes have recently been documented, including the Danian–Selandian transition (Bralower et al., 2002), the ELPE (Early Late Paleocene Event; Bralower et al., 2002; Petrizzo, 2005), the PETM (Paleocene Eocene Thermal Event; Kennett and Stott, 1991), the ELMO (Early Eocene Layer of Mysterious Origin; Lourens et al., 2005) and the X-event (Röhl et al., 2004, 2005). These are critical events which witness that intricate evolution in global climate and biota occurred during the early Paleogene. In order to document their possible interactions, we need a high-resolution age model as well as detailed documentation on changes in the biota. The integrated use of calcareous plankton biostratigraphy, magnetostratigraphy and astrono-cyclostratigraphy provides a detailed chronological framework that is essential for estimating the timing of the successive steps occurring before, during and after each of these “transient events”. Over the last two decades, calcareous nannofossil biostratigraphy has shown great improvement through the intensive studies on many deep-sea and on-land sedimentary sections. This refinement was essentially due to the improvement of data acquisition, based on: (1) increased biostratigraphic resolution, through the description of a greater range of biostratigraphic signals in addition to the classical markers; (2) integration and correlation to other stratigraphic data. This long-term combined effort has generated excellent results for various intervals of geologic time. Specifically, the

Neogene has been studied in detail, and the availability of complete sedimentary successions with robust chronology (based on magneto- and/or cyclostratigraphy), obtained for reference sections recovered in many ODP holes, has led to great improvement in accuracy and precision of age models (see review in Raffi et al., 2006). In contrast, little improvement has been made in Paleogene calcareous nannofossil biostratigraphy and biochronology (see review in Luterbacher et al., 2004). The known biochronologic data for calcareous nannofossils have been derived from calibrations to magnetostratigraphy of the biohorizons used in “standard” Paleogene Zonations (NP — Martini, 1971; CP — Okada and Bukry, 1980), that are mainly based on the work of earlier authors (e.g., Bramlette and Sullivan, 1961; Hay and Mohler, 1967; Bukry, 1973, 1975, 1978), with a synthesis presented in Berggren et al. (1995), who summarized, discussed and revised all available data. This synthesis shows that the bio-magneto-chronostratigraphic scheme available for the Paleogene is based on relatively poor magnetostratigraphies and by now outdated biostratigraphic data, and thus results in an obsolete biochronology. With the recent extension of astronomical calibration to lower Paleogene sedimentary successions (e.g., Pälike et al., 2001, 2004; Dinarès-Turell et al., 2002; Cramer et al., 2003; Röhl et al., 2003; Westerhold et al., 2007; Westerhold, personal communication), there is now an opportunity to improve the nannofossil biostratigraphy and its biochronologic resolution using astronomically dated sediment successions. A record of lower Paleogene cyclic sediments was recovered during ODP Leg 208 in the subtropical Southeast Atlantic, and provides one of the few complete composite sections available. The ODP Site 1262 is the deep end-member of a depth transect of sites drilled on the flank of Walvis Ridge (Fig. 1; Zachos et al., 2004). At this site, a composite, complete sedimentary succession was generated from drilling multiple holes, spanning from the Paleocene through the lower Eocene. Previous studies of calcareous nannofossils in the sedimentary successions of DSDP Sites 527 and 528, recovered in the same area (DSDP Leg 74, Moore et al., 1984), provided a detailed biostratigraphic and biochronologic framework for the upper Paleocene–lower Eocene (Backman, 1986). The section recovered at Site 1262 is thus suitable

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

217

Fig. 1. Three-dimensional diagram of the Leg 208 drill site locations (modified after Zachos et al., 2004). The small map projection indicates the location of ODP Leg 208 shown on paleogeographic map at 55 Ma (http://www.odsn.de/odsn/services/paleomap/paleomap.html).

to carry out a high-resolution study on calcareous nannofossil assemblages in order to improve the existing time scales through an update of biostratigraphy and biochronology, using orbital (Westerhold et al., 2007; Westerhold, personal communication), and magnetostratigraphic data (Bowles, 2006). Specifically, this paper is focused on the biostratigraphy, biochronology and evolutionary lineages of calcareous nannofossils in the interval from the upper part of Chron C27r to Chron C24n (∼62.5 to ∼52.5 Ma; NP4-CP3 to NP12-CP9b; Martini, 1971; Okada and Bukry, 1980), i.e., late early Paleocene–early Eocene times. The aims of the present paper are: (1) to document biostratigraphic markers and compare the data to those in the literature; (2) to update and improve the calcareous nannofossil biostratigraphic framework by means of quantitative evaluation of the abundance patterns of selected taxa, which include standard markers as well as new or poorly known index species, (3) to update and improve the existing calcareous nannofossil biochronology; (4) to use this revised framework to highlight the evolution of selected taxa of calcareous nannoplankton, with special attention to origination of new lineages and speciation within lineages.

2. Material and methods 2.1. Calcareous nannofossil quantitative distribution patterns The composite sedimentary section of ODP Site 1262 was sampled in the interval from 199.68 to 104.63mcd at an average spacing of 20 cm, except for the PETM interval in which samples were taken every 2–3 cm. Analyses were carried out on 558 smear-slides, prepared using the standard techniques, and observed in the light microscope at a magnification of 1250×. The calcareous nannofossil assemblages are rich and well diversified throughout the studied interval except for the clay sediments at the PETM, which are barren of nannofossils. The preservation varies from moderate to good and the dissolution and/or overgrowth seem to variably affect the assemblages. Specifically, the Paleocene sediments are usually characterized by a better preservation of the calcareous nannofossils, whereas the lower Eocene sediments show more overgrowth on the discoasterids and Rhomboaster– Tribrachiatus specimens, and dissolution in the rest of the assemblage. Though a modest dissolution is a

218

Table 1 Calcareous nannofossil biohorizons at Site 1262 NP

CP

Mean depth Mean age Err Mean age Err Chron % from Chron % from Age (Myr) relative Age (Myr) relative Chron % from Age (Ma) (mcd) (Ma) option1 (kyr) ± (Ma) option2 (kyr) ± top option1 top option2 to the PETM option1 to the PETM option2 top (GPTS1,2) (GPTS1,2)

1 Ellipsolithus HO 2 Chiprhagmalithus LCO 3 Discoaster lodoensis LCO 4 Girgisia gammation LO 5 Discoaster lodoensis LO 6 Discoaster multiradiatus HO 7 Sphenolithus villae LO 8 Sphenolithus orphanknollensis LO 9 Tribrachiatus contortus HO 10 Sphenolithus radians LO 11 Discoaster multiradiatus HCO 12 Sphenolithus editus LO 13 Tribrachiatus orthostylus LO 14 Tribrachiatus bramlettei HO 15 Tribrachiatus digitalis morphotype HO 16 Tribrachiatus bramlettei decrease 17 Tribrachiatus digitalis morphotype LO 18 Tribrachiatus contortus LO 19 Discoaster diastypus LO 20 Tribrachiatus bramlettei LO 21 Fasciculithus HO 22 R. calcitrapa gr. HO 23 Fasciculithus/Zygrhablithus CO 24 Fasciculithus decrease 25 Zygrhablithus bijugatus LCO 26 Campylosphaera eodela LCO 27 Tribrachiatus bramlettei LRO 28 Rhomboaster LO/R. calcitrapa gr. LO 29 P/E boundary 30 Fasciculithus alanii HO 31 Fasciculithus decrease in diversity 32 Campylosphaera eodela LO 33 Zygrhablithus bijugatus LO 34 Fasciculithus alanii LO 35 Ericsonia robusta HO

106.65 107.67 NP12 CP10 107.67 110.09 113.52

53.242

15

53.618

15

113.52 117.30 117.30

53.242 53.727 53.727

15 26 26

53.618 54.104 54.104

15 27 27

C24r.063 C24r.063

NP11 CP9b 118.09 118.72 119.38

53.795 53.849 53.898

9 9 7

54.173 54.229 54.278

9 9 7

120.01 120.67 121.30 123.88

53.945 53.995 54.042 54.223

8 8 8 7

54.325 54.375 54.422 54.603

124.87

54.292

7

125.08

54.307

CP9a 125.50 CP9a 127.45 133.34 135.87 139.72 139.80

− 2.288

− 2.312

C24n.1n? C24n.1n? C24n.1n? C24n.r? C24n.3n

52.374 52.525 52.525 52.796 53.111

C24r.056 C24r.056

− 2.288 − 1.803 − 1.803

− 2.312 − 1.826 − 1.826

C24n.3n C24r.035 C24r.035

53.111 53.435 53.435

C24r.085 C24r.102 C24r.118

C24r.078 C24r.096 C24r.112

− 1.735 − 1.681 − 1.632

− 1.757 − 1.701 − 1.652

C24r.056 C24r.073 C24r.090

53.489 53.532 53.578

8 8 8 7

C24r.133 C24r.149 C24r.164 C24r.222

C24r.127 C24r.143 C24r.158 C24r.216

− 1.585 − 1.535 − 1.488 − 1.307

− 1.605 − 1.555 − 1.508 − 1.327

C24r.107 C24r.125 C24r.142 C24r.211

53.621 53.666 53.709 53.886

54.672

7

C24r.244

C24r.238

− 1.238

− 1.258

C24r.237

53.954

7

54.687

7

C24r.249

C24r.243

− 1.223

− 1.243

C24r.243

53.968

54.336 54.477 54.924 55.109 55.455 55.466

7 8 8 8 1 2

54.716 54.857 55.308 55.488 55.857 55.871

7 8 8 7 1 3

C24r.258 C24r.303 C24r.446 C24r.505 C24r.616 C24r.620

C24r.252 C24r.297 C24r.442 C24r.499 C24r.618 C24r.622

− 1.194 − 1.053 − 0.606 − 0.421 − 0.075 − 0.064

− 1.214 − 1.073 − 0.622 − 0.442 − 0.073 − 0.059

C24r.254 C24r.307 C24r.464 C24r.532 C24r.636 C24r.638

53.997 54.131 54.535 54.708 54.972 54.978

139.80 139.82 CP8b 139.92 NP10 139.99 NP9b CP8b 140.02

55.466 55.469 55.483 55.492 55.496

2 1 11 2 1

55.871 55.875 55.893 55.904 55.909

3 1 14 3 1

C24r.620 C24r.621 C24r.625 C24r.628 C24r.629

C24r.622 C24r.624 C24r.629 C24r.633 C24r.635

− 0.064 − 0.061 − 0.047 − 0.038 − 0.034

− 0.059 − 0.055 − 0.037 − 0.026 − 0.021

C24r.638 C24r.638 C24r.641 C24r.643 C24r.644

54.978 54.979 54.986 54.991 54.993

140.13 140.15 140.15

55.530 55.514 55.514

10 1 1

55.930 55.932 55.932

10 1 1

C24r.640 C24r.642 C24r.642

C24r.640 C24r.642 C24r.642

0.000 − 0.016 − 0.016

0.000 0.002 0.002

– C24r.647 C24r.647

– 55.002 55.002

148.19 149.90 151.82 153.32

56.241 56.378 56.537 56.660

8 9 9 9

56.621 56.758 56.922 57.055

8 10 8 9

C24r.868 C24r.912 C24r.963 C25n.011

C24r.863 C24r.907 C24r.959 C25n.011

0.711 0.848 1.007 1.130

0.691 0.828 0.992 1.125

C24r.863 C24r.908 C24r.960 C25n.002

55.553 55.670 55.802 55.905

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

Biohorizon

NP9a

CP7

NP8

NP7

CP6

NP6

CP5

NP5

CP4

154.16 154.61 155.66

56.727 56.759 56.834

7 7 8

57.127 57.159 57.234

7 7 8

C25n.134 C25n.193 C25n.331

C25n.139 C25n.195 C25n.328

1.197 1.229 1.304

1.197 1.229 1.304

C25n.146 C25n.224 C25n.406

55.975 56.013 56.102

155.87 156.08 156.92 157.13 157.35 157.89 158.00 158.21 159.16

56.850 56.865 56.928 56.945 56.968 57.027 57.038 57.060 57.159

8 8 8 11 11 22 11 11 1

57.251 57.268 57.336 57.355 57.379 57.441 57.453 57.476 57.577

8 8 8 11 12 24 12 12 1

C25n.359 C25n.388 C25n.504 C25n.536 C25n.578 C25n.685 C25n.706 C25n.747 C25n.929

C25n.358 C25n.388 C25n.508 C25n.541 C25n.584 C25n.693 C25n.714 C25n.757 C25n.934

1.320 1.335 1.398 1.415 1.438 1.497 1.508 1.530 1.629

1.321 1.338 1.406 1.425 1.449 1.511 1.523 1.546 1.647

C25n.443 C25n.482 C25n.625 C25n.662 C25n.699 C25n.812 C25n.794 C25n.848 C25r.016

56.120 56.139 56.208 56.226 56.245 56.299 56.291 56.317 56.399

159.49

57.192

11

57.608

10

C25n.989

C25n.990

1.662

1.678

C25r.033

56.429

165.00 – 168.05 169.52

57.659 – 58.060 58.267

16 – 16 14

58.071 – 58.455 58.647

19 – 15 14

C25r.332 – C25r.619 C25r.768

C25r.336 – C25r.619 C25r.760

2.129 – 2.530 2.737

2.141 – 2.525 2.717

C25r.481 – C25r.728 C25r.847

56.950 – 57.237 57.376

169.52 169.95 170.15 171.50 171.71 171.92 172.13 173.22 173.93

58.267 58.327 58.356 58.545 58.591 58.636 58.671 58.842 58.995

14 15 14 20 23 21 16 12 10

58.647 58.708 58.737 58.929 58.975 59.020 59.055 59.232 59.408

14 15 14 20 23 21 16 14 11

C25r.768 C25r.811 C25r.832 C25r.968 C26n.004 C26n.171 C26n.297 C26n.925 C26r.046

C25r.760 C25r.804 C25r.826 C25r.967 C26n.003 C26n.165 C26n.289 C26n.915 C26r.053

2.737 2.797 2.826 3.015 3.061 3.106 3.141 3.312 3.465

2.717 2.778 2.807 2.999 3.045 3.090 3.125 3.302 3.478

C25r.847 C25r.882 C25r.899 C26n.054 C26n.169 C26n.285 C26n.401 C26n.998 C26r.032

57.376 57.417 57.436 57.573 57.614 57.656 57.697 57.910 58.009

174.05 174.44 175.18 175.39 175.81 184.91 188.03

59.021 59.107 59.254 59.306 59.369 60.460 60.902

16 21 26 22 15 8 14

59.438 59.536 59.692 59.746 59.799 60.848 61.274

19 25 27 21 13 8 14

C26r.055 C26r.084 C26r.135 C26r.152 C26r.174 C26r.549 C26r.700

C26r.063 C26r.097 C26r.151 C26r.169 C26r.188 C26r.549 C26r.696

3.491 3.577 3.724 3.776 3.839 4.930 5.372

3.508 3.606 3.762 3.816 3.869 4.918 5.344

C26r.038 C26r.056 C26r.089 C26r.099 C26r.118 C26r.504 C26r.672

58.025 58.079 58.179 58.208 58.265 59.426 59.934

190.76 190.76 191.72

61.224 61.224 61.339

12 12 25

61.602 61.602 61.719

13 13 25

C26r.811 C26r.811 C26r.851

C26r.810 C26r.810 C26r.850

5.694 5.694 5.809

5.672 5.672 5.789

C26r.796 C26r.796 C26r.840

60.306 60.306 60.437

193.92 196.85 201.04

61.593 61.948 62.530

11 13 15

61.973 62.322 62.911

11 13 16

C26r.938 C27n.569 C27r.444

C26r.938 C27n.569 C27r.449

6.063 6.418 7.000

6.043 6.392 6.981

C26r.940 C27n.638 C27r.488

60.738 61.147 61.873

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

36 Fasciculithus richardii gr. LO 37 Discoaster multiradiatus LO 38 Discoaster multiradiatus LRO 39 Discoaster okadai HO 40 Discoaster delicatus gr. LCO 41 Discoaster delicatus gr. LO 42 Discoaster okadai LO 43 Discoaster nobilis gr. LO 44 Fasciculithus tonii LO 45 Ericsonia robusta LO 46 Discoaster sp.1 HCO 47 Discoasteroides megastypus LO 48 Fasciculithus richardii gr. LRO 49 Discoaster sp.1 LO 50 Heliolithus riedeli LO 51 Heliolithus bukryi HO 52 Sphenolithus anarrhopus gr. LCO 53 Sphenolithus sp.1 LCO 54 Heliolithus kleinpellii HCO 55 Heliolithus bukryi LO 56 Discoaster LO/D. mohleri LO 57 Sphenolithus cf. conicus LO 58 Heliolithus cantabriae HO 59 Fasciculithus clinatus LO 60 Ellipsolithus LCO 61 Discoasteroides bramlettei gr. LO 62 Heliolithus kleinpellii LO 63 Sphenolithus anarrhopus LO 64 cf. Discoaster LO 66 Sphenolithus sp.1 LO 65 Heliolithus cantabriae LO 67 Fasciculithus billii LO 68 Fasciculithus tympaniformis gr. LO 69 Fasciculithus pileatus LO 70 Fasciculithus LO/F. ulii LO 71 Neochiastozygus perfectus LO 72 Sphenolithus LO 74 C. bidens gr. LO 75 T. pertusus LO

Biostratigraphic data are tied to magnetostratigraphy (Bowles, 2006) and calibrated both with cyclostratigraphy (Westerhold et al., 2007; Westerhold, personal communication) and GPTS1,2 (Cande and Kent, 1995; Lourens et al., 2004). Age (Myr) relative to the PETM of calcareous nannofossil biohorizons are proposed for two different options (Westerhold et al., 2007; Westerhold, personal communication). Biohorizons employed in standard Zonation (NP — Martini, 1971; CP — Okada and Bukry, 1980) are also emphasized.

219

220

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

Fig. 2. Abundance patterns of marker species employed in the Paleogene standard Zonations (NP — Martini, 1971; CP — Okada and Bukry, 1980) and other selected nannofossil taxa are plotted against magnetostratigraphy (Bowles, 2006) and biostratigraphy (Martini, 1971; Okada and Bukry, 1980). The position of the ODP Site 1262 cores (Zachos et al., 2004) is also reported. The X-axis values represent the number of specimens in a prefixed area (N/mm2), except when it is differently specified (%). The light grey shaded bars emphasize the ELPE, the PETM, the ELMO and the X-event. Dashed lines highlight the NP and CP Zonal boundaries.

persistent character of the sediments recovered at Site 1262, it becomes more intense in the calcareous nannofossil assemblages observed in the sediments deposited during the “transient events” of the ELPE, ELMO and X-event. The taxonomy used is that of Aubry (1984, 1988, 1989, 1990, 1999), Perch-Nielsen (1985) and Romein (1979), apart from the Rhomboaster–Tribrachiatus plexus for which we preferred to follow the taxonomic concept proposed by Raffi and coauthors (2005). We reconstructed the distribution patterns of index species and genera that characterized the early Paleogene calcareous nannofossil assemblages. Abundance of selected taxa was determined by counting the

number of specimens in a prefixed area (N/mm2 ) (Backman and Shackleton, 1983). Abundance patterns of the genus Discoaster was also obtained by counting a prefixed number of taxonomically related forms, i.e., 50–100 discoasterids (Rio et al., 1990). The Zonations adopted in this study are those of Martini (1971; NP Zones) and Okada and Bukry (1980; CP Zones). 2.2. Age model The cyclic sediments of ODP Site 1262 recovered during ODP Leg 208 at Walvis Ridge are an important part of the first complete astronomically calibrated

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

stratigraphic framework covering the entire Paleocene epoch and the early Eocene (Magnetochrons C30n through C24r). The sediment of Magnetochrons C24r and C25n yields a continuous cyclostratigraphy down to the precession cycle scale and provided the exact relative position of critical intervals like the PETM and ELMO events within Magnetochron C24r (Westerhold et al., 2007). A complete Paleocene cyclostratigraphy based on the identification of the stable long-eccentricity cycle (405-kyr) has been established by integrating highresolution records from Sites 1209, 1210, 1211 drilled during Leg 198 on the Shatsky Rise in the NW Pacific, and from Walvis Ridge Sites 1262 and 1267 (Westerhold, personal communication). Due to combined limits and uncertainties of current astronomical calculations (Laskar et al., 2004) and radiometric age constraints for this time interval (Machlus et al., 2004) no accurate and definite absolute age datums for the late Eocene and Paleocene can be provided at this moment (Kuiper et al., 2004, 2005; Lourens et al., 2005; Westerhold et al., 2007; Westerhold, personal communication). However, two alternative options correlating the early Eocene and Paleocene record to the Laskar et al. (2004) orbital eccentricity have been developed. Because the record is calibrated to the stable 405-kyr cycle the relative distance of biohorizons and the relative position to the magnetochrons is already very accurate and will be subject of only minor changes once a new early Paleogene Time Scale will be defined. The relative position of biohorizons within magnetochrons follows the system of Hallam et al. (1985) and the recommendation of Cande and Kent (1992) to use an inverted stratigraphic placement relative to the present. Table 1 provides the ages and relative positions of bioevents in relation to magnetochrons for two possible options of a new Paleocene stratigraphic framework for Site 1262 (Westerhold, personal communication). 3. Calcareous nannofossil biostratigraphy and biochronology: results and discussion Here we describe and discuss all the biohorizons of the Paleogene “standard” Zonations (Martini, 1971; Okada and Bukry, 1980) and several additional biohorizons which have been considered as useful for improving the biostratigraphic/biochronologic resolution in the time interval of interest. The distribution patterns of the biostratigraphic markers of NP and CP Zones are reported in Fig. 2. The considered biohorizons have been defined based on abundance patterns of the index species and recognized as follows: Lowest Rare Occurrence (LRO), Lowest Occurrence (LO), Lowest Common

221

Occurrence (LCO), Highest Common Occurrence (HCO), Highest Occurrence (HO) and Crossover (CO). For a detailed definition of different types of biohorizons see Raffi et al. (2006). Below, we comment on the identified biohorizons listed in stratigraphic order. Their positions, calibrations and biostratigraphic use are reported in Table 1 (see also Table A, Supplementary Data). Microphotos of standard markers as well as of several calcareous nannofossil taxa are also provided (Plates I–IV). The complete list of all species mentioned in the text is available in Appendix A. 3.1. The LO of Ellipsolithus macellus The LO of Ellipsolithus macellus (Fig. 2; Plate IV) is used to define the base of Zones NP4 and CP3 (Martini, 1971; Okada and Bukry, 1980). The reliability of this biohorizon has been much debated in the past (e.g., Berggren et al., 1995, 2000) because of the discrepancies affecting previous calibrations (see Table B, Supplementary data). The distribution of E. macellus is considered controlled by preservation (dissolution) (Monechi et al., 1985) or different paleoecological conditions (Backman, 1986; Monechi et al., 1985). At ODP Site 1262, E. macellus shows a rare and sporadic presence since the base of the studied section, therefore it was not possible to observe its LO and define the exact position of the base of NP4 (Fig. 2). 3.2. The LO of Chiasmolithus bidens group Abundance of Chiasmolithus bidens, Chiasmolithus edentulus and Chiasmolithus solitus has been evaluated under a single taxonomic group (i.e., Chiasmolithus bidens gr.) because distinctive features of the three species are sometimes blurred and specimens with intermediate features were consistently observed. The LO of Chiasmolithus bidens (Fig. 2; Plate IV) is used in the North Sea Zonations (Perch-Nielsen, 1979; Van Heck and Prins, 1987; Varol, 1989) and recorded within the Zone NP4 (e.g., Romein, 1979; Berggren et al., 1995, 2000). At Site 1262, the LO of C. bidens gr. is observed at the top of Chron C27n (Fig. 2; Table 1) and this position is consistent with Berggren and coauthors (2000), which reported a sporadic occurrence of the index species in Chron C27n, followed by an increase in the lower part of Chron C26r. 3.3. The LO of the genera Sphenolithus and Fasciculithus Previous studies have pointed out that the lowermost occurrences of these two important Paleocene taxa are

222

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

closely related. It is generally believed that the genus Sphenolithus appeared just below the genus Fasciculithus (e.g. Romein, 1979; Monechi, 1985; Backman, 1986; Berggren et al., 1995), although other authors observed a reversal in the relative stratigraphic position of the two

biohorizons (e.g. Varol, 1989; Berggren et al., 2000). At Site 1262 the lowermost presence (LRO) of the genus Fasciculithus, including the species Fasciculithus magnicordis and Fasciculithus magnus (Fig. 5; Plate II), is recorded below the LO of the genus Sphenolithus. The

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

LO of Fasciculithus, coincident with the LOs of Fasciculithus ulii and Fasciculithus pileatus, is recorded above the lowest occurrence of Sphenolithus, coincident with the LO of Sphenolithus moriformis group (Figs. 2, 5 and 9; Table 1). Now that the LOs of Sphenolithus and Fasciculithus are well delineated they can be utilized for further subdividing the Zone NP4. 3.4. The LO of Fasciculithus tympaniformis group The LO of Fasciculithus tympaniformis (Fig. 5; Plate II) is used to define the base of the Zones NP5 and CP4 (Martini, 1971; Okada and Bukry, 1980). Here, we consider the F. tympaniformis group, in which Fasciculithus involutus and Fasciculithus bobii are also included. Some uncertainty arises in precisely placing this biohorizon in the studied section (Fig. 2), because of the presence of transitional forms between F. tympaniformis and other fasciculith species, namely Fasciculithus ulii–F. chowii gr., Fasciculithus pileatus, and Fasciculithus billii–Fasciculithus janii gr. (Fig. 5; Table 1). Nevertheless, our data are in good agreement with previous age estimates (Table B, Supplementary data) and confirm the reliability of this biohorizon.

223

3.6. The LO of Sphenolithus anarrhopus and Sphenolithus sp.1 In the North Sea Zonation (Varol, 1989), the lowest occurrence of Sphenolithus anarrhopus approximates the base of Zone NP7 (Martini, 1971), while it has been recorded very close to the base of Zone NP6 elsewhere (e.g., Romein, 1979; Berggren et al., 1995, 2000). In the Site 1262 succession, the NP6–NP7 interval is characterized by the presence of two apical spine bearing sphenoliths, namely S. anarrhopus and Sphenolithus sp.1 (Fig. 4; Plate I). The distribution pattern of S. anarrhopus shows rare and discontinuous abundances at the beginning of its range, followed by a sharp increase (defined as Lowest Common Occurrence — LCO) (Fig. 4; Table 1). The LO of S. anarrhopus is close to the LO of Heliolithus kleinpellii (base of Zone NP6) in agreement with previous findings (Romein, 1979; Berggren et al., 1995, 2000; Table B, Supplementary data), while the LCO of S. anarrhopus closely follows the LO of genus Discoaster, in agreement with Varol (1989) and Wei and Wise (1992). Hence, these biohorizons are considered reliable at least at mid latitude locations.

3.5. The LO of Neochiastozygus perfectus

3.7. The LOs and HOs of Heliolithus cantabriae and Heliolithus kleinpellii

Neochiastozygus perfectus (Fig. 2; Plate IV) is utilized in the North Sea Zonations (Perch-Nielsen, 1979; Van Heck and Prins, 1987; Varol, 1989). Specifically, its LO is proposed to be used for approximating the base of Zone NP5 of Martini's scheme (Perch-Nielsen, 1985), since fasciculiths are very rare at the high latitudes. At Site 1262 the index species shows a rare and discontinuous abundance pattern (Fig. 2), but its distribution range is consistent with previous findings from both mid- and high latitudes sections (Perch-Nielsen, 1979; Van Heck and Prins, 1987; Schimtz et al., 1998; Bernaola and Nuño-Arana, 2006), since the LO of N. perfectus is interposed between the LO of Sphenolithus and the LCO of Fasciculithus.

The LO of Heliolithus cantabriae (Fig. 6; Plate II) marks the appearance of the genus Heliolithus (PerchNielsen, 1985) and predates the LO of H. kleinpellii (Fig. 6; Plate II). We observed transitional forms between H. cantabriae and H. kleinpellii that sometimes prevented the precise identification of this biohorizon (Backman, 1986; Wei and Wise, 1989; Varol, 1989). At Site 1262, the LO of H. cantabriae is a neat event, characterized by a sharp increase in abundance of the species, occurring in the upper part of Chron C26r (Fig. 6; Table 1). This result is in agreement with previous data from Walvis Ridge (Backman, 1986) and confirms the reliability of LO of H. cantabriae as additional biohorizon. By contrast, the low abundance of

Plate I. Microphotographs of calcareous nannofossil from the ODP Leg 208, Site 1262 in the middle Paleocene–early Eocene interval. All specimens × 1200. 1. Octolithus. Crossed nicols. Sample 208-1262B-20H-4W, 08. 2, 3. Octolithus–Sphenolithus intergrade; crossed nicols. Sample 208-1262B-20H4W, 08. 4–7. cf. Sphenolithus. Sample 208-1262B-20H-4W, 08. 4, 6. Crossed nicols 0°. 5, 7. Crossed nicols 45°. 8, 9. Sphenolithus Deflandre in Grassé. Sample 208-1262C-10H-6W, 02. 8. Crossed nicols 0°. 9. Crossed nicols 45°. 10, 11. Sphenolithus cf. conicus Bukry. Sample 208-1262B-18H-3W, 61. 10. Crossed nicols 0°. 11. Crossed nicols 45°. 12–17. Sphenolithus sp.1. Sample 208-1262B-17H-5W, 85. 12. Crossed nicols 0°. 13. Crossed nicols 30°. 14. Crossed nicols 45°. Sample 208-1262A-15H-3W, 44. 15. Crossed nicols 0°. 16. Crossed nicols 30°. 17. Crossed nicols 45°. 18–20. Sphenolithus anarrhopus Bukry & Bramlette. “Short” apical spine. Sample 208-1262B-17H-5W, 85. 18. Crossed nicols 0°. 19. Crossed nicols 30°. 20. Crossed nicols 45°. 21–23. Sphenolithus anarrhopus Bukry & Bramlette. “Long” apical spine. Sample 208-1262A-15H-4W, 107. 21. Crossed nicols 0°. 22. Crossed nicols 30°. 23. Crossed nicols 45°. 24, 25. Sphenolithus editus Perch-Nielsen in Perch-Nielsen et al. Sample 208-1262A-10H-3W, 86. 24. Crossed nicols 0°. 25. Crossed nicols 45°. 26, 27. Sphenolithus radians. Deflandre in Grassé. Sample 208-1262A-10H-3W, 86. 26. Crossed nicols 0°. 27. Crossed nicols 45°. 28–30. Sphenolithus villae Bown. Sample 208-1262A-11H-4W, 149. 28. Crossed nicols 0°. 29. Crossed nicols 30°. 30. Crossed nicols 45°.

224

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

H. cantabriae in the upper part of its range, and the discrepancies on the position of its HO among previous studies (Backman, 1986; Romein, 1979; Varol, 1989) suggest that this biohorizon should be used with caution for biostratigraphic classification.

The LO of H. kleinpellii is used to define the base of Zones NP6 and CP5 (Martini, 1971; Okada and Bukry, 1980). Controversial statements on the reliability of this biohorizon have been previously reported: while a possible diachrony was inferred for this event by Wei

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

and Wise (1989), other authors considered it a reliable event (Berggren et al., 1995). At Site 1262 the LO of H. kleinpellii is well delineated (Figs. 2, 6; Table 1), and its position is consistent with previous data by Backman (1986) and recent data from Shatsky Rise (Bralower et al., 2002). In the uppermost part of its distribution range, H. kleinpellii shows a sharp decrease in abundance, defined as Highest Common Occurrence (HCO). This biohorizon can be considered a good additional event, as already suggested by Backman (1986), though further studies would be necessary to validate its reliability. 3.8. The LOs of Discoasteroides bramlettei group and Discoasteroides megastypus Discoasteroides bramlettei group includes different morphotypes with analogous distribution ranges. In the studied section, D. bramlettei gr. shows a clear short stratigraphic range that coincides with that of H. kleinpellii (Fig. 6; Table 1; Plate III), as already observed by Backman (1986) in this oceanic area. At Site 1262, specimens belonging to Discoasteroides megastypus (Fig. 6; Plate III) show a discontinuous distribution and are generally very rare in the interval corresponding to Zones NP6 and NP7–NP8 (Fig. 6; Table 1) making the delineation of D. megastypus LO biohorizon difficult. It is here considered an unreliable biohorizon, in agreement with previous data which reported the event either in the upper part of NP8 or at the base of NP9 (Romein, 1979; Wei and Wise, 1992). It is noteworthy that we recorded an increase in abundance (LCO) of D. megastypus in the upper part of Zone NP8 at Site 1262 (Fig. 6). 3.9. The LO of Fasciculithus clinatus At ODP Site 1262, the LO of Fasciculithus clinatus (Fig. 5; Plate II) virtually coincides with the base of Zone

225

NP7–NP8 (Fig. 5; Table 1) in agreement with previous findings (Romein, 1979; Perch-Nielsen, 1985). The index species shows a continuous distribution all along the Paleocene interval of the studied section, and a relevant increase to relatively high abundance in correspondence with the lower part of Chron C24r. 3.10. The LO of genus Discoaster (= LO of Discoaster mohleri) The LO of Discoaster mohleri (Fig. 7; Plate III)is used to define the base of Zones NP7 (Martini, 1971) and CP6 (Okada and Bukry, 1980), and marks the appearance of the genus Discoaster. In the studied section, D. mohleri has a distinctive distribution in the lower part of the range, and its LO biohorizon is recorded very close to Chrons C26/C25 boundary (Fig. 7; Table 1). It is confirmed to be a reliable biohorizon because this finding at Walvis Ridge is consistent with data reported from different areas (Backman, 1986; Monechi and Thierstein, 1985; Monechi et al., 1985; Wei and Wise, 1989; Berggren et al., 1995, 2000; Table B, Supplementary data). 3.11. The LO of Heliolithus riedeli The LO of Heliolithus riedeli defines the base of Zone NP8 (Martini, 1971), but this biohorizon is not considered reliable. In fact, H. riedeli is affected by significant taxonomic ambiguities (Varol, 1989) and considered biogeographically controlled (Romein, 1979; Aubry, 1989). In addition, previous studies report significant differences in the stratigraphic range of H. riedeli at different locations (Okada and Thierstein, 1979; Berggren et al., 2000). At Site 1262, specimens with ambiguous taxonomic features prevented to confidently identify the index species and consequently it was not possible to differentiate Zones NP7 and NP8.

Plate II. Microphotographs of calcareous nannofossil from the ODP Leg 208, Site 1262 in the middle Paleocene–early Eocene interval. All specimens × 1200. 1. Fasciculithus magnus Bukry & Percival; crossed nicols. Sample 208-1262C-11H-6W, 17. 2. Fasciculithus pileatus Bukry; crossed nicols. Sample 208-1262C-10H-4W, 50. 3, 4. Fasciculithus ulii Perch-Nielsen; crossed nicols. 3. Sample 208-1262C-10H-5W, 100. 4. Transitional to Fasciculithus tympaniformis; sample 208-1262C-10H-4W, 50. 5, 6. Fasciculithus tympaniformis Hay & Mohler in Perch-Nielsen et al.; crossed nicols. 5. Sample 2081262C-10H-4W, 50. 6. Sample 208-1262C-10H-3W, 32. 7. Fasciculithus billii. Perch-Nielsen; crossed nicols. Sample 208-1262B-19H-4W, 58. 8. Fasciculithus clinatus Bukry; crossed nicols. Sample 208-1262B-17H-5W, 85. 9. Fasciculithus hayi gr. Haq; crossed nicols. Sample 208-1262B-16H4W, 45. 10. Fasciculithus lilianae Perch-Nielsen; crossed nicols. Sample 208-1262B-16H-4W, 45. 11, 12. Fasciculithus alanii Perch-Nielsen; crossed nicols. Sample 208-1262A-14H-5W, 44. 13. Fasciculithus richardii Perch-Nielsen; crossed nicols. Sample 208-1262A-14H-5W, 44. 14. Fasciculithus thomasii Perch-Nielsen; crossed nicols. Sample 208-1262B-15H-3W, 49. 15, 16. Fasciculithus. Base; Sample 208-1262A-14H-5W, 85. 15. Parallel light. 16. Crossed nicols. 17, 18. Zygrhablithus bijugatus (Deflandre in Deflandre & Fert); crossed nicols. Sample 208-1262A-12H-3W, 44. 19, 20. Octolithus sp. Crossed nicols. 19. Sample 208-1262B-12H-4W, 86. 20. Sample 208-1262A-10H-3W, 86. 21. Bomolithus sp. Parallel light. Sample 208-1262A-17H-3W, 136. 22–24. Heliolithus cantabriae Perch-Nielsen; crossed nicols. 22. Primitive form; Sample 208-1262A-17H-2W, 139. 23. Sample 208-1262B-18H-4W, 118. 24. Transitional to Heliolithus kleinpellii; Sample 208-1262B-18H-4W, 118. 25–28. Heliolithus kleinpellii Sullivan; crossed nicols. 25, 26. Sample 208-1262B-18H-4W, 78. 27. Small form; Sample 208-1262B-18H-3W, 61. 28. Small form; Sample 208-1262B-18H-3W, 61. 29, 30. Heliolithus bukryi Wei; Sample 208-1262A-16H-4W, 145. 29. Crossed nicols. 30. Parallel light.

226

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

3.12. The LO and HO of Heliolithus bukryi In a restricted interval above the highest occurrences of H. cantabriae and H. kleinpellii, we observed several morphotypes of Heliolithus that show rare abundance

and no clear distribution ranges (not reported in Fig. 6), and probably represent different steps in the evolutionary lineage within the genus Heliolithus. Among these Heliolithus morphotypes an exception is represented by Heliolithus bukryi (Fig. 6; Plate II), which is a distinct

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

species characterized by a short range that can potentially be used for refinement of the biostratigraphic framework (Fig. 6; Table 1) within Zone NP7 (CP6). 3.13. The LO and HCO of Discoaster sp.1 and Discoaster okadai In the studied section, an asterolith with 5–7 (commonly 6) free rays was observed within the lower part of D. mohleri range, and labeled as Discoaster sp.1 (Fig. 7; Plate III; see also Okada and Thierstein, 1979; Monechi, 1985). This form seems evolutionally related to D. mohleri. The distribution pattern of Discoaster sp.1 is characterized by a very distinct increase in abundance in correspondence with its lowest occurrence, that leads to a dominance of this form within the Discoaster assemblage, and by a final sharp decrease in abundance just below its HCO (Fig. 7; Table 1). At this level, transitional forms between Discoaster sp.1 and Discoaster okadai are present and suggest that an evolutionary relationship existed between the two species. The LO of D. okadai (Fig. 7; Plate III) is recorded within Chron C25r, in agreement with previous data (Backman, 1986; Berggren et al., 1995), and this result evidences the potential usefulness of these biohorizons for global correlations. 3.14. The LO of Discoaster nobilis group The Discoaster nobilis group, in which we include Discoaster falcatus, Discoaster limbatus and Discoaster nobilis, is considered to be related to D. mohleri (Romein, 1979). The LO of D. nobilis (Fig. 7; Plate III) defines the base of Zone CP7 (Okada and Bukry, 1980), but has a contradictory position with respect to magnetostratigraphy. In fact, some authors have detected the LO of D. nobilis in the upper part of Chron C25r (Monechi and Thierstein, 1985; Berggren et al., 1995; 2000), whereas others recorded it at the base of Chron

227

C25n (Backman, 1986; Pospichal et al., 1991; Table B, Supplementary data). The latter stratigraphic position is in good agreement with our data (Figs. 1, 7; Table 1), that show the consistent presence of specimens belonging to D. nobilis gr. starting within Chron C25n. This LO biohorizon follows an interval (within most of Chron C25r interval) in which only scattered, very rare and atypical specimens of D. nobilis are present. 3.15. The LO of Discoaster delicatus group Within the Discoaster delicatus group we include the species Discoaster delicatus and Discoaster lenticularis. At Site 1262, the lowest occurrence of D. delicatus gr. (Fig. 7; Plate III) is recorded in the lower part of Chron C25n (Fig. 7; Table 1), and precedes the evolutionary appearance of D. multiradiatus (Perch-Nielsen, 1985). This biohorizon is characterized by a distinct pattern, with a clear increase in abundance, that makes it a potential event useful for the improvement of biostratigraphic resolution in this interval. 3.16. The LO of Discoaster multiradiatus The LO of D. multiradiatus (Fig. 7; Plate III) defines the base of Zones NP9 of Martini (1971) and CP8 of Okada and Bukry (1980), and represents one of the better established Paleocene datum (e.g., Bramlette and Sullivan, 1961; Hay and Mohler, 1967; Radomski, 1968; Edwards, 1971; Romein, 1979; Backman, 1986; Wei and Wise, 1989; Berggren et al., 1995; 2000; Raffi et al., 2005; Table B, Supplementary data). This biohorizon has been recorded in Chron C25n in both deep-sea and onland sections in the Atlantic, Pacific and Mediterranean (Backman, 1986; Raffi et al., 2005; Müller, 1985; Monechi et al., 1985). Results of this study confirm the reliability of the LO of D. multiradiatus, observed within Chron C25n (Figs. 1, 7; Table 1).

Plate III. Microphotographs of calcareous nannofossil from the ODP Leg 208, Site 1262 in the middle Paleocene–early Eocene interval. All specimens ×1200. 1. Discoasteroides bramlettei Bukry & Percival; crossed nicols. Sample 208-1262B-18H-3W, 61. 2. Discoasteroides cf. bramlettei (Bukry & Percival) sensu Perch-Nielsen; crossed nicols. Sample 208-1262B-18H-3W, 61. 3, 4. Discoasteroides megastypus Bramlette & Sullivan; Sample 208-1262A-15H-3W, 44. 3. Crossed nicols. 4. Parallel light. 5, 6. cf. Discoaster; parallel light. 5. Sample 208-1262B-18H-5W, 34. 6. Sample 208-1262A-17H-2W, 118. 7, 8. Discoaster mohleri Bukry & Percival; parallel light. 7. Sample 208-1262A-16H-4W, 124. 8. Small form; Sample 208-1262A-16H-4W, 103. 9–11. Discoaster nobilis gr. Martini; parallel light. 9, 10. Sample 208-1262A-15H-3W, 44. 11. Sample 2081262A-14H-5W, 85. 12, 13. Discoaster sp.1; parallel light. Sample 208-1262B-17H-2W, 136. 14. Discoaster sp.1–Discoaster okadai intergrade; parallel light. Sample 208-1262B-16H-4W, 88. 15. Discoaster okadai Bukry; parallel light. Sample 208-1262A-15H-3W, 44. 16–18. Discoaster delicatus gr. Bramlette & Sullivan; Sample 208-1262B-16H-4W, 45. 16, 17. Parallel light; different focus. 18. Crossed nicols. 19–21. Discoaster multiradiatus Bramlette & Riedel; parallel light. 19. Sample 208-1262A-19H-5W, 85. 20. Sample 208-1262A-14H-5W, 44. Sample 208-1262B-15H3W, 49. 22. Discoaster araneus Bukry; parallel light. Sample 208-1262B-15H-3W, 49. 23. Discoaster salisburgensis Stradner; parallel light. Sample 208-1262B-13H-5W, 107. 24. Discoaster diastypus. Bramlette & Sullivan; parallel light. Sample 208-1262A-11H-1W, 128. 25, 26. Discoaster diastypus–Discoaster lodoensis intergrade; parallel light. 25. Nine-rays; Sample 208-1262A-10H-3W, 107. 26. Eight-rays; Sample 208-1262B-12H4W, 86. 27, 28. Discoaster lodoensis Bramlette & Riedel; parallel light. 27. Sample 208-1262A-11H-1W, 128. 28. Sample 208-1262B-12H-4W, 86. 29–30. Discoaster kuepperi Stradner; parallel light. Sample 208-1262A-12H-3W, 128. Sample 208-1262B-13H-4W, 44.

228

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

3.17. The LO and HO of Ericsonia robusta The taxonomy of the species Cyclolithus? Robustus (Fig. 2; Plate IV), described by Bramlette and Sullivan (1961), was previously revised by different authors: Perch-Nielsen (1985; fig. 23) and Bralower and

Mutterlose (1995; plate 4) referred this species to as Ericsonia cf. E. robusta, whereas Wise and Wind (1977; plate 14) ascribed this peculiar coccolith to genus Heliolithus (i.e., Heliolithus universus) (see also discussion in Raffi et al., 2005). In previous studies on cores from Walvis Ridge, Backman (1986) proposed the use of this

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

taxon, as Ericsonia robusta, for the biostratigraphic classification in the late Paleocene. At Site 1262, the total range of E. robusta provides a distinct biostratigraphic feature. Both the recorded positions of LO and HO biohorizons are in agreement with previous data from literature. In fact, LO of E. robusta shortly precedes the first appearance of D. multiradiatus (Fig. 2; Table 1) (e.g. Berggren et al., 2000), whereas the HO correlates with C24r/C25n boundary (Backman, 1986; Raffi et al., 2005). 3.18. The LRO and LO of Fasciculithus richardii group The Fasciculithus richardii taxonomic group consists of large, heavily calcified fasciculiths characterized by having a pentagon-like outline, and referred to the different species of Fasciculithus hayi, Fasciculithus mitreus, Fasciculithus richardii and Fasciculithus schaubii. At Site 1262, we observed a first rare and discontinuous occurrence (LRO of F. richardii) which precedes the LO of the group, that is more consistently distributed from the base of Zone NP9 (Figs. 5, 9; Table 1; Plate II). A similar biostratigraphic position was pointed out in previous studies (Romein, 1979; PerchNielsen, 1985). 3.19. The LO of Fasciculithus alanii In the studied section, Fasciculithus alanii (Fig. 5; Plate II) is rare but shows a distinctive distribution within Zone NP9. Specifically, the lowest occurrence of the index species is recorded at the base of Chron C24r (Figs. 5, 9; Table 1) and provides a potential biohorizon biostratigraphically useful to further subdivide the upper Paleocene time interval. Its position is consistent with other recorded distributions (Romein, 1979; PerchNielsen, 1985).

229

3.20. The LO and LCO of Zyghrablithus bijugatus At Site 1262, the first specimens of Zyghrablithus bijugatus (Fig. 3; Plate II) have been detected within Zone NP9 in the upper Paleocene interval. The distribution pattern of Z. bijugatus shows a rare but relatively continuous presence in the lower part of its range (Paleocene time), followed by a temporary virtual absence of the species during the PETM, and by an abrupt increase (LCO) that occurred simultaneously with the onset of the PETM δ13C recovery interval (Fig. 3; Table 1). The LCO of Z. bijugatus is considered a reliable biohorizon and corresponds to the cross-over in abundance with fasciculiths (see below). 3.21. The distribution range of the genus Campylosphaera, the LO and LCO of Campylosphaera eodela The LO of the species Campylosphaera eodela (Fig. 3; Plate IV) was proposed to define the base of Zone CP8b, together with the LO of Rhomboaster (Okada and Bukry, 1980). At Site 1262, the LO of the genus Campylosphaera has been observed in Zone NP9 (CP8a) and different stratigraphic ranges were obtained for the two index taxa. In fact, the lowermost specimens of C. eodela were observed in the upper Paleocene, while that of Rhomboaster lies with the lowermost Eocene sediments (Fig. 2). The distribution range of the index species shows a variable pattern throughout the studied interval that evidences scattered and low abundance in the upper Paleocene, followed by a sharp increase (LCO) in correspondence with the P–E transition (PETM), concomitantly with the LO of the genus Rhomboaster (Table 1). C. eodela evolves in Campylosphaera dela in the early Eocene, throughout a progressive increase of the central area and a thinning of the central structure (see Plate IV). The distribution

Plate IV. Microphotographs of calcareous nannofossil from the ODP Leg 208, Site 1262 in the middle Paleocene–early Eocene interval. All specimens ×1200. 1, 2. Rhomboaster cuspis Bramlette & Sullivan; parallel light. Sample 208-1262B-15H-3W, 49. 3, 4. Rhomboaster calcitrapa Gartner; parallel light. Sample 208-1262B-15H-3W, 49. 5, 6. Tribrachiatus bramlettei (Brönnimann & Stradner) Proto Decima et al.; parallel light. 5. Sample 208-1262B-15H-3W, 49. 6. Sample 208-1262A-12H-3W, 128. 7. Tribrachiatus bramlettei–Tribrachiatus contortus intergrade; parallel light. Sample 208-1262B-13H-4W, 44. 8. Tribrachiatus contortus (Stradner) Bukry; parallel light. Sample 208-1262B-13H-4W, 44. 9. Tribrachiatus cf. digitalis Aubry; parallel light. Sample 208-1262A-12H-3W, 44. 10, 11. Tribrachiatus orthostylus Shamrai; parallel light. Sample 208-1262A-11H1W, 149. 12–14. Chiphragmalithus Bramlette & Sullivan; sample 208-1262A-10H-3W, 86. 12, 13. Parallel light. 14. Crossed nicols. 15. Cruciplacolithus tenuis (Stradner) Hay & Mohler in Hay et al.; crossed nicols. Sample 208-1262C-11H-2W, 59. 16. Prinsius bisulcus (Stradner) Hay & Mohler; crossed nicols. Sample 208-1262A-14H-5W, 85. 17. Toweius pertusus (Sullivan) Romein; crossed nicols. Sample 208-1262A-14H-5W, 44. 18. Elipsolithus bollii Perch-Nielsen; crossed nicols. Sample 208-1262C-11H-2W, 59. 19. Elipsolithus macellus (Bramlette & Sullivan) Sullivan; crossed nicols. Sample 208-1262A-11H-1W, 149. 20. Chiasmolithus bidens (Bramlette & Sullivan) Hay & Mohler; crossed nicols. Small form; Sample 208-1262C-11H-2W, 59. 21. Chiasmolithus solitus (Bramlette & Sullivan) Locker; crossed nicols. Sample 208-1262A-14H-5W, 85. 22, 23. Neochiastozygus perfectus Perch-Nielsen; Sample 208-1262B-19H-4W, 58. 22. Parallel light. 23. Crossed nicols. 24, 25. Ericsonia robusta Bramlette & Sullivan; Sample 208-1262A-15H-3W, 44. 24. Crossed nicols. 25. Parallel light. 26, 27. Campylosphaera eodela Bukry & Percival; crossed nicols. Sample 208-1262A-12H-3W, 44. 28. Campylosphaera dela (Bramlette & Sullivan) Hay & Mohler; crossed nicols. Sample 2081262A-11H-1W, 149. 29, 30. Girgisia gammation (Bramlette & Sullivan) Varol; crossed nicols. Sample 208-1262A-10H-3W, 86.

230

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

Fig. 3. Abundance patterns of selected calcareous nannofossil genera and species from the ODP Site 1262 are plotted against magnetostratigraphy (Bowles, 2006) and biostratigraphy (NP — Martini, 1971; CP — Okada and Bukry, 1980). The position of the ODP Site 1262 cores (Zachos et al., 2004) is also reported. The X-axis values represent the number of specimens in a prefixed area (N/mm2). The light grey shaded bars emphasize the ELPE, PETM, ELMO and X-event. Dashed lines highlight the NP and CP Zonal boundaries.

pattern of the genus Campylosphaera in the early Eocene evidences variable abundance. The pattern in the upper part of its range is noteworthy: Campylosphaera has a decline in abundance with a temporary disappearance in correspondence with Subchron C24n.3n, and shows its final decline at the NP11/NP12 boundary. 3.22. The HO of Fasciculithus alanii In the studied section, the HO of Fasciculithus alanii (Fig. 5; Plate II) is recorded shortly below the Paleocene/ Eocene boundary (Fig. 5, Table 1), and seems to be consistent with previous data from the North-western

Atlantic and Tethys areas (Monechi et al., 2000; Dupuis et al., 2003; Agnini et al., 2007). Since the index species is consistently present along most of its range, its HO biohorizon can be considered a reliable event. 3.23. Decrease in diversity of the genus Fasciculithus Several Fasciculithus species underwent an abrupt extinction in correspondence with PETM interval at Site 1262 (Figs. 5, 9; Table 1). Among them there are the larger species of heavily calcified fasciculiths, here ascribed to the F. richardii gr. (see above) and F. tonii. This result is in agreement with previous data from PETM sections from Atlantic and Pacific oceans and

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

Tethys area (Monechi et al., 2000; Zachos et al., 2005; Raffi et al., 2005; Agnini et al., 2007), and represents a relevant evolutionary change. 3.24. Calcareous nannofossil biohorizons at the PETM At Site 1262, the 12-cm thick interval (between 140.14 and 140.02 mcd) at the Paleocene–Eocene transition (the onset of Carbon Isotope Excursion — CIE) is barren of nannofossils, reflecting the period of ocean acidification of the Paleocene–Eocene Thermal Maximum (PETM), when the Calcium carbonate Compensation Depth (CCD) had shoaled and dissolution was intense (Zachos et al., 2005). In the interval of gradual recovery of CaCO3 content in the sediments overlying the clay layer deposited during the PETM, we observed partially dissolved nannofossil assemblages, with high relative abundance of dissolution resistant taxa such as Fasciculithus and Rhomboaster. The Fasciculithus species that survived the PETM, F. tympaniformis and F. involutus, are consistently present for a short interval (Fig. 5), and finally decline in the lowermost Eocene (see below). We recorded the different species of genus Rhomboaster, that evolve and radiate within the PETM, together with specimens ascribable to Tribrachiatus (Romein, 1979). Specimens of R. calcitrapa group (sensu Raffi et al., 2005; fig. 8; plate 4) are dominant, and show high abundance as the result of dissolution in the PETM interval (between 139.95 and 139.9 mcd), within which we defined LO and HO biohorizons (Table 1; Plate IV). The peculiar forms of the asymmetrical discoaster species Discoaster araneus (Fig. 7; Plate III) and Discoaster anartios, typically restricted to the CIE–PETM interval in association with Rhomboaster (e.g., Bybell and Self Trail, 1995; Cramer et al., 2000; Kahn and Aubry, 2004; Tremolada and Bralower, 2004), were found only as very rare specimens at Site 1262 (Plate III). 3.25. The Fasciculithus/Zygrhablithus abundance cross-over The abundance cross-over (CO) between Fasciculithus and Zygrhablithus is considered a reliable biohorizon despite some inconsistencies in its chronology, since it is represented by a well-delineated pattern of change in relative abundance of these two genera, and has been recorded in different areas (Backman, 1986, Kelly et al., 1996, Monechi et al., 2000; Bralower, 2002; Tremolada and Bralower, 2004; Zachos et al., 2005; Agnini et al., 2006, 2007). This event is recorded during the δ13C main excursion (CIE) at DSDP Site 401 and

231

ODP Site 690 (Bralower, 2002; Tremolada and Bralower, 2004), whereas it occurs exactly at the end of CIE (onset of the δ13C recovery interval) at DSDP Site 213, ODP Sites 1262 and 1263 (Zachos et al., 2005; this study; Fig. 3; Table 1) and in many other on-land sections (Monechi et al., 2000; Tremolada and Bralower, 2004; Zachos et al., 2005; Agnini et al., 2006, 2007). 3.26. The HO of Fasciculithus The HO of Fasciculithus had been considered a useful biohorizon for recognizing the Paleocene–Eocene boundary (i.e., Monechi et al., 1985; Perch-Nielsen, 1985), before the recent re-definition of the base of the Ypresian at the onset of the CIE (Aubry et al., 2002; Ouda and Aubry, 2003). Actually, the final decline of fasciculiths occurs in the lowermost Eocene now that boundary has been so defined, i.e., namely it occurs clearly above the base of the CIE. At Site 1262, the HO of Fasciculithus is recorded ∼400 kyr after the PETM (Figs. 3, 5; Table 1), in good agreement with the chronology obtained in previous studies (Bralower, 2002; Raffi et al., 2005; Agnini et al., 2006, 2007; Table B, Supplementary data). 3.27. The LRO and LO of Tribrachiatus bramlettei In the studied material, the genus Tribrachiatus is strongly affected by pervasive overgrowth that sometimes hampered a precise determination of the different species. Moreover, the reconstructed abundance patterns display rare to scarce and scattered occurrences of all the index species except for Tribrachiatus orthostylus. Nevertheless, it was possible to delineate the wellknown progressive morphologic modifications within the genus, from T. bramlettei to Tribrachiatus contortus to T. orthostylus. The distribution ranges obtained at Site 1262 (Figs. 2, 8) for the different species are sufficiently reliable and consistent with the philetic evolution inferred for Tribrachiatus (Romein, 1979; Perch-Nielsen, 1985; Wei and Zhong, 1996). Calibrations of the LO of T. bramlettei (Fig. 8; Plate IV), used to define the base of Zone NP10 (Martini, 1971) are consistently different, show large offsets ones from each other (Backman, 1986; Berggren et al., 1995; Cramer et al., 1999; 2003, Raffi et al., 2005; Agnini et al., 2006, 2007; Table B, Supplementary data). These discrepancies are probably due to the dissolution that is often present in the basal (or lowermost) Eocene sediments, or could even reflect a real diachrony of the event in different areas. Nevertheless, recent high resolution studies in continuous and expanded sedimentary successions show that the earliest,

232

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

rare specimens of T. bramlettei occur just above the P–E boundary, within the CIE, and slightly above the LO of Rhomboaster (e.g., Agnini et al., 2006, 2007; Raffi, unpublished data at Site 1263; Mutterlose et al., in press; this study Figs. 2, 8, 9 and Table 1).

termediate morphotypes between T. contortus and T. orthostylus. These forms represent the progressive morphological modifications within the evolutionary lineage toward T. orthostylus. 3.31. The decrease in abundance and HO of T. bramlettei

3.28. The LRO and LO of Discoaster diastypus The LO of Discoaster diastypus (Fig. 7; Plate III) together with the LO of Tribrachiatus contortus define the base of Zone CP9 (Okada and Bukry, 1980). In the present study, we have ascribed to D. diastypus all specimens larger than 15 μm in size, with 9 to 20 rays. Its lowest occurrence (LO) is characterized by a sharp increase in abundance that follows an interval of discontinuous presence of rare specimens (LRO) (Figs. 2, 7; Table 1). The D. diastypus LO is a useful biohorizon to confirm the position of T. contortus LO biohorizon (Raffi et al., 2005; Agnini et al., 2006), when bad preservation (overgrowth) makes determination of T. contortus difficult. 3.29. The LO of Tribrachiatus contortus The LO of T. contortus (Fig. 8; Plate IV) defines the base of Zones NP10d (Martini, 1971, modified after Aubry, 1996) and CP9 (Okada and Bukry, 1980). Specimens with morphologic characters intermediate between T. bramlettei and T. contortus testify the morphologic changes within the evolutionary transition from a species to another. The distribution pattern of T. contortus (Figs. 2, 8, 9)) shows low abundance but a continuous presence and permitted to delineate the LO and HO biohorizons (Table 1). 3.30. The LO and HO of Tribrachiatus digitalis At ODP Site 1262, the LO of T. digitalis (Fig. 8; Plate IV) is recorded within the lowermost part of the range of T. contortus (Fig. 8), in agreement with previous findings of Raffi et al. (2005), in paleoequatorial Pacific and North Atlantic sections. At Site 1262 T. digitalis has a well-defined and short range, that lasted ∼80 kyr (Table 1). The results from Site 1262 do not agree with the distribution reported in the original description of the species (Aubry, 1996) and successive papers (e.g., Aubry et al., 1996). In fact these authors described T. digitalis as having a short total range within the range of T. bramlettei, and below that the LO of T. contortus. In the upper part of the range of T. contortus (Fig. 8), we observed specimens of Tribrachiatus resembling T. digitalis, which are considered to be in-

At ODP Site 1262, the HO of T. bramlettei (Fig. 8; Plate IV) occurs well above the LO of T. contortus, indicating a large overlap in the ranges of these two species (Raffi et al., 2005; Agnini et al., 2006), rather than a cross-over in abundance (Backman, 1986; Berggren et al., 1995) (Figs. 2, 8; Table 1). The definitive decline of T. bramlettei is characterized by an interval of rare presence, here defined as a decrease, that cannot be easily detected in sections where the index species is extremely rare and thus has a discontinuous range. 3.32. The LO of Tribrachiatus orthostylus and the HO of Tribrachiatus contortus The base of Zone NP11 is defined by the HO of T. contortus (Martini, 1971). At Site 1262, a cross-over in abundance is detected between T. contortus and T. orthostylus, consistent with previous data (Shackleton et al., 1984, Monechi et al., 1985; Backman, 1986; Raffi et al., 2005; Agnini et al., 2006; Figs. 2, 8; Table 1). This abundance cross-over biohorizon might be useful for biostratigraphic classification in sections where T. contortus is exceedingly rare. 3.33. The HCO and HO of Discoaster multiradiatus We did not use the HO of Discoaster multiradiatus (Fig. 7; Plate III) as a biostratigraphic marker event, although we documented the distribution pattern of this species in the upper part of its range. Discoaster multiradiatus shows a sharp decrease in abundance (HCO) just below the NP10/NP11 boundary (Raffi et al., 2005), followed by an interval of absence, and then a strongly reduced presence higher in the section (within Chron C24n.3n), which delineates the HO biohorizon (Figs. 2, 7; Table 1). This result is consistent with data from the western-central Tethys (Southern Alps–Possagno section, unpublished data) and suggests caution in using the HO of D. multiradiatus as a biohorizon. 3.34. The LO of Sphenolithus radians The LO of Sphenolithus radians (Fig. 4; Plate I) has been proposed as an alternative biohorizon to approximate

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

the base of Zone NP11 when the marker T. contortus is missing (Backman, 1986; Perch-Nielsen, 1985). At Site 1262, the LO of S. radians is virtually coincident with the LO of T. orthostylus (Fig. 4; Table 1), in agreement with data from the Pacific Ocean and western Tethys (Raffi et al., 2005; Agnini et al., 2006).

233

3.35. The LO and LCO of D. lodoensis and the presence of D. diastypus/D. lodoensis intergrade Discoaster lodoensis (Fig. 7; Plate III) is a classical calcareous nannofossil marker of the early Eocene, and its appearance defines the base of Zones NP12 (Martini,

Fig. 4. Evolutionary lineage of genus Sphenolithus is plotted against magnetostratigraphy (after Bowles, 2006) and biostratigraphy (Martini, 1971; Okada and Bukry, 1980). The position of the ODP Site 1262 cores (Zachos et al., 2004) is also reported. The X-axis values represent the number of specimens in a prefixed area (N/mm2). The light grey shaded bars emphasize the ELPE, PETM, ELMO and X-event. Dashed lines highlight the NP and CP Zonal boundaries.

234

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

1971) and CP10 (Okada and Bukry, 1980), respectively. In the studied section, the lowermost observed specimens of D. lodoensis (LO) occur and peaks in abundance ∼500 kyr before the common and continuous presence of D. lodoensis (LCO) (Figs. 2, 7; Table 1). This distribution pattern in the lower part of the D. lodoensis range is consistent with data from the central western Tethys (Agnini et al., 2006) and could explain the large inconsistencies affecting the previous calibrations of the LO of D. lodoensis (e.g., Backman, 1986; Berggren et al., 1995). Aubry (1984) suggested that an evolutionary relationship exists between D. diastypus and D. lodoensis, and we could document this possible evolutionary transition at Site 1262. Specimens of discoasters with 8–9 rays (Fig. 9; Plate III) with intermediate morphologic features between D. diastypus and D. lodoensis have been found close to both the LO and LCO of D. lodoensis (Fig. 7). 3.36. The LO of Girgisia gammation At ODP Site 1262, Girgisia gammation (Fig. 2; Plate IV) has its LO just above the LO of D. lodoensis (Fig. 2; Table 1) in agreement with data from westerncentral Tethys (Agnini et al., 2006). Primitive forms of G. gammation, most likely evolved from Toweius, are present slightly below, in coincidence with the LO of D. lodoensis. The range of G. gammation shows a clear pattern with a sharp increase in abundance following its LO, which is here considered a reliable biohorizon. 3.37. The LO of Chiphragmalithus The genera Neochiastozygus and/or Neococcolithes evolved into genus Chiphragmalithus (Fig. 3; Plate IV) through morphological modifications both in structure and outline that occurred in a temporally well constrained interval (NP11 and NP12 Zones; Perch-Nielsen, 1985). In the studied material, primitive elliptic specimens slightly precede circular specimens of Chiphragmalithus, that show a sharp increase in abundance (Fig. 3; Table 1) and provide a biostratigraphic feature useful for recognizing the base of Zone NP12. 3.38. The HO of Ellipsolithus At Site 1262, the HO of Ellipsolithus is recorded slightly above the LCO of D. lodoensis (Fig. 3; Table 1; Plate IV), and this result is consistent with data from Shatsky Rise and Walvis Ridge (Backman, 1986). These data indicate that the HO of Ellipsolithus is a potential additional biohorizon for approximating the base of

Zone NP12, but further detailed investigations are needed to test its reliability. 4. Highlights on early Paleogene calcareous nannofossil evolution The fossil record does not usually provide the opportunity to reconstruct ancient lineages of organisms in detail, being often sparse and discontinuous. Evolutionary lineages and relationships between taxa of calcareous nannofossils have been rarely documented, and most of their phylogenies are based on qualitative observations of affinities in simple morphologic features. Essential requirements for a rigorous study of evolutionary relationships is the availability of continuous sections with robust chronology, high resolution sampling, and the presence of forms that document the evolutionary transitions, possibly over wide geographic areas. In the past, few studies have investigated and described in much detail the phylogenetic relationships between Neogene and Paleogene calcareous nannofossil taxa (e.g., Romein, 1979; Theodoridis, 1984; Raffi et al., 1998). In analyzing Paleogene assemblages, Romein (1979) described the origin and phylogeny of some taxa, and provided convincing reconstructions of early Paleogene nannofossil families and genera phylogenies. During the early Paleogene, calcareous nannofossils generally increased in abundance within phytoplankton assemblages, and showed taxonomic diversification and innovation (Aubry, 1998; Bown et al., 2004). The K/T mass extinction, profoundly altered the ecological structure of the phytoplankton community, and the subsequently vacant ecospace was first occupied by opportunistic groups (e.g., the calcareous dinocyst Thoracosphaera). Calcareous nannofossils showed a two-phased recovery: first, the appearance of the postextinction Paleocene taxa (e.g., Cruciplacolithus primus and Coccolithus pelagicus) immediately after the K/T mass extinction, and second, the appearance of new Cenozoic taxa, including the genera Sphenolithus and Fasciculithus, ∼3–4 Ma after the K/T event, concomitantly with the final recovery of the open ocean ecosystem (D'Hondt et al., 1998; Fornaciari et al., in press). These two genera represent a structural novelty in the calcareous nannofossil assemblages. Studies focused on their origination (Romein, 1979; Perch-Nielsen, 1985; Aubry, 1998) indicate that, in the course of the Paleocene, additional calcitic structures, derived by progressive modification of the basic structure of the ancestral sphenoliths, led to the origination of the genus Fasciculithus, that subsequently evolved into Heliolithus, that in turn evolved into Discoaster.

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

The Paleocene–Eocene transition represented another critical interval for the evolutionary history of calcareous nannofossils. It is considered a time of intense innovation of assemblages, during which a pronounced turnover occurred over a relatively short time (∼1.1 Myr) (Aubry, 1998). Gibbs et al. (2005, 2006a,b) recognized a peak in origination and extinction rates and described it to be restricted to the very brief interval of the PETM. These authors claimed that the abrupt climate-environmental change controlled the evolutionary rates of nannoplankton, even if the sea surface water conditions were not detrimental to the survival of most calcareous nannofossil taxa. The well-dated and continuous lower Paleogene sedimentary succession of ODP Site 1262 presented a good opportunity to discuss patterns of evolution of some nannofossil taxa, and to investigate whether the above mentioned phylogenies could be confirmed in a detailed documentation. Our observations were limited to those taxa considered in the biostratigraphic study in the middle Paleocene to early Eocene interval (Fig. 9). The discussion does not consider evolutionary relationships within lineages of the Paleogene coccolithophores, that include speciation in the genera Toweius, Coccolithus, Cruciplacolithus and Chiasmolithus, despite the fact that some of them occurred in the studied interval. In the following chapters, we document and discuss evolutionary relationships among and within selected genera, taking into account the presence of forms with intermediate morphologic characters between distinct genera and species (the so-called “transitional” species/ morphotype). Moreover, we tentatively reconstruct diversification events within some lineages, that were already inferred by previous authors but were lacking of fossil documentation. 4.1. The origination of Fasciculithus and Sphenolithus Sphenolithus and Fasciculithus appeared during the Danian/Selandian transition. These taxa were forms with innovative calcitic structures, characterized by massive crystals, which became a common component of the calcareous nannofossil assemblage during the Paleocene. Both genera showed a relevant species diversification throughout their distribution ranges. Uncertainty exists to designate ancestor–descendant relationships for Fasciculithus and Sphenolithus: Cyclagelosphaera and Markalius, respectively, were considered the ancestors by Romein (1979), whereas Aubry (1998) designated Biantholithus as ancestor of both genera, and inferred a possible origination of Fasciculithus from Sphenolithus (fig. 10.5 in Aubry, 1998). In the studied section, the

235

finding of specimens showing common morphologic features with both the genera Octolithus and Sphenolithus (Plate I; Figs. 4, 9), and forms resembling primitive specimens of the genus Sphenolithus, recorded slightly below its LO, suggests an evolutionary relationship between the two taxa. Conversely, the phylogenesis of Fasciculithus still remains uncertain and our results do not provide details on its origination, but indicate a disjunctive origin of Sphenolithus and Fasciculithus because rare specimens of the latter genus (i.e., F. magnicordis gr.; Fig. 5) are present from the base of the studied section, thus preceding the lowest occurrence of Sphenolithus. 4.2. The evolution of Fasciculithus The Fasciculithus lineage extended from the Danian– Selandian transition to the lowermost Eocene, where the genus became extinct just above the P–E boundary. Fasciculiths are a common component of Paleocene calcareous nannofossil assemblages, and are represented by numerous species, among which F. tympaniformis gr. is the dominant species. Among this genus, major radiations subsequently occurred, have been documented in detail in the studied interval, and are characterized as follows: – First, a radiation in the lower part of Chron C26r, that occurred just above the lowest appearance of F. ulii gr., with origination of F. tympaniformis gr., F. billii gr. and F. pileatus (Figs. 5, 9). Several transitional forms among the four end-member species are present that differ one from each other in having minimal morphologic differences. – Second, a radiation in the late Paleocene, that occurred close to the base of Zone NP9 (Martini, 1971) with the origination of F. alanii, F. aubertae, F. hayi gr., F. lilianae and F. tonii. These species appeared close to each other in a short time interval and, although their abundance is low compared to that of dominant F. tympaniformis gr., they provided distinct features within fasciculith assemblages (Figs. 5, 9). These upper Paleocene species showed a high morphological variability that resulted in several transitional morphotypes between species. This great diversification could indicate ultra-specialized species which were better adapted to the more “stable” environmental conditions before the PETM event (Hallock, 1987). The profound environmental changes associated with the PETM were thus able to led almost all these species, that were probably more sensitive to modifications in the sea surface conditions, to the extinction. Finally, the Fasciculithus

236 C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248 Fig. 5. Evolutionary lineage of genus Fasciculithus is plotted against magnetostratigraphy (Bowles, 2006) and biostratigraphy (Martini, 1971; Okada and Bukry, 1980). The position of the ODP Site 1262 cores (Zachos et al., 2004) is also reported. The X-axis values represent the number of specimens in a prefixed area (N/mm2). The light grey shaded bars emphasize the ELPE and PETM. Dashed lines highlight the NP and CP Zonal boundaries.

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

lineage ended in the lowermost Eocene, just above the Paleocene–Eocene transition, with the final decline of F. tympaniformis gr., the taxon that survived after the marked decrease in diversification of Fasciculithus recorded at the onset of PETM. (e.g., Monechi et al., 2000, Raffi et al., 2005; Agnini et al., 2006, 2007). 4.3. The evolution of Sphenolithus The genus Sphenolithus evolved in the middle Paleocene, with the beginning of the lineage in correspondence with the lowest occurrence of S. moriformis gr. Two radiations are documented in the studied interval, the first one in the late Paleocene (lower Thanetian) and the second one in the early Eocene (Ypresian). S. moriformis gr., characterized by a dome-shape structure (PerchNielsen, 1985), is a major representative of the genus and shows a continuous and consistent distribution all along the Site 1262 succession (Fig. 4). The late Paleocene diversification generated sphenoliths with an apical spine, typically represented by the species S. anarrhopus. At Site 1262, the first diversification of apical spine-bearing sphenoliths has been documented by the expected consistent presence of S. anarrhopus as well as the co-occurrence of two morphotypes, labeled as Sphenolithus sp.1 and S. cf. conicus (Fig. 4; Plate I), characterized by a pronounced apical spine. The finding of these latter forms, with such a distinctive and innovative morphologic character, represents a novelty that improves the characterization of the diversification episode within the genus Sphenolithus. The early Eocene diversification led to the first appearance of several species at base of Zone NP11, namely S. editus, S. orphanknollensis, S. radians and S. villae, that were all characterized by a well-developed apical spine (Figs. 4, 9). The evolutionary pattern of sphenoliths during the Paleocene/Eocene time, as observed in the continuous sediment succession of Site 1262, shows that the lineage resulted from successive evolutionary pulses rather than from gradual evolutionary transition through time.

237

evolved from Fasciculithus. Our results from Site 1262 confirm this general overview (Fig. 9), and indicate that some additional observations on the lineage can be done. The genus Heliolithus was restricted to the middlelate Paleocene interval and underwent a pronounced species diversification. The evolution from H. cantabriae to H. kleinpellii was well established (Romein, 1979) and has been documented also in the studied section (Fig. 6). The consistent presence of morphotypes with intermediate characters (Plate II) and their variability suggest that the taxonomy of Heliolithus at species level could be further refined. The evolutionary transition from Heliolithus to Discoaster was previously inferred (Romein, 1979; PerchNielsen, 1985), although it was not delineated in detail due to taxonomic discrepancies. In fact, Romein (1979) proposed that Discoaster evolved directly from Heliolithus through the species Discoaster bramlettei and Discoaster megastypus, whereas Perch-Nielsen (1985) considered the two species as belonging to genus Discoasteroides. Based on our observations, we support the latter evolutionary relationship and consider Discoasteroides a distinct step in the morphologic evolution from Heliolithus to Discoaster. 4.5. The origination and evolution of Discoaster In the reconstruction of the Discoaster lineage in the Site 1262 succession, we recorded the presence of small asteroliths, labeled as cf. Discoaster in Fig. 3, well below the beginning of the lineage, namely below the first occurrence of Discoasteroides bramlettei, the inferred ancestor of Discoaster. We consider this occurrence as a first but unsuccessful attempt of evolutionary emergence of the genus Discoaster which, instead, originated later, through the morphologic modification of specimens of D. bramlettei into D. mohleri (Fig. 7). Following Romein (1979), two main groups are recognized within the genus Discoaster: group A, that includes D. mohleri, D. nobilis gr., Discoaster sp.1 and D. okadai, is considered to descend from D. bramlettei; and group B, that includes D. delicatus gr. and D. multiradiatus, is considered to evolve from Discoasteroides megastypus.

4.4. The origination of Heliolithus and Discoaster 4.6. The Rhomboaster–Tribrachiatus lineage The genera Heliolithus and Discoaster appeared in sequence in the middle Paleocene and are considered closely related to each other. Convincing evolutionary relationships linking these taxa have been proposed by Romein (1979) and confirmed by other authors (e.g., Perch-Nielsen, 1985; Aubry, 1998). Specifically, Discoaster is thought to evolve from Heliolithus, that in turn

The evolutionary patterns within the Rhomboaster– Tribrachiatus lineage are characterized by a gradual morphologic modification from a typically rhomboedral structure (genus Rhomboaster) to a symmetric triradiate structure (genus Tribrachiatus), in which the two triplets show a progressive rotation and flattening (Romein,

238

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

Fig. 6. Evolutionary lineage of genus Heliolithus is plotted against magnetostratigraphy (Bowles, 2006) and biostratigraphy (Martini, 1971; Okada and Bukry, 1980). The position of the ODP Site 1262 cores (Zachos et al., 2004) is also reported. The X-axis values represent the number of specimens in a prefixed area (N/mm2). The light grey shaded bar emphasizes the ELPE. Dashed lines highlight the NP and CP Zonal boundaries.

1979; Perch-Nielsen, 1985; Wei and Zhong, 1996; Raffi et al., 2005). The taxonomy within this lineage is the topic of an endless debate, owing to discrepancies among authors regarding recombination of most of the species within the genus Rhomboaster instead of maintaining two different genera (e.g., Romein, 1979, Perch-Nielsen, 1985; Bybell and Self Trail, 1995; Bybell and Self-Trail, 1997; Aubry, 1996; Aubry et al., 1996, 2000; Angori and Monechi, 1996; Wei and Zhong, 1996; von Salis et al., 2000; Raffi et al., 2005). We share here the reasoning of Wei and Zhong (1996), which implies the maintenance of two different genera, based on evolutionary and stratigraphic evidence as well as nomenclatural stability. Although in the studied material overgrowth on the observed specimens did not facilitate the analysis, unambiguous evolutionary relationships between the different species have been documented by the presence of transitional morphotypes (Figs. 8, 9; Plate IV). The first nannolith ascribable to the Rhomboaster– Tribrachiatus plexus originated during the PETM, whereas the last-member species, T. orthostylus, occurred up to the late early Eocene (e.g., Berggren et al., 1995) virtually in coincidence with the end of the EECO.

The representatives of the Rhomboaster–Tribrachiatus lineage are thus restricted to the early Eocene, a time characterized by a long-term warming trend, interrupted by hyperthermal transient events, that are the PETM, ELMO and X (Zachos et al., 2001). The end of the early Eocene climatic optimum coincides with a turning point both in the Earth's climate and calcareous nannoplankton evolution (Zachos et al., 2001; Agnini et al., 2006), resulting in a progressive pCO2 decrease as well as in important modifications of the sea surface dwellers. The data presented here point out that a complex interaction between evolution of the biota and evolution of the environmental system exists, even if the mode and tempo of the biota response are not still completely understood and more highly-resolved studied are needed. 4.7. Remarks on the evolving lineages and interactions between biota evolution and change in environmental conditions One of the most interesting issue in paleontology is to investigate the relationships between biotic evolution and environmental pressure. The evolutionary models

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

239

Fig. 7. Evolutionary lineage of genus Discoaster is plotted against magnetostratigraphy (Bowles, 2006) and biostratigraphy (Martini, 1971; Okada and Bukry, 1980). The position of the ODP Site 1262 cores (Zachos et al., 2004) is also reported. The X-axis values represent the number of specimens in a prefixed area (N/mm2), except when it is differently specified (%). The light grey shaded bars emphasize the ELPE, PETM, ELMO and X-event. Dashed lines highlight the NP and CP Zonal boundaries.

proposed are conflicting. Van Valen (1973; the “Red Queen” hypothesis) considered the biotic control (i.e., competition, grazing, predation, viral infections etc.) as overwhelming the abiotic ones, while Stenseth and Maynard Smith (1984) proposed that evolutionary change is dominantly driven by change in the external physical environment (the “Stationary” Model). The two models have sparked hot debates, but no conclusive evidence has yet emerged (Thierstein et al., 2004). Microfossils provide an excellent, well-documented and

continuous record with great temporal and spatial distribution that should be used in paleobiogeographic, paleoecologic, morphologic interpretation but also in testing evolutionary models/theories (Lipps, 1981). Specifically, calcareous nannofossils seem to have all the necessary qualifications (i.e., abundance, wide biogeographical distribution, rapid evolution through time) for being used in continuous and expanded sedimentary successions with a highly-resolved chronologic framework to develop/test/model evolutionary hypothesis.

240 C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248 Fig. 8. Evolutionary lineage of genus Rhomboaster–Tribrachiatus is plotted against magnetostratigraphy (Bowles, 2006) and biostratigraphy (Martini, 1971; Okada and Bukry, 1980). The position of the ODP Site 1262 cores (Zachos et al., 2004) is also reported. The X-axis values represent the number of specimens in a prefixed area (N/mm2). The light grey shaded bars emphasize the PETM, ELMO and X-event. Dashed lines highlight the NP and CP Zonal boundaries.

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

241

Fig. 9. Distributions and evolutionary lineages of early Paleogene calcareous nannoplankton are plotted versus astro-magneto-chronology and biostratigraphy (Martini, 1971; Okada and Bukry, 1980). Relative ages (Myr) from PETM are also provided. On the right side, standard and additional calcareous nannofossil biohorizons and chronostratigraphy are reported.

The distribution ranges of the early Paleogene nannofossil taxa, synthesized in Fig. 9, permit to reconstruct the major lineages and show patterns that are worth of consideration as regards their evolutionary history throughout the interval of interest. This partial picture of

early Paleogene calcareous nannofossil evolutionary lineages shows episodes of species and genera originations occurring repeatedly, as well as extinctions of taxa. Some of the changes observed within the evolving lineages are related somehow to periods characterized by

242

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

critical environmental conditions, while other significant modifications in the calcareous nannofossil assemblages seem to occur during “stable” conditions. A relation between the consecutive originations of the genera Heliolithus, Discoasteroides and Discoaster and the ELPE is evidenced. This evolutionary impulse seems related to either environmental perturbations leading to the onset of the ELPE (the emergence of Heliolithus), to the critical conditions during (the emergence of Discoasteroides) and after (the emergence of Discoaster) the transient event. The PETM event is associated with a significative, albeit transient, modifications in the calcareous nannofossil assemblage, among which a relevant, though not dramatic, evolutionary event turnover is represented by the decrease in species diversity of Fasciculithus and the appearance of Rhomboaster–Tribrachiatus plexus (e.g., Aubry, 1998; Monechi et al., 2000; Agnini et al., 2007). In addition, the decline in diversification and the concomitant decline of abundance of fasciculiths within the nannofossil assemblages seem to have provided a competitive advantage for other taxa such as Zygrhablithus, that displayed a significant increase in abundance during the recovery to the pre-event conditions. All these data suggest that a causal link exists between the PETM and the significant alteration observed in calcareous nannofossils, as recently pointed out by Gibbs et al. (2006a,b). Nevertheless, these changes result in temporary modifications in the relative abundance within the assemblages rather than in long-lasting changes (e.g., the appearance of Rhomboaster–Tribrachiatus lineage). In the early Eocene, we document that the radiation of a “2nd generation” of apical spine-bearing sphenoliths species (e.g., S. radians and S. editus), the emergence of T. orthostylus and the marked decline of D. multiradiatus slightly preceded the ELMO. Furthermore, we suggest that a correlation may exist between the onset of the transient X-event and the conspicuous and continuous presence of D. lodoensis (Fig. 9). In fact, the extinction of D. multiradiatus, the progressive decrease in the number of the rays of D. diastypus and the first continuous and abundant appearance of D. lodoensis represented an important step in the nannofossil assemblages. This is a noteworthy turnover in the Discoaster assemblage, which documents a neat decline of the major rosetteshaped discoasterids (i.e., D. nobilis gr., D. multiradiatus, D. diastypus) and emergence of more slender star-shaped forms (D. lodoensis). Other relevant episodes of evolutionary radiations documented in the present study seem to occur independently from the transient events, as the following: (a) the emergence of the genus Sphenolithus during the

Danian/Selandian transition; (b) the speciation within Fasciculithus lineage in the Selandian stage; (c) the speciation within Discoaster lineages in the Thanetian stage, and the concomitant extinction of a “1st generation “ of apical spine-bearing sphenoliths (S. anarrhopus gr.); (d) the second radiation among fasciculiths in the Thanetian stage; (e) the appearance of the structurally new asterolith D. diastypus in the Ypresian stage, concomitantly with the speciation within Tribrachiatus lineage. As emerged from our dataset, the calcareous nannoplankton evolution could occur independently from “stressed” environmental conditions, at least 5 significant evolutionary changes take place during periods that are considered to be stable (pink boxes in Fig. 9). Furthermore, a series of remarkable nannoplankton events are associated, but not coincident, with times of climate changes (Fig. 9 yellow boxes). The only exception is represented by the PETM that shows a cause–effect relationship between changes in environmental conditions and modifications in the calcareous nannofossil assemblages. The nannoplankton evolution develops both through gradual modifications or punctuated diversifications, but no evidence exists that a specific type of evolution could automatically be related with changes in physical environment rather than with an endogenous process. There is no obvious and/or systematic relationship between important steps in nannoplankton evolution and global environmental perturbations. However, a more detailed analytical approach than our qualitative observations is necessary to single out cause–effect relationships between environmental perturbations and evolutionary episodes. Moreover, it would require the identification of meaningful proxies for evolutionary dynamics, useful to explain and evaluate the factors and mechanisms controlling the evolutionary processes, for a comprehensive analysis which is far beyond the scope of this discussion. The exceptional time control available at Site 1262 is used to estimate tempos involved in evolutionary changes observed in calcareous nannoplankton during the early Paleogene. Our data reveal that gradual morphological modifications (e.g., T. bramlettei–T. orthostylus) as well as intrageneric diversifications (e.g.; fasciculiths and sphenoliths) are quite rapid relatively to the geologic point of view, in fact these significant evolutionary changes seem to completely develop in ca. 100–300 kyr. 5. Conclusions A high-resolution calcareous nannofossil biostratigraphy was carried out at Walvis Ridge (ODP Site 1262). More than 70 biohorizons have been defined in a ∼ 10 Myr time interval (between ∼ 62.5 Ma and

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

∼ 52.5 Ma), thus providing a biostratigraphic partitioning that improves the available “standard” middle Paleocene/early Eocene biostratigraphic resolution. Comparison between magnetobiochronology and astrobiochronology brings to light significant differences in age estimation of the calcareous nannofossil biohorizons, mainly derived from revised estimates for the duration of investigated magnetochrons. At Site 1262, we were able to accurately document the origin of several nannofossil genera as well as the presence of transitional morphotypes between end members representing distinct species. These intermediate forms confirm the phylogenetic relationships between the genera Fasciculithus, Heliolithus, Discoasteroides and Discoaster, as well as between Rhomboaster and Tribrachiatus. During the Paleocene/early Eocene times, the calcareous nannofossil evolution seems to develop both through branching of lineages via gradual, relatively rapid transitions documented by the presence of intermediate forms between the end-member ancestor and descendant forms, but also through spaced in time intrageneric diversifications that consist of rapid appearance of several species virtually all evolved from a single long-lived species (i.e., F. tympaniformis and S. morifomis gr.). Though placolith-bearing coccolithophorid algae could be very useful from biostratigraphic purpose, as for an instance in the Pleistocene, they seem to evolve very slowly during the investigated time interval. By contrast, the nannoliths, which include Sphenolithus, Fasciculithus, Heliolithus, Discoasteroides, Discoaster, Rhomboaster, and Tribrachiatus, show a rapid evolution that typically results in higher rates of evolution and significant changes in morphology through time. Our partial dataset has pointed out that the major modifications in the calcareous nannofossil assemblages during the early Paleogene often occur close to significant changes of the environmental conditions but the appearance of structural innovations and radiations within a single genus could also be associated with “stable” environmental conditions. Further efforts are needed to better investigate the relative timing between changes in the environmental conditions and evolution. Acknowledgements This research used samples and data provided by the Ocean Drilling Program (ODP). ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institution (JOI) Inc. CA, EF, IR, and DR were supported by MIUR/PRIN COFIN 2005–2007 coordinated by I. Premoli Silva (national), DR (Padova)

243

and IR (Chieti). University of Padova provided financial support to EF. Funding for this research was provided to UR and TW by the Deutsche Forschungsgemeinschaft (DFG). Helpful comments by Elisabetta Erba and Jörg Mutterlose greatly improved the manuscript. We are particularly grateful to Ellen Thomas and Jan Backman for fruitful discussions. We would like to thank Stefano Castelli for producing plates of calcareous nannofossil microphotographs and Lorenzo Franceschin for processing samples for micropaleontological analyses. Appendix A. Taxonomic list of calcareous nannofossils Species/genus

Author, year

Biantholithus Bomolithus Campylosphaera Campylosphaera dela

Bramlette & Martini (1964) Roth (1973) Kamptner (1963) Hay & Mohler (1967)

Campylosphaera eodela Chiasmolithus Chiamolithus bidens

Burky & Percival (1971)

Chiasmolithus edentulus Chiasmolithus solitus

Chiphragmalithus Coccolithus Coccolithus pelagicus Cruciplacolithus Cruciplacolithus primus Cruciplacolithus tenuis Cyclagelosphaera Discoasteroides Discoasteroides bramlettei Discoasteroides megastypus Discoaster Discoaster anartios Discoaster araneus Discoaster delicatus Discoaster diastypus

Original name, author, year

Coccolithes delus (Bramlette & Sullivan, 1961)

Hay, Mohler & Wade (1966) Hay & Mohler (1967) Coccolithus bidens (Bramlette & Sullivan, 1961) Van Heck & Prins (1987) Locker (1968)

Coccolithus solitus (Bramlette & Sullivan, 1961)

Bramlette & Sullivan (1961) Schwarz (1894) Schiller (1930) Coccosphaera pelagica (Wallich, 1877) Hay & Mohler (1967) Perch-Nielsen (1977) Hay & Mohler in Hay et al. (1967) Noël (1965) Bramlette & Sullivan (1961) Bukry & Percival (1971)

Heliorthus tenuis (Stradner, 1961)

Bramlette & Sullivan (1961) Tan Sin Hok (1927) Bybell & Self-Trail (1995) Bukry (1971) Bramlette & Sullivan (1961) Bramlette & Sullivan (1961) (continued on next page)

244

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

Appendix A (continued ) Species/genus

Author, year

Discoaster falcatus Discoaster lenticularis Discoaster limbatus Discoaster lodoensis Discoaster mohleri Discoaster multiradiatus Discoaster nobilis Discoaster okadai Ellipsolithus Ellipsolithus macellus

Ericsonia

Black (1964) emend. Haq (1971) Perch-Nielsen (1977)

Ericsonia robusta

Appendix A (continued ) Original name, author, year

Species/genus

Author, year

Bramlette & Sullivan (1961) Bramlette & Sullivan (1961) Bramlette & Sullivan (1961) Bramlette & Riedel (1954)

Fasciculithus tonii Fasciculithus ulii Girgisia Girgisia gammation

Perch-Nielsen (1971) Perch-Nielsen (1971) Varol (1989) Varol (1989)

Bukry & Percival (1971) Bramlette & Riedel (1954)

Heliolithus Heliolithus bukryi Heliolithus cantabriae Heliolithus kleinpellii Heliolithus riedeli Markalius Neochiastozygus Neochiastozygus perfectus Rhomboaster Rhomboaster calcitrapa Rhomboaster cuspis Sphenolithus Sphenolithus anarrhopus Sphenolithus cf. conicus Sphenolithus editus

Bramlette & Sullivan (1961) Wei (1989) Perch-Nielsen (1971)

Martini (1961) Bukry (1981) Sullivan (1964) Sullivan (1964)

Fasciculithus Bramlette & Sullivan (1961) Fasciculithus alanii Perch-Nielsen (1971) Fasciculithus Haq & Aubry (1971) aubertae Fasciculithus billii Perch-Nielsen (1971) Fasciculithus bobii Perch-Nielsen (1971) Fasciculithus Varol (1989) chowii Fasciculithus Bukry (1971) clinatus Fasciculithus hayi Haq (1971) Fasciculithus Bramlette & involutus Sullivan (1961) Fasciculithus janii Perch-Nielsen (1971) Fasciculithus Perch-Nielsen (1971) lilianae Fasciculithus Romein (1979) magnicordis Fasciculithus Bukry & Percival (1971) magnus Fasciculithus Gartner (1971) mitreus Fasciculithus Bukry (1973) pileatus Fasciculithus Perch-Nielsen (1971) richardii Fasciculithus Hay & Mohler (1967) schaubii Fasciculithus Hay & Mohler (1967) tympaniformis Fasciculithus Perch-Nielsen (1971) thomasii

Coccolithites macellus (Bramlette & Sullivan, 1961)

Cyclolithus? robustus (Bramlette & Sullivan, 1961)

Original name, author, year

Coccolithus gammation (Bramlette & Sullivan, 1961)

Sullivan (1964) Bramlette & Sullivan (1961) Bramlette & Martini (1964) Perch-Nielsen (1971) Perch-Nielsen (1971) Bramlette & Sullivan (1961) Gartner (1971) Bramlette & Sullivan (1961) Deflandre in Grasse (1952) Bukry & Bramlette (1969) Bukry (1971) Perch-Nielsen in Perch-Neilsen et al. (1978) Perch-Nielsen (1971)

Sphenolithus orphanknollensis Sphenolithus Bramlette & Wilcoxon moriformis (1967)

Sphenolithus radians Sphenolithus villae Thoracosphaera Toweius Toweius pertusus

Deflandre in Grasse (1952)

Tribrachiatus Tribrachiatus bramlettei

Shamrai (1963) Proto decima et al. (1975)

Tribrachiatus contortus

Bukry (1972)

Tribrachiatus digitalis Tribrachiatus ortostylus

Aubry (1996)

Bown (2005) Kamptner (1927) Hay & Mohler (1967) Romein (1979)

Shamrai (1963)

Nannoturbella moriformis (Brönnimann & Stradner, 1960)

Coccolithus pertusus (Sullivan, 1965) Marthasterites bramlettei (Brönnimann & Stradner, 1960) Discoaster contortus (Stradner, 1958)

Discoaster tribrachiatus (Bramlette & Riedel, 1954)

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248 Appendix A (continued ) Species/genus

Author, year

Zyghrablithus Zyghrablithus bijugatus

Deflandre (1959) Deflandre (1959)

Original name, author, year Zygolithus bijugatus (Deflandre in Deflandre & Fert, 1954)

Appendix B. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j. marmicro.2007.05.003. References Agnini, C., Muttoni, G., Kent, D.V., Rio, D., 2006. Eocene biostratigraphy and magnetic stratigraphy from Possagno, Italy: the calcareous nannofossil response to climate variability. Earth Planet. Sci. Lett. 241, 815–830. doi:10.1016/j.epsl.2005.11.005. Agnini, C., Fornaciari, E., Rio, D., Tateo, F., Backman, J., Giusberti, L., 2007. Responses of calcareous nannofossil assemblages, mineralogy and geochemistry to the environmental perturbations across the Paleocene/Eocene boundary in the Venetian Pre-Alps. Mar. Micropaleontol. 63, 19–38. doi:10.1016/j.marmicro.2006.10.002. Angori, E., Monechi, S., 1996. High-resolution calcareous nannofossil biostratigraphy across the Paleocene–Eocene boundary at Caravaca (southern Spain). Isr. J. Earth-Sci. 44 (1995), 197–206. Aubry, M.-P., 1984. Handbook of Cenozoic Calcareous Nannoplankton, book 1, Ortholithae (Discoaster). Am. Mus. of Nat. Hist. Micropaleontol. Press, New York. 263 pp. Aubry, M.-P., 1988. Handbook of Cenozoic Calcareous Nannoplankton, book 2, Ortholithae (Holococcoliths, Ceratoliths, Ortholiths and Other). Am. Mus. of Nat. Hist.Micropaleontol. Press, New York. 279 pp. Aubry, M.-P., 1989. Handbook of Cenozoic Calcareous Nannoplankton, book 3, Ortholithae (Pentaliths and Other), Heliolithae (Fasciculiths, Sphenoliths and Others). Am. Mus. of Nat. Hist. Micropaleontol. Press, New York. 279 pp. Aubry, M.-P., 1990. Handbook of Cenozoic Calcareous Nannoplankton, book 4, Heliolithae (Helicoliths, Cribriliths, Lopadoliths and Other). Am. Mus. of Nat. Hist.Micropaleontol. Press, New York. 381 pp. Aubry, M.-P., 1996. Towards an upper Paleocene–lower Eocene high resolution stratigraphy based on calcareous nannofossil stratigraphy. Isr. J. Earth-Sci. 44 (1995), 239–253. Aubry, M.-P., 1998. Early Paleogene calcareous nannoplankton evolution: a tale of climatic amelioration. In: Aubry, M.-P., Ouda, K. (Eds.), Late Paleocene–early Eocene Biotic and Climatic Events in the Marine and Terrestrial Records. Columbia University Press, New York, pp. 158–201. Aubry, M.-P., 1999. Handbook of Cenozoic Calcareous Nannoplankton, book 5, Heliolithae (Zygoliths and Rhabdoliths). Am. Mus. of Nat. Hist.Micropaleontol. Press, New York. 368 pp. Aubry, M.-P., Berggren, W.A., Stott, L., Sinha, A., 1996. The upper Paleocene–lower Eocene stratigraphic record and the Paleocene– Eocene boundary carbon isotope excursion. Implications for

245

geochronology. In: Knox, R.O., et al. (Eds.), Correlation of Early Paleogene in Northwest Europe. Geol. Soc. London, Spec. Publ., vol. 101, pp. 353–380. Aubry, M.-P., Requirand, C., Cook, J., 2000. The Rhomboaster– Tribrachiatus lineage: a remarkable succession of events from 55.5 to 53.2 Ma. GFF 122, 15–18. Aubry, M.-P., Ouda, K., Dupuis, C., Van Couvering, J.A., Ali, J.R., Berggren, W.A., Brinkhuis, H., Gingerich, P.D., HeilmannClausen, C., Hooker, J., Kent, D.V., King, C., Knox, R.W.O'B., Laga, P., Molina, E., Schimtz, B., Steurbaut, E., Ward, D.R., 2002. Proposal: Global Standard Stratotype-section and Point (GSSP) at the Dababiya section (Egypt) for the Base of the Eocene Series. Int. Union Geol. Sc., Int. Comm. Strat., Subcomm. Paleogene Strat. 59 pp. Backman, J., 1986. Late Paleocene to middle Eocene calcareous nannofossil biochronology from the Shatsky Rise, Walvis Ridge and Italy. Palaeogeogr. Palaeoclimatol. Palaeoecol. 57, 43–59. Backman, J., Shackleton, N.J., 1983. Quantitative biochronology of Pliocene and early Pleistocene calcareous nannoplankton from the Atlantic, Indian and Pacific Oceans. Mar. Micropaleontol. 8, 141–170. Berggren, W.A., Kent, D.V., Swisher III, C.C., Aubry, M.-P., 1995. A revised Cenozoic geochronology and chronostratigraphy. In: Berggren, W.A., et al. (Eds.), Geochronology, Time Scales and Global Stratigraphic Correlation, Soc. for Sediment. Geol. Tulsa, Okla., Spec. Publ., vol. 54, pp. 129–212. Berggren, W.A., Aubry, M.-P., van Fossen, M., Kent, D.V., Norris, R.D., Quillevere, F., 2000. Integrated Paleocene calcareous plankton magnetobiochronology and stable isotope stratigraphy: DSDP Site 384 (NW Atlantic Ocean). Palaeogeogr. Palaeoclimatol. Palaeoecol. 159, 1–51. Bernaola, G., Nuño-Arana, Y., 2006. Calcareous nannofossils assemblages across Mid-Paleocene. In: Bernaola, et al. (Eds.), The Paleocene and lower Eocene of the Zumaia section (Pasque Basin). Climate and Biota of the Early Paleogene 2006, Post conference Field trip guidebook, pp. 44–45. Billups, K., Rickaby, R.E.M., Schrag, D.P., 2004. Cenozoic pelagic Sr/Ca records: exploring a link to paleoproductivity. Paleoceanography 19, A3005. doi:10.1029/2004PA001011. Bowles, J., 2006. Data report: revised magnetostratigraphy and magnetic mineralogy of sediments from Walvis Ridge, Leg 208. Proc. Ocean Drill. Program Sci. Results 208, 1–24 (http://www-odp.tamu. edu/publications/208_SR/VOLUME/CHAPTERS/206.PDF). Bown, P.R., Lees, J.A., Young, J.R., 2004. Calcareous nannoplankton evolution and diversity through time. In: Thierstein, H.R., Young, J.R. (Eds.), Coccolithophores, From Molecular Processes to Global Impact. Springer Verlag, Berlin, Germany, pp. 481–508. Bralower, T.J., 2002. Evidence of surface water oligotrophy during the Paleocene–Eocene thermal maximum: nannofossil assemblage data from Ocean Drilling Program Site 690, Maud Rise, Weddel Sea. Paleoceanography 17 (2), 1029–1042. doi:10.1029/ 2001PA000662. Bralower, T.J., Mutterlose, J., 1995. Calcareous nannofossil biostratigraphy of ODP Site 865, Allison Guyot, Central Pacific Ocean: a tropical Paleogene reference section. Proc. Ocean Drill. Program Sci. Results 143, 31–72. Bralower, T.J., Premoli Silva, I., Malone, M.J. (Eds.), 2002. Extreme warmth in the Cretaceous and Paleogene: a depth transect at Shatsky Rise, Central Pacific. Proc. Ocean Drill. Program Sci. Results, vol. 198 (http://www-odp.tamu.edu/publications/198_IR/198ir.htm). Bramlette, M.N., Sullivan, F.R., 1961. Coccolithophorids and related nannoplankton of the Early Tertiary in California. Micropaleontology 7, 129–188.

246

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

Bukry, D., 1973. Low-latitude coccolith biostratigraphic zonation. Proc. Deep Sea Drill. Proj., Initial Rep. 15, 685–703. Bukry, D., 1975. New Miocene to Holocene stages in the ocean basins based on calcareous nannoplankton zones. In: Saito, T., Burckle, L.H. (Eds.), Late Neogene Epoch boundaries. Special Publication. Micropaleontoly Press, pp. 162–166. Bukry, D., 1978. Biostratigraphy of Cenozoic marine sediments by calcareous nannofossil. Micropaleontology 24, 44–60. Bybell, L.M., Self Trail, J.M., 1995. Evolutionary, Biostratigraphic and Taxonomic Study of Calcareous Nannofossils from a Continuous Paleocene–Eocene Boundary Section in New Jersey. U. S. Geol. Surv. Prof. Pap. 1554 (36 pp.). Bybell, L.M., Self-Trail, J.M., 1997. Late Paleocene and early Eocene calcareous nannofossils from three boreholes in an onshore– offshore transect from New Jersey to the Atlantic continental rise. Proc. Ocean Drill. Program Sci. Results 150X, 91–110. Cande, S.C., Kent, D.V., 1992. A new geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. J. Geophys. Res. 97, 13917–13951. Cande, S.C., Kent, D.V., 1995. Revised calibration of the geomagnetic polarity time scale for the Late Cretaceous and Cenozoic. J. Geophys. Res. 100 (B4), 6093–6096. doi:10.1029/94JB03098. Cramer, B.S., Aubry, M.-P., Miller, K., Olsson, R.K., Wright, J.D., Kent, D.V., 1999. An exceptional chronologic, isotopic, and clay mineralogic record of the latest Paleocene thermal maximum, Bass River, NJ, ODP 174AX. Bull. Soc. Géol. Fr. 170, 883–897. Cramer, B.S., Miller, K.G., Aubry, M.-P., Olsson, R.K., Wright, J.D., Kent, D.V., Browning, J.V., 2000. The Bass River Section: an exceptional record of the LPTM event in a neritic setting. Bull. Geol. Soc. Fr. 170, 883–897. Cramer, B.S., Wright, J.D., Kent, D.V., Aubry, M.-P., 2003. Orbital climate forcing of δ13C excursions in the late Paleocene–early Eocene (chrons C24n–C25n). Paleoceanography 18 (4), 1097. doi:10.1029/2003PA000909. Dinarès-Turell, J., Baceta, J.I., Pujalte, V., Orue-Etxebarria, X., Bernaola, G., 2002. Magnetostratigraphic and cyclostratigraphic calibration of a prospective Paleocene/Eocene stratotype at Zumaia (Basque Basin, northern Spain). Terra Nova 14, 371–378. D'Hondt, S., Donaghay, P., Zachos, J.C., Luttenberg, D., Lindinger, M., 1998. Organic carbon fluxes and ecological recovery from the Cretaceous–Tertiary mass extinction. Science 282, 276–279. Dupuis, C., Aubry, M.-P., Steurbaut, E., Berggren, W.A., Ouda, K., Magioncalda, R., Cramer, B.S., Kent, D.V., Speijer, R.P., HeilmannClausen, C., 2003. The Dababiya Quarry section: lithostratigraphy, geochemistry and paleontology. Micropaleontology 49 (Suppl. 1), 41–59. Edwards, A.R., 1971. A calcareous nannoplankton zonation of the New Zealand Paleogene. In: Farinacci, A. (Eds.), Proceedings II Planktonic Conference, Roma (1970), vol. 2, pp. 381–419. Fornaciari E., Giusberti, L., Luciani, V., Tateo, F., Agnini, C., Backman, J., Oddone, M., Rio, D., in press. An expanded Cretaceous–Tertiary transition in a pelagic setting of the Southern Alps (central-western Tethys). Palaeogeogr. Palaeoclimatol. Palaeoecol. Gibbs, S.J., Bralower, T.J., Bybell, L.M., 2005. Quantifying limits on extinction and diversification rates during rapid climate change events: calcareous nannoplankton at the Paleocene–Eocene Thermal maximum. Geol. Soc. Am., Salt Lake City, Abstr. Prog., vol. 37(7), p. 265. Gibbs, S.J., Bown, P.R., Bralower, T.J., 2006a. Ocean acidification and calcareous nannoplankton at the Paleocene–Eocene Thermal Maximum. In: Caballero, F., et al. (Eds.), Climate and Biota of the Early Paleogene, p. 51. Vol. Abstr., Bilbao.

Gibbs, S.J., Bown, P.R., Sessa, J.A., Bralower, T.J., Wilson, P.A., 2006b. Nannoplankton extinction and origination across the Paleocene–Eocene Thermal Maximum. Science 314, 1770–1773. Hallam, A., Hancock, J.M., LaBreque, J.L., Lowrie, W., Channell, J.E.T., 1985. Jurassic to Palaeogene magnetostratigraphy. In: Snelling, N.J. (Eds.), The Chronology of the Geological Record. Geol. Soc. London, vol. 10, pp. 118–140. Hallock, P., 1987. Fluctuations in the Trophic Resources Continuum: a factor in global diversity cycles? Paleoceanography 2 (5), 457–471. Hambrey, M.J., Ehrmann, W.U., Larsen, B., 1991. The Cenozoic glacial record from Prydz Bay continental shelf, East Antarctica. Proc. Ocean Drill. Program Sci. Results 119, 77–132. Hay, W.W., Mohler, H.P., 1967. Calcareous nannoplankton from early Tertiary rocks at Pont Labau, France, and Paleocene–early Eocene correlations. J. Paleontol. 41 (6), 1505–1541. Kahn, A., Aubry, M.-P., 2004. Provincialism associated with the Paleocene/Eocene Thermal Maximum: temporal constraint. Mar. Micropaleontol. 52, 117–131. Kelly, D.C., Bralower, T.J., Zachos, J.C., Premoli Silva, I., Thomas, E., 1996. Rapid diversification of planktonic foraminifera in the tropical Pacific (ODP Site 865) during the late Paleocene thermal maximum. Geology 24, 423–426. Kennett, J.P., Stott, L.D., 1991. Abrupt deep-sea warming, palaeoceanographic changes and benthic extinctions at the end of the Palaeocene. Nature 353, 225–229. Kuiper, K.F., Hilgen, F.J., Steenbrink, J., Wijbrans, J.R., 2004. 40 Ar/39Ar ages of tephras intercalated in astronomically tuned Neogene sedimentary sequences in Mediterranean. Earth Planet. Sci. Lett. 222, 583–597. Kuiper, K.F., Wijbrans, J.R., Hilgen, F.J., 2005. Radioisotopic dating of the Tortonian Global Stratotype Section and Point: implications for intercalibration of 40Ar/39Ar and astronomical dating methods. Terra Nova 17, 385–398. Laskar, J., Robutel, P., Joutel, F., Gastineau, M., Correia, A., Levrard, B., 2004. A long-term numerical solution for the insolation quantities of the Earth. Astron. Astrophys. 428, 261–285. Lipps, 1981. What, if anything, is micropaleontology? Paleobiology 7 (2), 167–199. Lourens, L.J., Hilgen, F.J., Shackleton, N.J., Laskar, J., Wilson, D., 2004. The Neogene period. In: Gradstein, F.M., et al. (Eds.), A Geological Time Scale. Cambridge University Press, Cambridge, UK, pp. 409–440. Lourens, L.J., Sluijs, A., Kroon, D., Zachos, J.C., Thomas, E., Röhl, U., Bowles, J., Raffi, I., 2005. Astronomical pacing of late Palaeocene to early Eocene global warming events. Nature 435, 1083–1087. Luterbacher, H.P., Ali, J.R., Brinkhuis, H., Gradstein, F.M., Hooker, J.J., Monechi, S., Röhl, U., Sanfilippo, A., Schimtz, B., 2004. The Paleogene time. In: Gradstein, F.M., et al. (Eds.), A Geological Time Scale. Cambridge University Press, Cambridge, UK, pp. 384–408. Machlus, M., Hemming, S.R., Olsen, P.E., Christie-Blick, N., 2004. Eocene calibration of geomagnetic polarity time scale reevaluated: evidence from the Green River Formation of Wyoming. Geology 32, 137–140. Martini, E., 1971. Standard Tertiary and Quaternary calcareous nannoplankton zonation. In: Farinacci, A. (Eds.), Proceedings of the II Planktonic Conference 2, Roma (1970), pp. 739–785. Monechi, S., 1985. Campanian to Pleistocene calcareous nannofossils stratigraphy from the northwest Pacific Ocean, Deep Sea Drilling Project Leg 86. Proc. Deep Sea Proj., Initial Repts. 86, 301–336. Monechi, S., Thierstein, H.R., 1985. Late Cretaceous–Eocene nannofossil and magnetostratigraphic correlations near Gubbio, Italy. Mar. Micropaleontol. 9, 419–440.

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248 Monechi, S., Bleil, U., Backman, J., 1985. Magnetobiochronology of Late Cretaceous–Paleogene and Late Cenozoic pelagic sedimentary sequences from the northwest Pacific (Deep Sea Drilling Project, Leg 86, Site 577). Proc. Deep Sea Proj., Initial Repts. 86, 787–797. Monechi, S., Angori, E., von Salis, K., 2000. Calcareous nannofossil turnover around the Paleocene/Eocene transition at Alamedilla (southern Spain). Bull. Soc. Géol. Fr. 171 (4), 477–489. Moore Jr., T.C., Rabinowitz, P.D., Borella, P., Boersma, A., Shackleton, N.J., 1984. Proc. Deep Sea Proj., Initial Repts. 74 894 pp. Müller, C., 1985. Biostratigraphic and paleoenvironmental interpretations of the Goban Spur region based on a study of calcareous nannoplankton. Proc. Deep Sea Proj., Initial Repts. 80, 573–599. Mutterlose, J., Linnert, C., Norris, R.D., in press. Calcareous nannofossils from the Paleocene–Eocene Thermal Maximum of the equatorial Atlantic (ODP Site 1260B): Evidence for tropical warming. Mar. Micropaleontol. doi: 10.1016/j.marmicro.2007.05.004. Okada, H., Bukry, D., 1980. Supplementary modification and introduction of code numbers to the low-latitude coccolith biostratigraphic zonation (Bukry, 1973; 1975). Mar. Micropaleontol. 5, 321–325. Okada, H., Thierstein, H.R., 1979. Calcareous plankton — Leg 43, DSDP. Initial Rep. Proc. Deep Sea Proj., Initial Repts. 43, 507–573. Ouda, K., Aubry, M.-P. (Eds.), 2003. Late Paleocene–early Eocene biotic and climatic events in the marine and terrestrial records. Columbia University Press, New York, p. 212. Pälike, H., Shackleton, N.J., Röhl, U., 2001. Astronomical forcing in late Eocene marine sediments. Earth Planet. Sci. Lett. 193, 589–602. Pälike, H., Laskar, J., Shackleton, N.J., 2004. Geologic constraints on the chaotic diffusion of the solar system. Geology 32, 929–932. Perch-Nielsen, K., 1979. Calcareous nannofossil zonation at Cretaceous/Tertiary boundary in Denmark. Proc. Cretaceous–Tertiary Boundary Events Symposium, Copenhagen, pp. 115–135. Perch-Nielsen, K., 1985. Cenozoic calcareous nannofossils. In: Bolli, H.M., et al. (Eds.), Plankton Stratigraphy. Cambridge University Press, New York, pp. 427–554. Petrizzo, M.R., 2005. An Early Late Paleocene event on Shatsky Rise, northwest Pacific Ocean (ODP Leg 198): evidence from planktonic foraminiferal assemblages. Proc. Ocean Drill. Program Sci. Results 198, 1–29 (http://www-odp.tamu.edu/publications/ 198_SR/VOLUME/CHAPTERS/102.PDF). Pospichal, J.J., Dehn, J., Driscoll, N.W., van Eijden, AJ.M., Farrell, J.W., Fourtanier, E., Gamson, P., Gee, J., Janecek, T.R., Jenkins, D.G., Kloothwijk, C., Nomura, R., Owen, R.M., Rea, D.K., Resiwati, P., Smit, J., Smith, G., 1991. Cretaceous–Paleogene biomagnetostratigraphy of ODP Sites 752–755, Broken Ridge: a synthesis. Proc. Ocean Drill. Program Sci. Results 121, 721–741. Radomski, A., 1968. Calcareous nannoplankton zones in the Paleogene of the Western Polish Carpathian. Rocz. Pol. Tow. Geol. 38, 544–605. Raffi, I., Backman, J., Rio, D., 1998. Evolutionary trends of tropical calcareous nannofossils in the late Neogene. Mar. Micropaleontol. 35 (1–2), 17–41. Raffi, I., Backman, J., Pälike, H., 2005. Changes in calcareous nannofossil assemblage across the Paleocene/Eocene transition from the paleo-equatorial Pacific Ocean. Palaeogeogr. Palaeoclimatol. Palaeoecol. 226, 93–126. Raffi, I., Backman, J., Fornaciari, E., Pälike, H., Rio, D., Lourens, L.J., Hilgen, F.J., 2006. A review of calcareous nannofossil astrobiochronology encompassing the past 25 Million years. Quat. Sci. Rev. 25, 3113–3137. doi:10.1016/j.quascirev.2006.07.007.

247

Rio, D., Raffi, I., Villa, G., 1990. Pliocene–Pleistocene calcareous nannofossil distribution patterns in the western Mediterranean. Proc. Ocean Drill. Program Sci. Results 107, 513–533. Röhl, U., Norris, R.D., Ogg, J.G., 2003. Cyclostratigraphy of the upper Paleocene and lower Eocene sediments at Blake Nose Site 1051 (western North Atlantic). In: Wing, S.L., et al. (Eds.), Causes and Consequences of Globally Warm Climates in the early Paleogene. Geol. Soc. Am., Spec. Paper, vol. 369. Geological Society of America, Boulder, Colorado, pp. 576–589. Röhl, U., Zachos, J.C., Thomas, E., Kelly, D.C., Donner, B., Westerhold, T., 2004. Multiple Early Eocene Thermal Maximums. Eos, Trans. – Am. Geophys. Union 85 (47) (Fall Meet. Suppl., Abstract PP14A-02). Röhl, U., Westerhold, T., Monechi, S., Thomas, E., Zachos, J.C., Donner, B., 2005. The third and final Early Eocene thermal maximum: characteristics, timing, and mechanisms of the “X” event. Geol. Soc. Am., Salt Lake City, Abstr. Prog., vol. 37(7), p. 264. Romein, A.J.T., 1979. Lineages in early Paleogene calcareous nannoplankton. Utrecht Micropaleont. Bull. 22 (231 pp.). Schimtz, B., Molina, E., von Salis, K., 1998. The Zumaya section in Spain: a possible global stratotype section for the Selandian and Thanetian stages. Newsl. Stratigr. 36, 35–42. Shackleton, N.J., Moore, T.C., Rabinowitz, P.D., Boersma, A., Borella, P.E., Chave, A.D., Duee, G., Futterer, D., Jiang, M.-J., Kleinert, K., Lever, A., Manavit, H., O'Connell, S., Richardson, S.H., 1984. Accumulation rate in Leg 74 sediments. Proc. Deep Sea Proj., Initial Repts. 74, 621–637. Stenseth, N.C., Maynard Smith, J., 1984. Coevolution in ecosystems: Red Queen evolution or stasis? Evolution 38, 870–880. Theodoridis, S., 1984. Calcareous nannofosssil biozonation of the Miocene and revision of the Helicoliths and Discoaster. Utrecht Micropaleont. Bull. 32 (271 pp.). Thierstein, H.R., Cortés, M.Y., Haidar, A.T., 2004. Plankton community behavior on ecological end evolutionary timescales: when models confront evidence. In: Thierstein, H.R., Young, J.R. (Eds.), Coccolithophores, From Molecular Processes to Global Impact. Springer Verlag, Berlin, Germany, pp. 455–480. Tremolada, F., Bralower, T.J., 2004. Nannofossil assemblage fluctuations during the Paleocene–Eocene Thermal Maximum at Sites 213 (Indian Ocean) and 401 (North Atlantic Ocean): palaeoceanographic implications. Mar. Micropaleontol. 52, 107–116. Van Heck, S.E., Prins, B., 1987. A refined nannoplankton zonation for the Danian of the Central North Sea. In: Stradner, H., PerchNielsen, K. (Eds.), Proc. Int. Nannoplankt. Assoc., Vienna Meeting, vol. 39, pp. 285–303. Van Valen, L., 1973. A new evolutionary law. Evol. Theory 1, 1–30. Varol, O., 1989. Paleocene calcareous nannofossil biostratigraphy. In: Crux, J.A., van Heck, S.E. (Eds.), Nannofossils and their applications. Ellis Harwood, Chichester, UK, pp. 265–310. von Salis, K., Monechi, S., Bybell, L.M., Self-Trail, J., Young, J.R., 2000. Remarks on the calcareous nannofossil genera Rhomboaster and Tribrachiatus around the Paleocene/Eocene Boundary. GFF 122, 138–140. Wei, W., Wise Jr., S.W., 1989. Paleogene calcareous nannofossil magnetobiostratigraphy: results from South Atlantic DSDP 516. Mar. Micropalentol. 14, 119–152. Wei, W., Wise Jr., S.W., 1992. Eocene–Oligocene calcareous nannofossil magnetobiochronology of the southern Ocean. Newsl. Stratigr. 26, 119–132. Wei, W., Zhong, S., 1996. Taxonomy and magnetobiochronology of Tribrachiatus bramlettei and Rhomboaster, two genera of calcareous nannofossils. J. Paleontol. 70 (1), 7–22.

248

C. Agnini et al. / Marine Micropaleontology 64 (2007) 215–248

Westerhold, T., Röhl, U., Laskar, J., Raffi, I., Bowles, J., Lourens, L.J., Zachos, J.C., 2007. On the duration of magnetochrons C24r and C25n and the timing of early Eocene global warming events: implications from the Ocean Drilling Program Leg 208 Walvis Ridge depth transect. Paleoceanography 22, PA2201. doi:10.1029/ 2006PA001322. Wing, S.L., Gingerich, P.D., Schmitz, B., Thomas, E. (Eds.), 2003. Causes and Consequences of Globally Warm Climates in the early Paleogene. Geol. Soc. Am., Spec. Paper, vol. 369. Boulder, Colorado. 614 pp. Wise Jr., S.W., Wind, F.H., 1977. Mesozoic and Cenozoic calcareous nannofossils recovered by DSDP Leg 36 drilling on the Falkland Plateau, southwest Atlantic sector of the Southern Ocean. Proc. Deep Sea Proj., Initial Repts. 36, 269–491. Zachos, J.C., Breza, J.R., Wise, S.W., 1992. Early Oligocene ice-sheet expansion on Antarctica: stable isotope and sedimentological evidence from Kerguelen Plateau, southern Indian Ocean. Geology 20, 569–573.

Zachos, J.C., Pagani, M., Sloan, L.C., Billups, K., Thomas, E., 2001. Trends, rhythms, and aberrations in global climate 65 Ma to present. Science 292, 686–693. Zachos, J.C., Wara, M.W., Bohaty, S., Delaney, M.L., Petrizzo, M.R., Brill, A., Bralower, T.J., Premoli-Silva, I., 2003. A transient rise in tropical sea surface temperature during the Paleocene–Eocene Thermal Maximum. Science 302, 1551–1554. Zachos, J.C., Kroon, D., Blum, P., et al., 2004. Early Cenozoic Extreme Climates: The Walvis Ridge Transect. Proc. Ocean Drill. Program, Initial Rep. 208 (http://www-odp.tamu.edu/publications/ 208_IR/208ir.htm). Zachos, J.C., Röhl, U., Schellenberg, S.A., Sluijs, A., Hodell, D.A., Kelly, D.C., Thomas, E., Nicolo, M., Raffi, I., Lourens, L.J., McCarren, H., Kroon, D., 2005. Rapid acidification of the ocean during the Paleocene–Eocene Thermal Maximum. Science 308, 1611–1615.