The Cenomanian–Turonian boundary event and dinocyst record at Ganuza (northern Spain)

Palaeogeography, Palaeoclimatology, Palaeoecology 150 (1999) 65–82

The Cenomanian–Turonian boundary event and dinocyst record at Ganuza (northern Spain) Marcos A. Lamolda a,Ł , Shaozhi Mao b a

b

Facultad de Ciencias–UPV, Campus de Lejona, Lejona 48940, Spain Dept. of Geology and Mineral Resources, China University of Geosciences, 29 Xueyuan Lu, Beijing 100083, China Received 5 June 1997; revised version received 17 July 1998; accepted 13 October 1998

Abstract An excellent record of the Cenomanian–Turonian transition zone occurs in an expanded section near the village of Ganuza, northern Spain. The sequence at Ganuza shows no evidence of stratigraphic discontinuities, and consists of an alternation of marl, marly limestone and limestone, which were deposited in a middle to outer shelf environment. Our results from palynological residues of 34 samples from the section indicate that number of cysts per gram of dry bulk sediment varies between a total of 7 and 236, and includes more than 130 taxa. Species diversity varies between 25 and 49 species and subspecies per sample. The Spiniferites ramosus group, together with two other important taxa, Trichodinium castanea and Exochosphaeridium phragmites dominate cyst assemblages. Other common taxa include Palaeohystrichophora infusorioides, Canningia reticulata, Pterodinium cingulatum, Coronifera oceanica, Florentinia mantellii, Cleistosphaeridium ?aciculare, Dapsilidinium laminaspinosum, and Xenascus ceratioides. Successive last occurrences of Florentinia cooksoniae, Litosphaeridium siphoniphorum and Epelidosphaeridia spinosa (uppermost Cenomanian), and the first occurrence of Senoniasphaera rotundata (lower Turonian) are used to characterize the transition from Cenomanian to the Turonian. Our results show that dinocysts occur continuously throughout the Ganuza section across the C=T boundary event marked by only a slight decrease in species diversity. The most significant variation in the Ganuzan section in relation to the C=T boundary event is a decrease in cyst abundance from Late Cenomanian to earliest Turonian. The decline occurred just above a level marked by a benthic foraminiferal turnover, as well as the extinction of Rotalipora cushmani. Dinocyst minimum abundances occurred slightly later than minimum abundances of nannofossils. Our dinocyst results are thus consistent with a decrease in productivity during the latest Cenomanian. A new cycle is marked by a strong recovery of dinocysts immediately above the boundary event, although a subsequent decrease recurred higher in the section. Dinoflagellates have a more general opportunistic behaviour than foraminifera and calcareous nannoflora, and such behaviour may offer a viable explanation to account for the different patterns of response observed between the dinocysts and the other microfossil groups.  1999 Elsevier Science B.V. All rights reserved. Keywords: dinocysts; bioevents; Cenomanian–Turonian; northern Spain

Ł Corresponding

author. Fax: C34 94464 8500; E-mail: [email protected]

0031-0182/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 9 9 ) 0 0 0 0 8 - 5

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1. Introduction Widespread carbon-rich sediments with a significant δ13 C excursion, and major biotic changes of both microplankton and microbenthos characterize the Cenomanian–Turonian (C=T) transition period. These changes have been interpreted as indicating a major oceanographic event since De Boer (1986) argued that low productivity was related to burial of organic matter and led to a critical reduction in the availability of nutrients. Subsequent studies of abundance data for various groups such as dinocysts, from Abbots Cliff and Akers Steps sections (Jarvis et al., 1988) and nannofossils (Lamolda et al., 1994) from Shakespeare Cliff, Dover in southeastern England, as well as nannofossils from two localities in northern Spain, Menoyo (Gorostidi and Lamolda, 1991; Paul et al., 1994) and Ganuza (Lamolda and Gorostidi, 1996) lend further credence to De Boer’s arguments by corroborating that lower productivity occurred during late Cenomanian times. Thus the worldwide biotic crisis that defines the Cenomanian–Turonian boundary event (CTBE) in the marine environment may have been caused by selective starvation due to reduced primary productivity (Lamolda et al., 1994), mainly zooplankton and suspension feeders (Paul and Mitchell, 1994). Changes in the benthic foraminiferal assemblages further indicate that unfavourable conditions associated with the decline in the oxygenation level of the bottom waters began at the end of the Rotalipora cushmani planktonic foraminifera Zone and lasted until the early part of the Whiteinella archaeocretacea Zone. The palaeontologic record of the Ganuza section indicates, however, that complete anoxia was not reached in the region during the transition period, and bottom waters remained instead dysaerobic (Lamolda and Peryt, 1995). Similar palaeoenvironments were also considered to characterize the Menoyo section, 70 km northwest of Ganuza (Peryt and Lamolda, 1996). Furthermore, such oceanographic conditions are also recorded in the Gamba section of southern Tibet where stronger anoxic conditions are inferred to have prevailed (Lamolda and Wan, 1996). The studied section is located to the west of Estella, at the locality of Ganuza in the Navarra Province of northern Spain (Fig. 1). The sedimentary

Fig. 1. Location of the Ganuza section in northern Spain.

succession consists of an alternation of marl, marly limestone and limestone, with marl becoming more common towards the top. Some silty beds occur in the base of the succession. This succession includes about fifteen cycles of alternating limestone and marl (Lamolda et al., 1997), similar to those found in the sections at Menoyo (Paul et al., 1994) and Dover, SE England (Lamolda et al., 1994). The Ganuza section represents deposition on a subsiding platform, because it includes more than 2000 m of Cenomanian and Turonian sediments (Lamolda, 1982). Ostracod and foraminifera assemblages are indicative of a middle to outer shelf environment (Colin et al., 1982; Lamolda and Peryt, 1995). Comparison of bed thickness between this section and coeval sections elsewhere has revealed that the Ganuza succession is of particular importance for detailed biostratigraphic analyses since it contains one most expanded and complete record of the C=T boundary known so far, e.g., the W. archaeocretacea Zone is about 45–50 m thick (Fig. 2). In this study additional data are presented on dinocyst assemblages, whose taxonomic content has been recently published (Mao and Lamolda, 1998). Previous studies on the succession include a summary of the stratigraphy and regional geology (Lamolda, 1982), and macrofaunal assemblages described by Wiedmann (1980) and Lamolda et al. (1989)

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from part of the section. Lamolda and Peryt (1995) also described the detailed biostratigraphy of the upper Cenomanian and foraminiferal response to the late Cenomanian event. Lamolda and Gorostidi (1996) studied nannofossil assemblages and their biostratigraphy in upper Cenomanian–lower Turonian sediments. Eventually, an integrated biostratigraphy (ammonites, inoceramids, nannofossils and planktonic foraminifera) has been recently proposed by Lamolda et al. (1997). Cenomanian and Turonian dinoflagellates have been studied intensively in western Europe, particularly in southern England and northern France (Clarke and Verdier, 1967; Davey, 1969, 1970; Foucher, 1980, 1982; Tocher and Jarvis, 1987; Jarvis et al., 1988; FitzPatrick, 1995, 1996), but little work has been done in Spain (Herngreen, 1980). The aims of the present study are to obtain semiquantitative data on dinocyst abundance, richness, dominance, etc., to characterize the Cenomanian–Turonian transition and to interpret their relationship to environmental change at the C=T boundary event at Ganuza. A fundamental assumption in the interpretation of the results is that primary producers such as calcareous nannoplankton and dinoflagellates appear to have had a similar response to the global CTBE, because during this critical interval no significant calcareous nannoplankton extinction is recorded. Courtinat et al. (1991) reported a decreasing frequency of dinocysts from the upper Cenomanian to the lower Turonian in sediments from Vergons, southeastern France. Although they did not provide quantitative values for the dinocysts decrease, it is clear that their minimum frequency occurred within an interval that corresponds to the first occurrence of Praeglobotruncana (D Helvetoglobotruncana) helvetica. This pattern is similar to nannofossil assemblage variations at Ganuza, as well as at other C=T sections, all of which are correlated with decreasing primary productivity (Lamolda and Gorostidi, 1996). Therefore, the dinocyst decline at Vergons may also be related to the CTBE.

Fig. 2. Composite section at Ganuza to show lithology, location of palynological samples and bioevents.

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2. Material and methods A total of 34 samples have been analysed for dinocysts from the Ganuza sections (21 from Section ‘G’, and 13 from Section ‘GZ’), all of them are subsamples of those studied by Lamolda et al. (1997). Their location and main bio-markers are shown in Fig. 2. Each sample weighted about 50 g and was processed by means of hydrochloric and hydrofluorid acid maceration and centrifuge techniques. Oxidation with nitric acid or Schultz solution was not used to avoid damage to the more delicate cysts. After chemical digestion, heavy liquid (mixture of potassium iodide and hydroiodic acid, with specific gravity around 2.1 g=ml), and a 20-µm mesh sieve were used to separate the organic residues and concentrate dinocysts. This concentrate was dispersed in about 1 ml of distilled water and the suspension agitated repeatedly. Five permanent strew slides (22ð22 mm) were prepared for each sample using glycerine jelly. Ten sample residues were used completely, and in 24 samples a small amount of residue remained. Total absolute abundance of dinocysts (number of dinocyst per gramme of dry bulk sediment) was computed counting specimens in randomly chosen fields from 10 lines (2 lines on each permanent slide at ð200 magnification — 20 lines to cover the whole area of each slide). When we used only a fraction of the residues, the number of dinocysts was calculated on that fraction and converted relative to the whole sample; therefore it is actually an estimate of absolute abundance. This number is supposed to be indicative of fossil absolute abundance for each individual sample if the sedimentation rate is assumed to be constant. In fact, the samples we studied for this paper have a similar nature (marl or marly limestone), thus the lithological influence on both abundance and diversity should be minimal, especially if known nannofossil abundances are used together. FitzPatrick (1996) also stated that both diversity and abundance indices for dinocyst are not lithologically controlled at the southern English localities studied. In spite of these uncertainties this computation is closer to actual abundance values than counting on one slide where assemblage fractionation could be relevant. Actually, absolute abundance data have been used in Quaternary and Recent studies (Davey

and Rogers, 1975; Wall et al., 1977), and rarely on Pre-Quaternary (Manum et al., 1989) since the real meaning would be distorted because of sedimentation rate changes. Only sample GZ 1 produced a small amount of residues which contain few dinocysts and a small number of bisaccate pollen grains (after re-checking it has since been realised that this sample was collected from a weathered outcrop, which may cause this trouble). More than 200 to 500 specimens or cysts were identified and counted for each sample, except for sample GZ 59, which contained too few dinocysts. This gives a 95 to 99% level of confidence, in most cases, of not overlooking any taxon present at 1% or more of the total assemblage (Dennison and Hay, 1967). Diversity — number of species per sample, relative abundance of species — percentage, and other indices are computed on counted specimens in each sample. Dominance is defined according to Goodman (1979) as the ratio between the number of specimens belonging to the two most frequent species to the total dinocyst figures in each sample assemblage. In general, for covariance analyses values of Student’s t have been computed. The dinoflagellate cyst taxonomy used in the present work generally follows Lentin and Williams (1993).

3. Results of analyses The Ganuzan dinocyst assemblages occur continuously across the Cenomanian–Turonian transition, although dinocyst abundance may fluctuate strongly; even some species become very rare or are absent (Fig. 3). They are diverse, accompanied by pollen grains and organic foraminiferal lining. The dinoflagellate cyst record shows quite large changes in frequency of both cysts and species — from 7 to 236 cysts=g of sediment, and 25 to 49 species and subspecies in one sample. From samples G 33 to G 8 abundance varies between 41 and 205 cysts=g (dry bulk sediment), with an average of 108 cysts=g. The abundance decreases to between 7 and 94 cysts=g (56 on average) from samples G 7b up to GZ 59, and eventually increases between 19 and 236 cysts=g (128 on average) from samples GZ 2b

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Fig. 3. Range chart for selected dinocyst species from the Ganuza section.

to GZ 8 (Fig. 4). The Ganuza section includes a critical interval, only 2.45 m thick, between samples GZ 59 and GZ 2b, the former with the lowermost abundance, 7, whereas sample GZ 2b has the highest abundance, 236, representing the main recovery in dinocyst assemblages throughout the section studied. Abundance remains high until the top, although

sample GZ 8 shows a significant decrease. There is another possible recovery between samples G 23 (abundance 53 cysts=g) and G 21 (205), but is not so remarkable. The total number of species and subspecies recorded in the upper Cenomanian sediments is 112, and 72 are recorded in the lower Turonian. Nonethe-

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Fig. 4. Absolute abundance, species diversity, relative abundance of selected dinocysts, and location of the CTBE at Ganuza.

less, average diversity is similar, reaching 37 (37.2) in the Cenomanian, whereas it is 36 (36.4) in the lower Turonian. Species diversity recovered quickly in lowermost Turonian rocks, from 25 in sample GZ 59 to 42 in sample GZ 5; similar values occur upwards, 40 in the topmost sample GZ 8 (Fig. 4). Therefore, although abundance and diversity show relatively similar trends, changes in abundance are more important than in species diversity. In fact, correlation between abundance and diversity is not high (r D 0:36, p < 0:025). Most commonly the assemblages are dominated by taxa of the Spiniferites ramosus group (S. ramosus and its subspecies, and S. multibrevis), together with

two other important groups, namely the Trichodinium castanea group (T. castanea, T. intermedium, T. ciliatum and Pervosphaeridium brevispinum, P. paucispinum, mostly the former species), and the Exochosphaeridium phragmites group (E. phragmites, E. bifidum and Pervosphaeridium pseudhystrichodinium, mostly the former species; Fig. 4). T. castanea is dominant in a few samples from the lower part of the succession. The species Achomosphaera ramulifera, Canningia reticulata, Cleistosphaeridium ?aciculare, Coronifera oceanica, Florentinia mantellii, Palaeohystrichophora infusorioides, Pterodinium cingulatum, and Xenascus ceratioides are relatively common, whereas Circulodinium distinctum, Cleisto-

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sphaeridium ?multispinosum, Dapsilidinium laminaspinosum, Kiokansium unituberculatum, Odontochitina operculata, and Oligosphaeridium complex are rare. It is also worth noting that the latter species only show sporadic occurrences. Florentinia spp. occur throughout the section studied, but in the lowermost part, below sample G 26 (Fig. 4), percentages are usually higher than 10%, whereas above that level they are around 5%. This species shows a similar trend to that of the T. castanea group but starts decreasing earlier. In contrast, Canningia reticulata occur throughout the section in small percentages (<5%), except in samples G 25, G 24 and GZ 7b, but both maxima are more apparent than real because they are coincident with minima of other main assemblage components (Fig. 4). Cleistosphaeridium spp. also show little change in absolute abundance throughout the section. Dominance is usually between 0.25 and 0.48 (one sample, GZ 6a up to 0.60); assemblages dominated by one or a few species are very rare. There is no special trend, except in the lower third of the column, where dominance values are usually higher than 0.4, whereas in the upper two thirds dominance is more variable and values are usually lower than 0.4 (Fig. 4). Dominance is not well correlated with abundance .r D 0:17/, nor with diversity .r D 0:08/. Peridinioid (or peridiniacean) cysts at Ganuza are minorities in assemblages, being composed of only species of the two genera Palaeohystrichophora and Subtilisphaera, mainly Palaeohystrichophora infusorioides, which show their highest values in lower Turonian samples (Fig. 4), but these highest values occur in samples where both the S. ramosus and T. castanea groups have relatively low percentages; thereby these high values could be more an artifact than an actual feature. In most samples the G=P ratio varies between 11.5 and 49, with four exceptions, which are: two very low values as 5.6 and 8.5 for respectively samples GZ 5 and G 25, in which relative abundances of P. infusorioides are the highest, and two extremely high values of 103.5 and 129 in respectively samples G 26 and G 29, both with relative low abundance of P. infusorioides (Figs. 4 and 5). The G=P index has a moderate correlation with pollen percentages (r D 0:39, p < 0:025). The Cyclonephelium group is a relatively important component of dinocyst assemblages at Ganuza,

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therefore the S=C index shows moderate changes throughout the section (maximum 4.06, minimum 0.43), particularly in respect to the Gonyaulacacean index. It shows a complex trend, somewhat cyclic (Fig. 5), with three decreasing intervals, whose minima are located at samples G 25–G 24, G 53–G 59, and G 8, although the last is not as conspicuous. The main one is around the C=T boundary. The S=C index has no correlation with the G=P index (r D 0).

4. Dinoflagellate assemblages and biostratigraphy In the present study the CTBE is taken to have a duration of approximately 250 to 270 ka, in accordance with the estimations from Dover (Lamolda et al., 1994) and Menoyo (Paul et al., 1994). Both Menoyo and Ganuza show expanded sections within the limits of the last occurrence of the planktonic foraminifera Rotalipora (R. cushmani) and the first occurrence of the nannofossil Quadrum gartneri, both of which are good bio-markers worldwide (Lamolda et al., 1997). Although the Ganuza section represents a shallower environment than Menoyo during the Cenomanian–Turonian transition period (Lamolda and Peryt, 1995; Peryt and Lamolda, 1996), both successions have similar expanded C=T transitions with an average deposition rate of about 9–11 cm per thousand years. Such a rate produces a twofold thickness as compared to the sections at Eastbourne and Vergons. Assemblage composition and abundance of relevant taxa seem to have been different during the CTBE, palaeoceanic conditions became uneven and coeval assemblages from different basins, or even within different sub-basins of a given basinal area, vary tremendously. In general, dinocyst assemblages at Ganuza show overlapping characteristics with widely different areas that are characterized by different taxa as dominant or important components (Table 1). The Ganuzan taxa are comparable, for instance, with those found in the Paris Basin, France (Foucher, 1982), as well as those found in the succession at Vergons, southeastern France (Courtinat et al., 1991). On the other hand, the Ganuzan taxa also show affinities with assemblages as far away as those found in the Pueblo Section, Colorado, Western Interior Basin of North America (Courtinat,

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Fig. 5. Changes in the Spiniferites=Cyclonephelium (S=C) and gonyaulacacean=peridiniacean (G=P) indices at Ganuza. Percentages of dinoflagellate cysts, pollen and foraminiferal linings. CTBE D Cenomanian–Turonian Boundary Event.

1993). For example, while Oligosphaeridium complex and Circulodinium distinctum are the dominant taxa in Dover (Jarvis et al., 1988), at other localities such as Pas-de-Calais, France (Foucher, 1980), Isle of Wight (Clarke and Verdier, 1967), and Devon (Tocher and Jarvis, 1987) Palaeohystrichophora infusorioides, Spiniferites ramosus and Trichodinium castanea are prevalent. The later two species are important at Ganuza, but are absent in the sequence at Dover. Most of the species and subspecies recorded in the assemblages at Ganuza are known to have a

worldwide occurrence between the late Early Cretaceous and the early Late Cretaceous, and some of these taxa occurred only between the Cenomanian and the Turonian. Nevertheless, a detailed comparison of zonations between the study area and other sections elsewhere is difficult because dinoflagellate assemblages became more provincial in the Late Cretaceous (Stover et al., 1996, p. 677). The Cenomanian=Turonian boundary was initially defined at Ganuza, as characterized by the stratigraphic horizon just below the first occurrence (FO) of the inoceramid Mytiloides kossmati kossmati

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Table 1 Comparison of dinocyst assemblages from the Cenomanian–Turonian transition at Ganuza and other localities

(sample GZ 57), and just above the FOs of the inoceramid Mytiloides submytiloides and the ammonite Kamerunoceras sp. (sample GZ 56; Lamolda et al., 1997). This level can be further characterized by successive occurrences of the calcareous nannofossils Microstaurus chiastius (last occurrence, LO; sample GZ 73) and Quadrum intermedium (FO; sample GZ a), and FOs of the foraminifera Helvetoglobotruncana praehelvetica (sample GZ 53) and calcareous nannofossil Nannoconus multicadus and Eprolithus octopetalus, both at sample GZ 58, whereas a suprajacent level is characterized by the FO of the calcareous nannofossil Quadrum gartneri, sample GZ 1 (Fig. 2) (Lamolda and Gorostidi, 1996; Lamolda et al., 1997). The record of dinocysts supports this boundary assignment (Fig. 3), based on the evidence discussed as follows. Epelidosphaeridia spinosa, which occurred consistently in the samples, has its LO in sample GZ 54. This species is known to range from late Albian to early late Cenomanian (Stover et al., 1996), elsewhere worldwide. Florentinia cooksoniae ranged

from late Albian to late Cenomanian in the Northern Hemisphere (Williams et al., 1993), and has its LO in sample GZ a. Litosphaeridium siphoniphorum is reported to range from late Albian to late Cenomanian in the Northern Hemisphere (Williams et al., 1993); it has its LO in sample G 51. Dodsworth (1996) concluded that the LO of L. siphoniphorum is a good interregional event to correlate late Cenomanian sediment. Furthermore, these successive LOs of F. cooksoniae and L. siphoniphorum were cited by Williams and Burjak (1985) as characteristic of the late Cenomanian. As shown in Fig. 3, the dinocyst data concur with previous assignments of a late Cenomanian age to the lower part of the Ganuza section. Furthermore, specimens of Senoniasphaera rotundata with its FO in sample GZ 5 allow further correlation of the upper part of the section with the early Turonian. In fact, the FO of S. rotundata has been used to define the early Turonian in the Paris Basin (Foucher, 1982), and also in the sequence of the Dover Chalk Fm., Dover, southeastern England (Jarvis et al., 1988). This age assignment is corrobo-

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rated by the FO of S. rotundata recorded just above the top of the Melbourn Rock Beds, Isle of Wight, UK (FitzPatrick, 1995), which is also of an early Turonian age. The FO of S. rotundata is used to distinguish its homonymous zone in the lower Turonian at Ganuza, whereas underlying beds belong to the L. siphoniphorum Zone.

5. Palaeogeography and palaeoecology Dinoflagellate cysts of various morphologies have long been used to interpret environmental parameters such as proximity to the shore line, salinity, surface water temperature etc. (Scull et al., 1966; Downie et al., 1971; Williams, 1977; Mao and Norris, 1988). Recently, Li and Habib (1996) further used the ratio between two cyst types with different length and complexity of processes to interpret sea-level changes during the late Cenomanian to the early Turonian in sequences of the Western Interior of the United States. One of the taxon groups used in the study includes the Spiniferites group, which encloses several species with long, complex-chorate cysts such as Achomosphaera spp., Spiniferites spp., Hystrichosphaeridium spp., and Oligosphaeridium spp., etc., all indicative of open marine conditions. In contrast, taxa such as the Cyclonephelium group, which displays proximate cysts with short, simple processes as found in Circulodinium distinctum, Cyclonephelium spp., Exochosphaeridium spp., Kiokansium spp., Cleistosphaeridium spp. and Sentusidinium spp., etc., are all representative of a near-shore and slightly less saline condition. The number of gonyaulacacean species=peridiniacean species, the G=P ratio, has been used extensively in the last decade as an indicator of proximity to shore, a higher ratio indicating a more open marine environment. The gonyaulacacean ratio was used by Harland (1973) to analyse upper Campanian dinocysts from Alberta (Canada). He computed the index as the ratio between the number of gonyaulacacean species and the number of peridinioid species. At Ganuza, the number of peridinioid species is very small, between 4 and 1, belonging to the genera Palaeohystrichophora and Subtilisphaera. Therefore, we have used instead the number of specimens for each group to compute that index, to

reduce the effect of possible non-random occurrence of peridinioid species. Gonyaulacacean index data are compared with the S=C index for every sample studied. Modern dinoflagellates are shelf dwellers that live in the shallow- to the medium-deep-water parts of the water column, perhaps in the upper slope (Wall et al., 1977; Dale, 1996). Their cysts tend to be more abundant in sedimentary deposits from the middle neritic to the upper bathyal zones; they often co-occur with pollens and spores in the middle shelf and with foraminera and calcareous nannoflora in outer shelf and upper slope (Stover et al., 1996). Although ecology of living and Quaternary dinocysts has been better studied than the palaeoecology of pre-Quaternary fossil cysts, there are still many unanswered questions concerning cyst distribution patterns, e.g.: Does cyst distribution in bottom sediments reflect the planktonic distribution of a species? Are distribution patterns revealed by cysts also typical for non-cyst-producing species? The resting cysts are only part of the life cycle of dinoflagellates, reflecting a period of non-motility following sexual reproduction (Evitt, 1985), and only 13–16% of living dinoflagellates species are believed to form cysts (Head, 1996). Furthermore, environmental factors combine to produce a cyst assemblage which does not necessary reflect accurately the characteristics of the planktonic dinoflagellate community in the overlying water column (Goodman, 1987), all of which make it more difficult to interpret palaeoenvironments solely based on dinocysts. However, in some cases we may reasonably interpret broad trends of environmental change. Perhaps, a better way of palaeoenvironmental interpretation is a multidisciplinary approach (Scull et al., 1966; Jarvis et al., 1988). According to Marshall and Batten (1988) dinoflagellate assemblages vary in composition with lithofacies and palynofacies. Evidence from nannofossils at Menoyo shows no significant correlation between lithology and nature of calcareous nannofossil assemblages (Paul et al., 1994), but abundance of nannofossils at Menoyo, Ganuza (Lamolda and Gorostidi, 1996) and at Shakespeare Cliff, SE England (Lamolda et al., 1994) share a similar pattern with those of non-calcareous dinocysts at the Abbots Cliff and Akers Steps section, Dover (2–3 km west

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of Shakespeare Cliff; Jarvis et al., 1988) in the coeval interval. These indicate that although the lithology may influence the detailed abundance and species diversity, it should not change their general trends. In addition, the samples we studied for this paper have a similar nature (marl or marly limestone), thus the lithological influence on both abundance and diversity could be minimized. Common methods such as fossil abundance, species diversity and dominance (Goodman, 1979), ratio of gonyaulacoid=peridinioid cysts (G=P ratio, Harland, 1973), relative abundance of dinoflagellates=pollen–spores=foraminifera=nannofossils and ratio of Spiniferites group=Cyclonephelium group (S=C ratio, Li and Habib, 1996) were employed to interpret palaeoenvironments of the studied time interval at Ganuza. Dinoflagellate cysts occur continuously across the Cenomanian–Turonian boundary in Ganuza, accompanied by pollen grains (from 1 to 30%) and organic foraminiferal linings (up to 7%). The latter have irregular, rare occurrences, except in the lowermost part of the section. In contrast, pollen grains are always present, but with quite variable abundance; their relatively high percentages throughout the two upper third parts of the section are noteworthy (Fig. 5). In accordance with the Stover– Williams’ model (Stover et al., 1996), such assemblages indicate that the study area was under medium shallow to moderately deep water of the outer continental shelf during the late Cenomanian to early Turonian. This depth assignment is also supported by the co-occurrence of abundant foraminifera and calcareous nannofossils (Lamolda et al., 1997). Additional evidence of the prevailing environmental conditions at Ganuza can be deduced from the similar species diversity and dominant taxa found in the Western Interior Basin of the United States (Li and Habib, 1996). The fact that the Circulodinium distinctum group is much less important throughout the Ganuza assemblages, suggests that during late Cenomanian to early Turonian time the study area never attained depths as shallow as those comparable to the inner continental shelf zone occupied by the site at Blue Point, Arizona, Western Interior Basin. The G=P ratio is relatively high, >10 to >50, which also confirms the above palaeodepth interpretation. The Ganuzan assemblages are dominated by

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gonyaulacoid cysts, including the three major groups and other common genera such as Cleistosphaeridium, Florentinia, and Coronifera. In contrast, tolerant thick-walled taxa such as Batiacasphaera euteiches and Odontochitina operculata are rare or even absent in some of the Ganuzan assemblages, whereas both taxa are abundant at Dover (Jarvis et al., 1988). We can further infer that during that time interval the palaeolatitude of Ganuza was lower than that of southern England, but was about the same as that of the Western Interior of the United States. In fact, typical cold-water dinoflagellate species such as Heterosphaeridium difficile and peridinioid cavate cysts (Eurydinium saxoniensis and Isabelidinium belfastense), which were found in southern England (FitzPatrick, 1995), are not found at Ganuza. These data suggest that warm tropical to subtropical conditions prevailed throughout the late Cenomanian to early Turonian. Previous reports based on high frequency of occurrence of the cool-water nannofossil species Eprolithus floralis (Lamolda and Gorostidi, 1996) had suggested an early Turonian ingression of northern temperate taxa in the area. The dinocyst species at Ganuza, therefore, show a different response to environmental conditions at that time compared with that of the nannoplankton. Li and Habib (1996) considered the Spiniferites group=Cyclonephelium group ratio (S=C ratio) to be a good indicator of environmental change related to both off-shore direction and eustatic sea-level fluctuations. Similarly, we used their method to calculate the S=C ratio in the Ganuza assemblages, and our results are, partially, in agreement with the earlier suggestion by Paul et al. (1994) that “the CTBE was initiated by a sudden fall in sea level, followed by a more gradual, but still relatively rapid rise, to a level higher than that before CTBE”. The smallest number of S=C ratio, of 0.88 (average number of samples G 7b to GZ 59, just spanning the C=T boundary) is interpreted to reflect a relatively low sea level. In contrast, the suprajacent beds with a higher value of S=C ratio of 1.25 (average number of samples GZ 2b to GZ 8) are inferred to indicate a rise of sea-level during the early Turonian (Fig. 5). These data agree with equivalent beds at Pueblo, where the S=C ratio from coeval beds shows values of 1.05 and 1.75, respectively (Li and Habib, 1996, table 2). Somehow, the Ganuza section has intermediate characteristics between Western In-

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terior Basin localities, as its lower third has the highest value of the S=C ratio (1.85). Comparison of species diversity at both Pueblo and Ganuza shows the same patterns of variations through the latest Cenomanian, and we can further recognize palynological cycles at Ganuza similar to those described for Pueblo, as well as for other localities from the Western Interior of the United States (Li and Habib, 1996). At Ganuza three palaeoenvironmental phases may be recognized during the C=T transition. (1) An early off-shore phase (interval from the base to sample G 14), characterized by high abundance, high S=C ratio, generally high G=P ratio, and high to moderate diversity (49 to 33), together with moderate to low dominance (usually >0.4). In addition, there are 1 to 16% (average 7.5) pollen and 0 to 7% (average 2.0) organic foraminiferal linings accompanied by 80 to 99% (average 90.5) dinocysts in the palynological residue (>20 µm) of each sample (Figs. 4 and 5). All these data indicate that when this lower interval was deposited a more off-shore palaeoenvironment prevailed. (2) A middle in-shore phase (interval between samples G 13 and GZ 59), characterized by low abundance, moderate to low S=C ratio, fluctuating G=P ratio, and moderate (–low) diversity (44 to 25), together with low (–moderate) dominance (0.47 to 0.23). Pollen grains occur constantly in each sample and increase the proportions considerably (10 to 30, average 18%), together with increased terrigenous debris in palynological residues, whereas foraminiferal linings commonly disappear from the residues (Figs. 4 and 5). (3) A late off-shore phase (interval between samples GZ 2b and GZ 8), characterized by high to moderate abundance, moderate S=C ratio, fluctuating G=P ratio and moderate diversity (30 to 42), together with moderate (–low) dominance (usually 0.3 to 0.4). Pollen grains occur continuously but decrease frequency slightly (9–25, average 16.5%), whereas foraminiferal linings occur more often than in the middle phase (Figs. 4 and 5). In addition, terrigenous debris and pollen increase topwards. All these indicate a more off-shore trend during the early Turonian when the upper interval was deposited and an another in-shore trend towards the end of this phase, late early Turonian.

Species diversity trends at Ganuza look similar to data of Li and Habib (1996), with two successive minima in samples G 8 and GZ 59. Furthermore, their ‘species diversity inflexion point’ (SDIP) is recognizable in our sample GZ 53 (latest Cenomanian), which is in good agreement with their general model (their fig. 10). We infer for the Ganuza section a palaeogeographical location intermediate between the Blue Point and Carthage localities of Li and Habib (1996).

6. The response to the CTBE The CTBE is defined at Ganuza by correlation with Menoyo (Paul et al., 1994). Therefore, its base is characterized by the extinction of dominant epifaunal foraminifera (Lamolda and Peryt, 1995; Peryt and Lamolda, 1996), very close but below the LO of R. cushmani, late Cenomanian. Its top is marked by the FO of the nannofossil Quadrum gartneri, earliest Turonian (Fig. 4). Dinocyst abundance decreases distinctly at the lowermost part of the CTBE, remaining low throughout it, and recovered soon after the CTBE, and then decreased again afterwards (Fig. 4). The variations indicate a pattern of high dinocyst abundance during the late Cenomanian, a sharp decrease during the CTBE, and another cycle with high abundance during the earliest Turonian, similar to that of the late Cenomanian (pre-CTBE), which is followed by another decrease. Species diversity shows a more complex trend, especially a clear maximum during the CTBE and no evident decrease during the early Turonian, although its minima show a good correspondence with total dinocyst abundance (Fig. 4). Comparison with nannofossil abundances shows a similar general pattern, except in lower Turonian sediments (Lamolda and Gorostidi, 1996, fig. 4); where the two clear maxima are shown by dinocysts cannot be recognized. In contrast, the lower minimum for dinocysts in sample GZ 59 also occurs in the nannofossil record. Neither of low abundance values for dinocysts in the uppermost Cenomanian sediments occur in the calcareous nannofossil data. The response of the dinoflagellate flora to the CTBE was less pronounced and subsequent recovery was quicker at Ganuza than at Dover, where there is

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a 5-m barren zone spanning the C=T boundary and dinocyst abundance remained dramatically decreased above the CTBE. This is possibly taphonomic due to the very low deposition rate of the lowermost Turonian nodular chalks (C.R.C. Paul, pers. commun.). The Dover, Akers Steps, sequence shows a progressive increase in abundance and species diversity upwards, in the lower Turonian, with assemblages from the top of the succession typically consisting of 50 to 100 cysts=sample (FitzPatrick, 1996). At Ganuza, only 2.45 m includes the interval of recovery between samples GZ 59 and GZ 2b, the former with the lowest abundance, 7, indicating the effect of the CTBE. Sample GZ 2 has the highest abundance, 236, representing the recovery after the CTBE. Abundance remains high up to the top of the section, although sample GZ 8 shows a significant decrease. Similarly, species diversity recovered quickly after the C=T event from 25 in sample GZ 59 to 42 in sample GZ 5, and remains at similar levels upwards: 40 in the topmost sample GZ 8. These trends in the Turonian rocks at Ganuza are similar to those at Holywell Steps, Sussex, southern England (FitzPatrick, 1996). Dinocyst response at the Western Interior Basin is somewhat different, most species disappearing temporarily. However, the two minima (in both abundance and species diversity) are similar, except that the younger minimum is the more important at Ganuza, whereas the opposite is true at Blue Point, Arizona (Li and Habib, 1996, p. 20). Marshall and Batten (1988) recognized two main dinocyst associations closely related to the CTBE. The one dominated by species of Spiniferites, with S. ramosus being the most abundant species, typically occurs in light grey marls and chalks, which reflect well-oxygenated, open marine conditions. When the same association occurs in dark organic-rich marls, it has been interpreted as indicating deposition in an environment of the oxygen-depleted zone — at the sea bottom — in a stratified water column developed in isolated fault-bounded basins lying on the continental shelf. The other dinocyst association is represented by Cyclonephelium=Eurydinium, dominated by the Cyclonephelium compactum–C. membraniphorum complex and the peridinioid Eurydinium saxoniensis. This second association typically occurs in organic-rich marls or ‘black shales’,

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indicating a stressed environment with low level of oxygen extending throughout the water column. Marshall and Batten (1988) also proposed depositional models for interpreting alternating occurrences of the two associations. Although the peridinioid Eurydinium saxoniensis does not occur in the Ganuza assemblages, the other taxa are sufficiently significant to allow appropriate interpretation of the environment of deposition of the sequence, which is characterized by the absence of ‘black shales’. The dinocyst assemblages are dominated by Spiniferites ramosus, which implies that the site of deposition of the Ganuza sequence was well oxygenated, although an oxygen-depleted zone developed during the CTBE. At the same time in samples from G 7b upwards specimens of Spiniferites ramosus tend to reduce their size and wall thickness and to develop thinned and shortened processes, which may indicate an unfavourable stagnant or dysaerobic environment. All of these facts demonstrate that Ganuza, having more land-influence, was slightly more in-shore, perhaps, under slightly dysaerobic conditions during the middle interval of deposition. This phase coincides with the CTBE and all these changes indicate a response to the CTBE. Perhaps, the final period of the early off-shore phase was the onset of an unfavourable palaeoenvironment, evidenced by the decrease of abundance and S=C ratio. The life cycle of cyst-producing dinoflagellates is composed of motile planktonic thecate and non-motile benthic resting or dormant cyst stages. Regardless of the controversy about the function of dinocysts as survival strategies, Dale (1983) argued that these organisms have evolved an alternation of generations, one planktonic and one benthic, the combination of which offers one viable ‘life strategy’. Considered from this perspective, many of the ‘survival’ aspects may be adaptations toward survival of the cysts in a particularly rigorous benthic environment. Records of dinocyst at Ganuza from the CTBE interval may offer evidence to support Dale’s statement, that is, to reduce the population and develop some morphological modification to adapt the adverse CTBE environments. During the transition period, however, oxygen depletion did not cause complete anoxic conditions at Ganuza. Instead, dysaerobic conditions developed at the end of the late Cenomanian and per-

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sisted into the early Turonian. These conditions are well documented by data from benthic foraminifera, where assemblages record a major change (sample G 18) close to the LO of the planktonic foraminifer Rotalipora cushmani (Lamolda and Peryt, 1995; Peryt and Lamolda, 1996), including lower primary productivity (nannofossils; Lamolda et al., 1994). According to De Boer’s model previously cited, a dysaerobic environment could severely impede the benthos and cause extinction, whereas most of the planktonic organisms such as planktonic foraminifera, calcareous nannoplankton, and dinoflagellates living in the well-oxygenated upper water column, could survive during nutrient-reduced conditions by decreasing their reproduction to cope with such unfavourable environments. It appears that this was the case for the Ganuza sequence, where there is a distinct decline in fossil abundance accompanied only by a slight decrease in species diversity. Palaeoecological interpretation of dinocyst assemblages by FitzPatrick (1996) is based on the importance of three main dinocyst groups (the Gonyaulax, Odontochitina and Cyclonephelium groups), and used to infer sea-level changes. His method is not directly applicable to the Ganuza section because of its different palaeogeographic location compared with FitzPatrick’s localities. Ganuza was more off-shore than Eastbourne, Sussex, and more open marine than Dover. The dinocyst abundance maximum in the lower Turonian (Fig. 4) and the relatively important percentages of pollen grains in the upper lower Turonian samples (Fig. 5) have no correspondence in southern English localities. Direct use of off-shore=in-shore, or deep-water=shallow-water, is not reliable at the Ganuza section as G=P and S=C indices show no correlation at all (see Section 3). The usual climatic interpretation of rich cavate cyst assemblages as being characteristic of cold water has been challenged by Courtinat et al. (1991). They found at Vergons, southeastern France, that dinocyst assemblages are dependent upon lithology, in particular that assemblages rich in Cyclonephelium spp. and Leberidocysta spp. are characteristic of ‘black shales’ facies. They proposed a relationship of cavate cysts with disphotic relatively deep water, and an oxygen-level control on the main cavate cysts; e.g., in their model Palaeohystrichosphora infusorioides should be characteristic

of well oxygenated water, but Cyclonephelium spp. and Leberidocysta spp. should be from disaerobic water. Spiniferites spp., and chorate cysts in general, in contrast should live in the well oxygenated upper photic zone. This explains their occurrence in both normal oxygenated facies with P. infusorioides and in ‘black shale’ facies with Cyclonephelium spp. and=or Leberidocysta spp. At Ganuza there is no ‘black shale’, but during the CTBE a disoxic environment of deposition occurred (Lamolda and Peryt, 1995). Therefore, the model of Courtinat et al. (1991) is useful to explain the decreasing S=C index at Ganuza, which from a simple palaeogeographic point of view would be contradictory — in a deepening trend the S=C index should increase. In fact, during the disoxic CTBE the Cyclonephelium group increased and the S=C index shows lower values. However, the trend in the Ganuza G=P index is not well explained by the model of Courtinat et al. The peridinioid taxa are P. infusorioides and Subtilisphaera spp., all of which are characteristic of well oxygenated disphotic water. Therefore during the disoxic CTBE the G=P index should increase, but it decreases with respect to the underlying sediments (Fig. 5). Peridinioid taxa are always a minor component of the Ganuza assemblages, so that changes among gonyaulacoid taxa could be more important than among peridinioid taxa, but this is not directly shown in the G=P index values. Dinoflagellates, like other planktonic organisms, survived the adverse conditions at the CTBE, and the dinocyst record shows a higher resolution cyclicity of relatively shorter periods than the record left by both foraminifera (Peryt and Lamolda, 1996) and nannofossils (Lamolda and Gorostidi, 1996). As pointed out previously, the differences shown by the record of the dinocyst groups can be correlated with sealevel changes, and higher species diversity occurred during the early phase of transgression (third-order cycles; Habib et al., 1992) but decreased later. The overall distinct response of the dinoflagellates to the CTBE, even in the case of such a long period of low productivity (as recorded by both nannofossil and foraminifera assemblages), most likely reflects the opportunistic behaviour of dinoflagellates relative to the other two groups discussed. In addition, it is remarkable that there is no major extinction among the dinoflagellates throughout

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the CTBE. This could be related to the above-mentioned survival strategy of cyst production in dinoflagellates, which enabled them to avoid those unfavourable environmental conditions.

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ments, which have improved the manuscript. This is a contribution to project No. PB95-0505-C02-01 of the Spanish government (MEC).

Appendix A 7. Conclusions We report for the first time the occurrence of abundant and diverse dinoflagellate cyst assemblages from Ganuza, northern Spain, which further characterize the CTBE as recorded on a mid to outer continental shelf of a subtropical latitude. The dinocyst record provides additional correlative evidence to define the C=T boundary by these successive bioevents: the last occurrences of Florentinia cooksoniae, Litosphaeridium siphoniphorum, and Epelidosphaeridia spinosa (uppermost Cenomanian), and the first occurrence of Senoniasphaera rotundata (lower Turonian). The absolute abundance of dinocysts, as well as their S=C index, and species diversity are comparable to palynological cyclicity recorded in the C=T transition at Pueblo. The dinocyst data also show short-range fluctuations (third-order cycles) superimposed on the longer-term cycle of low productivity that characterized the CTBE. We interpret this difference in taxonomic response as reflecting the more opportunistic behaviour of dinoflagellates relative to foraminifera and calcareous nannoflora. A simple palaeogeographic explanation for changes in the S=C index at Ganuza is not reliable. However, the S=C index trend throughout the upper Cenomanian and lowermost Turonian rocks at Ganuza agrees with the model of Courtinat et al. (1991) of changes in oxygen levels.

Acknowledgements The financial support for this project No. SAB94= 0303 to Mao Shaozhi was provided by a research grant of the DGICYT, Spanish MEC. The assistance of the Palynological Laboratory, Department of Geology and Mineral Resources, China University of Geosciences at Beijing is acknowledged. We are indebted to Graag Herngreen, Han Leevereld, Florentin Maurrasse, Chris Paul and Bruce Tocher for their com-

List of dinocyst species and subspecies recorded from the Ganuza Section, which are arranged alphabetically by genera and species. Achomosphaera ramulifera (Deflandre, 1937) Evitt, 1963 Achomosphaera sagena Davey and Williams, 1966 Achomosphaera sp. Apteodinium deflandrei (Clarke and Verdier, 1967) Lucas-Clark, 1987 Apteodinium granulatum Eisenack, 1958 Apteodinium maculatum Eisenack and Cookson, 1960 Apteodinium sp. cf. A. granulatum Eisenack, 1958 Areoligera sp. cf. A.. coronata (O. Wetzel, 1933) Lejeune-Carpentier, 1938 Areoligera sp. cf. A. senonensis Lejeune-Carpentier, 1938 Batiacasphaera euteiches (Davey, 1969) Davey, 1979 Batiacasphaera macrogranulata Morgan, 1975 Batiacasphaera sp. Callaiosphaeridium asymmetricum (Deflandre and Courteville, 1939) Davey and Williams, 1966 Canningia reticulata Cookson and Eisenack, 1960 Canningia senonica Clarke and Verdier, 1967 Canningia sp. Canningopsis colliveri (Cookson and Eisenack, 1960) Backhouse, 1988 Chlamydophorella sp. Circulodinium brevispinosum (Pocock, 1962) Jansonius, 1986 Circulodinium distinctum (Deflandre and Cookson, 1955) Jansonius, 1986 Circulodinium distinctum longispinatum (Davey, 1978) Lentin and Williams, 1989 Cleistosphaeridium?aciculare Davey, 1969 Cleistosphaeridium armatum (Deflandre, 1937) Davey, 1969 Cleistosphaeridium clavulum (Davey, 1969) Below, 1982 Cleistosphaeridium diversispinosum Davey, Downie, Sarjeant and Williams, 1966 Cleistosphaeridium flexuosum Davey, Downie, Sarjeant and Williams, 1966 Cleistosphaeridium huguoniotii (Valensi, 1950) Davey, 1969 Cleistosphaeridium?multispinosum (Sinhgh, 1964) Brideaux, 1971 Cleistosphaeridium radiculopse Mao and Norris, 1988 Codoniella campanulata (Cookson and Eisenack, 1960) Downie and Sarjeant, 1965 Cometodinium sp. Coronifera minus (Yu and Zhang, 1980) Mao and Norris, 1988 Coronifera oceanica Cookson and Eisenack, 1958 Cribroperidinium sp. Cyclonephelium clathromarginatum Cookson and Eisenack, 1962 Cyclonephelium compactum Deflandre and Cookson, 1955

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Cyclonephelium membraniphorum Cookson and Eisenack, 1962 Cyclonephelium paucispinum Davey, 1969 Dapsilidinium laminaspinosum (Davey and Williams 1966) Lentin and Williams, 1981 Dapsilidinium?pumilum (Davey and Williams, 1966) Lentin and Williams, 1981 Dapsilidinium simplex (White, 1842) Bujak, Downie, Sarjeant and Williams, 1980 Disphaeria munda (Davey and Verdier, 1973) Norvick (in Norvick and Burger) 1976 Ellipsodinium rugulosum Clarke and Verdier, 1967 Epelidosphaeridia spinosa (Cookson and Hughes, 1964) Davey, 1969 Exochosphaeridium arnace Davey and Verdier, 1973 Exochosphaeridium bifidum (Clarke and Verdier, 1967) Clarke et al., 1968 Exochosphaeridium muelleri Yun, 1981 Exochosphaeridium phragmites Davey, Downie, Sarjeant and Williams, 1966 Exochosphaeridium sp cf. E. phragmites Davey, Downie, Sarjeant and Williams, 1966 Florentinia sp. cf. F. berran Below, 1982 Florentinia clavigera (Deflandre, 1937) Davey and Verdier, 1973 Florentinia cooksoniae (Singh, 1971) Duxbury, 1980 Florentinia deanei (Davey and Williams, 1966) Davey and Verdier, 1973 Florentinia laciniata Davey and Verdier, 1973 Florentinia mantellii (Davey and Williams, 1966) Davey and Verdier, 1973 Florentinia radiculata (Davey and Williams, 1966) Davey and Verdier, 1973 Florentinia resex Davey and Verdier, 1976 Florentinia sp. Fromea sp. Gonyaulacysta sp. cf. G. cassidata (Eisenack and Cookson, 1960) Sarjeant, 1966 Gonyaulacysta helicoidea (Eisenack and Cookson, 1960) Sarjeant, 1966 Heslertonia striata (Cookson and Eisenack, 1960) Norvick in Norvick and Burger, 1976 Heterosphaeridium?heteracanthum (Deflandre and Cookson, 1955) Eisenack and Kjellstro¨m, 1971 Hystrichodinium pulchrum Deflandre, 1935 Hystrichosphaeridium bowerbankii Davey and Williams, 1966 Hystrichosphaeridium raritanianum Kimyai, 1966 Hystrichosphaeridium tubiferum (Ehrenberg, 1838) Deflandre, 1937 Kallosphaeridium?helbyi Lentin and Williams, 1989 Kallosphaeridium?ringnesiorum (Manum and Cookson, 1964) Helby, 1987 Kiokansium unituberculatum (Tasch, 1964) Stover and Evitt, 1978 Kleithriasphaeridium eoinodes (Eisenack, 1958) Davey, 1974 Kleithriasphaeridium loffrense Davey and Verdier, 1976 Kleithriasphaeridium readei (Davey and Williams, 1966) Davey and Verdier, 1976 Laciniasphaeridium sp. cf. L. orbiculata McIntyre, 1975

Leberidocysta defloccata (Davey and Verdier, 1973) Stover and Evitt, 1978 Leptodinium sp. Litosphaeridium siphoniphorum (Cookson and Eisenack, 1958) Davey and Williams, 1966 Nematosphaeropsis sp. Nexosispinum hesperum Davey, 1979 Odontochitina costata Alberti, 1961 Odontochitina operculata (O. Wetzel, 1933) Deflandre and Cookson, 1955 Odontochitina sp. cf. O. singhi Morgan, 1980 Oligosphaeridium albertense (Pocock, 1962) Davey and Williams, 1969 Oligosphaeridium complex (White, 1842) Davey and Williams, 1966 Oligosphaeridium prolixispinosum Davey and Williams, 1966 Operculodinium operculatum (Sah, Kak and Singh, 1970) Jain, 1982 Palaeohystrichophora granulata Mao and Norris, 1988 Palaeohystrichophora infusorioides Deflandre, 1935 Palaeoperidinium sp. Pervosphaeridium brevispinum (Norvick in Norvick and Burger, 1976) Below, 1982 Pervosphaeridium cenomaniense (Norvick in Norvick and Burger, 1976) Below, 1982 Pervosphaeridium paucispinum (Eisenack and Cookson, 1960) Jan du Cheˆne and Fauconnier, 1986 Pervosphaeridium pseudhystrichodinium (Deflandre, 1937) Yun, 1981 Pervosphaeridium truncatum (Davey, 1969) Below, 1982 Polysphaeridium sp. Prolixosphaeridium conulum Davey, 1969 Protoellipsodinium densispinum Morgan, 1980 Protoellipsodinium spinosum Davey and Verdier, 1971 Pseudoceratium sp. Pterodinium cingulatum (O. Wetzel, 1933) Below, 1981 Pterodinium cingulatum reticulatum (Davey and Williams, 1966) Lentin and Williams, 1981 Pterodinium cornutum Cookson and Eisenack, 1962 Pyxidiella sp. Rhynchodiniopsis sp. Scriniodinium?obscurum Manum and Cookson, 1964 Senoniasphaera rotundata Clarke and Verdier, 1967 Sentusidinium capillatum (Davey, 1975) Lentin and Williams, 1981 Sentusidinium cf. eisenackii (Boltenhagen, 1977) Lentin and Williams, 1981 Sentusidinium sp. Spiniferites katatonos Corradini, 1973 Spiniferites membranaceus (Rossignol, 1964) Sarjeant, 1970 Spiniferites multibrevis (Davey and Williams, 1966) Below, 1982 Spiniferites porosus (Manum and Cookson, 1964) Harland, 1973 Spiniferites ramosus (Ehrenberg, 1838) Mantell, 1854 Spiniferites ramosus ?angustus (W. Wetzel, 1952) Lentin and Williams, 1973 Spiniferites ramosus gracilis (Davey and Williams, 1966) Lentin and Williams, 1973

M.A. Lamolda, S. Mao / Palaeogeography, Palaeoclimatology, Palaeoecology 150 (1999) 65–82 Spiniferites scabrosus (Clarke and Verdier, 1967) Lentin and Williams, 1975 Spiniferites sp. Subtilisphaera hyalina C. Singh, 1983 Subtilisphaera perlucida (Alberti, 1959) Jain and Millepied, 1973 Surculosphaeridium?longifurcatum (Firtion, 1952) Davey, Downie, Sarjeant and Williams, 1966 Systematophora sp. Taleisphaera hydra Duxbury, 1979 Taleisphaera sp. Tanyosphaeridium variecalamum Davey and Williams, 1966 Thalassiphora bononiensis Corradini, 1973 Trichodinium castanea (Deflandre, 1935) Clarke and Verdier, 1967 Trichodinium ciliatum (Gocht, 1959) Eisenack, 1964 Trichodinium intermedium Eisenack and Cookson, 1960 Trichodinium pellitum Eisenack and Cookson, 1960 Trichodinium sp. Trithyrodinium suspectum (Manum and Cookson, 1964) Davey, 1969 Valensiella reticulata (Davey, 1969) Courtinat, 1989 Xenascus blastema (Davey, 1970) Stover and Helby, 1987 Xenascus ceratioides (Deflandre, 1937) Lentin and Williams, 1973 Xenascus plotei Below, 1981 Xenascus sarjeantii (Corradini, 1973) Stover and Evitt, 1978 Xiphophoridium alatum (Cookson and Eisenack, 1962) Sarjeant, 1966

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