Chitinozoan bio-event in the Frasnian-Famennian boundary beds at La Serre (Montagne Noire, Southern France)

PALAEO, ELSEVIER

Palaeogeography, Palaeoclimatology, Palaeoecology 121 (1996) 131- 145

Chitinozoan bio-event in the Frasnian-Famennian boundary beds at La Serre (Montagne Noire, Southern France) Florentin Paris a, Catherine Girard b, Raimund Feist b, Theresa Winchester-Seeto ° a Laboratoire de PalOontologie et de Stratigraphie, URA 1364 du CNRS, UniversitO de Rennes I, 35042 Rennes-cedex, France Institut des Sciences de l'Evolution, URA 327, UniversitO de Montpellier II, Place E. Bataillon, 34060 Montpellier Cedex, France c School o f Earth Sciences, Centre for Ecostratigraphy and Palaeobiology, Macquarie University, NSW, 2109, Australia Received 23 March 1995; revised and accepted 4 September 1995

Abstract

Chitinozoans and other palynomorphs have been investigated in Frasnian-Famennian boundary beds from trench C at La Serre (Montagne Noire, Southern France). This section is located some 30 km from the Coumiac quarry containing the Global Stratotype Section and Point (GSSP) for the base of the Famennian. At La Serre, the FrasnianFamennian ( F - F ) boundary falls within interbedded anoxic dark shales and limestones. This lithology proves to be more suitable for palynological investigations than the reddish nodular limestones of the F F stratotype in the Coumiac section, which are virtually devoid of organic matter. At La Serre, the uppermost meter of Frasnian deposits, corresponding to the Upper Kellwasser Event level, contains only a few chitinozoans but other palynomorphs, especially tasmanaceans (Maranhites) proliferate in some horizons. Conversely, the most basal Famennian bed yields an exceptional number of chitinozoans (up to 19,000 specimens per gram of rock), a few tracheids, but virtually no tasmanaceans. The absence--or the extremely low n u m b e r s - - o f miospores and acritarchs in these F - F boundary beds seems related to the distal position of the deposition site. We have compared the fluctuation of the chitinozoan abundance both with the abundance of the other palynomorphs, and with that of the conodonts present in the same samples. These data, collated with sedimentological and palaeoecological information document the relationships between the anomalous abundance of the chitinozoans and the latest Frasnian-earliest Famennian mass extinction. It is concluded that the exceptional chitinozoan concentration in the most basal Famennian bed at La Serre is not related to a sorting process generated by turbidity currents or by a temporary increase in the hydrodynamic energy at the water-sediment interface. It corresponds to the conjunction of a high production of chitinozoans, a very low rate of destruction of the vesicles and minimal dilution of the deposit by terrestrial mineral input. The environmental factors leading to such an exceptionally high record of chitinozoans are discussed with a particular attention to consequences of the late Frasnian mass extinction.

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132

E Paris et al./Palaeogeography, Palaeoclimatology, Palaeoecology 121 (1996) 131 145

1. Introduction The only Late Devonian chitinozoans known in southern Europe were recovered from the southeastern Montagne Noire "l~cailles de Cabribres" area. They occur in dark marly mudstones of the La Serre Formation that characterises the oxygen deficient offshore basinal environment during Frasnian through early Famennian times in that area (Feist, 1985). This local facies development contrasts sharply with that of the contemporaneous, well oxygenated red calcilutites in the Montagne Noire nappe area where the stratotype of the Frasnian Famennian boundary is defined at Coumiac, some 30 km to the SE of Cabribres (Klapper et al., 1993). The La Serre Formation is best exposed in several artificial trenches on the southern slope of La Serre hill, at 2.5 km S of Cabribres, in the vicinity of La Roquette farm (Fig. 1). Preliminary investigations in relation to the search for the Middle-Upper Devonian Series boundary had already revealed the presence of chitinozoans in the basal part of the formation (Paris and Feist, 1983). We report here on the discovery of palynomorphs from beds across the Frasnian Famennian boundary in trench C of La Serre, which exposes the higher part of the formation (Fig. 2). Section C has been the subject of intense biostratigraphic investigations by the Subcommission on Devonian Stratigraphy (SDS/ICS) in order to challenge the F F stratotype section at Coumiac (House et al., 1985; Schindler, 1990; Feist, 1990; Becket, 1993). Recently, Girard (1994) carried out a high resolution conodont biostratigraphic study based on population dynamics, especially trends in diversity and abundance. According to these data the level of the terminal Frasnian mass extinction and the position of the F F boundary are precisely documented in a complete succession without a sedimentary gap. According to the definition agreed upon by SDS, the base of the Famennian coincides with the first appearance, at the base of bed 14 f, of Palmatolepis triangularis, i.e. the index of the earliest Famennian conodont zone. The unexpected abundance of chitinozoans in this bed, right above the boundary, is of particular interest with regard to the F - F mass extinction and its

aftermath. The interval of the section considered for this palynological study (i.e. beds 13 15, Fig. 2) starts with light grey calcilutites rich in pyrite and benthic biotas such as small rugose corals, brachiopods and trilobites (beds 12a 14a). These are abruptly overlain by a sequence of alternating dark brownish and black, fissile argilires and platy thin-bedded bituminous marly limestones (beds 14a e), that are considered to be equivalent to the Upper Kellwasser interval (Schindler, 1990). At La Serre, the fine-grained laminated pattern of the sediments that contain exclusively pelagic, necto-planktonic biotas of minute size such as homoctenids, conodonts, entomozoan ostracods, buchiolid bivalves, but in which cephalopods have not been observed, contrasts with that of the normally much coarser grained, cephalopod-rich Kellwasser limestone. The La Serre equivalent may constitute a rather distal and basinal substitute for the typical Kellwasser limestone which is deposited on submarine rises. In contrast to sections such as Coumiac and Steinbruch Schmidt, where the typical Kellwasser horizon is interbedded in a homogeneous succession of oxygenated limestones, the hypoxic environment of the terminal Frasnian at La Serre lasted far beyond the F F boundary, being replaced by red griotte limestone no earlier than in late early Famennian rhomboidea time. Unlike other European sections, the facies development of La Serre is more comparable to that of SE Morocco (Wendt, 1985; Schindler, 1990).

2. Palynological investigations Sixteen samples were collected bed by bed from a 1.60 m-thick sequence of late Frasnian early Famennian micrites exposed in trench C at La Serre (Fig. 2). Palynological investigations were carried out on all these samples using the technical procedure described by Paris (1981) for chitinozoans and other organic-walled microfossils of Palaeozoic age. The average sampling interval of about 10 cm enables an accurate record to be made of the fluctuations in the abundance of the recorded palynomorphs. In addition, such closelyspaced sampling allows the precise resolution of

F. Paris et aL/Palaeogeography, Palaeoclimatology, Palaeoecology 121 (1996) 131 145

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any event perturbing the settling out of these organic-walled microfossils. Particular attention is paid to the quantification of the palynologic data, e.g. the abundance of each group of palynomorphs is expressed in numbers of specimens per gram of rock according to the counting procedure described by Baudu and Paris (1995). The unusu-

ally high abundance recorded for some palynomorphs groups (e.g. chitinozoans in bed 14fl) made it necessary to restrict the counting to a fraction (e.g. one thousandth in the case of sample 14f) of the whole palynological residue recovered from 20 g of rock. As a consequence, the precision of the abundance estimate is slightly modified.

F Paris et al.,/Palaeogeograph), Palaeoclimatolog),, Palaeoecology 121 (1996) 131 145

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However, based on several tests, the resulting error is less than 10%. This does not have real impact on the general conclusions, since the a n o m a l o u s concentrations observed in some beds are about three orders o f magnitude higher than the usual values reported in the literature for limestone samples. Where there is very low chitinozoan abundance (i.e. close t o - - o r less t h a n - - o n e specimen per g r a m o f rock) within a palynological residue extremely rich in other p a l y n o m o r p h s o f similar size (e.g. tasmanaceans, microforaminiferal linings), it is necessary to carefully investigate the

whole residue so as not to miss the few chitinozoans hidden a m o n g the other palynomorphs. The observed abundances o f p a l y n o m o r p h s in the rock samples collected in section C at La Serre prove to be highly variable. Some lithologies, especially the pinkish-beige micritic limestone o f bed 13, are unsuitable for p a l y n o m o r p h studies and, for example, only a single scolecodont was recorded in sample 13b. Highly weathered material is also very unfavourable. In fact, the chocolatecoloured marls (samples 14a, 14b2, 14d2, 14e, 15a) interbedded between the limestone beds are barren

F Pariset al./Palaeogeography,Palaeoclimatology,Palaeoecology121 (1996) 131-145 (Fig. 2). The organic matter here has been destroyed by oxidation due to sub-recent weathering processes. On the other hand, after removal of their partly-weathered external surfaces, the black micritic limestone beds can yield abundant organic-walled microfossils (e.g. samples 14bl, 14cl, 14c2, 14dl, 14fl-3, 14g and 14h).

3. Palynological results Only the abundances of chitinozoans, scolecodonts and tasmanaceans have been quantified (Fig. 2). Microforaminiferal linings are very abundant in some samples (e.g. several thousand per gram of rock in sample 14dl). However, they are not counted because they are usually difficult to identify in permanent palynological slides. For tracheids, woody debris, and various "organic tubes", only a simple indication of occurrence is given because any recorded fragment may result from the breakage of coarser debris during laboratory treatment. In such cases, a precise estimate of abundance has no real meaning. The preservation of the palynomorphs is fairly poor; the material has been affected by low-grade thermal alteration rendering the organic membranes and walls opaque. However, the chitinozoan vesicles are still preserved in full relief in the limestones. Hydrocarbon generation during the early diagenetic history of these anoxic deposits appears to have led to the coating of some microfossils by bituminous matter that simulates geometrical ornamentation (e.g. casts of calcite crystals). The specific diversity of the palynomorphs is extremely low. As regards the chitinozoans, only Angochitininae were collected in these beds. With the unique exception of the possible chitinozoan fragments in sample 14b, and a few Ramochitina sp.1 observed in sample 14g, all the chitinozoans recorded in the investigated material belong to the genus Angochitina. However, clear differences exist between the latest Frasnian forms from sample 14dl and Angochitina sp. 1, the earliest Famennian chitinozoan form composing the mono-specific assemblage of bed 14f. From its outline, Angochitina sp. 1 resembles the specimens described under the name of

135

Sphaerochitina nodulosa var. hispida by Taugourdeau and de Jekhowsky (1960) from the Upper Devonian of the Bg-1 drill-hole (depthreading: 866 m) in the south-western Algerian Sahara. The original diagnosis of this variety and its description under the light microscope is unfortunately insufficient to enable a precise comparison with the present material which, due to its opacity, was studied by scanning electron microscopy. It is noteworthy that the spiny ornamentation of some of the studied specimens tends, in some cases, to show an alignment, sometimes with thick spines developing from short stretched ridges parallel to the axis of the chamber (Plate I, 8-9). This feature was not recorded by Taugourdeau and De Jekhowsky (1960). Thus, a review of the Saharan material is necessary before attempting a precise determination of the specimens from La Serre. Tasmanaceans are represented in beds 14c2, 14dl, 14d2, 14f, 14g and 14h by the genus Maranhites (i.e.M. brasiliensis or M. britoi), but the opacity of the material prevents a precise determination. Certain authors (Tappan, 1980) classify these lenticular microfossils with the prasinophyceans (unicellular green algae). The chemical composition of the tasmanaceans is favourable to hydrocarbon formation and these microfossils are commonly associated with peaks in radioactivity measured in oil industry drill holes in the Silurian and Devonian of the Sahara (Combaz, 1966). 3.1. Biostratigraphy and chronostratigraphy Taxonomically useful data on Upper Devonian chitinozoans is sparse. Towards the north of Gondwana, particularly in the eastern Algerian Sahara and western Libya, where palynological studies have been the most numerous (see references in Paris, 1996), a sedimentary hiatus cuts out a large part of the Upper Devonian, leading to a lack of information on chitinozoans from the Frasnian-Famennian boundary (AbdesselamRouighi, I986; Boumendjel et al., I988; MoreauBenoit et al., 1993). In Morocco, late Frasnian and early Famennian chitinozoans have been reported in the E1 Atrous section (Tafilalt) where they are mixed with late Ordovician to early Devonian reworked forms (see discussion in Paris

F. Paris et al./Palaeogeography, Palaeoclimatology, Palaeoecology 121 (1996) 131 145

136

PLATEI

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F Pariset al./Palaeogeography, Palaeoclimatology,Palaeoecology121 (1996) 131-145 and Elaouad-Debbaj, 1989). In E1 Atrous, specimens practically identical to Angochitina sp. 1 are recorded in the upper part of bed " H " , in association with Ramochitina sp. 1, another chitinozoan taxon present in bed 14g at La Serre (early Famennian). Bed " H " at E1 Atrous contains P. triangularis, the conodont index for the basal Famennian (Becker et al., 1988). In Belgium, the Matagne-like shales representing the upper part of the Frasnian near Frasnes yield some acritarchs and abundant chitinozoans (Martin, 1982). These latter make up more diverse assemblages dominated by Hoegisphaera glabra Staplin and forms attributed by Martin (1982) to Sphaerochitina cf. S. sphaerocephala and Angochitina sp. A. This last form is very close to the latest Frasnian Angochitina form recorded in sample 14dl at La Serre, but exhibits fairly distinct differences (less globose chamber and less robust ornamentation) to Angochitina sp. 1 from bed 14f at La Serre. In Senzeilles section, exposing the old Belgium stratotype of the Frasnian Famennian boundary (i.e. before the definition of the GSSP at Coumiac), Angochitina sp. 1 is probably present (specimens labelled Gotlandochitina sp. 1 by Martin, 1990). Typical examples of H. glabra, a species first described from the Frasnian of Alberta (Staplin, 1961), have been observed in the Montagne Noire (France) from beds 24a and 24b of section A' at La Serre (Paris and Feist, 1983). These beds belong to the lower Frasnian (Lower asymmetricus Zone

137

of the Montagne Noire conodont zonation; i.e. Zone 4 of Klapper, 1988, with Palmatolepis transitans). The species is also present in section C at La Serre in samples taken at 6.27, 6.07, 5.57, 3.87, 3.27 and 2.73 m below the F - F boundary (Winchester-Seeto and Paris, 1995). Additional reports of H. glabra are from the Upper asymmetricus Zone in the Bois Member of the Ferques Formation, Boulonnais area, northern France (Taugourdeau, 1965; Paris, 1988). Consequently H. glabra is typically a Frasnian species that does not range beyond the Frasnian-Famennian boundary. In the absence of precise stratigraphic data concerning coeval assemblages, the chitinozoans found in the basal Famennien of section C at La Serre currently offer no basis for correlation with other sequences, excepted with the E1 Atrous and Senzeilles sections (see above). Independent stratigraphic control is provided by the conodonts present in the section from Montagne Noire (Fig. 3) and in E1 Atrous. It therefore seems possible to make use of Angochitina sp. 1 as a biostratigraphic marker in other sections that lack conodonts (e.g. Senzeilles). A m o n g the phytoplankton, tasmanaceans contribute very little to accurate biostratigraphic dating. The genus Maranhites is found in the Upper Devonian (especially the Frasnian) of Western Europe, N o r t h Africa and Central Africa, as well as in Australia and particularly South America, where it was first described by Brito

PLATE I Chitinozoan taphonomy in bed 14f of section C at La Serre (Montagne Noire, southern France). 1. Thin-sectioncut perpendicularly to the bedding showing the lamination (x 3; the bar represents 10 ram). 2,3. Scanningelectron microscopephotos of chitinozoan taphonomy in bed 14f. 2, (× 20; the bar represents 1 mm) etched polished surface cut perpendicularly to the bedding; the square surrounds the chitinozoan specimens illustrated in fig. 3 (lower right quadrant) and 7. 3, (x 50; the bar represents 500 ~m) detail of the etched polished surface shown in fig. 2; IGR 53166. 4,5. etched bedding plane; note the random orientation of the chitinozoan vesicles. 4, (x 50; the bar represents 500 ~m). 5, (x 70; the bar represents 500 ~m); IGR 53167. 6. (x 350; the bar represents 100 ~tm)fragment of tracheids extracted from bed 14f; IGR 53170 (M.40). 7. ( x 300; the bar represents 100 ~tm)two specimens of Angochitina sp. 1 in situ in micritic limestone; detail of fig. 3; IGR 56166. 8. (× 350) extracted specimen ofAngochitina sp. l; I(3R 53165 (M.36). 9. ( x 700; the bar represents 25 p,m) lateral view showing a detail of the spiny ornamentation of the flanks of Angochitina sp. 1; IGR 53165 (0.36.2). The illustrated material is preserved in the collections of the "Institut de G6ologie de Rennes" (IGR 53165 to 53170). England-Finder co-ordinates are used to locate the extracted specimens on the palynologicalslides.

138

F Paris et al./Palaeogeography, Palaeoclimatologv Palaeoecology 121 (1996) 131 145

La Serre Section C 15=

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Fig. 3. Distribution and abundance of the recorded conodont genera in the Frasnian Famennian boundary beds in section C at La Serre (Montagne Noire, southern France) (from Girard, 1994). Legend as in Fig. 2.

(1965) (see references in Le H6riss6 and Deunft, 1988). 3.2. Palaeoenvironement control of the palynomorphs at La Serre The most striking feature of the palynological study is the anomalously high abundance of chitinozoans in bed 14f, just above the Frasnian Famennian boundary ( F - F ) . Quantification of chitinozoan abundance across the Frasnian Famennian boundary in section C from La Serre reveals a remarkable concentration of these microfossils, with nearly 19,000 specimens per gram of rock at the base of bed 14f. This concentration is

actually six times greater than the highest value recorded so far, which is 3000 specimens per gram of rock in basal Llandovery black shales of Saudi Arabia (Paris et al., 1995). In general, the abundance of chitinozoans in Devonian carbonates of the Gondwanan realm is of the order of several tens to about one hundred individuals per gram of rock (Paris, 1981; Baudu, 1994; WinchesterSeeto, 1993, 1994). By comparison, the concentration of chitinozoans in bed 14f is 2 3 orders of magnitude greater than the average. This exceptional abundance could equally reflect a selective sorting of the chitinozoans, condensed deposits, a mass extinction, extremely high primary production and/or a lack of predators. Clues as to

F. Paris" et al./Palaeogeography, Palaeoclimatology, Palaeoecology 121 (1996) 131-145

reasons for this unexpected result can be gleaned from all aspects of the environment in which the organisms lived, died and were preserved. In the rock record such information is revealed by studying the composition of the fauna, the sedimentary features, and from evidence of global events registered in the F - F boundary beds. The organic residue recovered after palynological treatment of a sedimentary rock sample is the result of an interaction between numerous competing factors (Fig. 4). Some of these factors favour the concentration of organic particles (e.g. slight increase of the hydrodynamic regime; low terrestrial or biogenic mineral input in condensed deposits) and their preservation (e.g. low grade metamorphism; lack of weathering; anoxic conditions both in the water column and on the sea bottom preventing destruction of the palynomorphs by endofauna or aerobic microorganisms; absence or low density of suspensivorous or planktic feeders). Others, on the contrary,

139

hinder its deposition while bringing about the selective sorting of certain taxa (e.g. moderate to medium hydrodynamic regime), or lead to the decomposition of organic remains during their transfer through the water column (e.g. well oxygenated marine environment with numerous planktic feeders) or after their sedimentation (e.g. fungi, bacteria, mud-dwellers) (see discussion in Baudu, 1994; Paris, 1996). A quantitative estimate of the composition of a palynological residue can thus yield some information on the history of the host sediment and the environmental conditions that prevailed during its deposition. By using independent controls, such as those provided by sedimentological or ichnological criteria, it is possible to determine the influence of certain limiting factors including, for example, the hydrodynamic regime operating during deposition of the palynomorphs, or the oxygenated or anoxic environment existing on--or close to--the sea bottom.

"Planktonic rain"

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Processing Metamorphism Weathering Fig. 4. Schematic diagram illustrating the main factors controlling the sedimentation and the preservation of Palaeozoic palynomorphs. The arrows oriented outside symbolise the depleting factors and the two arrows oriented inside correspond to extra input of palynomorphs.

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E Paris et aL/Palaeogeography, Palaeoclimatology, Palaeoecology 121 (1996) 131 145

3.3. Composition of the fauna across the F-F boundary beds The palynomorph assemblages recovered close to the F - F boundary in section C at La Serre display several distinctive characteristics (Fig. 2): (1) an almost total lack on both sides of the F F boundary of the lighter or more delicate types of palynomorphs, whether marine phytoplankton (acritarchs) or miospores of continental origin, despite the fact that these forms proliferated in other parts of northern Gondwana during the Upper Devonian (Abdesselam-Rouighi, 1986; Boumendjel et al., 1988; Loboziak et al., 1991, 1993; Moreau-Beno~t et al., 1993; Martin, 1993 for references on acritarchs); (2) the occurrence of a small amount of plant debris (i.e. tracheids, "organic tubes") in the palynological residues, especially in the lowermost Famennian beds (e.g. beds 14f, 14h); (3) the very low abundance of chitinozoans in contrast to a proliferation of tasmanaceans and microforaminiferal linings, just below the F - F boundary (e.g. in bed 14d); (4) the exceptional abundance of chitinozoans in bed 14f, varying from 19,000 specimens per gram of rock at the base to 13,000 specimens per gram in the middle part of the same bed, but the virtual lack of tasmanaceans and microforaminiferal linings. These characteristics are discussed in order below.

Paucity of small thin-walled palynomorphs close to the F-F boundary The scarcity of small, thin walled palynomorphs close to the F F boundary may result either from the weathering, from a too high hydrodynamic regime, or from another palaeoenvironmental control. The intense sub-recent meteoric weathering affecting the shale interbeds in section C at La Serre has completely destroyed the organic membranes by oxidation. Hence, material from these shaly beds cannot be used to determine the factors influencing the distribution and abundance of the palynomorphs. In the carbonate beds, on the other hand, it is hardly realistic to invoke weathering, low-grade metamorphism, or oxidation effects to explain the lack of the more delicate organicwalled microfossils, since other members of the palaeoplankton are present in the rock, sometimes

in extremely high concentrations (e.g. chitinozoans in bed 14f or tasmanaceans in bed 14dl; see Fig. 2). Other explanations need to be put forward, while successively testing the effects of the principal environmental parameters, i.e.: energy regime of the depositional environment, palaeobathymetry, distance from the coast. The primary production of marine phytoplankton, and the preservation of the palynomorphs during their transfer through the water column and after their sedimentation must also to be explored. Samples of black micritic limestone from this section show no evidence of a flow regime sufficiently strong to prevent the sedimentation of small-sized palynomorphs (see discussion below for bed 14f). Fine-grained shales usually provide crucial information on the effects of depositional environment on palynomorph abundance. Unweathered marls and interbedded black micritic limestones in the Silurian Devonian stratotype section at Klonk (Bohemia) have been used to document the usual abundance of the palynomorphs in these two lithologies, because at La Serre the shales are too weathered and therefore cannot be used to assess the energy of the hydrodynamic regime. Study of the sedimentary facies at the Bohemian locality indicates little change in the hydrodynamic regime between the marly and anoxic micrite intervals. In fact, the composition of the palynomorph assemblages remains fairly constant despite the lithological variation (Paris et al., 1981 ); no significant increase in acritarch or miospore abundance is observed in the marly intercalations at Klonk (Deunff, 1980; Richardson, 1984). Since the absence or extreme paucity of very small palynomorphs in the material from La Serre cannot be due to agitation of the sedimentary environment, other types of explanation should be sought. In general, the primary production and diversity of Palaeozoic phytoplankton (including acritarchs) was at a maximum in open shelf type environments (Dorning, 1981; Al-Ameri, 1983; Wicander and Playford, 1985; Le H6riss6, 1988). The nearer shore deposits display lower phytoplankton abundances and diversities, while more distal facies are characterised by a considerable reduction in ornamented forms at the expense of smooth forms with

F Paris et al./Palaeogeography, Palaeoclimatology, Palaeoecology 121 (1996) 131-145

thick walls (e.g. tasmanaceans). Although this palaeoenvironmental pattern needs to be further refined, its broad outlines are now generally accepted (Richardson and Rasul, 1990). By applying this model to the material from La Serre, the lack of ornamented acritarchs, taken together with the proliferation of Maranhites and the lack of benthic faunas, suggests a relatively deep distal sedimentary environment (i.e. outer shelf/upper slope). The distance from sources of continentally-derived debris is supported by the almost total lack of miospores in the palynological residues.

Occurrence of plant-debris The presence of a few small organic "tubes" and tracheid fragments (length less than 1 mm; Plate I, 6) in beds 14f, 14g and 14h would imply a modification of this interpretation. It should be stressed that this type of plant micro debris is normally extremely abundant in lacustrine or nearshore deposits of late Devonian age. In this context, about ten fragments per gram of rock were recovered in sample 14h, which is so far one of the richest samples among the investigated material from La Serre. Consequently, this section should be regarded as very poor in plant material in comparison to other micro debris-bearing deposits (e.g. deltaic deposits). An increase in the abundance of vegetal debris is usually taken as indicating the relative proximity of a continental source (Richardson and Rasul, 1990). In the Montagne Noire area, the general structural setting, the discontinuous exposure of the Upper Devonian and its position in nappes or thrust slices prevent detailed reconstruction of the palaeogeographic setting at the time. On the other hand, there is no reason to suppose that the Frasnian Famennian deposits of the upper member of the La Serre Formation were not laid down relatively far from the coastline. The minute plant debris (tracheids and "organic tubes") could represent drifted remains (e.g. roots) of near-shore plants washed out during a regressive marine phase that took place during the basalmost Famennian (Johnson et al., 1985). Such a hypothesis is supported by the significant increase of Icriodus in bed 14f (Fig. 3), as a high ratio of this conodont

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genus is usually regarded as an indicator of marine shallowing (see discussion in Girard, 1994).

The proliferation of Maranhites The abundance of tasmanaceans in the anoxic beds corresponding to the Upper Kellwasser event is not surprising as they have been regarded as cysts of green algae (Tappan, 1980), which would have developed in the photic zone, above the upper limit of anoxia, before they sank. Conversely, the anoxic environment would have favoured preservation of Maranhites cysts by preventing their oxidation at the sediment-water interface, or consumption by mud-dwellers (limivorous organisms). A similar depth/anoxia control explanation may also account for the great abundance of the microforaminiferal linings in the same samples. The relative rarity of chitinozoans in the same beds may result from the high concentration of the organic matter (Maranhites, microforaminiferal linings, amorphous organic matter) acting as a dilution factor, obscuring a more normal production of chitinozoans.

Sedimentary features of the ~ F boundary beds In order to investigate the fine fabric of the rock making up bed 14f in section C at La Serre, polished surfaces were cut both perpendicular and parallel to the bedding and then treated for 5 rain with 2% formic acid. This revealed the stratification, with laminae ranging from 0.01 to 0.5 mm in thickness (Plate I, 1 5). Although found elsewhere in the rock, the chitinozoans are generally concentrated close to the carbonaceous laminae, with their longitudinal axes lying parallel to the microlamination (Plate I, 2,3). Other laminae, devoid of chitinozoans, are picked out by thin layers of pelagic entomozoid ostracods (frequently disarticulated valves) (Plate I, 1,2). The association of laminae with an exceptional concentration of chitinozoans in bed 14f does not appear to have arisen by chance. Therefore, the origin of the lamination should be discussed. Deposition from a turbidity current can be ruled out due to the lack of grading of the bioclasts within the bed. Other hypotheses may be proposed, e.g. (1) temporary increase of the hydrodynamic energy resulting in a selective sorting of the chitino-

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zoans and calcareous bioclasts; (2) condensed sedimentation with episodic deposits of small bioclasts (i.e. disarticulated valves of entomozoid ostracods). According to the first hypothesis, during accumulation of bed 14f, there was a perfect match between the energy of the flow regime and the hydrodynamic behaviour of the chitinozoans (i.e. removal of muddy material by winnowing out of very fine-grained particles leading to a concentration of the chitinozoans). This energy must have been temporarily relatively high since ostracods also are concentrated within certain laminae. This sudden change in the energy of the depositional environment could have resulted from an event such as a major storm, or alternatively a tsunami following an earthquake or the impact of a large meteorite. In either case, the effect was very distal since the microlaminae are parallel and planar (Plate I, 1). Concerning this last factor, however, evidence of proximal effects of very high energy events are missing in most of the contemporaneous sections (see discussion in Goodfellow et al., 1988). Breccias reported in Steinbruch Schmidt in Germany and from Devils Gate, Nevada, considered as storm or tsunami deposits (Sandberg et al., 1988), may be reinterpreted as local synsedimentary or gravitational flow phenomena. On the other hand, deep erosion processes generating the reworking of Ordovician to Devonian chitinozoans in E1 Atrous section, Morocco (Paris and ElaouadDebbaj, 1989) are related to tectonic instability and block tilting (Wendt, 1985). The hydrodynamic explanation thus seems unlikely. The second hypothesis demands a very low terrigenous input preventing the dilution of chitinozoan vesicles in the sediment. The concentration of the chitinozoans depends on the duration of the phases of lull in bioclastic supply (i.e. ostracod valves). Such a condensation of the sedimentation could be registered in the neighbouring section at Coumiac ( F F GSSP) by the occurrence of a thin iron crust at the base of bed 32a (Feist, 1990), which yields the first P. triangularis. However, extremely low rates of sedimentation are not corroborated by any obvious increase of the conodont abundance in bed 14f (Girard, 1994). The arrangement of the chitinozoans (i.e. long

axes of the vesicles parallel to the bedding) must be controlled by very quiet nepheloid currents that were not able to introduce any obvious linear orientation of the vesicles (Plate I, 4 5). The almost total lack of tasmanaceans in this bed is surprising. This absence could be related either to a specific temporary nutriment deficit, to the absence of encystment or to total consumption of these unicellular algae by phytoplankton feeders. Beneath the Frasnian Famennien boundary, conditions were again more favourable for the proliferation of Maranhites than of chitinozoans, whereas miospores and acritarchs remained absent. Global events recorded in the F - F boundary beds' The occurrence of chitinozoan mortality on a massive scale in bed 14f appears unlikely since chitinozoans are rare or virtually absent in the immediately underlying beds (Fig. 2), and extinction in other groups (e.g. homoctenids) is recorded in bed 14d (Feist, 1990). The biology of chitinozoans is not yet understood. In particular, it is not known whether they were subject to sudden bursts in proliferation or blooms, as observed in the case of dinoflagellates, or mass releases as exhibited in the spawning of certain organisms. The moderate fluctuations in chitinozoan abundance measured in sequences from the northern Gondwana Province rather militate against such an overproduction hypothesis. Nevertheless, it is possible that a mass extinction of other organisms (e.g. homoctenids) acting as predator on the chitinozoan vesicles or to their parent-organisms (hypothesis of chitinozoophorans, in the sense of Grahn, 1982) could have allowed the preservation of almost all the chitinozoan vesicles, or produced a vacant ecological niche that was immediately colonised by a chitinozoan (or a chitinozoophoran) species. In such a context, the chitinozoans (or chitinozoophorans) would have behaved in a highly opportunistic manner. This type of ecological control has already been invoked for the recolonization of shallow marine environments after a meteorite impact during the Ordovician in Scandinavia (Grahn et al., 1996). However, the abundances reported in such cases are incomparably lower than those found at La Serre. Moreover, no exceptional chi-

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tinozoan concentrations have been noted in contemporaneous beds at E1 Atrous or Senzeilles. Therefore, the factor controlling the proliferation of the chitinozoans may be a local amplification of a wider regional or global event. Concerning the chitinozoan production, it should be noted that during Ordovician and Silurian times the chitinozoan abundance is significantly greater in the high latitude deposits of northern Gondwana than in low latitude sediments of Laurentia or Baltica (Paris and Robardet, 1990). Cold water seems therefore more favourable for the proliferation of these microfossils. A cooling tendency of the sea water is documented in basal Famennian (Copper, 1986). This cooling succeeded the warm climate prevailing during most of the Frasnian which was responsible, in connection with the late Frasnian transgression (Johnson et al., 1985), for the anoxic conditions of the Upper Kellwasser event (Lethiers and Raymond, 1991; Lethiers and Feist, 1991). The cooling trend of deep oceanic water continued, and was even accentuated, during Famennian time (Copper, 1986; Lethiers and Raymond, 1991; Lethiers and Crasquin-Soleau, 1991) while the abundance of the chitinozoans decreased immediately above the first Famennian bed, remaining fairly normal up to the end of the Famennian, when the group finally became extinct. Therefore, the drop of temperature cannot be regarded as the single factor inducing the exceptional abundance of chitinozoans in bed 14f at La Serre. The cooling, if involved in the chitinozoan burst, must have been amplified by additional factors such as a temporary cessation of the pressure of predators or competitors, or exceptional nutriment supplies.

4. Conclusions Variations in the concentration of different palynomorph groups in the F F boundary beds at La Serre reflect drastic and rapid palaeoenvironmental changes. The warm water conditions prevailing during the late Frasnian Upper Kellwasser event were unfavourable for the proliferation of the chitinozoans whereas the temperature was not a limiting factor for Maranhites. Maranhites,

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regarded as cysts of unicellular green algae, were very abundant in the late Frasnian as well as in the early Famennian anoxic deposits at La Serre. The most striking feature is the exceptional abundance of a single chitinozoan species in the basalmost Famennian, coupled with a lack of other minute palynomorphs and tasmanaceans except for the occurrence of a very small amount of plant debris. Because rock preservation (weathering and low-grade metamorphism) as well as the hydrodynamic regime are identical in all the processed anoxic micritic samples, these factors do not explain the abnormal chitinozoan concentration in bed 14f. Selective post-sedimentary destructions by mud-dwellers or bottom micro-organisms also appear not to play a role in the exceptional abundance of chitinozoans. Thus, the most plausible explanation calls upon factors intervening higher up in the water column, before the settling of the vesicles. We favour the hypothesis of a high productivity of chitinozoan vesicles, possibly related to a temporary lowering of the ocean temperature, and concomitant with a very low activity of the usual specific predators. The latter were probably drastically affected by the mass mortality registered in many groups in the topmost Frasnian. Restricted terrestrial input during a condensed sedimentation that was mostly supplied by the persistant "rain" of organic particles (mainly chitinozoans), with periodic influx of pelagic ostracod valves (blooms?), favoured the concentration of the chitinozoans. The virtual absence of acritarchs and spores is interpreted to reflect the distance from the shore rather than a hydrodynamic sorting effect. The occurrence of rare plant debris in an environment fairly remote from emerged lands may be explained by the seaward drift of plants fragments uprooted from littoral swamps during the basalmost Famennian regression. The exceptional concentration of chitinozoan vesicles in the most basal Famennian bed at La Serre would hardly been caused by physical mechanisms alone. This anomalous abundance of a single taxon points to an event-related opportunistic burst. Among several cumulative factors, the most amplifying one seems to be a temporary disappearance of chitinozoan predators or competitors after

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the mass mortality registered in many invertebrate groups at the end of the Frasnian.

Acknowledgements The authors thank Francis Lethiers (University of Paris VI), Alain Le Hdriss6 (University of Brest) and Franqois Guillocheau (University of Rennes) for valuable discussions and comments. Dr. M.S.N. Carpenter translated the manuscript. This work benefited greatly of review by Bernard Owens (British Geological Survey, Nottingham) and Jan Jansonius (Calgary). Funding for this research was provided by the French National Research Centre (CNRS, URA 1364 a n d 327. Contribution ISEM 1995/083).

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