global ordovician vertebrate biogeography

Global Ordovician vertebrate biogeography

Alain Blieck a;�, Susan Turner b;c

a Universite¤ des Sciences et Technologies de Lille, Sciences de la Terre, Laboratoire de Pale¤ontologie et Pale¤oge¤ographie du Pale¤ozo|«que(LP3), UMR 8014 et FR 1818 du C.N.R.S., 59655 Villeneuve d’Ascq Cedex, France

b School of Geosciences, Monash University, Clayton, Vic. 3088, Australiac Queensland Museum, PO Box 3300, South Brisbane, Qld 4101, Australia

Received 17 April 2002; received in revised form 22 October 2002; accepted 25 January 2003

Abstract

Cambrian^Ordovician vertebrate and supposed vertebrate occurrences have been repeatedly claimed duringrecent decades, with confirmed taxa bearing mineralized tissues with a vertebrate histomorphology still relatively rare.The only biogeographic province that we can presently recognize is the Gondwana Endemic Assemblage (GEA) withpossible Late Cambrian fragmentary remains from Australia but more definite Early Ordovician (Arenigian) to earlyLate Ordovician (Caradocian) arandaspids (i.e., Sacabambaspis, Arandaspis) and other taxa known from SouthAmerica and Australia. Certain chondrichthyans (‘sharks’, at first without teeth and which might not constitute amonophyletic group) might have originated in East Gondwana province and then are found in the Late Ordovicianand Early Silurian of Mongolia, Tarim, and South China. The GEA fauna proper disappears by middle^lateCaradocian and vertebrates do not reappear in Gondwana until mid Late Silurian. Late Ordovician (3455 Myr orearlier) vertebrates are also known with certainty from Laurentia, viz., North America (pteraspidomorphs Astraspis,Eriptychius, and various gnathostome-like taxa including chondrichthyan-, placoderm- and acanthodian-like remains),and Siberia (astraspid-like microremains with an unusual histology, which might correspond to a new group of lowervertebrates) as well as scales from putative loganiid and thelodontidid thelodonts from North America and Russia(Timan^Pechora, the Severnaya Zemlya archipelago and Siberia). This is defined as the Laurentia^Baltica^SiberiaAssemblage (LBSA). We also mention one enigmatic reference to a Late Ordovician anaspid in South Africa. There isno clear association of taxa between the GEA and LBSA despite a small overlap in time. Various recentpalaeogeographic models published for the Ordovician are critically analyzed and considered within four groups: thearchetypal palaeogeographic reconstructions, two alternative solutions, and a compact version. Habitats ofvertebrates in mostly BA1 (marine intertidal) to BA3 (shallow subtidal) environments, and their dispersal capabilitiesare evaluated with regard to those models. The main feature of Ordovician vertebrate biogeography is endemism.Furthermore, the present lack of complete descriptions of most taxa, which are often represented only by isolatedmicroremains, and the need for a thorough phylogenetic analysis preclude any phylogenetic palaeobiogeographicstudy. In such a framework, we also evaluate possible links between external, physical factors and the Ordovicianradiation of vertebrates. The Late Proterozoic deposition of oceanic phosphate and the Early Cambrian increase inoxygen on Earth might have been the spur for vertebrate evolution before the phase when hard tissues appeared. Thesharp decline of the marine strontium isotope ratio during the Middle to Late Ordovician transition, interpreted ashaving been controlled primarily by continental collisional tectonics and its associated erosion and weathering, has

0031-0182 / 03 / $ ^ see front matter A 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0031-0182(03)00301-8

* Corresponding author. Tel. : +33-3-20434140; Fax: +33-3-20436900.E-mail address: [email protected] (A. Blieck).

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Palaeogeography, Palaeoclimatology, Palaeoecology 195 (2003) 37^54

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been proposed as the consequence of a possible mantle superplume event which could have caused the prominentCaradocian transgressive phase. This might have been a factor in the changeover from a Gondwanan to a Laurentianfocus for vertebrates.A 2003 Elsevier Science B.V. All rights reserved.

Keywords: Agnatha; Gnathostomata; Gondwana; Laurentia; Baltica; Siberia

1. Introduction

Ordovician vertebrates have been known sincethe late 19th century, with notable discoveriesmostly from recent decades including articulatedOrdovician ¢sh from western USA, Bolivia andcentral Australia (Turner et al., in press a,b).However, little work has been attempted to placethese records in a global context or to assess theirpalaeobiogeographic signi¢cance (Blieck et al.,1991, 2001; Elliott et al., 1991; Webby et al.,2000). Only recently has this become possiblethrough more detailed biostratigraphic work (e.g.,Young, 1997; Sansom et al., 1996, 1997, 2001;Turner et al., in press a,b). What is most signif-icant is the now-recognized widespread recordof Ordovician vertebrates in a circum-equatoriallocation. Possible Late Cambrian, Early Ordo-vician to early Late Ordovician (Caradocian)taxa are known from Gondwana, and constitute

our Gondwana Endemic Assemblage (GEA, Fig.1) (Blieck et al., 2001; Turner et al., in press a).This fauna disappears by middle^late Caradocianand vertebrates do not reappear in Gondwanauntil mid Late Silurian (e.g., Pickett et al., 2000).By 3455 Myr (or earlier) a wide range of taxafrom several groups: pteraspidomorphs, puta-tive loganiid and thelodontidid thelodonts, andgnathostomes including chondrichthyan-, placo-derm- and acanthodian-like remains, occur inLaurentia (Colorado, Wyoming, and Ontario).Late Ordovician to earliest Silurian vertebrates(astraspid? Tesakoviaspis, thelodonts) are re-stricted to Laurentia (Arctic Canada, Wisconsin,and Que¤bec), Baltica (Timan^Pechora) and Sibe-ria (including western Mongolia) ; we have pro-posed the name of Laurentia^Baltica^Siberia As-semblage (LBSA) for this series of faunas (Fig. 1)(Karatajute-Talimaa, 1998; Blieck et al., 2001;Turner et al., in press a). Almost everywhere a

Fig. 1. Stratigraphic distribution of the Gondwana Endemic Assemblage (GEA) and the Laurentia^Baltica^Siberia Assemblage(LBSA) of vertebrates in the Ordovician. Chronostratigraphic scale of Webby et al. (in press).

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latest Ordovician to earliest Silurian (earliestRhuddanian) gap in the vertebrate record exists,which we called the Talimaa’s Gap (Turner et al.,in press a).

Sadler and Cooper (in press) provided a cali-bration of global Series intervals of 17, 11.5, and17.5 Myr for the Early, Middle, and Late Ordo-vician respectively. During this comparativelyshort time span true mineralized vertebrates ap-peared in the Arenigian (see Turner et al., in pressa, for our de¢nitions of vertebrate status). Fromthe Darriwilian onward, however, the record be-comes ¢rmer and diversity increases rapidly espe-cially on the LBS blocks. By the Caradocian itappears that all major vertebrate clades arepresent and the Darriwilian to Caradocian wasthe time of their major expansion. Signi¢cant isthe disappearance of the GEA at ca. 3453 Myrwith a slight overlap with the incoming of verte-brates in North America. The last 3 Myr of theOrdovician show an upsurge of new taxa andpossibly new clades of thelodonts and pteraspido-morphs. All of these evolutionary events in thevertebrate record had an underpinning of thetimes the animals lived through. Here we havesought to analyze recent palaeogeographic modelsto test against the actual evidence of the verte-brate record.

This paper is a partial answer to the challengefor ‘a new evolutionary synthesis’ put forward byCarroll (2000), which we stress is necessary forevery geological period, and in particular for thegreat Ordovician biodiversi¢cation event. Carroll(2000, p. 27) de¢ned a program of research asfollows:

(1) To increase knowledge of the fossil recordand get accurate geological dating: this is nowmore clearly demonstrated for Ordovician verte-brates (Turner et al., in press a,b).

(2) To de¢ne plate tectonics/continental driftin£uence on climate and capacity of dispersal oforganisms as major forces in driving evolutionarychange: this is the major topic of the present pa-per.

(3) To elaborate on phylogenetic systematics:this has still to be properly made as there is noconsensus on basal vertebrate relationships (com-ments in Turner et al., in press a).

(4) To obtain the contribution of molecular de-velopmental biology: we are partly concernedwith this because most clades are extinct, andno molecular data are available on Ordoviciantaxa (‘ostracoderms’ sensu Janvier, 1996b, 2001,and gnathostomes). Carroll (2000, p. 28, and citedreferences) observed that ‘largest scale change T

between the cephalochordates and early verte-brates’ occurred ‘when the number of Hox clus-ters duplicated twice, resulting in four clusters bythe time early bony ¢sh (Osteichthyes) appearedsome 415 million years ago’, i.e., in the Silurianaccording to Carroll’s (2000, ¢g. 1) geologicaltime scale. This cannot be tested yet on fossilcephalochordates and ‘cyclostomes’ (hag¢shes andlampreys) which are virtually unknown in theCambrian^Ordovician record, except for puta-tive cephalochordates and allied basal chordatesfrom the Early and Middle Cambrian of Chinaand British Columbia (references and commentsin Smith et al., 2001), and a putative lampreyfrom the Caradocian, Harding Sandstone of Col-orado (new genus B of Sansom et al., 2001, ¢g.10.4e^f). Neither can it be tested on ‘ostraco-derms’ and earliest known gnathostomes (placo-derm-, chondrichthyan- and acanthodian-like inOrdovician times, Turner et al., in press a,b,and references therein).

As already assessed by various authors (amongthem, e.g., Carroll, 1997, pp. 349^350), the incom-pleteness of the early fossil record of vertebratesprecludes any detailed view of their phylogeneticrelationships, ¢rst adaptive radiation, and tempoand mode of evolution. Some recent analyses ofmolecular phylogeny of living vertebrates estimatethe time of divergence between cephalochordatesand early vertebrates at about 3751 R 30.9 Myr(Hedges, 2001), that is, nearly 262^277 Myr be-fore the earliest veri¢ed fossil record of mineral-ized vertebrates, depending on whether the latestCambrian to earliest Ordovician remains are con-sidered as vertebrates or not, conodonts being ex-cluded from the early vertebrate record (Turner etal., in press a,b); and nearly 221 Myr before theearliest putative vertebrates from the mid LowerCambrian of China (Shu et al., 1999). The hy-pothesis of an early radiation of vertebrates inrelation to the end-Proterozoic glacial events

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A. Blieck, S. Turner / Palaeogeography, Palaeoclimatology, Palaeoecology 195 (2003) 37^54 39

(the Neoproterozoic Refugia Model in Hedges,2001) is clearly in contradiction with our hypoth-esis of early radiations in the Ordovician (¢rst inthe Early^Middle Ordovician and mainly duringthe Darriwilian, second in the Late Ordovician,that is, mostly during the Caradocian: Turner etal., in press a). However, if Donoghue et al.(2000) and Smith et al. (2001) are right and cer-tain conodonts are true vertebrates, then there is amuch longer record to account for.

This paper is complementary to Smith et al.’s(2002) and Young’s (in press) papers on EarlyPalaeozoic and Siluro^Devonian vertebrate bio-geography respectively: what are the conditionsneeded during and at the end of the Ordovician,which could explain the situation at the beginningof the Silurian (see, e.g., Blieck and Janvier, 1991,1999; Turner, 1999)? All three papers will thusgive the ¢rst general overview of early vertebratebiogeographic dispersal.

2. Material studied

The material on which our review is based ispublished in two separate papers, one on the in-terpretation of biodiversity (Turner et al., in pressa) and one on the database of CambrianÔrdovi-cian vertebrates (Turner et al., in press b). As al-ready said elsewhere about global palaeobiodiver-sity analyses (e.g., Foote and Sepkoski, 1999), ourdata can only be based on what is known (pub-lished or not), which is the result of ¢eld explora-tions. It appears that most data come from coun-tries where ¢nancial means in terms of employedpeople and ¢eld expeditions could be mounted, sothat the various data are clearly from restrictedareas, mostly North America, former USSR(European and Asian, Siberian parts), and Aus-tralia. Some data also come from Gondwananregions other than Australia, that is, South Amer-ica (Bolivia and Argentina) and South Africa (a¢rst and problematic record of an anaspid). Sev-eral vast areas are still totally devoid of informa-tion, i.e., Europe ^ except Timan^Pechora; non-Russian Asia ^ except perhaps China; Africa êxcept perhaps South Africa; and Antarctica.This severely a¡ects any interpretation of the dy-

namics of origin and evolution of earliest verte-brates, and thus their palaeobiogeographic inter-pretation. The other point that we want to stressis that (palaeo)biogeography is a historical sci-ence, and that any biogeographic scenario is pro-visional, as pointed out by many other authors(e.g., Smith et al., 2002).

3. Habitat and dispersal of Ordovician ¢shes

McKerrow et al. (2000, p. 10) have recentlysummarized various organisms commonly pre-served as fossils into ¢ve categories (aê) of in-creasing biogeographic utility in distinguishing re-gions which were approaching collision (alsoYoung, in press). Fishes were included into cate-gories ‘a’ [free swimming (or pelagic) nekton],possibly ‘b’ (benthic, but with pelagic larvalstage), and ‘d’ [restricted to non-marine or brack-ish environments (or with reduced marine in£u-ence) such as the Old Red Sandstone sediments](ibid.). However, as concerned with ¢sh assem-blages of the Old Red Sandstone type, ‘analysisof empirical data for some well-known fossil ¢shoccurrences T has not produced unequivocal evi-dence of a strictly freshwater habitat’ (Young etal., 2000, p. 211). Category ‘a’ is of little helpfor studying the palaeogeographic relationshipsof palaeocontinents and their shelves (but of pa-ramount importance in long distance biostra-tigraphic correlations). This category probablyhad a reduced capacity of fossilization because,inter alia, most Palaeozoic oceanic £oors havedisappeared. We thus remain with two main cat-egories of ¢shes: (I) benthic taxa on the continen-tal shelves, but with pelagic larval stages allowinggreat dispersal capacity in marine environments,this characterizing very widespread taxa; these arenot very di¡erent from pelagic taxa; (II) taxa withvery limited distribution, apparently con¢ned bymarine barriers (e.g., wide oceanic areas, longitu-dinal oceanic currents), either on continentalshelves in true marine environments, or restrictedto non-marine-brackish environments.

I and II correspond respectively to the ‘oceanic’and ‘continental’ groupings of Rosen (1974, p.323; also Young, in press). Group II, ‘continen-

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tal’ is the most appropriate to test the palaeopo-sition of continents through time. In Ordoviciantime, because of the very incomplete fossil record,most taxa seem endemic and linked to very re-stricted geographic areas, and would correspondto category II. All pteraspidomorph agnathanshave indeed been collected in marine benthic as-semblages BA1 (marine intertidal) to BA3 (shal-low subtidal) sensu Boucot and Janis (1983), andthey do not seem to have been capable of trans-oceanic migrations (Gagnier et al., 1986; Elliott etal., 1991; Gagnier and Blieck, 1992). Similarly,many thelodont taxa are typical of BA1 to BA3(Turner, 1999). A few taxa only might correspondto category I, ‘oceanic’, viz., chondrichthyan-like,?mongolepids and, perhaps, some thelodonts, butnot in Ordovician (references in Turner et al., inpress a,b); more likely their more widespread dis-tribution is explained by their having pelagic lar-val dispersal.

4. A global palaeogeographic problem

Ideas regarding the palaeogeographic relation-ships of the various ‘northern’ landmasses and thesouthern Gondwana supercontinent for Ordovi-cian time are still in a state of £ux. This problemwas treated brie£y in a previous review of Ordo-vician vertebrates by Elliott et al. (1991), whoconcluded that there was an inadequate ¢t be-tween the known fossil record of vertebrates andproposed global palaeocontinental reconstruc-tions. This same situation has prevailed in latestsymposia on Early Palaeozoic palaeogeographies.The most crucial point that Elliott et al. (1991)observed was the inconsistency of the wide oceanhypothesized between Laurentia (for the NorthAmerican Ordovician localities) and Gondwana(for the Bolivian and Australian localities), whichmitigated against any direct migratory relationbetween the two palaeocontinents. This problemcan also be stressed with regard to Baltica andSiberia, from where Ashgillian vertebrates arenow known. Most Ordovician vertebrate-bearinglocalities are interpreted as corresponding to ma-rine benthic assemblages BA1 to BA3 (see above),and Ordovician vertebrates do not seem to have

been capable of transoceanic migrations (referen-ces ibid.). So, to explain the occurrence of verte-brates on both extremities of Gondwana andother Ordovician landmasses, we need palaeogeo-graphic reconstructions based upon geologicaland palaeomagnetic data which show a ratherclose connection between those palaeocontinents,at least during some time slices of the OrdovicianPeriod. Or else we have to conclude in a diphy-letic origin for vertebrates, which is against thecurrent universally accepted concept of mono-phyly of the group.

Reconstructions with too wide oceanic areaspreclude direct biogeographic relations betweenthe vertebrate localities of the continents. This istrue in any biogeographic model. In a migratorymodel, we would hypothesize migrations of verte-brates (or their juveniles or larvae) from the LateCambrian^Early Ordovician neritic platforms ofAustralia to the Middle^Late Ordovician ones ofArgentina-Bolivia, then to the Late Ordovician(Caradocian) ones of N. America, and ¢nally tothe latest Ordovician (Ashgillian) sites of Siberiaand Baltica. (This model has been named the ‘outof Gondwana model’ by Smith et al., 2002.) Inthis case, we need a close relationship (even if nota direct contact) between Gondwana and Lauren-tia, which is not the case on some of the proposedreconstructions. If we do not hypothesize ances-tor^descendant relationships between the variousOrdovician vertebrates, but consider the problemwithin the framework of a cladistic analysis (El-liott et al., 1991, ¢g. 5A; Gagnier, 1995, ¢g. 7;Janvier, 1996a, ¢g. 9.1) (see Section 6), we need ageneralized track covering Gondwana+Laurentiaor Laurentia+Baltica+Siberia with a close rela-tionship between those various blocks. (For alter-native palaeocurrent scenarios, see below in Sec-tion 6.)

Smith et al. (2002) did consider the biogeo-graphic dispersal of early vertebrates under thesupport of a cladistic scheme, and concluded towhat may be named an ‘out of Laurentia model’,where ‘the latest common ancestor of all ‘ostraco-derms’ and jawed vertebrates was Laurentianrather than Gondwanan (contra Elliott et al.,1991)’ (Smith et al., 2002, pp. 76^77). This result,however, is largely dependent upon the fact that

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they consider (1) the conodonts as the sister groupof ‘ostracoderms’+gnathostomes, and (2) the ar-andaspids as a derived (rather than basal) groupof pteraspidomorphs (their ‘heterostracomorphs’),so that the cosmopolitan distribution of cono-donts, and the Laurentian distribution of basalpteraspidomorphs, earliest thelodonts, and chon-drichthyans would favor a Laurentian track forthe early dispersal of mineralized ‘non-conodontvertebrates’ (sensu Smith et al., 2002). Neverthe-less, several data and interpretations are againstthis model : conodonts may not be vertebrates(our hypothesis) ; arandaspids are generally con-sidered as basal pteraspidomorphs; the possibleCaradocian chondrichthyans of Australia, andthe putative Ashgillian anaspid of South Africado not ¢t the model ; the latest Cambrian possiblevertebrate from the Gola Beds of Australia(Young et al., 1996) is not taken into considera-tion by Smith et al. (2002); the Cambrian^Ordo-vician taxon Anatolepis, even if endemic to Lau-rentia, is not placed within their cladistic scheme.So, we treat the problem quite di¡erently.

Several di¡erent solutions have been proposedfor Ordovician global palaeogeographies. Here wewill consider four groups of solutions, viz. (1)what is now classically known as the ‘archetypal’solution of, e.g., Scotese and others; (2) the ‘alter-native palaeogeographic approach’ of Dalziel andothers; (3) another alternative solution of Lewan-dowski ; and (4) the ‘compact version’ of Boucotet al.

In the archetypal palaeogeographic reconstruc-tion of Scotese (1986; Scotese and McKerrow,1990, 1991; also Trench and Torsvik, 1992;McKerrow and Cocks, 1995; Scotese, 1997; Lew-andowski, 1998) (archetypal sensu Torsvik et al.,1995, p. 284), rather wide oceanic areas arehypothesized between western Gondwana andLaurentia on one hand, and between Siberia(+Kazakhstania) and eastern Gondwana on theother hand. This is true for both Early Ordovicianreconstructions (Arenigian: Trench and Torsvik,1992, ¢g. 1; McKerrow and Cocks, 1995, ¢g. 2;Torsvik and Rehnstro«m, 2003, ¢g. 6A; Tremado-cian to Arenigian/Darriwilian: Tait et al., 1997,¢gs. 3,4) and Late Ordovician reconstructions(‘Llandeilo’^Caradocian: Scotese and McKerrow,

1991, ¢g. 4; Trench and Torsvik, 1992, ¢gs. 1,2;Tait et al., 1997, ¢g. 5; Torsvik and Rehnstro«m,2003, ¢g. 6B).

A rather di¡erent solution has been proposedby Neuman (1984; also Nowlan and Neuman,1995) who hypothesized numerous ‘insular ter-ranes’ between the major landmasses (viz., Lau-rentia, Baltica, NW Gondwana) in Arenigian^Darriwilian time. These intermediate elements(mostly volcanic) might have worked as ‘stagingposts for the dispersal of shallow-marine biota’(sensu Talent, 1985, ¢g. 1; Talent et al., 1987, p.92 and ¢g. 1), and the vertebrates might havemigrated under a process called ‘sweepstake mi-gration’ by Simpson (1969, p. 70: ‘route decourses d’obstacles’ in French; also Blieck andJanvier, 1991, p. 378; Blieck et al., 2002). Sucha pattern with intermediate volcanic arcs in theIapetus Ocean is also hypothesized by Mac Nio-caill et al. (1997, ¢g. 2 and p. 161: ‘complex ge-ometry, resembling, in many ways, the tectoniccomplexity of the modern southwest Paci¢c’ ;also Van Staal et al., 1998; the complex historyof Avalonia, the Armorican Terrane Assemblage(ATA) and Moldanubia in Franke, 2000; and thenumerous volcanic arcs in Cocks, 2001, ¢g. 1 forthe Arenigian). This pattern would allow directrelationships between Laurentia, Avalonia, andNW Gondwana. However, on this model, Siberiais too far away from Laurentia.

In another solution based on palaeomagneticand palaeoclimatic data, Golonka et al. (1994,¢gs. 10^12, ‘Llandeilo’) retain wide oceans be-tween Gondwana and the ‘northern’ landmasses,but the latter are grouped together in an equato-rial to southern tropical location. This could sat-isfy a generalized track for the LBSA of LateOrdovician vertebrates (Turner et al., in pressa). Such a close connection between LBS is alsoreconstructed for Late Ordovician time by Tait etal. (2000, ¢g. 3b), based on palaeomagnetic data.

In the alternative palaeogeographic approach ofDalziel (1997, ¢gs. 4,15,16), Laurentia is recon-structed in an equatorial location just north ofthe South American margin of Gondwana, withan intermediate palaeocontinental element in be-tween, called the ‘Texas Plateau’. This modelwould solve some of the problems concerning

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the vertebrates, that is, by providing a rather closecontact between Gondwana and Laurentia. Itwould also be acceptable to explain other biogeo-graphic relationships such as the distribution ofEarly Palaeozoic benthic foraminifers collectedboth in the SE USA and Bolivia (Vachard, inGagnier et al., 1996). Erdtmann (1998, ¢g. 1)also hypothesized a drift of Laurentia along thewestern margin of Gondwana, with a rather closerelation between Laurentia and S. America inEarly Ordovician time. However, on Dalziel’smodel, Siberia is once again too far away fromLaurentia. This problem is perhaps solvable if wedisplace Siberia westwards in a closer relation toLaurentia (but of course with the same latitudinalposition). This is the case in the model of Torsviket al. (1995, ¢g. 11d^f) which shows a close rela-tion between S. America, N. America, and Sibe-ria. However, a comment has to be made aboutthe conclusions of Torsvik et al. (1995). In theirabstract, they say, ‘T a tight continental ¢t (be-tween Laurentia and S. America) during the entireOrdovician is contradicted by biogeographic data’although, in their text (p. 280), they say, this ‘T isapparently contradicted by some of the biogeo-graphic evidence’. The biogeographic evidence ishere taken from papers by McKerrow, Cocks andFortey (references in Torsvik et al., 1995), and isused in the archetypal model explained hereabove. We submit here that other biogeographicevidence (vertebrates and foraminifers at least asoutlined above) do support the alternative ap-proach of Torsvik et al. (1995) and Dalziel(1997), the latter being the only one tested bySmith et al. (2002).

We also place in this group of models that ofBenedetto (1998) who reconstructed a series ofinsular, partly volcanic, terranes including thePrecordillera terrane (Cuyania) of Argentina be-tween Laurentia and S. America in Early Ordovi-cian time (Arenigian/Llanvirnian). This Precordil-lera terrane is considered as having been driftingfrom a Laurentian location to a West Gond-wanan location, based on mostly benthic faunalevidence (microplate hypothesis of Benedetto,1998, ¢g. 6); it was thus accreted to S. Americaat least in the Late Ordovician (Benedetto et al.,1999, ¢g. 6).

Another alternative solution has been proposedby Lewandowski (1993, ¢g. 27) for the Tremado-cian^Arenigian. Apart from the problem of thelocation of the Holy Cross Mountains in Poland,and of Cadomia (including the Armorican Massifof France), which is treated in Lewandowski’spaper, his model shows a rather close relationshipbetween Laurentia, Siberia and the Australianmargin of Gondwana, with narrow oceanic areasin between. This could well be another good so-lution for explaining the vertebrate distribution inOrdovician time, except that the migratory routefrom Australia, through Siberia, to Laurentia isnot in agreement with the ages of the fossil rec-ord: Early to early Late Ordovician in Gondwa-na, Ashgillian in Siberia and Baltica, Caradocianin N. America (given that the fossil record is com-plete, which is probably not the case). Lewandow-ski’s (1993) model also requires a (much too) rap-id clockwise rotation of the whole Gondwana forbringing West Gondwana towards Laurentia inpost-Ordovician time.

Finally, the palaeoclimatically based compactversion of Boucot et al. (1995; also Scotese etal., 2001, ¢gs. 3,4) for Ibexian (Early Ordovician)and post-Ibexian^pre-Hirnantian (Middle^LateOrdovician) time puts the neritic platforms closetogether in a southern latitudinal position, en-abling migratory relations between Laurentia,Baltica, West and East Gondwana, but excludesSiberia, which is reconstructed in rather lownorthern latitudes. However, this model doesnot take into account the traditionally recon-structed oceanic areas such as the Iapetus Oceanbetween Laurentia and Baltica (e.g., Vannier etal., 1989; Erdtmann, 1991; Oliver et al., 1993)or the Tornquist Sea between Baltica and NWGondwana (‘Armorica’) (same references; alsoServais and Fatka, 1997) (without entering thedebate on the reality of the Tornquist Sea; see,e.g., Paris and Robardet, 1990; vs. Servais andFatka, 1997.) At the same time, in the model ofBoucot et al. (1995), no oceanic area seems tohave been intercalated between the various ter-ranes of East Asia, SE Asia and East Gondwana,in agreement with the solution of Metcalfe (1999,¢g. 3).

In such a confusing situation, which solution to

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select? A model that would associate close palaeo-geographic relationships between the major con-tinents (that is, Gondwana, Laurentia, Balticaand Siberia as concerned with Ordovician verte-brates) has to be preferred. A combination withintermediate smaller elements such as exotic ter-ranes, drifting microplates, and/or volcanic arcsmight also be favored. As none of the recentlypublished reconstructions ful¢ll all requirements,we choose here one of the latest, that is, Li andPowell’s (2001) series of global Ordovician models(also in Webby et al., 2000) where Gondwana andLaurentia are rather closely related in the Early

Ordovician (their ¢g. 9), and LBS relativelygrouped in the intertropical zone of the Late Or-dovician (their ¢g. 10), with three major subduc-tion zones in between and their probable associ-ated insular terranes.

5. Fish distribution patterns in the Ordovician

As stressed in our review of Cambrian^Ordovi-cian vertebrate diversity (Turner et al., in press a),Ordovician vertebrates are highly endemic. Foreach selected time slice, only a few spots with

"Tesakoviaspis"

Astraspidida?

Acanthodii n. gen.A

"Tesakoviaspis"

Sandivia

Stroinolepis

Porophoraspis

Arandaspis

lamprey? (nov. gen.B)

chondrichthyan A

of. Sinacanthus

thelodont (nov. gen. C)

loganiid

Astraspis

mongolepid?

chondrichthyans? (nov. gen. A,F)

Skiichthys

thelodontid

loganiid?

Eriptychius

Sacabambaspis

Areyongalepis

Apedolepis

indet. vert.Assemblage 3

indet. vert.Assemblage 4

A B

DC

BAL

SCBPo

GON

Pi

PCLAU

IVF

GONPC

SS

LAU

IAP

AVA

LAU

IAP

AVA

BAL

SIB

Ac?

GON

An?

LAU

IAP

AVA

Sa?

PC

SGON

Fig. 2. Fish distribution pattern in four time slices of the Ordovician. (A) Lower Arenigian vertebrates on the reconstruction ofLi and Powell (2001, ¢g. 9) at ca. 3480 Ma (Tremadocian). (B) Darriwilian vertebrates on Li and Powell’s reconstruction ofWebby et al. (2000, ¢g. 7). (C) Caradocian vertebrates on the reconstruction of Li and Powell (2001, ¢g. 10) at ca. 3450 Ma(latest Caradocian). (D) Ashgillian vertebrates on the same latest Caradocian reconstruction. Abbreviations for palaeocontinentalelements: AVA ^ Avalonia, BAL ^ Baltica, GON ^ Gondwana, LAU ^ Laurentia, PC ^ Precordillera, SCB ^ South ChinaBlock, SIB ^ Siberia; and for palaeocean: IAP ^ Iapetus. Abbreviations for taxa: F ^ Fenhsiangia, Po ^ Porophoraspis, IV1 ^ in-det. vertebrate Assemblage 1, Pi ^ Pircanchaspis, S ^ Sacabambaspis, Sa? ^ cf. Sandivia, An? ^ anaspid?, Ac? ^ acanthodian?

PALAEO 3072 9-5-03


Ordovician vertebrate localities are known, andeach palaeocontinent bears a totally di¡erent taxicassemblage.

In pre-Arenigian time, the record is very frag-mentary and problematic, and is not examinedhere. In the early Arenigian (time slices 2a^2c ofWebby et al., in press), Gondwana only is colon-ized by taxa with questionable vertebrate a⁄n-ities, viz., Porophoraspis and an ‘indet. vertebrateAssemblage 1’ in the Amadeus Basin of Australia,and Pircanchaspis in Bolivia. Additionally, we canmention the phosphatic fragments Fenhsiangiafrom the South China Block (time slices 1d^2a)which would represent the only non-Gondwananrecord for both the Lower and Middle Ordovi-cian, if we reject the fragment of ‘plate’ fromthe lower Middle Ordovician of Inner Mongolia,which probably corresponds to a non-vertebraterecord (references in Turner et al., in press a,b)(Fig. 2A).

The Darriwilian record is more abundant andalso restricted to Gondwana: Porophoraspis, Ar-andaspis and Sacabambaspis in 4b; Sacabambaspisalone in 4c, and on to early Caradocian 5a, thetwo former genera being from the Amadeus Basinof Australia, and Sacabambaspis from Bolivia andArgentina in the Precordillera terrane (Fig. 2B).This record identi¢es what Young (in Webby etal., 2000, ¢g. 7) called a ‘Sacabambaspis fauna’,and Turner et al. (in press a) a ‘Gondwana cladeassemblage’ based upon the arandaspidiform ag-nathans Sacabambaspis and Arandaspis. This isthe major constituent of our GEA. Sacabambaspisis still recorded in the Caradocian (5b^5c) of theAmadeus Basin together with Areyongalepis, Ape-dolepis, an ‘indet. vertebrate Assemblage 3’, andan ‘indet. vertebrate Assemblage 4’. They repre-sent the last record of vertebrates on the Gond-wana supercontinent from where they disappearuntil they re-occur in Silurian time (Pickett et al.,2000) ^ unless we except the claimed anaspid re-corded from the late Ashgillian (6c) of SouthAfrica (references in Turner et al., in press a,b).

In Caradocian time, the most diversi¢ed recordis found on Laurentia with the bulk of the assem-blage from the Harding Sandstone of Colorado(5a^5c) and its numerous equivalents all overUSA and Canada. Nearly 13 di¡erent taxa are

now known there, among them the pteraspido-morph agnathans Astraspis and Eriptychius werethe ¢rst to be described. Both genera have beenrecorded from 5a^5c to 5d. The other elements ofthe Harding Sandstone fauna (i.e., the Laurentiancomponent of our LBSA) include several prob-lematic taxa, i.e., a lamprey?, thelodonts, Skiich-thys (osteostracan? or placoderm?), cf. Sinacan-thus (acanthodian? or shark?), and chondrich-thyans (Fig. 2C). This de¢nitely settles the occur-rence of both agnathans (lamprey? and ostraco-derms sensu Janvier, 1996b) and gnathostomes inthe Ordovician. It also corresponds to a probableradiation event, as far as we can judge from theexisting fossil record. Lastly, further uncertainthelodont and acanthodian records are knownfrom 5c^6a.

The Ashgillian record is poorer than the Cara-docian, and mostly outside Laurentia, except theproblematic acanthodian-like spine from Girvan,in the Midland Valley of Scotland (6b). Thismight signify a second possible extinction eventon Laurentia where both agnathans and gnathos-tomes ‘come back’ in Silurian time (Blieck andJanvier, 1991, 1999; Turner, 1999). Other Ashgil-lian vertebrates are known from Baltica (mostlythe Timan^Pechora region in the NE part ofEuropean Russia) with the enigmatic ‘Tesakovias-pis’, and thelodonts Sandivia and Stroinolepis (6c-Llandovery), and from Siberia (including Tuvaand Mongolia) with again ‘Tesakoviaspis’ and As-traspididae? (6c-Llandovery), and ‘Acanthodiin.g. A’ (6c) (Turner et al., in press a,b). Thiswould identify a ‘Tesakoviaspis fauna’, i.e., theBaltica^Siberia component of our LBSA (Fig.2D).

6. Problems and interpretation

The Ordovician is characterized, inter alia, by(a) widespread epeiric seas with extensive carbon-ate accumulations in low latitude palaeoconti-nents, (b) a generally high sea level, particularlyin the Early and Late Ordovician with a drasticdrop in the latest Ashgillian, (c) wide dispersal ofpalaeocontinents, and (d) a greenhouse state cli-mate during most of the period, with a deteriora-

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PALAEO 3072 9-5-03


tion toward an icehouse state in the Ashgillian(e.g., Ross and Ross, 1992; Barnes et al., 1996;Morrow et al., 1996). In such a global context, allthe vertebrate localities, as plotted on Li andPowell’s (2001) reconstructions, fall within the in-tertropical zone of the Ordovician (Fig. 2), that iscertainly in warm water conditions (Spjeldnaes,1979, ¢g. 10), even if the Earth was rotating fasterin Ordovician times than today, with climaticzones nearer to the equator (references in Chris-tiansen and Stouge, 1999). This being stated, howto interpret the strong endemism of vertebrate as-semblages?

First problem : Cambrian vertebrates (excludingconodonts once again) are very poorly known,and might be represented by ‘naked agnathans’in China and a single mineralized taxon fromAustralia. The latter has super¢cial similaritieswith Porophoraspis, known later in the Ordovi-cian of Australia as well (Fig. 2A,B). Does thismean that vertebrates may have originated in‘eastern’ Gondwana (in fact northern Gondwanaon the Cambrian palaeocontinental reconstruc-tion of Eldridge et al., in Brock et al., 2000), oris it simply due to a strong sampling artefact? Thelatter seems more probable. Sampling of all po-tential Cambrian limestones in Australia is recom-mended. Acceptance of this hypothesis refutes the‘out of Laurentia’ theory of Smith et al. (2002).

Second problem : in our review of Cambrian^Ordovician vertebrates (Turner et al., in pressa), we considered some Anatolepis records as pos-sible vertebrate-like occurrences from the latestCambrian to late Early Ordovician of Wyoming,Utah and Texas (Nitecki et al., 1975, ¢g. 3; Re-petski, 1980, ¢gs. e^f; Smith et al., 1996, ¢gs. 1^3). If really vertebrates, these ‘Anatolepis’ recordscould serve as a test for the Texas Plateau hypoth-esis of Dalziel (1997), as pointed out by Smith etal. (2002). The only problem is that no vertebrate-

like Anatolepis is presently known from Gondwa-na, where the GEA starts in the late Early Ordo-vician.

Third problem : what connection/relation can wehypothesize between the GEA on Gondwana andthe LBSA on LBS? Apparently none for the timebeing: there is no common taxon between GEAand LBSA. However, at the very least, somewhereeither on Gondwana or on LBS (preferably Lau-rentia where the oldest record of the LBSA isknown), at least one speciation event is neededas an origin for the LBSA taxa.

Let us take the pteraspidomorph vertebrates asan example. They are known ¢rst in Australia(Late Cambrian?^Early Ordovician), then inSouth America (Middle Ordovician), in NorthAmerica (Caradocian), and ¢nally in Baltica^Si-beria (Ashgillian). We might thus be tempted toimagine a faunal spreading for pteraspidomorphsfrom Gondwana as a center of origin, to Lauren-tia, and then Baltica and Siberia (the ‘out ofGondwana’ theory). But, what are the phyloge-netic relationships between the GEA and LBSApteraspidomorphs, that is, between arandaspidsand Astraspis, Eriptychius, or between Astraspisand ‘Tesakoviaspis’? Most recent cladistic analy-ses, however, have resulted in no consensus con-cerning basal vertebrate relationships, and partic-ularly pteraspidomorphs (see short comments inTurner et al., in press a). For instance, Eriptychiuswas supposed to be the sister group of Astraspisby Gagnier (1995, ¢g. 7) in the following topology[((((Pteraspidiformes, Cyathaspidiformes) (Eripty-chius, Astraspis)) (Arandaspis, Sacabambaspis))petromyzontids) myxinoids], rooted on cephalo-chordates (Fig. 3A); or the sister group of gna-thostomes by Donoghue et al. (2000, ¢gs. 14,17)in the following topology [Myxinoidea (Petromy-zontida (Conodonta ((Astraspis (Arandaspida,Heterostraci)) ((Anaspida (Jamoytius, Euphaner-

Fig. 3. Several recent schemes of relationships among early vertebrates, with details of pteraspidomorphs and their geographicaldistribution, from (A) Gagnier (1995, ¢g. 7); (B) Janvier (1996a, ¢g. 9.1); (C) Janvier (1996b, ¢g. 5B); (D) Janvier (1997, ¢g.1A); (E) Janvier (1998, ¢g. 7); (F) Donoghue et al. (2000, ¢g. 14). Solutions A, B, D ¢t better both the geographical and strati-graphical records of pteraspidomorphs. Abbreviations for palaeocontinents as in Fig. 2. H is for Heterostraci, and P for Pteraspi-domorphi (polyphyletic in solution F).

PALAEO 3072 9-5-03


ops)) (Loganellia ((Eriptychius, Gnathostomata)(Osteostraci, Galeaspida)))))))], rooted on Tunica-ta and Cephalochordata (Fig. 3F). Note, how-ever, that in Donoghue et al. (2000) 85% ofdata was missing for Eriptychius, and a numberof characters were miscoded. As for Janvier(1996a,b, 1997, 1998), nothing seems de¢nitelysettled as he placed Eriptychius either as the sistergroup of Arandaspida+Heterostraci (Janvier,1996b, ¢g. 5B; nearly similar to Smith et al.,2002, ¢g. 4) (Fig. 3C), of Heterostraci+?other‘Thelodonti’ (Janvier, 1997, ¢g. 1) (Fig. 3D), orof Heterostraci alone (Janvier, 1996a, ¢g. 9.1;Janvier, 1998, ¢g. 7) (Fig. 3B,E). We can justnote that some of these proposals are in disagree-ment with the fossil record, leading to long-stand-ing ghost lineages as, e.g., supposed early astras-pids, anaspids, and basal taxa of the clade(Osteostraci, Galeaspida, ?Pituriaspida) in Do-noghue et al.’s (2000, ¢g. 17) hypothesis. On thecontrary, the ghost lineage for basal chon-drichthyans, as in one of Janvier’s (1998, ¢g. 7)hypotheses, is now partly ¢lled with the discoveryof Ordovician taxa as early as the Caradocian,and the more classical hypothesis of Janvier(ibid.) may be considered as ¢tting better thestratigraphical record. (Except for myxinoidsand lampreys, nearly unknown as fossils ^ exceptthe Chinese, Early Cambrian taxa of Shu et al.(1999), and a Caradocian ?lamprey mentioned bySansom et al. (2001), and much later Carbonifer-ous taxa; and thus, with very long-lasting ghostlineages in all analyses.) [This problem of ghostlineages is more developed by Smith et al. (2002).]The problem is also that ‘scale taxa’ such as ‘Te-sakoviaspis’ have still to be accurately described,and they have never been introduced in any clad-istic analysis.

In such conditions, any scenario of spreading ofpteraspidomorphs, based upon stratigraphy andgeography is nothing else than an author’s opin-ion, where exotic terranes and/or microcontinentssuch as the Texas Plateau of Dalziel (1997), drift-ing between or connected to Laurentia and Gond-wana, might have played a signi¢cant role forvertebrate dispersal in Ordovician time. In anoth-er model, this role would be played by either west-£owing equatorial ocean currents between Siberia,

Laurentia and Gondwana (in the apparent wrongdirection as concerned with vertebrates), or by aneast-£owing equatorial counter current betweenGondwana, Laurentia and Siberia (this time ina perceived accurate direction for vertebrates),which would have transported pelagic larvae be-tween the widely separated Ordovician palaeocon-tinents (Webby et al., 2000). This just means thatwe do not know. The palaeocurrent hypothesis isan ad hoc scenario, inferred from the supposeddistribution of landmasses vs. oceans (e.g., Wilde,1991; also Webby et al., 2000; vs. Christiansenand Stouge, 1999), and the distribution of mostlybenthic marine faunas (trilobites, molluscs, gas-tropods, brachiopods, and echinoderms), includ-ing encrusting/reef building organisms (bryozo-ans, sponges, and stromatoporoids), and somenektonic taxa (conodonts and nautiloids) (Chris-tiansen and Stouge, 1999; Webby et al., 2000),which cannot be tested by the currently knownOrdovician pteraspidomorph zoogeographicalpattern.

Nevertheless, at least one regional problemseems to be solved. The Precordillera microplatemodel of Benedetto (1998; Benedetto et al., 1999)would explain the occurrence of the same aran-daspid genus Sacabambaspis both in the Precor-dillera (Argentina) and West Gondwana (Bolivia)in the Darriwilian (references in Turner et al., inpress a) during what Benedetto et al. (1999) calledthe pre-accretion stage (Llanvirnian^Caradocian)of the Precordillera terrane (Fig. 2B). Saca-bambaspis would have been able to colonize thePrecordillera from Gondwana at a time whenboth elements were not too far away from eachother.

Moreover, how to integrate the other Ordovi-cian vertebrates (thelodonts, and gnathostomes)in this scheme? Siberia has traditionally beenseen as a point of origin for thelodonts (Blieckand Goujet, 1978; Karatajute-Talimaa, 1978;Turner, 1999), which are, however, now knownfrom Timan^Pechora (Baltica), with the earliestSilurian and earlier Ordovician taxa in Laurentia(Sansom et al., 2001; Turner et al., in press a).Thelodonts (Loganellia sp.) had even dispersed asfar as Mongolia by late Llandovery (e.g., Turner,1999). Once again, in absence of a detailed phy-

PALAEO 3072 9-5-03


logenetic analysis of thelodonts, and taking intoaccount stratigraphy and palaeogeography, thelo-donts would have originated in Laurentia beforespreading to Baltica and Siberia, thus parallelingthe pteraspidomorph pattern.

Fourth problem : what other kinds of external,physical factors can explain the strong radiationof Ordovician vertebrates? Can¢eld (1998; alsoCarroll, 2000) noted ‘substantial increases in theavailability of atmospheric oxygen, which wouldhave enabled the achievement of larger body sizesand the formation of calcareous or phosphaticskeletons in many lineages’ in the Early Cambri-an. However, such an increase in oxygen values isnot observed for Ordovician time by Berner et al.(2000) who identi¢ed a rather long stable oxygencurve from the Cambrian to the Devonian. Never-theless, some comments must be made as con-cerned with oxygen. One problem is that theoceanic oxygen content can only be indirectlydetermined quantitatively by, e.g., the isotopiccomposition of sedimentary sul¢des which is in-£uenced also by other factors. Especially for Pa-laeozoic oceans, the range of mechanisms in£u-encing the oxygenation of ocean and atmosphereare not yet fully understood. Oceanic surfacewater was more or less directly connected to theatmosphere. The large reservoir of the deepocean, however, remains unknown due to the sub-duction of Palaeozoic deep-sea sediments. For ex-ample, many authors suggest that during theuppermost Ordovician (Hirnantian glaciation)the global oceanic circulation was similar to themodern one (with an oxygenated deep ocean)(e.g., Brenchley et al., 1994, 1995). This assump-tion seems questionable because at that time theedges of Gondwana extended well into the sub-tropics, thereby prohibiting a deep-water convec-tion as it is known in the modern ocean (Mun-necke et al., 2003, this issue).

So, the Early Cambrian increase in oxygen,plus the great deposition of phosphate in theLate Proterozoic (at ca. 3750 Myr, e.g., Cookand Shergold, 1986), which might or might nothave been related to the breakup of the proposedsupercontinent Rodinia (Li et al., 1996; Li andPowell, 2001), might have been the spur for verte-brate evolution before the phase when hard tis-

sues appeared. For example, Halstead (1974)postulated a link between the phosphate cycle,apatite-based skeletons and mitochondrial activityin vertebrates, but did not know of the evidenceof past times of superabundance of phosphate insea water which would satisfy many requirements.Nevertheless, this hypothesis is in contradictionwith the fact that phosphatic skeletons occur inmany invertebrate groups early in the Cambrian,and not only in the latest CambrianÔrdovicianas for vertebrates.

Strontium isotope analysis of Ordovician andSilurian brachiopods and conodonts by Qing etal. (1998, ¢g. 6; also Holser et al., 1996, ¢g. 2,on apatite and carbonate) shows a gradual de-crease in 87Sr/86Sr from Tremadocian to late Llan-virnian, a sharp decline during the late Llanvir-nianêarly Caradocian, little change duringCaradocianÂshgillian, and a steady rise throughthe Silurian. These variations have been inter-preted by Qing et al. (1998) as controlled primar-ily by continental collisional tectonics and its as-sociated erosion and weathering. In particular,they hypothesized that the rapid decline near theLlanvirnian/Caradocian boundary ‘suggests astrong hydrothermal £ux likely due to increasedsea-£oor spreading and a possible superplumeevent’ which might ‘have caused the prominenttransgressive phase, the largest in the Phanerozo-ic’ (also Barnes et al., 1996; Barnes, in press).This series of physical e¡ects might have been afactor in the changeover from a Gondwanan to aLaurentian focus for vertebrates.

7. Conclusion

The evolutionary and biogeographic history ofOrdovician vertebrates is perforce earlier than anddi¡erent from that of Silurian^Devonian verte-brates, because in Ordovician times: (1) agna-thans seem to predominate over gnathostomes interms of species diversity; and (2) vertebrateswere undergoing strong adaptive radiation, with-out apparent major extinction events, except per-haps in Gondwana during the Caradocian (wherethey only appear again in the Silurian), and inLaurentia during the late Ashgillian (where they

PALAEO 3072 9-5-03


occur again in the Silurian) (Pickett et al., 2000;Turner et al., in press a).

Our review shows a strong endemic pattern forOrdovician vertebrates, with a GEA in MiddleOrdovician time, and a LBSA in Late Ordoviciantime: a Laurentian fauna in the Caradocian, andBaltica^Siberia faunas in the Ashgillian, with nocommon genera between those palaeocontinentalblocks. This pattern re£ects the currently pro-posed palaeogeographical reconstructions for theOrdovician, when Gondwana, Laurentia, Balticaand Siberia are supposed to have been widely sep-arated (e.g., Li and Powell, 2001).

This essay may be read as complementary toYoung’s (in press) paper in the sense that it setsthe scene at the end of the Ordovician, before(1) the enigmatic Talimaa’s Gap at the very begin-ning of the Silurian, and (2) the Silurian radiationevents of vertebrates from either the ‘northernrealm’ (LBS) (sensu Young, in press) and/or thesouthern supercontinent (Gondwana). It proposesalso an interpretation which is di¡erent from thatof Smith et al. (2002), who hypothesized thatmost Cambrian^Ordovician vertebrates, still un-known to science, were ecologically di¡erentfrom their Middle Palaeozoic relatives, and mightalso be found in deeper-water lithic facies. How-ever, there are as yet too many uncertainties inthe fossil record of Ordovician vertebrates and,therefore, our interpretations should be regardedwith caution. As Lieberman (2002) says, aftera simulation analysis of the e¡ects of palaeonto-logical incompleteness on phylogenetic biogeo-graphic analyses: ‘Phylogenetic biogeographicstudies of fossil taxa should avoid groups withlow diversity and a poor fossil record; these stud-ies should also avoid time periods or regions witha poor fossil record’, which is exactly the presentsituation for Ordovician vertebrates.

Publication of recently discovered taxa, andnew discoveries will certainly modify the imageproposed here. Nevertheless, what is absolutelyneeded is a new research program to discoverand describe (publish) Early Palaeozoic macrore-mains and microremains altogether, as well astheir morphology/anatomy and palaeohistology,the latter especially using and illustrated by thesame techniques.

Acknowledgements

This is a contribution to IGCP Project No.410: The Great Ordovician Biodiversi¢cationEvent. We are indebted to Drs. M. Legrand-Blain(Gradignan, France), O.H. Walliser (Go«ttingenUniversity, Germany), J. Lazauskiene (GeologicalSurvey of Lithuania, Vilnius), B. Meyer-Berthaud(Montpellier University, France), S. Steyer (whileat Lille University, France), D.E. Can¢eld(Odense University, Denmark), C.R. Barnes (Uni-versity of Victoria, Canada), T. Servais (LilleUniversity), M.G. Carrera (Cordoba University,Argentina), and the librarians of the Socie¤te¤Ge¤ologique de France in Paris and the Queens-land Museum for help in ¢nding references. Wethank the organizers of both the IGCP 421 meet-ing in Frankfurt am Main (May 2001) and theIGCP 410 meeting in Riverside (June 2001) forallowing us to meet and discuss with many col-leagues, and in particular Dr. G.C. Young (Aus-tralian National University, Canberra). We bene-¢ted from discussion on geochemistry with Dr. A.Munnecke (Erlangen University, Germany). Wealso o¡er this paper as a contribution to IGCP440: Breakup of Rodinia, as a mark of respectand thanks to the Late Professor Chris Powell.A.B. thanks IGCP 410: The Great OrdovicianBiodiversi¢cation Event, the French IGCP Na-tional Committee (PICG France), and the FrenchNational Committee of Geology (CNFG) for ¢-nancial support. S.T. thanks the Australian IGCPCommittee for ¢nancial support to attend IGCP410 and 421 meetings in 2000^2001, and theCNRS for a three-month fellowship at USTL inlate 2001. Our referees C.R. Barnes, D.K. Elliott,and T. Servais helped to improve our paper.

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