Jurassic ammonite faunas from Nepal and their bearing on the palaeobiogeography of the Himalayan belt

Journal of Asian Earth Sciences 17 (1999) 829±848

Jurassic ammonite faunas from Nepal and their bearing on the palaeobiogeography of the Himalayan belt Raymond Enay a,*, Elie Cariou b a b

Centre des Sciences de la Terre, Universite Claude Bernard-Lyon 1, 27±43 Bd du 11 Novembre 1918, F-69622, Villeurbanne, Cedex, France Laboratoire de GeÂologie, Biochronologie, PaleÂontologie humaine, Universite de Poitiers, 40 av. du Recteur Pineau, F-86022, Poitiers, France

Abstract From the Upper Bathonian up to the Tithonian±Berriasian, six main faunas and twelve basic faunal assemblages within them are distinguished in Nepal. The successive faunas show (1) low taxonomic diversity and (2) the dominance of a small number of genera and the subordinate place of the associated taxa. The assemblages include: (1) strictly Tethyan (e.g., Mediterranean or European Tethyan) species and/or genera, very few in number and occurring as isolated individuals or discontinuous faunal horizons; (2) Indo-Malagasian components, some scattered, others with a wide occurrence in the SW Paci®c, some as far as Antarctica and/or Patagonia; (3) indigenous genera endemic for the Himalayas and the SW Paci®c region. Faunas of the same age for the Sula Islands, Papua-New Guinea, Australia, New Zealand, Antarctica and South America are also considered. In spite of common components, the Himalayan faunas contrast with the relatively higher diversity of the Indo-Malagasian faunas. Low diversity and dominance of indigenous genera mean that the faunas extending from the Himalayas to Antarctica and Patagonia represent an actual biogeographical unit, the Indo Paci®c (faunas and) Realm. Indo Paci®c and Tethyan faunas show a less marked contrast than the Tethyan and Boreal. Transitional or mixed faunas of subaustral type developed in the Indo-Malagasian and Andean regions. This is explained by the absence of a geographical trap comparable to the land-locked palaeogeography of the Arctic Basin. The palaeogeography of the Arctic ampli®ed the role of the other environmental factors. Among these the high latitude seasonal e€ects are likely to have resulted in environmental instability, controlling trophic resources and therefore the structure of the ecosystems, for instance low diversity and high density of the high latitude ammonite faunas. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Ammonites; Jurassic; Himalayas; Nepal; Palaeobiogeography; Indo Paci®c; Austral fauna

1. Introduction Brie¯y illustrated ®rst by Oppel (1863), Jurassic Himalayan ammonites have been famous since `The Faunas of the Spiti Shales' were monographed by Uhlig (1903±1910). However, the faunal succession was not accurately determined at that time and even now, a re®ned complete biostratigraphy of the Spiti type-area remains to be carried out (Cariou et al., * Corresponding author. Tel.: +33 4 7244 8223; fax: +33 4 7244 8382. E-mail address: [email protected] (R. Enay).

1996; Enay and Cariou, 1997). So, the correlation with European biochronological standards is insucient and still doubtful. Meanwhile, Himalayan Jurassic ammonite faunas were identi®ed as original and distinct from those of the Mediterranean Tethys (Neumayr, 1872, 1883; Uhlig, 1911) and the Himalayan Province was proposed ®rst by Uhlig. Within the more recent biogeographical scenarios proposed, the position and signi®cance given to the Himalayan faunas and Province are quite di€erent. Discrepancies result from controversy about whether to use the term and the extent of a third Paci®c (or Indo Paci®c) Realm, as

1367-9120/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 7 - 9 1 2 0 ( 9 9 ) 0 0 0 1 2 - 7

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well as the other two, Boreal and Tethyan, in respect to the Tethyan Realm. There is probably no scheme which can be useful for the whole Jurassic as the history and distribution patterns of the Jurassic ammonites changed over time (Arkell, 1956). These showed alternation of times of increasing provinciality with others showing a tendency to faunal homogenization (Enay, 1980; Westermann, 1993). Increasing provinciality resulted from more or less long term faunal isolation which increased faunal divergence, even endemic forms of high taxonomic rank. Reciprocally, when e€ective isolation disappeared or did not exist, disparities between faunas were not so strong, the distinction being at generic or speci®c level only. From the Upper Bathonian to the Upper Tithonian±Berriasian we have identi®ed six main faunas and twelve basic faunal assemblages in the Jurassic strata of the Tethyan Himalaya in Nepal. They bring some enlightenment both for the biostratigraphy and the biogeography of the Himalayan faunas through the association of taxa from various origins or relationships; some are strictly Tethyan (e.g., Mediterranean or European Tethys); others are IndoMalagasian but occur sometimes farther east as far as the SW Paci®c, together with indigenous Himalayan or Indo Paci®c taxa. New data obtained in Nepal (and Spiti, India) on the Himalayan faunas and recent studies on faunas from the Sula Islands, Papua-New Guinea, New Zealand, Antarctica and South America provide a better picture of the faunal succession and distribution patterns. So, it has become possible to revise and propose a new scheme for the Jurassic palaeobiogeography for the peri-gondwanan regions. Again the question concerning the appearance of a true austral fauna during the Jurassic arises; this has already been proposed for the ammonites as early as the Tithonian (Enay, 1972), and is generally accepted from the Lower Cretaceous onwards (Fleming, 1967; Stevens, 1967, 1971b, 1977; Stevens and Clayton, 1971). 2. The Tethys Himalaya in Nepal Previous works on Jurassic rock stratigraphy in Nepal include Bordet et al. (1964, 1967, 1971), Gradstein et al. (1989, 1991, 1992), Gradstein and von Rad (1991), Gibling et al. (1994) which tried to set out a faunal succession. Other works by Ryf (1962), Helmstaedt (1969), Kamada et al. (1982), Matsumoto and Sakai (1983) are palaeontological studies including descriptions of new species, but with either inaccurate or absent stratigraphic support. Bassoullet et al. (1986), Krishna (1983a,b), Krishna and Pathak (1995) and Westermann and Wang (1988) also contribute in-

directly by comparing Nepal with the other areas of the Indian subcontinent. The Western Himalayas (Lahul-Spiti and GarhwalKumaon areas) still remain the inevitable reference for Himalayan biostratigraphy and faunas because of Uhlig's impressive monograph on the Spiti Shale fauna (1903±1910, 1910). However, in spite of recent studies by Krishna et al. (1982), Pathak (1993), Pathak and Krishna (1995), modern detailed biostratigraphy is lacking. Faunal succession in the type-area of Spiti reaches up to the Lower Cretaceous (Valanginian± Hauterivian), well beyond the highest beds exposed in Nepal. Unlike Nepal, the Lower Spiti Shale member (or Belemnites gerardi beds), just below the Paraboliceras beds and fauna, contains few fossils, except locally. Field collections during summer 1995 proved a Callovian age for most of the Lower Spiti Shale member, probably up to the Oxfordian Belemnites Green Sandstone (Cariou et al., 1996). In Nepal, at least in the Muktinath basin, the Spiti Shale facies does not extend upward as in the typearea. Owing to local regulations, both ®nancial and administrative, we did not visit Mustang where possibly a more complete sequence with more distal environmental conditions might be exposed (Hagen, 1959). In the studied area, the Spiti Shale facies ends just above the Blanfordiceras and Proniceras beds bounded by the plant-bearing sandstones of the Dangardzong Formation (Garzanti and Pagni, 1991; non Dangar Formation Cariou et al., 1994;=GreÁs continentaux du Wealdien, Bordet et al., 1967, 1971;=Chukh Unit of the Chukh Group, Gradstein et al., 1989, 1991, 1992; Gradstein and von Rad, 1991). Unlike Spiti, the mayaitid beds, with rich and diverse faunas, and the Kossmatia beds, which are virtually lacking in the Spiti area, are best developed here. Tectonic complications have been the main problem in the setting up of a suitable and complete sequence of rocks and faunas in the Spiti Shale Formation or Nupra Formation (a local name proposed by Gradstein et al., 1989 and a junior synonym, following Garzanti and Pagni Frette, 1991). First, the contact with the more competent underlying Ferruginous Oolite Formation is often disturbed by tectonic detachment and disharmonic folding. Then, microstructures and large folded and synform or antiform shaped nodules prove the Spiti Shale Formation itself is folded. Owing to tectonic and plant or drift cover, outcrops are scattered and there are no signi®cant facies changes or marker beds to allow easy correlations between di€erent sections or exposures. However, constant renewal of outcrops results from active erosion by the Kali Gandaki river and its tributaries. Consequently, fresh shale and nodules in situ are widely exposed. Most of the nodules contain fossils, mainly ammonites and far fewer belemnites or bivalves

Fig. 1. Correlation of ammonite faunas in the Peri-Gondwanan or austral regions. Sources of data including taxonomic revision or opinions of the authors are given or argued in the text. () In the two columns on the Himalayas, the asterisks show the probable position of the Hybonoticeras beds. (1) The Himalayas±Spiti column revises only the Callovian±Kimmeridgian part according to Enay and Cariou (1996) and Cariou et al. (1996); the Tithonian part is adapted from Enay (1972), Krishna et al. (1982), Krishna (1983a), Pathak (1993) and Pathak and Krishna (1995); (2) Faunas numbered as in the text. Early Oxfordian and Late Callovian faunas not numbered because they are ¯oat collections. (3) Mayaites in Francis and Westermann (1993) have now been identi®ed as Callovian (cf. Iniskinites ) or belated Oxfordian Eurycephalitinae (Westermann, 1996a,b; (4) Epimayites (a€. transiens ) in Thomson (1982b) now unclearly suggested to be of Latest Bathonian±Early Callovian age in agreement with the recently discovered ``external mould of an ammonite body chamber probably from a species of Stehnocephalites or Xenocephalites'' (Thomson and Damborenea, 1993).

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(Retroceramus ). Sometimes several specimens occur in the same nodule. We have gathered a large collection of several hundred pieces, the main part in situ, from more than ten localities. The new results concerning the faunal succession of the Middle and Upper Jurassic (Cariou et al., 1994, 1996; Enay and Cariou, 1996, 1997) were achieved in Nepal during four years of ®eldwork, supplemented by work in the Spiti area during 1995. Eleven ammonite assemblages are identi®ed from in situ collections, well situated in the succession. The successive faunas are dominated by only one genus or a limited number of genera in the same group or family, which have been used to de®ne and characterize each fauna. From bottom to top (Fig. 1): . The Ferruginous Oolite Formation yielded three macrocephalitid faunas: Late Bathonian M. bifurcatus (1) and M. apertusmantataranus (2) faunas, Early Callovian with M. triangularis fauna (3). . Nine distinct faunas in the Spiti Shale Formation: Oxfordian mayaitid faunas in the Lower Spiti Shale; associated perisphinctids allow us to divide this into: Lower mayaitid beds with Tethyan and indigenous perisphinctids (4), Upper mayaitid beds with di€erent indigenous perisphinctids (5). Kimmeridgian Paraboliceras assemblage, which is also divided into: Lower Paraboliceras beds together with the last mayaitids (6), Upper Paraboliceras beds (7). Early Tithonian Kossmatia assemblage (8) with a large range of species (or variability) as yet without an equivalent outside Nepal. Late Tithonian Virgatosphinctes faunas which include two well-de®ned assemblages or horizons: V. broilii-raja group and Aulacosphinctoides Horizon (9), V. densiplicatus Horizon (10) associated with oppeliid-rich beds (including Hildoglochiceras ), Latest Tithonian-?Berriasian Blanfordiceras faunas to be divided into: Blanfordiceras assemblage or Horizon (11), Blanfordiceras and Proniceras assemblage or Horizon (12).

More details on this succession and individual sections or outcrops containing the faunas are available

in two recent reports by Enay and Cariou (1997; in prep.). Apart from the dominant components that we have used to characterize each fauna, other associated forms are locally numerous. These associated forms are always subordinate, whether they are autochthonous or are the result of episodic exotic arrivals. The dominant genera as well as the associated subordinate components are known outside the Himalayas, westwards as far as the Mediterranean Tethys and the Indo-Malagasian enbayement (or corridor), and around Eastern Gondwanaland, in the Indo SW Paci®c regions (Sula Islands, Papua-New Guinea, New Zealand), Antarctica and Patagonia. 3. The West Mediterranean Tethyan components Jurassic faunas in Nepal yielded some exotic components identical or closely related to taxa known in the Mediterranean Tethys. Here, two provinces are classically distinguished, the Mediterranean (or Tethyan s.st.) and the Submediterranean (or Subtethyan). They appear ecologically controlled, the former occuring on the slope and oceanic heights and the latter on the outer shelf of the North and South Tethyan margins. Tethyan stragglers occur as discontinuous horizons resulting from short episodic arrivals. They are useful for correlation because they are well connected to stratigraphically and phyletically better constrained groups. 3.1. Late Bathonian±Early Callovian macrocephalitid assemblages These assemblages yielded sporadic specimens of relatively numerous Submediterranean Tethyan species. They con®rmed the suggested Late Bathonian age of the Apertus Zone (Westermann and Callomon, 1988; Sukamto and Westermann, 1992; Hillebrandt von et al., 1992a; Cariou et al., 1994). These West Tethyan taxa, currently under study include: Late Bathonian Prohecticoceras, Late Bathonian and Early Callovian Homeoplanulites (Paracho€atia ) and two Early Callovian species, ®rst described in Kachchh (India), but closely related to other European species of the genera Cho€atia and Indosphinctes. Although Indosphinctes has been based on Indian species from uppermost Early Callovian beds, the genus probably originated in the Submediterranean Europe where Indosphinctes is more diverse and shows a larger vertical range reaching the basal Middle Callovian. Other West Tethyan taxa have been collected from loose or nearly in situ specimens: latest Middle Bathonian Wagnericeras and Late Callovian Collotia (so, these do not appear or are not numbered in the Fig. 1).

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3.2. Oxfordian mayaitid assemblages The Lower mayaitid beds contain perisphinctids closely related to Submediterranean species. Besides the microconch Dichotomosphinctes, already mentioned (Bordet et al., 1971; Gradstein et al., 1989, 1992), we identi®ed the large macroconch Arisphinctes in several outcrops, occurring in two beds very close to each other, probably a single horizon, about 15 m above the base. A large complete specimen of Passendorferia and several Euaspidoceras con®rm a Middle Oxfordian age and episodic Tethyan arrivals. 3.3. Kimmeridgian Paraboliceras assemblages The Submediterranean Tethyan forms are scarce in the Paraboliceras beds. In the lower part, several specimens might be ascribed to the classic Himalayan (or Indo Paci®c) genus (or subgenus) Uhligites, but are closer to Streblites, a Kimmeridgian genus in Southern Europe. Near the top, we collected a fragmentary Hybonoticeras, a genus which characterizes the beds around the Kimmeridgian±Tithonian boundary, also known in Spiti (Pathak, 1993; Enay and Cariou, 1996) and the Indo-Malagasian area (Futterer, 1894; Collignon, 1959b; Verma and Westermann, 1984; Pathak, 1989 and Pandey, 1993, unpublished theses; Schweigert et al., 1996), the evolution of which might be studied in Submediterranean Europe (Schweigert et al., 1996). 3.4. Early Tithonian Kossmatia assemblage A single species with Submediterranean relationship has been referred to the aspidoceratid Schaireria. 3.5. Late Tithonian Blanfordiceras-Proniceras assemblages Proniceras, a Late Tithonian genus has a large extent from Southern America to the Himalayas. But nowhere are the richness and diversity so great as in the Mediterranean (or European) Tethys (DjaneÂlidzeÂ, 1922). The only other equivalents are those described from Malagasy (Collignon, 1960). Proniceras is the earliest genus of the subfamily Spiticeratinae, whose type-genus, Spiticeras, includes a wide range of forms in the Spiti type-area and also in the Berriasian of the Mediterranean Tethys. The most valuable evidence that would point to the inclusion of Proniceras among Submediterranean Tethyan components is its probable origin in the Simoceratinae, following the unsupported suggestion by Oloriz and Tavera (1979a,b), that subfamily is extensively developed only in the Mediterranean Tethys. To summarize, the Submediterranean or closely re-

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lated components of the Jurassic faunas in Nepal are relatively numerous from the Upper Bathonian to the Oxfordian (macrocephalitid and lower mayaitid assemblages); they are more rare later (upper mayaitid, Paraboliceras and Kossmatia beds); and they are missing in the Virgatosphinctes and Blanfordiceras beds, apart from the Blanfordiceras and Proniceras beds, even if Aulacosphinctes and Paraboliceras are misidenti®ed with European Parapallasiceras (Oloriz and Tintori, 1990) and Lemencia (Gibling et al., 1994) respectively. 4. The components indigenous from or also occurring in the Indo-Malagasian region The Indo-Malagasian enbayement (Eastern Africa, Malagasy, Western Indian margin) which was to become a seaway in Tithonian time, was open on the southern Tethyan margin, between Western and Eastern Gondwana. Indo-Malagasian faunas show a relationship with the Submediterranean Tethyan faunas, but they also include taxa missing in the European Tethys and occurring in and beyond the Himalayan belt, around Eastern Gondwana. So, strict Indo-Malagasian components and those with a larger extent are dealt with separately. 4.1. Strict Indo-Malagasian components These include the perisphinctid genera Torquatisphinctes st. s., Pachysphinctes and Katroliceras. All have been de®ned on species from Eastern Africa or Kachchh (India) and species of these genera are common and numerous in Eastern Africa (Dietrich, 1925; Verma and Westermann, 1984), Malagasy (Collignon, 1959b) and Kachchh (Waagen, 1873±1875; Spath, 1927±1933), where a faunal succession and a new zonal scheme have been described recently (Pathak, 1989 and Pandey, 1993, unpublished theses; Krishna and Pathak, 1993). In the Himalayas, Torquatisphinctes st. s., Pachysphinctes and Katroliceras occur mainly in the Paraboliceras beds, at several levels. They are never numerous in Nepal but more frequent in Spiti where some exposures yielded specimens of these genera almost exclusively. This evidence supports the hypothesis that they occur in the rock sequence as faunal horizon(s), as do the Mediterranean Tethyan genera mentioned above. The highest frequency of these genera in Spiti agrees well both with its situation nearest to the Western Indian margin, and with the IndoMalagasian origin of these in¯uxes to the Himalayas (Enay and Cariou, 1996). Similarly, in the Lower Spiti Shale member of the type-area, some Callovian Indo-Malagasian taxa

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Fig. 2. Distribution maps of Oxfordian Mayaitids and the genus Araucanites around Gondwanaland. Palaeocontinental reconstruction by Vrielynck (in Enay and Cariou, 1997). Palaeogeography from Dercourt et al., 1993 (Atlas Tethys), Riccardi (1991) and Stevens (1990). Stippled: land areas, including ¯uvial deltaic deposits and evaporite platforms; blank: marine continental platforms and oceans, the hypothetic boundary being marked. From Enay and Cariou, 1997 (modi®ed).

(Idiocycloceras, Obtusicostites, Hubertoceras ), which are unknown eastwards (Cariou et al., 1996) also occur. 4.2. Components occurring both in the Indo-Malagasian area and around Eastern Gondwana We place here somewhat numerous taxa, some of them ®rst described from the Himalayas, which also extended westward in the Indo-Malagasian enbayement. Late Bathonian and/or Early Callovian (Westermann and Callomon, 1988; Cariou et al., 1994; Enay and Cariou, 1997) Macrocephalites of the M. triangularis group are well known in Kachchh (Spath,

1927±1933; Krishna and Westermann, 1987) and Malagasy (Collignon, 1958). They have been found recently in Nepal (Cariou et al., 1994), but not farther east. Mayaitids (Fig. 2), very rare or missing in Spiti, but a prominent group in the mayaitid beds of Nepal, also occur in the Indo-Malagasian area, in Eastern Africa (Tornquist, 1893; Kapilima, 1984), Malagasy (Collignon, 1959a) and Kachchh (Waagen, 1873±1875; Spath, 1927±1933), as well as in the Sula Islands and Papua-New Guinea (Boehm, 1908; Francis and Westermann, 1993). Among the Nepal fauna (currently under study) species described and ®gured from Kachchh and others from New Guinea occur together. So deciding whether mayaitids are an Indo-Malagasian

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Fig. 3. Distribution of Upper Tithonian-? Berriasian Blanfordiceras around Gondwanaland. Palaeocontinental reconstruction by Vrielynck (in Enay and Cariou, 1997). Palaeogeography from Dercourt et al., 1993 (Atlas Tethys), Riccardi (1991) and Stevens (1990). Stippled: land areas, including ¯uvial deltaic deposits and evaporite platforms; blank: marine continental platforms and oceans, the hypothetic boundary being marked. From Enay and Cariou (1997).

or Himalayan-Papuan (or Indonesian) group is not easy (Fig. 2). The Oxfordian in New Zealand was thought to be missing or represented by non-marine strata (Fleming, 1970), but according to Hudson et al. (1987), the Oxfordian ammonite Epimayaites is present together with Late Callovian dino¯agellates, and Mayaites has been recently listed by Francis and Westermann (1993) but now identi®ed as a Callovian (cf. Iniskinites ) or belated Oxfordian eurycephalitine (Westermann, 1996a,b). Although mayaitids would be missing in New Zealand, a large perigondwanan extension was assumed from the occurrence of Epimayaites in Antarctica (Thomson, 1982b) and Araucanites, a genus

(or subgenus) close to Mayaites described from the Middle Oxfordian of Argentina (Stipanicic et al., 1975; Riccardi et al., 1992). But recently, Thomson and Damborenea (1993) on the one hand, Westermann and Riccardi (1985) and Westermann (1996a,b) on the other hand challenged the hypothesis that they are either Callovian eurycephalitines or belated representatives of the same group. Virgatosphinctes of the V. densiplicatus group, which include a large number of species are quite numerous in the Indo-Malagasian area (Lemoine, 1911; Spath, 1927±1933; Collignon, 1960), unlike the V. broilii-raja group. Following Westermann (G.E.G., personal communication, 1997) the V. densiplicatus group occurs in

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Fig. 4. Distribution map of the Late Bathonian Macrocephalitids of the M. apertus group (square) and Eurycephalitins (circle), here typi®ed by the genus Xenocephalites ), around Gondwanaland. Eurycephalitines extended northwards on the Northwest America during Late Bathonian± Early Callovian time. Palaeocontinental reconstruction by Vrielynck (see text). Palaeogeography from Dercourt et al., 1993 (Atlas Tethys), Riccardi (1991) and Stevens (1990). Stippled: land areas, including ¯uvial deltaic deposits and evaporite platforms; blank: marine continental platforms and oceans, the hypothetic boundary being marked. From Enay and Cariou (1997).

Papua-New Guinea and perhaps also in Australia from a text reference in Brunnschweiler (1954). On the contrary, the V. densiplicatus group is well illustrated from Antarctica, as well as Aulacosphinctoides and virgatosphinctids of the V. broilii-raja group or forms closely related to Southern American species (Thompson, 1976). Blanfordiceras (Fig. 3) exhibits the same distributional pattern: the genus was ®rst described from, and is now well documented in the Spiti area (Uhlig, 1903± 1910), as in Nepal (Helmstaedt, 1969; Mouterde, 1971; Gradstein et al., 1989, 1991, 1992), Malagasy (Collignon, 1960), Papua-New Guinea (Boehm, 1904; Gerth, 1965; Westermann and Getty, 1970; Sato, 1975; Sato et al., 1977; Helmcke et al., 1978; Westermann,

1981), Antarctica (Crame and Howlett, 1988; Howlett, 1989; Whitham and Doyle, 1989), Patagonia (Feruglio, 1936±1938; Leanza, 1967) and Argentina (Steuer, 1897; Uhlig, 1903±1910; Riccardi et al., 1990, 1992). The genus is missing between Papua-New Guinea and Antarctica (Fig. 3).

5. Indigenous or Indo-Paci®c components Several taxa are unknown to the west of the Himalayas, either in the Indo-Malagasian area and in the Mediterranean Tethys. These Indo-Paci®c taxa are distributed more or less widely eastward, in PapuaNew Guinea and as far as Antarctica and Patagonia.

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Fig. 5. Distribution map of Kimmeridgian Paraboliceras (circle) and Tithonian Kossmatia (square) around Gondwanaland. Palaeocontinental reconstruction by Vrielynck (in Enay and Cariou, 1997). Palaeogeography from Dercourt et al., 1993 (Atlas Tethys), Riccardi (1991) and Stevens (1990). Stippled: land areas, including ¯uvial deltaic deposits and evaporite platforms; blank: marine continental platforms and oceans, the hypothetic boundary being marked. From Enay and Cariou (1997).

Where they are present, they also dominate the fauna and give evidence of endemism. So, at the present time, although progress has been made, correlations with the standard zonal schemes for the IndoMalagasian and Tethyan areas are dicult and hazardous. In the Upper Bathonian, the Macrocephalites bifurcatus and M. apertus-mantataranus groups (Fig. 4), recently discovered in Nepal (Cariou et al., 1994), are currently being monographed (Cariou and Enay, in prep.). They display peculiar faunas as yet occurring only in the Sula Islands (Boehm, 1911) and PapuaNew Guinea (Westermann and Getty, 1970; Westermann and Callomon, 1988). The New Guinea Apertus Zone also yielded rare Xenocephalites, which

are more numerous together with Lilloetia in New Zealand where Macrocephalites is missing. Eurycephalitines Xenocephalites and Lilloetia show East Paci®c distribution from Yukon to Argentina and Chile (Thierry, 1976; Riccardi, 1985; Westermann and Callomon, 1988). So, when referring to eurycephalitines the specimens or species from Antarctica and Patagonia, ®rst described as Epimayaites (Thomson, 1982b; Thomson and Damborenea, 1993) or Araucanites (Stipanicic et al., 1975; Riccardi et al., 1992) would provide the link between Southern America and New Zealand (Fig. 4). The Oxfordian mayaitid beds yielded numerous specimens of two peculiar perisphinctid groups, including both microconchs and macroconchs. They are quite

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Fig. 6. Distribution of (Upper) Tithonian Virgatosphinctes and Aulacosphinctoides around Gondwanaland. Palaeocontinental reconstruction by Vrielynck (in Enay and Cariou, 1997). Palaeogeography from Dercourt et al., 1993 (Atlas Tethys), Riccardi (1991) and Stevens (1990). Stippled: land areas, including ¯uvial deltaic deposits and evaporite platforms; blank: marine continental platforms and oceans, the hypothetic boundary being marked. From Enay and Cariou (1997).

apart from the Submediterranean Tethyan genera mentioned above (Dichotomosphinctes, Arisphinctes, Passendorferia ) and indigenous for the Himalayas and Indo-Paci®c area. The ®rst one is closely related to the ``Perisphinctes'' sularus-moluccanus group, described from the Sula Islands (Boehm, 1908) and Papua-New Guinea (Francis and Westermann, 1993; Westermann, 1996a) and for these Oloriz and Westermann (1998) just proposed the new genus Sulaites. The second group includes specimens previously referred to Idoceras or Kossmatia, because of morphological resemblances. It is probably one of the reasons why in New Zealand the ®rst appearance of Kossmatia is assumed to occur earlier and below Paraboliceras

(Stevens, 1968, 1978, 1997; Westermann, 1996a) in contrast with Nepal. Perhaps the true (Himalayan) Kossmatia originated from these early Kossmatia-like forms, but this is not established with certainty. Besides the age di€erence, they show enough peculiar features to be provisionally and informally separated as ``pseudokossmatia''. In Nepal, Paraboliceras (including Paraboliceratoides ) below, Kossmatia just above, are two prevailing genera in the two corresponding assemblages (Fig. 5). They are missing in the Indo-Malagasian area and the Tethys, although the Tethyan species richteri was misidenti®ed for a long time as Kossmatia until Avram (1976) created the new genus Richterella (=Richteria

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Oloriz, 1978). Paraboliceras and Kossmatia are probably phyletically related and indigenous in the IndoPaci®c area: Papua-New Guinea (Francis and Westermann, 1993), New Zealand (Stevens, 1978, 1981, 1990) and Antarctica (Thomson, 1975, 1983; ? Whitham and Doyle, 1989, as Lithacoceras sp.). According to Stevens (1978, 1990), Paraboliceras and Kossmatia would occur also in New Caledonia. Kossmatia is also mentioned from Northwestern Australia (Arkell, in McWhae et al., 1958; Brunnschweiler, 1954, 1960), but this is disputable following Francis and Westermann (1993) and they would be best referred to Oxfordian perisphinctids of the P. sularus-moluccanus group (Fig. 5). Virgatosphinctids of the Virgatosphinctes broilii-raja group and the microconch Aulacosphinctoides prevail in the Upper Tithonian Virgatosphinctes and Aulacosphinctoides horizon (Fig. 6). They were fully described and illustrated from Spiti (Uhlig, 1903± 1910), also in Nepal (Helmstaedt, 1969; Mouterde, 1971; Matsumoto and Sakai, 1983) and they occur in Papua-New Guinea (Francis and Westermann, 1993), New Zealand (Boehm, 1911; Stevens, 1978, 1981), Antarctica (Thomson, 1979, 1983; Howlett, 1989) and the Magellanes Basin (Thomson, 1982a). The `virgatosphinctids' from Southern America (Burckhardt, 1903; DouvilleÂ, 1910; Weaver, 1931; Indans, 1954; Leanza, 1980, 1981) look like the Himalayan and/or IndoPaci®c species; however, they are older (Early Tithonian, Mendozanus Zone) and therefore, the style of ribbing is di€erent; they have already been separated as `Virgatosphinctes' gr. mendozanus (m)-burckhardti (M) (Enay, 1972, 1973) (Fig. 6). Malagasian Aulacosphinctoides from the Late Tithonian Kobelli Zone (Collignon, 1960), and the `virgatosphinctids' (other than those species of the densiplicatus group), identi®ed or compared to Uhlig's species, are also quite distinct from the Himalayan species. Most of the ®gured specimens (studied in Dijon University) are wholly septate inner whorls of large sized species the full-grown aspect of which is unknown. Whorl sections of the Malagasian species are not so thick, barely depressed, with coarse, strong ribbing. They display an Indo-Malagasian o€-shot parallel to the Himalayan types. Unpublished theses by Pathak (1989) and Pandey (1993) illustrate closely related forms from Kachchh which have been confused with Uhlig's species. 6. Biogeographical scheme Our survey of the Himalayan ammonite faunas from the Late Bathonian to the Tithonian-?Berriasian shows that the biogeographical pattern cannot be understood without taking into account the broader framework of

839

the Peri-Gondwanan faunas. These inhabited the platforms around Eastern Gondwana and geographically they were located on the margins of both the Tethys and the Indo-Paci®c part of the Paci®c ocean. A long time ago, Himalayan Jurassic ammonite faunas were identi®ed as original and distinct from those of the Mediterranean Tethys, especially those well known from the Spiti Shale facies (Neumayr, 1872, 1883; Uhlig, 1911). Later studies give various schemes, generally within a vast Tethyan Realm contrasting with the Boreal Realm of lesser extent. But the geographical extent of the Tethys ocean and the extent ascribed to the Tethyan biogeographical Realm should be distinguished. Sometimes, part of the Eastern Tethys has been separated as the Indo-SW Paci®c province (Westermann and Riccardi, 1985; Enay, 1980) or subrealm (Westermann and Hudson, 1991; Westermann, 1993 and in Hillebrandt et al., 1992b); on the contrary, the Tethyan Realm exceeds widely the geographical extent of the Tethys, including the western margin of the Paci®c. Consequently, the Uhlig Himalayan Province (1911) received various di€erent interpretations: . Uhlig's original Himalayan Province included the Indo-Malagasian area as a subprovince. Cariou (1973) used Indo-Malagasian Province with the same broad meaning. . The Himalayan and Indo-Malagasian regions are also brought together in the larger Indo-East African Province (Krishna, 1983a, 1987) which is therefore a junior synonym of the Uhlig Himalayan Province. The former is also used by Westermann (1992, 1993) as a province (or subprovince) within his Indo-SW Paci®c Realm (or Province). . On the contrary, the Indo-SW Paci®c Province of Stevens (1967, 1971a,b, 1977, 1980a,b) and Mutterlose (1986, 1992a,b) includes only the Himalayan regions excluding the Indo-Malagasian and corresponds to the Himalayan (or Indo-Paci®c) Province of Enay (1973). . Finally, the Himalayan Province of Enay (1972) was restricted to the Himalayan regions and was a part of a wide peri-Gondwanan (=Austral) Realm which also included the Ethiopian (=Indo-Malagasian) and Andean Provinces. The numerous and varied pictures and names can be explained by the place of the Himalaya regions in the borderland between the Tethys and the SW Paci®c. They also express the peculiar and variable features of the faunas that are more or less conspicuous depending on the time and the fossil group considered. The biogeographical schemes also depend on the biogeographical meaning given to some taxa; this is a subjective opinion owing to the absence of a good knowledge of phyletic relationships. The lack of data

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also limits the value of the interpretation; it results from collection failure, the absence of marine strata, a gap in the geological record or a true absence. Various genera occur both in the Indo-Malagasian regions and the Himalayas as well as in Papua-New Guinea from the Late Bathonian to the Kimmeridgian and during the Late Tithonian. Concerning the macrocephalitid-mayaitid lineage, almost uninterrupted occurrence from the Indo-Malagasian regions to Papua-New Guinea supports the hypothesis that it was an indigenous group whose origin is disputed: (1) according to Thierry (1976), macrocephalitids evolved from the East Paci®c eurycephalitines while they migrated to the Southern Tethys, but the Late Bathonian opening of the Indo-Malagasian trough is very unlikely; (2) Westermann and Callomon (1988) assumed they evolved from the sphaeroceratines (genus Satoceras ) in the Western Paci®c (Sula-New Guinea Subprovince of Westermann, in Hillebrandt et al., 1992b) and expanded westwards on the southern margin of the Tethys. The discovery of the eurycephalitines Xenocephalites and Lilloetia in New Zealand (Westermann and Hudson, 1991) could provide a satisfying compromise between the two hypotheses. Whatever the genus or the group from which macrocephalitids evolved and the center of origin or the migration route, there are no inconsistencies with the indigenous character of the macrocephalitid-mayaitid lineage. Moreover, the Late Bathonian age of the earliest macrocephalitids in the Sula Islands (Boehm, 1911), Papua-New Guinea (Westermann and Getty, 1970; Westermann and Callomon, 1988; Westermann, 1992; Sukamto and Westermann, 1992; Hillebrandt et al., 1992a) and Nepal (Cariou et al., 1994; Cariou and Enay, in prep.) supports Westermann and Callomon's hypothesis. No de®nite Bathonian macrocephalitids are known in Malagasy. The contrast between the Himalayan and the IndoMalagasian faunas mainly shows itself the higher diversity of the latter. The high diversity results from both the higher frequency or greater abundance of Tethyan components and indigenous genera often of Tethyan origin which enabled numerous authors to distinguish the Indo-Malagasian Province. Tethyan in¯uences seem to have been greater on the African margin and Malagasy and they contrast with the Western Indian shelf (Kachchh). Here, recent studies show that the exotic stragglers occur as episodic discontinuous `horizons' (Cariou and Krishna, 1988; Krishna et al., 1995; Pathak, 1989 and Pandey, 1993, unpublished theses; Krishna and Pathak, 1995). Tethyan in¯uxes, like the Indo-Malagasian taxa, are scarcer in Spiti and Nepal. However, Oxfordian Peltoceratoides occurs as far as the Sula Islands (Boehm, 1908) and New Guinea (Kruizinga, 1926). But, Nepal is the farthest limit eastwards so far known

for Hybonoticeras (Enay and Cariou, 1996) and Middle Oxfordian Submediterranean Tethyan perisphinctids (Dichotomosphinctes, Arisphinctes, Passendorferia ) from Thakkhola (Mouterde, 1971; Kamada et al., 1982; Gradstein et al., 1989, 1992; Enay and Cariou, 1997), except for a badly preserved specimen from Australia (Brunnschweiler, 1960). IndoMalagasian taxa (Torquatisphinctes, Pachysphinctes, Katroliceras ) cannot be ascertained on the basis of text references in Papua-New Guinea (Francis and Westermann, 1993) and New Zealand (Stevens, 1978, 1997). In the Himalayas they are rare in Nepal and more frequent in Spiti which agrees with the palaeogeographical location of the Spiti area between Kachchh and Nepal (Enay and Cariou, 1996). Low diversity of Himalayan faunas is correlated with the domination of only one genus or a small number of related genera in the successive assemblages. The survey of the Peri-Gondwanan ammonites shows a number of peculiar taxa, more or less widely distributed, from the Himalayas to Antarctica and Patagonia, approximatively above 308 south in latitude. Paraboliceras, Kossmatia and Virgatosphinctes of the V. broilii-raja group (Figs. 5 and 6) are clearly peri-Gondwanan, although in New Zealand the stratigraphic range of Paraboliceras and Kossmatia would not be the same as in Nepal (Stevens, 1978, 1997), perhaps because in the respective areas we see only fragments of the total ranges. Dominating genera in the other assemblages show limited (e.g., mayaitids, Fig. 2) or disjunct distribution (e.g., Virgatosphinctids of V. densiplicatus group and Blanfordiceras, Fig. 3). Since the mayaitid occurrences in Antarctica and Patagonia have been questioned (Thomson and Damborenea, 1993; Westermann and Riccardi, 1985; Westermann, 1996a), the Peri-Gondwanan distribution of the mayaitids is doubtful. But no one can argue from the assumed absence of mayaitids in New Zealand (according to Westermann, 1996a,b) because the Oxfordian was thought for a long time to be missing or represented by non-marine strata (Fleming, 1970; Stevens, 1997). Also, the opening of the transgondwanan seaway through the Indo-Malagasian regions is not the only possible explanation for the absence of Virgatosphinctes of the V. denseplicatus group and Blanfordiceras between Papua-New Guinea and Antarctica. The uppermost Jurassic strata do not outcrop or have restricted marine or non-marine facies, without ammonites. For instance, in New Zealand the latest ammonite-bearing beds contain the Aulacosphinctoides fauna (Stevens, 1978, 1997). Beyond Antarctica and Patagonia, the Andean Province also shows Late Jurassic ammonite faunas with relatively higher diversity. As well as the indigenous taxa of Tethyan origin or relationship, Tethyan exotic arrivals are more regular and numerous during

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the Jurassic from the Oxfordian time onwards when the formerly limited Hispanic Corridor widened. The consequence was the end of the East Paci®c Province or Subrealm which reached its height during the Late Bajocian±Early Callovian (Westermann, 1981; Taylor et al., 1984). The frequence of Tethyan components increases from south to north along the Paci®c margin of South America (Enay, 1972; Callomon in Hillebrandt et al., 1992b; Enay et al., 1996) and Damborenea (1993) commented in a similar way concerning benthic bivalves. So, the Peri-Gondwanan ammonite faunas, from the Himalayas to Antarctica and Patagonia, are considered as a single biogeographical unit, probably without such sharp limits as the Boreal and the Tethyan, but enough to be accepted as a third realm, the IndoPaci®c Realm. Our meaning of Indo-Paci®c is di€erent and not so limited as has been the frequently used term Indo-SW Paci®c Province and corresponds to the Indo-Paci®c Province of Stevens (1963, 1965) and Mutterlose (1986, 1992a,b). However, the Indo-Paci®c Province of Stevens (1973) was more extensive and included the whole Paci®c margin of South America.

7. Controlling factors Being at the borderland between the Tethyan and Indo-Paci®c Realms, Andean and Indo-Malagasian faunas show mixed characters. They are comparable to the mixed faunas which, several times during the Jurassic, developed between the adjacent parts of the Tethyan and Boreal Realms, in the Submediterranean and Subboreal Provinces respectively. When adjacent faunas (or biotas) overlap (and mix together) it becomes dicult to divide the two provinces and within the overlapping area the characteristics of the local faunas are time- or spatially-controlled by environmental conditions, which favour either Subboreal or Submediterranean taxa (Hantzpergue, 1991, 1993, 1995). At every age during the Jurassic, Tethyan and IndoPaci®c faunas exhibited less marked contrast than the Tethyan and Boreal faunas. However the low diversity of the Indo-Paci®c faunas, especially those of the highest latitude, contrasts with the relatively high diversity of the faunas at the boundaries of the Tethyan belt (subaustral faunas). Such a low diversity is rather similar to that of the Boreal faunas, whose contrast with the more diverse Tethyan faunas has been recognized for a long time. But in the southern hemisphere the contrast is not so marked and this has justi®ed: (1) rejection of an austral fauna and still more an Austran Realm, as a counterpart of the Boreal Realm; (2) the biogeographical placement of the whole Peri-

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Gondwanan Indo-Paci®c faunas within the Tethyan Realm. Although the contrast between the Indo-Paci®c and Tethyan Realms is not so marked, from the above palaeobiogeographical picture, there results a latitudinal distribution of the Jurassic ammonite faunas, somewhat parallel to those of some other invertebrates and microfossils. The previous survey by Enay (1972) shows a bipolar distribution of the bivalves Buchia s.l. and was supported by bipolarity of shelf assemblages of Jurassic foraminifera (Gordon, 1970) and bipolar distribution of belemnites using the data of Stevens (1963, 1965), supplemented by the intertropical distribution of calpionellids. Later, new examples of bipolar distribution have been described for bivalves, again Buchia s.l. (Crame, 1983, 1986; Sha and FuÈrsich, 1994), as well as `pectinaceans' (Damborenea, 1993) and radiolarians (Pessagno et al., 1993; Kiessling and Scasso, 1996). Crame (1986) accepts only an austral fauna, but an Austral Province (Damborenea, 1993) or Realm (Sha and FuÈrsich, 1994; Pessagno et al., 1993; Kiessling and Scasso, 1996) is also distinguished. Features of the Indo-Paci®c faunas lead again to the question of the existence of a true austral ammonite fauna already proposed for intervals as early as the Tithonian (Enay, 1972), which has long been accepted for the Lower Cretaceous (Fleming, 1967; Stevens, 1971a,b; 1977, 1980a,b). We think that the austral Indo-Paci®c fauna and Realm were di€erentiated from Late Jurassic onwards, after the Late Bajocian±Early Callovian East Paci®c Province or Subrealm (of the Tethyan Realm) disappeared (Westermann, 1981; Taylor et al., 1984). Di€erentiation of the austral Indo-Paci®c fauna and the controlling factors are detailed in Enay and Cariou (1997) and here we give only a summary. The ®rst concern is the less marked contrast between Tethyan and Indo-Paci®c faunas compared with the Tethyan and Boreal Realms. Copious literature deals with the early appearance during the Jurassic of the Boreal Realm and the di€erentiation has been attributed to various causes, especially climate, but this is not the only factor concerned. Following Arkell (1956) and Imlay (1965), physical or palaeogeographical control could have played a prominent part in accounting for the contrast between the Boreal and Tethyan. Indeed, the best-de®ned boreal faunas (Late Bajocian± Early Callovian and Kimmeridgian±Berriasian) resulted ®rst from more or less long-term isolation of pandemic, East-Paci®c or Tethyan organisms trapped in the Arctic Basin during times of extensive shelf ¯ooding in sea-level highstands. That is the `geographic trap e€ect' (Tuchkov, 1973; Pozaryska and Brochwicz-Lewinski, 1975), which resulted from the geography of the Arctic Basin, almost completely enclosed inside emerged lands. And the more or less

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pronounced isolation of the Arctic Basin strengthened the action of the other controlling factors put forward (cooling of surface water, incident radiation, season, illumination changes). Such a land-locked pattern never existed in the Antarctic region. The Peri-Gondwanan platforms were always freely open to the Panthalassa and/or the Tethys. The route from west Tethys towards the eastern Paci®c was fully open only from the Middle-Late Callovian or Oxfordian on and the consequence was the end of the East-Paci®c Province or Subrealm (Westermann, 1981; Taylor et al., 1984). The absence of a geographical trap comparable to that of the Arctic Basin explains why the Peri-Gondwanan faunas do not show as sharp a contrast with the Tethyan faunas as the Boreal faunas. Furthermore, the palaeolatitudinal model of Pessagno et al. (1993) has been modi®ed to accommodate asymmetrical radiolarian distribution north and south of the palaeo-equator (Kiessling and Scasso, 1996). Moreover, the greater extent of the platforms Peri-Gondwanan faunas inhabited explains why these are more diverse, for example between Eastern and Western Gondwanan (e.g., Himalayan-Papuan and Andean faunas). A similar poleward decrease in faunal diversity of the Indo-Paci®c faunas, in spite of less marked contrast with the Tethyan, as well as bipolar distributions, supports latitudinal control, often referred to as climatic (Enay, 1972; Crame, 1986, 1993). Climatic control is fully discussed by Hallam (1973, 1975, 1977, 1984, 1985, 1993, 1994), Crame (1986, 1993) and Doyle (1987). As well as the study of biological or sedimentary palaeoclimatic markers, palaeoclimate simulations or models have been developed (Parrish and Curtis, 1982; Moore et al., 1992a,b; Valdes and Sellwood, 1992; Valdes, 1993). Valdes and Sellwood (1992) only look at the question of ice deposits or ice shields in the Southern Hemisphere. These would require: (1) compression of snow into ice (implying permanent snow cover); (2) estimated predictions of mountains (height and extent) in Australia and Antarctica. There is no evidence for either and Valdes and Sellwood conclude ``snow does not accumulate in high latitudes and low altitudes, despite the low temperatures''. Nearly all authors agree that (1) climatic contrasts were less pronounced in the Jurassic than today with a lower latitudinal temperature gradient; (2) there was polewards cooling of surface sea water; and (3) when faunal distributions are considered, the restricted role of climatic zonation with, especially, the temperature gradient was not the most signi®cant controlling factor. Thus, other factors besides latitudinal control, climatic or not, appear to have controlled diversity. The model of environmental stability (Sanders, 1968; Valentine, 1971; Hallam, 1971, 1973, 1975; Doyle,

1987) is founded on decreasing diversity from more to less stable environments. The less stable environments, owing to constraining life conditions, favour eurytopic and opportunist organisms, often with high density but low diversity. As far as one can determine from the deposits, platforms of divergent margin type from the Himalayas to Papua-New Guinea, as well as the euxinic basin of the eastern Falklands Plateau, covered a wide area with quite unchanged environments. Fine-grained sediments, often organic-rich and with nodule beds, or sapropelic claystones were deposited. Models of Jurassic atmospheric and/or oceanic circulation and upwelling (Parrish and Curtis, 1982; Valdes and Sellwood, 1992; Valdes, 1992; Cottereau, 1992; Cottereau and Lautenschlager, 1994; De Wever et al., 1994) predict seasonal upwelling areas on the southern Tethyan margin, especially the North Indian platform and Northwest shelf of Australia. Deposits are thicker, more distinctly terrigenous and clastic-rich, in New Zealand (Stevens and Speden, 1978) and Antarctica (Crame and Howlett, 1988; Taylor et al., 1979; Thomson, 1972, 1979) situated along an active continental margin. Apart from local palaeogeographic situations such as in the eastern Falklands Plateau related in some way to the initial fragmentation of Gondwanaland and the opening of the South Atlantic (Thompson, 1976; Barker et al., 1976), Jurassic organic-rich deposition around Gondwanaland probably depended on more general conditions such as: seasonal upwelling on the Southern Tethyan margin (the Himalayas±New Guinea belt), high latitude position for the areas in a near-polar or subpolar situation (New Zealand, Antarctica and Patagonia). In Nepal a new study of the organic-rich Spiti Shales facies has established that: (1) ``the total organic carbon is of marine rather than terrestrial origin'', in contrast to the conclusions of Gradstein et al. (1989, 1991, 1992) and Gradstein and von Rad (1991); (2) ``the source of the organic carbon (if planktonic) was not linked with productivity of organisms such as coccolithophorids, but was rather a cellulosic or chitineous phytoplankton (dinocysts) or even siliceous plankton (radiolaria, silico¯agellates)'' (F. Baudin, personal communication, 1992); (3) and ``the palynofacies does not con¯ict with the high production of marine organic carbon derived from cellulosic material and with high primary productivity'' (E. Masure, personal communication, 1992). The organic carbon of Jurassic deposits in the Himalayas±NW Australia±New Guinea is a good indicator of a rich planktonic life in high primary productivity areas, but the presence of plankton in the deposits depends on di€erent burial conditions. The depositional environment of the Spiti Shales in Nepal, in the deep distal part of the platform or the top of

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the slope (Gradstein et al., 1989, 1991, 1992; Gradstein and von Rad, 1991) can explain why the organic carbon was preserved on an anoxic sea bottom associated with upwelling, high planktonic productivity and thus high oxygen consumption. Recent modelling exercises of Jurassic atmospheric circulation, upwelling, and organic-rich rocks concerning Volgian by Parrish and Curtis (1982, Figs. 1 and 2) or Tithonian (Cottereau, 1992; Cottereau and Lautenschlager, 1994; De Wever et al., 1994) and based on the reconstructions of Dercourt et al. (1993), predicted upwelling areas, situated either on the Arabian platform and the Northwest Shelf of Australia during the austral summer, two areas with well known oil resources or potential, whose source rocks are thought to be Upper Jurassic, or on the southern margin of the Tethys, especially on the north Indian shelf. The `High latitude seasonal factors e€ect' interpretation was proposed by Reid (1973) to explain the `Origin of the Mesozoic `Boreal' realm`. Except that in the Arctic Basin these are magni®ed by the land-locked palaeogeography, southern high latitudes are also concerned. Reid (1973) assumed that `a polar zone of seasonal darkness would presumably also have an in¯uence on minimum `Boreal' temperatures' and the `southern temperatures would also have been higher than northern ones'. Palaeoclimate modelling for the Kimmeridgian stage indicates temperatures below zero in the winter hemisphere, especially in the Southern Hemisphere and Eastern Gondwanaland. This low temperature resulted from the large Gondwanan continental land mass and was enhanced by the orography (Valdes and Sellwood, 1992; Valdes, 1993). Reid's hypothesis is directly connected with the importance of trophic resource stability (Valentine, 1971; Hallam, 1972). If the Earth's rotational axis had the same tilt in Mesozoic times as today the seasonal illumination changes would have had the same pattern as present. Seasonal change increased away from the tropics towards the polar zones independently of the global temperature. These ¯uctuations included both: (1) incident radiation reaching high latitudes; (2) the pattern of alternative light and darkness changing from diurnal alternation to seasonal alternation. The two factors directly in¯uenced the whole marine ecosystem because of the e€ects on plankton, and hence all other higher elements of the food chain. Hallam (1973) drew attention to the views expressed by Valentine (1971) and Hallam (1972) on the importance of trophic resource stability as the prime factor controlling latitudinal faunal distributions. The ``high latitude seasonal e€ects'' of Reid (1973) may not alone explain all the faunal distribution patterns; however they played a limiting role for the whole biomass and the structure of the di€erent trophic levels, for instance

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the low diversity of the high latitude ammonite faunas of higher trophic level.

Acknowledgements We are extremely grateful to the people who kindly assisted us in this study. We acknowledge the contributions of Drs B. Vrielynck, who prepared the basic maps, Drs E. Masure and F. Baudin for studying the palynofacies and organic content of the Spiti Shales samples from Nepal, all of the Department of Sedimentary Geology, University P. & M. Curie, Paris. Professor M. Colchen kindly helped us with tectonic problems during the 1991 ®eld trip. Dr G.R. Stevens of the Institute of Geological and Nuclear Sciences of New Zealand kindly supplied xerox copies of some ®gures of his monograph on `The Late Jurassic Ammonite Fauna of New Zealand' before it was out of print. We would also like to thank the two reviewers, Professor F. Cecca (Urbino) and M. Weiss, whose constructive criticisms contributed to better expose and clarify many ideas. Mrs Glynis Leyshon carefully checked the language of the ®nal version. The ®gures were drawn by Mrs A. Armand of the Centre des Sciences de la Terre, Lyon University. The research has been supported by the French `Centre National de la Recherche Scienti®que' and the Universities of Lyon and Poitiers.

References Arkell, W.J., 1956. In: Jurassic Geology of the World. Oliver & Boyd, Edinburgh 806 pp. Avram, E., 1976. Les fossiles du ¯ysch eÂocreÂtace et des calcaires tithoniques des hautes valleÂes de la Doftana et du Tõà rlung (Carpates Orientales). In: Contributions aÁ la paleÂontologie du Jurassique terminal et CreÂtace des Carpates, vol. 24. MeÂmoire Institut GeÂologie GeÂophysique, Bucarest, pp. 5±73. Barker, P., Dalziel, I.W.D., Dinkelman, M.G., Elliot, D.H., Gombos Jr, A.M., Lonardi, A., P¯aker, G., Tarney, J., Thompson, R.W., Tjalsma, R.C., Borch von der, C.C., Wise Jr, S.W., 1976. Evolution of the southwestern Atlantic Ocean Basin, result of the leg 36, Deep Sea Drilling Project. Initial Report Deep Sea Drilling Project 36, 877±891. Bassoullet, J.P., Enay, R., Mouterde, R., 1986. La marge nord-himalayenne au Jurassique. In: Le Fort, P., Colchen, M., Montenat, C. (Eds.), Evolution des domaines orogeÂniques d'Asie meÂridionale (de la Turquie aÁ l'IndoneÂsie), Livre jubilaire Pierre Bordet, Sciences Terre, Nancy, MeÂm. 47, 43±60. Boehm, G., 1904. BeitraÈge zur Geologie von NiederlaÈndisch-Indien. IÐDie SuÈdkuÈsten der Sula-Inseln. Palaeontographica, Stuttgart Suppl. 4, 11±46. Boehm, G., 1908. Geologische Mitteilungen aus dem Indo Australischen Archipel. 6(c) Jura von Rotti, Timor, Babar und Buru. Neues Jahrbuch Mineralogie Geologie PalaÈontologie 25, 324±338. Boehm, G., 1911. Grenzschichten zwischen Jura und Kreide von

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