FAUNAL CONTENT OF THE CEPHALOPOD LIMESTONE BIOFACIES IN THE VARISCAN TERRANES

Tinduf Basin (Northwestern Sahara) [A]

Ratschiller (1970-71) in the northeastern part of Western Sahara (former Spanish Sahara) described two formations: the Dammia and Afreiyat Formations. He writes (pp. 44-46), "The boundary between the two formations is lithologically very distinct. The Dammia shale contains interbedded thin siltstone layers that are dark red on exposed surfaces at the contact between greyish green and red shale of the Dammia Fm and massive, predominant greyish-blue calcarenite beds of the Afreiyat Fm and surfaces. The Afreiyat calcarenite contains abundantly 'Orthoceras'. These beds exhibit a characteristic shimmer and are observable for long distances along the Afreiyat Fm of the southern flank of the Tinduf Basin. An angular unconformity of a few degrees (2°-3°) seems to be present between the two formations along the area of the Tifariti basement. It is not well observable because of extensive fluvial deposit." Ratschiller continues (p. 45), "Based on its fossil content the Afreiyat Fm. belong to the Silurian. In the graptolitic limestone of Rag Bularsag the following graptolites were collected: Pristiograptus (Saetograptus) chimaera (Barrande), Monograptus? unguiferous (Perner), Pristiograptus dubius (Suess), Monograptus flemingi (Salter)." These index graptolites correspond approximately to the following conodont zones or time intervals [Štorch, personal commun., 1995 (Geol. Inst. AS CR, Prague, Czech Republic)]:

Pristiograptus (Saetograptus) chimaera (Barrande) to Ancoradella ploeckensis Upper Gorstian such as the scanicus zone (Middle Ludlow);

Monograptus? unguiferous (Perner) to the Lower Gorstian;

Pristiograptus dubius (Suess) to Ozarkodina boh. bohemica (Wenlock-Ludlow);

Monograptus flemingi (Salter) to O. sagitta sagitta (Middle Wenlock).

Inasmuch as the cephalopod limestone biofacies in this area are of special importance, the following is reported from Ratschiller (1970-71, pp. 45-56, fig. 2 and enclosures 1-4) ad litteram.

AFREIYAT FORMATION

Lithology: "In the type section area the top of the formation is a key horizon (thickness 5 m) of calcarenite, with abundant "Orthoceras." compact of finely crystalline, and grayish-red. Below the calcarenite greenish-red shale predominates. It is thin bedded to laminate, with some interstratified, thin, gray limestone beds (thickness 73 m). Below the shale is another red, calcarenite horizon, with abundant orthocones (maximum length 40 cm, diameter 7-8 cm). This calcarenite overlies 57 m of green or variegated shale, laminated, with crystals of limonitized pyrite and thin, interbedded, graptolitic, sandstone beds at the bottom." In the lithostratigraphic description of section "Rio 21" at the base of the "upper section" of the Southern flank of Tinduf Basin near Tifariti, Ratschiller reported the presence in silty limestone of Dawsonoceras annulatum (Sowerby 1818).

The writer was kindly invited by Dr. Ratschiller visit his personal fossil collection in the late spring of 1995, and had an opportunity to study a cephalopod slab, naturally polished by wind abrasion (Fig. 2) from the boundary of the Dammia and Afreiyat formations. This slab contained very abundant Arionoceras submoni-liforme (Meneghini 1857), a few specimens of A. affine (Meneghini 1857), and scattered Michelinoceras (M.) michelini (Barrande 1866). [Note: A. affine could represent the sexual dimorph (female) of A. submoniliforme (Meneghini 1857) (see Gnoli 1998)].

This slab is from 26°32'30" North and 9°16'26" West along the road that cuts toward the northeast boundary of the Afreiyat and Dammia formations. Biostratigraphically, its age belongs to the Ancoradella ploeckensis conodont Biozone. The cephalopod and conodont fauna it contains represents typical, monospecific populations probably due to the particularly high-energy depositional environment of this nautiloid-bearing calcarenite. Wave motion dominated this microfacies as evidenced by the presence of a normally packed oolite consisting of single-coated, rounded grains in which the nuclei are minute fragments of cephalopod shells. This is a good example of the "Standard Microfacies 15" that today is very common in tidal bars (see Wilson 1965).

Anti-Atlas (Morocco) [B]

Knowledge of the cephalopod limestone biofacies developed here is reported several times and can be summarized by the existing literature on this topic. The Tafilalt structural platform can be traced over an area approximately 80 kilometers in a north-south direction and 30-40 kilometers in an east-west direction. It is characterized by a thin succession (5-50 m in the center) of Upper Frasnian/Lower Famennian, cephalopod-bearing, limestone strata, showing distinct lateral facies variation and frequent hiatuses. To the north and south, Devonian rocks are overlain by Upper Cretaceous strata. A possible continuation of the platform towards the north, however, may be inferred from the reappearance of Upper Devonian rocks of similar facies in the eastern Meseta, about 200 kilometers to the northwest (Hollard 1967; Bensaid,1974). An extremely reduced (3 m), isolated, Upper Devonian limb of a platform is found in the north Jebel Rheris area. Because Devonian rocks are eroded towards the north and east, the extent of this platform remains uncertain (Wendt et al. 1984). These authors speak of slow, shallow pelagic deposition of the Upper Devonian cephalopod limestone biofacies on the structural Tafilalt-platform. The same type of depositional pattern of this biofacies occurs in southwestern Sardinia, also during the Lower Devonian (Gnoli 1983, 1985).

The author also had the opportunity to see a polished slab of Silurian nautiloid cephalopod dark limestone from Tazzarine in which several orthoceratid species were recognized, including: Michelinoceras (M.) michelini (Barrande 1866), Kopaninoceras jucundum (Barrande 1870), among Geisonoceratidae Arionoceras affine (Meneghini 1857), Arionoceras aff. capillosum (Barrande, 1868), Ar. submoniliforme (Meneghini 1857), and Harrisoceras cf. vibrayei (Barrande 1859). This fauna is similar to those of southwestern Sardinia and Central Bohemia (Perunica). The age assignment--cross-checked by conodonts--is lowermost Ludfordian, Ludlow [P. siluricus Biozone (Serpagli, personal commun., 1995)]. A comparable slab is also stored in the Museum at Modena (Fig. 3). In addition, Dawsonoceras dulce (Barrande 1868) was recovered by Termier and Termier (1952) from the Silurian at Amdekhelni.

Ougarta Chains Aulacogen (sensu Wendt 1995) Southern Algeria [C]

These are approximately north of the Ahnet Basin and south of Beni Abbes locality, South Algeria, "Due to excellent outcrops in all this area, the number of available data is virtually unlimited" Wendt (1995, p. 163). The exposed shallow and marginal basin limestones extend from the northwest to southeast for more than 200 kilometers, the central part of which is at 31° 30' north and 1° 6' west (Wendt 1995, fig. 1).

Nautiloid cephalopod assemblages from this area remain poorly known. Massa (1965, p. 48) reports a Silurian-Lochkovian sequence cropping out in some formations (e.g., Qien Ali, Zemlet, Saheb el Djir) described by Alimen et al. (1952), Poueyto (1950, 1952), and Menchikoff (1933). In the Tafilalet-Maïder Basin and associated platform, Massa (1965) reports: Calorthoceras? pseudocalamiteum fide Engeser "Data Retrieval System Nautiloidea", 1997-2002 in CD-ROM, after courtesy of the Author.(Barrande in Quensted 1851)–already regarded as Anaspyroceras by various authors, Chen in Chen et al. (1981, p. 98) systematically revised the above reported Barrande taxon as type species of the newly proposed genus Calorthoceras.-- Lamellorthoceras coralliforme Le Maìtre, 1952 (revised as belonging to Coralloceras by Zhuravleva 1961), L. vermiculare Termier and Termier, 1955, various Orthoceratidae spp. ind., and Gomphoceras sp. sensu Massa, 1965. The remnant reported cephalopod fauna mainly contains various genera of ammonoids (e.g., Agoniatites, Foordites, Blacheoceras, Cabrioceras, Cymaclymenia, Cyrtoclymenia, Curvites, Dimeroceras, Discoclymenia, Gonioclymenia, Kosmoclymenia, Koenetites, Lobotornoceras, Maenioceras, Manticoceras, Mimosphinctes, Pharciceras, Pinacites, Prionoceras, Platiclymenia, Sobolewia, Sellanarcestes, Sporadoceras, Sphenoclymenia, Subanarcestes, Sympharciceras, Timanites, Tornoceras, Trochoclymenia, Wedekindella, and Wocklumeria) linked to `Kellwasser facies'. Consequently, it is not possible to compare these ammonoid assemblages with those of the so-called `Clymeniae Limestone' of Sardinia which is pelagic.

Additional data on the Silurian Ougarta Chains (Aulacogen) is provided by Legrand (1981). Orthocone nautiloids occur in South Algeria in profusion and their sedimentology and taphonomy have been investigated by Wendt et al. (1993) and Wendt (1995).

Western Pyrenees [D]

According to Depéret and Loutrel (1908-9, p. 111) the best outcrops of fossiliferous dark limestone included nodules of various dimensions in which Orthocycloceras? fluminese (Meneghini 1857) (a senior synonym for O. bohemicum Barrande 1866, see Serpagli and Gnoli 1977), Arionoceras? severum (Barrande 1868), Parakionoceras originale (Barrande 1868), Protobactrites styloideum (Barrande 1866), Columenoceras cf. agassizi (Barrande 1866), Metaspyroceras subannulare (Münster 1840) (may belong to Metaspyroceras, see Zhuravleva 1978), and Plagiostomoceras pleurotomum (Barrande 1866) occur. These represent a Silurian nautiloid cephalopod faunule that occurs near the village of Castelnau together with the bivalves Maminka sp. ind., Panenka cf. subaegualis, a pygidium of the trilobite Arethusina, the graptolite Monograptus priodon, and the brachiopod Atrypa cf. philomela.

Spain - Catalunya [E]

Vidal (1875) reports Orthoceras regularis (Schlotheim 1820) and Orthoceras bohemicum Barrande, 1868 [senior synonym of Orthocycloceras? fluminese (Meneghini 1857)] from the central region of Lérida Province and characterizing the Prídolí (see also Serpagli and Gnoli 1977). The presence of the latter was confirmed by Barrois (1901). Faura Hy Sans (1913) notes "Orthoceras" limestone with the bivalve Cardiola interrupta (?Wenlock-Ludlow). Gómez-Alba (1988) reports from Catalunya: Columenoceras grande (Meneghini 1857) and Parakionoceras originale (Barrande 1868), and Vericeras? barroisi (Vidal 1914). Note that Orthoceras barroisi Vidal 1914 is here regarded as dubiously belonging to Vericeras Kolebaba because Vidal in the description of his newly proposed species writes: "y se distinguen por las finas y rectas costillas longitudinales que con mucha regularidad cubren la concha. Esta, en vez de aparecer todas iguales, como en el O. originale, presentan diferente relieve, aunque no mucho, distiguiéndose costillas principales y otras secundarias". These features are peculiar to the Kolebaba genus Vericeras. It is suspected that the correct identification is Vericeras cf. ambigena (Barrande 1867).

Eastern Pyrenees - Aquitaine [F]

These areas are geologically linked by the collision that allowed subduction of the southern Iberian Plate (Ebro pro-foreland basin) below the northern European Plate (Aquitaine retro-foreland basin) beyond the eastern Pyrenean axial zone (Morris, in press) in which Silurian sedimentary terranes were mixed.

Vidal (1886) in the Gerona Province and 1914 in the Upper Silurian (? Prídolí of the Catalan Pyrenees) records Orthocycloceras? fluminese (Meneghini 1857), ?Orthoceras distans Barrande (probably ?Orthoceras distans Barrande, see Sowerby in Murchison 1839), ?Rayonnoceras giganteum (Sowerby, 1819), Cycloceras striatum (Sowerby 1840), O. laterale Phillips, 1836, O. tenuis Wahlenberg, 1821, Mimogeisonoceras timidum (Barrande 1866), O. placidum Barrande 1868, Parakionoceras originale (Barrande 1868), Armenoceras nummularium (Sowerby 1839).

Armorican Massif [G]

 From this area Babin (1966) described and figured the following Silurian and Early Devonian nautiloid cephalopods: Endoceras? sp., Ormoceras puzosi (Barrande 1866), Ormoceras multicamerata Babin, Kopaninoceras jucundum (Barrande 1870), Arionoceras repetitum (Barrande), Arionoceras cf. repetitum (Barrande, 1866), Orthocycloceras? cf. fluminese (Meneghini), Temperoceras temperans (Barrande 1867), Harrisoceras vibrayei (Barrande 1859), Murchinsoniceras murchisoni (Barrande 1867), Dolorthoceras occidentale Babin, Dolorthoceras n. sp., Mimogeisonoceras cf. barbarum (Barrande 1870), Geisonoceratoides? bicingulatum (Sandberger and Sandberger 1852), Geisonoceroides? sp. 1, 2, Spyroceras pulchrum (Barrande 1866), Spyroceras calamiteum (Münster 1840), Calorthoceras pseudocalamiteum (Barrande in Quensted1851), Spyroceratinae gen. indet. reluctans Barrande, 1868, Arthrophyllum vermiculare (Termier and Termier 1955), Arthrophyllum gracile (Termier and Termier 1955), Jovellania buchi (de Verneuil 1850), Jovellania cf. kochi (Kayser, 1878, revised by Zhuravleva 1974 as belonging to Tripleuroceras), Cyrthoceratites? zeilleri (Bayle 1878), and Cyrthoceratites ? sp. 1-3.

In the Prídolí of 'la Meignanne' Kríz and Paris (1982) reported the Cheiopteria bridgei and Snoopya insolita assemblages, including Orthocycloceras? fluminese (Meneghini 1857), Parakionoceras sp. and Orthoceratida indet. div. spp. Babin et al. (1979) also described the nautiloid cephalopods Arionoceras arion (Barrande 1868) [= A. affine (Meneghini 1857)] and Parakionoceras sp. from the Prídolí of the `Vouned Section'.

Montagne Noire [H]

 In the southern part of the Montagne Noire M.-C. Chaubet (1937, pp. 84, 175) reports that at "Col de l'Orte" and "Combe d'Yzarne" the following nautiloids occur: Sphooceras truncatum (Barrande 1860), "Orthoceras" pyrenaicum Leymerie, Vericeras barroisi (Vidal 1914), Michelinoceras? perstrictum (Barrande 1867), M. amoenum (Barrande 1867), Orthocycloceras? fluminese (Meneghini 1857), Parakionoceras originale (Barrande 1866), the Dawsonoceras group of annulatum (Sowerby 1816), the Kopaninoceras? group of cavum (Barrande 1867), Protobactrites aff. styloideum (Barrande 1866), Geisonoceras aff. gryphus (Barrande 1868), Chryzoceras? or Mimogeisonoceras aff. timidum (Barrande 1866), Metaspyroceras aff. subannulare (Münster 1840), Geisonoceras aff. inchoatum (Barrande 1868), and Arionoceras chaubete (Ristedt 1968). Kríz (1996) also reported from this area and the nearby Mouthoumet Massif the bivalve-dominated Cardiola agna figusi Community, Cardiola donigala Community, Cardiola docens Community and the Snoopya insolita Community. Following Chaubet (1937) the ostracodes Bolbozoe? bohemica (Barrande), Bolbozoe? lanceolata Canavari, Kloedenia sp., several Entomis (Richteria) migrans (Barrande) and some telsonal parts of the phyllocarid Ceratiocaris bohemica (Barrande) are also present.

Southwestern Sardinia [I]

 Upper Silurian nautiloid cephalopods of the Fluminimaggiore Formation (Gnoli et al. 1990) have been studied by Serpagli and Gnoli (1977), and Gnoli (1987, 1990, 1992c) who recognised the following nautiloids, most already described by Barrande in Central Bohemia. The following species are all described by Barrande unless otherwise noted. Akrosphaerorthoceras gregale Ristedt, 1968, Hemicosmorthoceras laterculum Ristedt, 1968, H. aff. serratulum, H. semiannulatum, Hem. sp. ind. sensu Ristedt. 1968, Kopaninoceras jucundum, Kopaninoceras? thyrsus, Michelinoceras currens, M. (Shaerorthoceras) beatum (Ristedt 1968, see also see Kiselev and Gnoli 1992), Michelinoceras (Shaerorthoceras) teicherti (Ristedt 1968), Michelinoceras (S.) curvum (Ristedt 1968), Plagiostomoceras gruenewaldti, Plagiostomoceras cf. pleurotomum, Calorthoceras? pseudocalamiteum, Orthocycloceras? cf. lynx, Kionoceras doricum, Parakionoceras originale, Harrisoceras vibrayei, Orthocycloceras? fluminese (Meneghini, 1857), Metarmenoceras meneghinii Serpagli and Gnoli 1977, Oocerina abdita, Oonoceras plebeium, Arionoceras affine (Meneghini 1857), A. submoniliforme (Meneghini 1857), A.? repetitum, Merocycloceras declive Ristedt, 1968, Kosovoceras? sandbergeri, Ormoceras richteri, Pseudocycloceras transiens, Hemicosmorthoceras aff. serratulum, Mericoceras? cf. simois, M.? cf. sericatum, Plagiostomoceras culter, "Parasphaerorthoceras" sp. ind. B sensu Ristedt 1968, "P." sp. ind. H sensu Ristedt, 1968, "P." sp. ind. J sensu Ristedt. 1968, "P." sp. ind. K sensu Ristedt, 1968, Cryptocycloceras? cf. deludens, Vericeras ambigena, Columenoceras grande (Meneghini 1857), C. agassizi, C? degener, C.? intermixtum, Geisonoceras? cf. socium, Murchisoniceras? calamoides, Sphooceras truncatum, Temperoceras temperans, Phragmoceras broderipi subleve, Ph. cf. labiosum, and Protophragmoceras minus. In addition, according to Teichmüller (1931) some other Barrande species (e.g., Kionoceras bronni, Dawsonoceras dulce, and Oonoceras potens) represent the southwestern Sardinian Silurian nautiloid fauna.

From these data it is evident that there is an intimate cephalopod faunal relationship with Bohemia. This link was pointed out by Serpagli and Gnoli (1977), and Gnoli (1990, 1993) also describes other faunal components (e.g., phyllocarids [Ceratiocaris bohemica (Barrande 1872), C. cornwallisensis damesi Chlupáč 1963, C. grata Chlupáč 1984, Echinocaris sp.], eurypterids [Pterygotus divers-Eurypterus pugio (Barrande 1872)] and the problematic organism Kolihaia sardiniensis Gnoli (Gnoli and Serpagli 1984; Gnoli 1992a; 1992b). To sum up, during the Silurian, as already remarked by Kríz (1996, pp. 36-39) "good faunal affinities permit to state intimate relationships between Bohemia, Carnic Alps, southwestern Sardinia and Montagne Noire." These affinities aid reconstruction of the Northern Margin of Gondwana during the Silurian.

From the Early Devonian of southwestern Sardinia, Gnoli (1983) described the following from various outcrops belonging to the Mason Porcus Formation (Gnoli et al. 1990): Hemicosmorthoceras semimbricatum Gnoli, 1983, Kopaninoceras floweri, Gnoli, 1983, K. sp. ind. A and B sensu Gnoli, 1983, Mericoceras karagandense Zhuravleva, 1978, Michelinoceras currens (Barrande, 1870), M. (M.) michelini (Barrande 1866), M. (S.) effrenatum (Ristedt 1968), M. (S.) sp. ind. A and B sensu Gnoli, 1983, Mimogeisonoceras cf. liberum (Barrande 1870), Mimogeisonoceras cf. timidum (Barrande, 1870), Arkonoceras? cf. adjectum Zhuravleva, Plagiostomoceras? squamatulum (Barrande), P. sp. ind. sensu Gnoli, 1983, Akrosphaerorthoceras cf. gregale Ristedt 1968, Orthocycloceras pseudoextensum Gnoli, 1983, Murchisoniceras? subnotatum (Barrande 1868), Disjunctoceras sp. ind. sensu Gnoli and Kiselev, 1994, and Jovellania buchi (de Verneuil 1850).

The lack of modern taxonomic revision and unsolved problems of synonymy prevent direct comparison of the Sardinian fauna with coeval assemblages from central and southern Europe (Gnoli, 1983, p. 75). However, the beginning of the Devonian seems to signal a break in the close Silurian relationship between the Sardinian and Bohemian faunas.

Prague Basin (Czech Republic) [J]

Quite recently Kríz (1998) recognized eight recurrent events in account seven areas (ibid. figs. 1, 5) from the Middle Wenlock to the Lower Devonian. Cephalopod limestones occur in the Testograptus testis, Colonograptus colonus, upper Saetograptus chimaera, lower S. linearis, Monograptus fragmentalis and M. ultimus, upper M. transgrediens, and lower M. uniformis uniformis biozones, plus another level recovered by Š. Manda (Kriz, personal commun., 2002) at the base of the Saetograptus chimaera Biozone.

Kríz (1998, pp. 184, 197) wrote that cephalopod limestone biofacies "usually mark a significant change in sedimentation in a generally low-energy environment in sequences dominated by shale with micritic limestone intercalations. Deposition of the cephalopod limestone facies took place in the upper photic zone below wave base, where surface currents reached the bottom and aligned the shells of nektobenthic cephalopods... indicating eustatic lowstands, of which most are in agreement with the standard Silurian sea-level curve (Johnson 1996). Other occurrences may be interpreted as lowstands caused by synsedimentary tectonics." This area remains in a northern position if geographically compared to already reported localities and, furthermore belongs to the `Perunica' micro paleocontinent (Havlícek et al. 1994; Havlícek 1999).

The Prague Basin has benefited from the extensive work of Barrande (1860, 1865-1874) who first described and figured more than a thousand Silurian-Lower Devonian nautiloid cephalopods. Many scientists, from the nineteenth century to now--including the author in 1999--have tried to carry out systematic revision of this giant work. However, lack of topotype Barrandian material has, thus far, prevented the development of a truly phyletic classification. The original material represents only comparable type-material useful to "routine" taxonomical work. Without judging the taxonomic validity of a number of Barrande's taxa, the summary of Barrandian revised taxa are presented in Table 1.

For details see Gnoli (1999). In addition, there are a further dozen species revised by Marek (1998) and Manda (1999, 2001). A more complete report of valid genera is beyond the scope of this investigation. For example, Cyrtoceras Goldfuss 1832 is now divided into more than 30 cyrtochoanitic-type genera. Moreover, with few exceptions, in the Barrandian literature it is very difficult to find a nautiloid cephalopod taxon not already reported in any of the Gondwanan Europe and North Africa Terranes.