New and rare Jurassic ammonites have been found in Oman. A latest Bajocian Arabian Platform-type species was discovered in the Haushi-Huqf Massif autochthon of southwestern Oman, and Bajocian species typical of the Mediterranean Tethys and northwestern Europe were found in the Kawr-Misfah exotic unit of the Hawasina Nappes in the Oman Mountains.
The dates provided by the new fauna have resulted in a reinterpretation of the geologic history of the containing rocks, and of their paleoecology and paleobiogeography. It is significant that ammonites from shallow-marine environments of the Arabian Platform are in close proximity to species from open-sea environments of the Mediterranean Tethys and northwestern Europe. This shows that endemism of the Arabian Province resulted from ecological isolation, whereas open-marine environments on the Oman margin, especially the pelagic seamounts off the margin, form part of a migration route between western and eastern Tethys (or Indo-Southwest Pacific), and perhaps far beyond. The occurrences among the Tethyan and pandemic components of ammonite faunas in the Canadian Pacific Cordillera of most of the taxa of the open-marine environments on the Oman margin reopens the question of Pacific biogeography during the Early Jurassic before the Hispanic oceanic corridor was completely open. Among the proposed models, the Pantropic Distribution Model of Newton is examined in the light of the Cretaceous paleobiogeography, with particular reference to rudists.
During the last twenty years, important progress has been made in the understanding of the geology of Oman. Previously, work was directly connected to petroleum exploration and had, as its major interest, the study of lithostratigraphic facies and structures, whereas chronostratigraphy was poorly constrained. Paleontologic and biostratigraphic data were scarce, especially in relation to the macrofaunas. This situation changed as a result of regional geologic mapping by the French Bureau de Recherches Géologiques et Minières (BRGM) for the Ministry of Petroleum and Minerals of the Sultanate of Oman.
From 1982 to 1984, BRGM geologists mapped the mountains of northern Oman at 1:100,000-scale and, from 1990 onwards, they mapped the Haushi-Huqf region, east of Dhofar (Figure 1). Concurrently, data from the field work were used in doctoral theses and papers, of which those by Béchennec (1988), Béchennec et al. (1988, 1989, 1990) and Roger et al. (1992) relate to the areas that are the subject of this paper. Also taken into account are the works of geologists from the universities of Paris 6 and Lausanne concerning the exotic blocks of the Hawasina Nappes (Baud et al., 1990a, b; Pillevuit, 1993; Pillevuit et al., 1997).
During the field work, rare Jurassic ammonites were discovered that have allowed the dating of units with few other biostratigraphic markers. The new dates were used in the 1:250,000-scale geologic maps compiled by BRGM. The maps covered the Haushi-Huqf autochthon of the Mafraq quadrangle (Roger et al., 1992) and the Khaluf quadrangle (Dubreuilh et al., 1992) and the nappes and the autochthons of the mountains of northern Oman that crop out in a tectonic window in the Seeb quadrangle (Béchennec, Roger, Le Métour, and Wyns, 1992) and the Ibri quadrangle (Béchennec, Roger, Le Métour, Wyns and Chevrel, 1992) (Figure 1).
The new dates have contributed to an improved knowledge of the Jurassic evolution of the exotic units of the Hawasina Nappes. However, the ammonite finds have an importance that goes beyond simple improved dating. Although they are not numerous, the Oman ammonite fauna shows adaptations to the conjunction of two quite different environmental conditions. On the one hand, shallow-marine environments on the Arabian Platform were inhabited by ammonite species that were often endemic, whereas the more open-water environment in which the rocks of the Hawasina Nappes were deposited favored ammonites that are identical to those known from the same environments in the Submediterranean Tethyan margins and northwestern Europe.
Recently discovered Jurassic ammonites of Oman and their paleoecology and paleobiogeography are the subject of this paper. These ammonites, discovered by geologists from BRGM and Lausanne University, Switzerland, are used in the development of a new paleobiogeography that is based on paleogeographic and evolutionary frameworks established by the authors listed above.
GENERAL STRUCTURAL AND PALEOGEOGRAPHIC FRAMEWORK
For an historical account of the geology of the area under consideration, reference should be made to the doctoral theses of Béchennec (1988) and Pillevuit (1993) and to the relevant geologic maps of Oman at 1:100,000 and 1:250,000 scales and their explanatory notes.
The mountains of Oman are composed of nappes overlying relative autochthonous rocks and parautochthonous units (the Sumeini Group). According to Pillevuit (1993), thrusting of between 5 and 40 km has taken place. The whole area is overlain discordantly by neoautochthonous units of Middle-Late Maastrichtian to Early Miocene age.
Autochthonous Rocks. The autochthonous rocks crop out within two tectonic windows in the core of the Oman Mountains at Saih Hatat and Jabal Akhdar, and in the Musandam Peninsula to the north (Figure 1). Autochthonous rocks also form small isolated outcrops in the foreland of the nappe structures, and more widely in the Haushi-Huqf Massif of southwestern Oman (Figure 1).
Nappes. The two nappes are the Semail ophiolitic nappe (not dealt with in this paper) and the Hawasina Nappes formed of rock units defined on lithologic, sedimentologic and biostratigraphic criteria by various authors; for example, Béchennec, 1988; Béchennec, Roger et al., 1992; Béchennec, Roger, Le Metour and Wyns, 1992; Pillevuit, 1993; and Pillevuit et al., 1997.
The Hawasina Nappes originated in the Hawasina Basin (Béchennec, 1988) and were thrust onto the margin of the Arabian Platform at the time of the general ophiolite obduction in the Campanian – Maastrichtian. Figure 2 is taken from Pillevuit et al. (1997) and shows diagrammatically the nappes structure and their palinspastic reconstruction, as modified from the work of BRGM.
Paleogeographic interpretation.Glennie et al. (1974) proposed two models for the evolution of the Oman Arabian margin: the Hawasina Nappes would be a structural pile of sedimentary units that accumulated either on oceanic crust (model 1) or on continental crust (model 2).
Model 1 was adopted by Pillevuit (1993) and Pillevuit et al. (1997), with slight changes. In this interpretation, the exotic blocks (or units) were islands built on oceanic crust. Differences between the sedimentary successions in the various units were assumed to be facies changes within a single basin and the proponents did not accept that the so-called Hamrat Duru, Al Aridh and Umar basins were true “basins bounded by topographic swells” (Figure 3a).
BRGM favored model 2. The sedimentary units of the exotic units were interpreted as horsts (Ba’id, Misfah-Kawr) and basins (Hamrat Duru, Al Aridh and Umar) on continental crust, resulting from Triassic extension (Figure 3b).
THE JURASSIC GEOLOGY OF THE AMMONITE LOCALITIES
The ammonites that are the basis of this paper were found in two quite different areas; namely, the autochthonous rocks of the Haushi-Huqf Massif in southwestern Oman, and in the Kawr Group at Jabal Kawr and Jabal Hamrat al Hasan in the Oman Mountains. In addition, Blindinger (1995) mentioned the discovery of two ammonites from the Ba’id exotic unit (or Al Buda’ah Group) of Pillevuit (1993) and Pillevuit et al. (1997), but the author was unable to obtain these for study during the present investigation.
Jurassic rocks of the autochthonous Haushi-Huqf Massif
The autochthonous rocks that crop out in a tectonic window beneath the Hawasina Nappes did not yield Jurassic macrofaunas. In general, the Haushi-Huqf Massif proved to be much less fossiliferous than similar rocks in central Saudi Arabia where more than 1500 ammonites were studied by Énay and Mangold (1985, 1994), Énay et al. (1986) and Énay, Le Nindre et al. (1987).
Jurassic autochthonous succession
This is relatively thin succesion of about 100 to 200 m that is bounded by two discontinuities. It is divided into the Mafraq and Dhruma Formations; in Oman, ammonites are found only in the Mafraq Formation:
The Mafraq Formation disconformably overlies sandstones of the Minjur Formation that is assumed to be of Triassic age but its upper part could be Early Jurassic (Figure 4). The discontinuity between the Mafraq and the underlying Minjur Formation is probably linked to a sedimentary hiatus of unknown duration.
The overlying Dhruma Formation is truncated by an erosion surface and overlain by transgressive shallow-marine deposits of the Thamama Group. No ammonites have been found in the Dhruma Formation in Oman. The Thamama Group is a lateral counterpart of the Kamak Group in the Mountains of Oman that contains a pelagic facies dated by calpionellids as lowermost Cretaceous (Berriasian – Valanginian) (Béchennec, 1988).
The two formations consist of shallow-marine Arabian Platform deposits. The Mafraq Formation is about 50 m thick. The lower part consists of four sequences of thin basal marls grading into argillaceous limestones, each bounded at the top by a bored hardground. Above, are calcarenitic beds interbedded with argillaceous limestones. The Dhruma Formation is formed of two sequences each 30 m thick. At its base is an alternation of micritic limestones, in places with marls, and fine grained calcarenites that grade upward into a massive body of coarse-grained ochre-colored limestone. The basal part of both sequences contain dolomitic beds, and bioturbated layers and cherts are present throughout the formation.
Macrofauna and ages
The fauna is similar to that found in Saudi Arabia but is less rich and diverse.
Lower Mafraq Formation. In the Haushi region, immediately above the highest hardground, a single ammonite, plus brachiopods, bivalves (oysters) and corals (samples SC 1458B and JPP 400B) were found (Figure 4). The ammonite, Thambites planus Arkell, is a species frequent in, and characteristic of, Unit D3 of the Dhruma Formation in central Saudi Arabia (Énay et al., 1986), and is found also in Egypt (Sinai) and Morocco (Énay, Mangold et al., 1987). The age is latest Bajocian, approximately the Parkinsoni Zone in northwestern Europe (Énay and Mangold, 1985, 1994; Énay et al., 1986; Énay, Le Nindre et al., 1987) equivalent to the Planus Zone in the zonal scale proper for the Arabian Province (Énay and Mangold, 1994) (Table 1). The same age was used by Manivit et al. (1990). The brachiopods, for example, Burmirhynchia sp. and Daghanirhynchia sp., were studied by A. Boullier (written communication, 1992) but few are chronologically significant. The exception is Eurysites transversus Cooper, a species described in Saudi Arabia from Unit D3 of the Dhruma Formation up to the Callovian Tuwaiq Mountain Limestone (Cooper, 1989).
Dhruma Formation. The formation was dated as Bathonian to Callovian by Roger et al. (1992) on the basis of the numerous and diverse echinoids, corals and benthic foraminifers in the calcarenite beds. Brachiopods (A. Boullier, written communication, 1992) are common in the upper part of the formation in the Haushi region (sample JPP 2586) and include Burmirhynchia moulani Alméras and Cererithyris wylliei Weir, two species known from central Saudi Arabia in the faunal (brachiopod) Assemblage 3 of Alméras (1987) and Alméras et al. (2010). Daghanirhynchya cf. subversabilis Weir is a brachiopod species transitional to Assemblage 4. It has an extended vertical range but is not common above the Micromphalites Beds that are equivalent to the Clydocromphalus Zone (Énay and Mangold, 1994) of Unit D5 of the Dhruma Formation dated as Early Bathonian. In the Saiwan region Burmirynchya decorticata Cooper (sample JPP 413F) gives the same age. The species assumed to be possibly Heteromychus magnificus Cooper (sample JPP 546) (Roger et al., 1992; Dubreuilh et al., 1992) could be of a later age as the figured specimens from Saudi Arabia from Unit D6 and the lower part of Unit D7 (Atash Member) are of Bathonian age. According to Alméras (1987) and Alméras et al. (2010), H. magnificus is equivalent to B. nazeri, one of the species characteristic of Assemblage 4, but A. Boullier (written communication, 1992) suggests that the Oman specimens are likely to be included in Lessiniella Vörös, a new genus of Callovian age from the southern Alps of Europe. Therefore, a Late Bathonian – Middle Callovian age for the Dhruma Formation is probable. A complete discussion on nomenclature, taxonomy and stratigraphy of the brachiopod fauna of Saudi Arabia, including the Cooper species, may be found in the recent monograph by Alméras et al (2010).
Autochthonous units of the central part of the Oman Mountains (Jabal Akhdar). The Oolitic Member of the Sahtan Group yielded a brachiopod fauna that was dated as Early Bathonian to Middle Callovian by Rousseau et al. (2005). The fauna includes the two species Daghanirhynchya subversabilis Weir and Cererithyris bihiniensis Weir that are transitional to Assemblage 4 of Alméras (1987) and have large vertical ranges. However, whereas D. subversabilis is rarely found above unit D5 (Micromphalites Beds), the most widespread development of C. bihiniensis is in unit D7 (Hisyan Member). A third species, D. daghaniensis Muir-Wood is almost totally and mutually exclusive of D. subversabilis (Y. Alméras, oral communication) and it is one of the species characteristic of Assemblage 4 of Alméras. This assemblage fits with the renewal of brachiopod fauna related to the reversal toward more open-marine conditions controlled by a rise in relative sea level during Late Bathonian to Callovian times. Rousseau et al. (2005) interpreted this layer to be the maximum flooding surface of their middle-scale genetic sequence II. It is correlated with Maximum Flooding Surface (MFS) J30 of Sharland et al. (2001) that is dated Early Bathonian – Middle Callovian in Central Saudia Arabia. It is probable that the sea-level rise took place in Late Bathonian to Middle Callovian times. In the upper part of the Oolitic Member of sequences III and IV of Rousseau et al. (2005), Septirynchia sp., a brachiopod genus known from Tunisia to the Arabian Peninsula from the Middle Callovian onward, is common together with benthic foraminifera.
The evidence suggests that the onset of marine conditions on the central Oman platform occurred later than in Central Saudi Arabia where it took place as early as the Early Toarcian (Bouleiceras beds). In central Oman, the maximum flooding of the transgressive phase began in the Early Bajocian (T/R7 Cycle of Jacquin et al., 1998; Hardenbol et al., 1998; and MFS J20 of Sharland et al., 2001). Its occurrence was later compared to northern Oman where it started even earlier than in Saudi Arabia. In central Oman, the Pliensbachian Lithiotis Limestone, the lowest unit of the Jurassic Sahtan Group, is overlain by the Toarcian – Bajocian (in part) Terrigenous Member that shares similarities with the upper Mafraq Formation in southwestern Oman, and the Oolitic Member that is of Bajocian to part-Callovian in age.
The Dhruma Formation in southwestern Oman corresponds to the Early-Middle Bathonian regressive phase, in accordance with the age given by brachiopods. An important pre-Late Tithonian erosion is responsible for the absence of Late Jurassic beds from ?Callovian upward.
Jurassic of the Kawr and Misfah exotic blocks
The Kawr Group (Béchennec, 1988) was included in the Oman exotics of Glennie et al. (1974). The Kawr and Misfah exotic blocks (as well as Jabal Hamrat al Hasan and others) are allochthonous units of platform and pelagic deposits of Late Triassic to Late Jurassic – Early Cretaceous age that overlie oceanic volcanics. Following Pillevuit (1993), the Kawr Group defined by Béchennec (1998) includes a basal Misfah Volcanic Unit and five formations; from the base up they are the Subayb, Misfah, Fatah, Nadan and Safil Formations.
Jurassic ammonites were found at two localities in the Kawr Group; namely, Jabal Kawr and Jabal Hamrat al Hasan (also known as Hawrat al Asan). The samples were collected by Baud and Marcoux during 1990 and by Béchennec and Chiron in December 1991 (see Baud et al., 1990b; Béchennec, Roger, Le Métour and Wyns, 1992; and Pillevuit et al., 1997). The beds in which the ammonites were found were ascribed firstly to the base of the Nadan Formation (Béchennec, 1988) and dated by calpionellids as Late Tithonian to lowermost Valanginian; later they were attributed to the upper part of the Misfah Formation (Béchennec, Roger, Le Métour and Wyns 1992), whose upper member is dated as Late Triassic. As a result of the later age designation, Pillevuit et al. (1997) proposed a new formation that they named the Fatah Formation.
Fatah Formation (Figure 5)
The lower contact with the underlying Misfah Formation is a manganiferous hardground containing belemnites and ammonites that locally displays erosive features (microkarst). Passage to the overlying Nadan Formation is transitional and Pillevuit (1993) defined it at the first development of cherty nodules or discontinuous layers in well-bedded (5–20 cm) light-grey micritic limestones containing radiolarians and aptychus, that are similar to the “Maiolica” facies (Béchennec, 1988; Béchennec, Roger, Le Métour and Wyns 1992) or the “Calcari ad Aptici” facies (Pillevuit, 1993).
The Fatah Formation has an average thickness of 1 to 6 m. Its characteristic Tethyan Rosso Ammonitico lithofacies distinguishes it from the underlying Misfah Formation. It commonly displays alternations of grey or pink limestones, and condensed surfaces containing eroded and generally unrecoverable ammonites. Locally, the formation is reduced to a breccia 1 m thick consisting of limestone fragments 1 to 2 cm in size together with belemnites within a red calcareous matrix in microkarst overlying the Misfah Formation (Figure 6). Elsewhere, the formation consists of graded beds, and microbreccias or well-laminated red limestones with lithoclasts.
Fauna and depositional conditions. The macrofauna of belemnites and ammonites, the microfauna of sacoccomidae, and the lithofacies indicate that the Fatah Formation is a condensed deposit affected episodically by non-depositional events. The depositional environment was pelagic, but not necessarily deep water. Ammonites are preserved as internal moulds as the shells were destroyed during diagenesis as is common in carbonate deposits of the Tethys and its margins, including shallow environments. Partial removal of the upper part of the ammonite shell that Pillevuit (1993) put forward as evidence supporting a location, “probably (…) very near the aragonite dissolution limiting value”, is a normal feature (e.g. subsolution) well known in the Rosso Ammonitico facies [cf. Farinacci and Elmi, 1981], at the seawater/sea bed contact. The Tethys environmental maps of Dercourt et al. (1993) show that Rosso Ammonitico facies occurs on distal platforms or pelagic swells and deep basins above the Carbonate Compensation Depth.
Age of the Fatah Formation. The age of the formation is well defined by ammonites that were instrumental in defining it as a new formation, as follows:
The single specimen from Jabal Hamrat al Hasan is a species that appears similar to Docidoceras cf. transiens (Bremer) (see Figures 5 and 7e) of Early Bajocian age (Laeviuscula and Propinquans Zones)1 (see Table 1)
The Jabal Kawr fauna, Ptychophylloceras sp. aff. chronomphalum (Vacek), Polyplectus discoides Zieten, Pseudomercaticeras sp. aff. frantzi (Reynès) (Figures 7b, c, and d, respectively), and Phymatoceras sp., occurs at the boundary between the Bifrons Zone and the Thouarsense and Insigne Zones of the Middle and Late Toarcian (Elmi et al., 1997) (Table 2).
The Jabal Kawr area was visited later by the late J. Marcoux and P. Moix, but both the facies and content of the specimens collected by them prove that the sampled outcrop was not the one first visited by Béchennec in 1991. Fragmentary ammonites are numerous but of little use for dating purposes except for a few phylloceratids and lytoceratids and two or three specimens with falcoid ribbing of harpoceratid type from which a middle to late Lower Jurassic age was inferred. From a single specimen with compressed whorl, flat sides and carinate-bisulcate venter, similar to the Fuciniceras or Protogrammoceras genera, a more precise Late Pliensbachian – Early Toarcian age was obtained.
JURASSIC AMMONITES FROM THE HAUSHI-HUQF MASSIF AND THE OMAN MOUNTAINS (JABAL KAWR AND JABAL HAMRAT AL HASAN)
The ammonites found in Oman belong to well-known and well-studied species. Although few in number compared to discoveries in Saudi Arabia, their importance is related to their affinities to either Arabian Province or European-type fossils. Because of this, they are described in detail below. The present investigation has resulted in the revision of many of the previous, provisionary paleontological determinations.
Arabian Province fauna from the Haushi-Huqf Massif
Thambites planus Arkell (Figure 7a)
A single specimen of Thambites planus Arkell (JC 1458B) was collected by Stéphane Chevrel from the Mafraq quadrangle in the northern part of the Haushi-Huqf Massif in southwestern Oman. Although Roger et al. (1992) in the explanatory notes to the geology of the Mafraq quadrangle mentions only “impressions” of the species, the specimen from the Mafraq Formation examined in the present study (FSL 162837) is nearly complete. One side is well preserved, retaining a quarter whorl of the living chamber but without the peristome.
The genus consists of two species, T. planus and T. oxynotus that were first described by Arkell (1952) from Jabal Tuwaiq in Central Saudi Arabia and the genus is also known from Egypt (Sinai), Levant and Morocco. New and more numerous specimens have been collected from Arabia by Énay et al. (1986), Énay, Le Nindre et al. (1987) and Énay and Mangold (1994). The Oman specimen belongs to the planus species that is distinguished from T. oxynotus Arkell by a narrower ventral furrow at the same diameter and a subradial rectiradiate suture line (convex in T. oxynotus). The lack of ribs or growth lines, emphasized by Arkell, was the result of the poor preservation of the original specimens, and they are present in the new material from Saudi Arabia and in the Oman specimen.
In Saudi Arabia, the Thambites fauna characterize unit D3 of the Dhruma Formation (Énay et al., 1986; Énay, Le Nindre et al., 1987; Manivit et al., 1990). Following modification of the biochronological standard used by Arkell (1952) and Imlay (1970), Thambites has been reappraised as belonging to the Late Bajocian Planus Zone that is approximately equivalent to the Parkinsoni Zone in northwestern Europe (Énay and Mangold, 1994) (Table 1).
European-type fauna from the Fatah Formation at Jabal Kawr
All the studied specimens are derived from a single sample (FRE 3312A) collected by François Béchennec in red limestone from the hardground overlying platform carbonates of the Norian Misfah Formation (see Figure 5). The sample showed only one ammonite (Polyplectus discoides). However, while the specimen was being prepared, other ammonites were discovered and the assemblage now consists of six complete or fragmentary specimens of which five are specifically identifiable. This concentration within one rock sample may mean that more intensive examination of the outcrop might have yielded an even richer assemblage. The author recommended additional work on the outcrop in a report to BRGM dated January 22, 1992, but the suggestion was not followed up. The late J. Marcoux and P. Moix visited the area in 2008 but did not find the outcrop.
Ptychophylloceras (Tatrophylloceras) cf. chronomphalum (Vacek) (Figure 7b)
One fragmentary specimen (FSL 162831) and one incomplete specimen (FSL 162832) were examined. The latter has only half a whorl and one side preserved, the opposite side of which bears the imprint of Polyplectus discoides (see Figure 7c; as described later). Suture lines are partly discernible on the more complete specimen beneath the thin black layer of possible manganese that encrusts the internal mould.
Coiling is involute and the umbilicus is narrow, the whorl section compressed, the sides flat or slightly convex without conspicuous umbilical margin, and the ventral area is rounded. The internal mould has four to five shallow furrows that are inclined forward without any backward inflexion on the whorl side and that are bordered by a labial ridge or flare on the ventral side and in the ventral area. The general shape is similar to P. tatricum (Push), the first proposed specific name, but it corresponds more to P. chronomphalum (Vacek), especially with respect to the rectiradiate lateral constictions. Moreover, Joly (2000) ascribed to the Vacek species the specimen figured by Rulleau (see Alméras et al., 1998, pl. 21, fig. 3) as P. aff. tatricum (Push). According to recent reviews by Joly (2000) and Rulleau (1991; in Alméras et al., 1998), P. tatricum is a species restricted to the Aalenian (Opalinum and Murchisonae Zones) whereas the vertical range of P. chronomphalum is greater, from Middle-Upper Toarcian to Early Aalenian.
Polyplectus discoides (Zieten) (Figure 7c)
Specimen FSL 162833 is a well-preserved, almost complete example of Polyplectus discoides. It has a current diameter of 68 mm, the two sides are well preserved and it is almost totally chambered but shows the beginning of the living chamber.
Description. The general shape is discoidal and involute, and it has a narrow umbilicus. The whorl section is tightly compressed and lanceolate, the side convex from the umbilical margin, the venter acute and bordered by two flat parts bounding a keel showing a subtle creneling where well preserved. The sides bear falcoid ribs that are numerous (30 on the last quarter whorl) and delicate. They are as broad as the dividing space between them, scarcely or poorly visible on the inner half of the side, and strongly projected near the outer margin where they give rise to subtle creneling on the ventral keel. On the median side, bulges including several ribs form wide undulations on the shell surface. An elaborate and highly complex suture line is partially visible on the side opposite to the one shown in Figure 7c.
Variability and polymorphism. Accepting the assumption by Geczy (1967) that the ornament on the inner half of the whorl and the shell surface undulations are “irrelevant characters to P. discoides”, the Oman specimen would be P. apenninicus (Haas). However, it seems that Geczy was unaware of the work of Dubar and Mouterde (1965) who concluded that P. apenninicus (Haas) and P. discoides (Zieten) were variants of the same species. They distinguished the following forms:
A “paucicostate form”, with “flat and numerous ribs” (62 ribs at d = 68 mm), without shell surface undulations (Dubar and Mouterde, 1965, pl. 1, fig. 2). The form includes the Zieten type refigured by Howarth (1992, pl. 28, fig. 7) and previously illustrated by d’Orbigny (1845, pl. 115, fig. 1) and a new figure by Mouterde and Gabilly in Fischer et al. (1994, pl. 35, fig. 3). This form also includes specimens from the Canadian Pacific Cordillera (Jakobs, 1997, pl. 4, fig. 14);
A “multicostate form” (Dubar and Mouterde, 1965, pl. 1, figs. 3–6; Guex, 1975, pl. 7, fig. 8) that is more similar to P. apenninicum (Haas), having subtle and dense ribbing (135 ribs at d =100 mm). Radial shell surface undulations are common but as they could “appear or disappear in the same specimen”, Dubar and Mouterde (1965) did not attach to them the same value as Geczy (1967).
The Oman specimen is of the multicostate variety.
Distribution of the genusPolyplectus. The genus contains species known from the Tethyan region For example, P. pleuricostatus (Haas) from the Early and Middle Toarcian has a Mediterranean distribution that ranges from Portugal to the Tyrol (Dubar and Mouterde, 1965). According to Howarth (1992) it is also present in the Pacific Coast Ranges of North America. He assigned to the species the specimen identified as ‘P.’ aff. subplanatus (Oppel) by Hall (1987, pl. 3, fig. A–C) and Hillebrandt (1987, pl. 8, fig. 2–4). Previously, Spath (1936) identified from Pakistan (Baluchistan), “a specimen near of the type species P. discoides and varieties”, that he described as similar to the older species of the genus. This would agree with the joint fauna of Phylloceratids, Lytocerid, Bouleiceras, Dactylioceras, Fuciniceras and Protogrammoceras mentioned by Spath. Arkell (1952) identified another specimen from Baluchistan as similar to Early Toarcian specimens from Portugal first assumed to be P. discoides by Renz (1912) that were later assigned to P. pleuricostatus by Dubar and Mouterde (1965). Yin and Zhang (1996, pl. 1, fig. 1) attributed to P. discoides a probable wholly septate specimen from the Nyalam area of southern Tibet. A more recent and better photograph has shown that the specimen is, indeed, P. discoides. The Nyalam specimen is closely related to the “paucicostate” form, whereas a more recently published specimen from northern Tibet (Hui-nan and Wen-kai, 2001, pl. 3, fig. 20–21) is similar to the multicostate form.
The widest distribution within the genus Polyplectus is of the type species P. polyplectus. The genus was acknowledged as being pandemic by Dommergues (1994). It is present on the northern Tethyan margin in Europe. It is found in England (Howarth, 1992) and in the Poitou region of France (Gabilly, 1976a, b) but is not common. It is more widespread in Portugal (Dubar and Mouterde, 1965) and in the limestone plateaus of France in the Causses region of the Massif Central (Monestier, 1931; Guex, 1975), and in the Lyons area (Rulleau, 1989; in Alméras et al., 1998).
The species is also found in the coastal ranges of North America (Jakobs, 1997; Jakobs et al., 1994a, b) and South America (Hillebrandt, 1987; Riccardi et al., 1990) and possibly in Japan. In Oregon, USA, Polyplectus cf. subplantus figured by Imlay (1968) was identified as Polyplectus sp. by Howarth (1992). The situation with regard to its presence in Japan is confused. Sato and Westermann (1991) placed within the Polyplectus genus the species described as Harpoceras okadai Yokoyama (Yokoyama, 1904; Sato, 1957, 1964; Hirano, 1971). However, the type specimen of H. okadai, when studied by Hirano (1971, pl. 20, fig. 1) together with another better preserved specimen (Hirano, 1971, pl. 20, fig. 2), are too evolute for inclusion within the Polyplectus genus. Other figured specimens, especially those of Yokoyama (1904, pl. 20, figs. 3, 5 and 9) and a large one of Sato (1957, pl. 2, fig. 20) do not belong to H. okaidi. Probably the assignment to Polyplectus was based on these specimens being closely related, if not conspecific, with P. discoides. These included Dubar and Mouterde’s “paucicostate form” (Hirano, 1971, pl. 20, fig. 5) and the “multicostate form” (Hirano, 1971, pl. 20, fig. 3 and 9; Sato, 1957, pl. 2, fig. 10).
Stratigraphic range (Table 2).P. discoides, as a member of the subfamily Harpoceratinae, has an exceptionally long stratigraphic time range throughout the Late Toarcian. The longest accurately known stratigraphic range is from the uppermost Thouarsense Zone to the lowermost Aalensis Zone (Rulleau, 1989 and in Alméras et al., 1998; Rulleau and Elmi, 2001) in the Lyons area of France. In the Causses region of France (Guex, 1975), P. discoides ranges from the Middle Thouarsense Zone (Thouarsense (in part), Fascigerum and Fallaciosum Subzones) up to the lowermost Dispansum Zone (Insigne and Gruneri Subzones) (Table 2). Also in France in the Poitou region, P. discoides is present in the Pseudoradiosa Zone, Levesquei Subzone (Gabilly, 1976ab), and in England it occurs in the Thouarsense Zone, Fallaciosum Subzone and the Levesquei Zone, Dispansum Subzone (Howarth, 1992). According to Elmi et al. (1986), P. discoides would be present earlier and especially numerous in the Variabilis Zone and its Submediterranean counterpart, the Gradata Zone, at the top of the Middle Toarcian. However, at the same stratigraphic level in the High Atlas of Morocco, Dubar and Mouterde (1965, p. 88) described specimens with a morphology intermediate between P. pleuricostatus (Haas) and P. polyplectus (Zieten).
In the Canadian Pacific Cordillera, subject to correlation with the northwestern European standard zonation, P. discoides (Jakobs, 1995, p. 14, fig. 14) would occur at the Middle/Late Toarcian boundary, in the Crassicosta and Hillebrandti Zones that are approximatively coeval with the Variabilis/Gradata and Thouarsense Zones (Table 2). In Japan, the age of the specimens assumed to be close relatives of P. discoides has not been established with confidence. According to Sato (1964) and Sato and Westermann (1991), they would occur in Lower Toarcian – lower Middle Toarcian beds, with a possible range to Upper Toarcian. Tibetan specimens (Yin and Zhang, 1996; Hui-nan and Wen-kai, 2001) are ascribed a Toarcian age but without supporting evidence.
Pseudomercaticeras sp. aff. frantzi (Reynès) (Figure 7d)
The single specimen (FSL 162834) was first named as Podagrosites sp. (Béchennec, Roger, Le Metour and Wyns, 1992; Béchennec, Roger, Le Métour, Wyns and Chevrel, 1992). It is a little more than half an individual with a quarter whorl belonging to the living chamber. It displays only one well-preserved side together with the impression of a whorl fragment identified as Phymatoceras sp.
Coiling is evolute, the umbilicus is wide and rather deep, the umbilical area is steep and is well defined by an angular umbilical margin. The whorl section is thick (E/H = 0.99 at d = 54 mm) and subquadrate, but the thickness decreases towards the venter. The sides are convergent ventrally and the venter is carinate bisulcate with two well-defined lateral furrows. Ribs are simple, rectiradiate and strongly developed on the inner whorls, and falcoid on the outer whorl or living chamber. They are poorly visible on the dorsal part of the side walls. The bundled or fasciculate ribs are stronger and project onto the ventral area.
The Oman specimen is best identified with the Mediterranean genus Pseudomercaticeras rather than with Podagrosites. However, problems with the taxonomy of hildoceratids at the Middle/Late Toarcian boundary are made more difficult, according to Elmi et al. (1986), because of associated homeomorph ontogenic stages together with strong intraspecific variability. Elmi et al. (1986) noted that the characteristic features of Pseudomercaticeras are ventrolateral furrows that are well pronounced on the living chamber and are responsible for the tricarinate bisulcate whorl section, together with poorly pronounced ornamentation on the inner part of the side wall and many intercalatory ribs. Taking these features into account, the Oman specimen is equated with P. frantzi (Reynès) (Elmi et al., 1986, pl. 1, fig. 7). Except for its larger size, the specimen belongs to a group known from the Rosso Ammonitico of the central Appennines in Italy that were studied by Merla (1933) and Venturi (1972). Among the figured specimens, those ascribed to P. cf. frantzi (Reynès, 1868) (Merla, 1933, pl. 5, fig. 12) and P. rotariesMerla (1933, pl. 5, fig. 4–6; and Venturi, 1972, pl. 41, fig. 6, 8, 10, 14 and 18) are the closest to specimen FSL 162834.
The genus Pseudomercaticeras is especially common in the uppermost Mercati Zone of the zonal scheme used by Venturi (1972). The Mercati Zone corresponds to the Bifrons Zone (Middle Toarcian; Table 2) of the standard accepted in the Mediterranean Province and in northwestern Europe (Elmi et al., 1994; 1997) except that the genus in northwestern Europe is less common and occurs in the Variabilis Zone (see Table 2). P. cf. frantzi from the Canadian Pacific Cordillera (Jakobs, 1997, pl. 13, fig. 4–7) occurs in the Crassicosta Zone that is more or less equivalent to the upper Middle Toarcian Variabilis/Gradata Zones (Jakobs et al., 1994).
? Phymatoceras sp.
The large fragmentary specimen (FSL 162835) was found on the reverse of the specimen, Pseudomercaticeras sp. aff. frantzi. It has only one side preserved, the ventral part is absent, and the side slightly convex and without an umbilical margin. Seven rectiradiate, broad and top-rounded ribs are preserved, which tend to disappear towards the ventral area. The specimen is similar to the adult morphology of the large Phymatoceras, but as its ventral area is lacking, its generic attribution remains uncertain.
European-type fauna at Jabal Hamrat al Hasan
Docidoceras (Docidoceras) cf. transiens (Bremer) (Figure 7e)
This is the only ammonite found in the Rosso Ammonitico facies beds near the base of the Fatah Formation at Jabal Hamrat al Hasan (see Figure 5). The specimen was previously mentioned by Pillevuit et al. (1997) as Emileia sp.
The specimen (FSL 162836) is oval, having been slightly distorted, and is divided into two parts by a calcite vein and the two are slightly displaced. It is complete and although the peristome is crushed it is well discernable. The living chamber is more than a whorl long. Preservation is not good. Most of the ribbing is obliterated on the side shown in Figure 7e and is hardly better preserved on the middle whorls of the opposite side.
The inner whorls are cadicone, thick and depressed and the umbilicus is relatively large and deep. The middle and outer whorls are more evolute, with an excentric living chamber. The whorls section are at first depressed, then more compressed and subcircular toward the end of the living chamber. The ribs are numerous and fine and regularly branch just above mid-side. Rib thickenings or tubercles are not obvious at the bifurcation points, but preservation is not good enough to make a considered judgement and it is the same concerning the number of secondary ribs.
The Oman specimen, first placed among the densely and finely ribbed Emileia genus, is very similar in appearance to the specimen figured by Sandoval (1983, pl. 1, fig. 2) as Docidoceras (D.) transiens (Bremer). One of the features distinguishing Docidoceras from Emileia is the evolute coiling. The specimen figured by Sandoval and the one from Oman are clearly more involute than the other species of Docidoceras and so are closer to Emileia, for instance E. aff. contrahens Buckman (in Sandoval, 1983, pl. 8, fig. 1). Although it is not clearly expressed, the generic choice by Sandoval could have resulted from the stratigraphic position (Aalenian – Bajocian transition beds) assumed for the two specimens that have been found outside the described sections. Another possibility is that the specimen could be an older form of Emileia.
Within the Docidoceras-Emileia lineage, Docidoceras is especially common at the base of the Bajocian in the uppermost Discites Zone, Subtectum Subzone and as far as the Laeviuscula Zone, Ovalis Subzone (see Table 2). From there it is replaced progressively by Emileia that reaches its maximum frequency and dies out in the Propinquans Zone.
Conclusions regarding the ages of the Haushi-Huqf and Oman Mountains ammonites
In conclusion, the few ammonite data available from Oman give precise ages of:
(a) Late Bajocian for the single Arabian Platform ammonite from the Haushi-Huqf Massif.
(b) Latest Middle – Late Toarcian at Jabal Kawr and Early Bajocian at Jabal Hamrat al Hasan, that relate to the Tethyan and northwestern Europe types of the Hawasina Nappes.
(c) With less certainty, Late Pliensbachian to Early Toarcian for the most recently discovered but poorly preserved specimens from the Jabal Kawr area.
In addition to their dating significance, the Oman ammonites are important because of the juxtaposition of genera and/or species that inhabit quite different environments. On the one hand are those from the Arabian Platform and Arabian Province, and on the other those from the Kawr/Misfah exotic unit that are taxa of the Mediterranean Tethys and northwestern Europe.
ENDEMISM OF ARABIAN PLATFORM FAUNAS AND THE OFF-SHORE MIGRATION ROUTE TOWARD THE EASTERN TETHYS
The particular features of the total Jurassic fauna (not only ammonites), from the Arabian Platform allowed the identification of an Arabian Province (also known as the Ethiopian or Trans-Erythrean Province), from as early as the Toarcian Bouleiceras fauna. Differences with the European faunas are so great that even today there is no clearly defined equivalence with Mediterranean and Submediterranean standards (Énay et Mangold, 1994; Énay et al., 2007, 2009).
Concurrently, a migration route along the southeastern margin of the Tethys was necessary as faunas with Tethyan features or origin are present in the Indo-Malagasian embayment (eastern Africa, Malagasy, Kachchh on the western edge of the Indian Plate) and as far east as the Himalayas and Papua New Guinea. In contrast, within the eastern Mediterranean Tethys, the Tethyan faunas are not found beyond Sicily, the Ionian Islands and the Malta fault scarp. However, from the Malta fault scarp to southeastern Turkey (Hezan-Abdulaziz), outer-platform environments during the Jurassic were almost totally absent.
Ecological control of Arabian Province fauna
In order to explain these prominent faunal differences, some authors assumed that Jurassic Arabian faunas were isolated from Tethyan faunas of the same age by a geographical or ecological barrier (Gill et al., 1985). For example, Thierry (1976) in a study of the global paleobiogeography of the Middle-Late Jurassic Stephanoceratacea identified a large landmass extending from the south of the Arabian Peninsula as far as southern Turkey that isolated Central Saudi Arabia from the eastern Arabian margin and the Zagros Mountains.
The numerous data on the Jurassic rocks of the Arabian Plate obtained from outcrops and subsurface (Manivit et al., 1990; Sharland et al., 2001) run counter to a possible physical barrier responsible for the isolation and endemism of the Arabian faunas. Jurassic paleoenvironmental maps of the Tethys (Dercourt et al., 1993) and Peri-Tethys (Dercourt et al., 2000) show that shallow-carbonate or detrital platform environments graded into distal platforms and beyond the continental slope to deep-basin conditions.
Ecological control is one explanation for the isolation and endemism of the Arabian faunas, well illustrated in Oman by faunas cohabiting in quite different environments. Endemism of the Arabian faunas resulted from the persistence during the Jurassic (and beyond) of very shallow platform environments to which the Arabian taxa were adapted. For example, during Toarcian to Kimmeridgian times when several successive endemic ammonite populations evolved, open-water environments did not exceed distal infratidal conditions (Early Bajocian) and more commonly consisted of protected middle (outer lagoon) to proximal infralittoral conditions (Énay et al., 1986; Énay, Le Nindre et al., 1987; Le Nindre et al., 1990).
The lack of a physical barrier and the importance of ecological (or environmental) control are proved by the episodic occurrences of Submediterranean European taxa associated with the prevailing Arabian forms and, conversely, the presence of Arabian taxa as far west as Sicily (Galacz, 1999), Algeria and Morocco (Énay, Mangold et al., 1987), Spain (Fernandez-Lopez, 2000) and France (Énay, 1993; Énay et al., 2001).
Such cross influxes of faunas improved the correlations between the faunal records and biochronological scales for each area. Such faunal exchanges were probably made easier by sea-level changes, and these correspond well with highstands in the Early Toarcian, Early Bajocian and Middle Callovian, which resulted in the enlargement of platform environments and the development of shallow-marine deposits (Énay, 1980; Westermann, 1993). With respect to Arabian faunas adapted to very shallow-marine platforms, their development, normal or episodic, was dependant of the prevailing environmental conditions (Énay, Mangold et al., 1987; Énay, Le Nindre et al., 1987). Such episodic expansions proved they were no impediments for migration out the Arabian Province other than control by the environments that may or may not have favored their expansion. It was the permanence of shallow-marine environments on the Arabian Platform during the whole of the Jurassic Period that was responsible for the endemism and particular features of Arabian faunas (for example, the maintenance process of Rosen, 1992).
Migration route to the Eastern Tethys off the Arabian Platform
The Kawr/Misfah exotic unit is important because of its European-type fauna. It emphasizes the faunal contrast versus paleoenvironments and the effects of ecological control on populations. It was also a milestone along the migration route between the Mediterranean and the Eastern Tethys, in particular during the Middle Toarcian (or earlier) to Early Bajocian, when outer-platform environments were suitable for the expansion of ammonites. Indeed, recent data on the Jurassic rocks in the Kawr/Misfah exotic unit have resulted in the revision of the paleoenvironmental maps of the Toarcian and Callovian of the Tethys and Peri-Tethys (Dercourt et al., 1993, 2000), as well as, but to a lesser extent, the Early Kimmeridgian. By Toarcian times the deep-marine depositional conditions of the Fatah and Nadan formations was established with an intermediate transitional phase (see Figure 5). As a result, the symbol “pelagic rise” should be substituted for “exposed land” and “non-deposition or erosion on exposed land” on the Toarcian map and for “shallow carbonate platform” on the Callovian map. Similarly, “pelagic carbonate with cherts” should replace “submarine hiatus” in the Early Kimmeridgian.
The recognition of the Kawr/Misfah exotic unit as a new milestone on the migration route adds to the data, partly unpublished (at least concerning the ammonite faunas) from southeast Turkey. But the paleogeographic situation (Figure 8) of the allochthonous units of Hezan-Abdulaziz (Hz) was less similar to the Kawr/Misfah exotic unit (Mf) and more in line with another Oman exotic, the Ba’id exotic (Al Buda’ah Group) (see Figure 3).
Milestones on the continental margin in Turkey and Oman
In southeastern Turkey, the allochthonous unit of Hezan-Abdulaziz (Figure 8) crops out through a tectonic window beneath the ophiolite suite of the Koçali nappe (Fontaine, 1981; Fontaine et al., 1989; Fourcade et al., 1991; Gunay et al., 1992). Nodular limestone beds (Rosso Ammonitico facies) yielded ammonites from the Middle Bathonian to the Late Tithonian (Cadomites, Bullatimorphites Bathonian; Gregoryceras, Lithacosphinctes Oxfordian; Nebrodites, Pseudosimoceras, Hybonoticeras Kimmeridgian; Micracanthoceras, Djurjuriceras, Durangites, Proniceras Late Tithonian) that are identical with those known from the Submediterranean and/or northwestern Europe. They were described by Gunay et al. (1992), but the poor reproduction of the original negatives made the plates unusable and they will be republished at a later date.
Units of the Hezan-Abdulaziz windows are interpreted as fragments of the northern edge of the Arabian Platform that evolved as a passive margin and were subjected to faulting and tilting either since the Bajocian (Fontaine, 1981; Fontaine et al., 1989) or Bathonian (Fourcade et al., 1991; Gunay et al., 1992). Bathonian – Late Tithonian ammonite-bearing beds were deposited on the distal platform or deep basin above the Carbonate Compensation Depth. The Kawr/Misfah exotic contains condensed pelagic sequences deposited on swells corresponding to the “distal pelagic swell areas” of Marques et al. (1989) and Oloriz et al. (1991), also referred to as “pelagic carbonate platforms” in Santantonio (1993, 1994).
A similar situation occurred on the Oman margin concerning the Ba’id exotic, or Al Buda’ah Group, of Pillevuit (1993) and Pillevuit et al. (1997) that Béchennec (1988) included in the Kawr/Misfah exotic block. Blendinger (1995) also described these beds in the Ba’id area (the Wadi Alwa meggabreccia) and isolated blocks near Jabal Safra (see Figure 1). The units were interpreted as a horst by Pillevuit (1993) and Pillevuit et al. (1997) or as a seamount (Blendinger, 1995) that corresponded to the upper part of a tilted block on the Oman continental margin in a proximal position compared to the Kawr/Misfah exotic. The drowning of the swell as a result of subsidence and/or a relative sea level rise was during the Early Triassic (Dienerian), underlined in the sedimentary record by the change from platform limestone to pelagic facies of Hallstadt-type limestone. Local occurrences of redeposited beds including limestone with Hettangian (Schlotheimia sp.) at Jabal Safra and Sinemurian – Pliensbachian (Juraphyllites sp.) ammonites at Wadi Alwa (see Figure 1) prove that the swell existed during the Early Jurassic and that ammonite-bearing open-marine sediments were deposited on the Oman continental margin well before the earliest Jurassic faunas of Late Pliensbachian – Early Toarcian age known from the Kawr/Misfah exotic.
Seamounts and oceanic platforms off the Arabian Plate
Two interpretations have been proposed for the origin of Kawr/Misfah exotics. Béchennec (1988) and Béchennec et al. (1988, 1990) interpreted them as a horst-and-swell structure on the continental crust of the Arabian passive margin (see Figure 3b). This was similar to the interpretation of the Hezan-Abdulaziz units in southeastern Turkey by Fontaine (1981) and also of the Al Buda’ah Group from central Oman that Béchennec (1988) equated with the Kawr/Misfah exotics (see Figure 3b).
In contrast, the paleogeographical reconstructions by Pillevuit (1993) assumed a simple Hawasina Basin (see Figure 3a) on the oceanic crust, following Model 1 of Glennie et al. (1974). The proposed interpretation was original because it accepted that some atypical parts of the oceanic crust were rising topographically like volcanic oceanic plateaus in the present oceans. Pillevuit (1993) compared the Kawr/Misfah exotic to existing seamounts and developed arguments that allowed him to conclude, “that [the] exotic is a remnant of volcanic rise of atoll and/or guyot type”.
The lithologic record of the Kawr Group (see Figure 3a) would therefore represent a build up of volcanic deposits on the Late Permian to Middle-Late Triassic oceanic crust. These were later affected by thermal subsidence comparable with that of the present-day mid-ocean ridges, as indicated by the formation of deeper water deposits. These accumulated, firstly, on a carbonate platform during the Triassic (Ladinian-Carnian to Norian); secondly, by the late Triassic, the break up of the platform deposits was probably due to emergence; latterly, emergence was followed by a transgression to deeper-water conditions. The pelagic conditions began in the Late Pliensbachian to Middle Toarcian with the deposition of the nodular limestone of the Rosso Ammonitico facies of the Fatah Formation; an increase in water depth is shown also by the deposition of the cherty micritic limestone of the later Nadan Formation.
As shown by the Tethys and Peri-Tethys paleoenvironmental maps of Dercourt et al. (1993, fig. 12) and Dercourt et al. (2000, map 10), the Kawr/Misfah pelagic rise was isolated from the platform edge by 150 km in Toarcian to Callovian times and by at least 300 km during the Kimmeridgian and Tithonian. This poses questions about dispersal. Ammonites, like all cephalopods, but unlike other molluscs such as gastropods and bivalves, may not have had a planktotrophic larval stage. Therefore, according to Newton (1988) and Moyne et al. (2004), passive dispersion opportunities would have been restricted. However, examples from present-day cephalopods (Boletzky, 1974, 1999) and the study of the earliest growing stages of ammonites (Landmann et al., 1996) support the hypothesis that the formation of the ammonoidea protoconch was related to a (pseudo) planktotrophic growing process. This included a non-larval juvenile stage (the ammonitella) with a transitory pelagic lifestyle that favored dispersal. Thus, the larval dispersion of ammonites is widely assumed in paleobiogeography (Mangold, 1989; Énay, 1980, 1993; Westermann, 1996) and is probably how the ammonite populations reached the Kawr/Misfah rise; the other requisite condition being the availablity to take advantage of suitable environmental conditions.
The Kawr/Misfah exotics and its ammonites illustrate the part that this pelagic rise provided as a milestone on the migration route along the southeastern margin of Tethys. But, could not these pelagic rises have provided potential stepping stones for more distant trans-oceanic dispersions? This is one of the proposed interpretations of some features of Pacific paleobiogeography during the Jurassic.
FROM THE KAWR/MISFAH PELAGIC RISE TO JURASSIC PACIFIC BIOGEOGRAPHY
Toward the end of the 1970s, mobilistic paleogeography came to prominence and paleobiogeography was revived. Consequently, the biogeography of the Pacific (part of the Panthalassic Ocean) that formed contemporaneously with Pangea in the Late Paleozoic, was subject to various interpretations. However, the results were similar and accounted for the occurrence in the Pacific Ranges of North America of taxa that are identical or comparable to the so-called Tethyan fossils of Newton (1988) that lived in intertropical and/or tropical seas. The ages of these Tethyan fossils range from the Paleozoic to Mesozoic. In particular, the Tethyan faunas in the eastern Pacific are of Early and Middle Jurassic age.
In spite of the small number of species from the European Tethys found in Oman, several of them also occur in the American Cordillera (Figure 9). During the Toarcian, Polyplectus (with the two species described from Europe, P. discoides and P. pleuricostata), Pseudomercaticeras cf. frantzi and Phymatoceras sp. are present in the Canadian Pacific Cordillera (Hall, 1987; Jakobs, 1997), and P. pleuricostatus is present in the Chilean Andes according to Howarth (1992). Dommergues (1994), in a paleobiogeographic survey of the Pacific ammonite faunas, named Polyplectus and Phymatoceras as pandemic genera.
Details of Pseudomercaticeras from the Canadian Cordillera were not available, but the related genus Mercaticeras is of Tethyan type. Docidoceras is known only from the Bajocian of eastern Oregon in the USA (Imlay, 1973), but Westermann (1969) described a particular subgenus, Pseudodocidoceras, in Alaska. Emileia is also present in eastern Oregon (Imlay, 1973) and in Alaska (Imlay, 1964) together with a close relative of the western European species, E. polyschides (Hall and Westermann, 1980), and in Mexico, a single fragmentary specimen occurs in strata associated with the Siquisique ophiolite, whose tectonic and paleogeographic position is disputable (Bartok et al., 1985).
Thus, one interpretation of the Pacific biogeography relies heavily on the part played by faunal populations settling on oceanic rises, so allowing trans-oceanic dispersal. Were the Kwar/Misfah pelagic rises (and also those of Pakistan) able to play a part?
Interpretative models of Tethyan influxes into the eastern Pacific
A list of references and the discussion on the various models proposed are available in Taylor et al. (1984), Smith and Tipper (1986), Hallam (1986), Newton (1988), Dommergues (1994), Aberhan (1998), Cecca (2002) and Moyne et al. (2004). A revised list of the ammonite genera occurring in the Pacific Coast Ranges of North America was compiled by Dommergues (1994) with additions by Jakobs (1997).
Marine Corridor Model / Faunal Transit Model
This model by Newton (1988) and Dommergues (1994), respectively, is the most frequently accepted interpretation of migration. The model supposes a migration route between the Tethys and the eastern Pacific through the central (or Tethyan) Atlantic and what is now Central America. That is the concept of the “Hispanic Corridor” of Smith (1983, 1998), Westermann (1993), Smith and Tipper (1986), Damborenea (1987), Nauss and Smith (1988), and Aberhan (1998, 2001), or the “Atlantic seaway” of Damborenea and Mancenido (1979), Schmidt-Effing (1980), Énay (1980, 1985), Hillebrandt and Schmidt-Effing (1981), Westermann (1981), Hallam (1983) and Elmi (1993).
Evidence for an early Hispanic Corridor is based on the following paleontologic and paleobiogeographic data:
Tethyan Sinemurian ammonites from Huayacocotla in Mexico (Schmidt-Effing, 1980); Bajocian ammonites from the Siquisique ophiolite in Venezuela (Bartok et al., 1985); and Callovian ammonites from Oxoaca in Mexico (Westermann and Riccardi, 1976; Westermann, 1981, 1993).
The geographic distribution of the Tethyan bivalve genera Lithiotis (Nauss and Smith, 1988) and Weyla during Pliensbachian – Toarcian times (Damborenea and Mancenido, 1979); of Early Jurassic bivalves (Hallam, 1983; Damborenea, 1987; Aberhan, 1998, 2001); and of Early and Middle Jurassic ostracods (Boomer and Ballent, 1996).
The presence of sea-floor basalt proves that the complete opening of the oceanic Hispanic Corridor took place in the Oxfordian (Ogg et al., 1983). Before then, only episodic exchanges of fauna were possible through shallow channels formed by cratonic tectonic grabens aided by a relative rise in sea level (Du Dresnay, 1987; Énay, 1980, 1985; Westermann, 1993).
However, the reality of such earlier (pre-Oxfordian) episodic migration routes was not demonstrated by either geologic or geophysical data.
At site DSDP 534 drilled off Florida, marine Callovian sediments rest on oceanic basalt. Dated as Middle Callovian by calcareous nannofossils and dinokystes (Gradstein and Sheridan, 1983), the Jurassic deposits at site 534 were the origin for the proposal by Cariou (1984) and Cariou et al. (1985) that Tethyan Reineckeinae ammonites evolved from eastern Pacific Neuqueniceratinae. Similarly, Énay et al. (1993) noted the occurrence in Cuba of marine deposits containing Middle Callovian trigoniid bivalves intercalated within the deltaic San Cayetano Formation (Krommelbein, 1956).
The earliest Jurassic beds are Middle Bathonian (Baumgartner and Matsuoka, 1995) based on radiolarian data, and are approximatively the same age as the earliest Jurassic deposits overlying oceanic crust in the northwestern Pacific (Matsuoka, 1995). The coincidence is related to an increasing rate of sea-floor spreading that increased the rate of fragmentation of Pangea and initiated the formation of the Pacific microplate (Bartolini and Larson, 2001). However, it does not prove that the Hispanic Corridor was open (see Cecca (2002) for a complete discussion).
Bassoullet et al. (1993) maintained that, “there is not enough constraint (paleontological, sedimentological, tectonic) to state positively that the Hispanic corridor was open in Toarcian time”. However, Damborenea (2000) assumed that a possible seaway through the Hispanic Corridor as early as the Sinemurian, but Aberhan (2001) and Dommergues et al. (2004) ruled out any possibility of exchange before the Pliensbachian. Moyne et al. (2004) accepted that the Hispanic Corridor could have been open, “at least during Bajocian or during Early Bathonian”.
Displaced Terrane or Suspect Terrane Model
This model by Monger and Ross (1971) is also termed “horizontal or longitudinal displacement”.
The concept originally related to exotic units in the Alpine or Tethyan ranges. The model is a tectonic or displacement concept (Newton, 1988) and the allochthonous character of the terranes containing the Tethyan fossils explains their occurrence in the Pacific Coast Ranges of North America. There is little evidence of continental origin as nearly all the constituent rocks represent former seamounts, island arcs, or slivers of the deep ocean floor (Schermer et al., 1984). However, some authors assume that the allochthonous units would have been displaced long distances from central or western Pacific or even eastern Asia. When the terranes were accreted to the North American Craton they would have juxtaposed faunas that were far distant in origin. This follows the so-called Noah’s Ark Model of McKenna (1973), but also takes into account the evolution of successive faunas during the traveling time of the terrane according to his “Viking Funeral Ship Model”.
Objections to the proposed model relate to the scale of the displacements and to the concept of terranes as vehicles for faunal distribution, and are as follows:
The scale of the displacements, already in doubt for the Triassic, is even more dubious in relation to the Early Jurassic. According to Taylor et al. (1984), Tethyan Hettangian and Early Pliensbachian fossils, and particularly the more pandemic Sinemurian, Toarcian, Aalenian and Bajocian genera, occur in deposits of the North American craton and related terranes. Hence, they concluded that the craton and the terranes were close enough geographically to allow faunal exchanges. The only obvious major displacements, known from paleomagnetic data and paleobiogeography, are northward latitudinal movements that are sufficient to account for Tethyan fossils in boreal regions, but do not solve the question of their provenance.
With regard to the Jurassic, researchers tend to dismiss terranes as as a vehicle for the distribution of Tethyan Jurassic taxa in relation to the Hispanic Corridor (e.g. Taylor et al., 1984; Westermann, 1981; Hallam, 1986; Aberhan, 1998, 1999, 2001; Smith et al., 2001). Whatever the original location of terranes, even if they lay more to the west than is generally thought, the whole width of the Pacific divided the Tethys Ocean from North American terranes containing Tethyan Jurassic fossils. Therefore, the question of the colonization process remains open.
Pantropic Distribution Model
The model was proposed by Newton (1988) in order to balance the perceived deficiencies in the other models. The new model refers to the present Pacific Ocean and assumes a marine pantropic fauna that gathers together benthic and pelagic biotas. Newton emphasized the numerous analogies between the Tethyan Jurassic faunas and the present Indo-West Pacific biota whose species extend as far as the eastern Pacific, and some even reach islands off Central America and its continental margin.
Among the controlling factors of the Newton (1988) model are:
The Cromwell Current (or Pacific Equatorial Undercurrent) is a relatively shallow eastward-flowing Pacific tropical undercurrent whose part in the dispersal of the Indo-Southwest Pacific biota taxa is more important than the effects of the westward-flowing North and South Equatorial surface currents;
The part played in the trans-oceanic dispersal westward of populations that settled on pelagic rises of “the stepping stone effect of the cluster of volcanic islands and atolls, rises of the plateaus (….) in eastern Pacific” (Dommergues, 1994);
During the Phanerozoic, there was a link between the geographic ranges of tropical taxa and climatic equability. Periods of relatively equable climate favored a wide dispersal; in contrast, were periods of non-equable climate, as today.
The pantropic distribution model was discussed by Smith and Westermann (1990), two defenders of the Hispanic Corridor Model, and Stanley and Yancey (1990), supporters of the Terranes Model. With reference to the Jurassic, the main criticisms are as follows:
Comparison between the present and Pacific (or Panthalassa Ocean); and
The supposed analogy between the present-day Indo-Southwest Pacific biota and Tethyan faunas.
Proponents of the Pantropic Distribution Model are Dommergues (1994) and Dommergues et al. (2004). Dommergues (1994) assumed that the Pantropic Distribution Model of Newton (1988) was more neutral and allowed a wider application than the other models that relate more to a narrative concept of paleobiogeography. Dommergues (1994) tested the various models in relation to the Hettangian – Toarcian interval. His critical analysis of the faunal content on the eastern Pacific margin used five types of taxa: Pandemic, Tethyan sensu lato, Euroboreal, West Tethyan-Lusitanian, and East Pacific. His conclusions tended to favor the Newton model, or emphasized some aspects not accounted for by the other models.
Dommergues et al. (2004) reexamined the problem by studying a Sinemurian ammonite fauna from southeastern Ecuador. As a result, they rejected a seaway through the Hispanic Corridor and made a general survey of the theoretical potential migration routes discussed by Hallam (1977, 1983). Without being definitive, because of the subjective nature of the evidence, they suggested that Newton’s Pantropic Model, “at the present time gives the best possible alternative to the Hispanic corridor”.
More recently, Moyne et al. (2004) objected to the Pantropic Distribution Model of Newton. They stated it was, “unlikely concerning cephalopods as were ammonites with a direct development and therefore with less dispersion ability”. However, they did not discuss the possible part that seamounts and oceanic islands play as milestones for dispersion and migration.
Paleoceanography and paleobiogeography of the modern and ancient Pacific Ocean
In the critical examination of Newton’s Pantropic Distribution Model, Smith and Westermann (1990) and Stanley and Yancey (1990) emphasized the width of the Jurassic Pacific (or Panthalassa Ocean) that was much wider than at present. As Winterer (1991) pointed out, subduction has destroyed most Jurassic oceanic sediments (except for terranes included in the North American Pacific Coastal Ranges) and the volcanic islands, atolls, oceanic rises and plateaus that may have provided evidence of potential stepping stones for transoceanic dispersal in the Pantropic Distribution Model of Newton. The Jurassic oceanic crust preserved at the present time in the northwestern Pacific, the earliest being dated as Middle Bathonian (Bartolini and Larson, 2001), covers only about 10 percent of the present-day floor of the Pacific. Only a few sites have been drilled where Jurassic sediments directly overlie oceanic crust (Winterer, 1991).
Paleobiogeography of the modern Pacific and the Eastern Pacific Barrier
The present-day Eastern Pacific Barrier of Ekman (1953) is commonly described as the most effective controlling factor in the decline of faunal diversity eastward in the tropical Pacific (Grigg and Hey, 1992). The Barrier is the expanse of the tropical Pacific Ocean where no islands exist that separates the Indo-West Pacific Province from the Eastern Pacific Province. Grigg and Hey (1992) discussed its present effectiveness and its effect in the past, for example, during the Cretaceous when the width of the ocean was about the same as it is now although the Pacific Ocean was about 30 percent larger than at present.
As the Pacific Ocean today has no connection with the central Atlantic, the Eastern Pacific Barrier is more effective in limiting dispersal westward (the Indo-West Pacific Province) than eastward (the Eastern Pacific Province) across the Pacific. This is attributed both to external and internal factors. For example, although the major surface currents flow from west to east, currents also flow periodically stronger eastward, and many of the diverse Indo-West Pacific species are adapted for long-distance dispersal or eurytopic attached adults. Grigg and Hey (1992) illustrated their conclusions by presenting distribution patterns of tropical shelf echinoderms and reef-building corals. When questioning the influence of El Nino-Southern Oscillation events on oceanic circulation and dispersal in the tropical Pacific, as well as the antiquity of the events, they concluded that the East Pacific Barrier may have been more effective in the past than it is today because the Indonesian seaway was not then closed.
In contrast, Belasky and Runnegar (1993) questioned the effectiveness of the Eastern Pacific Barrier with regard to the dispersal and decreasing diversity of Indo-West Pacific corals. They reduced its influence when dealing with paleolongitude estimates based on the diversity gradients of corals. They assumed that the distance between eastern Pacific islands cannot be considered the primary factor because of the following factors:
The Indo-West Pacific coral fauna decreases eastward by a factor of two even in areas with a high concentration of islands; for example, between Indonesia and French Polynesia; and
Coral diversity is everywhere high throughout the Indian Ocean even though it has only a few islands in its central part but several archipelagos in the west.
Belasky and Runnegar (1993) attributed the tropical diversity in the eastern Pacific as being much more affected by long-lived global phenomena such as temperature gradient and patterns of ocean circulation than the shifting mosaic of island stepping stones. Their findings fit with some aspects of the Pantropic Distribution Model of Newton (1988).
In deciding if the Eastern Pacific Barrier was as effective now as in the past, the larger width of the Pacific is not the only factor to consider, as Newton pointed out in response to the comments of Smith and Westermann (1990). Ignoring the Triassic and older Periods, Smith and Westermann (1990) and Stanley and Yancey (1990), who are opponents of the Newton model, made a comparison mainly between the present-day Pacific and the Jurassic Pacific. No comparison was made with the Cretaceous Pacific where oceanic sediments are well preserved and which could provide evidence on Jurassic paleogeography and paleobiogeography, especially with relation to corals and rudist on Pacific atolls and seamounts.
Dispersal of Cretaceous and Tertiary coral genera
Using the data from the revised list of Durham (1966), Grigg and Hey (1992) searched for coral genera that favored dispersal between the Indo-West Pacific Province, the Eastern Pacific Province, and the western Atlantic during Cretaceous and Tertiary times. The greatest affinities were found between western Atlantic and eastern Pacific genera, and this was explained because they were geographically close and a connection existed between them until the Late Pliocene. The number of genera that reached one or another of the two American provinces (Eastern Pacific and Western Pacific), particularly from the Indo-West Pacific Province toward the Eastern Pacific Province, is higher than in dispersal from or toward the Indo-West Pacific Province, but it is higher than between the latter and the Eastern Pacific Province.
With regard to the Cretaceous, Grigg and Hey (1992) considered that their conclusions were strengthened by the results of model studies of surface paleocirculation in the Pacific by Barron and Peterson (1989). The model experiment gave the following results:
Circulation in the Tethys was dominated by a clockwise, gyre-type flow, and a predominant westward flow through the Tethys (as previously hypothesized) did not occur;
Although westward flow occurs throughout the tropical Pacific, the concept of a westward circumglobal equatorial current must be re-assessed.
Being aware of the need to test the model by comparison with observations, Barron and Peterson (1989) assumed that the Tethyan affinities of the fauna of mid-Pacific guyots could have had a western Caribbean provenance because they are in a region of westward oceanic flow. Nevertheless, Newton (1990) in response to the comments of Smith and Westermann (1990), and referring to the experiment of Barron and Peterson (1989), reiterated that the “predominant circulation in the Tethyan seaway was found to be eastward- rather than westward-directed”. The model simulation does, in fact, show an eastward-directed flow at about 30°N and 30°S, that is, perhaps, of great significance. Curiously, the model also indicates an equatorial westward flow with no eastward-flowing counter current.
Rudist bivalves, especially Late Cretaceous forms, provide additional information that is perhaps more reliable because evolutionary trends of rudists are better known than for corals.
Pacific atolls and seamounts having Cretaceous rudist-bearing deposits
The oldest seamounts in the present-day Pacific occur in the Mid-Pacific Mountains and are dated as Barremian, the same age as is ascribed to other northwestern Pacific rises (Winterer, 1991). Several of these seamounts are drowned atolls bearing Barremian to Aptian rudist reefs of the same age as rudist formations in the Middle East or those of the Urgonian Formation limestones in the Submediterranean Tethys. Similarly dated rudists reefs are widespread in the Atlantic on the Blake-Bahamas Plateau and in the Gulf of Mexico, and in Baja California, at a time when a seaway through Central America linked the Atlantic and Pacific oceans. The disappearance of rudists during the latest Albian to Middle Turonian coincides with two successive major rudist crises according to Philip (1998, 2000); the first was in the Mid-Aptian, and the second at the Cenomanian/Turonian boundary. These Barremian – Aptian rudist formations prove that dispersal and larval settlement on submerged oceanic rises or seamounts was possible, but they have no bearing on trans-Pacific dispersal or the direction of dispersal. The situation was much different during the Late Cretaceous, as discussed below.
The Campanian – Maastrichtian interval was a major expansion phase of reef and carbonate-platform development on oceanic seamounts. Rudists have been identified on the Darwin Guyot (Philip, 1982), Japanese oceanic seamounts, Mid-Pacific Mountains, at Lime Island, Mauru Basin (Philip, 1998) and in the Marshall Islands (Philip, 1998, 2003). In addition, they are present in Oman, eastern Africa, southern India, Misol in the Sula Islands of Indonesia, Peru and the Caribbean. Distinct Arabian-African-Indian and Caribbean rudist provinces were distinguished by Philip (2003), and there was a wide opening at this time between the Atlantic and Pacific oceans through Central America; neither favored Pacific transoceanic dispersal and migration. However, the question of transoceanic dispersal and migration is emphasized by disjunct distributions of rudist genera occurring both in Oman and the Caribbean that are not known in the remnant parts of the Tethys (Skelton, 1985; Philip and Platel, 1987).
The genus Torreites is known from Oman and the Caribbean. Because of the older age of the genus in the Caribbean, the distribution of the genus was first interpreted by Skelton and Wright (1987) and by Skelton (1988) as resulting in a westward dispersal in conjunction with the westward surface flow of the North and South Equatorial Currents. However, the discovery in Oman of Praetorreites, the most primitive of the Torreitidae, dated as Late Santonian to earliest Campanian and about the same age as the earliest Caribbean Torreites, allows (among various hypotheses) for a possible eastward migration route (Philip and Platel, 1994, 1995; Philip, 1998) and affirmed by Philip (2003).
The genus Macgyllavrya also displays a disjunct distribution in Oman and the Caribbean. The Oman species are older, Late Santoniann to earliest Campanian, compared with earliest Late Campanian for the Caribbean varieties. This favors a dispersal from Oman toward the Caribbean via Pacific seamounts, although occurrences of Macgyllavrya have yet to be found on the seamounts (Philip, 2003).
Paleobiogeography: from the Cretaceous Pacific to the Jurassic
A study of Cretaceous rudist paleobiogeography showed that:
Different biotic provinces occurred on both sides of the Pacific; namely, the Arabian-African-Indian Province and the Caribbean Province; and
There were genera with disjunct distribution patterns.
That the disjunct genera occur in both provinces increases the probability of transoceanic dispersal or migration by way of intra-oceanic island stepping stones. Previously, Newton (1988) emphasized that rudists as well as corals included a large number of pantropic taxa and that several Cretaceous gastropod groups also have an extended longitudinal distribution. Therefore, the existence of two biotic provinces does not preclude the occurrence of taxa with pantropic distribution. On the one hand, during the whole of the Cretaceous the wide passage between the central Atlantic and the eastern Pacific, combined with the relatively narrow expanse of the southern Atlantic, favored the influx of Tethyan taxa. Conversely, the eastward decreasing gradient of taxa diversity was similar to that in the present-day Pacific; and the oceanic width during the Cretacous was sufficient to favor species distribution and the diversification of the fauna. According to Grigg and Hey (1992), “genera would be more widely distributed than species because they have had more evolutionary time for dispersal, and they are genetically more variable”, in the eastern Pacific.
Referring back to the Jurassic, although the eastern Pacific faunas include numerous pandemic or Tethyan genera common with those of the Tethys, at species level they are commonly either different or closely related but not identical. However, Polyplectus, which numbers only two species in the Tethys seems to be known by the same two species in the eastern Pacific, of which P. discoides displays a wide distribution and a long vertical range. Is the occurrence of P. discoides in the North American Pacific Coast Ranges the result of transoceanic dispersal from Oman, Baluchistan and Tibet? Dispersal could have been realized in easy steps during the species long biochron, via the numerous volcanic islands and atolls, oceanic rises and plateaus of the tropical Jurassic Pacific Ocean (Figure 9).
The situation during the Early Jurassic was different from that in the Cretaceous. This was because the Early Jurassic Pacific Ocean was larger and there was no easy communication through Central America (even assuming an episodic epicratonic seaway). Nevertheless, the pantropic distribution model of Newton (1988) assumed a relationship between climatic equability and the geographic range of tropical taxa that formed an equilibrium throughout the wide extent of the Pacific.
According to Grigg and Hey (1992), the El Nino-Southern Oscillation events in the present-day Pacific Ocean modify the equatorial oceanic circulation and have resulted in:
The occurrence of Indo-West Pacific fish and molluscs in the Galapagos and Cocos islands that were previously unknown in the eastern Pacific, and the presence of tropical species off California and Peru as a result of dispersal pulses during El Nino events; and
The widespread mortality of established reefs in the eastern Pacific induced by elevated temperatures has sometimes persisted for prolonged periods. As a result, large, old coral colonies may not survive exceptionally strong El Nino events.
Thus, El Nino-Southern Oscillation events might play an important part in the recruitment of eastern Pacific taxa by increasing larval dispersal and creating space available for colonization when adult mortality rates are high.
The effects of El Nino-Southern Oscillation events during the Jurassic is hypothetical. The closure during the Pliocene of the Panama seaway would have increased the outcome of these events, a situation that compared with that existing during the Early Jurassic. However, a more important factor would be the closure of the seaway through Indonesia to the Indian Ocean, which established a western boundary for equatorial circulation. Hence, Grigg and Hey (1992) assumed that the Eastern Pacific Barrier was a more effective barrier. The Jurassic was a non-glacial period with a low latitudinal temperature gradient, whereas the intertropical climatic zone was wide. These brought together favorable conditions for widely extended dispersal opportunities, especially for long-lived pseudo-planktotrophic larvae and/or juveniles, subject to them finding the necessary stepping stones, such as pelagic seamounts, oceanic plateaus, guyots and atolls.
Newton (1990) underlined the lack of quantitative models of oceanic circulation for the Early Jurassic. Simulations by Cottereau (1992), Cottereau and Lautenschlager (1994) and De Wever et al. (1994) related only to the latest Jurassic and was focused on Tethyan circulation and control by Mediterranean highs; for example, the Apulian micro-plate/Italian-Dinaric unit. The connection between the Tethys and the eastern part of the Pacific was open and the world oceanic circulation was very similar to the simulation by Barron and Peterson (1989) of the surface currents during the Middle Cretaceous. Global paleogeographies were similar, except that South East Asia was situated farther north in the Early Jurassic. Strong surface equatorial currents flowed westward across the Panthalassa Ocean toward the coast of eastern Africa.
These results are not inconsistent with ocean currents empirically predicted in the Pliensbachian by Parrish (1992) and by Ager (1975) that were based on a slightly different version of Pangea. The two authors considered that an equatorial countercurrent flowed from west to east; according to Parrish (1992) it was strong because no connection between western Tethys and the Pacific existed, and thus the east-west thermal gradient was weak. He suggested that, “the countercurrent could have completely overcome the effects of the easterlies, suppressing even the coastal upwelling zones, and keeping the Jurassic Pacific Ocean in a permanent El Nino-like state”.
Considering the reality or otherwise of Jurassic milestones useful for dispersal, which is an important parameter of the pantropic model, the small amount of Jurassic ocean floor preserved in the present-day northwest Pacific does not help in determining the paleo-oceanography of the Jurassic Pacific. Present-day seamounts date from the Cretaceous to Eocene according to Heezen and Fornari (1976). Nevertheless, except for the increased width of the ocean during the Jurassic, the present distribution of oceanic rises and plateaus in the Pacific (Ben Avraham et al., 1981; Nur and Ben Avraham, 1982) could be a good illustration of what the Jurassic Pacific might have looked like.
New discoveries of Jurassic ammonites were made in the autochthonous Haushi-Huqf Massif of central Oman, and also in allochthonous units of the Hawasina Nappes, from the mountains of northern Oman. The new data have had a profound affect on the ages of the host units. In the Haushi-Huqf autochthon, the lower beds of the Mafraq Formation, the original age of which was based on insignificant brachiopods, are now dated as Late Bajocian. They occur in the Planus Zone of the biochronological scale for the Arabian Province, which is correlated with the Parkinsoni Zone in the Northwest Europe Standard scale. The dating is consistent with, and confirms indirectly, the age of the overlying beds of the Dhruma Formation dated as Bathonian – Callovian by corals, echinoids and benthic foraminifera. With regard to the allochthonous units of the Oman Mountains, the new data have resulted in a lithostratigraphic re-ordering and the definition of a new formation, the Fatah Formation, which is separate in terms of facies (Rosso Ammonitico) and age from the underlying Late Triassic Misfah Formation. After revision, the Fatah Formation was dated as Middle to Late Toarcian, perhaps even Late Pliensbachian – Early Toarcian at Jabal Kwar or Early Bajocian at Jabal Hamrat al Hasan.
In terms of paleoecology, the Jurassic faunas from Oman are a juxtaposition of endemic Arabian faunas and others peculiar to the Mediterranean Tethys and northwest Europe. Although they are of different ages when each grouping is considered separatively, they give witness to a global environment and show the contradistinction between the Arabian faunas dependant on shallow-marine environments and the faunas with European characteristics from open-marine environments. There is no evidence that they were separated by a geographic barrier. Endemism of the Arabian faunas resulted from the continuation during the entire Jurassic (and farther back) of constraining environmental conditions. Faunas with northwest European features from open-marine environments are related to the swells (cratonic horsts or seamounts) isolated off the Arabian Platform. They are landmarks on the route from the Mediterranean Tethys to the eastern Tethys that were present during the Jurassic in Malagasy, Kachchh and as far east as Indonesia.
The paleobiogeographic importance of the Oman faunas is emphasized by the wide distribution of several taxa. These are present on the Indo-West Pacific (Pakistan) and northern Tethyan margins (southern Tibet and Japan) and as far as the Pacific Coast Ranges of Canada and Argentina.
The paleobiogeography of the Jurassic Pacific, has been attributed to the following three models:
(1) Marine corridor or faunal transit;
(2) Displaced (or suspect) terranes; and
(3) Pantropic distribution.
Most of the Jurassic ocean floor in the Northwest Pacific was destroyed by subduction. Thus, there is no evidence of the volcanic islands, atolls, oceanic rises and plateaus that may have served as potential “stepping stones” during transoceanic dispersion. This leaves the pantropic distribution model of Newton (1988) as offering examples of trans-Pacific dispersion based on Cenozoic corals and Cretaceous rudists. These examples do not prove that wide-scale dispersion was the origin of Jurassic Tethyan ammonite populations of the American Pacific Coast Ranges, but from Cretaceous examples the oceanic rises and seamounts of the present-day Pacific gives a good picture of the supposed Jurassic Pacific.
To conclude, although the pantropic model of faunal distribution is merely an hypothesis, it is here assumed, as did Dommergues (1994), that the model of Newton (1988) is presently the best possible alternative to correct the apparent inadequacy of other models.
Firstly, I thank J. Béchennec and J. C. Chiron of the French BRGM, the late J. Marcoux (Paris VI University, France) and A. Baud (Lausanne University, Switzerland) who entrusted to me the Oman ammonites that are the basis of this paper. In particular, I commend the late J. Marcoux, who from the beginning showed an interest far beyond merely dating the fauna and who attempted to rediscover the Jabal Kwar outcrop first sampled by J. Béchennec. I thank J. Philip (Provence University, Marseille, France) for acquainting me with data and references on Cretaceous rudist biogeography. Similarly, I thank my friend J. Yin (Beijing Geosciences University, China) for assisting me with the English translation of Chinese papers and for providing an improved photograph of the specimen of Polyplectus discoides from southern Tibet. I am grateful to L. Rulleau (Chasselay, Rhône, France) who provided information on the taxonomy of the Toarcian genera Podagrosites and Pseudomercaticeras. Mme S. Passot (Claude Bernard-Lyon University, France) helped me to solve several computer problems. This manuscript was published in the Revue de Palaeobiologie in French. Christian Meister is thanked for obtaining the permission of the Revue to publish the manuscript in English in GeoArabia. I thank David Grainger for his editorial assistance and support, and GeoArabia designer Nestor “Nino” A. Buhay for preparing the graphics and layout.
ABOUT THE AUTHOR
Raymond Énay is Emeritus Professor in Earth Sciences at Claude-Bernard-Lyon 1 University, which he entered in 1956. He received his Natural Science “Agregation” in 1957 and his DSc in 1967. His scientific work focused on Jurassic ammonite paleontology, biostratigraphy and paleobiogeography, mostly along the Jurassic South Tethyan margin. He has worked in the Jura Mountains and SE France, Spain, Morocco, western Algeria, Tunisia, Turkey, Egypt (Sinai), Saudi Arabia, Yemen, Nepal and India. He has published more than 180 papers, and has contributed extensively to the applied geology of the French Jura Mountains. Raymond was elected twice as Chairman of the International Subcommission on Jurassic Stratigraphy (October 1989 to October 1996) and during his chairmanship two International congresses were held: one on Jurassic Stratigraphy, in Poitiers, France (1991) and the other in Neuquen-Mendoza, Argentina (1994). In his retirement, Raymond continues to be involved in publishing data on faunas collected during his active employment.