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Palaeocommunities, diversity and sea-level change from middleEocene shell beds of the Paris Basin

Stefano Dominici1* & Martin Zuschin2

1 Museo di Storia Naturale, Universit di Firenze, Via La Pira 4, 50121 Firenze, Italy2 Department of Palaeontology, University of Vienna, Althanstrae 14, A-1090 Wien, Austria*Correspondence: [email protected]

Abstract: The middle Eocene interval at some Paris Basin localities was studied through high-resolution stratigraphy.Abundance data (499 species, 37 719 individuals) on the distribution of molluscs, collected at 12 shell beds of the middleLutetian and lower Bartonian, formed the basis for a palaeoecological study. The middle Lutetian succession is subdivided intoseveral elementary depositional sequences (EDS) interpreted as the product of relative sea-level change. Species-abundancedistributions arebettercorrelated with EDS than with geographical locality, suggesting that sea level played an important role inthe distribution of palaeocommunities. Diversities were compared with analogous data from modern subtropical and warm-temperate intertidal and subtidal communities. We found that sea-level variation is responsible for a major change in the upper

part of the middle Lutetian succession, leading from high- to low-diversity palaeocommunities. From base to top sampledpalaeocommunities indicate a transition from high-energy and mesotrophic (EDS 2) to oligotrophic low-energy conditions of asandy lower shoreface (EDS 4) to an upper shoreface (EDS 5 and lower Bartonian), the last with mangroves and a seagrasscover. Notwithstanding the Lutetian cooling, we found that subtropical conditions reached as far north as the Paris Basin. Ourstudy suggests that climatic fluctuations might be obscured by facies control.

Received 24 November 2015; revised21 February 2016;accepted 9 March 2016

The middle Eocene of the Paris Basin belongs to one of the mostintensely studied stratigraphic intervals of geology. Shell beds therehave attracted scholars such as Guillaume Bruguiere, Jean-BaptisteLamarck, Grard Deshayes and Alcide dOrbigny, who contributedto establishing the principles of modern stratigraphy and the tenets

of macroevolution (Rudwick 2005). As a corollary result of themany efforts, the lists of its macro- and microfossils are mostlycompleted (Merle 2008, and references therein; Courville et al.2012; Lozouet 2014), but so far no species-abundance data ofquantitative bulk samples have been used to perform palaeocom-munity and diversity analyses, apart from a recent study devoted toone particular shell bed (Sanders et al. 2015). Such analyses,however, are of major interest because fossil assemblages of theParis Basin show characters not known from benthic communitiesliving today at analogous latitudes, where many families and generahave meanwhile undergone an almost complete turnover inimportance (Lozouet 2014;Tomasovchet al. 2014). For instance,the Paris Basin was located around 3035N within the warm-temperate belt at a time of general cooling with respect to the EarlyEocene climatic optimum (EECO). Nonetheless, overall speciesrichness calculated at the stage level suggests values similar to thoseof modern tropical hotspots (Huygheet al. 2012a;Lozouet 2014;Sanderset al. 2015).

The thickest part of the middle Lutetian succession is composedof unlithified fine-grained sandstones, with a large component ofbiogenic carbonates. Original aragonitic shells and residual colourpatterns are finely preserved and resemble much younger Neogeneshells (Merle 2008; Caze et al. 2011). Other well-studiedPalaeogene examples (e.g. the Pyrenees: Dominici & Kowalke2014; SE Europe:Oppenheim 1901; the Middle East: _Islamogluet al. 2011; Harzhauser et al. 2012), whether siliciclastic orcarbonate-dominated, are poorly preserved. Similar good preserva-

tion is found only in some unlithified siliciclastic Bartoniansuccessions of the US Gulf Coast, including the Gosport Formation(e.g. CoBabe & Allmon 1994) and the Stone City Formation

(e.g.Zuschin & Stanton 2002). Sedimentary structures, mudstonepartings and stratal surfaces are subtle, and massive bedding isfrequent, so that field correlations between Paris Basin outcropstraditionally are not based on facies contrast but on macrofaunalcontent (Gly 1996). The main bioclastic components are mollusc

shells, in concentrations of both sedimentological and biogenicorigin. Large foraminifera are also abundant, although theiraccumulations do not take the form of the nummulitic banks thatare common in most coastal and shelfal Eocene successions ofnearby seas (Pyrenees:Dominici & Kowalke 2007;Huygheet al.2012b). If the Eocene is a strange old world (Clyde 1999), itsexpression in the Paris Basin appears an utter mystery.

Paris Basin Palaeogene stratigraphy is relatively well understood,with isopachous relations across long distances (Gly & Lorenz1991; Gly 1996). This allows a good correlation of localsuccessions with regional subsidence patterns and global events.Available studies recognize tectonic stability from the Danian to lateBartonian (Robinet al. 2003). During this time, sedimentation wascontrolled by major sea-level fluctuations, typically in the range ofthird-order sea-level cycles (i.e. lasting 0.55 myr) and fourth-ordersea-level cycles, which are well framed in a chronostratigraphicframework (Gly 1996; Huyghe et al. 2015). The third-orderLutetian cycle, for example, was formed by three fourth-ordersea-level cycles. During the major transgression, which lasted morethan 3 myr (Huygheet al. 2015), the coastline moved in a southerndirection in a stepwise fashion, with maximum flooding occurringin the middle Lutetian, apparently without superimposed high-frequency fluctuations (Gly 1996).

To explore the ecological structure of middle Eocene shell bedsand shed further light on the history of sea-level changes, we havereconsidered three classical Lutetian localities, well known at leastsince the 18th century: Ferme-de-lOrme and Grignon, both near

Paris, and Fleury-la-Riviere, near Reims (Merle 2008; Courvilleet al. 2012). In addition to the Lutetian localities, we included thelower Bartonian succession at La Guepelle, to the NE of Paris, also

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Research article Journal of the Geological Society

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one of the type areas for the definition of this stage (Pomerol 1981;Fig. 1). The significance of sedimentary facies was reconsideredand, to explore the nature of the Paris Basin biota, fossilassemblages were bulk-sampled in some key units. Abundancedata on the distribution of molluscan species were evaluated andused for a multivariate analysis and to compute sample-leveldiversity. This allowed us to carry out for the first time apalaeocommunity analysis of the Paris Basin marine macrofauna,which provides information on the trophic structure of the benthicecosystem. A key question is whether the diversity of the Paris Basinsamples is comparable with that of modern tropical or warm-temperate environments. The sample diversity of the fossilassemblages, also studied for the first time in the Paris Basin, wastherefore compared with that of modern assemblages from two well-studied shallow marine settings, the northern Adriatic Sea and thenorthern Red Sea, in warm-temperate and tropical climates,respectively (Zuschin & Hohenegger 1998; Sawyer & Zuschin2010). Species-level abundance data and the sample-level quanti-tative comparisonopened a new approach to interpreting Paris Basinshell beds. The influence of sequence architecture is also of majorinterest because most changes in first and last occurrences of

species, and widespread changes in species abundance andbiofacies, occur at sequence boundaries and at major transgressivesurfaces (Holland 2000). This calls for evaluation of changes intaxonomic diversity in the context of environmental bias to ensurethat these changes are not simply driven by sequence architecture(Smith 2001).

Specifically, this approach allowed us to reconstruct Paris Basinbenthic palaeocommunities at a time of gradual change during themiddle Eocene and through climatic thresholds. The joint input fromsedimentary facies analysis and palaeoecology sheds light on externalfactors controlling the record. Thisstudy allows a better understandingto be obtained of the effects of global sea-level fluctuations in drivingboth the structure and the composition of benthic communities.

Sedimentary facies

Paris Basin strata are roughly horizontal and isopachous (Gly1996;Merle 2008), connecting outcrops as far as 160 km from oneanother. Sections are oriented along depositional strike, which is thereason for little lateral facies change within the studied succession(Fig. 1). The most detailed sedimentological description of theFaluniere de Grignon, where the Middle Lutetian is thickest andmost complete, has been given by Jean-Pierre Gly and DidierMerle (cited byGuernetet al. 2012;Sanders et al. 2015). Thoseresearchers described and numbered the sedimentary units from 1 to8, starting from the top (i.e. lower numbers represent younger units),a basic framework adopted here. In this phase of our research,

middle Lutetian palaeoenvironments are interpreted based on recentpalaeontological studies carried out at Grignon (Guernetetal. 2012;Huygheet al. 2012a) and Fleury-la-Riviere (Courvilleet al. 2012;see alsoTomasovch et al. 2014) and from our own fieldwork.Starting from the oldest, the sedimentary units of Gely & Merle(cited byGuernetet al. 2012) are summarized as follows.

Fig. 1.Middle Lutetian at Ferme-de-lOrme, Grignon and Fleury-la-Riviere, and lower Bartonian at La Guepelle, in the Paris Basin, with position of bulk

samples. Small numbers and arrows at Grignon represent the extent of sedimentary units as defined by Guernetet al. (2012). Large numbers in circlesrepresent fifth-order depositional sequences as recognized in this paper, with main transgressive surfaces correlated according to the literature (Gly 1996;Merle 2008). Description of the succession at La Guepelle is from B. Pattedoie (pers. comm., 2007). A major unconformity separates the middle Lutetianfrom the lower Bartonian (Huygheet al. 2015; not all intervening strata are shown here). Insets show explanation of symbols and location map of the studysites. Labels in rectangles refer to bulk samples collected in main shell beds.

S. Dominici & M. Zuschin

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Units 7 and 8. Bioturbated calcareous sandstone with abundantglauconite and quartz and sparse shell material. Thickness 34 m.Based on field evidence, the association is characterized byCardium (Orthocardium) subporulosum dOrbigny 1850. Atabular shell bed in the upper part is marked by sparse gravels andTurritella shell debris. Fine gravels in the lower part of the Fleurysection, within sedimentological shell concentrations (Fig. 2a)associated with bones of marine vertebrates, are here correlated withthe preceding shell bed. Units 7 and 8 have a complex stratigraphy,at least in their lower part at Grignon. Samples PB27 and PB16 werecollected in the lower part of the Fleury-la-Riviere section, indeposits correlative with the upper part of unit 7 at Grignon (Merle2008;Courvilleet al. 2012).

Unit 6. Cross-bedded, locally bioturbated calcareous sandstones,rich in quartz and glauconite, about 2 m thick. No samples wereevaluated from this unit.

Unit 5. Highly bioturbated massive calcareous sandstone withabundant quartz and glauconite, about 1.5 m thick. In thelower part ofthe unit, loosely packed shell beds are characterized byCampanilopagiganteum(Lamarck 1804), which dominates the local biogenic shellconcentration with its high biomass (Fig. 2b). The upper shell beds,

with a species-rich mollusc association, are instead dominated bySigmesalia intermedia (Deshayes 1832) at Fleury-la-Riviere(Courvilleetal.2012). Sample PB9 was collected in unit 5 atGrignon.

Unit 4. Massive calcareous sandstone with shells in loosely(Fig. 2c) or densely packed shell beds (Fig. 2d), or also resting onirregularerosional surfaces in thebasal part of the unit,in event bedsof sedimentological origin. Glauconite is rare. Thickness is about2 m. The association is characterized in the field by Chamalamellosa Lamarck 1806 and abundantOrbitolites complanatusLamarck 1801. Sample PB6 was collected at Grignon.

Units 2 and 3. Lithified massive calcareous sandstone about2.5 m thick with very rare glauconite, sparse shell beds andabundant Orbitolites in the lower part. In the upper part, theassociation is dominated bySeraphs sopitus (Solander cited byBrander 1776) and Avicularium lithocardium (Linn 1771), andsome colonial corals occur (Fig. 2f). Samples PB1 and PB3 werecollected at Grignon, and PB10 at Ferme-de-lOrme.

Unit 1. Massive calcareous sandstone, about 0.5 m thick, withthin laterally continuous shell beds (Fig. 2e, sample PB14). Themollusc association is dominated in the field by Potamideslapidorum (Lamarck 1804), Vicinocerithium echidnoides(Lamarck 1804) and Saxolucina saxorum (Lamarck 1806), andmiliolids dominate among the benthic foraminifera.

The higher content of glauconite in older units may reflect lowersedimentation rates, but the decreasing quartz content in the up-section direction gives evidence of the contrary.

Our investigations of the lower Bartonian succession croppingout at La Guepelle, also known as Guepelle Sandstone Fm andBeauchamp Sandstone Fm (biozone NP16:Huyghe et al. 2015),are based on an unpublished description by B. Pattedoie (pers.comm., 2007). The third-order sequence boundary at the base of the

Bartonian succession (Fig. 1; see Huyghe et al. 2015) was notobserved in the field. Sedimentary facies at La Guepelle include anumber of massive sandstone units a few metres thick. They arecharacterized by a sparse or loosely packed bioclastic fabric andthinner calcareous intervals in the middle part of the succession,topped by an important transgressive surface marked by boreholesof bivalves (Fig. 1). These characters led previous researchers to theinterpretation of a lower shoreface environment, comparable withthat sampled in the middle Lutetian (e.g.Huyghe et al. 2015). Acoarsening-upward facies sequence above the transgressive surface,

Fig. 2.Middle Eocene shell beds of theParis Basin. (a) Sedimentological shellconcetration at Fleury-la-Riviere, near anunconformity separating the middleLutetian from the Ypresian (EDS 2). (b)Campanilopa giganteumbiogenic shell

bed in plan view (Fleury-la-Riviere, EDS2). (c) Loosely packed bioclastic fabric atGrignon (EDS 4): disarticulated bivalvesinterspersed with shell hash. (d) Densely

packed bioclastic fabric (Grignon, EDS 4;height of outcrop about 20 cm):disarticulated and nested bivalve shellsconcentrated in pockets. (e)Potamideslapidorum shell bed at Ferme-de-lOrme

(EDS 5): laterally continuous shell bedwithin shelly sands. (f) Shell bed in planview, with colonial corals and bivalves(upper part of EDS 4, Grignon). For scale:

pencil, 16 cm; coin, 1.5 cm.

Paris Basin Middle Eocene shell beds



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topped by coarse-grained sandstones with cross-bedding, indicatesthe passage to a high-energy upper shoreface seafloor. Otherwise,as for the middle Lutetian, a better palaeoenvironmental recon-struction is expected from a palaeoecological analysis of sampledshell beds.

Sequence stratigraphy

The Palaeogene Paris Basin infilling is subdivided into five large-scale depositional sequences of million-year duration, bounded bymajor unconformities (third-order sequences in the sense ofHaqet al. 1987) and formed by the stacking of smaller units (Gly &Lorenz 1991; Huyghe et al. 2015). The Lutetian third-ordersequence, of about 8 myr duration, has been divided into threefourth-order sequences: sequence V (comprising the Glauconiegrossire Fm, the Calcaire Nummulites laevigatus Fm and theCalcaire grossier Fm), sequence VI (Marnes et caillasses infrieuresFm and Falun de Foulangues Fm) and sequence VII (Marnes etcaillasses suprieures Fm). These units approximately correspond tosequences A, B and C of Gly (1996). Sequence V is of longduration (about 5 myr) and comprises one sea-level fall dated

around the middle of biozone NP15 of calcareous nannoplanktonbiostratigraphy, within Polarity Chron C20, followed by a new,rapid transgressive event and a long, stepwise fall, until reaching amajor lowstand at the boundary C20C19 (Kominzet al. 2008).Evidence for one episode of these global fluctuations is provided bythe fourth-order sequence boundary at the top of NP15 (VVIunconformity ofHuygheet al. 2015).

The middle Lutetian localities under study here belong to thehighstand systems tract (HST) of sequence V. Around Paris, thesesediments sharply overlie glauconitic strata of the lower Lutetian,whereas furthest west, near Reims, they rest on lower Eocenedeposits (Gly 1996), indicating an important erosion preceding themajor middle Lutetian transgression (Chron C20). The middleLutetian is formed by elementary units A6A10, interpreted asparasequences (Gly 1996;Huygheet al. 2012a). As an alternativehypothesis, we propose that beds and bedsets are instead stacked toform small-scale sequences bounded by subtle unconformities,expressing low-amplitude fifth-order cycles and forming thebuilding blocks of the whole succession (elementary depositionalsequences: EDSs, numbered 16) (Fig. 1). Analogous high-frequency fluctuations, similarly expressed by metres-thick units,are recognized in Lutetian carbonates of the Spanish Pyrenees,increasing in amplitude and thickness in the Bartonian (Huygheet al. 2012b).

Facies changes justifying the grouping of bedsets into EDSsinclude the following: (1) the presence of fine-grained gravels(lower part of EDS 2 at Grignon and Fleury:Fig. 2a); (2) sharp-

based coarse-grained strata with sedimentological shell concentra-tions(lower part of EDS 3 at Grignon and Fleury; lower part of EDS4 at Grignon); (3) enrichment in carbonates in the middle part ofEDS 4; (4) occurrence of a shallow subtidal fauna in EDS 5. EDSs24 are stacked to form a retrogradational pattern, with calcareoussands of EDS 4 possibly representing a relatively deeper setting,although not exceeding the lower limit of influence of wave actionduring storms (i.e. lower shoreface). All 10 middle Lutetian bulksamples belong to biozone NP15 of calcareous nannoplanktonstratigraphy (Huyghe et al. 2015). Samples were collected in thetransgressive tract of EDSs 25 (EDS 1 and EDS 6 were notsampled), in biogenic shell beds from upper or lower shorefacedeposits (corresponding to nearshore and inner shelf samples,respectively, ofTomasovchet al. 2014).

A superficial comparison suggests that also the Bartonian at LaGuepelle may be subdivided in a number of EDSs. Two sequenceboundaries are therefore proposed, with some transgressive surfacesintervening in the succession (Fig. 1). Two bulk samples were

collected in the lower EDSs, which were possibly deposited in anupper shoreface environment.

Most of the 12 samples were collected in a shell-rich andbioturbated, massive sandstone lithofacies, indicative of low-energysubtidal conditions below fair-weather wave base. Shallowerconditions were inferred from the presence of dispersed finegravels (PB27), abundance of intertidal gastropods (Potamides:PB14), and position in a coarsening-upward succession (PBN7,PBG). High glauconite content indicates low sedimentation rates.Samples PB1 and PB3 were collected in a calcareous lithofacieswith denser accumulation of biogenic shell beds (Fig. 2d),suggesting a further lowering of sedimentary input with respect tounderlying and overlying strata.

Analytical methods

Bulk samples (0.5 l) were washed through a series of sieves (1.0, 2.0and 4 mm mesh-sizes). Species were separated and identified usingthe monograph of Cossmann & Pissarro (19041913) and bycomparison with museum collections hosted at the University ofFlorence, following modern taxonomic updates (Merle 2008). All

recognizable mollusc specimens in the fractions were counted andcombined for the purpose of this study. Raw abundances persample, for a total of 37 719 specimens and 499 species distributedin 12 samples, and data on locality and stratigraphic interval servedas the basis for a Q-mode multivariate analysis. Cluster analyses(Wards method) and ordination techniques (non-metric dimen-sional scaling, NMDS) and analysis of similarity (ANOSIM) wereapplied to a matrix of square-root transformed per cent abundancedata. The similarity matrix for NMDS and ANOSIM is based on theBrayCurtis Index. NMDS results are based on a 2D plot (stress =0.1553), with Axis 1 explaining 47.42% and Axis 2 20.37% of thevariance. In the 3D plot, stress lowers to 0.115, with Axis 3explaining only 7.52% of variance. In particular, ANOSIMprovides a way to test statistically whether there is a significantdifference between two or more groups of sampling units, based onstratigraphy or geography.TheR-values of the ANOSIM, which canvary between zero (no difference between samples) and unity, wereused to evaluate relations between samples. Once groupings amongsamples, recognized by cluster analysis and NMDS, wereestablished, the fossil fauna was used to explore the nature of co-occurrences by way of an R-mode analysis, carried out on asimplified version of the dataset. This was obtained by includingonly those species that contributed at least 2% of the total abundanceof at least one sample, and the species that co-occurred in at leastfour samples, no matter what their frequency. By doing so, thespecies list was downgraded to lessthan one-fifth of the original list,and the total abundance decreased to just two-thirds (S= 91,n= 23

592). Results were expressed as frequency of a given species in theoriginal dataset. Per-sample, species-level diversity was measuredon the original dataset (S = 499, n = 37 719) through rarefiedspecies richness and evenness. Richness was calculated with 95%confidence intervals at two values:n = 79, corresponding to theminimum number of individuals (in sample PB3), and n = 623,corresponding to the second lowest number of individuals (sample94-6, from the Red Sea). Evenness was measured through theShannonWiener index. These values were compared with those ofmodern analogues from the warm-temperate Adriatic Sea (Sawyer &Zuschin 2010) and the subtropical Red Sea (Zuschin & Hohenegger1998), which are taxonomically and taphonomically comparable(i.e. the fauna consists mostly of molluscs) and were sieved on 1 mmmesh-size. Mean values for the Red Sea, Adriatic Sea and ParisBasin were measured for both values of richness and evenness. Thisallowed us to evaluate the climate affiliation of Paris Basin shallowmarine biota, which during the middle Eocene inhabited latitudestoday associated with cool-temperate waters. All statistical analyses




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were carried out with the software package Primer 6.0 (Clarke &Warwick 2001) and PAST 3.10 (Hammeret al. 2001)

Results

Multivariate statistics

In the Q-mode cluster analysis, two groups are recognized at asimilarity level of 15% (Fig. 3a). One larger group comprises allmiddle Lutetian samples (cluster be), excluding PB14, theyoungest Lutetian sample in our analysis, which grouped with the

lower Bartonian sample PBG (cluster a). The second sample fromthe lower Bartonian, PBN7, stands as an outlier. Within the middleLutetian cluster, three samples from EDS 4 at Grignon form asmaller and tighter subset (cluster c: similarity above 40%). Theother three clusters within the large Lutetian group consist of twosamples each, which are always from different EDSs and differentlocalities (clusters ce).

The dataset includes four localities and five EDSs of chrono-stratigraphic significance, four from the middle Lutetian, at c.42 48 Ma, and one from the lower Bartonian, at 3940 Ma. Thethree samples from Fleury-la-Riviere, all collected within a 5 mthick sandstone succession and located as much as 160 km fromthe other section, are scattered in three groups. This suggests that

geographical position played a minor role in structuring thedistribution of the Paris Basin mollusc biota. Similarly, the twosamples collected at Ferme-de-lOrme, a section also only a fewmetres thick, are also widely separated in the sample ordination plot(Fig. 3c). The Q-mode clusters make more sense when interpreted interms of chronostratigraphic units. In the NMDS ordination, all EDS4 samples, although from two different localities, plot close together(Fig. 3d), with no overlap with those from other stratigraphic units.EDS 2 and EDS 3 can be similarly separated. Displaced from theseare the sample from EDS 5 and the lower Bartonian samples. PB14,from the middle Lutetian of Ferme-de-lOrme, plots relatively farfrom the other middle Lutetian samples, and has affinities with thelower Bartonian sample PBG as found in the cluster analysis(Fig. 3a). This result is mirrored by the position of PB14 and PBG at

high values alongAxis1 of the NMDS plot (Fig. 3d). The ANOSIMshows a lower Global R when the samples are grouped bylocality (GlobalR = 0.626; Table 1), than by EDS (Global R =0.733;Table 2).

The R-mode cluster analysis highlighted eight main groups ofspecies contributing to the similarities of samples within eachEDS (Fig. 4, clusters labelled AH). Group A is the largest andcomprises 41 species, which preferentially occur in samplescollected from EDS 4 at Grignon and Ferme-de-lOrme. Thebivalve Paraglans calcitrapoides is abundant and, together withthe gastropods Vermicularia conica and Collonia striatus,ubiquitous and exclusive of this interval. Cluster D also mainlycontributed to the grouping of samples from EDS 4. Some of the

species of clusters A and D, although not ubiquitous, were foundonly in samples from this chronostratigraphic interval, includingStriarca decipiens, S. quadrilatera, Barbatia barbatula,Cyclocardia squamosa, Plagiocardium granulosum, Emarginulacostata, Lapparentia fischeri and, in cluster D, Hipponixcornucopiae, Sigmesalia trochoides and Siphonaliopsis minuta.Species groups A and D, even when broken down to smallerclusters, are composed of a mixture of trophic groups. Thesecomprise many suspension-feeding, shallow infaunal bivalves(Carditidae, Cardiidae, Veneridae, etc.), many infaunal andepifaunal carnivore gastropods (Natica, Vexillum, Crassispira,etc.), some microherbivores (Tricolia, Pusillina) and parasites(Tenuiscala). Cemented forms are rarer than in other clusters.Among the species of cluster D, Eopleurotoma plicaria, Naticaepiglottina and Haustator mitis are particularly characteristic ofsample PB3 and also occur in sample PB16, from EDS 2; thisexplains in part the grouping of these two samples in Q-modeanalyses (Fig. 3).

Fig. 3.Multivariate analysis. (a) Cluster analysis. (b) Plot of NMDS;samples grouped by clusters shown in (a). (c) Plot of NMDS; samplesgrouped by locality. (d) Plot of NMDS; samples grouped bychronostratigraphic unit (EDS).




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Cluster B comprises species occurring mostly in a sample fromEDS 3 (PB9, from Fleury-la-Riviere), such as suspension-feeding(Haustator imbricatarius) and carnivorous gastropods(Olivancillaria mitreola). Glycymeris pulvinata (a suspensionfeeder) andFustiaria circinata(a deposit feeder) are ubiquitous inolder Lutetian samples, disappearing up-section (EDS 5,Bartonian). Overall, the suspension-feeding bivalves form the bestrepresented trophic group.

Clusters C and F contain species particularly abundant in EDS 2,none of them being exclusive to this interval. Species belong to avariety of trophic niches, with abundant microherbivorous gastro-pods Bittium semigranosum and B. transenna and many suspen-sion-feeding bivalves and gastropods (e.g. Cardiidae, Veneridae,Corbulidae, Turritellidae). Carnivores are less diversified orabundant than elsewhere.

Clusters E and H contain species abundant in lower Bartonianshell beds, the first with species of sample PBN7, the second withspecies of PBG. Cluster H, characterized by the herbivorousgastropods Potamides lapidorum and an unidentified rissoid, is

exclusive of younger samples (comprising EDS 5). Cluster E, inturn, contains shallow infaunal suspension-feeding bivalves such asCallocardia nitidulaand Bicorbula gallica, and chemosymbioticlucinids (Parvilucina pusilla) that range from EDS 2 to the lowerBartonian. Cubitostrea cubitus (cluster E) is found in both lowerBartonian samples, whereas it is absent in the Lutetian. Finally,cluster G groups species characteristic of sample PB14 from EDS 5at Ferme. Here, characteristic species point to a vegetated bottom,with abundant herbivorous gastropods Bittium duchasteli andTympanotonos semicoronatus. The chemosymbiotic lucinidbivalve Parvilucina turgidula from cluster G points to organic-rich bottoms with local sulphuric conditions, possibly associatedwith the presence of seagrass.

A few species were found throughout the study interval, but themajority are either middle Lutetian or lower Bartonian.Potamideslapidorumis abundant and exclusive of EDS 5 and lower Bartoniansamples, whereas no species in the restricted dataset (S= 91) isexclusive of thelower part of themiddle Lutetian (EDS 2 or EDS 3).

Richness and evenness comparisons

Owing to differences in taxonomic and taphonomic conditions andsampling design, there are not many modern examples that can becompared with our fossil dataset. The two case histories that wehave selected are taxonomically and taphonomically comparable

with the Paris Basin study because they both consider the wholebenthic molluscan shelly fauna, including aragonitic shells, whichis well preserved in the Paris Basin collections. Sampling designsare also comparable, with bulk samples sieved through the samesieve sizes across the three studies that are being compared. The twomodern settings examined, the Red Sea and the Adriatic, likethe Paris Basin are from vast shallow seas enclosed by land masses.The wide range of habitats included in both modern settings allowsfor a meaningful comparison with the middle Eocene subtidal fossilassemblage. In the end, we expect differences in composition,evenness and abundance of singlesamplesto be under the control ofclimate, with the Red Sea and the Adriatic serving as templates forsubtropical and temperate settings, respectively. To facilitate

comparison, the information on bottom facies and water depth forall samples is provided on a per-sample basis (Table 3).The analysis confirms the uniqueness of samples from EDS 4:

this unit has the highest values of rarified species richness andevenness among all samples, most notably PB1 and PB10, collectedin calcareous sandstones at Grignon and Ferme-de-lOrme,respectively. The lowest evenness is present in lower Bartoniansamples at La Guepelle (PBN7 and PBG) and the upper part of themiddle Lutetian at Ferme-de-lOrme (PB14). Another sample withlow evenness is PB16, from the upper part of EDS 2 at Ferme. Ourdiversity study therefore confirms a stratigraphic trend alreadyindicated by multivariate analysis: low or intermediate evenness insamples from EDS 2 and EDS 3, very high values in EDS 4, and adrop of evenness in EDS 5 and in the lower Bartonian. When Paris

Basin rarefied data are compared with their modern analogues, wefind that values encountered in EDS 4 are similar to those measuredin assemblages from modern coral sand and reef slope habitats; thatis, possibly the highest diversities encountered in the tropical Red

Table 1.Results of analysis of similarity (ANOSIM), factorlocality

Groups Rstatistic P-value Possible permutations Actual permutations Number observed

Grignon, Ferme 0.491 0.095 21 21 2Grignon, Fleury 0.508 0.018 56 56 1

Grignon, Guepelle 0.982 0.048 21 21 1

Ferme, Fleury 0.5 0.1 10 10 1Ferme, Guepelle 0.75 0.333 3 3 1

Fleury, Guepelle 0.917 0.1 10 10 1

Sample statistics (GlobalR): 0.626. Number of permutations: 999 (random sample from 83 160).

Table 2.Results of analysis of similarity (ANOSIM), factorEDS

Groups Rstatistic P-value Possible permutations Actual permutations Number observed

EDS 4, EDS 3 0.600 0.095 21 21 2EDS 4, EDS 5 1.000 0.167 6 6 1

EDS 4, EDS 2 0.836 0.048 21 21 1EDS 4, Lower Bartonian 1.000 0.048 21 21 1

EDS 3, EDS 5 0.000 0.667 3 3 2

EDS 3, EDS 2 0.750 1.000 3 3 3

EDS 3, Lower Bartonian 0.500 0.333 3 3 1EDS 5, EDS 2 1.000 0.333 3 3 1EDS 5, Lower Bartonian 0.000 0.667 3 3 2

EDS 2, Lower Bartonian 1.000 0.333 3 3 1

Sample statistics (GlobalR): 0.733. Number of permutations: 999 (random sample from 83 160).




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Fig. 4.R-mode cluster dendrogram (analysis conducted on a subset of the original dataset) compared with sample composition, in stratigraphic order, withfrequencies expressed semiquantitatively.




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Sea (Table 3;Zuschin & Hohenegger 1998). In contrast, in EDS 5and the lower Bartonian shell beds the palaeocommunities show thehigh dominance typical of intertidal assemblages from the modernwarm-temperate Adriatic Sea (Table 3;Sawyer & Zuschin 2010).

Middle Eocene molluscan palaeoecology

Thequantitative analysis of middle Eocene Paris Basin shell beds hashighlighted a change in the composition of mollusc palaeocommu-nities that parallels and expands knowledge based on lithology andstratigraphy. Evenness and complexity in trophic relationshipsgenerally increase from sandstones of EDS 23 to calcareoussandstones of EDS 4, dropping to a minimum in calcareoussandstones of EDS 5 and lower Bartonian sandstones. This changeis also reflected in the relative position of samples in ordination space(Fig.3), althoughrelationships here aremore complexto interpret anda simple gradient cannot be extrapolated. The following discussion

highlights three possible end-members of a continuum, where factorssuch as nutrient level, water depth and t rophic interaction with plantscontribute to shaping macrofaunal benthic communities.

Mesotrophic sandy bottoms

The lowermost sample PB16, from the middle Lutetian (EDS 2),and PBN7, from the lower Bartonian, show low evenness (Table 3),but they occupy different positions in the NMDS plot (Fig. 3).Dominant species of either assemblage, however, belong to thesuspension feeders. These include a turritelline gastropod,Haustator mitis (32% of frequency in PB16), and two infaunalvenerid bivalves, Calloocardia nitidula and Callista elegans(respectively 32% and 23% in PBN7). Suspension feeders can actas opportunist species within shallow marine benthic communities,taking advantage of high quantities of seston and capable of facingrapid changes of environmental conditions. Accordingly, high

Table 3.Comparison of rarefied species richness and evenness of molluscan faunas from samples collected in the modern Red Sea (Zuschin & Hohenegger1998), the modern Adriatic Sea (Sawyer & Zuschin 2010) and in the middle Eocene of the Paris Basin (this study)

Bottom facies Water depth (m) Sample label EDSRarefied species richness

ShannonWienerIndex

Richness (n= 79) 95% CI Richness (n= 623) 95% CI Shannon_H

Red Sea

Coral sand 10 94-1-a 40.55 1.11 130.40 0.93 4.12Coral sand 10 94-1-b 41.24 1.09 132.57 0.92 4.15Coral sand 10 94-1-c 37.31 1.12 119.43 0.89 3.90

Coral sand 10 94-1-d 39.60 1.10 127.14 0.90 4.04

Muddy sand 23 94-3-a 25.83 1.17 75.63 0.47 2.91Muddy sand 23 94-3-b 23.60 1.19 68.80 0.63 2.75

Mud 39 94-4-a 17.62 1.00 37.22 0.92 2.65

Mud 39 94-4-b 19.95 1.08 45.86 0.93 2.83Reef slope 19 94-5 43.30 1.05 135.43 0.79 4.18

Mangrove


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dominance of turritelline gastropods is frequently associated withdeltaic and upwelling zones, where seasonally driven nutrientsabound (Allmon 1976). As upwelling is unlikely, given thepalaeogeographical setting of the Paris Basin, some form ofdeltaic influence can be hypothesized for the PB16 palaeocommu-nity. In contrast, rapid burrowers such as the dominant veneridbivalves in PBN7 are abundant in high-energy shallow subtidalbottoms, where they can withstand the vagaries of a rapidly shiftingsandy substrate and take advantage of the abundant suspendedorganic matter (Stanley 1977). The separation between PB16 andPBN7 in the ordination plot can thus be explained by two different,but similarly unpredictable subtidal environments. An importantspecies-level turnover has, however, taken place during the severalmillion years that separate the two. This calls for also consideringevolutionary change in explaining the distance of PBN7 from allother samples. This is not the case for PB3, also from the middleLutetian (EDS 4), which shares with PB16 abundantHaustatormitis (eight individuals, 32% in PB16), explaining their proximityin ordination space. Rarefied species richness and evenness of thissample are relatively high, but have to be interpreted with cautionbecause of the small size of the sample (n= 79).

Oligotrophic sandy bottoms

All other middle Lutetian samples are plotted in the middle part ofthe NMDS. Samples from EDS4, including PB1, PB4 and PB6from Grignon and PB10 from Ferme-de-lOrme, have the highestevenness, comparable with that of open marine coral sands ofsubtropical settings such as the Red Sea (Table 3). Similarly, we canassume that the upper half of EDS 4 was deposited underoligotrophic conditions in an ecosystem characterized by narrowniches. Accordingly, the palaeocommunity is evenly composed ofherbivores (Emarginula costata,Collonia stratus,Ptychocerithiuminabsolutum, Vicinocerithium sp., Tricolia sp., Pusillina nana,Lepidochitona grignonensis), carnivores (Tenuiscala cloezi,Payraudeautia perforata, Lapparentia fischeri, Volvarinella spp.,Eopleurotoma plicaria, Cryptoconus spp.), some deposit feeders(Rimella fissurella), and suspension feeders. The last are in turnevenly represented by byssate epifauna (Barbatia barbatula,B. angusta,Striarca decipiens), cemented epifauna (Vermiculariaconica, Vermetia sp.) and infauna (Paraglans calcitrapoides,Crassatina triangularis, Varicorbula minuta, Plagiocardiumgranulosum, etc.). These characters suggest a subtidal settingdeeper and furthest from terrestrial inputs with respect to all otherpalaeoenvironments considered.

Among samples from EDS 4, PB10 shows many similarities toPB22 from EDS 3, including the abundance of the herbivoreBittiumtransenna, the suspension-feeding Turritellidae (Sigmesalia inter-

media and Haustator funiculosus) and the Veneridae Callistaelegans and Cyclocardia serrulata. Because the last two areabundant also in PBN7 from high-energy subtidal bottoms, andbecause the Turritellidae more often prefer high-nutrient settings,we infer that Ferme-de-lOrme and Fleury-la-Riviere, where PB10and PB22 were collected, respectively, were shallower and closer toterrestrial inputs with respect to Grignon. The last two remainingassemblages to consider, PB27 and PB9 from EDS 2 and 3,respectively, can also be interpreted as representing moreunpredictable palaeoenvironments than those encountered atGrignon within EDS 4. Evenness is not as high as in the latter,and opportunistic suspension feeders often abound at the expense ofother trophic groups; this is the case with Haustator funiculosus(23% in PB27),Cubitostrea plicata (21% in PB9, 17% in PB27)andGlycymeris pulvinatus(20% in PB9, 2% in PB27).

Analogies of Paris Basin rarefied diversities with those measuredin modern shallow marine mollusc communities in the Red Seastrengthen the above palaeoecological interpretation. Modern coral

sands are good analogues of samples PB1, PB4, PB6 and PB10(EDS 4, excluding PB3) because they have even proportions of themain trophic groups (herbivores, suspension feeders and carni-vores), and because opportunistic suspension feeders such asTurritella orCorbulaare missing (Zuschin & Hohenegger 1998).The lack of chemosymbiotic bivalves in EDS 4 palaeocommunities,although they are diversified and abundant in Red Sea subtidal coralsands, can be explained by the lackof a seagrass cover in ParisBasinbottoms during this part of the middle Lutetian.

Mangroves and seagrasses

Younger samples PBG and PB14 occupy the lower right quarter ofthe ordination, forming a loose cluster (Fig. 3a). Their trophiccomposition justifies, for both, the interpretation of palaeocommu-nities from a very shallow subtidal sandy bottom, with very highorganic content from the decomposition of vegetal matter. Thesimplest palaeocommunity is represented by lower Bartoniansample PBG with a high dominance of herbivorous gastropods ofthe families Potamididae (Potamides lapidorum, Tympanotonossemicoronatus), Cerithiidae (Ptychocerithium lamellosum) and

Rissoidae (Pseudotaphrus buccinalis and an undeterminedspecies), followed by the carnivores (Amauropsina canaliculata, anaticid) and some suspension feeders, either byssate- (Trinacriamedia) or cemented-epifaunal (Cubitostrea cubitus), with only rareinfauna (Cyclocardia pulchra).

Next in composition comes sample PB14 from the upper part ofthe middle Lutetian (EDS 5). It shares with PBG the dominance ofcerithioidean herbivorous gastropods, mostly belonging to the samespecies (Potamides lapidorum, Tympanotonos semicoronatus,Ptychocerithium lamellosum), but also the cerithiids Bittiumduchasteli and B. semigranosum. Rissoidae, however, are absent;their ecological role is played in part by a Pseudomelaniidae(Bayaniasp.), and trophic complexity and evenness are consider-ably higher than in PBG. In particular, the suspension feeders aremuch more diverse and equally distributed within the epifauna (e.g.Trinacria deltoidea,Striarca quadrilatera,Barbatia angusta) andthe infauna (e.g.Cyclocardia serrulata,Gari dutemplei,Venerellasecunda, Katelysia texta). Alongside the suspension feeders, theinfauna is occupied by abundant chemyosymbiotic lucinid bivalves(Parvilucina turgidula), indicative of a substratum rich in organiccontent. Carnivores are represented by rare Oliviidae(Olivancillaria mitreola) and Marginell idae (Volvarinellaeburnea).

Modern Potamididae show parallel distribution with mangrovetrees along the tropical intertidal belt (for the relations betweenEocene and modern potamidids, seeReidet al. 2008;Dominici &Kowalke 2014), but the presence of Rissoidae (PBG) and miliolid

foraminifera (PB14), together with lucinid bivalves, points also tothe presence of seagrass. The comparison of rarefied richness andevenness for these two samples with those from the Red Sea pointsto a species-abundance distribution similar to that of a modernmangrove environment (Table 3). We therefore suggest a veryshallow subtidal environment, closely associated with a mangroveecosystem, possibly covered with sparse seagrass.

Discussion

Several aspects complicate a sequence stratigraphic interpretation:thethinness of the sections (515 m) and the slow net sedimentationrates they imply, the pervasive bioturbation that hampers thedevelopment of a facies model, the paucity of outcrops and theorientation of the section along depositional strike. However,available data on the sequence architecture of low-accommodationCenozoic siliciclastic successions allow our sequence stratigraphicinterpretation to be considered as a viable working hypothesis. In




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fact, data-rich studies from similar closed marine embaymentsdeveloped along passive continental margins allow for therecognition of similarities. A very good analogue is the middleEocene Clairborne Group, in the onshore US part of the Gulf ofMexico Basin (Hackley 2012). Claiborne formations contempor-aneous with those here under study are the Welch Formation(middle Lutetian), the Sparta Sand and the Cook MountainFormation (lower Bartonian). Similarly to their Paris Basinanalogues, they represent shallow marine sediments and are onlya very fewmetres thick, glauconitic, richly fossiliferous and laterallypersistent for tens of kilometres (Stenzel 1940; Gimbrede 1962;Yancey & Davidoff 1994), implying low sedimentation rates and aneven topography. A younger analogue is found in the Miocene ofMaryland, further north along the US east coast. This ischaracterized by laterally continuous and rich shell beds croppingout for tens of kilometres in a strike direction, with unconformity-bounded regressivetransgressive cycles stacked to form a retro-gradation succession lasting more than 10 myr, but only about 30 mthick (Kidwellet al. 2015).

The sedimentary facies analysis and palaeoecology of middleEocene Paris Basin shell beds allow an interpretationof the nature of

the shallow marine benthic ecosystem and its abiotic factors duringa peculiar time in the Cenozoic. The doubthouse comprised aperiod between the Early Eocene greenhouse climate, with thehighest temperatures of the Cenozoic, and the Oligoceneicehouse,featuring a sharp decrease in global temperatures and the formationof the Antarctic ice sheet. This period was characterized by a globaltemperature decrease interrupted by a temporary ameliorationduring the lower Bartonian (the Middle Eocene ClimaticOptimum; MECO) lasting about 500 kyr (Bohaty & Zachos2003). Palaeoenvironmental change during the middle Lutetianhas been recently evaluated by geochemical and palaeontologicalstudies conducted on the Grignon succession, which are directlycomparable with our approach (Fig. 5). The isotopic study onmollusc shells (Huyghe et al. 2012a, 2015) and the study onostracod assemblages (Guernetet al. 2012) suggest that maximumdepths were reached in the middle part of their parasequence A6,corresponding to the lower part of EDS 2, in our study representedby sample PB27 from Fleury-la-Riviere (Figs 1 and 5). In ourinterpretation, however, PB27 is representative of a shallowing withrespect to underlying sediments (Glauconie grossiere Fm:Huygheet al. 2015; EDS 1) because of its palaeoecological signal (i.e.relatively low evenness and high abundance of opportunisticsuspension feeders). Sedimentary facies analysis and the palaeo-ecology of mollusc shell beds suggest instead that the middleLutetian (fourth-order) maximum flooding surface (MFS) coin-cided with the lower part of shelly calcareous sandstones in theupper part of the section, at the MFS (fifth-order) of EDS 4 (Fig. 5).

This interval is also marked by the richest ostracod assemblage(Guernetet al. 2012). This fourth-order MFS would thus pinpointthe beginning of the sea-level highstand recognized in global sea-level curves at around 45.0 Ma (Kominzet al. 2008). The sequenceboundary separating EDS 4 and EDS 5 would therefore mark thelong and stepwise sea-level drop occurring between about 44 and43 Ma. This resulted in a major change in shell bed compositionrecorded during the ensuing transgression, which is correlated withthe global rise occurring at about 42 Ma (Kominz etal. 2008), whenonly very shallow subtidal depths were attained (sample PB14,collected at Ferme-de-lOrme). A short-lived sea-level fluctuation isrecorded at Ferme-de-lOrme at the base of a thin muddy interval(possibly deposited in an intertidal environment), overlain by uppershoreface deposits dominated in the field by the turritellidSigmesalia (nutrient-rich shoreface environment;Fig. 1). The sealevel then fell again, in coincidence with the pre-MECO climaticcooling. During this time, the third-order sequence boundary wasformed, separating the Lutetian from the Bartonian (Huygheet al.

2015). The La Guepelle succession can be correlated with thegeneral transgression occurring during the MECO, centred ataround 40.0 Ma (base of Chron C18n.2n: Huyghe et al. 2015).Similarly to the transgressive systems tract of EDS 5 and EDS 6,depths did not attain the same magnitude as in the middle Lutetian.

Cooling during the later part of the middle Lutetian, and the furthertemperature drop preceding the MECO (Bohaty & Zachos 2003),may explain the species-level turnover registered at the stage level(Cossmann & Pissarro 19041913). Our sample-level data,however, point to a facies shift at the sequence boundary betweenEDS 4 and EDS 5, which controls mollusc diversity. We thereforesuggest that sequence stratigraphical architecture controls patternsof faunal change at this regional scale, as has been shown for severalother regions and time intervals (e.g.Brett 1998; Patzkowsky &Holland 1999;Smithet al. 2001;Zuschinet al. 2011).

The middle Eocene history of mollusc benthic communitiesrecorded in the Paris Basin is just one step in the long andcomplex march leading from early Eocene greenhouse shallowmarine ecosystems to the modern fauna (Lozouet 2014). In thistime interval, the macroevolutionary turnover was particularlysevere at intertidal and shallow subtidal depths (Tomasovchet al.2014). Middle Lutetian communities, however, are similar tomodern tropical assemblages such as those living in coral sands

Fig. 5.Middle Lutetian stratigraphic units recognized in this study,compared with units described by previous researchers. All middleLutetian samples used in our palaeoecological analysis were projectedonto the Grignon succession, taken as a template of this time interval.




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and seagrass bottoms of the Red Sea, sharing high evenness andcomplex trophic relationships (Table 3; Zuschin & Hohenegger1998), and unlike warm-temperate faunas of the Adriatic Sea,which are dominated by fewer species at all depths (Table 3;Sawyer & Zuschin 2010). This evidence confirms previoussuggestions (e.g. Huyghe et al. 2012a) that, even during themiddle Eocene cooling, subtropical conditions of the Tethyanrealm (Lozouet 2014) reached as far north as the Paris Basin. Thesame situation was not found in the later part of the middleLutetian, nor during the MECO, notwithstanding an importantglobal climatic amelioration. Before concluding that shell beds ofEDS 4 record the last evidence of tropical affinity among mid-latitude shallow marine communities in Europe, younger assem-blages from a comparable facies must be sought.

Conclusions

Important middle Eocene successions of the Paris Basin werestudied, starting from a re-evaluation of sedimentary facies andthrough new bulk-sampling in mollusc shells beds. The studyinterval comprises the middle Lutetian and the lower Bartonian,

which areseparated by a major depositional sequence boundary on aregional scale. Our study reached the following conclusions.

(1) Both intervals can be subdivided into small-scaleunconformity-bounded units (i.e. metre-scale elementarydepositional sequences; EDSs), interpreted as the product ofhigh-frequency (fifth-order) cycles of sea-level change. Weidentified five EDSs in the middle Lutetian and two in thelower Bartonian. Sequence boundaries were recognized at thebase of sandstones with fine gravels or cross-bedding, withinsuccessions dominated by bioturbated fine-grained shellysandstone and shelly calcareous sandstone.

(2) The species abundance distribution of molluscan assemblagesfrom 12 bulk samples of major shell beds, used for

multivariate analysis and diversity analysis, helpedreconstruct the composition and structure of the benthiccommunities living at this peculiar time of Palaeogene climatetransition. Palaeocommunities from middle Lutetian EDS 14are marked by high evenness and trophic complexity, whichpeaked in EDS 4. These indicators dropped in EDS 5, possiblyin EDS 6 (not sampled), and in lower Bartonian shell beds.

(3) When compared with modern assemblages, middle Lutetiancommunities strongly resemble mollusc species from coralsands at subtropical latitudes, but less so warm-temperateshallow marine communities. This is counterintuitive if ParisBasin palaeolatitudes are considered. It does, however, agreewith previous knowledge on stage-level species richness,calculated after more than two centuries of research on thesesuccessions. We therefore confirm that the middle LutetianParis Basin constitutes the remnant of a former early Eocenegreenhouseworld, which disappeared as the icehouse worldof the EoceneOligocene boundary was approached.

(4) The steps with which the climatic transition occurred cannotbe understood unless the appropriate facies is sampled inyounger sediments. This was not possible at the study sitesbecause EDS 5, EDS 6 and Bartonian assemblages are fromshallower water depths. The vertical stacking of Paris Basinpalaeocommunities suggests that the peak of tropical diversityof EDS 4 coincides with a fourth-order sea-level highstandrecorded on a worldwide basis.

Acknowledgements and Funding

The following people ensured the completion of this paper: D. Merle,B. Pattedoie, P. Legrand, A. Tomasovch and M. Stachowitsch. We are gratefulto S. Holland for a thorough review of the paper. This work was supported by the

Austrian Science Fund (FWF) project P19013-Bio and MIUR/PRIN 20102011(Past Excess CO2 worlds: biota responses to extreme warmth and oceanacidification).

Scientific editing by Quentin Crowley

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