Faunal invasions as a source of morphological constraints and … · 2011-06-19 · (Ammonoidea;...

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Paleobiology, 31(1), 2005, pp. 98–116

Faunal invasions as a source of morphological constraints andinnovations? The diversification of the early Cardioceratidae(Ammonoidea; Middle Jurassic)

Nicolas Navarro, Pascal Neige, and Didier Marchand

Abstract.—Multivariate analysis of shell characters and quantification of morphological diversity(morphospace occupation and disparity) are used here to investigate the modes of morphologicaldiversification of ammonites. We define five events in early cardioceratid history that connect geo-graphical changes causing emigration or immigration phases with biodiversity dynamics: (1) theinitial colonization of the Arctic Basin by the Cardioceratidae at the end of the Bajocian, MiddleJurassic; (2) the first appearance of the Kosmoceratidae clade in the Boreal Realm during the Bath-onian; (3) the ensuing expansion phase of this clade in the Boreal Realm; (4) the first phase ofmigration of the Cardioceratidae (early Callovian) through Eastern Europe, Western Britain andthe Yukon corridor; and (5) the second unrelated migration phase in the Western Interior only.Analysis of spatial occupation shows that acquisition of this field occurs essentially by replacementor subdivision of preexisting peaks of occupation. These replacements seem to follow differentpatterns: progressive trend, saturation, iteration, and apparent preferential extinction. We describethese patterns and suggest different factors that may have shaped them, including a morphologicaldifferentiation that has been interpreted by various authors as sexual dimorphism. Another factorthat could cause disparity modification is fluctuations in the ammonites’ proximal environment.The effect of immigrating faunas is a third (and preponderant) factor that is prominent in the stud-ied example: immigration phases of the Cardioceratidae lead to increased morphological diversity,whereas the spread of nonindigenous species reduces it and is contemporaneous with a morpho-logical shift in the native clade. We thus demonstrate here that geographical constraints play asignificant role in the expression of innovation and may be seen as a major factor in macroevolu-tionary dynamics.

Nicolas Navarro, Pascal Neige,* and Didier Marchand. UMR CNRS 5561 Biogeosciences, Centre des Sci-ences de la Terre, Universite de Bourgogne, 6 boulevard Gabriel, F-21000 Dijon, France.E-mail: [email protected], E-mail: [email protected]

*Corresponding author

Accepted: 26 May 2004

Introduction

Global climate change, biodiversity, andtheir interaction are fundamental scientifictopics. Climate change may remove or repo-sition geographical barriers, allowing faunalexchanges between adjacent areas. Expansionof geographic species ranges may or may notlead to species invasions in adjacent regions.Invasion is successful when a spreading phaseof nonindigenous species occurs, and it is re-lated to various factors during the precedingestablishment stage (e.g., competition withnative species). Where invasion is successful,the integration of the invasive species maybring about many changes in ecology and bio-diversity such as niche shifts or extinction(Moyle and Light 1996). Even in the present-day ecosystem, many aspects of the tempoand mode of biological invasions are not yet

understood. Rapid phenotypic changes areexpected to occur in invasive species in re-sponse to new environmental conditions(Hanfling and Kollmann 2002). However,long-term patterns (i.e., those found in the fos-sil record) of response of native and invasivespecies, especially in terms of morphology, arelargely unknown. This paper is a pilot studyof ammonite biodiversity and its fluctuationsin a context of species emigration and immi-gration.

The Jurassic Boreal Realm was a complexset of epeiric basins that were episodically iso-lated from adjacent provinces. Episodic lan-dlocking was a primary factor in the evolu-tionary isolation of its life forms and in theirdifferentiation from Tethyan faunas (Pozarys-ka and Brochwicz-Lewinski 1975; see Enayand Cariou 1997 for a synthesis). This geo-

99INVASIONS AND MORPHOLOGICAL CHANGES

graphical control compounded the effects ofother environmental factors (Enay and Cariou1997) such as the incidence of solar radiation,seasonal fluctuations of illumination (Reid1973), and the cooling of surface seawater byrestricted exchange and heat transfer betweenwater masses (Imlay 1965; Hallam 1985). Theincidence of solar radiation and seasonal fluc-tuations of illumination affect the stability oftrophic resources (Valentine 1971) and seem tocontrol the structure of the ecosystem (Enayand Cariou 1997). Thus, the configuration ofthe Arctic Basin was a major factor in creatingand maintaining specific environmental con-ditions suitable for Boreal taxa (Enay and Car-iou 1997). As a result Boreal ammonite faunasdisplay low taxonomic diversity throughoutthe Jurassic (Enay and Cariou 1997), althoughconsiderable morphological variation occurswithin species (cf., for Triassic, Dagys andWeitschat 1993). The Arctic Region becamelandlocked in Jurassic times largely as a resultof North Sea doming. This regional tectonicevent (Underhill and Partington 1993) formeda physical barrier preventing any communi-cation between the Boreal Realm and thenorthwest European province from Aaleniantimes onward. Similarly nonmarine detritalfacies on the Russian shelf prevented com-munication with the Tethyan region. The Arc-tic Basin was less effectively sealed off, how-ever, in North America and northern Siberia.The Cardioceratidae lineage is one of the best-studied faunas of the Boreal Realm. The firstforms appeared in this previously ammonite-free area at the onset of the late Bajocian: thelast reported faunas in the area before the ad-vent of the Cardioceratidae are dated to theearly Toarcian in Greenland (Dam and Sur-lyck 1998; Surlyck and Noe-Nygaard 2000)and to the early Bajocian in the Arctic area ofNorth America and Siberia (Poulton et al.1992; Sey et al. 1992).

A conceptual and methodological frame-work of disparity (morphological componentof biodiversity) developed over the last decadeprovides a fresh view of biodiversity and theway it is structured over time. Disparity hasproved to be a useful tool when exploringlarge-scale (i.e., in both temporal and taxo-nomic senses) evolutionary dynamics (e.g.,

Saunders and Swan 1984; Foote 1991a,b, 1995,1999; Wills et al. 1994; Dommergues et al.1996; Wagner 1997; Eble 2000a). A few studieshave also applied this tool to other fields, e.g.,phenotypic variation (Smith 1998), short timeintervals with high time resolution (Neige etal. 2001), or developmental disparity (Eble2002). However, examples of empirical studiescombining a small taxonomic scale with highstratigraphic resolution (i.e., biozone-scale)are still lacking. The wealth of data collectedover the last 100 years (see Appendix) makesammonites prime examples for an empiricalstudy of this type.

It is in this context that the present study, byquantifying the morphological component ofbiodiversity, sets out (1) to monitor morpho-logical modifications in the initial (strictly bo-real) phase of the Cardioceratidae, (2) to eval-uate the consequences of their first ingressionsinto peripheral faunal provinces, and (3) toevaluate the impact on cardioceratid diversi-fication of competing immigrant ammonitesin the Boreal Realm (i.e., the Kosmoceratidaeclade).

Early Cardioceratid History

To explore biodiversity dynamics, we definehere five separate events in early cardioceratidhistory associated with geographical changesthat gave rise to phases of emigration or im-migration (see Figs. 1, 2). The rootstock spe-cies of the Cardioceratidae family is Cranoce-phalites borealis (Callomon 1985, 1993; Marc-hand in Dommergues et al. 1989). This speciesderived from sphaeroceratids, closely relatedto Chondroceras oblatum, found in the lower Ba-jocian (Oblatum zone) of the northeast Pacificterranes and Canadian Western Interior (Poul-ton et al. 1992). The first event considered hereis the initial colonization of the Arctic Basin bythe Cardioceratidae (E1) at the beginning ofthe Borealis zone (end of the Bajocian, MiddleJurassic). The Bering region seems to havebeen colonized first probably via the Yukonstrait (Fig. 1) (Callomon 1985). The rootstockspecies then spread throughout the Arctic(Callomon 1985, 1993; Marchand in Dom-mergues et al. 1989). The Cardioceratidaewere virtually the only ammonites in the Bo-real area, to which they were confined. The

100 NICOLAS NAVARRO ET AL.

FIGURE 1. Major events of early Cardioceratidae history. Position of present-day coastline, tectonic faults and oce-anic crusts are taken from the Bajocian map of Owen (1983); the simplified outline of emerged land is drawn afterVinogradov (1968), Poulton (1987), and Ziegler (1988).

second event (E2) was the FAD of the Kos-moceratidae clade in the Boreal Realm (earli-est record is in the Yukon), at the beginning ofIshmae zone, during the Bathonian (Poulton,1987). After this first sporadic record, thisclade came to form the second major ammo-nite stock in the Boreal Realm. The third event(E3) was the expansion phase of this clade inthe Boreal Realm, beginning in the Cranoce-phaloides zone. From this time on, these am-monites were a major component of ammo-nite biodiversity and presumably in competi-tion with the Cardioceratidae. The onset of theCallovian saw the first sporadic connections

between the Arctic Basin and adjacent re-gions. However, the Cardioceratidae re-mained confined to the Arctic Basin until theKoenigi zone (early Callovian) when perma-nent seaways opened up between the BorealRealm and Eastern Europe, Western Britainand Yukon (see Fig. 1). At this time (Goweri-anus subzone, Fig. 2), the Cardioceratidae mi-grated through these corridors, giving rise tonew taxa (Cadochamoussetia and Chamoussetia).These migration phases were the fourth event(E4). A second independent migration phaseoccurred during the Coronatum zone (Fig. 2)but in the Western Interior only (see Fig. 1),


FIGURE 2. Composite zonation used in this study, major events, and stratigraphic range of the non indigenousspecies (NIS). Subboreal zonation is after Callomon et al. (1988); Thierry et al. (1997). Boreal zonation is after Cal-lomon (1993). Numbers from 1 to 18 show time divisions considered in this study (see text for explanations of zonalgrouping or subdivisions).

giving rise to Stenocadoceras. It was the fifthevent (E5).

A second phase of southward expansion(not studied here) began in the late Callovian,peaking in the early Oxfordian (Cariou et al.1997; Taylor et al. 1984). Then, the final epi-sode extended from the mid-Oxfordian to lateKimmeridgian, when the Cardioceratidae re-sumed a more northerly distribution. Thefamily died out without descendants in thelate Kimmeridgian at the base of Autissiodor-ensis zone (Callomon 1985). In all, the Cardi-oceratidae, a monophyletic family (followingCallomon 1985; Donovan et al. 1981), occu-pied the Boreal Realm for 21 Myr without in-terruption (after ages of Gradstein et al. 1994,1995 and assuming constant duration ofzones), spreading sporadically into other re-

gions. In most cases, this phenomenon ofsouthward expansion went along with pro-nounced morphological modifications (aswith Chamoussetia, Longaeviceras, and the Car-dioceratinae).

Materials and Methods

Sampling. In most studies of disparity, thesampling method involves studying just onespecimen per genus. However, this techniqueis effective only when comparing high-leveltaxa. Ammonite species exhibit widely differ-ing morphologies (e.g., sexual dimorphism[Makowski 1963; Callomon 1963]; develop-mental polymorphism sensu Matyja [1986,1994], and marked intraspecific variability[e.g., Dagys and Weitschat 1993]). In addition,our study focuses on a single lineage with lit-


tle taxonomic diversity. Accordingly, onespecimen per morph (sexual, thick/com-pressed or others) was sampled regardless ofthe taxonomic interpretation of the authors,and 163 specimens were used in all. Threeforms of Chondroceras oblatum were included toallow for the initial morphological differenti-ation between the early Cardioceratidae andtheir putative ancestor (Callomon 1985; Marc-hand in Dommergues et al. 1989). Nineteenspecimens of nonindigenous species (i.e., non-Boreal species collected from locations nearthe boundary between the Boreal and otherregions), and nine specimens of Kosmocera-tidae are also included. The kosmoceratidspecimens correspond to Cranocephaloidforms from the Calyx zone. Just a single spec-imen is available for the Ishmae zone display-ing a typical morphology (Poulton 1987).Thus, we expanded the stratigraphic range ofthe nine sampled kosmoceratid specimens tothe Ishmae zone. All are described from illus-trations in various monographs or publica-tions about the Arctic regions or about theCardioceratidae lineage (see Appendix).

Stratigraphic Resolution. The Boreal Realmin mid-Jurassic times displayed a clear ten-dency to provincialism and at times to com-plete endemism. The Boreal ammonites werefirst figured by Keyserlying (1846) for the Pet-shora Basin, by Newton (in Newton and Teall1897) and then Whitfield (1907) for the islandsof Franz Joseph Land, by Madsen (1904) andthen Spath (1932) for the Jameson Land Basin(eastern Greenland), and by Salfeld and Fre-bold (1924) for New Zembla. Since then, workin the Jameson Land Basin (see essentiallystudies by Callomon from 1959 to 1993) hasled to the development of a biostratigraphicstandard for the Arctic area. This successionfor the period considered here (late Bajocianto mid-Callovian) is based on the identifica-tion of 37 faunal horizons grouped into 14zones, 11 of which are strictly Boreal (Callo-mon 1993). Studies of other Arctic areas(North Sea [Callomon 1975, 1979] and Sval-bard [Rawson 1982]) for the same period showsimilar faunas to Greenland. In North Amer-ica (Callomon 1984; Poulton 1987; and for syn-thesis see Poulton et al. 1992) and Siberia (Me-ledina 1973, 1977; for synthesis see Sey et al.

1992), faunas were initially similar to those ofGreenland but then became distinct, and soregional scales have been constructed forthese two areas. Correlations between thesedifferent regions of the Boreal Realm and be-tween this realm and non-Arctic regions ofNorth America (‘‘Western Interior,’’ Terranes)are based on studies by Callomon (1984,1993), Poulton (1987), and Hillebrandt et al.(1992). Correlations between the northwestEuropean province and the Boreal Realm arebased on the work of Dietl and Callomon(1988), Callomon and Wright (1989), and Cal-lomon (1993).

To make our reference scale both homoge-neous and operational several established bio-zones have been grouped or split and a com-posite zonation constructed (Fig. 2): the earlypart corresponds to zonation for Greenland(from Borealis to Nordenskjoeldi: late Bajo-cian to early Callovian) whereas the late partcorresponds to the northwest European stan-dard zonation (from Koenigi to Coronatum:early Callovian to mid-Callovian). This latterzonation is that of Great Britain (Callomon etal. 1988; see also synthesis of Thierry et al.1997). Time divisions are based on the biozonescale. The study period (late Bajocian–mid-Callovian) covers some 11.5 Myr (according tothe data of Gradstein et al. 1994, 1995, adjust-ed assuming constant duration of zones).

Morphospace Definition. Currently, dispari-ty is mainly quantified from empirical mor-phospaces (i.e., from ordination of actualforms). This approach has been vigorouslycriticized by McGhee (1999) as its sample de-pendence induces non-comparability and in-stability of empirical morphospaces. Eble(2000b) reviewed these various points, em-phasizing bias and contributions from thesetwo approaches and suggested a possiblecombined approach (Eble 2002).

Since Raup’s characterization (1966, 1967) ofcoiled shell shapes, there has been a tendencyto produce increasingly complex theoreticalmodels (e.g., Bayer 1978; Okamoto 1988; Ack-erly 1989; Savazzi 1990; Schindel 1990; seeMcGhee 1999 for a review). However, suchmodels have become barely serviceable asmorphometric descriptors of actual forms. Aswith theoretical models, the empirical quan-


FIGURE 3. Morphometric measurements of shell shape.The derived shape ratios used in this study are: D 5 c/d; S 5 b/a; RH 5 a/T; W 5 (d/e); RW 5 b/T and AH5 f/a.

tification of ammonite shape has tended to useincreasing numbers of descriptors. This trendis also marked by the increasing use of dis-crete characters (Saunders and Swan 1984;Dommergues et al. 1996; Neige et al. 2001).One so far unresolved problem is that thesediscrete characters, although allowing mor-phologies to be more exhaustively described,may involve redundancy and misunderstoodcovariation.

The present study is based on six parame-ters (Fig. 3): three from Raup (1967: D, W andS; T 5 0 in the Cardioceratidae) and three in-dices (RW, RH, and AH). Measurements aremade at the end of the adult phragmoconethereby ensuring homologous measurementsbetween specimens and so obviating ontoge-netic effects.

The Cardioceratidae morphospace was ob-tained by normalized principal componentanalysis (PCA) of the six tube parameters us-ing the 163 specimens strictly belonging to thecardioceratids. Other taxa have been plottedon the morphospace as additional data not in-volved in the computation of the new axes. Be-cause some shape ratios contain similar mea-surement variables, a direct algorithmic rela-tionship may exist between them. In order toobtain a better representation of shape ratioon PCs, a Varimax transformation was per-formed. A density analysis was performed onthe overall morphospace. This procedure wasfirst used by Raup (1967) and has been adopt-ed in various morphological studies of ceph-alopod shells (Ward 1980; Chamberlain 1981;Saunders and Swan 1984). Under the original

procedure, density was calculated by countingpoints per cell. We quantified morphospacedensity by a binned approximation to the 2Dkernel density estimate (Wand 1994; Wandand Jones 1995). This analysis was performedwith the bkde2D function of the KernSmoothpackage vs. 2.22–4 (B. Ripley) under R free-ware (Ihake and Gentleman 1996). Observeddifferences in morphospace density may re-flect various factors such as historical contin-gency, historical constraints, distribution ofbiomechanical optima, developmental con-straints (Eble 2000b, 2002), and clusters ob-served may result from complex interactionsamong these factors.

Quantification of Disparity. Considerationof the factorial plane as a morphospace im-plies that coordinates on an axis can be un-derstood as quantifying a complex morphol-ogy. Thus each axis can be seen as a compositemorphological character combining the covar-iant part of the initial morphometric param-eters. Under this hypothesis, the dispersionand position parameters of distribution onany axis can be viewed as a simple morpho-logical character. These two types of metric(dispersion and position) are used here toquantify the modification of morphologicaldiversity.

Numerous methods have been proposed forquantifying morphospace occupation: variouspairwise distances or between specimen andcentroid (e.g., Foote 1991a, 1993, 1994a, 1995,1996, 1999), morphospace occupation countedas cells occupied in subspace (Foote 1992;Dommergues at al. 1996), sums or products ofvariances or ranges (e.g., Foote 1991b; Wills etal. 1994; Eble 2000a), hypersphere or hyper-ellipsoid volume (Wills et al. 1994; Foote1999), convex hull volume (Foote 1999) and,for discrete characters, character-state combi-nations (Foote 1995, 1999). Here, dissimilaritybetween forms at each stratigraphic intervalwas measured as the mean Euclidean pair-wise distance and the area of a bidimensionalconvex hull is used to quantify the amount ofmorphospace occupation.

Spatial Modifications in Morphospace Occupa-tion. Most recent works on changes in mor-phological diversity over time concentrate onquantifying variations in dispersion. In these


TABLE 1. Loadings of the two first rotated principalcomponents of the cardioceratid dataset.

PC 1 PC 2

% Variance explained 48.3 32.7D 20.21 0.95W 0.60 20.12S 20.91 0.30RW 20.91 20.04RH 0.37 20.90AH 0.54 0.73

studies, changes in distribution within a mor-phospace over time have received little atten-tion (Foote 1994a,b, 1995). In this perspective,Dommergues et al. (1996) proposed to tracechanges of position using cumulative Euclid-ean distances between successive centroids ofsubspace over time. However, such measure-ment gives only an indication of the magni-tude of displacement; the directional dimen-sion of the change is totally lost.

Alternatively, various distributional param-eters (mean, median, skews, minimum andmaximum) have been used to detect and dis-tinguish long-term trends in the evolutionaryhistory of clades (McShea 1994; Wagner 1996).Modes too have been used to study body-sizepatterns (Roy et al. 2000). These different pa-rameters are able to reveal patterns relative tocentral trends: i.e., stasis, unidirectionaltrends within the same or different overallmorphospace occupation, or shifts in optimalshape. However, morphospace occupation isnot always continuous. Thus mean and some-times median shapes may lie in unoccupiedmorphospace zones and so model a non-ex-istent morphology for the time interval con-sidered. Conversely, minima and maximaquantify the extreme peaks of morphospaceoccupation. Accordingly, they reveal expan-sion or contraction of space occupation and socorrespond to morphological innovations inrelation to the appearance of new species orloss of shape resulting from the extinction ofspecies. In this study, the latter two parame-ters (minimum and maximum) are used tomodel the spatial occupation of univariatecomponents of the morphospace.

Correction of Indices. The crude value of in-dices is sample-dependent and is biased bythe experimental procedure. An estimator andthe associated standard error may be comput-ed by the bootstrap procedure (Efron 1979; Ef-ron and Tibshirani 1993): data are resampledrandomly with replacement, and the meanand standard deviation then calculated. Stan-dard deviation then provides an estimate ofstandard error, which, in paleobiological stud-ies, essentially reflects an estimate of analyti-cal error (Foote 1993; Eble 2000a). Moreover,both convex hull area and maximum and min-imum are sensitive to sample size. The rare-

faction procedure was used to correct samplesize dependence for these indices at each timeinterval (see Foote 1991b, 1992). The rarefac-tion procedure was carried out by resamplingof k individuals (k , n) with replacement(Foote 1992; Eble 2000a). The bootstrappingprocedure used for rarefaction yields the samestatistical structure of error for all indiceswhether rarefied or not (Eble 2000a). Fivehundred replicates were used for various in-dices whether rarefied or not. Because boot-strap sample statistics are normally distrib-uted, z-tests are computed here (and see Foote1993; Eble 2000a) to test for differences asso-ciated with critical events (E2 to E5).

These procedures were performed usingthe MDA package (Navarro 2003) running onMATLAB software and available at http://www.u-bourgogne.fr/BIOGEOSCIENCE/NNavarro/MDApprgm.html.

Morphospace through Time

Overall Morphospace. Principal componentanalysis (PCA) of the six shape parameters de-scribing the shell tube summarizes 81% of thetotal variance on the first two axes alone (Table1, Fig. 4A). In order to clearly illustrate whichshapes the morphospace includes, we haveadded some theoretical ammonite shapes (seecrosses on Fig. 4A) corresponding to the mainmorphological peak as defined by Wester-mann (1996). This will help later (see Discus-sion) when interpreting shape changes interms of habitat changes. PC 1 (48.3% of var-iance, Table 1) essentially reflects variation inthe shape of the whorl section (S), the relativewidth (RW), and to a lesser extent variation ofthe whorl expansion rate (W). PC 2 (32.7% ofvariance) essentially reflects variation in thedistance of the generative curve (D), relative


FIGURE 4. Morphospace structure for the Cardioceratidae. Ordination is based on principal component analysiswith Varimax rotation. The first factorial plane represents 81% of total variance. A. Cardioceratid morphospace withaddition of Westermann’s shape triangle (Westermann 1996: Fig. 1). B. Taxonomic information. White and light greydots for the Arctocephalitinae (respectively to macro- and microconchs); black and dark grey dots for the Cado-ceratinae; white, black and grey triangles correspond respectively to Cadochamoussetia, Chamoussetia and their mi-croconchs; black stars correspond to the Stenocadoceras subgenus. C. Structure of the cardioceratid morphospace.Contour plot is based on binned approximation to the 2D kernel density estimate. D. Cardioceratid spectrum withnon indigenous species (NIS) and invasive clade (Kosmoceratidae). E: Eurycephalitinae; P: Perisphinctidae; C: Ca-domites; K: Kosmoceratidae; Ph: Phylloceratidae; O: Oppelidae. The gray arrow represents the phylogenetic rela-tionship between Cadomites and the kosmoceratids. The more positive group of kosmoceratids corresponds to themicroconch morphs.

height (RH), and aperture overlap (AH). Axispolarization (from negative to positive peak)exhibits on PC 1 a decrease in shell thickness(S, RW) with its negative covariation withwhorl expansion (W) from extreme cadicone

to oxycone forms. PC 2 shows a decrease inshell involution with its negative or positivecovariation with relative height (RH) and re-spectively with apertural height (AH) fromdiscocone to serpenticone forms.


Morphological dispersion seems to be dis-tributed into three subgroups (Fig. 4B). Thefirst is situated on positive values (near zero)of PC1 and on negative values (also near thezero value) of PC 2. This peak clusters the Arc-tocephalitinae pars (the primitive family) andthe genera Cadochamoussetia (in part), Cham-oussetia and Stenocadoceras (derived genera).The second subgroup forms an uninterruptedcluster with the first group but has its peakdensity at high positive values of PC 1 and PC2. This group comprises microconch mor-phologies of Articoceras and Cadoceras. The fi-nal, less structured, group is localized on neg-ative values of PC 1 and corresponds essen-tially to Cadoceratinae macroconchs with afew thick Arctocephalitinae macroconchs.Thus the Cardioceratidae morphospace has amultipolar style of preferential occupation.These zones of preferential occupation corre-spond either to a particular morph (case of mi-croconchs) or to a particular genus (Cadoceras)associated with certain specific forms (thickmorphs). The first peak is the most heteroge-neous taxonomically as it includes primitiveforms (Arctocephalitinae) as well as severalderived forms (Cadochamoussetia in part,Chamoussetia and Stenocadoceras).

Morphospace through Time. After an initialcontraction (Indistinctus zone), the pattern ofmorphospace occupation (Fig. 5) exhibits aprogressive increase in occupation followedby individualization of new morphologicalpeaks: two with the differentiation of micro-conchs up to the Gowerianus subzone (#12),then three with the appearance of Cadocham-oussetia and Chamoussetia (E4) up to the Eno-datum subzone (#16) and the extinction ofChamoussetia. The appearance of Stenocadocer-as (E5) in the mid-Callovian (#17/18) leads torecolonization of positive values of PC 1whereas the most negative value on PC 1 dis-appears. The appearance of the clade and thecolonization of the Boreal Realm (E1) did notresult in any marked morphological differen-tiation from the hypothesized ancestor (Chon-droceras oblatum). Morphospace occupationseems unmodified between the Greelandicuszone (#05) and the Ishmae zone (#06), whereasthis period corresponds to the appearance ofthe kosmoceratids in the Yukon region (E2).

Conversely, a large morphological shift to-ward the cadicone morphs occurred parallelto the geographic clade-range expansion ofthe kosmoceratids (E3). The geographic clade-range expansion of the cardioceratids in theEuropean area (E4) and North Eastern Pacific(E5) seems to have allowed the recolonizationof unoccupied morphospace by species origi-nation.

A broken-down picture of this temporal oc-cupation can be assessed using location indi-ces. Maximum values on PC 1 (Fig. 6A) in-crease progressively from the Borealis (#1) toIshmae (#6) zones (late Bajocian to mid-Bath-onian), reflecting a trend toward more com-pressed forms. Subsequently the maximumdecreases markedly corresponding to the ex-tinction of compressed forms of Arcticoceras(survivors exhibit a thicker morphology), con-temporaneous with Event 3. Immediately af-ter that values recover, rising to the previousmaximum, and then remain stable. This re-covery and stabilization is due to microconchsoccupying a morphological peak. From thePompeckji zone (#03) to the Gowerianus sub-zone (#12) and after an initial contraction ofvariability, minimum values (correspondingto thicker forms) display a marked tendencyto decrease until a minimum in the Goweri-anus subzone. However, we note a slight in-crease in the Ishmae zone followed by a de-crease immediately after. The origination ofthe first Cadoceras (cadicone morphology) inthe Variabile zone (here associated with theCranocephaloides and Calyx zones; #07–09) isencompassed in this pattern of decrease.Thereafter the minimum rises progressively(i.e., less extreme cadicone morphs), with amore marked shift at the beginning of the LateCallovian (Jason and Coronatum zones, #17–18), contemporaneous with Event 5.

Initially the position of the PC 2 maximumis near zero (Fig. 6B). During a first phase (Bo-realis to Greenlandicus zone: late Bajocian tomid-Bathonian; #01–05), minimum and max-imum follow a similar pattern, shifting to-ward more positive values and then decreas-ing. Subsequently the two parameters aremore or less independent. In the Ishmae zone,the maximum shifts markedly toward highpositive values (corresponding to platycone


FIGURE 5. Pattern of morphospace occupation in the Cardioceratidae from the late Bajocian to mid-Callovian (11.5m.y.). Same convention as in Figure 4. See Fig. 2 for key to zone numbers.


FIGURE 6. Patterns of spatial occupation from the lateBajocian to mid-Callovian. Spatial occupation is mea-sured by minimum and maximum values of coordinateson principal components. Error bars were generated bybootstrapping (500 replicates) and samples are rarefiedto n 5 7. Values for the Indistinctus zone are for infor-mation only (poor sample with n 5 3). Thick lines in-dicate significant difference at 5% (tested only for events2 to 5).

morphology) and then becomes relatively sta-ble, although a decrease is clearly markedfrom the Apertum to Gowerianus zones. Thisnew morphological peak corresponds to thedifferentiation of microconchs appearing inthe Ishmae zone and remaining present there-after. Thus PC 2 reflects rapid saturation ofthis maximum value by differentiation of mi-croconchs. After this event, drastic modifica-tion of morphospace occupation is only pos-sible by modification of minimum values. Theminimum shows a clear increase from the

Greenlandicus to Cranocephaloides zone(#05–07), corresponding to the loss of disco-cone morphology and contemporaneous withEvents 2 and 3. Minimum values recover inthe Gowerianus subzone (#12) with the orig-ination of Cadochamoussetia and Chamoussetia(Event 4) and the re-occupation of the disco-cone morphology. Subsequently, the mini-mum remains constant up to the extinction ofChamoussetia in the Enodatum subzone(#16).

Patterns of Disparity. Temporal sequencesof analyzed disparity indices (rarefied area ofconvex hull and mean pairwise distance, Fig.7) exhibit quite similar patterns: they are dif-ferent only at the end of the period understudy (Enodatum subzone (#15), end of earlyCallovian). We observe a progressive and sus-tained increase until the Ishmae zone (#06;mid-Bathonian) which can be attributed to (1)the progressive recovery of thick forms (trendof the minimum values on PC 1) and (2) theindividualization of microconchs inducing amajor increase in morphospace occupation.Next, the two metrics exhibit, contemporane-ous with Event 3, a decrease (but not statisti-cally significant, see Fig. 7) corresponding tothe extinction of compressed forms of Arcti-coceras. Earlier values are restored in the Aper-tum zone. Thus, it seems that disparity reach-es a first plateau from the Ishmae to Norden-skjoeldi zones reflecting a disparity levelbased on two morphological peaks of occu-pation. This plateau is modified in the Gow-erianus subzone (#12), where a major increaseoccurs. A second plateau is reached and main-tained until the Calloviense subzone (#15).This arrangement is based on three occupa-tional peaks and corresponds to the origina-tion of Cadochamoussetia and Chamoussetia(Event 4), which recovered morphologicalniches lost. Thereafter the two metrics followdifferent patterns. The amount of morphos-pace occupation decreases markedly beforestabilizing at values comparable with those ofthe Nordenskjoeldi subzone (#11), while themean dissimilarity between species declinesprogressively to values comparable with thosefound for the Nordenskjoeldi subzone. Theseparation of these two parameters reflects theextinction of Chamoussetia which was a pe-


FIGURE 7. Patterns of morphological disparity from thelate Bajocian to mid-Callovian. Disparity is measuredby mean pairwise distance. An alternative metric is thearea of convex hull that quantifies the amount of mor-phospace occupied. Error bars were generated by boot-strapping (500 replicates) and for convex hull area sam-ples were rarefied to n 5 7. Values for the Indistinctuszone are for information only (poor sample with n 5 3).Thick lines indicate significant difference at 5% (testedonly for events 2 to 5).

ripheral form and so preferentially affectedthe amount of occupation rather than the dis-similarity. The origination of Stenocadoceras inthe mid-Callovian is not expressed here as itis masked by the extinction of extreme cadi-cone forms (negative values of PC 1) offsettingthe potential increase that origination wouldhave caused. Recovery of values similar tothose preceding the Gowerianus subzone(#12) reflects a return to the initial plateau andso a return to a morphospace structuredaround two occupational peaks.

Discussion

The conceptual and methodological frame-work of disparity, although developed in re-lation to major evolutionary radiations, hasproved a transposable approach with greatpotential for the study of biodiversity in vari-ous contexts: geographic patterns, geographicrange expansion, taxon sorting, evolutionarydynamics, extinction events, and scale effects.However, in developing this new methodology,the spatial component of morphospace occu-pation has been largely overlooked. Alongsidethe quantification of dispersion, the use of in-dices based on the position of the minimumand maximum values on each axis (to decipherthe extreme peaks of space occupied) is highlyinformative (1) for quantifying temporal se-quences of occupied morphospace and notjust simply observing this occupation and (2)for understanding the various components ofchanges in disparity. In contrast to the ap-proach involving tests of extreme modifica-tions, such as that used by Foote (1994a,b,1995), our approach provides temporal se-quences that can be compared directly withtemporal sequences of indices based on dis-persion of shapes in the morphospace. How-ever, this method is a non-synthetic one (eachaxis is treated separately). Moreover, for theminor factorial axis, the covariant part of char-acters becomes less intuitive and noise be-comes more important. However, it may besuggested that only major axes need be ob-served since they summarize the essentialpart of dispersion and so display the more in-teresting patterns. On the other hand, indicesbased on dispersion possess a synthetic qual-ity. However, using dispersion without con-trol of position would make it difficult to in-terpret events involving disparity. The meth-od used here, involving analysis of positionand dispersion using similar metrics, allowsus (1) to compensate the detachment of dis-parity indices with group centroids; (2) to in-terpret disparity modifications in morpholog-ical terms; and (3) to detect significant varia-tions (i.e., trend, saturation of morphologicalpeak, location of extinction or origination) interms of morphology and not just of disper-sion.


Cardioceratidae shells occupied a largemorphological field similar to that of all plan-ispiral ammonites. Differential filling showsthat their morphospace is structured aroundthree peaks corresponding to specific groups.One corresponds to derived microconchs. Theother two correspond essentially to macro-conchs: one combining the majority of theprimitive subfamily and several derived gen-era; the second corresponding to Cadocerati-nae and some thick forms of the primitive sub-family. Analysis of spatial occupation showsthat acquisition of this field occurs essentiallyby the replacement or subdivision of preexist-ing peaks of occupation. These replacementsseem to be produced by different patterns:—progressive trend: (1) progressive increase inthickness from the Pompeckji (#03; late Bajo-cian) to Nordenskjoeldi zones (#11; early Cal-lovian) and then a decrease through the sec-ond part of early and mid-Callovian;—satu-ration: acquisition of platycone morphologyby microconchs in the Ishmae zone (#06; mid-Bathonian) and its subsequent conservation;—iteration: re-acquisition of the discocone mor-phology presents from Indistinctus to Ishmaezone (#02–06), then in the early Callovian(Chamoussetia: northwest European platformand Russian platform; #12–15) and in the mid-Callovian (Stenocadoceras: North America;#18);—apparent preferential extinction: (1) ex-tinction of compressed forms (late Bathonian;#07); (2) extinction of discocone morphologyat the end of the early Callovian (#16); (3) ex-tinction of extreme cadicone morphology inthe mid-Callovian (#17).

After observing these patterns, we can sug-gest different factors that may have shapedthem. One of these factors corresponds to amorphological differentiation that has beeninterpreted by various authors as sexual di-morphism. After its acquisition, the micro-conch morphological peak shows a ‘‘regionalmorphologic stasis’’ (McGhee 1999). This sub-stantial morphological differentiation be-tween microconchs and macroconchs isthought to reflect sexual segregation in modeof life.

Another factor that could cause disparitymodification is fluctuations in ammonites’proximal environments. Such relationships

between morphology and environment arefrequently observed in ammonites (Batt 1989,1993; Bayer and McGhee 1984; Jacobs 1992; Ja-cobs et al. 1994; Tintant et al. 1982; Wester-mann 1996). For example, changes in shellshape are related to sea-level changes (e.g.,Dommergues et al. 1996; Neige et al. 1997).This is because the shell is the hydrostatic ap-paratus of the ammonite. This apparatus wasin direct contact with the physical conditionsof the environment and the initial assumptionis that the two may be related. However, suchcorrelations may be misleading because a cor-relation between environmental and morpho-logical curves does not necessarily reflect atrue connection between them and such cor-relations may just be random features (Mc-Kinney 1990; Alroy et al. 2000). Nonethelessammonite shells potentially harbor informa-tion about environment and mode of life. Thestudy of shell distribution initiated by Ziegler(1967) and the study of hydrostatic propertiesof shells such as aperture orientation (True-man 1941; Saunders and Shapiro 1986) haveallowed habitat and mode of life models to bedeveloped (see synthesis of Westermann1996). However, there are many exceptions tothese major trends (Elmi 1993), and thus mod-els have to be seen as very general guidelinesof ammonites’ habitats. Moreover, the hypoth-esis about body extension outside the bodychamber (Chamberlain 1980; Jacobs andLandman 1993) challenges relationships be-tween body chamber length and aperture ori-entation. This capability would permit totalcontrol over orientation. The study by Elmi(1993) of hydrodynamic properties (e.g., drag)shows that the relationship between shell andfluid can be more complex. Parameters such asshape of ventral area or shape of umbilicusplay a preponderant role. Ornament can makeup for an unfavorable general morphology.Thus it appears that (1) shells contain infor-mation about ammonite mode of life althoughthat information remains difficult to extract bymodeling except for general features; (2) un-derstanding the mode of life for a group in-volves, above all, collecting facts about faunaldistribution. In our case, a particular patternmay be interpreted in the light of such shape/environment relationships. It concerns the ini-


tial and progressive increase in disparity fromthe Indistinctus zone to Greenlandicus zone.This phenomenon is due to a modification ofmorphospace occupation toward negative val-ues on PC 1 (see Fig. 5), and reflects a faunalassemblage modification: initially strictlycomposed of discocone shapes then progres-sively composed of discocone but associatedwith cadicone shapes (see Fig. 4). FollowingWestermann’s model (1996: Fig. 1), this sug-gests a modification from nektonic ammonitesonly to nektonic and planktonic ammonites.Boreal signals of relative sea level exist butthey are derived from tectonically controlledbasins. Due to this local control, curves pre-sent numerous discrepancies, and thus arenot, a regional (Boreal Realm) proxy of envi-ronmental modifications. Thus, unfortunately,our signals of regional morphological diver-sification cannot be compared with these localcurves.

A third preponderant factor is related togeographical modifications and potentially il-luminated by the previous considerations. In atheoretical perspective (see Neige 2003 for adraft model), geographical modifications mayaffect disparity (1) when the modifications of-fer the possibility of emigration events for aclade, the so-called faunal transgressions,leading to an expansion of geographic clade-range and (2) in turn when these modifica-tions offer the possibility of immigrationevents for another clade. This latter may dis-play a diversification in its new geographicrange and thus compete with native clades.Emigration events can be driven by adult em-igration or by juvenile dispersal. Adult mor-phology can be a key to emigration or on thecontrary emigration can yield morphologicalchanges (e.g., Hellberg et al. 2001). In our ex-ample, emigration of Cardioceratidae from theBoreal Realm over the Russian and northwestEuropean platforms (Event 4), inducing theorigination of Cadochamoussetia and Chamous-setia, led to the colonization of a new part ofthe morphospace (i.e., a new peak) in coexis-tence with the initial one causing increaseddisparity. This marked morphological differ-entiation at the time of faunal transgressionsis thought to reflect acquisition of a new eco-logical niche, much as Marchand in Dom-

mergues et al. (1989) interpreted the origina-tion of Chamoussetia by acquisition of a nec-tobenthic mode of life. On the contrary, a sim-ilar emigration event, the origination ofStenocadoceras associated with the colonizationof the Western Interior in the mid-Callovian(Event 5), is not related here with increaseddisparity. However, as explained before, this ismainly because its effect (see Fig. 5, stars indiagram #17/18) is masked by the extinctionof extreme cadicone forms.

The effect of immigrating faunas is alsoclearly marked in the studied example. TheKosmoceratidae first arrived in the Ishmaezone (Event 2), but clearly expanded theirgeographical range and invaded the BorealRealm just after in the Cranocephaloides zone(Event 3). This diversification is clearly appar-ent in the decreased disparity of the Cardi-oceratidae clade (see Fig. 7). Early kosmocer-atids (Kepplerites) have discocone morpholo-gy and thus present a large morphological dif-ferentiation from their putative ancestor fromthe Cadomites genus, which has cadicone mor-phology. When plotting kosmoceratid mor-phologies as additional points, this clade lieswithin the cardioceratid morphospace (Fig.4D). Other nonindigenous species occurredepisodically in one border region (Yukon, Si-beria, Greenland) but unlike the kosmocera-tids did not invade the entire Boreal Realm. Inthis case, all species are located in the periph-eral region of the cardioceratid morphospace(Fig. 4D). It can be noted that the only cladeto have invaded the Boreal Realm is also theonly one to occur within the cardioceratidmorphospace. Moreover, when the Kosmocer-atidae originated in the Ishmae zone (#06)their morphospace location (discocone peak)was at the same time occupied by the Cardi-oceratidae (Fig. 8). Immediately after, in theCranocephaloides zone (#07), Cardioceratidaeunderwent a large morphological shift fromdiscocone to cadicone morphology, giving riseto the Cadoceras genus. However, whereas thetwo clades have the same shell morphology,their costal patterns are very different. Earlykosmoceratid species have numerous thin ribswhereas cardioceratids have only a few flat,rough ribs. Considering the hydrodynamicmodels, a similar shell shape implies similar


FIGURE 8. Patterns of morphospace occupation for theCardioceratidae compared with the location of the Kos-moceratidae (top: Cranocephaloides to Calyx zones,bottom: Ishmae zone). Kosmoceratidae (labeled K) areadded as additional points and are not included in thefactor analysis process. White and light gray dots forArcticocephalitinae (respectively macro- and micro-conchs) and black and dark gray dots for Cadoceratinae(respectively macro- and microconchs). Kosmoceratidforms of the Ishmae zone are considered to be the sameas in the latter zone (see text for additional discussion).

hydrodynamic capabilities. However, costalpatterns and umbilical shape could vastly im-prove these capacities (Elmi 1993). It may bethat their costal pattern favored Kosmocerati-dae by improving their hydrodynamic capac-ities. Consequently, when the two clades oc-cupied what was probably the same ecologicalniche, the Kosmoceratidae temporally exclud-ed the native clade of the discocone peak andinduced the morphological shift of Cardiocer-atidae to the cadicone peak. The discoconepeak was recovered only when some cardi-oceratid species immigrated to adjacent re-gions.

The overall type of disparity pattern canthus be described as follows: first, an increase(maybe due to environmental change andsubsequent ammonite adaptations to a near

planktonic mode of life) toward a plateau;then once this plateau was reached, fluctua-tions occurred around it corresponding to ex-tinction and origination events. These eventsmay be seen as second order ones as they donot drastically modify disparity. The plateaucorresponds to a specific structure of the mor-phospace (e.g., number of distinct morpholog-ical peaks). To actually modify this plateauand not merely generate minor fluctuationsaround it requires a drastic reorganization ofthe morphospace. A new morphospace struc-ture is obtained either by destruction of theinitial arrangement (e.g., extinction of somepeak), or by modification of the initial ar-rangement by acquisition of a new peak andits superimposition on the former one (e.g.,origination of a new peak). At the very onsetof the early Callovian, expansion of the geo-graphic area by ingression into new provincesentailed a reorganization of the morphospacewith the origination of a new peak in associ-ation with the two existing ones. This reor-ganization allowed a new disparity plateau tobe reached. Similarly, a diversification phaseof an immigrant clade (Kosmoceratidae) dur-ing the Bathonian also caused a reorganiza-tion (see suppression of shapes of the positivePC 1—negative PC 2 corner on Fig. 5) of themorphospace. Geographic constraints thusplayed a preponderant role in maintaining ormodifying the state of disparity. Removal ofthose constraints allowed faunas to encounternew environmental conditions (e.g., Russianand European platforms) while retaining theirinitial Boreal niche. Finally, just a few branch-ing events entail substantial increases in dis-parity. These events coincide with faunaltransgressions to other provinces and theirconfrontation with different environmentalconditions (Russian and northwest Europeanplatforms). Nevertheless, the Cardioceratidaeacquired a large morphological field equal tothe near complete field of major morphologi-cal types among planispiral ammonites (Raup1967, Ward 1980). This spectrum was obtainedby progressive shifting of the whole cladeover time (so without any marked increase indisparity). In fact, while they were exclusivelyor nearly exclusively confined to the BorealRealm, the Cardioceratidae never displayed


genuine diversification and only the openingof the Arctic trap permitted diversificationwith marked morphological differentiation:expansion of the geographic clade-range al-lowed confrontation with a new environmen-tal niche while conserving the existing oneand so, a new morphological peak was ac-quired without the initial peak being lost, in-ducing major increases in Cardioceratidaedisparity. On the other hand, the invasion ofthe Boreal Realm by another clade induced amajor morphological shift and probably theloss of an initial ecological niche.

Acknowledgments

The material used in this study was for alarge part compiled from two ammonite da-tabases: one by D.M. and the other by J. Thier-ry. We thank the latter for making his databaseavailable and for his comments on paleoge-ography. Thanks also to J. Callomon for an-swering our inquiries about Cardioceratidaetaxonomy. We are grateful to G. Eble and L.Villier for discussion of the disparity aspect ofthe study, and to D. K. Jacobs and A. Mc-Gowan for constructive reviews. This work isa contribution to the ATI 2001 (CNRS) re-search program and to the Equipe C ‘‘Mac-roevolution et dynamique de la biodiversite’’of UMR CNRS 5561 Biogeosciences.

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