Cretaceous Research 28 (2007) 1017e1032www.elsevier.com/locate/CretRes

Ammonite-based correlations in the Cenomanian-lower Turonianof north-west Europe, central Tunisia and the Western Interior

(North America)

Claude Monnet*, Hugo Bucher

Palaontologisches Institut und Museum, Universitat Zurich, Karl-Schmid Straße 4, CH-8006 Zurich, Switzerland

Received 28 October 2005; accepted in revised form 24 January 2007

Available online 25 July 2007

Abstract

The biochronology of Cenomanian-early Turonian ammonite faunas from three key stratotype areas (north-west Europe, central Tunisia andthe Western Interior of North America) has been analysed and revised by utilizing the unitary association method. This review is promptedby the huge amount of biostratigraphic data published during recent decades and by a taxonomic homogenisation of the ammonite faunasfrom these key areas. The Cenomanian and lower Turonian of Tunisia comprise twenty-four Unitary Association zones and the middleCenomanian-lower Turonian of the Western Interior Basin twenty-three such zones. The unitary association method means a two-fold increasein resolution of these ammonite zonations compared to the standard, empirical schemes. Central Tunisia and the Western Interior are correlatedwith north-west Europe by constructing a zonation including all taxa common to these areas. These correlations highlight the variable complete-ness and resolution of the faunal record through space and time, and reveal a significant number of diachronous taxa between the three areas.These correlations enable the designation of a new global marker for the middle/upper Cenomanian boundary, which is characterised by thedisappearance of the genera Turrilites, Acanthoceras and Cunningtoniceras and by the appearance of Eucalycoceras, Pseudocalycoceras andEuomphaloceras. The only synchronous datum known is the last occurrence of Turrilites acutus, which may thus be used as a marker forthe middle/upper Cenomanian boundary, provided that it does not turn out to be diachronous in the light of any new data.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Ammonites; Biochronology; Unitary Associations; Correlations; Cenomanian; Turonian; Europe; Tunisia; Western Interior; Diachronism

1. Introduction

The marine Cenomanian-Turonian is one of the best-studied stratigraphic intervals of the Cretaceous. Such focushas been prompted mainly by several, more or less interwo-ven, biotic and abiotic events, such as a moderate mass extinc-tion (Raup and Sepkoski, 1982; Hallam and Wignall, 1997),the highest sea-level of the Mesozoic (Hancock and Kauffman,1979; Haq et al., 1988) and a global oceanic anoxic event(Schlanger et al., 1987). In order to better understand the globalevents and their consequences during the mid-Cretaceous(e.g., Reyment and Bengtson, 1986; Cotillon, 1989), a wealth

* Corresponding author.

E-mail address: claude.monnet@pim.unizh.ch (C. Monnet).

0195-6671/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.cretres.2007.01.007

of biostratigraphic data for this critical time interval has beengenerated. Hence, the biostratigraphic distribution of majorammonite genera and species during the Cenomanian-Turonian is relatively well known. In addition, similar ammo-nite zonations have been established for this interval in suchdistant basins as north-west Europe, central Tunisia and theWestern Interior (e.g., Robaszynski et al., 1982, 1994, 1998;Cobban, 1984; Robaszynski, 1984; Wright and Kennedy,1984; Kennedy and Cobban, 1991; Gale et al., 1996; Kennedyet al., 2004).

Achieving the best possible biochronological resolution bymeans of exact and robust correlations has direct implicationsfor various geological, geochemical, palaeoclimatic and evolu-tionary hypotheses. Current efforts to define a ‘Global bound-ary Stratotype Section and Point’ (GSSP) have prompted

1018 C. Monnet, H. Bucher / Cretaceous Research 28 (2007) 1017e1032

a revision of empirical biochronological zonations. Hence, thefirst aim of the present paper is to revise the ammonite zonalsequences of central Tunisia and the Western Interior by utiliz-ing the quantitative, unitary association method (Guex, 1991).Secondly, these newly revised zonations are then correlatedwith north-west Europe, which has already been processedsimilarly by us (Monnet and Bucher, 1999, 2002). Finally,these quantitative zonations are compared to the standard, em-pirical ones.

2. Palaeogeographical setting

The present study encompasses three major areas whereammonite-bearing rocks of Cenomanian age are well devel-oped (Fig. 1), namely north-west Europe, central Tunisia andthe Western Interior of North America. These are all typeregions (old stratotypes and recent boundary stratotypes) forthe Cenomanian and Turonian stages (e.g., Juignet, 1980;Robaszynski et al., 1982, 1990, 1994; Robaszynski, 1984;Kennedy and Cobban, 1991; Gale et al., 1996).

North-west Europe was situated at the northern Tethyan mar-gin, at an average palaeolatitude of 35�N. The study area en-compasses three epicontinental basins: the Vocontian Basin(south-east France), the Munster Basin (north-west Germany)and the Anglo-Paris Basin. Central Tunisia, belonging to thesouthern Tethyan margin, was located at a palaeolatitude ofabout 15�N (Philip et al., 1993). The Kalaat Senan area, selectedhere for discussion, was situated at the transition between a car-bonate platform to the south and a basin to the north (Robaszyn-ski et al., 1994). The Western Interior Basin is an epicontinental,complex foreland basin, elongated along a north-south axis(Kauffman, 1977, 1984; Pratt et al., 1985) and bounded in thewest by the Cordilleran thrust belt and by a stable cratonic plat-form in the east. During the Early Cretaceous, this seaway wasrestricted in the north, whereas non-marine deposition tookplace in the southern part of the basin (Kauffman, 1984; Lucaset al., 1998), where the oldest Cenomanian marine rocks yieldammonites of middle Cenomanian age.

3. Taxonomy

The data used for biochronological analysis have beencompiled from multiple sources (from the literature, as wellas from unpublished records for the Vocontian Basin), inwhich taxonomic concepts may be at variance. In order to con-struct reliable correlations and for the sake of consistency,a standardized taxonomy has first been established. The useof a species name must be consistent throughout the entiredata set, and it must also account for intraspecific variabilityand for ontogenetic changes. Note that inherent limitations tothis standardization are obviously imposed by the quality of avail-able taxonomic data (i.e., plots of occurrences against logs and il-lustration of intraspecific variability and ontogeny). It is worthnoting that we have used a population approach (rather than thetypological concept) to identify species (e.g., Monnet andBucher, 2005), because ammonites may display large intraspe-cific variability and covariation of both whorl shape and coilingwith strength of ornament (Buckman’s first law of covariation).Forms of a single species usually range from compressed, invo-lute and weakly ribbed to more depressed, more evolute andmore strongly ribbed (e.g., Reeside and Cobban, 1960; Callo-mon, 1985; Dagys and Weitschat, 1993; Checa et al., 1997; Mon-net and Bucher, 2005; Hammer and Bucher, 2005). For furtherdetails on taxonomic standardization of Cenomanian and Turo-nian ammonites, reference is made to Monnet (2005).

4. Methods

4.1. Zonation

The chronological component of the fossil record of ammo-nites is extracted here by utilizing the unitary associationmethod (Guex, 1991). This has the following advantages: (i)it is a quantitative and deterministic method based on the co-existence of species; (ii) it produces discrete biozones inagreement with the discontinuous nature of the fossil record;(iii) it preserves the integrity of the original data set (i.e., all

WI

0°

30°

AP

TU

M

V

Fig. 1. Palaeogeographical location of study areas; WI, Western Interior Basin; TU, central Tunisia; AP, Anglo-Paris Basin; M, Munster Basin; V, Vocontian Basin.

Oceanic surface currents (arrows) after Lloyd (1982) and palaeogeography modified after Davis et al. (1999).

1019C. Monnet, H. Bucher / Cretaceous Research 28 (2007) 1017e1032

raw documented associations of taxa e coexistence in space eare preserved), contrary to other biochronological methodsbased on probabilistic and multivariate treatment of local firstand last appearance datums (for discussion, see Baumgartner,1984; Boulard, 1993); (iv) its efficiency in resolving compli-cated biochronological problems has been demonstrated withtaxonomic groups that have a much less favourable recordthan ammonites, e.g., radiolarians (Baumgartner et al., 1995),micromammals (Guex and Martinez, 1996), nannoplankton(Boulard, 1993); (v) it usually leads to a significant improve-ment of biochronological resolution, even in the case of ammo-nites (e.g., Monnet and Bucher, 2002), which have traditionallybeen acknowledged as one of the leading groups for dating Me-sozoic marine rocks; (vi) it allows an posteriori and objectiveassessment of the diachronism of taxa studied and the choiceof actual characteristic taxa of each zone; and, last but not least,(vii) it has been demonstrated that the unknown duration ofdiscrete biochronozones produced by the unitary associationmethod does not involve ceteris paribus, a methodologicalbias when computing fluctuations of taxonomic richnessthrough time (Escarguel and Bucher, 2004).

The unitary association method (UAM) constructs zona-tions composed of a sequence of discrete, association zones,called UA zones. These are maximal sets of intersectingranges of taxa and are the most resolved and laterally repro-ducible subdivisions derived from the association concept.Fundamentally, Oppel Zones, Concurrent Range Zones, As-semblage Zones and Unitary Associations Zones are closelyrelated, because they are all based on the coexistence of spe-cies. The unitary association method differs from other associ-ation methods in that it parsimoniously exploits conflictingbiostratigraphic relationships that commonly occur among firstand last occurrences of taxa to infer virtual associations (i.e.,coexistence in time but not in space). It should be notedhere that a strict association zone, such as those produced bythe unitary association method, is either characterised by thetaxa occurring only within this zone, or by the intersectingranges of taxa observed within the zone. Unitary associationmethod has been automated by the BioGraph computer pro-gram (Savary and Guex, 1991, 1999). Edwards and Guex(1996) and Monnet and Bucher (2002) summarised the majorprinciples of this deterministic method. Here, reference ismade to Guex (1991), Angiolini and Bucher (1999) and Mon-net and Bucher (2002) for exhaustive methodological explana-tions of the unitary association method. Monnet and Bucher(1999, 2002) also developed an optimization procedure thatautomatically increases the accuracy of the results. Thesame procedure is used in the present study.

4.2. Correlation

In order to minimise the pitfalls of diachronism and endemictaxa, the revised zonal sequences are correlated by creating a zo-nation which considers all taxa common to the basins studied.Note that species with a poorly constrained age are not included.This ‘common taxa zonation’ is achieved by utilizing the unitaryassociation method in conjunction with the previously cited

optimization procedure at the species and genus levels. Thecommon taxa zonation is composed of ‘global’ zones (at the pa-laeogeographical scale of the present study), which have a widerscope than the local, basin-scaled zones which are restricted toa single basin. These ‘common taxa zonations’ enable the datingof each local zone by determining which local zone documentsthe characteristic or age-diagnostic species and/or genera (sin-gletons or pairs of intersecting ranges) of each global zone ofthe common taxa zonation. These ages are listed in a ‘correlationtable’ which corresponds to the dating of each local zone in thecommon taxa zonation. It thus makes it possible to assign to eachof the local zones of the studied basins a ‘common age’, whichthen helps to construct correlations between the local zonations.

The interest of this approach is twofold. First, it enablesa more objective and precise method of correlation of studyareas. Indeed, common taxa zonation allows for the correlationof local sequences with greater confidence, because it is basedon all of their common taxa rather than just a few taxa selectedas index guides. Hence, this method determines if a local zoneis an exact correlative of another local zone, if it correlateswith a group of zones, or if it has no correlative at all in theother basin. Secondly, it permits quantification of the dia-chronism of taxa common to the basins studied, a frequentlyoverlooked aspect of ammonite biochronology.

5. Results

5.1. North-west Europe

A modern ammonite biostratigraphic scheme of the Euro-pean Cenomanian was first established by Hancock (1960)and Kennedy (1971), who provided the basis for all subse-quent biochronological studies. The taxonomic revision ofCenomanian ammonites from England by Wright and Ken-nedy (1981, 1984, 1987, 1990, 1995, 1996) has enableda more refined, ‘standard’ scheme. Subsequently, Gale andFriedrich (1989), Gale (1995, 1996) and Robaszynski et al.(1998) have emended the robust scheme of Wright and Ken-nedy by adding several interval subzones. These changeswere rejected by Monnet and Bucher (2002) on the basis ofthe incompatibility between interval and assemblage zones.Finally, Monnet and Bucher (2002) completely revised theammonite sequence of the European Cenomanian-lower Turo-nian and proposed a zonation covering the Anglo-Paris,Vocontian (south-east France) and Munster (north-west Ger-many) basins. Their revised zonation is in good agreementwith the widely used standard zonation of Wright and Ken-nedy (1984), but its resolution is three times higher, andincludes in excess of a hundred ammonite species distributedover thirty biochronological units for the Cenomanian-lowerTuronian interval. The zonation of Monnet and Bucher(2002) and its associated range chart are adopted here withonly minor changes in the faunal lists, on account of a few tax-onomic changes (for further details, see Monnet, 2005) and ofthe inclusion of new data (e.g., Gale et al., 2005). The revisedzonation and its associated range chart are shown in Fig. 2.

1020 C. Monnet, H. Bucher / Cretaceous Research 28 (2007) 1017e1032

Stoliczkaia (Lamnayella) juigneti Wright & Kennedy, 1978Stoliczkaia (L.) sanctaecatherinae Wright & Kennedy, 1978Mantelliceras couloni (d'Orbigny, 1850)Mantelliceras saxbii (Sharpe, 1857)Mantelliceras mantelli (Sowerby, 1814)Mantelliceras lymense (Spath, 1926)Mantelliceras cantianum Spath, 1926Mantelliceras picteti Hyatt, 1903Mantelliceras dixoni Spath, 1926Sharpeiceras laticlavium (Sharpe, 1855)Acompsoceras inconstans (Schlüter, 1871)Acompsoceras renevieri (Sharpe, 1857)Cunningtoniceras inerme (Pervinquière, 1907)Cunningtoniceras cunningtoni (Sharpe, 1855)

Cunningtoniceras lonsdalei (Adkins, 1928)Cunningtoniceras diadema (Spath, 1926)Cunningtoniceras arizonense Kirkland & Cobban, 1986Acanthoceras rhotomagense (Brongniart, 1822)Acanthoceras jukesbrownei (Spath, 1926)Protacanthoceras tuberculatum Thomel, 1972Protacanthoceras arkelli Wright & Kennedy, 1980Protacanthoceras proteus Wright & Kennedy, 1980Protacanthoceras bunburianum (Sharpe, 1853)Calycoceras (Gentoniceras) spp.Calycoceras (N.) asiaticum (Jimbo, 1894)Calycoceras (N.) planecostatum (Kossmat, 1897)Calycoceras (N.) hippocastanum (Sowerby, 1826)Calycoceras (P.) picteti Wright & Kennedy, 1990Calycoceras (P.) guerangeri (Spath, 1926)Calycoceras (C.) bathyomphalum (Kossmat, 1895)Calycoceras (C.) naviculare (Mantell, 1822)Eucalycoceras pentagonum (Jukes-Browne, 1896)Eucalycoceras gothicum (Kossmat, 1895)Eucalycoceras rowei (Spath, 1926)Pseudocalycoceras harpax (Stoliczka, 1864)Pseudocalycoceras angolaense (Spath, 1931)Sumitomoceras conlini Wright & Kennedy, 1981Nigericeras gadeni (Chudeau, 1909)Thomelites sornayi (Thomel, 1966)Thomelites? serotinus Wright & Kennedy, 1981Neocardioceras juddii (Barrois & Guerne, 1878)Neocardioceras tenue Wright & Kennedy, 1981Watinoceras praecursor Wright & Kennedy, 1981Watinoceras devonense Wright & Kennedy, 1981Watinoceras amudariense (Arkhanguelsky, 1916)

Metoicoceras geslinianum (d'Orbigny, 1850)Spathites (Jeanrogericeras) subconciliatus (Choffat, 1898)Spathites sulcatus Wiedmann, 1960Mammites nodosoides (Schlüter, 1871)Metasigaloceras rusticum (Sowerby, 1817)Lotzeites aberrans (Kossmat, 1895)Euomphaloceras euomphalum (Sharpe, 1855)Euomphaloceras septemseriatum (Cragin, 1893)Euomphaloceras irregulare (Cobban, Hook & Kennedy, 1989)Euomphaloceras costatum Cobban, Hook & Kennedy, 1989Paramammites polymorphus (Pervinquière, 1907)Morrowites? mohovanensis (Böse, 1918)Morrowites? michelobensis (Laube & Bruder, 1887)

Vascoceras diartianum (d'Orbigny, 1850)Vascoceras costatum Barber, 1957Vascoceras gamai Choffat, 1898Vascoceras obessum (Taubenhaus,1920)Vascoceras kossmati (Choffat, 1897)Fagesia catinus (Mantell, 1822)Choffaticeras spp.Thomasites gongilensis (Woods, 1911)Thomasites rollandi (Thomas & Peron, 1889)Neoptychites xetriformis Pervinquière, 1907

Forbesiceras beaumontianum (d'Orbigny, 1841)Forbesiceras largilliertianum (d'Orbigny, 1841)Forbesiceras obtectum (Sharpe, 1853)Hyphoplites campichei Spath, 1925Hyphoplites falcatus (Mantell, 1822)Hyphoplites curvatus (Mantell, 1822)Hyphoplites costosus Wright & Wright, 1949Schloenbachia spp.Desmoceras latidorsatum (Michelin, 1838)Pachydesmoceras denisonianum (Stoliczka, 1865)Puzosia mayoriana (d'Orbigny, 1841)Puzosia dibleyi Spath, 1922Parapuzosia austeni (Sharpe, 1855)Lewesiceras peramplum (Mantell, 1822)Phylloceras seresitense Pervinquière, 1907Zelandites dozei (Fallot, 1885)Tetragonites spathi Fabre, 1940Puebloites corrugatus (stanton, 1894)

Scaphites obliquus Sowerby, 1813Scaphites equalis Sowerby, 1913Anisoceras plicatile (Sowerby, 1819)Idiohamites alternatus (Mantell, 1822Algerites ellipticus (Mantell, 1822)Allocrioceras annulatum (Shumard, 1860)Sciponoceras roto Cieslinski, 1959Sciponoceras baculoides (Mantell, 1822)Sciponoceras gracile (Shumard, 1860)Sciponoceras bohemicum (Fritsch, 1872)Hamites simplex d'Orbigny, 1841Mariella cenomanensis (Schlüter, 1876)Neostlingoceras carcitanense (Matheron, 1842)Turrilites scheuchzerianus Bosc, 1801Turrilites costatus Lamarck, 1801Turrilites acutus Passy, 1832Mesoturrilites boerssumensis Schlüter, 1876Hypoturrilites mantelli Sharpe, 1857Hypoturrilites tuberculatus Bosc, 1801Hypoturrilites gravesianus (d'Orbigny, 1842)

UA 5

UA 8

UA 14

UA 28

UA 17

UA 13

UA 18

Mam

mites

nodosoid

es

W. colo

radoense

Neocardio

ceras

juddii

Metoic

oceras

geslin

ianum

Caly

coceras

guerangeri

Acanthoceras

jukesbrow

nei

Acanthoceras

rhotom

agense

Mantellic

eras

dix

oni

Mantellic

eras

mantelli

UA 7

UA 12

UA 11

UA 16

UA 10

UA 9

UA 6

UA 31

UA 30

UA 34

UA 33 UA zones (WE)

UA 32

UA 29

UA 21

UA 27

UA 26

UA 20

UA 25

UA 24

UA 23

UA 22

UA 19

UA 15

CENOMANIAN TURONIANLower Middle Upper Lower Revised standard zones

for north-west Europe

(modified after

Monnet & Bucher, 2002)

1021C. Monnet, H. Bucher / Cretaceous Research 28 (2007) 1017e1032

5.2. Central Tunisia

Since the pioneer work of Pervinquiere (1907) in centralTunisia, Robaszynski et al. (1990, 1994) thoroughly resampledthe ammonite successions in this classic area, and the taxon-omy was revised by Francis Amedro. Robaszynski et al.(1990, 1994) proposed a new refined zonation (Fig. 3) whichbasically reflects the traditional scheme originally defined inEurope by Wright and Kennedy (1984). The Cenomanian-lower Turonian interval comprises thirteen interval zones,i.e. zones defined by the first occurrence of an index taxon(Interval Zone), by the exact range of an index taxon (TotalRange Zone) or by the last occurrence of an index taxon(Partial Range Zone).

In order to revise this zonation, raw ammonite biostrati-graphic data have been compiled from Robaszynski et al.(1990, 1994), Accarie et al. (1996), Chancellor et al. (1994)and Amedro et al. (2005). This data set includes seventy-four taxa, distributed over sixteen sections. The flow chart ofthe biochronological revision can be summarized as follows:selection of raw data (i.e., sections with vertical distributionof taxa), taxonomic standardization (see Monnet, 2005), itera-tive optimization procedure (see Monnet and Bucher, 2002)and construction of the zonation.

The biochronological processing of Tunisian data has led tothe construction of twenty-four unitary association zones forthe Cenomanian-lower Turonian interval (Fig. 3). The faunalcontent of each zone is listed in Fig. 4. Note that a strict asso-ciation zone (e.g., a UA zone) is characterised either by thetaxa occurring only within this zone (e.g., Mantellicerascobbani in UA zone 3), or by the intersecting ranges of taxaobserved within the zone (e.g., Sciponoceras roto and Hypo-turrilites schneegansi for UA zone 3). The results of this revi-sion are congruent with the interval zones of Robaszynskiet al. (1990, 1994), being based on almost the same data,but show a twofold increase in resolution (Fig. 3). This dem-onstrates once more the benefits of using the unitary associa-tion method to construct biochronological zonations. Note thatthe Watinoceras sp. Zone documented by Amedro et al. (2005)is not recognised here on account of the absence of a diagnosticassociation of ammonite taxa (Watinoceras also being presentin the underlying zone).

5.3. Western Interior

Cobban (1984) proposed a ‘standard’ ammonite zonationfor the middle Cenomanian to upper Turonian of the WesternInterior Basin. Subsequently, Cobban et al. (1989), Kennedyand Cobban (1990a, 1991) and Kirkland (1991) have providedadditional minor modifications. This standard zonation (Fig. 5)comprises nineteen assemblage zones and subzones for theCenomanian-lower Turonian interval. The bases of zones aremarked by the first appearance of a variety of ammonites,

including the index species, which is usually, but not invari-ably, restricted to its zone (Kennedy and Cobban, 1990a).Monnet (2005) fully standardised the taxonomy of thesedata with those of north-west Europe and central Tunisia.

Although this zonation is very detailed, raw data (logs withtaxa plotted) of North American Cenomanian ammonites arerarely made available in the literature. This seriously hampersbetter correlations and diversity analyses. Thus, our data are

TU 25

TU 23

TU 18

TU 19

TU 20

TU 15

TU 16

TU 17

TU 11

TU 12

TU 13

TU 14

TU 7

TU 8

TU 5

TU 6

TU 9

TU 10

TU 4

TU 2

TU 3

TU 1

CEN

OM

ANIA

NTU

RO

NIA

N

Upp

erM

iddl

e

UppermostALBIAN

Low

erLo

wer

Mantelliceras

azregensis I.Z.

Cunningtoniceras

inerme I.Z.

Acanthoceras

rhotomagense I.Z.

Paraconlinoceras

barcusi I.Z.

Euomphaloceras

septemseriatum I.Z.

Pseudaspidoceras

pseudonodosoides

Pseudaspidoceras

pseudonodosoides

Mantelliceras

cobbani T.R.Z.

Acanthoceras

amphibolum T.R.Z.

Mantelliceras

mantelli P.R.Z.

Eucalycoceras

pentagonum P.R.Z.

Mantelliceras

dixoni I.Z.

Mortoniceras

sp. I.Z.

Watinoceras

sp.

Metoicoceras

geslinianum

Eucalycoceras

pentagonum

UA zones

(this study)

Interval

Zones s. l.

(Robaszynski

et al.,

1990, 1994)

Interval

Zones s. l.

(Amédro

et al., 2005)

Assemblage

Zones

(Chancellor

et al., 1994)

TU 26

TU 22

TU 24

TU 21

Pseudaspidoceras

flexuosum I.Z.Pseudaspidoceras

flexuosum

Choffaticeras

spp. I.Z.Thomasites

rollandi

Kamerunoceras

turoniense I.Z.

Mammites

nodosoides T.R.Z.Mammites

nodosoides

Pseudaspidoceras

flexuosum

Fig. 3. Correlation between various ammonite zonations for central Tunisia;

I.Z., interval zone; P.R.Z., partial range zone; T.R.Z., total range zone.

Fig. 2. Revised zonation for north-west Europe and associated range chart (modified after Monnet and Bucher, 2002); black squares denote synchronous occur-

rences; ‘x’ stands for diachronous occurrences, and ‘?’ signifies probable occurrences. For references to original papers in which taxa shown have been named,

please consult literature items listed in the References.

1022 C. Monnet, H. Bucher / Cretaceous Research 28 (2007) 1017e1032

Stoliczkaia (Stoliczkaia) spp.Paracalycoceras melleguensis (Sornay, 1955)Mantelliceras spp.Mantelliceras azregensis Amédro, 1993Mantelliceras lymense (Spath, 1926)Mantelliceras saxbii (Sharpe, 1857)Mantelliceras cobbani Amédro, 1993Mantelliceras mantelli (Sowerby, 1814)Mantelliceras dixoni Spath, 1926Sharpeiceras schlueteri Hyatt, 1903Sharpeiceras laticlavium (Sharpe, 1855)Acompsoceras suzannae (Pervinquière, 1907)Acompsoceras renevieri (Sharpe, 1857)Cunningtoniceras inerme (Pervinquière, 1907)Acanthoceras rhotomagense (Brongniart, 1822)Texacanthoceras amphibolum (Morrow, 1935)Paraconlinoceras barcusi (Jones, 1938)Calycoceras spp.Calycoceras (Newboldiceras) spp.Calycoceras (N.) tunetanum (Pervinquière, 1907)Calycoceras (N.) asiaticum (Jimbo, 1894)Calycoceras (Proeucalycoceras) spp.Calycoceras (P.) guerengeri (Spath, 1926)Calycoceras (Calycoceras) spp.Eucalycoceras spp.Eucalycoceras pentagonum (Jukes-Browne, 1896)Pseudocalycoceras angolaense (Spath, 1931)Watinoceras spp.

Metoicoceras geslinianum (d'Orbigny, 1850)Mammites spp.Mammites nodosoides (Schlüter, 1871)Lotzeites aberrans (Kossmat, 1895)Euomphaloceras septemseriatum (Cragin, 1893)Euomphaloceras costatum Cobban, Hook & Kennedy, 1989Pseudaspidoceras spp.Pseudaspidoceras pseudonodosoides (Choffat, 1898)Pseudaspidoceras flexuosum Powell, 1963Paramammites polymorphus (Pervinquière, 1907)Morrowites depressus (Powell, 1963)Kamerunoceras turoniense (d'Orbigny, 1850)

Vascoceras spp.Vascoceras durandi (Peron, 1890)Fagesia spp.Fagesia superstes (Kossmat, 1895)Fagesia tevesthensis (Peron, 1897)Choffaticeras spp.Choffaticeras meslei (Peron, 1897)Choffaticeras luciae Pervinquière, 1907Thomasites spp.Thomasites rollandi (Thomas & Peron, 1889)Thomasites jordani Pervinquière, 1907Neoptychites cephalotus (Courtiller, 1860)Wrightoceras munieri (Pervinquière, 1907)

Forbesiceras beaumontianum (d'Orbigny, 1841)Forbesiceras obtectum (Sharpe, 1853)Hysteroceras helveticum (Renz, 1968)Algericeras boghariense (Coquand, 1879)Algericeras proratum (Coquand, 1879)Euhystricoceras nicaisei (Coquand, 1862)Neosaynoceras gazellae (Pervinquière, 1907)

Anisoceras spp.Sciponoceras roto Cieslinski, 1959Mariella bergeri (Brongniart, 1822)Mariella cenomanensis (Schlüter, 1876)Ostlingoceras spp.Ostlingoceras rorayense (Collignon, 1964)Neostlingoceras carcitanense (Matheron, 1842)Turrilites costatus Lamarck, 1801Turrilites scheuchzerianus Bosc, 1801Turrilites acutus Passy, 1832Hypoturrilites spp.Hypoturrilites gravesianus (d'Orbigny, 1842)Hypoturrilites betaitraensis Collignon, 1964Hypoturrilites schneegansi Dubourdieu, 1953

Central Tunisia

UA zones (TU)

Fig. 4. Biostratigraphic ranges of Cenomanian-early Turonian ammonite species from central Tunisia.

mainly based on faunal lists (Cobban, 1971, 1987, 1988aec;Cobban and Kennedy, 1990a,b, 1991; Kennedy and Cobban,1990a,b; Kirkland, 1991), with the exception of a few metresstraddling the Cenomanian/Turonian boundary for which com-plete data (i.e., published sections) are available (Cobban,1985; Kennedy and Cobban, 1991; Kennedy et al., 1999).Data on the middle Cenomanian-lower Turonian of the West-ern Interior Basin allow the recognition of twenty-three uni-tary association zones, based on one hundred and twentytaxa (Fig. 5). The faunal content of each zone is shown inFig. 6. A comparison between the standard zonation and theunitary association zonation is illustrated in Fig. 5.

Note that, for reasons outlined above, the zonation for theWestern Interior Basin is less optimal than other basins stud-ied, mainly due to very few available raw data for the middleCenomanian. Indeed, the North American empirical assem-blage zones have a broad definition and sometimes includetaxa which are not strictly associated. For example, the Mam-mites nodosoides Zone can be subdivided into three strictassociation zones (UA zones 21 to 23; see Fig. 5), as demon-strated by the published stratotype section at Pueblo (Colorado),which gives exact distributions of ammonite taxa (Cobban,1985; Kennedy and Cobban, 1991; Kennedy et al., 1999). Un-fortunately, broadly defined zones such as the M. nodosoides

1023C. Monnet, H. Bucher / Cretaceous Research 28 (2007) 1017e1032

Zone may lead to distorted biodiversity counts. Indeed, theM. nodosoides Zone as a whole yields twelve taxa, whereasthe three successive UA zones into which it can be brokendown, yield nine, ten and five taxa, respectively (see Fig. 6).Hence, the species richness of zones for which detailed rawdata are not available may be overestimated in the WesternInterior Basin.

5.4. Correlations

As described above, the zonal sequences of north-westEurope, central Tunisia and the Western Interior have been

Watinoceras devonense

Pseudaspidoceras flexuosum

Vascoceras birchbyi

Watinoceras

coloradoense

Neocardioceras

juddii

Metoicoceras

mosbyense

Sciponoceras

gracile

Vascoceras diartianum

Euomphaloceras

septemseriatum

Euomphaloceras irregulare

Neocardioceras juddii

Nigericeras scotti

Dunveganoceras problematicum

Dunveganoceras albertense

Dunveganoceras conditum

Calycoceras canitaurinum

Plesiacanthoceras wyomingense

Collignoniceras woollgari

Acanthoceras amphibolum

Acanthoceras granerosense

Conlinoceras tarrantense

Acanthoceras bellense

Acanthoceras muldoonense

Mammites nodosoides

UA zones

(this study)

Assemblage Zones

(Cobban, 1984; Cobban et al.,

1989; Kirkland, 1991)

WI 24

WI 19

WI 20

WI 23

WI 21

WI 22

WI 15

WI 16

WI 17

WI 18

WI 11

WI 12

WI 13

WI 14

WI 7

WI 8

WI 5

WI 6

WI 9

WI 10

WI 4

WI 2

WI 3

WI 1

CEN

OM

ANIA

NTU

RO

NIA

NU

pper

Mid

dle

Low

er

Fig. 5. Standard ammonite zonation for the Western Interior Basin (after Cob-

ban, 1984; Cobban et al., 1989; Kirkland, 1991) and equivalent unitary asso-

ciations (the present study).

correlated by constructing a common taxa zonation betweenthese basins. In our case, the three basins studied are nottreated all together but are processed in a pairwise fashionon account of the very small number of taxa in commonbetween these three areas. Utilizing these common taxa zona-tions make it possible to propose correlations shown here inFig. 7 between north-west Europe, central Tunisia and theWestern Interior Basin. Although there is no one-to-one corre-spondence between the zones in each area, Fig. 7 illustratesmore detailed correlations than the ones previously proposed(e.g. Robaszynski et al., 1994, fig. 32). It also underlinesthat correlations are not straightforward and are usually im-possible below the zonal level. For example, the unitary asso-ciations of each basin in the middle Cenomanian are verypoorly constrained. Based on data currently available, all dem-onstrated correlations on the one hand, and all remaining un-certainties on the other, are therefore graphically representedby boxes in Fig. 7. For instance, the local unitary associationzone WE27 is not documented in central Tunisia; while thelocal unitary association zone TU20 (Pseudaspidoceras flexuo-sum Zone) is not documented in north-west Europe. Thebiochronological resolution during a given time interval mayalso vary across the basins. This is well illustrated duringthe early Cenomanian (Fig. 7). Such changes in resolutionon a large palaeogeographical scale may essentially resultfrom the combined effects of selective preservation andecological controls. Moreover, some time intervals are poorlycorrelated because of increased faunal endemism. This is wellexemplified by the early Cenomanian where European faunasare characterised by hoplitids, schloenbachiids and gaudrycer-atids; these taxa are all absent in Tunisia. These two basinsalso differ in having different species of heteromorphs andacanthoceratids.

The ‘common taxa zonation’ of the north-west Europe/cen-tral Tunisia data set is shown in Fig. 8. Eleven unitary associ-ation zones are recognised for nineteen species in commonand thirteen unitary association zones for twenty-nine generain common. The ‘common taxa zonation’ of the north-westEurope/Western Interior data set is shown in Fig. 9. Nine uni-tary association zones each are recognised for twenty-fourcommon species and also for twenty-seven common genera.The ‘common taxa zonation’ of the Western Interior/centralTunisia data set is shown in Fig. 10; it comprises eight andnine unitary association zones for sixteen and twenty-sixcommon species and genera, respectively.

The ‘correlation tables’ associated with the common taxazonations (Figs. 8e10) enable dating of each local zone byusing the temporal sequence of common taxa. For example,the north-west European, local unitary association zoneWE11 contains the association of Acompsoceras renevieriwith both Mantelliceras saxbii and Mariella cenomanensis,which constitutes a characteristic association of the ‘global’(i.e., at the palaeogeographical scale of the present study)unitary association zone 4 (Fig. 8A). On the other hand, thecentral Tunisian, local unitary association zone TU8 containstaxa which do not belong to a single global zone, but ratherto zones 3 to 5. This relative imprecision indicates that TU8

Acompsoceras landisi (Cobban, 1971)Cunningtoniceras cunningtoni (Sharpe, 1855)Cunningtoniceras inerme (Pervinquière, 1907)Cunningtoniceras lonsdalei (Adkins, 1928)Cunningtoniceras johnsonanum (Stephenson, 1955)Cunningtoniceras arizonense Kirkland & Cobban, 1986Conlinoceras tarrantense (Adkins, 1928)Conlinoceras gilberti (Cobban & Scott, 1972)Conlinoceras sp. indet.Paraconlinoceras barcusi (Jones, 1938)Paraconlinoceras leonense (Adkins, 1928)Texacanthoceras granerosense (Cobban & Scott, 1972)Texacanthoceras muldoonense (Cobban & Scott, 1972)Texacanthoceras bellense (Adkins, 1928)Texacanthoceras amphibolum (Morrow, 1935)Plesiacanthoceras wyomingense (Reagan, 1924)Plesiacanthoceras bellsanum (Stephenson, 1953)Plesiacanthoceratoides fisherense (Cobban, 1987)Plesiacanthoceratoides vetula (Cobban, 1987)Plesiacanthoceratoides hosei (Cobban, 1987)Plesiacanthoceratoides alzadense (Cobban, 1987)Calycoceras (N.) asiaticum (Jimbo, 1894)Calycoceras (P.) canitaurinum (Haas, 1949)Calycoceras (C.) rubeyi Cobban, 1988Calycoceras (C.) obrieni Young, 1957Calycoceras (C.) naviculare (Mantell, 1822)Eucalycoceras pentagonum (Jukes-Browne, 1896)Pseudocalycoceras templetonense (Cobban, 1988)Pseudocalycoceras angolaense (Spath, 1931)Nigericeras scotti (Cobban, 1971)Dunveganoceras pondi Haas, 1949Dunveganoceras problematicum Cobban, 1988Dunveganoceras albertense (Warren, 1930)Dunveganoceras conditum (Haas, 1951)Tarrantoceras sellardsi (Adkins, 1928)Tarrantoceras cuspidum (stephenson, 1953)Tarrantoceras flexicostatum Cobban, 1988Sumitomoceras bentonianum (Cragin, 1893)Sumitomoceras conlini Wright & Kennedy, 1981Neocardioceras uptonense Cobban, 1988Neocardioceras laevigatum Cobban, 1988Neocardioceras minutum Cobban, 1988Neocardioceras densicostatum Cobban, 1988Neocardioceras sp. A Cobban, 1988Neocardioceras juddii (Barrois & Guerne, 1878)Watinoceras praecursor Wright & Kennedy, 1981Watinoceras depressum Wright & Kennedy, 1981Watinoceras devonense Wright & Kennedy, 1981Watinoceras sp. indet.Watinoceras coloradoense (Henderson, 1908)Watinoceras hattini Cobban, 1988Quitmaniceras reaseri Powell, 1963

Metoicoceras swallovi (Shumard, 1860)Metoicoceras latoventer Stephenson, 1953Metoicoceras praecox Haas, 1949Metoicoceras sp. A Kennedy & Cobban, 1990Metoicoceras frontierense Cobban, 1988Metoicoceras mosbyense Cobban, 1953Metoicoceras geslinianum (d'Orbigny, 1850)Mammites nodosoides (Schlüter, 1871)

Euomphaloceras euomphalum (Sharpe, 1855)Euomphaloceras septemseriatum (Cragin, 1893)Euomphaloceras irregulare (Cobban, Hook & Kennedy, 1989)Euomphaloceras costatum Cobban, Hook & Kennedy, 1989Burroceras clydense Cobban, Hook & Kennedy, 1989Paraburroceras minutum Cobban, Hook & Kennedy, 1989Pseudaspidoceras pseudonodosoides (Choffat, 1898)Pseudaspidoceras flexuosum Powell, 1963Kamerunoceras puebloense (Cobban & Scott, 1972)Morrowites wingi (Morrow, 1935)Morrowites subdepressus Cobban & Hook, 1983Collignoniceras woollgari (Mantell, 1822)

Vascoceras diartianum (d'Orbigny, 1850)Vascoceras gamai Choffat, 1898Vascoceras cauvini (Chudeau, 1909)Vascoceras sp. A Kennedy & Cobban, 1991Vascoceras sp. B Kennedy & Cobban, 1991Vascoceras silvanense (Choffat, 1899)Vascoceras proprium (Reyment, 1954)Vascoceras harttii (Hyatt, 1870)Vascoceras birchbyi Cobban & Scott, 1972Fagesia catinus (Mantell, 1822)Choffaticeras pavillieri (Pervinquière, 1907)Thomasites sp. indet.Neoptychites cephalotus (Courtiller, 1860)

Forbesiceras conlini Stephenson, 1953Forbesiceras chevillei (Pictet & Renevier, 1866)Anagaudryceras involvulum (Stoliczka, 1865)Borissiakoceras compressum Cobban, 1961Borissiakoceras orbiculatum Stephenson, 1955Borissiakoceras reesidei Morrow, 1935Johnsonites sulcatus Cobban, 1961Metengonoceras dumbli (Cragin, 1893)Metengonoceras acutum Hyatt, 1903Moremanoceras straini Kennedy, Cobban & Hook, 1988Moremanoceras sp. indet.Moremanoceras scotti (Moreman, 1927)Desmoceras japonicum Yabe, 1904Metaptychoceras reesidei Cobban & Scott, 1972Placenticeras cumminsi (Cragin, 1893)Rubroceras spp.Puebloites corrugatus (Stanton, 1894)Puebloites spiralis Cobban & Scott, 1972Puebloites greenhornensis Cobban & Scott, 1972Tragodesmoceras bassi Morrow, 1935

Anisoceras plicatile (Sowerby, 1819)Allocrioceras annulatum (Shumard, 1860)Allocrioceras larvatum (Conrad, 1855)Sciponoceras gracile (Shumard, 1860)Baculites yokoyamai Tokunaga & Shimizu, 1926Hamites cimarronensis (Kauffman & Powell, 1977)Hamites? pygmaeus Cobban, Hook & Kennedy, 1989Neostlingoceras kottlowskii Cobban & Hook, 1981Turrilites dearingi Stephenson, 1953Turrilites costatus Lamarck, 1801Turrilites scheuchzerianus Bosc, 1801Turrilites sp. indet.Turrilites acutus Passy, 1832Worthoceras gibbosum Moreman, 1942Worthoceras vermiculum (Shumard, 1860)

Western Interior Basin

UA-zones (WI)

Fig. 6. Ranges of Cenomanian-early Turonian ammonite species in the Western Interior Basin.

1025C. Monnet, H. Bucher / Cretaceous Research 28 (2007) 1017e1032

Upp

erM

iddl

eLo

wer

Low

er

CEN

OM

ANIA

NTU

RO

NIA

N

ALBIAN

North-west EuropeWestern Interior Central Tunisia

WE 27

WE 23

WE 24

WE 25

WE 26

WE 19

WE 20

WE 21

WE 22

WE 34

WE 28

WE 29

WE 17

WE 18

WE 11

WE 12

WE 13

WE 14

WE 7

WE 5

WE 6

WE 8

WE 9

WE 10

WE 31

WE 32

WE 33

WE 30 ?

Mantelliceras

mantelli

M. s.

N. c.

Collignoniceras woollgari

Watinoceras coloradoense

Stoliczkaia dispar

Mammites

nodosoides

Mantelliceras

dixoni

Acanthoceras

jukesbrownei

Acanthoceras

rhotomagense

Metoicoceras

geslinianum

Calycoceras

guerangeri

WE 15

WE 16

?

TU 16

TU 26

TU 22

TU 24

TU 25

TU 19

TU 20

TU 21

TU 15

TU 12

TU 13

TU 14

TU 7

TU 8

TU 5

TU 6

TU 9

TU 4

TU 2

TU 3

Mantelliceras azregensis I.Z.

Cunningtoniceras inerme I.Z.

Acanthoceras

rhotomagense I.Z.

Paraconlinoceras barcusi I.Z.

Euomphaloceras

septemseriatum I.Z.

Pseudaspidoceras flexuosum I.Z.

Choffaticeras

spp. I.Z.

Kamerunoceras turoniense I.Z.

Mantelliceras cobbani T.R.Z.

Acanthoceras

amphibolum T.R.Z.

Mammites

nodosoides T.R.Z.

Mantelliceras

mantelli P.R.Z.

Eucalycoceras

pentagonum P.R.Z.

Mantelliceras

dixoni I.Z.

Mortoniceras sp. I.Z.

TU 23 ?

TU 10

TU 11

Neocardioceras

juddii

Sciponoceras

gracile

V. diartianum

E. septemseriatum

E. irregulare

N. juddii

N. scotti

D. problematicum

D. albertense

D. conditum

Metoicoceras

mosbyense

Calycoceras canitaurinum

Plesiacanthoceras wyomingense

Collignoniceras woollgari

Acanthoceras amphibolum

Acanthoceras granerosense

Conlinoceras tarrantense

Acanthoceras bellense

Acanthoceras muldoonense

Mammites

nodosoides

WI 24

WI 23

WI 21

WI 22

WI 15

WI 16

WI 17

W. devonense

P. flexuosum

Watinoceras

coloradoense

WI 19

?

?

V. birchbyi

WI 18

WI 12 TU 17

WI 20

WI 13

WI 14

WI 11

WI 7

WI 8

WI 9

WI 10

WI 5

WI 6

WI 4

WI 2

WI 3

WI 1

TU 18

1

2

3

5

4

6

7

8

9

10

1112

1314

15

16

Neocardioceras juddii

Fig. 7. Correlation of revised Cenomanian-lower Turonian zonations for north-west Europe, central Tunisia and the Western Interior Basin. Grey numbers refer to

subzone numbers used in the north-west Europe succession (see Fig. 11).

can only be attributed to any of these global zones withoutfurther precision. This means that the characteristic faunasof TU8 are mostly biogeographically restricted to centralTunisia and are only of regional significance. It appears thata local zone rarely correlates exactly with another localzone, but rather, and more easily, with a group of zones. Forexample, WE5-8 correlate with TU2-7, and WE23-25 corre-late with TU18 (Fig. 7). Some zones may also have no correl-ative in the other basin, such as TU9, which is characterised bya Mantelliceras-Calycoceras association. Hence, it should benoted that the correlation table serves as a guide for correlationand not directly as a definitive result because of relativeendemism of the diagnostic taxa of each local zone.

Finally, these new correlations allow an assessment ofdiachronism of the taxa common to the basins studied. Dia-chronism is summarised in Fig. 11, which shows that veryfew of the common taxa have synchronous first and last occur-rences. For example, the first occurrence of Turrilites costatusand T. scheuchzerianus are younger in central Tunisia than innorth-west Europe, while the first occurrence of Metoicocerasis older in the Western Interior compared to the two otherbasins. This significant diachronism must therefore be takeninto account when designating an index or characteristictaxa for a zone. Diachronism is crucial to stratigraphicconstraints used in phylogenetic analyses as well. It shouldbe noted that assessment of diachronism by means of interval

1026 C. Monnet, H. Bucher / Cretaceous Research 28 (2007) 1017e1032

111131 2 3 4 5 6 7 8 9 0 1 2

111 2 3 4 5 6 7 8 9 0 1

Stoliczkaia (Stoliczkaia)

Ostlingoceras

Mariella

Anisoceras

Sharpeiceras

Mantelliceras

Hypoturrilites

Sciponoceras

Neostlingoceras

Forbesiceras

Turrilites

Acompsoceras

Calycoceras (Newboldiceras)

Acanthoceras

Cunningtoniceras

Calycoceras (Proeucalycoceras)

Calycoceras (Calycoceras)

Eucalycoceras

Euomphaloceras

Lotzeites

Vascoceras

Thomasites

Watinoceras

Paramammites

Mammites

Fagesia

Neoptychites

Choffaticeras

Morrowites

Section C. Tunisia 25: 12 – 13 24: 13 – 13 23: 12 – 12 22: 12 – 12 21: 12 – 12 20: 12 – 12 19: 10 – 11 18: 10 – 11 17: 9 – 11 16: 9 – 15: 9 – 14: 6 – 13: 5 – 12: 6 – 11: 6 – 10: 6 – 9: 5 – 8: 4 – 7: 3 – 6: 3 – 5: 3 – 4: 3 – 3: 3 – 2: 2 – 1: 1 –

9966666553333321

Common taxa zonation Correlation table

Mariella bergeri

Sharpeiceras schlueteri

Forbesiceras beaumontianum

Sciponoceras roto

Neostlingoceras carcitanense

Hypoturrilites gravesianus

Mantelliceras saxbii

Mariella cenomanensis

Mantelliceras mantelli

Mantelliceras lymense

Mantelliceras dixoni

Turrilites scheuchzerianus

Acompsoceras renevieri

Turrilites costatus

Forbesiceras obtectum

Cunningtoniceras inerme

Acanthoceras rhotomagense

Turrilites acutus

Calycoceras (Newboldiceras) asiaticum

Eucalycoceras pentagonum

Thomelites sornayi

Lotzeites aberrans

Calycoceras (Proeucalycoceras) guerengeri

Euomphaloceras septemseriatum

Pseudocalycoceras angolaense

Metoicoceras geslinianum

Euomphaloceras costatum

Mammites nodosoides

Paramammites polymorphus

Thomasites rollandi

Section C. Tunisia

25: 11 – 11

24: 11 – 11

23: 11 – 11

22: ? –

21: ? –

20: ? –

19: 10 – 10

18: 9 –

17: 9 –

16: 8 –

15: 8 –

14: 6 –

13: 6 –

12: 6 –

11: 6 –

10: 6 –

9: 5 –

8: 3 –

7: 2 –

6: 2 –

5: 2 –

4: 2 –

3: 2 –

2: 2 –

1: 1 –

?

?

?

9

9

8

8

6

6

6

6

6

6

5

4

4

2

2

2

2

1

Section W. Europe

34: 11 – 11

33: 11 – 11

32: 11 – 11

31: 11 – 11

30: 11 – 11

29: 11 – 11

28: 10 – 10

27: ? –

26: 9 –

25: 9 –

24: 9 –

23: 9 –

22: 8 –

21: 8 –

20: 7 –

19: 7 –

18: 6 –

17: 6 –

16: 6 –

15: 6 –

14: 5 –

13: 5 –

12: 4 –

11: 4 –

10: 3 –

9: 3 –

8: 2 –

7: 2 –

6: 2 –

5: 2 –

4: 1 –

3: 1 –

2: 1 –

1: 1 –

?

9

9

9

9

8

8

7

7

6

6

6

6

5

5

4

4

3

3

2

2

2

2

1

1

1

1

Section W. Europe

34: 12 – 13

33: 12 – 13

32: 13 – 13

31: 13 – 13

30: 13 – 13

29: 12 – 12

28: 11 – 11

27: 11 –

26: 10 –

25: 10 –

24: 9 –

23: 9 –

22: 9 –

21: 8 –

20: 8 –

19: 7 –

18: 6 –

17: 6 –

16: 6 –

15: 6 –

14: 4 –

13: 4 –

12: 4 –

11: 4 –

10: 4 –

9: 4 –

8: 3 –

7: 3 –

6: 3 –

5: 3 –

4: 1 –

3: 1 –

2: 1 –

1: 1 –

11

10

10

10

10

9

8

8

7

6

6

6

6

5

5

5

4

4

4

3

3

3

3

4

1

1

1

Common taxa zonation Correlation table

Species levelA

Genus levelB

?

?

?

??

?

1027C. Monnet, H. Bucher / Cretaceous Research 28 (2007) 1017e1032

zones is almost impossible in this case, unless a first or lastoccurrence can be demonstrated to be synchronous withrespect to an instantaneous abiotic event of global extent. Dia-chronism has also been recognised among planktonic forami-nifera whose biozones are usually and typically defined as‘total range zones’ (see Caron et al., 2006).

6. Discussion

6.1. Correlations between north-west Europe and centralTunisia

Some discrepancies appear between the correlations andzonations proposed by Robaszynski et al. (1994) and thosepresented here. Robaszynski et al. (1994) placed the middle/upper Cenomanian boundary between their Acanthocerasamphibolum and Eucalycoceras pentagonum zones (Fig. 3).This substage boundary is here believed to be slightly olderfor the following reasons. Robaszynski et al. (1994) definedthis boundary by the last occurrence of ‘Acanthoceras’ am-phibolum, which is now better referred to as Texacanthoceras(see Cooper, 1998; Monnet, 2005). Moreover, the top of theamphibolum Zone (here TU15) yields typical late Cenomanianammonites such as Eucalycoceras pentagonum and Lotzeitesaberrans. Because the lower part of the amphibolum Zone(here TU14) yields typical middle Cenomanian ammonitessuch as Turrilites acutus and Calycoceras asiaticum, the mid-dle/upper Cenomanian boundary is better placed within theiramphibolum Zone. This clearly stresses the difficulties inher-ent to the use of some interval zones in comparison to thatof association zones.

6.2. Correlations between north-west Europe and theWestern Interior

The correlations proposed here between the Western Inte-rior Basin and north-west Europe are at variance with thosealready published by other workers (e.g., Hancock et al.,1993). Note that for the Metoicoceras geslinianum Zone andyounger zones, the correlations are fairly similar, even if a bet-ter precision is achieved by using unitary associations. Moregenerally, the geslinianum to nodosoides zones are recognisedmore or less throughout most of the northern hemisphere andindicate a time interval during which biogeographical barrierswere weak. Some correlations are also confirmed by chemo-stratigraphic data. For example, the boundary between theEuropean Calycoceras guerangeri/Metoicoceras geslinianumzones and the American Metoicoceras mosbyense/Sciponoce-ras gracile zonal boundary both correspond to the start of

the late Cenomanian carbon isotope positive excursion(Kennedy and Cobban, 1991; Morel, 1998).

The discrepancies between correlations proposed here andthose of Hancock et al. (1993) have a basically dual origin:(i) the position of the middle/upper Cenomanian boundaryand (ii) the correlative of the European Acanthoceras juke-sbrownei Zone in the North American record. Indeed, Cobban(1984) and subsequent authors placed the middle/upper Ceno-manian boundary between the Plesiacanthoceras wyomin-gense and Calycoceras canitaurinum zones in the WesternInterior (Fig. 5). This substage boundary is here believed tobe older for the following reasons. Despite the presence ofendemic faunas in the Western Interior Basin, the P. wyomin-gense Zone (here WI7) contains typical late Cenomanian gen-era such as Metoicoceras and Eucalycoceras, whereas middleCenomanian genera such as Acanthoceras, Cunningtonicerasand Turrilites have already disappeared (Fig. 6). Hence, com-pared to the European faunas, the middle/upper Cenomanianboundary is better placed between the amphibolum and wyo-mingense zones (Fig. 7). Although the wyomingense Zone ishere included in the late Cenomanian, its exact correlationwith the lower part of the European guerangeri Zone remainsunclear. The few taxa in common between north-west Europeand the Western Interior Basin span several other zones; hencethe wyomingense Zone could also be intercalated between theEuropean jukesbrownei and guerangeri zones. Unfortunately,the high proportion of endemic faunas at this time precludesbetter correlations.

The correlation problem of the European jukesbrowneiZone (upper middle Cenomanian) is mainly a matter of ende-mism as well. As shown by the correlation table (Fig. 9), theNorth American Conlinoceras tarrantense to Acanthocerasmuldoonense zones are equivalent to the European rhotoma-gense Zone. Hancock et al. (1993, p. 467) also noted that thesezones, ‘fall into the lower part of the middle Cenomanian’.Note that this correlation relies mainly on the occurrence ofCunningtoniceras inerme. A problem remains with the Acan-thoceras bellense and ‘Acanthoceras’ amphibolum zones(here WI4-6), which belong to the middle Cenomanian, butwithout further precision if only the common taxa are takeninto account. Indeed, the diagnostic taxa of the jukesbrowneiZone in Europe have a limited palaeobiogeographical distribu-tion (e.g., Acanthoceras jukesbrownei) or have diachronousdatums with the Western Interior Basin (e.g., Calycoceras (Ca-lycoceras) or Calycoceras (Proeucalycoceras); see Fig. 11). Itappears that Hancock et al. (1993) based their correlationmainly on the occurrence of Turrilites acutus, and therefore,equated the bellense and amphibolum zones with the EuropeanTurrilites acutus Subzone (upper rhotomagense Zone). How-ever, Monnet and Bucher (2002) clearly demonstrated that

Fig. 8. Common taxa zonation and correlation table of Cenomanian-lower Turonian zonations for north-west Europe and central Tunisia at the species [A] and

genus levels [B]; ‘x: y - z’ means that the age of the local zone X is comprised within the time interval bounded by the global zones Y and Z in the common taxa

zonation. For example, local unitary association zone 26 in north-west Europe and local unitary association zone 19 in central Tunisia both correlate with global

unitary association zone 9, which is characterised by the occurrence of Euomphaloceras septemseriatum. Local unitary association zone 9 in central Tunisia does

not yield sufficiently characteristic species for assignment to a single global zone, but correlates with the interval bounded by global unitary association zones 5 and

6, without further precision.

1028 C. Monnet, H. Bucher / Cretaceous Research 28 (2007) 1017e1032

Desmoceras

Acanthoceras

Cunningtoniceras

Turrilites

Forbesiceras

Neostlingoceras

Calycoceras (Newboldiceras)

Anisoceras

Sciponoceras

Hamites

Calycoceras (Proeucalycoceras)

Calycoceras (Calycoceras)

Eucalycoceras

Metoicoceras

Neocardioceras

Euomphaloceras

Pseudocalycoceras

Vascoceras

Allocrioceras

Nigericeras

Thomasites

Watinoceras

Morrowites

Mammites

Fagesia

Neoptychites

Choffaticeras

Section W. Europe 34: 9 – 9 33: 9 – 9 32: 9 – 9 31: 9 – 9 30: 9 – 9 29: 8 – 9 28: 7 – 7 27: 7 – 7 26: 5 – 5 25: 5 – 5 24: 4 – 5 23: 4 – 5 22: 4 – 4 21: 3 – 3 20: 3 – 3 19: 3 – 3 18: 2 – 2 17: 2 – 2 16: 1 – 2 15: 1 – 2 14: 1 – 2 13: 1 – 2 12: 1 – 2 11: 1 – 2 10: 1 – 2 9: 1 – 2 8: 1 – 3 7: 1 – 3 6: 1 – 3 5: 1 – 1 4: 1 – 1 3: 1 – 1 2: 1 – 1 1: 1 – 1

Section W. Interior 24: 9 – 9 23: 9 – 9 22: 9 – 9 21: 9 – 9 20: 9 – 9 19: 9 – 9 18: 8 – 8 17: 7 – 7 16: 6 – 6 15: 6 – 6 14: 6 – 6 13: 5 – 5 12: 5 – 5 11: 3 – 5 10: 3 – 5 9: 3 – 4 8: 3 – 3 7: 3 – 4 6: 1 – 1 5: 1 – 1 4: 1 – 2 3: 1 – 2 2: 1 – 2 1: 1 – 2

Common taxa zonation Correlation table

Cunningtoniceras cunningtoni

Cunningtoniceras inerme

Turrilites acutus

Turrilites costatus

Turrilites scheuchzerianus

Calycoceras (Newboldiceras) asiaticum

Anisoceras plicatile

Euomphaloceras euomphalum

Vascoceras diartianum

Calycoceras (Calycoceras) naviculare

Eucalycoceras pentagonum

Pseudocalycoceras angolaense

Metoicoceras geslinianum

Sciponoceras gracile

Euomphaloceras septemseriatum

Allocrioceras annulatum

Vascoceras gamai

Burroceras irregulare

Neocardioceras juddii

Euomphaloceras costatum

Watinoceras praecursor

Watinoceras devonense

Fagesia catinus

Mammites nodosoides

Section W. Interior23: 9 – 922: 9 – 921: 9 – 920: 8 – 819: 8 – 918: 7 – 716: 6 – 615: 5 – 514: 4 – 413: 3 – 312: 2 – 211: 1 – 610: 1 – 6 9: 1 – 6 8: 1 – 1 7: 1 – 1 6: 1 – 1 5: 1 – 1 4: 1 – 1 3: 1 – 1 2: 1 – 1 1: 1 – 1

Section W. Europe34: 9 – 933: 9 – 932: 9 – 931: 9 – 929: 7 – 728: 6 – 627: 5 – 526: 3 – 325: 2 – 324: 2 – 323: 2 – 322: 2 – 321: 2 – 320: 2 – 219: 2 – 318: 1 – 117: 1 – 116: 1 – 115: 1 – 114: 1 – 113: 1 – 112: 1 – 111: 1 – 110: 1 – 1 9: 1 – 1

Common taxa zonation Correlation tableSpecies levelA

Genus levelB

?

?

?

?

?

1 2 3 4 5 6 7 8 9

1 2 3 4 5 6 7 8 9

Fig. 9. Common taxa zonation and correlation table of Cenomanian-lower Turonian zonations for northwest Europe and the Western Interior at the species [A] and

genus levels [B].

the subdivision of the rhotomagense Zone into the Turrilitescostatus and Turrilites acutus subzones is not justified in termsof association. Moreover, T. acutus is shown to occur through-out the entire middle Cenomanian (Fig. 11; see Monnet and

Bucher, 2002). Thus, this species cannot be used to distinguishthe rhotomagense Zone from the jukesbrownei Zone. As shownby the common taxa zonation, the only synchronous datumsof taxa between the Western Interior Basin and north-west

1029C. Monnet, H. Bucher / Cretaceous Research 28 (2007) 1017e1032

Acanthoceras

Paraconlinoceras

Cunningtoniceras

Forbesiceras

Turrilites

Neostlingoceras

Calycoceras (Newboldiceras)

Anisoceras

Sciponoceras

Texacanthoceras

Calycoceras (Proeucalycoceras)

Eucalycoceras

Pseudocalycoceras

Metoicoceras

Calycoceras (Calycoceras)

Euomphaloceras

Vascoceras

Pseudaspidoceras

Watinoceras

Mammites

Fagesia

Neoptychites

Choffaticeras

Thomasites

Kamerunoceras

Morrowites

Section W. Interior24: 9 – 923: 9 – 922: 9 – 921: 9 – 920: 8 – 819: 8 – 818: 8 – 817: 8 – 816: 7 – 715: 6 – 714: 6 – 713: 6 – 612: 6 – 611: 5 – 610: 5 – 6 9: 5 – 5 8: 4 – 4 7: 4 – 5 6: 2 – 2 5: 2 – 2 4: 1 – 1 3: 1 – 1 2: 1 – 1 1: 1 – 1

Section C. Tunisia25: 9 – 924: 9 – 923: 9 – 922: 8 – 921: 8 – 820: 8 – 819: 7 – 718: 6 – 716: 5 – 515: 4 – 414: 3 – 313: 1 – 112: 1 – 111: 1 – 110: 1 – 2 9: 1 – 3

Common taxa zonation Correlation table

Cunningtoniceras inerme

Turrilites scheuchzerianus

Turrilites costatus

Paraconlinoceras barcusi

Turrilites acutus

Calycoceras (Newboldiceras) asiaticum

Texacanthoceras amphibolum

Eucalycoceras pentagonum

Euomphaloceras septemseriatum

Metoicoceras geslinianum

Pseudocalycoceras angolaense

Euomphaloceras costatum

Pseudaspidoceras pseudonodosoides

Pseudaspidoceras flexuosum

Neoptychites cephalotus

Mammites nodosoides

Section C. Tunisia25: 8 – 824: 8 – 823: 7 – 822: 7 – 820: 7 – 719: 6 – 618: 5 – 517: 5 – 515: 4 – 414: 3 – 313: 2 – 212: 2 – 211: 1 – 110: 1 – 1 9: 1 – 2

Section W. Interior23: 8 – 822: 8 – 821: 8 – 820: 7 – 719: 7 – 716: 6 – 614: 5 – 513: 5 – 512: 4 – 5 8: 3 – 3 7: 3 – 3 6: 3 – 3 5: 3 – 3 4: 3 – 3 3: 1 – 1 2: 1 – 1 1: 1 – 1

Common taxa zonation Correlation table

Species levelA

Genus levelB

?

?

?

?

1 2 3 4 5 6 7 8 9

1 2 3 4 5 6 7 8

Fig. 10. Common taxa zonation and correlation table of Cenomanian-lower Turonian zonations for the Western Interior and central Tunisia at the species [A] and

genus levels [B].

Europe are the last occurrence of C. inerme and of Turrilitesscheuchzerianus. The distribution of the latter species in NorthAmerica suggests that the bellense and amphibolum zones maycorrespond to the European jukesbrownei Zone. In spite of thistenuous evidence, this is the only known objective ammonitecorrelation between Europe and North America to date.

6.3. Middle/upper Cenomanian boundary

Finally, these correlations allow us to propose a better sub-stantiated marker for the middle/upper Cenomanian boundary,which may still be amenable to further improvement as newdata may become available. Indeed, ever since Hancock(1960) proposed the threefold subdivision of the CenomanianStage, the middle/upper Cenomanian boundary has remainedsomewhat unclear. For example, Kennedy (1986) noted that

this boundary is marked by the disappearance of the genusAcanthoceras and the diversification of the genus Calycoceras.Although the Global Boundary Stratotype Section and Pointfor the base of the Cenomanian Stage has now been formallyestablished (Kennedy et al., 2004), there currently is no agree-ment on the definition of the middle/upper Cenomanianboundary. Different markers have been proposed (Troger andKennedy, 1996), and are as follows: (i) first occurrence ofAcanthoceras jukesbrownei; (ii) last occurrence of A. juke-sbrownei; (iii) first occurrence of Calycoceras (Proeucalycoce-ras) guerangeri; (iv) first occurrence of Calycoceras(Calycoceras) naviculare; and (v) first occurrence of Eucaly-coceras pentagonum.

The zonations and their correlations as revised in the pres-ent study allow an alternative, more robust solution. Betweenthe Western Interior Basin, north-west Europe and central

1030 C. Monnet, H. Bucher / Cretaceous Research 28 (2007) 1017e1032

Mantelliceras lymenseMantelliceras mantelliMantelliceras saxbiiMantelliceras dixoniSharpeiceras laticlaviumAcompsoceras renevieriCunningtoniceras inermeCunningtoniceras cunningtoniAcanthoceras rhotomagenseParaconlinoceras barcusiTexacanthoceras amphibolumCalycoceras (N.) asiaticumCalycoceras (P.) guerangeriCalycoceras (C.) naviculareEucalycoceras pentagonumPseudocalycoceras angolaenseNeocardioceras juddiiWatinoceras praecursorWatinoceras devonense

Metoicoceras geslinianumMammites nodosoidesLotzeites aberransEuomphaloceras septemseriatumEuomphaloceras euomphalumEuomphaloceras irregulareEuomphaloceras costatumPseudaspidoceras pseudonodosoidesPseudaspidoceras flexuosumParamammites polymorphus

Vascoceras diartianumVascoceras gamaiFagesia catinusThomasites rollandiNeoptychites cephalotus

Forbesiceras beaumontianumForbesiceras obtectum

Sciponoceras rotoSciponoceras gracileNeostlingoceras carcitanenseHypoturrilites gravesianusMariella cenomanensisTurrilites scheuchzerianusTurrilites costatusTurrilites acutusAnisoceras plicatileAllocrioceras annulatum

X -> western Europe % -> western Europe & central Tunisia O -> central Tunisia @ -> central Tunisia & Western Interior + -> Western Interior # -> Western Interior & western Europe

-> western Europe & central Tunisia & Western Interior

SharpeicerasMantellicerasAcompsocerasCalycocerasCalycoceras (Newboldiceras)Calycoceras (Proeucalycoceras)Calycoceras (Calycoceras)AcanthocerasCunningtonicerasEucalycocerasEuomphalocerasLotzeites

WatinocerasParamammitesMammites

MorrowitesMetoicocerasNeocardiocerasPseudocalycocerasNigericerasParaconlinocerasTexacanthocerasPseudaspidocerasKamerunoceras

VascocerasThomasitesFagesiaNeoptychitesChoffaticeras

TurrilitesMariellaHypoturrilites

Fig. 11. Biostratigraphic ranges and diachronism, with respect to the north-

west European subzone succession (see Fig. 7 for subzone numbers), of spe-

cies and short-ranging genera common to north-west Europe, central Tunisia

and the Western Interior Basin. Symbols indicate in which basin taxa are pres-

ent during each zone; e.g., Cunningtoniceras inerme occurs in the three basins

during the Acanthoceras rhotomagense Zone (unitary association zone 5) and

is a synchronous taxon, while the diachronous genus Euomphaloceras appears

earlier in north-west Europe.

Tunisia, the major common datums around the middle/upperCenomanian boundary are the following: last occurrences ofTurrilites, Acanthoceras, and Cunningtoniceras, first occur-rence of Eucalycoceras, Euomphaloceras, and Pseudocalyco-ceras. Among these, only the last occurrence of Turrilitesand of T. acutus are synchronous between the three areas.Therefore, the last occurrence of T. acutus is here chosen tomark the middle/upper Cenomanian boundary, since thisspecies is also abundant and widely distributed. Any furtherdata could still potentially provide a test for the synchronismof the last occurrence of T. acutus.

7. Conclusions

Analysis of ammonite biostratigraphic data recently pub-lished in the literature by utilizing the unitary associationmethod leads to refined zonations for central Tunisia and theWestern Interior Basin. The Cenomanian-lower Turonian ofTunisia can be subdivided in 24 unitary association zones,which correlate well with the thirteen empirical interval zonesof Robaszynski et al. (1994). Hence, the unitary associationmethod yields a twofold increase in resolution of the ammo-nite zonation. The zonation of the Western Interior Basincomprises twenty-three unitary association zones, coveringthe middle Cenomanian-lower Turonian interval. The faunalcontent of these zonations are summarised in Figs. 4 and 6,which account for all observed and deduced co-occurrencesof taxa. Correlations of these two revised zonations withnorth-west Europe are determined by constructing a zonation,which takes into account all the taxa common to these basins.This method leads to more precise and robust correlations thanthose previously published and reflect the discontinuous natureof the fossil record (Fig. 7). These correlations also underlinethe diachronism of a high percentage of taxa. This emphasisesthe risk of making erroneous correlations when using onlya few, index taxa for correlation across distant basins. Finally,these correlations highlight a slight time difference around themiddle/upper Cenomanian boundary between north-west Eu-rope and both central Tunisia and the Western Interior Basin.In the two last-named basins, the middle/upper Cenomanianboundary is one unitary association zone older. Because therecurrently is no agreement on the definition of the middle/upperCenomanian boundary, the correlations proposed here canhelp to improve this definition. There is only a single specieswhich has a common and apparently synchronous datum be-tween the three studied basins: the last occurrence of Turrilitesacutus. Therefore, if one marker for the middle/upper Ceno-manian boundary is to be chosen, the last occurrence ofT. acutus is here proposed since this species is also abundantand widely distributed.

Acknowledgements

We thank Peter Bengtson, Francis Amedro and FrancisRobaszynski for constructive reviews of an earlier typescript.John Jagt is also thanked for improving the English text.

1031C. Monnet, H. Bucher / Cretaceous Research 28 (2007) 1017e1032

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