microbial reefs from the northern Paris Basin — facies, palaeoecology and palaeobiogeography

microbial reefs from the northern Paris Basin — facies, palaeoecology and palaeobiogeography

ELSEVIER Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139-175 Late Jurassic coral/microbial reefs from the northern Paris B a s i n ...

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ELSEVIER

Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139-175

Late Jurassic coral/microbial reefs from the northern Paris B a s i n - facies, palaeoecology and palaeobiogeography Markus Bertling a,,, Enzo Insalaco

b

a Geologisch-Palgiontologisches Institut, Corrensstrasse 24, D-48149 Miinster, Germany b School of Earth Sciences, Edgbaston, GB, Birmingham B15 2TT, UK Received 11 November 1996; received in revised form 4 September 1997; accepted 16 September 1997

Abstract

During the late middle Oxfordian, patch reefs grew on the northern margin of the Paris Basin. According to the facies analysis of the reef and inter-reef sediments, the environment was a warm, clear and agitated sea with highly episodic sedimentation. The bioherms were a short-lived phenomenon during the third phase of regional reefal development. Sequence stratigraphically, they are associated with a highstand system tract. Volumetrically and trophically dominant organisms were microbes now represented by massive clotted leiolite; 'stalactitic' hemispheroids with purely thrombolitic texture are restricted to open caves. Corals were of structural, reef-building importance due to their rapid upward growth. The patch reefs are characterised by thickets of ramose corals which developed a very open framework. In the vicinity of these patch reefs, though in hydrodynamically higher-energy environments, grew thickets of more stoutly branched corals; however, they are rarely preserved in situ and are generally represented as abundant coral rubble. The reef taxa are characterised by the notable absence of several groups (e.g. oysters, serpulids, bryozoans, pectinids) occurring at other localities where reefs of similar age developed in similar environmental conditions. The reefs also have strikingly modern aspects to them, in particular the presence of cryptic elements within caves and a sponge-dominated borer association. Dwellers belong to various life-form types although encrusting taxa are exceedingly rare. This may be explained by the presence of soft microbial films on most surfaces. The palaeoecological analysis suggests that the major controls on faunal composition and high diversity were elevated nutrient levels, highly episodic sedimentation and probably seasonal environmental disturbances. Structural and functional aspects of the reef community (grazers trigger framebuilders, borers trigger binders, binders hamper borers) allow ecological comparisons to be made with contemporaneous, as well as Recent, reefs. The unique combination of ecological factors resulted in a specialised, previously undescribed, community which differs from both Tethyan and northern localities in various aspects; these include cavities with cryptofauna, prominence of grazing gastropods and high faunal diversity in a microbially dominated build-up. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Jurassic; France; reefs; scleractinia; palaeoecology; carbonate; platforms; palaeobiogeography

Corresponding author. Fax: +49 251 8333968. E-mail: [email protected] 0031-0182/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0031-0182(97)001 25-9

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M. Bertlmg, E. lnsalaco / Palaeogeography, Palaeoclimatoh~gy, Palaeoecoh~gy 139 (1998) 139 175

1. Introduction

1.1. Previous work The Late Jurassic reefs south of the Ardennes have received very little scientific attention. Munier-Chalmas (1894) noted the occurrence of corals and gave a very general description. The reefs were first reliably dated by ammonites by Bonte (1938), followed by Beauvais (1964) with very much the same result. More recently, these findings were confirmed and regionally correlated by Enay and Boullier (1981). Although Beauvais' (1964) work was a monographic study of the coral fauna around the Paris Basin, the Ardennes' reefs were not studied taxonomically. Thus, the sampled localities are hitherto virtually unknown from a palaeoenvironmental and compositional point of view.

1.2. Aims of work This paper aims to demonstrate interactions between sedimentation, ecology of reef organisms and taxonomic composition of the reefs studied. The approach is to relate facies to thorough palaeobiological and synecological evaluation. This includes considering habitat and trophic preferences as well as direct individual and indirect group interactions. Differing from many other studies of fossil reefs, particular attention is given to bioeroders and reef dweller autecology. Thus the work leads to a graphic model of growth and structure of these reefs, allowing to compare them with other Late Jurassic reefs.

1.3. Location and stratigraphy The study localities are situated in northern France in the Department Ardennes, northeastern Champagne. The exact location is an area of three adjacent quarries called l'l~pine, approximately 12km north-northeast of Rethel and 1300m northeast of the village of Novion-Porcien at 4°25'E, 49°24'N (Fig. 1). It may be found on sheet 2910 O of the topographical map 1:25,000 with R 0233000 and H 5512600. Geologically, the

outcrops are situated on the northern margin of the Paris Basin. Bonte (1938) found several perisphinctid ammonites in the region, with Perisphinctes (Dicho-tomosphinctes) falculae Ronchadze indicating middle Oxfordian age ('Argovien, Rauracien') at l'l~pine. His dating is corroborated by our find of a Perisphinctes (Arisphinctes) cf. plicatilis (Sowerby). Beauvais (1964) narrowed reef growth to the upper "Argovien'. However, these established French "stages' are essentially facies units, and are thus of limited value for a detailed chronostratigraphy. In modern chronostratigraphic usage, the reefs are placed in the antecedens subzone of the plicatilis zone, or in the transversarium zone, both defining the later part of the middle Oxfordian; the reefs in the adjacent Meuse region are roughly of the same age (Enay and Boullier, 1981 ). According to our own field observation, however, the reefs studied correspond to the o61ites capping the Lorraine reefs, thus representing a third, though less intense, phase of regional framework construction (Insalaco, 1996a). Extensive fresh quarry outcrops allow the reconstruction of the reef and its adjacent environments three-dimensionally. There is also evidence for the presence of another reefal unit situated stratigraphically below the reef facies studied at outcrop. This comes in the form of isolated limestone blocks with a coral fabric which is clearly distinct from that present at outcrop. These blocks may be found in topographically low parts of the area and a river deposit south of the quarries. They contain a coral assemblage very different from the reef fauna exposed: platy forms of Dimorpharea, Microsolena, Comoseris, lsastrea and Thamnasteria are dominant. The composition and biofacies of this underlying unit is very similar to the microsolenid biostromes in the lower (first) reefal units which are followed by branching coral thickets in many other areas (second reefal unit) (e.g. Roniewicz and Roniewicz, 1971; Errenst, 1990; Leinfelder et al., 1994, p. 37; Nose, 1995, pp. 23, 107: Insalaco, 1996b). The timing and succession of these reefal units are almost identical to that of the departments of the Yonne and Nibvre (Menot, 1974); the facies suite is also similar to that of the Upper Jurassic of the upper Meuse valley (Geister

M. Bertling, E. Insalaco / Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139-175

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and Lathuili~re, 1991), which also exhibits a marked shallowing-upward trend. In the Ardennes, however, the exact position of the 'lower reefal unit' has yet to be located. From their sequence stratigraphic context, these lower reefal units are closely associated with the maximum flooding surface, marking the begin of a shallowing-upward sequence in which an environmentally driven species replacement of framebuilders took place (Insalaco, 1996a).

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1.4. General palaeogeography

The northern Champagne region was situated in subtropical latitudes (about 35°N) during the Late Jurassic (Smith et al., 1981). A shallow carbonate ramp dipped southward to the Tethys, with an island formed by the London-Brabant Massif lying some tens of kilometres to the north (Fig. 2). Isolated reefs grew along its southern

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M. Bertling, E. lnsalaco / Palaeogeography, Palaeoclirnatoh~gy. Palaeoeco/ogy 139 (1998) 139 175

margin, truncated by a hemigraben between the Ardennes and the Armorican Massif ( Rioult et al., 1991 ). During this time, reef development reached considerable importance only further to the southeast, in the northeastern Paris Basin. This area, nowadays comprising Lorraine and Switzerland, has been well studied taxonomically (e.g. Buvignier, 1852; Thurmann and l~tallon, 1864: Beauvais, 1964), as well as other occurrences further south into the Tethys. Palaeoceanographic reconstructions suggest an equatorial tropical current flowing westward across the southern margin of the London-Brabant Massif, This current was then partly deflected northwards due to the Armorican Massif to form an Atlantic proto-Gulf stream (Jansa, 1986; Oschmann, 1988: Leinfelder, 1993).

2. Materials and methods

2.1. Type of preservation The most striking feature regarding preservation of fossils in these reefs is the complete dissolution of all aragonite skeletons. In numerous coral cavities, a marginal veneer of secondary, millimetresized, clear calcite crystals occurs. Scleractinians, gastropods and most bivalves thus are represented only by external moulds; steinkerns of bivalves are frequent. The only exception of this rule are very thin, leafy colonies of Thamnasteria, occurring in the cavities described below. Their coralla have been replaced by calcite, presumably due to early diagenetic aggrading neomorphism. The calcitic hard parts of echinoids are preserved recrystallised but still with their original mineralogy. Oysters and brachiopods have retained their microstructure. A virtually identical pattern of preservation has been described by Ftirsich et al. (1994) from bivalve reefs of Dorset; most probably, in both cases the passage of meteoric water undersaturated in hydrogencarbonate is responsible for the phenomena. The lack of selectivity in aragonite (non-)preservation suggests that no mixed-water diagenesis took place (see Walter, 1985 ). Aragonite dissolution may also take place because of lowered

pH due to the decomposition of organic matter under oxygen-deficient conditions (Purser and Schroeder, 1986). This may also have been possible (see Section 4.2). The micritic sediment that forms the external moulds gives perfect impressions of the external morphology of any hard parts; fine details of corallite morphology and gastropod surface sculpture are always visible. Borings in corals are filled by micrite which was cemented before the skeleton was dissolved. This makes boring traces stand out three-dimensionally in full detail in the coral dissolution cavities.

2.2. Data collection For the facies analysis, thin sections and polished slabs of rock samples were prepared. The relative proportions of microfacies constituents were estimated using the comparison charts of Bacelle and Bosselini (1965). The relative importance of reef builders was visually estimated in outcrop sections. A test for accuracy with more quantitative methods, such as working with quadrats or mapping coral colonies on photos, yielded favourable comparability (Insalaco, 1996a). No fully quantitative data could be obtained because of the rock hardness and the rugged outcrop faces. Reef and associated macrofauna were recorded unbiasedly from all facies; neither state of preservation nor number of specimens previously taken had an influence on collecting. Large coral colonies could not be secured complete in most instances, however. Which facies each specimen came from was noted. Additional material was collected from loose rubble in order to increase the number of specimens (mostly of borings and juvenile gastropods) available for taxonomic identification; they were not included in the main study. In most cases the corals could be identified in the field: several scleractinian species have a highly characteristic corallite arrangement and calyx morphology, thus allowing their assignment to a genus because of the safe assignment to a species. In debatable cases, as well as for all molluscs, latex pulls of external moulds were prepared. Boring bivalves were prepared from their holes by carefully removing their encasing micrite.

M. Bertling, E. lnsalaco / Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139 175

3. Facies analysis and framework construction

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143

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3.1. Sedimentary environments Three main reef and reef-related facies have been identified in outcrop: mobile carbonate sand, reef rubble, and in-situ-reef framework. These three facies are interbedded with each other and can occur in any sequence (Fig. 3). Taphonomy was considered in detail only for the latter. IT',,,"

3.1.1. Reef rubble This is the most widespread of the two coral facies and dominates the sequence in all three of the sections studied, especially in the westernmost quarry 3. The sediment is well bedded on a metre scale but the style of bedding varies from planar parallel to non-parallel. It is packed with reef debris which is clearly not in situ, giving these units a distinctively rubbly, rudstone appearance. The composition of this material is essentially reefal in origin; in places, corals can constitute up to 80% of the rock volume (generally around 30-60%), and hence the facies may appear reefal. Vast amounts of dense, hard, peloidal micrite bind the reef debris together. This has not been transported far since (a) much of the material is composed of large coral fragments and occasionally whole coral colonies, (b) the material shows very little evidence of abrasion, with even the most delicate calicular structures remaining intact, and (c) most of the shell material is still articulated. Hence this material can be considered as parautochthonous reef debris which has been deposited at, or very close to, the site of reef development. Many of the coral branches as well as nerineid gastropods show a preferred orientation suggesting significant current action. The bases of these debris sheets are generally erosive, and well-developed scour troughs are common. Although the material has not travelled far it clearly represents a mixed assemblage, composed of reef (dominantly coral) and off-reef (dominantly mollusc) faunas. The fossil association can therefore be described as a spatially averaged fauna. On a microfacies scale the sediment is a bioclastic wackestone, locally packstone (biopelmicrite) with approximately 30-50% bioclasts. The matrix

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is a heterogeneous dense peloidal micrite, with a vague wavy lamination in areas. The peloidal material is polygenic, comprising faecal pellets,

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M. Bertling, E. lnsalaco / Palaeogeography, Palaeoclimatology, Palaeoecolo~,Lv 139 (1998) 139 175

highly micritised bioclasts, lithoclasts and peloids of presumed microbial origin, It has subsequently been bound by further microbial peloidal material. The bioclasts consist of angular, very poorly sorted grains ranging from less than 0.01 to 10 mm in size (average approximately 1 mm). They show a considerable taxonomic richness, with corals, gastropods, bivalves, serpulids, red algae, echinoids, calcified cyanobacteria and foraminifera being particularly abundant. Many components are highly micritised and possess well-developed spongiostromate coatings up to 3 mm thick. This suggests that the bioclastic material was not rapidly buried but resident on the sediment surface for a considerable amount of time. Thus a low background sedimentation is envisaged.

3.1.2. Carbonate sand facies Well-developed carbonate sandwaves with large fining-upward avalanche foresets suggest effective unidirectional flow. Their architecture, an impoverished benthic fauna and lack of bioturbation indicate constantly shifting, unstable carbonate shoals. However, the mature nature of the carbonate grains suggests that the grains had a high residence time on the sediment surface, allowing them to become well rounded, highly micritised and coated. This again points at a contemporaneous more stable, lower-energy source area with low background sedimentation rates. The sands are well-washed bioclastic grainstones (biosparites). Carbonate grains constitute 20-60% (average 50%) of the rock volume and are set in a sparitic cement, with micritic matrix generally less than 10%. The grains are moderately to well-sorted, well-rounded and generally coarse-grained (finer bands: 0.2-0.5 mm up to 1 mm; coarser bands: 0.5-1.6 mm up to 3 mm). The carbonate grains include bioclastic material (15%), highly micritised and coated grains (50%) as well as intraclasts (15%). Bioclastic material includes foraminifera which are extremely common (numerically most abundant group), bivalves, gastropods, and small rounded coral fragments. There is little evidence of compaction which perhaps implies early cementation.

3.1.3. In-situ reef ln-situ-reef development occurs in small localised patches throughout the sections. The preserved reef patches tend to be domal in form, of relatively small size (approximately 11 m high and 20 m across), and dominated by thinly branching ramose corals. Details of the facies, framework construction and palaeoecology of the in-situ reef are given in subsequent sections. The fact that the sections are dominated by reef rubble facies suggests that in-situ-reef development was far more extensive than the present extent of reef framework indicates. 3.2. Facies relationships and transitions The field relationships between the three facies are shown in Figs. 3, 4 and 5a. The boundary between in-situ-reef framework and clean carbonate sand changes throughout the growth of the reef. During the initiation and early stages of reef growth, the lateral transition from the carbonate sand to in-situ reef is gradational over approximately 3 to 4m. From carbonate sandwave to in-situ reef there are increases in the proportions of oncoids, micrite relative to sparite, micrite relative to grains, peloidal intraclasts, coral bioclasts, and grain angularity. During the later stages of reef development, reef growth accelerated and/or the rate of sandwave encroachment decreased, which resulted in the development of moderate reef relief (?1 to 5 m). This led to an onlap of carbonate sands onto the reef in the later stages of reef growth (Figs. 4 and 5a). The vertical transition from carbonate sand to reef facies at the base of the reef follows the same pattern as lateral, though it is far more rapid, occurring over 15 cm. This transition represents the stabilisation of the sand facies. The reefs are terminated by bioclastic material and reef rubble. 3.3. Framework construction and cavity development Reef growth commenced during phases of reduced sandwave mobility, perhaps during doldrum calms. The loose carbonate sands were stabilised by the swift development of microbial mats

M. Bertling, E. lnsalaco / Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139 175

145

Fig. 4. Field sketch showing the relationships between the three main facies in westernmost outcrop (3) at L'l~pine, traced from photomontage.

over the static sediments. The subsequent cementation of the microbial film then provided a suitable substrate for coral colonisation. Reef development thus occurred on stabilised sediments as well as in areas slightly protected by sand bars. The reef framework is dominated by various microbial rocks which constitute approximately 70% of the total volume. Corals, although representing only 10-15% (remaining percentages are debris) and thus volumetrically of secondary importance, are nevertheless essential for the development of these reefs for three reasons: (1) they provided the 'skeleton' which the microbes encrusted; (2) they acted as the main component of vertical growth and hence defined the reef's form; and (3) they were primarily responsible for the formation of numerous cavities (see below). Reef construction was therefore a continuous twostep process: firstly, the growth of ramose corals provided the primary open framework which subsequently acted as a site for the precipitation of large volumes of peloidal micrite. Secondly, its early cementation bound the material together and gave the reef structural rigidity. Throughout the development of the reefs a number of caves were created by the process of local, small scale 'roofing-over' due to fusion of their walls. These cavities, generally less than 1 m across, are relatively frequent, accounting for approximately 10% of the total reef volume. Extending from the roofs of these cavities are welldeveloped 'pseudostalactites' and 'pillows' of dense hard thrombolite (Fig. 6); the former is the more

common of the two forms. These are virtually identical in form to those described from lower Kimmeridgian reefs at Le Chay, Charente, France (Taylor and Palmer, 1994) and numerous Upper Jurassic reef localities in Portugal (Leinfelder et al., 1993; Schmid in Leinfelder et al., 1994). According to the classification of Schmid (1996), these hemispheroids texturally are pure clotted thrombolites; the main volume of microbial rocks within the bioherms consists of clotted leiolite, however. The surface of the microbial pseudostalactite is made up of numerous accreted torpedo-shaped increments ('microbial pendants', Fig. 6). Each pendant is about 2-3 cm long and 1 cm in diameter. The shape of these increments is defined by the very early cementation of the micrite and gravitational forces pulling down on the free growing microbial films. Fine sediment must have adhered readily to the surface of the microbial mass (by trapping in mucus films (Burne and Moore, 1987; Reitner, 1993) if the steep sides of the projecting knobs were to keep their shape. Bioclastic material constitutes 10 to 20% of the thrombolite and is made up predominately of finegrained (less than 0.5 mm), angular, poorly sorted coral fragments. Large bioclasts up to 50 mm across are occasionally incorporated into the pseudostalactite. A section through the hemispheroids reveals a typical clotted thrombolitic fabric, with mesoclots of approximately 2-4 mm in diameter (Fig. 7). Around the external region, there are numerous primary pores with similar dimensions and shapes

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M. Bertlinlz, E. lnsalaco / Palaeogeo~,,raphy, Palaer~climatoh>g:v, Palaeoecolo~4y 139 (1998) 139 175

Fig. 5. Reef and its relationship with the surrounding facies; quarry 3, L+l~pine. (a) Facies relationship between the reel', peloidal sandwave and reef rubble facies. Note that at the top of the reef the sandwave facies onlaps onto the reef but the lateral margins are gradational; person (right centre) 1.8 m in height. (b) Reef rubble facies built up of successive sheets and channels of reef debris. The base of the sheets and channels are often, though not always, clearly erosive: hammer 32 cm in length. (c) Detail of the reef rubble fabric. The sheets tend to be packed with the broken branches of Dendrohelia coalescens: the bright material in-between the coral debris is hard leiolite; part of hammer showing is 22 cm+ (d) Detail of the reef fabric which is dominated by branching ramose forms as the main framebuilders. Note the very thin, widely spaced branches of Thamnasteria dendroidea (type A) developing open colonies: part of hammer showing is 30 cm.

M. Bertling, E. lnsalaco / Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139-175

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Fig. 6. Field morphology o f thrombolites in reef caves in quarry 3, L'l~pine. (a) Cluster of pseudostalactites hanging from a palaeocave ceiling; width of frame 100 cm. (b) Detail o f a pseudostalactite. Below the pseudostalactite a chalky foraminiferal biomicrite is visible representing the passive sediment fill of the cave; part of hammer head showing is 17 cm. (c) Pillow-shaped microbial masses which have developed in some of the primary cavities of the reef framework; hammer head is 19 cm across. (d) Detail of the outer surface of a pillow in (c) with typical thrombolitic fabric. Note almost complete lack of encrusters; part of hammer head showing is 10 cm across. (e) Detail of the surface of a microbial pseudostalactite covered by numerous accreted microbial pendants. Note the boring bivalve (top centre).

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M. Bertling, E. &salaco : Pahteogeography, Palueoclimatoh~Kv. Palaeoecoh~gy 139 (1998) 139 175

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Fig. 7. Internal structure of microbial pseudostalactite: quarry 3, L'Epine; scale with mm division for both samples. (a) Longitudinal section: note elongate mesoclots. (b) Transverse section: dark clots in the main body of the pseudostalactite are primary pores that have been infilled by fine detrital material, peloids and sparite.

as the clots. These voids can constitute up to 30% of the surface area in the outer regions of the pseudostalactite. They gradually decrease in number towards the centre of the structure which is still clotted but lacks open voids (Fig. 7). The darker clots in the core represent infill by silt-sized skeletal debris and peloids in a sparitic cement. Commonly they are only partially filled geopetally, with the remaining space being occupied by sparite. Its very porous internal structure is therefore a consequence of the growth of the pseudostalactite. The voids in its centre were exposed long enough to be filled, whereas the pores in the external regions, being much younger, could not be closed prior to the termination of microbial growth and therefore remained open. The space created by these caves was infilled either passively by fine-grained bioclastics or actively by layered leiolite growing from the floors, walls and ceilings; the latter was the main mechanism of cave infill. The sediment reminds of very fine-grained, poorly cemented chalk. Bioclastic material is rare, fine (grain size 0.1 0.5 mm), angular, and often highly micritised. The matrix is a detrital fine lime mud with some local sparite cements. Foraminifera are extremely abundant.

3.4. Depositional environment The sediments were deposited on a clean carbonate platform with no siliciclastic influx. The carbonate sands represent active, constantly shifting subtidal shoals. Erosional and storm blow-out channels were abundant and shifted across the depositional area. Debris sheets rich in reefal material were deposited following storm erosion and surf disintegration of the reefs in the more exposed areas. The rubble between isolated reef frameworks contained a rich mollusc fauna. Current and wave action was generally strong and sufficient to drive sandwave migration, develop channels, and prevent deposition of mud and silt (Fig. 8). All sediments were deposited in shallow water, no more than a few metres, and hence the facies belonged to the upper subtidal zone. Conditions were fully marine, as indicated by the stenohaline fauna (see Section 4Section 6.1.2 ). The depositional environment therefore appears to be very similar to the back-reef and reef flat environments of the Florida shelf. The area may be divided into a number of broadly zoned subenvironments (Enos, 1977; Multer, 1977; Shinn, 1980) including:

M. Bertling, E. Insalaco / Palaeogeography, Palaeoclimatology,Palaeoecology139 (1998) 139-175

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hydrodynamically exposed, higher-energy region

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"back-reel', lower-energy region Ncdon preNrved in

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Fig. 8. Schematicmodel (tens of metres vertically,hundreds of metres horizontally)for the developmentof the upper reefal units in the Dept. Ardennes (see text): The debris derived from the reef-fiat(not observed in outcrop) was deposited as sheets in the 'backreef' areas. These gave way to carbonate sands, channels and local patch reefs. The lower reefal unit is inferred from isolated blocks, it has not been located in outcrop.

( 1 ) a deep seaward coral rubble zone; (2) a spur-and-groove zone; (3) a zone of landward orientated Acropora palmata, constructing the reef crest; (4) the reef flat, composed of unorientated A. palmata with much reef rubble; (5) the back-reef zone, consisting of scattered colonies of A. palmata, thickets of A. cervicornis, and large heads of Montastrea and Diploria. Reef rubble and lime sand are locally abundant; and (6) back-reef lime sands with local patch reefs and cut by tidal channels. Zones (5) and (6) of the Florida shelf therefore provide a broad Recent analogue for the autochthonous patch reefs whereas zones (3) and (4) provide a possible analogue for an inferred seaward framework responsible for the rubble units in the more exposed areas of the reef complex. Ketcher and Allmon (1993) have presented criteria to distinguish various taphonomic histories of coral rubble beds. Several characteristics of the L'l~pine rubble facies suggest accumulation over several decades followed by sudden burial: the strongly varying degree of bioerosion (indicator of varying exposure time), the coarse particle size of faunal elements and matrix (winnowing), the presence o f intact, small corals ('recently' buried) as

well as micritisation of matrix grains (longer exposure), The main processes responsible for the formation of the reef rubble were storms which produce very similar sedimentary results in modern shallow seas.

4.

Reef fauna

4.1. Corals 4.1.1. Facies distribution of coral fauna Two coral units can be identified: reef rubble facies and in-situ-reef facies. The distinction between these is based primarily on the presence or absence of in-situ framework and the depositional style of the unit. However, closer examination of the two facies reveals that their coral composition is different (Table 1 ). The reef rubble is overwhelmingly dominated by branches of Dendrohelia coalescens (Goldfuss), which constitutes up to 80% of the total coral skeletal volume. Within the in-situ-reef framework, dominance patterns are not so marked; Dendrohelia coalescens, although present, is generally subordinate to Thamnasteria dendroidea (Lamouroux) and other branching forms.

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M. Bertling, E. lnsalaco , Palaeogeut{raph)', Palaeoclimatology, Palaeoecology 139 (1998) 139 175

Table 1 Coral fauna of the in-situ reet~ and reel rubble tacies Principal framebuilders

Dwellers

Ramose forms Thamnasteria dendroidea ( f o r m B)

Dendraraea sp. Dendrohelia coalescens AIlocoenia sp. Thamnasteria dendroidea (form A )

Massive forms ~- ~+ +

+ +

Phaceloid forms

Latomeandra sp. (Tadophyllia sp. Thecosmilia sp, Calamophylliopsis sp. So,Iosmilia sp.

+ + + + + +

Thamnasteria sp. I Microphyllia sp. Cvathophora sp. Fungiastraea sp. Stephanastraea sp. lsastrea sp. Microsolena sp. Diplocoenia sp. Stylina sp. Mean¢h'ophyllia sp. Dermoseris sp. Mixastrea sp. Mesomorpha sp.

+ + + + + + + +

Encrusting f o r m s

Thamnasteria sp. lsastrea sp.

+

Solitary forms

Epistreptophyllum sp. Haplaraea sp. Montlivaltia sp. Five-step scale from + + to

- indicates relative importance.

Because the reef rubble facies is made up of autochthonous to proximal parautochthonous material (see Section 3.1.1 ), a taphonomic concentration of D. coalescens derived from reefs of similar composition to those preserved in the sections may be ruled out. Taphonomic features of the reef rubble suggest that two reef types have to be recognised: ( 1 ) observed reef patches not dominated by D. coalescens: and (2) inferred reefs dominated by D. eoalescens. 4.1.2. Coral assemblage Taking both reefal facies into account, these reefs are taxonomically very rich, with 25 genera having been identified (Table 2 ). Compositionally, the coral fauna is typically 'Tethyan', containing many of the genera that are common in the Upper Jurassic reefs of Lorraine, Burgundy and the Swiss Jura (Bourgeat, 1887; Beauvais, 1964; Pttmpin, 1965; Geister and Lathuili6re, 1991; lnsalaco, 1996a). The in-situ patch reefs are dominated by loose

open colonies of ramose forms (Fig. 9), which constitute over 70% of the principal framebuilders. Four branching ramose forms have been identified: Dendrohelia coalescens (Goldfuss), Thamnasteria dendroidea (Lamouroux), Allocoenia sp. and Dendrarea racemosa (Michelin). Thamnasteria dominates, though Allocoenia and Dendrohelia are also common. Two forms of T. dendroidea have been recognised: form A, which is by far the more abundant of the two, is a very thinly branched lbrm with a branch thickness of generally less than 8 ram. This growth type generally developed very loose open thickets with sinuous, widely spaced branches reaching a height of 1.5 m. Form B has far thicker branches (generally more than 15 mm) and a significantly higher branch packing density. It developed colonies of similar dimensions to form A. Although there may be some gradation between the two forms in terms of branch packing density, there is a clear bimodal distribution of values around 6ram (form A) and 15 mm (form B) in terms of branch diameter. These two forms

M. Bertling, E. lnsalaco / Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139-175

151

Table 2 Details of growth form, integration, size and habit of the coral fauna from both reef rubble and in-situ reefs Coral

Growth form

Integration

Growth habit and modal dimensions

Allocoenia

branching ramose

cerid

CalamophyHiopsis

branching

phaceloid

Cladophyllia Cyathophora Dendraraea Dendrohelia coalescens

branching massive branching ramose branching ramose

phaceloid plocoid thamnasteroid plocoid

Dermoseris Diplocoenia Epistreptophyllum Fungiastraea Goniocora

branching massive to subbranching ramose cylindroid massive branching

phaceloid ceroid-subplocoid solitary thamnasteroid phaceloid

Loosely packed thickets with widely branches. Sinuous branches with a thickness ofl cm. Colony height 60 cm. Loosely packed thickets with widely spaced branches. Colony height 40 cm. Densely packed thickets. Colony height 50 cm. Small dome-shaped colonies. Colony width 10 cm; height 6 cm. Branch thickness 1.5 cm. In situ colonies not seen. Large densely packed thickets. Branch thickness over 2cm; colony height up to 2 m. In situ colonies not seen. Small dome-shaped colonies, colony width 10 cm; height 6 cm. Some colonies show a sub-branching ramose form.

Haplaraea lsastraea

cylindroid massive

solitary cerid

Latomeandra Meandrophyllia Mesomorpha Microphyllia Microsolena Mixaslraea Montlivaltia Stephanastraea Stylina Stylosmilia Thamnasteria

branching phaceloid massive massive massive massive massive cylindroid massive massive branching massive encrusting

phaceloid meandroid thamnasteroid meandroid thamnasteroid thamnasteroid solitary cerioid plocoid phaceloid thamnasteroid thamnasteroid

Thamnasteria dendroidea branching ramose

thamnasteroid

Thetvsmilia

phaceloid

branching

have almost identical calicular structure, although t h e d i s t a n c e b e t w e e n a d j a c e n t c a l i c u l a r c e n t r e s is s i g n i f i c a n t l y less in f o r m A ( 1 - 1 . 5 m m in f o r m A ; 2 - 3 m m in f o r m B). W h e t h e r t h e s e t w o f o r m s r e p r e s e n t d i f f e r e n t species o r s i m p l y d i f f e r e n t g r o w t h f o r m s r e m a i n s u n c l e a r until m o r e r i g o r o u s s y s t e m a t i c a n a l y s i s is u n d e r t a k e n . D. coalescens f o r m s d e n s e t h i c k e t s o f r o b u s t l y b r a n c h i n g c o l o n i e s w h i c h c a n r e a c h 1.7 m in h e i g h t

Small dome-shaped colonies. Colony width 10 cm; height 6 cm. Large loosely packed thickets with steep-angled lateral budding. Colonies up to 1 m. Dome- to plate-shaped colonies. 10cm thick and 10 20cm across. ? Very thin foliaceous plates 2 mm thick and up to 12 cm across. Small colonies with few branches; height 15 cm. Small dome-shaped colonies. Colony width 10 cm; height 6 cm. Small dome-shaped colonies. Colony width 5 cm; height 3 cm. Small dome-shaped colonies. Colony width 8 cm; height 3 cm. Small domal to platy colonies. Colony width 10 cm; height 4 cm. Small dome-shaped colonies. Colony width 4 cm; height 3 cm. Small dome-shaped colonies, colony width 4 cm; height 3 cm. Dome-shaped colonies. Colony width up to 10 cm; height 10 cm. Loose open thickets; colony height 80 cm. Small dome-shaped colonies. Colony width 5 cm: height 3 cm. Very thin foliaceous plates. Colony thickness 2 mm and up to 12 cm across. Form A Mainly as very loose thickets with thin, widely spaced branches. Branch thickness generally <0.8 mm. Colony height over 1 m. Form B Densely packed thickets with thick, densely packed branches. Branch thickness generally over 15 mm. Colony height over 1 m. Loose open colonies. Colony height 50 cm.

a n d possesses t h i c k b r a n c h e s ( g e n e r a l l y m o r e t h a n 2 c m thick). P h a c e l o i d f o r m s , a l t h o u g h v o l u m e t r i cally m u c h less i m p o r t a n t , still c o n t r i b u t e to f r a m e w o r k c o n s t r u c t i o n a n d are r e p r e s e n t e d by s o m e seven g e n e r a , Calamophylliopsis, Cladophyllia, Thecosmilia, Goniocora a n d Stylosmilia b e i n g t h e m o s t a b u n d a n t . T h e s e o c c u r as l o o s e l y p a c k e d t h i c k e t s o c c a s i o n a l l y r e a c h i n g 60 c m in height. Massive and solitary forms contributed very

152

M. Bertling. E. lnsalaco / PalaeogeoL,raphy. Palaeoclimatology, Palaeoecology 139 (1998) 139 175

| i

| C!?m

Fig. 9. Ramose coral taxa from the reef facies: quarry 3, L'l~pine: latex peels from natural casts. All scales in mm. (a) Branch of AIlocoenia sp. (b) Branch of Thamnasteria dendroide~t (Lamouroux) (type A). (c) Branch of Demlrohelia coalescens (Goldfuss). (d) Calicular details of Allocoenia sp. te} Calicular details of Thamnasteria dendroidea (type A). (f) Calicular details of Dendrohelia coalescens.

little to c o n s t r u c t i o n a n d are best interpreted as s e c o n d a r y f r a m e b u i l d e r s a n d dwellers. The massive colonies are generally d o m e s h a p e d and rarely exceed 5 cm in height or 7 cm across, whilst the c y l i n d r o i d forms are a p p r o x i m a t e l y 1 0 c m high a n d a few cm across. A l t h o u g h they constitute a small p r o p o r t i o n o f the coral skeletal volume as a consequence o f their size, they are a b u n d a n t and

diverse. Diplocoenia, Fungiastraea, Stylina, Latomeandra, Thamnasteria a n d Microphyllia are p a r t i c u l a r l y characteristic o f the fauna. W i t h i n the caves, very thin encrusting a n d foliaceous forms o f Thamnasteria a n d Isastraea? occur, in fact the only corals to d o so. Their colonies are less t h a n 2 m m thick, a n d up to 12 cm across. In general they encrust the roofs a n d walls, t h o u g h

M. Bertling, E. lnsalaco / Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139 175

occasionally occurring as foliaceous outgrowth directly off the walls of the caves. In summary, the sections studied suggest the existence of two reef types (Fig. 8): (1) dense thickets dominated by D. coalescens developed in exposed places further offshore. Their branch architecture increased their mechanical strength, thus allowing the colonisation of wave-controlled environments. These frameworks produced vast amounts of reefal debris deposited as sheets and channels; these represent both storm erosion and surf disintegration. (2) Small patch reefs developed in more protected, but still high-energy environments. In these reefs T. dendroidea (form A) was the main framebuilder. Phaceloid forms and small dome-shaped forms are common.

4.1.3. Autecology of corals The two forms (A, B) of Thamnasteria dendroidea (Lamouroux) occupy different niches. Form A appears to be restricted to shallow-water environments dominated by sand shoals, whereas form B has also been documented from a number of other similar facies (Insalaco, 1996a). The peculiar growth form of T. dendroidea type A has been noted by Beauvais et al. (1974), although no reasons for the development of this morphology were suggested. It may be related, however, to its faster linear growth (i.e. calcification) rate. In moderately high-energy environments, the ability to cope with mobile sediments and episods of rapid sedimentation must have been important. If an increased linear growth rate is assumed as advantageous for this coral in such environments, then this could have been achieved in two ways: (1) by retaining the same growth form and habit though having increased the rate at which it could produce calcium carbonate, which would have gone directly toward increasing its linear growth rate; or (2) by retaining the same rate of calcium carbonate production but having concentrated deposition in a linear direction, thereby making the branches long and thin. Light levels were not significantly different between environments where the two forms of T. dendroidea occurred (Insalaco, 1996a). Since light is one of the main controls on coral calcification, there was no obvious way for T. dendroidea to

153

increase the rate at which it could deposit carbonate from environments where form B grew to environments where form A grew [i.e. mechanism ( 1) of above]. Therefore it can be speculated that the thin branches of type A form represent a vertically faster growing form than type B [mechanism (2) of above]. However, due to the fact that the preservation of T. dendroidea type A is invariably mouldic, no growth bands have been retrieved to corroborate this. Nevertheless high growth rates are typical of modern pioneer species, which may compensate for unstable substrates in this way (Sheppard, 1982), and coral assemblages in such environments are often rich in thin ramose corals. Indeed all the branching corals (including the phaceloid forms) in these reefs developed very loosely branched colonies with wide interspaces, which would also be better suited to higher sedimentation rates. Moreover, in these reefs high linear growth rates in the coral would have been promoted by the fast-growing microbial mats, which had similar effects on vitality (smothering) and larval settlement as sedimentation. Large massive forms are rare, though small dome-shaped forms are abundant (Table 2). This suggests that the corals died before they could grow to a large size. This may be a consequence of their comparatively slower growth rates, as a result of which they were not able to cope with periods of high sedimentation rates and mobile substrates, and relatively rapid framework growth. D. coalescens formed dense thickets of colonies up to 1.7 m in height. The inferred presence of Dendrohelia coalescens-dominated frameworks and rubble units in the more exposed areas of the reef complex studied suggests that they were adapted to higher-energy environments. This may be by virtue of their thicker branches (in a similar fashion to Acropora-dominated frameworks of the presentday Florida shelf; see Section 3.4). However, the comparison of Upper Jurassic branching corals with Modern Acroporidae (e.g. Eli~ovfi, 1981; Leinfelder, 1992; Nose, 1995, p. 107) should not be taken too far since these Jurassic corals did not rely on breakage as a method of reproduction (no regenerated branches have been observed in several hundred specimens). Moreover these Jurassic corals were unable to grow freely without support

154

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Table 3 Associated fauna of in situ-reefs and reel" rubble Associated molluscs occurring in...

Rubble

Gastropoda Pleurotomarioidea

Fissurelloidea Patelloidea Trochoidea

Neritoidea Subulitoidea Littorinoidea Cerithioidea

Stromboidea N aticoidea Nerineoidea

('onom,utria cf. guirandi ( Loriol ) Trochotoma (Discotoma) amata (d'Orbigny) 7)'ocholoma (Discoloma) mastoktea (l~talton) Rinudopsis sp. Helcion {Heh'ion) V(I(/TneIL',#; Loriol .,lngaria (Angari~t) hiswlhzta (Rollier) Muricolrochu,~ dae~hdus {d'Orbigny) A.vwri/la .~le/lata (Buvignier) (71ih~donta (Chilo~hmla) clathrata (Etallon) :Veritopsis (Neritol~si.~) huchini Guirand and Ogerien Bourguetia sacmanni (Oppel) Purlmroidea mululaw ( Young and Bird ) Pm?mroidea moreana (Buvignier) Brach)'lrenla (Pctersia) hucchloidca (Buvignier) .~v.~lrella? msctdpta (Buvignier) Vvstrc/ht struckmanni ( Loriol ) ILw'li.~'sa of. ,k,emmelaroi ( Loriol ) Rhat~hwollms humtwrtinum (Buvignier) Spinigcra or Qmuh'mervus cf. alta [courtesy N. Morris] Glohuhu'ia (Glohuhtria) amata (d'Orbigny) Tvlostoma ]~ondcro.~utll Zittel ()'.VI~IOI~hJCIIS(l('l~res,sll,v Vollz ('ossl~l~llln~'~l stq~rafltrcn.sis (Voltz) "Nur#lea" e/oltgala Vollz • .\,'erim, a" sulwlegan.~ (Etallon) Plygmalis orllala (d'Orbigny)



• • • • • • • • • • • • • • • • • • • •

Bivalvia Arcoidea

Area {Eomn,icukt) auti.~sio~h,'e,.s'i,~ Cotteau

Barhatia (Barhalia) clvtia Loriol Barhalia (Barhatial .~uhtcxata ( Etallon) Grammalodon (Grammalodon) motllanal'ensi.v ( Loriol ) Gl'anllllatO~h)n (GranlnlaloLh.ql) ehmgatum (Sowerby) Mytiloidea

Pterioidea Limoidea

Ostreoidea Lucinoidea Crassatelloidea Tellinoidea Glossoidea Gastrochaenoidea Pholadoidea Megalodontoidea Hippuritoidea

• • • •

Graltllllalo~bn ((Jralllltlalodon) cora//Jvorllt, (Buvignier) ,lrcopcma? ~m'iali.~ ( Loriol )

Litlwldulga antica (Buvignier) Modiolus (Modio/us) hmgacl'us Contejean Pterop(,rna polvo~hm (Buvignier) H.vpotrema sp. /Inliquilima (('temdh,a) aequilatera (Buvignier) P]agiostonla rathierana (Cotteau) P/agiostoma ltml&kz (Roemer) Lioslrca san&tlina (Goldfuss) F#nhria ~lvoni.vea (Buvignier) Lucinidae gen. indet. Pressastarte stth.~triala ( R oemer) Astartopsis polita (Buvignier) Quenstedtia? n. sp.? Ccratomyol~sis cf. rulwllensis (d'Orbigny) Gaslrochaena (Rocellaria)flora { Loriol ) .hmancttia (Jouanettia) gelvana (Buvignier) PterocardM huvignicri (Deshayes) Diceras cf. ariet#mm Lamarck [courtesy of N. Morris]

• • • • • • • • • • • • • •

Reel"

M. Bertling, E. Insalaco / Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139-175

155

Table 3 (continued) Associated fauna of in situ-reefs and reef rubble Associated molluscs occurring in...

Rubble

Reef

Ammonoidea

Perisphinctoidea

Perisphinctes (Arisphinctes) cf. plicatilis (Sowerby)

Articulata

Terebratuloidea

Juralina etalloni (Douvil6)

Echinozoa

Cidaroida Pedinoida Hemicidaroida Phymosomatoida

Paracidaris sp. (spines) Phymopedina marchamensis (Wright) Hemieidaris sp. (spines) Hemieidaris erenularis (Lamarck) Gymnocidaris? sp. (spines) Phymechinus mirabilis (Agassiz)

Polyehaeta

Serpulimorpha

Glomerula gordialis (Schlotheim) Pomatoceros sp. Spirorbis sp. Vermiliopsis sp.

Crustacea

Dromioidea

Pithonoton (Pithonoton) sp. [courtesy N. Morris]

Borings (by foraminifers) (by clionid sponges) (by clionid? sponges) (by clionid? sponges) (by sponges?) (by phoronids) ( by polychaetes) (by polychaetes) (by spionids) (by eunicids) ( by eunicids?) (by acrothoracics) (by Lithophaga) (by Arcoperna?) (by Gastroehaena) (by Jouanettia) (by sipunculids?)

Dendrina cf. jodicans l~tallon Entobia ret([brmis Bromley and D'Alessandro Entobia cervieornis Ftirsich, Palmer and Goodyear Entobia cf. laquea Bromley and D'Alessandro Uniglobites? n. isp. Talpina bromleyi F0rsich, Palmer and Goodyear Cunctichnus probans FCirsich, Palmer and Goodyear Spirichnus spiralis Fiarsich, Palmer and Goodyear Meandropolydora sulcans Voigt Caulostrepsis cretacea Voigt Caulostrepsis n.isp. Conchotrema eanna Price Rogerella pattei Saint-Seine Gastrochaenolites torpedo Kelly and Bromley Gastrochaenolites lapidicus Kelly and Bromley Gastroehaenolites dijugus Kelly and Bromley Gastrochaenolites orbicularis Kelly and Bromley Trypanites weisei M~igdefrau

Other trace fossils

(polychaete burrow) (echinozoan bite)

Arachnostega gastroehaenae Bertling Gnathichnus isp.

(see Sheppard, 1982), as suggested by their absence as in-situ colonies on palaeosurfaces o f the reef rubble facies which appear to have been colonised only by small (?free-living) d o m a l forms.

4.2. M i c r o b i a l m a t s

The pure thrombolite has not yielded definite skeletal remains o f its producers; however, preser-

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M. Bertling, E. lnsalaco / Palaeogeography, Palaeoclimatoh)gy. Palaeoecology 139 (1998) 139-17.5

vation of microbial cells cannot be expected because fast calcification normally destroys the tissue during crystal growth (Chafetz, 1986). Relics of organic matter are bituminous substances which may be smelt around the freshly broken rock. The preservation of these substances is only possible with incomplete degradation which occurs in oxygen-deficient environments. This has been observed in the older parts of modern reef caves (Reitner, 1993) where the redox layer is positioned just below the surface of microbial mats. This implies that there is no need to assume poorly oxygenated seawater tbr the preservation of the organics because they probably formed in a restricted micro-environment below the mat surface. Unfortunately, there is no definite clue to the systematic status of the producers: both autotrophic cyanobacteria and heterotrophic bacteria normally occur together in a stratified system with strongly differing chemical and nutrient environments (Chafetz and Buczynski, 1992). Judging from the trophic conditions, it may be speculated, however, that the 'pseudostalactites' in the dark caves were mainly produced by heterotrophic bacteria whereas the bulk of intra-reef leiolite formed due to the activity of cyanobacteria. The autecological demands of Late Jurassic thrombolite-forming procaryotes have been clarified by Leinfelder et al. (1993): The organisms do not tolerate high sedimentation rates but are otherwise euryoecous. Poor oxygenation is tolerated, and elevated or fluctuating nutrient levels are not harmful, if not required. They mostly occur in calm water below normal-wave base but may also be found in shallow turbulent environments if reefal debris is exported (Leinfelder, 1992; Nose, 1995). Various processes of growth have been demonstrated for rocks of microbial origin (Burne and Moore, 1987), the most important being direct metabolic action of heterotrophic bacteria, such as concentration of Ca 2+ and Mg 2+ ions in the cell walls (Chafetz and Buczynski, 1992). By this means, the walls may be calcified by regular calcite crystals within a few days (Chafetz, 1986; Reitner et al., 1994). Bacteria may also apply a different, 'abiogenic' process by trapping automicrite in their

gelatinous polysaccharoid mucus sheaths (Burne and Moore, 1987; Reitner, 1993). The micrite can be microbial in origin, or derived from borers' action (aphanitic) or peloids ( peloidal ). Cementation may occur very early (e.g. Reitner et al., 1994) but may also be significantly delayed in very similar environments, allowing the development of a thick soft top layer (Fezer, 1988). In microbial mats which are typically composed of both eyanobacteria and bacteria, lithification does not take place on the surface but only in several millimetres to centimetres depth: the calcifying bacteria are excluded from the surface by the faster growing photoautotrophic cyanobacteria, which are not cemented (Chafetz and Buczynski, 1992).

4.3. Borers

Borings are an important aspect of these reefs not only by virtue of their abundance and hence bioerosion intensity but also by their taxonomic diversity (Table 3). This may be a manifestation of the favourable type of preservation, but there is no question that it also reflects an originally higher degree of bioerosive activity than in many contemporaneous reefs in central and western Europe (M.B., unpublished own studies). In the reefs as well as in the rubble the distribution of borings is very patchy; consequently strongly bored corals may occur next to completely unaffected ones. Within the reef, even adjacent branches of the same colony may be bored to a different extent. In this context the obviously preferred attack on ramose colonies in the rubble facies is notable. Taking into account the taphonomy of the rubble beds (see Section 3.4), the best explanation of this phenomenon is that the coral branches formed a dead though fresh substrate whereas most of the massive or encrusting colonies were living on the rubble. Despite their better spatial homogeneity they could not be bored as easily as the recently killed corals because of their protective cover of living tissue. The possibility of specific coral/borer associations may not be ruled out completely but this interaction is always restricted to the species level

M. Bertling, E. lnsalaco / Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139 175

and does never include complete families (e.g. Kleemann, 1980). In addition it has been shown by several studies that neither skeletal density nor extent of the substrate are important factors governing bioerosion (Bromley, 1978; Scott, 1987; Peyrot-Clausade and Brunel, 1989). On the other hand, numerous authors have found length of exposure to be one of the main controlling factors (e.g. McCloskey, 1970; MacGeachy and Stearn, 1976; Hutchings and Peyrot-Clausade, 1989; Peyrot-Clausade and Brunel, 1989; PeyrotClausade et al., 1992; Kiene and Hutchings, 1994). For these reasons, it is probable that non-bored branches within the reefs were killed by, or at least immediately before, the microbial encrustation and hence the exposure time of the coral skeleton to bioerosion was very short resulting in low bioerosion intensity. In a few cases, the bivalve producers of the traces could be located within their bore holes whereas other boring organisms may only be inferred by comparing their cavities with Modern analogues. Due to the highly significant domicile character ('domichnia') of the trace fossils, most producers could be identified at the family level. Five different borings of colonial taxa (mostly sponges) and twelve traces of solitary forms (mainly bivalves and polychaetes) have been identified (Table 3). A single species of sponges (producer of Entobia cf. laquea) volumetrically dominates the borer association. This may be due to their feeding demands: the adjacent bacterial mats supplied abundant nutrition. It is interesting to note that microbial encrustation of corals is a prominent feature in the Bajocian of Morocco as well, where sponges again are the most important borers (Warme, 1977). Modern clionids are an extremely eurybathic family, occurring from the upper subtidal to the bathyal zones (Volz, 1939; Bromley, 1978; Wilkinson, 1983; Bromley and D'Alessandro, 1990). They are frequently found in Modern branched corals where they may form long galleries, from dead patches even extending into the living parts of the colonies (Pang, 1973; Risk and MacGeachy, 1978). All depend on low sedimentation rates in order not to smother the ostia.

157

Polychaetes in general are typical shallow water borers, with most Eunicidae (producers of Caulostrepsis) in particular requiring high water energy. Spionidae (responsible for Meandropolydora, and others?) have less narrow ecological demands, having repeatedly been found at bathyal depths as well (Bromley, 1978; Bromley and D'Alessandro, 1990). Larval recruitment is extremely variable between years, favouring a highly patchy distribution (Hutchings, 1985; Hutchings and Peyrot-Clausade, 1989). The Sipunculida which may have been responsible for Trypanites are restricted to turbulent shallow water and can tolerate significant sedimentation (Rice and Macintyre, 1982; Bromley, 1994). Their low number in comparison with contemporaneous reefs is remarkable. Bivalves (ichnogenus Gastrochaenolites) are equally typical of shallow water environments, mostly the upper subtidal. However, they avoid extreme turbulence. Most species of Gastrochaena are able to colonize living corals (Kiihnelt, 1933; Riedl, 1966; MacGeachy and Stearn, 1976; Bromley, 1978, 1994; Peyrot-Clausade and Brunel, 1989). The rarer bivalve Lithophaga preferably attacked dead corals in Jurassic reefs. The ecological demands of Phoronida (producer of Talpina) are too poorly known to justify environmental interpretations; Acrothoracica prefer current-exposed water and also attack living substrates (Seilacher, 1969). Summarising, the stenoecous groups among the borers recorded are all restricted to or at least prefer agitated shallow water. At the same time, they are able to bore into living coral substrates; dead-coral bioeroders were mostly observed in the reef rubble facies.

4.4. Associated fauna Despite the high species richness, the fauna associated within the reefs is not very diverse ecologically (Table 3); most species belong to molluscs (Fig. 10). Because of the rather small size of the reefs it can hardly be stated where exactly individual species lived; no molluscs were found in the caves, however. One may rather assume an

158

M. Bertling, E. h~salaco / Pahteogeography, Palaeoclh~atology, Palaeoecology 139 (1998) 139 17.5

Fig. 10. Important elements of the associated fauna in various types of preservation: all specimens whitened with MgO, collection of Geologisch-Palgontologisches Museum, MOnster: all figures x 1.3. Gastropoda: (a) ()motomaria of. guirandi (Loriol) (latex cast): (b) Trochotoma (Discotoma) amata (d'Orbigny) (mould): (c) Helci(m (Heleion) valfinensis Loriol (negative print): (d) Muricotroehus daedalus (d'Obigny) (latex cast): (e) ChihMonta (('hilodo,ta) clathrata (l~tallon) (latex cast): (f) Brachytrema (Petersia) buccinoidea (Buvignier) (latex cast): (g) Crrptoplocus depressus Voltz (latex cast). Bivalvia: (h) Barbatia (Barbatia) clytia Loriol (mould): (i) Grammatodon (Grammatodon) montanayensis (Loriol) (latex cast): (j) Modiolus (Modiolus) hmgaevus Contejean (mould): (k) Plagiostoma rathierana (Cotteau) (mould). Echinoidea: (I) Hemicidari.~ crenularis (Lamarck). Borings: (m) Meandropolydora suh'ans Voigt in periphery of Gastrochaenolites isp,; (n) Entohia cf. laquea Bromley and D'Alessandro phases A to C, and Uniglobites? isp. in mid-foreground: (o) Entobia retilbrmis Bromley and D'Alessandro phase B in Dendrohelia (infill of calices visible): (p) Gastroehaenolites dijugus Kelly and Bromley filling coral branch.

e x p o r t o f reef f a u n a to the r u b b l e facies t h a n an i m p o r t o f elements f r o m the a d j a c e n t h a b i t a t s into reef pores, especially given the elevation o f the structures. Therefore, the d i s t r i b u t i o n o f reef t a x a

(not: t a x a f o u n d in the r u b b l e facies) in the rock record should m a t c h their ecological distribution. A c c o r d i n g to the criteria o f F i s c h e r a n d Salvat ( 1971 ), the general faunal c o m p o s i t i o n , especially

M. Bertling, E. Insalaco / Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139-175

of the reef rubble, indicates a high-energy environment dominated by mobile substrates. The fauna of the rubble facies is probably composed of spatially and temporally averaged (heterotopic and heterochronous) taphocoenoses, including burrowers from loose carbonate sands and byssally attached forms from the reefs.

4.4.1. Gastropoda The gastropods contain many mobile epifaunal species, mainly herbivorous raspers. Detailed accounts of their ecology have been given by e.g. Demond (1957), Janicke (1970), Taylor (1971) and Bandel and Weiler (1987): Pleurotomarioidea are typical reef dwellers in many Late Jurassic reefs; at l'l~pine, two of the three species identified occur only in the in-situ-reef facies. Fissurelloidea, Patelloidea and Trochoidea are restricted to hard substrate, and in Modern environments they are not found below 15 m depth. The Neritoidea appear to provide even better environmental indicators, since today they are restricted to wellagitated intertidal zones. Cerithioidea have a wide depth range and a highly variable ecology including deposit feeding. The species encountered in these reefs lack columellar folds and carry a distinct sculpture thus suggesting epifaunal grazing habit (see Signor and Kat, 1984). Numerous species in Modern reefs prefer somewhat protected habitats such as pools or crevices. Summarising, the fauna of the in-situ reefs - - with all reservations regarding the transferability of Modern gastropod ecology to the Mesozoic - - suggests a turbulent environment in very shallow water of the upper subtidal zone with water depths no more than a few metres, although there were also calmer microenvironments. The environment was characterised by hard substrate with a microbial cover on which most gastropods fed. The gastropod fauna of the reef rubble facies contains numerous infaunal elements which may have been transported from regions further seaward: Naticidae are mostly burrowing hunters occurring down to the lower subtidal zone. The smoothly sculptured species of Cerithioidea were probably infaunal as well, according to the criteria of Signor and Kat (1984). They wandered around

159

in sandy areas between the reefs where hydrodynamic energy was lower, just as their Modern counterparts. Aporrhaids are dwelling in soft substrate; they are suspension feeding by means of a mucus net expanded on the sea floor. A similar feeding mode is generally assumed for Neri-neoidea; however, they are reported from various substrates and seem to prefer shallow water with low sedimentation rates and moderate turbulence (Vogel, 1968; Wieczorek, 1979; Sirna, 1995). Remarkably, the spined trochoideans do not occur on the reef proper: they might simply have fallen off or the water may have been too agitated for them.

4.4.2. Bivalvia Bivalves at l'l~pine exhibit a variety of life forms; the reef fauna, however, consists almost exclusively of epibyssate taxa (see Kauffman, 1969). The morphology of most nestling Arcidae and crevicedwelling Limidae, as well as the species of Modiolus, is typical of an epifaunal mode of life (see Stanley, 1972). Elongated species of Grammatodon may also have been semi-infaunally byssate, suggesting that there were partially unconsolidated patches on the reef surface. Modiolus was found in its life position, apex down between coral branches. Hypotrema and Liostrea were directly cemented to their hard coral substrate. All taxa indicate a necessity to shelter from strongly agitated water. Similar bivalve assemblages under similar conditions can be found in Modern environments (see Taylor, 1971). Other forms required soft substrate since they had a shallow-infaunal (Pteroperna, Astartidae, Ceratomyopsis) or deep-infaunal (Fimbria, Lucinoidea) burrowing habit. Notably they occur only in the reef rubble which also contains some infaunal gastropods. Compared with Modern reefs (Fischer and Salvat, 1971), the fauna is well adapted to mobile substrates without macroalgal overgrowth. There are many ecotypes in addition to other subtidal hard substrates (see Taylor, 1971); this suggests faunal mixing with adjacent soft mobile bottoms. This was also the habitat of large free-lying forms with strong beaks (Pterocardia, Diceras) which require hard or at

160

M. Bertling, E. lnsalaco / Palaeogeography, Palaeoclimatolog.v, Palaeoecology 139 (1998) 139 175

least firm substrate. Their absence from the reefs is conspicuous and may be a consequence of the hydrodynamic regime which was too turbulent for these non-anchored taxa, or they may have avoided the reefs because of the likelihood of microbial overgrowth. Truly enigmatic, however, is the complete absence of Pectinidae despite their presence in virtually every other Late Jurassic reef; taphonomic reasons alone are an unlikely explanation.

4.4.3. Other taxa The non-mollusc fauna associated with the reefs is dominated by echinoderms (apart from bioeroders discussed above). Only regular sea urchins have been found in and around the reefs, presumably a reflection of the lack of extended stable soft substrate. The crustacean Pithonoton is very rare; its occurrence yields little environmental information. More interesting is the scarcity of organisms which were diverse in other contemporaneous reefs such as bryozoans, thecidean and terebratulid brachiopods, and serpulids; in the reefs at l'l~pine these organisms are represented by very few species and individuals (very low species diversity and abundance). Serpulids have only been found encrusting protected habitats such as the inner walls of empty bivalve shells in the reef rubble facies and the microbial pseudostalactites in the reefs. Terebratulids were confined to the reef rubble. One of the most likely reasons for all this is the inferred soft surface of microbial mats which hindered other encrusters.

4.5. Coelobites

Reef cavities of any size support an array of organisms which permanently live there; these are termed coelobites (Ginsburg and Schroeder, 1973) or cryptobionts (Kobluk, 1988). In the reef studied, the most prominent structures are the 'microbial pendants' described in Section 3.3. They also occur in several Kimmeridgian coral-microbial reefs of the western Tethys, for example southeastern Spain, Lusitanian Basin in Portugal and Charente Maritime, France (Leinfelder, 1992;

Leinfelder et al., 1993, 1994; Taylor and Palmer, 1994). Downward-growing microbial crusts are also known from caves in the older parts of Modern reefs (Reitner, 1993). They grow very slowly and are restricted to reefs of high-energy environments, independent of the sea level (Zankl, 1993). To our knowledge, the occurrence reported here is the stratigraphically lowest so far. Coelobites other than the microbial films are very scarce, although thin blade-like corals (Thamnasteria, ?Isastrea) can be conspicuous. This association is remarkably similar to the exposed surface and forms the first record of fossil hermatypic corals in reef cavities (see Kobluk and Lysenko, 1987). The corals in the caves at l'l~pine are occasionally bored and can be encrusted by thecideans, serpulids, foraminifers and bryozoans; however, the numbers of these sciaphilous individuals are very low. This is striking, especially considering their abundance in otherwise identical caves in Charente Maritime (Taylor and Palmer, 1994; E.I., pers. obs.). This suggests, at least with regard to pseudostalactites in the caves, that the paucity of these faunal elements is not simply a function of the former presence of microbial films but may be due to other ecofactors. Modern analogues for these Jurassic cryptic scleractinians are provided by occurrences in cave habitats at depths in excess of 80 m off Jamaica (Jackson et al., 1971; Kobluk and Lysenko, 1987) and the waters off Madagascar, where foliaceous hermatypic corals still thrive at 5% of the surface light levels. They can do so by switching to different metabolic pathways without reducing calcification rates; the morphology is always laminate or encrusting (Jaubert and Vasseur, 1974; Sheppard, 1982). The restricted coral fauna of these caves is probably a result of the extremely low light intensities within the cave. However, the absence of zooxanthellate species that are particularly tolerant to low light levels, such as the Microsolenidae (Leinfelder, 1994; Insalaco, 1996b), may suggest that light levels were sufficiently low to exclude all zooxanthellate corals. If this is the case, these cavedwelling species of Thamnasteria and ?Isastraea may have been non-zooxanthellate.

M. Bertling, E. lnsalaco / Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139-175

5. Synecological discussion 5.1. Trophic structure The trophic structure of the reef community can be evaluated from an examination of the trophic demands of extant relatives of the individuals in the fossil assemblage (see Riedl, 1966, pp. 384-386). There is no evidence to suggest a change in feeding behaviour of the taxa in question; their ecological constraints have remained the same since the Late Jurassic. Corals are mostly microcarnivores preying upon zooplankton by passive filtering. However, their nutritional requirements can be subsidised to varying degrees in some genera by temporary ingestion of their symbiotic zooxanthellae and/or the use of their photoassimilates (Porter, 1976). Thus the feeding mode of corals is highly variable; even direct utilisation of dissolved proteins has been observed. However, at least Thamnasteria seems to have had azooxanthellate species (Stanley and Swart, 1995). Corals may therefore best be regarded as mixotrophs (with an uncertain degree of primary production), since they function at every trophic level (Porter, 1976). Microbial mats were important primary producers because of their partly cyanobacterial origin. However, their bacterial parts have to be considered as decomposers despite the production of carbonate. Both types of microbes directly sustained primary consumers, i.e. herbivorous grazers among gastropods and echinoids. In Recent reefs, strong niche partitioning between grazers exists, e.g. by mode of grazing, by substrate choice or by activity periods (Hatcher, 1983). Very few carnivorous hunters as secondary consumers have been recorded in the reefs proper at l'l~pine (the crustacean Pithonoton and the Naticoidea); this may be due to taphonomic loss. Regular sea urchins are omnivores with herbivorous preferences and are active at night. Remarkably, all herbivorous gastropod superfamilies recorded from the l'l~pine reefs have at least some Recent species which feed on cyanobacteria, 'microbes' or 'microalgae' (Steneck and Watling, 1982). They may be envisaged as crawling across the reef surface, rasping off parts of the microbial mats. Other trophic types among the gastropods

161

have only been found in the adjacent rubble facies but not in the reef itself: Nerineoidea were suspension feeders (Vogel, 1968), whereas Naticoidea were carnivorous. All reef-dwelling bivalves were suspension feeders, but little information is available concerning the preferred particles digested. Lucinidae which are restricted to the rubble facies are known to consume bacteria from the suspended detritus they inhale. The only probable deposit feeder among bivalves is the tellinoidean Quenstedtia? which has not been found in the proper reef either. The borer taxa observed possess very varied trophic demands but there are remarkably few plankton feeders. Clionid sponges actively filterfeed (sensu stricto) suspended microbes and minor phytoplankton, whereas phoronids capture microbes on the surface (Riedl, 1966, p. 384). A similar stratagem is applied by the spionid genus Polydora among polychaete borers; it collects resuspended detritus, algae and zooplankton semipassively (Hempel, 1957). Other members of the Spionidae are deposit feeders while out of their boring; Eunicidae are omnivorous but otherwise display the same behaviour. The single most important group of boring bivalves, Gastrochaenidae, lives on bacteria resuspended with detritus (Carter, 1978, p. 25) whereas Lithophaginae prefer phytoplankton, especially dinoflagellates (McCloskey, 1970; Mokady et al., 1993). Only the Acrothoracica which appear in low numbers are semipassive filterers of zooplankton, similar to corals but with smaller prey size. In summary, the reefal foodweb seems to be strongly sustained by the microbial mats (Fig. 11 ), mainly by converting their cell production into animal biomass directly in the case of grazers and borers. Another source of energy was zooplankton (including the larvae of reef biota) utilised by corals and several associated bivalves. Generalising, a situation with higher nutrient levels (relative to oligotrophic regimes) has to be postulated for several reasons: firstly it is unlikely that the cryptic corals could gain enough nutritional energy from autotrophic symbionts (if they had them at all); secondly, strong bioerosion is indicative of at least mesotrophic conditions (e.g. Hallock and Schlager, 1986; Wood, 1993); and

162

M. Bertling, E. lnsalaco / Pa/aeogeographv, Palaeoclimatoh)gv Palaeoecology 139 (1998) 139 175

Patch Reef Trophic Web (Oxfordian, Novion-Porcien) _

m

suspension fesdens

carnivores

Naflcoidea

1 ===oOO " O pdmary producers

-

-

-

crustaceans

2. cyanobacterla

}

:'

I e

corals

(rhodophyta, ?chlorophyta)

Fig. 11. Schematic reconstruction of suggested trophic relationships in the autochthonous reefs studied: arrows indicate nutritional pathways. Corals occur at various levels because of their mixed style of energy uptake and their autotrophic symbiotic zooxanthellae.

thirdly the growth of heterotrophic microbes into 'pendants' in the reef caves is improbable in normal or even depleted waters. The photoautotrophs among the mat builders were also triggered by raised nutrient levels. According to Hallock (1988), nutrient excess may lead to coral hypertrophication resulting in intensive mucus production by the corals. This mucus may host oxygen-consuming bacteria during its decomposition. Nutrient-enriched waters should also have triggered large plankton populations which in turn serves organisms with planktotrophic larvae, as well as suspension feeders in general. Although corals are suspension feeders they are, at least at present, especially adapted to low nutrient levels (oligotrophic regimes), thus avoiding competition with encrusting procaryotes and algae. Combining the various considerations above, a mesotrophic regime is suggested, provid-

ing conditions in the ecological pejus for corals but sustaining the rest of the reef community. 5.2. MorphoJunctional groups The impact of a benthic organism's growth form on the habitat has been stressed by Fagerstrom (1987). He distinguishes several morphofunctional groups (builders, binders, bafflers, dwellers, destroyers) irrespective of trophic status. Despite its irritating terminology~ i.e. misuse of the ecological term 'guilds' as defined by Root (1967), and much criticism (e.g. Precht, 1994), this concept can be valuable when applied solely to characterise a species' growth form effect on its surrounding. All morphological groups have their representatives at the locality studied but true reef builders are volumetrically sparse. To act as framebuilder, an organism has to be equipped with a rigid

M. Bertling, E. lnsalaco / Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139 175

skeleton growing mainly vertically. Corals do grow vertically here and despite their thin branches and loose open organisation they formed a wave-resistant structure. The l'l~pine reefs only could attain their relief because of the presence of the microbial mats which functioned as basal binders of autogenic micrite. Corals additionally strengthen the microbial mound protecting it from wave energy. Most molluscs were constructionally neutral and thus are best interpreted as dwellers. They may be subdivided into (hemi)sessile nestlers (most bivalves) and vagile strollers (mainly gastropods, but also echinoids). The gastropod grazers tend towards destructive action by their rasping; however, removal of the thin organic film on top of the microbial rock obviously had little long-term harmful effect. The boring molluscs (bivalves) were clearly destroyers and they weakened the coral skeleton. However, these bivalves and other borers could only infest less than half the coral branches. Thus their impact on the corals was not ubiquitous, and only some colony breakage related to boring is recorded in the debris around the patch reefs.

5.3. Function of the reef community Energy utilisation, as expressed in the trophic structure, and the utilisation of space, as expressed in the morphofunctional groups, are the main functional aspects of most benthic communities. The large species numbers of corals, borings and associated fauna might have resulted in strong competition among the members of some morphological and trophic groups. In order to avoid this competition, niche diversification beyond different growth forms must have been distinct. The community is thus strongly influenced by the microbial activities on the one hand, whereas the codominant branching corals belong to a different trophic group. This pattern of only one or two species dominating within atrophic group is also obvious for the filter-feeders (dominant group member: sponges), detritus feeders (dominant group member: Spionidae) and carnivore hunters (dominant group member: Naticidae) while it is not as clearly expressed in the grazers. However, each prominent group contributed to the community in

163

some way resulting in a complex web of mutual interactions (Fig. 12). Coral growth promoted the diversification of other organisms: borers found a substrate, and dwellers were protected against dislocation by turbulence. Corals were also important for the microbial mats by significantly reducing water energy and by providing shelter for the initially soft, gelatinous organic coating. On the other hand, the binding microbes helped stabilise the bases of the coral colonies by their embedding, encrusting nature. However, the growth of the microbes is also likely to have been detrimental for corals by the suffocating effect of the encroaching films along the branches. Nevertheless this combination of a coral community dominated by branching Thamnasteria species with relatively high growth rates (at least 10-14mm/a; Insalaco, 1996c), together with the early cementing leiolite, resulted in a biotic association capable of developing a rigid framework with relatively high accretion rates in strongly agitated water. Such construction is rare in the Late Jurassic and still rarer to be preserved in the geological record (see Leinfelder, 1992, 1993). A very similar interaction has been documented from shallow settings in Tahiti where stromatolites significantly contribute to acroporid buildups (Montaggioni and Camoin, 1993). The relationship between borers and binders is likewise complex. Due to the rapid growth of microbial mats, boring organisms only had limited time to bore into coral skeleton before it was encrusted by microbes. This microbial hindering of coral bioerosion was beneficial for the maintenance of the reef fabric. On the other hand, since most of the identified borers directly feed on the microbes, the abundance of micro-organisms sustained a rich boring fauna. This abundance of borers in turn resulted in the production of fine carbonate material, especially carbonate chips, which could be immediately trapped by the organic mats, thus ensuring their continued growth via provision with this type of automicrite. Reef dwellers, especially grazers, were supplied with food by the microbes; their rasping obviously had little detrimental effect on the crusts. On the other hand, modern grazers may produce much aphanitic micrite by scraping off skeletal parts

164

M. Bertling, E. lnsalaco I Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139 175

Functional Interactions in Reefs Oxfordian, Novion-Porcien dwellers

binders

:.

• framebuilders

~ bioeroders

Modem Scleractinian Reefs

triggering caus......... e target •

I hemperlng

dwellers

binders

).

• framebuilders

bioeroders Fig. 12. Sketch of (mostly inferred or postulated) interactions between morphofunctional groups in the autochthonous reefs studied (above), compared with the situation in typical Modern scleractinian reefs (below).

with the surface tissue (Sammarco et al., 1987). Jurassic grazers, in this case, did not contribute much to the growth of microbial mats via this pathway because they fed on the microbes with a soft underlayer rather than hard corals. Instead, they promoted coral growth by keeping the microbial mats, to a certain degree, in check (see

Sheppard, 1982), and also helped borers by extending the exposure time of the coral skeleton. There naturally can be no direct evidence for this suggestion because potential grazing traces will have been overgrown again very soon, thus precluding fossilisation. The diversity and abundance of grazers, however, together with Modern interactions of the

M. Bertling, E. lnsalaco / Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139-175

ecological groups involved, strongly support this hypothesis. 5.4. Composition and diversity Several notable features of the reefs at NovionPorcien emerge when these reefs are compared with other broadly contemporaneous reefs. They will be discussed in this subsection after comparisons with other coral-microbial reefs of the same age. 5.4.1. Comparison with Late Jurassic coral microbial reefs A striking characteristic of the l'l~pine bioherms is the dominance of leiolite in the framework which seems to be a widespread phenomenon only in Tethyan reefs, e.g. in Iberia, eastern North America and southern Germany (Baria et al., 1982; Leinfelder, 1992, 1993; Leinfelder et al., 1993, 1994; Nose, 1995; Schmid, 1996). Microbial rocks are not abundant in England though certainly present (Insalaco, 1996a), and play a minor role in the reefs of northern Germany (Bertling, 1993a). The reason for the absence in higher latitudes (see Schmid, 1996) is probably the low sediment tolerance of the microbes (see Section 4.2). An alternative explanation might be the lack of alkalinity in the northern regions which is essential for thrombolithe formation (Keupp et al., 1993; Zankl, 1993). However, alkalinity may be increased by terrestrial runoff rich in hydrogencarbonate (through meteoric dissolution of carbonates), as suggested by Keupp et al. (1993) and Reitner (1993). Thus, if their hypotheses were correct, the situation encountered in the Late Jurassic of northern Europe should trigger rather than hamper formation of microbial rocks. A simple control by sedimentation appears more probable. Leinfelder et al. (1993) and Nose ( 1995, pp. 91, 133) have divided Jurassic microbial reefs into several groups; the reefs at Novion-Porcien may be classified as 'coral-bearing thrombolites' which are frequent in eastern and southeastern Spain (Fezer, 1988) and parts of the Lusitanian Basin (Nose, 1995, p. 23). There, thrombolites carry numerous microencrusters which are very sparse

165

in the l'l~pine reef. In addition, microbial mats commonly include macro-encrusters such as oysters, serpulids and bryozoans (Faure-Marguerit, 1920; Leinfelder, 1994). The paucity of encrusting invertebrates at Novion-Porcien is best explained by the presence of organic microbial films which hindered larval settlement (see Fezer, 1988), and perhaps other ecofactors. Judging from the ramose growth form of the corals and the low debris content, the reef type at l'l~pine does not occur in Iberia (see Nose, 1995, p. 78). In addition, the inferred water depth for the Iberian reefs is much higher (Nose, 1995, p. 79). Schmid (1996, p. 124) also states the preference of leiolites for deeper water. Shallow-water microbial rocks should have stromatolitic texture which is not the case at the study sites, however. The coral morphologies and reconstructed palaeobathymetry of the l'l~pine reefs appear to be very similar to the eastern American coral thrombolites described by Baria et al. (1982). Leinfelder et al. (1993) suggested the absence of microbial reefs from the eastern Paris Basin but contradicting observations have been made by Faure-Marguerit (1920) and Ptimpin (1965). However, insufficient detail published currently precludes thorough comparisons. Gygi (1992) describes microbial crusts around massive coral colonies from the Oxfordian of the Swiss Jura: they form small pillars which may also grow downward, but their volumetric importance is rather low. A similar reef complex to l'l~pine, regarding the importance of microbes, existed around the village of Ota in the Lusitanian basin (Leinfelder, 1992): the reefs developed caves, grew in high-energy settings and their morphology could only be maintained when strong (microbial) binding was present. Otherwise reefs tended to drown in their own debris since other efficient binders (such as the corallinaceans) were not important in the Late Jurassic. Hence the microbes appear to have played a similar binding role to the corallinaceans in present-day reefs (Leinfelder, 1992, 1993). 5.4.2. Comparison with other Late Jurassic coral reefs In a comparison of the l'l~pine reefs with other contemporaneous reefs, various general features,

166

M. Bertling, E. lnsalaco : Palaeogeography, Palaeoclbnatology, Palaeoecology 139 (1998) 139 175

such as facies, reef type and morphology, have to be taken into account. Numerous widely differing reef types are known from the Late Jurassic (see Leinfelder et al., 1993, 1994, for review: Insalaco~ 1996a) but the composition of their associated fauna at low taxonomic level has yet to be considered. Detailed descriptive information on this can only be found in the older literature, despite numerous recent studies dealing with corals. In the following account, growth form, systematics and the diversity of the builders is discussed first, followed by a comparative assessment of diversity and composition of binders, borers and dwellers. Coral patch reefs may occur in many facies types but are structured differently (Leinfelder, 1993; Leinfelder et al., 1993, 1994). Generally in high-energy environments, reef frameworks which were not bound by vast amounts of microbial micrite~ were quickly disintegrated and deposited as reef rubble beds (see above). The taxonomic comparison (family level) of numerous localities yields a biogeographic pattern difficult to interpret, probably because local facies was a more effective control than geography. The coral richness at Novion-Porcien is only slightly less than in the reefs in Iberia and southern Poland (Roniewicz and Roniewicz, 1971) but much higher than at adjacent localities to the west, in England and the Boulonnais (Tomes, 1884: Arkell, 1928). Similarities are most pronounced with the east; the number of common genera is greatest in the Meuse area and Swiss Jura ( Pfimpin, 1965; Geister and Lathuili6re, 1991 ). Binders other than microbial mats only had limited success in Late Jurassic reefs but oysters, bryozoans and serpulids could have a binding effect in settings where sedimentation rates were low and hydrodynamic levels were not too high (for example the Upper Jurassic reefs of Pomerania, England, Swabia, Algarve, northern Germany and the Lusitanian Basin; Schmidt, 1905; Arkell, 1928; Rosendahl, 1985; Bertling, 1993a; Leinfelder, 1994). The same association is also found on the underside of platy corals in deeper water with stronger clastic influx (Roniewicz and Roniewicz, 1971; Bertling, 1993a; lnsalaco, 1996b), indicating its low tolerance of sedimentation.

Bioerosion is very difficult to compare since detailed descriptions of the situation in other Jurassic reefs have rarely been published. Nevertheless the detailed study of Tithonian bivalve reefs from Dorset (England) by Ffirsich et al. (1994) yields numerous borings in common with those from the l'l~pine reefs, as do the Oxfordian reefs of Pomerania (Pisera, 1987). However, in both cases, bivalve borings (Gastrochaenolites) are dominant over sponge borings (Entobia)~ which is the reverse of the situation encountered at Novion-Porcien (although the estimated volume of skeletal mass removed by borers from the Tithonian reefs is similar; around 40%). Other areas with Late Jurassic reefs cannot be reliably evaluated because of the lack of quantitative or semi-quantitative data. 'Strong' bioerosion is reported from the Oxfordian reefs of the Swiss Jura (Ptimpin, 1965 ), England, Spain and northern Germany (due to bivalves; Arkell, 1928; Errenst, 1990; Bertling, 1993a) as well as in the Kimmeridgian kusitanian Basin (mainly due to bivalves and sponges; Leinfelder, 1994). No quantitative or semi-quantitative mention of bioerosion may be found in the literature on other European reefs. The only other documented case of clionid dominance over other borers is from the Moroccan Bajocian ( Warme, 1977 ). A change of the bioerosional pattern with water depth has been recognised by e.g. Leinfelder (1993) as well but the issue is much more complex (see review by Hutchings, 1986). Hydrodynamics may play an important role but this is unclear even in Modern reefs. The associated fauna of Late Jurassic reefs may consist of numerous and different elements which can currently only be compared at the (super)family level; taxonomic revision of genera and species in older literature is far beyond the scope of this study. Even at this high systematic level, comprehensive data sets seem to be lacking for some areas, e.g. for Spain, southern France, Slovenia and Poland. Highest degrees of similarity exist with the localities to the east, as is the case for the reef builders; the Meuse region is more diverse but the general faunal composition (including several species) is identical (see Buvignier, 1852). The Swiss Jura is also similar although the presence of Pectinidae, Opinae and Ostreoidea there contrast

M. Bertling, E. lnsalaco / Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139-175

167

Table 4 Relative richness of the reef-associated f a u n a in E u r o p e a n Late Jurassic coral reefs Bivalvia: England N. G e r m a n y Pomerania S. G e r m a n y Normandy Swiss J u r a French Jura Moravia Savoy Algarve Gastropoda: England N. G e r m a n y Pomerania S. G e r m a n y Normandy Swiss J u r a French J u r a Moravia Savoy Algarve

Mytiloidea

Pterioidea

Limoidea

Ostreoidea

Heterodonta

Hippuritoidea

+ + + + + + _+ + +

+ + +

+

+ + + + _+ _+ + + _+ -

+ + + + + + _+ +

+ + -+ + _+ + + + + +

_+ + + + + + _+ +

Trochoidea

Cerithioidea

Naticoidea

Nerineoidea

+

+

+ + + + + + + + + ?

+

+

+ + + + ?

+ + ± + + + -

+ +

Brachiopoda:

+ + + _+ + + + + + + + + +

Terebratulida

+ + +

+ + + + + +

Five-step scale from + + to - - . In all regions v a r i o u s echinoids, A r c o i d e a and Pectinoidea as well as several P l e u r o t o m a r i o i d e a are present ( c o m p i l e d after T h u r m a n n and I~tallon, 1864; Von Zittel, 1873; S t r u c k m a n n , 1878; Boehm, 1883; Bourgeat, 1887; Schmidt, 1905; J o u k o w s k y a n d Favre, 1913; F a u r e - M a r g u e r i t , 1920; Arkell, 1928, 1929&[; Geyer, 1954; P a m p i n , 1965; Beauvais et al., 1974; R o s e n d a h l , 1985; L a u x m a n n , 1991 ).

to their absence at l'l~pine (Thurmann and l~tallon, 1864; Pi~mpin, 1965). All other occurrences differ in several aspects, highlighting the peculiar ecology of the reefs at l'l~pine (Table 4). However, some general patterns emerge: Echinoids are ubiquitous and most frequent in turbulent settings (e.g. Arkell, 1928; Turngek et al., 1981; Lauxmann, 1991). Among molluscs, Arcoidea and Pectinoidea are always present in typical coral reefs (not mentioned in Table4 for this reason), as are Limoidea, Pleurotomarioidea and Trochoidea. This is also the faunal emphasis at l'l~pine, suggesting that these coral-microbial reefs were ecologically comparable to other reefs. Nerineoidea are common in shoal facies with agitated waters and low sedimentation rates (Wieczorek, 1979; Sirna, 1995). Diceratidae (Hippuritoidea) become more important to the south and may thus be called a Tethyan element; however, they also appear to need turbu-

lent conditions between the reefs to be abundant (Buvignier, 1852; Turn~ek et al., 1981; Geister and Lathuili6re, 1991 ). Summarising, it is very difficult to compare in detail the overall diversity of these reefs with other areas because of the lack of published taxonomical data.

5.4.3. Maintenance of diversity The diversity of a community is controlled by its productivity and the predictability of the environment (Colinvaux, 1984, p. 661). At l'l~pine, high rates of primary production and decomposition are suggested by the abundance of microbial mats. This must mean raised nutrient levels in the seawater in addition to the reasons outlined above (Section 5.1). High production usually sustains high population densities of herbivores and decomposers. However, at l'l~pine these could not endanger the position of the microbial mats, since the

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(cyano-)bacteria were neither space limited (they simply grew on one another and extended their surface with the growing corals) nor energy limited (nutrients were plentiful ). This in turn means that there is no need to assume a physical or predator control of the various herbivores. Large animal populations normally cause strong competition eventually leading to low diversities, but this was obviously not the case at l'l~pine: at the community level, each trophic and morphological group is rather well represented and not completely dominated by any other. This conclusion is independent of the situation within trophic groups where dominance may be marked. Two explanations of the observed pattern are possible: in a predictable environment predators increase the prey diversity in a kind of biological accommodation according to the 'cropping principle' (Colinvaux, 1984, p. 652). Strongly differing from the Modern situation, this seems unlikely at l'l~pine because very few predators appear to have been present. The other and more probable way to increase diversity is via frequent but moderate environmental disturbances, as formulated by the 'intermediate disturbance hypothesis' (Connell, 1978). Seasonal or episodic variations in physical parameters, for example, turbidity, hydrodynamic energy and sedimentation, reduce the number of individuals of both strong and weak competitors, thereby diminishing competition effects. No species is completely expelled nor is one able to dominate; instead, fluctuations of population densities favour the intrusion of opportunists, all leading to a significant increase in diversity. The community thus never reaches its climax, and several stages of succession are present synchronously in different sub-habitats. Environmental perturbations must not be too harsh, however, and sufficient supply of nutrients was essential in order to keep up productivity. Summarising, the l'l~pine reef environment was one of high productivity though low predictability and this sustained medium to high diversities with varying population densities. This means nutrients were not a main factor controlling diversity. Also, spatial control is important only on hard substrates (Jackson, 1977): this was very rare here because of the microbial mats. For the same reason hardly

any clonal organisms (e.g. calcareous sponges or bryozoans) could exist. Thus substrate availability had no effect on diversity either; rather the adaptation of the community as a whole to intermittent disturbances seems to be paramount. Regarding diversity at the level of morphofunctional groups, too little is known about the ecological demands of the various molluscs except their life habit (Section4.4) and trophic status (Section 5.1). Even in Modern reefs, ecospace utilisation is poorly understood; for these reasons, the dwellers are not considered here. Much more may be said about the destroyers: the remarkable numerical dominance of aclonal taxa (bivalves and polychaetes) strongly suggests an unstable environment (e.g. Jackson and Hughes, 1985). Live coral borers (sponges, Gastrochaenidae, Spionidae) dominate but sponges never reached maturity. This suggests a rapid covering of the coral surface with microbial films leading to suffocation of the solitary taxa. For the same reason, the taxa appearing late during succession, such as bivalves and sipunculids had little success (see e.g. Risk and MacGeachy, 1978; Hutchings and Peyrot-Clausade, 1989; PeyrotClausade et al., 1992); instead, the opportunist sponges were favoured, especially considering their bacterial diet. Generally, intense bioerosion was possible due to several factors. Firstly, high nutrient levels were essential (e.g. Pang, 1973; Rose and Risk, 1985; Hallock, 1988). Furthermore, the exclusion of encrusting competitors by the microbes (e,g. Otter, 1937: MacGeachy and Stearn, 1976; Hallock, 1988; Peyrot-Clausade and Brunel, 1989), and the complete absence of vigorous grazers such as parrotfish or surgeonfish (in Modern reefs; yet to evolve in the Jurassic) eased bioerosion significantly (see Bromley, 1978; Sammarco et al., 1987; Kiene, 1989; Kiene and Hutchings, 1993). To some extent, low sedimentation rates might have contributed to the strong bioerosion (Leinfelder et al., 1994). Another effective mechanism for diversity increase via competitive exclusion within the destroyer group is tiering of borings (Bromley and Asgaard, 1993): Apart from the superficial microborings, the topmost tier down to 2 mm depth is made up by Uniglobites?, Rogerella and Dendrina. Entobia

M. Bertling, E. Insalaco / Palaeogeography, Palaeoclimatology, Palaeoecology 139 (1998) 139 175

and Caulostrepsis achieve about 8 mm depth, and the deepest tier of more than 10 mm consists of Trypanites and Gastrochaenolites. Surface traces and deep borings are absent because of the preservation and type of the substrate, respectively.

6. Conclusions and implications

6.1. Ecological conditions of reef growth

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a function of a number of mechanisms, in particular, sediment bypass which is characteristic of fully aggraded carbonate platforms where accommodation space has been almost completely filled, and the absence of siliciclastic influx. Sediment export thus prevailed over the negligible sedimentary input, part of which was microbially bound to the reefal framework. In general, physical factors, except for the shifting inter-reef sediments, were beneficial to the establishment and growth of reef organisms.

6.1.1. Physical ecofactors The physical environment in which these reefs developed can be inferred with confidence from the bio- and lithofacies analysis (Fig. 13). The water was warm but with seasonal variations (although temperature alone is not the factor governing reef structure) (Fagerstrom, 1987, p. 40; Bertling, 1993b). Moderate seasonality is likely considering the latitudinal position of reef development and the faunal diversity pattern. The hydrodynamic regime was rather highly energetic as is suggested by the reefal debris and the presence of several mollusc superfamilies. Sedimentological and additional palaeoecological data suggest that water depth during reef growth did not exceed 5 m. Sedimentation rates are likely to have been very low; hence the water was clear most of the time. The low sedimentation rates were probably

6.1.2. Chemical ecofactors Important chemical factors for biogenic carbonate production are salinity, nutrients, oxygenation and alkalinity. The salinity was euhaline as demonstrated by the various fully marine families amongst the gastropods, bivalves and borers, as well as the presence of echinoderms. Very little variation may have occurred since this would have resulted in the larvae failing to metamorphose, which in turn would have resulted in low numbers of individuals; this is clearly not the case. Trophic environment is less easy to evaluate with the same degree of confidence. Much can be inferred from the presumed effects that eutrophication has on the reef biota, but many of these cause-and-effect relationships are of a qualitative nature and have not been rigorously tested.

omission

/l",, / / ',,,",,, d~vr~[.y

shallow agitated water

borer diversity

microbial mats

raised nutrient level

Fig. 13. Control of prominent reef synecological features by critical physical ecofactors; arrows indicate triggering effects.

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M. Bertling, E. hlsahtco

Palaeogeography, Palaeoclimutology, Palaeoecology 139 (1998) 139 175

Furthermore the limiting effects of excess nutrients on zooxanthellate corals is suggested to be temperature dependent, with its effects being more significant at lower temperatures (Hallock, 1988). Nevertheless there are a number of features of the reef fauna that are suggestive of raised (with respect to an oligotrophic stand point) nutrient conditions: ( 1 ) inferred presence of heterotrophic bacteria: (2) diversity plus numerous individuals of the associated fauna: (3) need of a more heterotrophic feeding strategy in cryptic corals due to the inactivity of zooxanthellae in poorly illuminated environments ( Fagerstrom, 1987, p. 24); (4) high diversity of destroyers (Hallock, 1988). However, the limited presence of bivalves and polychaetes among borers is difficult to explain since both classes have planktotrophic larvae. The reason may be that nutrient levels were not as high (eutrophic) as to trigger strong larval recruitment (Tomascik and Sander, 1985: Hallock and Schlager, 1986; Hallock, 1988). The various criteria presented by Wood (1993) to estimate palaeonutrient levels suggest that the waters in which the l'l~pine reefs developed were neither oligotrophic nor eutrophic but rather mildly mesotrophic. Possible sources for the nutrients include terrestrial run-off, upwelling, and to a very limited extent, ocean turnover (e.g. Berger, 1991; Leinfelder, 1993; Wood, 1993). Moreover, the effects of terrestrial nutrient influx would extend beyond the range of active siliciclastic sedimentation (Crossland, 1983: Hallock and Schlager, 1986; Birkeland, 1988), thus the fact that there is no evidence of siliciclastic sediment within the intrareefal sediment, does not necessarily imply that nutrients were not being introduced into the ecosystem from a terrestrial source. With regard to upwelling and oceanic turnover it is questionable whether these mechanisms would have been significant in the inner parts of such epicontinental seas. There are a number of examples where microbial reefs have been shown to grow under oxygen deficiency conditions (e.g. some of the reefs in the Lusitanian Basin: Leinfelder, 1993) and therefore the seawater in which the l'l~pine reefs grew cannot be automatically assumed to have been well oxygenated. However, Lusitanian reefs developed

below the storm-wave base and are thus not comparable. Moreover, the abundance of the metazoans in the l'l~pine reefs strongly suggests agitated waters with good aeration. However, slight (seasonal'?) variations cannot completely be discounted. Hallock and Schlager (1986) suggested that the growth of bacteria feeding on excess coral mucus might cause a reduction of oxygen if plenty of nutrients are available.

6.1.3. Biological eco&ctors Biological interactions (competition, interference, opponency, symbiosis, commensalism, etc.) may very rarely be demonstrated in the fossil record. Nevertheless biological interactions must have occurred in these reefs and therefore in this respect it is pertinent to speculate on such interactions. Competition was probably rather intense between various groups (Section 5.3) but none can be demonstrated to have controlled the reef development. Moreover the quantitative role of bioerosion remains to be investigated.

6.2. Palaeobiogeography The faunal composition and diversity at l'l~pine is considered here in the context of 'faunal provinces'. Since the work of Arkell (1928), the European Late Jurassic has been divided into a northern 'Boreal' and a southern 'Tethyan' faunal realm, based on ammonites. Hallam (1971, 1975) noted that the boundary between these two realms is rather diffuse, especially before late Oxfordian times. F~irsich and Sykes (1977) introduced a "Subboreal' province for the transition area (including the study site) between the two realms; this area corresponds to a subtropical to warmtemperate climatic zone (Bertling, 1993b). Because the l'l~pine reefs occur within the transitional Subboreal province their composition includes several Tethyan elements, whereas other taxa characteristic of the southern realm are absent; similarities with more northerly localities also exist. As a general trend, species numbers in groups such as corals, bivalves and gastropods tend to decline from the Tethyan to localities further north (Arkell, 1935; Ziegler, 1965). However, with the exception of the corals, species

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richness increases again towards the Arctic with environmental stability (Ftirsich and Sykes, 1977; Bertling, 1993b). The lack or absence of several Tethyan groups may be explained biogeographically: Chaetetida and Stromatoporida were restricted to more southerly tropical climates and thus could not survive in the less predictable subtropical environment of the northern Paris Basin. Some taxa which Hallam (1971, 1975) has claimed to be typically Tethyan also occur far more north; for example, Nerineoidea, the gastropod Purpuroidea and Diceratidae have all been reported from northern Poland and northern Germany (Struckmann, 1878; Schmidt, 1905). These taxa are all present at l'l~pine although diceratids are very rare. This is because the latter reach the northwestern border of their distribution here (Ziegler, 1965). Seasonal environmental variations of a variety of ecological factors governed the distribution of many marine invertebrates during the Late Jurassic (Hallam, 1971; Ffirsich and Sykes, 1977; Bertling, 1993b). No single factor was of overriding importance, especially not temperature. An important factor, however, was the intensity and type of sedimentation which is siliciclastic-dominated in the north and carbonate-dominated in the south. The study area, however, received almost no siliciclastics, as shown by the very pure nature of the carbonates. Since several Tethyan reef building taxa were not tolerant of siliciclastic sedimentation they are generally absent from many northern Boreal and Subboreal localities, but are present at l'l~pine because of the lack of siliciclastic influx. Others which were controlled by different ecological factors, such as e.g. temperature variation, are lacking. The l'l~pine assemblage may thus be considered as a southern Subboreal fauna, though with the absence of various Tethyan taxa which could not extend their range so far north. The combination of medium nutrient concentrations and marked environmental variations was not present elsewhere in Europe during the Late Jurassic: the northern Subboreal was too strongly influenced by terrigenous elastics to permit strong microbial growth, whereas Tethyan regions in both carbonate and siliciclastic settings did not experience perturbations frequent enough for the

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maintenance of a high diversity under these circumstances.

6.3. Reef evolution The Late Jurassic was one of the phases of Earth history when microbial reefs were important (also see Section 5.4.1 ). They generally developed below storm-wave base in palaeoenvironments with calm water. Microbial reefs appear to be very rare in high-energy settings and have hitherto not been studied in detail. Nevertheless a Recent analogue developing within a similar facies has been described by Montaggioni and Camoin (1993). All these microbial reefs are similar in that they bound, or presently bind, fine-grained particles to the coral framework. However, the l'l~pine reefs differ from other Upper Jurassic microbial reefs by virtue of their high overall diversity (especially regarding accompanying fauna and borers) and the prominence of grazing gastropods. They also contrast to other Jurassic reefs in that they possess structural characteristics commonly associated with Recent coral reefs. These modern features include others than the ones mentioned by Leinfelder (1992): the development of relatively large reefal cavities, caves containing a cryptic fauna (Section 4.5), and the dominance of sponges among borers (Section 4.3). The latter is particularly significant; older strong bioerosion by sponges has only been documented from a single Middle Jurassic occurrence (Warme, 1977). Considering the intensity of study of Jurassic reefs across Europe, this paucity of sponges is likely to represent a primary absence at this time rather than a reflection of the incompleteness of the geological record or taphonomic reasons. It appears, with the exception of the present study, that sponges only begin to become important reefal bioeroders from the Kimmeridgian onwards. Although there are some aspects shared with Recent reefs, this similarity should not be overstressed and the l'l~pine reefs should not be described as analogous to Modem reefs. This is because there are a number of important Holocene characteristics that had yet to evolve (Fagerstrom, 1987). In particular, the main predators in Recent reefs (fish) as well as their main primary producers

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(Corallinaceae and Chlorophytes) were insignificant or absent before the Late Cretaceous or even Oligocene (Fagerstrom, 1991 ); moreover extensive reefal cryptos only developed in the Miocene (Kobluk and Lysenko, 1987). The lack of efficient predators such as the fish in the Jurassic is especially significant since in present-day reef ecosystems diversity is typically predator-controlled (in its broadest sense). Instead, nutrients could exert stronger influence on Jurassic reefs because blooming diatoms which buffer excess nutrients did not become important before the Palaeogene (Wood, 1993). Soft-bodied taxa such as tunicates or various sponges might have been present in ancient reefs but have too little preservation potential to be evaluated. These contrasts between Jurassic and Recent reefs demonstrate that Jurassic reefs, as palaeoecosystems, should not be seen in a fully actualistic way, despite a few identical traits. Reefs in similar physicochemical settings today are very different in terms of both taxonomic composition and ecological structure because not only reef builders were evolving.

Acknowledgements E.I. gratefully acknowledges NERC for the funding of his part of this study. Noel Morris (BMNH) kindly identified some fossils for us. We thank Steve Kershaw, Reinhold Leinfelder and Finn Surlyk for their critical reviews which significantly improved the manuscript.

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