Upper Jurassic

The late Middle Jurassic witnessed a phase of deepening seas accompanied by marine invasion of the earlier Middle Jurassic lagoons and deltas. The resulting environmental changes led to the deposition of more uniform marine sediments over much of northwestern Europe and provided sharp environmental contrasts with those that had preceded them. Although major marine invasion began in the late Middle Jurassic, as defined by ammonite zones, for convenience we have included these latest Middle Jurassic facies within our Upper Jurassic section because of environmental comparability.

The initial phase of deepening produced a fairly widespread clay facies, the Oxford Clay, but this was succeeded by a temporary shallowing that produced sands and coral-rich limestones during the Upper Oxfordian (the Corallian Facies). Later, renewed deepening and further transgression spread relatively uniform clay facies, the Kimmeridge Clay, once more over much of Britain and northern France. During the latest period of the Jurassic a further and substantial regressive phase occurred and this led to uplift in northern Britain and to the development of extremely varied environments in the south that were comparable in many respects to those that had developed during the Middle Jurassic in central Britain.

Animals in stagnant basins (bituminous mud associations) (not illustrated)

Several times during the Upper Jurassic, bituminous mud facies spread extensively over north-west Europe. The facies is very similar to that developed in the Lower Jurassic (Fig. 63, p. 210), and are similar enough for a separate diagram to be unnecessary. However, it is worth while mentioning here the main differences between the Upper and Lower Jurassic bituminous mud associations.

The faunas were dominated by plankton, epiplankton and nekton with an absence of benthic forms. Whole bedding surfaces are often crowded with Bositra (a bivalve which may have been an active swimmer) and ammonites that have inevitably been crushed flat during compaction.

At times, and particularly toward the late Upper Jurassic, the minute planktonic algae called coccolithophorids were sufficiently abundant to produce coccolithic limestones. In the laminated and bituminous shales, where anaerobic bottom conditions excluded burrowing organisms, coccolithic laminae could sometimes develop. Coccolithic sediments were sometimes concentrated on submarine highs and this material was occasionally carried by turbidity currents into the stagnant basins.

Examples of Upper Jurassic bituminous shales are found in Britain on the Dorset coast, in Yorkshire and in west Scotland. In east Scotland, a series of Kimmeridgian sandy bituminous shales occur interbedded with spectacular conglomerates and breccias. These conglomerates contain a shallow water shelly benthos mixed with large fragments of Devonian and older rocks brought from further north; the conglomerates are slumped masses of shallow and near-shore sediments which slipped from the north down the scarp of a fault which was active during Kimmeridgian times.

83 Muddy Sea Floor Communities

Restricted clay communities were similar to their counterparts in the Lower Jurassic. Besides thin-«helled pectinids, protobranchs and lucinoids, they also contained certain specialized deep-burrowing infaunal bivalves (like Thracia) which, although suspension feeders, became adapted to life in environments with a low food supply from which other infaunal suspension feeders were excluded. Cerithid gastropods were still common as surface or shal-low-infaunal detritus feeders and were often accompanied by Dicroloma, an aporrhaid gastropod with an inflated flange around its aperture. In modern aporrhaids this flange separates the inhalent and exhalent currents while the snails forage as scavengers or detritus feeders on the sea floor.

Some species of Pinna also spread into this facies while the large and inflated Gryphaea dilatata, and other oysters, were sometimes common. The Gryphaea, as well as being able to clap their valves free of mud, probably had a large gill area capable of driving powerful currents through their mantles. This would enable them to derive adequate food supplies from these tranquil waters while the inflated mantle margin efficiently separated their inhalent and exhalent currents.

During phases of slow deposition in relatively deep shelf waters, the sea floor often became colonized by thickets of sponges (Rhaxella and Pachastrella). These organisms were epi-faunal suspension feeders consisting of a bag of cells supported by a siliceous frame and were able to live in the deeper waters from which other filter feeders were excluded by the low concentrations of food material. It is also possible that dense clumps of these sponges consumed so much of the food material suspended in the sea that other filter feeders were often excluded. After death, the

Ioturbation Infauna

b Yoldia (Mollusca: Bivalvia: Palaeotaxodonta — nuculoid)

c lucinid (Mollusca: Bivalvia: Veneroida) d Thracia (Mollusca: Bivalvia: Anomalodesmata) e cerithid (Mollusca: Gastropoda: Mesogastropoda)

f Dicroloma (Mollusca: Gastropoda: Mesogastropoda)

g Pinna (Mollusca: Bivalvia: Mytiloida) h Gryphaea (Mollusca: Bivalvia: Pterioida — oyster)

i Rhaxella (Porifera: Hyalospongea)

j Rhizocorallium (trace-fossil of crustacean) k Chondrites (trace-fossil of annelid)

1 Amoeboceras (Mollusca: Cephalopoda: Ammonoidea) m belemnite guards (Mollusca: Cepholopoda: Coleoidea)

n Exogyra (Mollusca: Bivalvia: Pterioida — oyster)

sponges collapsed and their siliceous spicules dropped down to the sediment which then constituted a spiculite.

Often, the only other organisms associated with these spiculites were deposit-feeding burrowers, particularly crustaceans which produced meandering horizontal 'U' burrows [Rhizo cor allium) and mined the particularly nutritious layers of sediment for food (Seilachcr, 1967).

Where hard substrates were afforded by the shells of dead ammonites, either clusters of small Exogyra (oysters) became established or knotted 'skeins' of the serpulid worm Glomerula produced small mounds on the sea floor before being overwhelmed with mud.

Examples of these clays are exposed in Britain on the Dorset coast, and in central England, Yorkshire and Scotland.

84 Marine Sand and Muddy Sand Communities

These communities bore some similarities to their earlier Jurassic counterparts and they often had a rather limited benthos. However, the sediments were often intensively bioturbated, particularly by crustaceans that produced Rhizocorallium and Thalassinoides burrows. These burrow systems made by infaunal suspension feeders were constructed within cross-bedded and rippled sands and this particular combination of burrows and sedimentary structures indicates deposition under relatively shallow, agitated water conditions. Occasionally another, very distinctive burrow-type was present — Ophiomorpha — a complex crustacean burrow similar in many respects to Thalassinoides, but with its various galleries lined with pellets that provide a nodular grape-like texture to the surfaces of the tubes after fossilization. In modern sediments, the shrimp Callianassa produces comparable burrow systems, particularly in inshore and shallow subtidal environments. In our diagram, we have not been able to indicate the fact that these burrows often extend over a metre down into the sediments. The vertical 'U' burrow Diplocraterion, which housed an infaunal suspension feeder may also be present.

Associations of rippled sands with clay laminations indicate alternate turbulent and slack water episodes. Such associations are found today in some tidal areas and Wilson (1968) has interpreted some of these Upper Jurassic associations in this light. It is probable that high rates of sedimentation in strongly fluctuating environmental conditions inhibited colonization of these substrates by a diverse shelly benthos. The sand depicted in our diagram represents one of the low-diversity assemblages. With slower rates of sedimentation, however, the sediments became more diversely

Bivalve Fossil Diagram

Fig. 84 Marine Sand and Muddy Sand Communities a Ophiomorpha (trace-fossil)

produced by b a 'ghost shrimp' (Arthropoda: Crustacea); seldom preserved c Diplocraterion (trace-fossil of annelid or crustacean; annelid shown)

Fig. 84 Marine Sand and Muddy Sand Communities a Ophiomorpha (trace-fossil)

produced by b a 'ghost shrimp' (Arthropoda: Crustacea); seldom preserved c Diplocraterion (trace-fossil of annelid or crustacean; annelid shown)

inhabited. Deep-burrowing bivalves like Pholadomya and Goniomya appeared accompanied by the byssally-fixed Gervillella, which, in the uncemented beds, may all occur as moulds where the shell material has been dissolved.

Thalassinoides burrows may contain abundant small oysters [Liostrea and Exogyra) and serpulids (Glomerula) that may have lived within the burrows alongside the shrimps, or were perhaps collected as debris by the crustacean from the sediment surface. Other bivalves that lived in these slightly more diverse sand environments included the semi-infaunal Pinna, large Liostrea, the thick-shelled Ctenostreon cemented to other shells and the pecti-nids Chalmys and Camptonectes. The more diverse communities also included scavenging and detritus-feeding gastropods like Pseudomelania together with Natica which today is a carnivore.

Heart-urchins such as Nucleolites lived in shallow depths within the sediment and the fully marine characteristics of the higher diversity communities are confirmed by the presence of ammonites and belemnites.

Examples of these sands occur on the Dorset coast, in central England and Yorkshire and compose the reservoir of the Piper oil field in the North Sea.

85 Calcarenite Community

In the Upper Jurassic of England, clays, sands and limestones commonly occur in coarsening upward cycles which have often been interpreted as shallowing sequences. The limestones capping these cycles are often oolitic (Fig. 85a), and like those in the Middle Jurassic, many oolites contain few indigenous organisms. Burrows are sometimes present, particularly the straight vertical Skolithos which is perpendicular either to dune surfaces or to individual cross-bedded laminae. These burrows, along with occasional Diplocrat-erion, were produced by suspension-feeding organisms inhabiting very shallow turbulent waters. Another type of vertical 'U' burrow, Arenicolites, which lacks reworked spreite between the tubes, represents the burrow of a detritus feeder. Examples of these true oolite communities occur in Dorset and in central England.

Transported ooliths, however, are a common component in other Upper Jurassic limestones which developed adjacent to the sites of oolite formation, and which accumulated under more stable conditions and thus contain a richer fauna. Sometimes the ooid sand dunes became stabilized and were then colonized by an abundant and diverse fauna. Epifaunal bivalves took advantage of such surfaces, and these bivalves included large cemented oysters, the pectinid Camptonectes and the byssally attached clam Isognomon, this last being well adapted to life in turbulent conditions, first because of its thick prismatic shell and second because of its streamlined shape. In modern seas, this bivalve lives swinging freely from plants (guessed at in our diagram) and other exposed features on the sea floor (Kauffman, 1969). These epifaunal shells are seldom preserved in their original articulated form, but have usually been dispersed and broken by currents.

Patches of coral and clusters of the serpulid worm Glomerula also sometimes became established, while grazers were represented by the gastropod Aptyxiella and other snails.

Soft-bodied animals burrowed in the sediment, as did the bivalves Myophorella and Trigonia. These bivalves were sluggish shallow burrowers; although they lacked long siphons they were infaunal suspension feeders with a heavily ornamented shell which aided their burrowing and anchorage within the sediment (Stanley, 1970). Clusters of coral sometimes became established temporarily (not shown here) and these patches of coral along with shell debris were usually bored both by algae (on a fine scale) and by the mytiloid Lithophaga. Algal borings penetrated the surface of much of the shell material, as in modern clear tropical seas where algal infestation rapidly creates patinated surfaces on both living and dead shell material. The quick-burrowing Thracia and other siphon-ate suspension-feeding bivalves also indicate shallow water conditions with stabilized substrates.

Mollusca Bivalvia

Fig. 85 Calcarenitc Community a Rhaxella (Porifera: Demospongea)

b Isognomon (Mollusca: Bivalvia: Pterioida)

c Trigonia (Mollusca: Bivalvia: Trigonioida)

d Camptonectes (Mollusca: Bivalvia: Pterioida — pectinid)

e serpulids (Annelida)

f conjectural algal fronds g Arenicolites (trace-fossil of annelid)

Photos Jurassic Bivalves

Modern oysters are tolerant of variable salinity, and Barthel (1969) has suggested that even Jurassic trigonids and Pholadomya were able to withstand salinity fluctuations (and particularly hypersaline conditions). In the highest Jurassic limestones of southern England, although the enormous titanitid ammonites occur, belem-nites are never found, and it may well be that many of these limestones were deposited under hypersaline conditions to which some ammonites had adapted themselves.

A contribution to the sediment is made by the spicules of two kinds of siliceous sponge: Rhaxella and Pachastrella. These animals are seldom found intact, but when alive they may have acted as sediment baffles and trapped material. Sometimes whole areas became so infested with sponges that other suspension feeders were unable to compete.

Examples of this community occur in Britain on the Dorset coast, in central England and in Yorkshire.

BIOHERM COMMUNITIES (86-87)

Reefs are organically produced mounds which are resistant to wave and current action. To recognize an old reef we not only have to show that a patch of fossils produced a mound, but also that the mound was resistant to erosion. In the Upper Jurassic we can recognize a number of bioherms (organic mounds), some of which were predominantly corals and others that were produced by groups of organisms not usually thought of as reef builders (for instance sponges, bivalves and algae); some of these mounds were wave-resistant, others were not.

86 Oyster-algal Bioherms Community

Stabilized surfaces of ooid sand were sometimes colonized by communities of oysters {Liostrea) which, once established, often grew upon each other, either spreading out over the sea floor or upwards, thus producing shelly mounds. These thick-shelled epi-faunal bivalves with their extremely high tolerance of salinity fluctuations were capable of producing bioherms of up to 2m in height. The oyster shells were often encrusted with other epifaunal suspension feeders that took advantage of the hard substrates. These other organisms included the oyster-like scallop Plicatula, serpulid worms and bryozoans. Red algae such as Solenopora were sometimes associated with these oyster bioherms, and the growth of algae and other encrusting organisms helped to bind the oyster mounds together. The crevices between the shells were filled with the debris derived from the shell and algal masses either by erosion or by predators feeding upon the shells.

The crevices with their fillings provided a new habitat for organisms including thecideidine brachiopods (too small to be shown). Lithophage and pholadid bivalves are commonly found in crypts penetrating both shells and laminated algal masses, and presumably the detritus provided by these borers was incorporated in the crevice fillings. The shells of grazing gastropods (Aptyxiella andPleurotomaria) which fed upon the Solenopora and other algae occur mixed with the shell debris.

Surprisingly, although the foundation sediment for the bio-herms was often sand grade oolite, the non-shelly interstitial sediment within each bioherm was commonly composed of lime mud (micrite). Modern oysters, like some other epifaunal bivalves, collect mud grade material as they feed and the discarded material is aggregated into sand grade pseudofaeces which rapidly decompose after deposition to produce mud. The micritic matrix in these

Fig. 86 Oyster-algal Bioherms Community a Liostrea (Mollusca: Bivalvia: Pterioida — oyster)

b Solenopora (Algae)

c Lithophaga (Mollusca: Bivalvia: Mytiloida)

d Isognomon (Mollusca: Bivalvia: Pterioida)

e Pleurotomaria (Mollusca: Gastropoda: Archaeogastropoda)

f Aptyxiella (Mollusca: Gastropoda: PMesogastropoda)

g Plicatula (Mollusca: Bivalvia: Pterioida)

Fig. 86 Oyster-algal Bioherms Community a Liostrea (Mollusca: Bivalvia: Pterioida — oyster)

b Solenopora (Algae)

c Lithophaga (Mollusca: Bivalvia: Mytiloida)

d Isognomon (Mollusca: Bivalvia: Pterioida)

e Pleurotomaria (Mollusca: Gastropoda: Archaeogastropoda)

f Aptyxiella (Mollusca: Gastropoda: PMesogastropoda)

g Plicatula (Mollusca: Bivalvia: Pterioida)

Jurassic oyster-rich mounds may have been formed in a similar way.

Sometimes the perimeters of the bioherms were marked by shell debris from free-swinging epifaunal suspension feeders like Isognomon which were attached to the surface of the shell mounds. However, it is by no means clear whether these mounds were wave-resistant or not.

Examples in Britain of these oyster-algal mounds occur in Dorset.

87 Coral-algal Patch-reef Community

This community was dominated by massive and branching corals that produced an unbedded rock. The main genera of corals are Thecosmilia and Isastrea (with its Thamnasteria growth form) producing individual masses up to 1.5m in diameter. Like modern corals, these genera were adapted to life in shallow, well lit wave agitated waters. Comparisons between these Jurassic scleractinian corals and their modern descendants are fairly reliable, and it is quite probable that they were hermatypic, with zooxanthellae algae living within their tissues in a symbiotic relationship. Modern polyps are wholly carnivorous and feed upon microscopic zooplankton which they paralyse with the aid of stinging cells upon their tentacles. The coral polyps were never preserved as fossils, although the massive calcareous structures which they secreted are commonly found. However, their skeletons were often made of aragonite, and this mineral form of calcium carbonate is easily dissolved during diagenesis. Thus, rocks that were originally coral-rich are now cavernous or the cavities have subsequently been filled with sparry calcite, with the corals as mounds.

Modern corals are particularly intolerant of reduced salinities but can withstand high salinities (over 40° /00) and although some larger polyps can remove irritating sand grade sediment with their tentacles many modern genera are unable to withstand very turbid habitats. The tropical reef-builders also require warm (18—36° C) waters and firm substrates on which to grow. Diverse coral communities are found in shallow situations fringing the open sea and communities become more restricted towards lagoonal environments where food supply is less and where the seasons are most strongly marked by environmental fluctuations.

The organisms associated with the Jurassic patch reefs included many forms with modern analogues. The coral heads were often

Jurrassic Water Organisms

Fig. 87 Coral-algal Patch-reef Community a Thecosmilia (Coelenterata: Anthozoa: Scleractinia)

b Isastrea (Coelenterata: Anthozoa: Scleractinia)

c Thamnasteria arachnoides (Colenterata: Anthozoa: Scleractinia) d Thamnasteria concinna (Colenterata: Anthozoa: Scleractinia)

e Rhabdophyllia (Coelenterata: Anthozoa: Scleractinia)

f Lopha (Mollusca: Bivalvia: Pterioida)

g trochid (Mollusca: Gastropoda: Archaeogastropoda)

h Chlamys (Mollusca: Bivalvia: Pterioida — pectinid)

i Cidaris (Echinodermata: Echinozoa — sea urchin)

j terebratulids (Brachiopoda: Articulata: Terebratulida)

k bryozoan (Bryozoa: Ectoprocta)

1 Cladophyllia conybeari (Coelenterata: Anthozoa: Scleractinia)

Fig. 87 Coral-algal Patch-reef Community a Thecosmilia (Coelenterata: Anthozoa: Scleractinia)

b Isastrea (Coelenterata: Anthozoa: Scleractinia)

c Thamnasteria arachnoides (Colenterata: Anthozoa: Scleractinia) d Thamnasteria concinna (Colenterata: Anthozoa: Scleractinia)

e Rhabdophyllia (Coelenterata: Anthozoa: Scleractinia)

f Lopha (Mollusca: Bivalvia: Pterioida)

g trochid (Mollusca: Gastropoda: Archaeogastropoda)

h Chlamys (Mollusca: Bivalvia: Pterioida — pectinid)

i Cidaris (Echinodermata: Echinozoa — sea urchin)

j terebratulids (Brachiopoda: Articulata: Terebratulida)

k bryozoan (Bryozoa: Ectoprocta)

1 Cladophyllia conybeari (Coelenterata: Anthozoa: Scleractinia)

drilled by boring bivalves (Lithophaga), algae, bryozoans and sponges (clionids). Other organisms, particularly pectinid bivalves and thecideidine brachiopods inhabited fissures between the coral heads. Nests of terebratulid brachiopods occasionally became established, probably growing within crevices amongst the older, dead portions of the coral patches. This phenomenon has been observed in modern coral associations by Jackson et al. (1971).

In modern reefs and patch reefs, waves and predators (particularly fish) attack the corals and produce sand grade sediment which helps to fill the fissures between the coral heads. The Jurassic coral patches were similarly attacked and both fissures and patch-reef margins were marked by coral detritus. These fragments were frequently encrusted by suspension-feeding serpulids and Exogyra oysters which could not establish themselves on the living coral heads because the carnivorous coral polyps preyed upon their spat.

The dead portions of modern patch reefs are frequently overgrown with red and blue-green algae and bryozoans which help to bind the frame of the bioherm. In the Jurassic patch reefs much of the shell material is riddled and sometimes overgrown with algal infestations.

Other common predators on modern reefs are asteroid starfish, but although known in the Jurassic these organisms have a low preservation potential and are found only exceptionally as fossils. However, algal grazers are frequently represented by the thick-shelled gastropod Bourgetia and by trochids.

The detrital sands derived from reef erosion provided habitats similar to those of the stable ooid sands (Fig. 86) and in them a variety of infaunal burrowers are found, including the carnivorous sea-urchins Cidaris, Nucleolites and Pygaster. These beds, unlike the massive patch-reef limestones, sometimes contain ammonites.

The diversity of coral communities generally increased southwards into Europe and decreased northwards into Scotland where only Isastrea occurred. This distribution probably reflected a combination of climatic and ecological factors including increasing turbidity, salinity fluctuations and climatic instability towards the north. Influxes of volcanic ash sometimes killed off fauna on parts of the sea floor and the resulting clay blankets help to show the low relief of the sea floor even when it was covered by flourishing growth.

Examples of the patch-reef communities occur in Britain in Dorset, central England and Yorkshire and in east Scotland blocks of slumped coral occur in basinal Kimmeridgian shales.

Oyster Lumachelle Community (not illustrated)

Oyster communities which did not form part of patch reefs occurred high in the Jurassic and sometimes mark the base of the Cretaceous (Casey, 1971) in southern Britain. These communities were dominated by Liostrea which occur either fragmented into single disarticulated valves or as complete individuals in position of growth. Assemblages dominated by oysters often occur in reduced salinity and also indicate deposition at very shallow depths (Kauffman, 1969). These epifaunal suspension feeders occur in beds where there is a matrix of limey clay between the shells, and this sediment is probably the remains of pseudofaeces deposited by the animals (see Fig. 86, p. 267).

Oysters often comprised more than 90 per cent of the fauna; other organisms included encrusting suspension feeders such as serpulids, and infaunal mobile bivalves like Trigonia and Protocardia. Thick spines of the regular echinoid Hemicidaris are sometimes found and this scavenger probably lived upon the material from dead bivalves. The presence of Trigonia and Hemicidaris indicates that salinity was probably no lower than in normal marine environments, but the scarcity of other marine organisms suggests that they were affected by salinity, perhaps hypersalinity. The oyster beds often show internal low-angle cross-bedding which may be the original growth surfaces. The topmost surfaces of the beds are richer in articulated valves. Epifaunal bivalves rapidly disintegrate after death as the ligament rots, and it is not entirely unexpected that many individuals in these beds are often lacking their upper valves. Many specimens have also suffered disturbance of their lower valves after death, either from current or biological action, but although some movement of the valves has occurred, often the ratio of left to right valves in a single unit is approximately the same, indicating only limited disturbance.

As we have seen earlier (Fig. 86), Exogyra beds may occur in deeper water clay-rich sediments where substrate conditions restricted the other faunas. The beds in which these deeper water Exogyra communities occurred were frequently dolomitic (Townson, 1975), and heavy brines flowing down from shallow water may also have played a part both in restricting the fauna and in causing dolomitization of the sediment.

Examples of these oyster communities occur in Britain in Dorset and also, rather poorly exposed, in central England.

LAGOONAL COMMUNITIES (88-90)

In southern Britain, the Jurassic ended with a phase of Iagoonal conditions in which a variety of communities lived in habitats ranging from dominantly marine or hypersaline to wholly freshwater. Some of the oyster and oolite communities already considered are also relatively restricted.

88 & 89 Intertidal and Subtidal Algal Mat Communities

Some Late Jurassic formations contain laminated lime mud resulting from the growth of blue-green algae (e.g. Girvanella). These algae trapped and bound mud grade carbonate in sheet-like mats.

Laminated lime muds (micrites) contain scattered shells, particularly those of minute bivalved crustaceans (ostracodes) which must have teemed in the shallow water above the mats. Often, the mats were full of burrows, particularly Thalassinoides produced by crustaceans, and the burrowing was sometimes so intense that only remnants of algal laminations remain. Fragments of the burrowing crustacean Callianassa are found associated with Thalassinoides. Modern intertidal algal mats (Fig. 88) are high stress environments, and burrowing animals that destroy algal laminations are progressively eliminated towards the highest intertidal regions, particularly in arid climates.

Where the lamination is best preserved, a series of polygonal cracks may be seen on the bedding surfaces (Fig. 89e), and these desiccation cracks indicate exposure to the sun at the time of deposition. The algal laminations often contain gypsum pseudo-morphs now replaced by calcite (Fig. 89f). The original gypsum crystals were probably also produced during phases of desiccation.

Gastropod, bivalve and, rarely, ammonite shells are embedded within the porcellaneous sediment. The bivalves include the in-faunal deposit feeder Nucula and the infaunal siphonate suspension feeder Pleuromya. Weakly siphonate or non-siphonate mobile suspension feeders included Trigonia, Isocyprina and Protocardia while the epifaunal bivalves consisted of pectinids (Camptoncctes), rare oysters and byssally fixed Mytilus. Grazers and scavengers were represented by gastropods [Procerithium and Aptyxiella).

This association of organisms contains too many marine forms for the habitat to have been isolated from the sea; however, the association of marine organisms, including ammonites, witli sedimentary features indicating exposure (sun cracks and gypsum pseudomorphs) requires some explanation. There are few normal marine environments in which desiccation occurs; in general the only marine environments which habitually suffer exposure are intertidal ones. Where carbonate deposition occurs on modern coasts algal mats are a feature of the intertidal zone (Ginsburg, 1975). Here, the mats below the low tide level commonly suffer disruption by Callianassa whereas in the intertidal and supratidal zones, little burrowing occurs, particularly if evaporation is causing concentration of marine salts.

In the well laminated Jurassic micrites, the shells are usually disarticulated and often occur in layers, which implies that they were drifted. However, in the bioturbated micrites the bivalves are frequently preserved in their growth positions. Thus it may be that the Upper Jurassic laminated micrites are inter- and supratidal algal flat deposits which were periodically inundated by storm tides that carried and dumped reworked shells. The bioturbated micrites probably indicate the tidal channel and shallow subtidal zones from which the shells were derived. The gastropod grazers, like their modern counterparts, may have been able to withstand limited phases of desiccation.

The aragonite-shelled fauna has mostly been dissolved, leaving only a rock with cavities showing both the internal and external moulds of the gastropods, bivalves and ammonites.

Examples of this community are found in Britain on the island of Portland and in other parts of Dorset (e.g. Purbeck).

Hypersaline Lagoon Communities

The fauna and flora of the hypersaline lagoons were extremely restricted. The sediments consist of laminated, stromatolitic lime mud which may have been associated with evaporite minerals. The salts have often been removed by solution, sometimes resulting in collapse of the beds (West, 1960, 1964, 1975). As well as the laminations produced by blue-green algae (Fig. 89c, e and f), there were large numbers of small pellets probably produced by grazing gastropods and crustaceans (Fig. 89a and d). In the diagrams we show a general view of the reconstructed environment with broad low-lying hinterlands basking under a blazing sun. The supratidal area would have been very similar to that of today in the Persian Gulf (Fig. 88). Even if tidal movements were small, the low relief would have provided wide intertidal areas which would have been colonized by blue-green algae. Cutting through this intertidal area would have been tidal channels (Fig. 89). The insets show various surface features found in modern sabkha environments in the intertidal zone and their general location in the pattern: the entrance to a crab burrow (a) with a mound of pellets (a, d); an algal surface near a channel margin with the trails produced by grazing cerithid gastropods (b) ); the entrances to Callianassa burrows from

Fig. 88 View from the sea of a salt flat (sabkha) coast near Abu Dhabi, on the south shore of the Persian Gulf. The background is several km from the viewer

Fig. 88 View from the sea of a salt flat (sabkha) coast near Abu Dhabi, on the south shore of the Persian Gulf. The background is several km from the viewer

Sabkha Picture Persian Gulf

the same zone (b2); the irregular surface of the algal flat with the leathery algal crusts partially broken by contraction during desiccation (c and e); while f shows the type of internal structure obtained on digging a section through the mat. All the surface pictures are based upon photographs from the present-day sabkha

Persian Gulf Sabkha

Fig. 89 A channel (a few metres wide) on a sabkha coast, with locations of insets a to f a entrance to crab burrow — trace of Callianassa (Crustacea) b[ trails of grazing gastropods on an algal surface b2 entrances to 'ghost shrimp' burrows c surface of crinkled algal mat; blue-green alga (Prokaryota — stromatolite) d surface covered with crustacean faecal pellets e] pustular algal mat ej algal mat cut by desiccation cracks f stromatolitic laminations with gypsum pseudomorphs and vugs

Fig. 89 A channel (a few metres wide) on a sabkha coast, with locations of insets a to f a entrance to crab burrow — trace of Callianassa (Crustacea) b[ trails of grazing gastropods on an algal surface b2 entrances to 'ghost shrimp' burrows c surface of crinkled algal mat; blue-green alga (Prokaryota — stromatolite) d surface covered with crustacean faecal pellets e] pustular algal mat ej algal mat cut by desiccation cracks f stromatolitic laminations with gypsum pseudomorphs and vugs at Abu Dhabi on the Trucial Coast while the section (f) is based upon a specimen from the Purbeck. Structures comparable to the modern ones (a— e) can be seen in parts of the Purbeck Formation in Dorset.

90 Freshwater Lagoon Communities

As the marine influence decreased, so the lagoons of the Upper Jurassic began to contain a variety of bivalves, gastropods, ostra-codcs and vertebrates which were adapted to life under freshwater conditions. The faunas often have modern counterparts and descendants and their modes of life can be interpreted fairly confidently. The dominant benthic animals were the molluscs, particularly the suspension-feeding bivalve Unio which, like its ancestors in the Middle Jurassic (Fig. 73, p. 235) and its modern descendants, probably lived free on stream and lake beds. Unio was a mobile bivalve which lacked siphons and its modern descendant is usually found slightly covered by deposited sediment. It has a thick pro-teinaceous outer covering to the shell (the periostracum) which protects the shell from solution by running fresh water.

Other molluscs associated with Unio were mostly gastropods. These included the mucosal filter feeder Viviparus (see Fig. 73) and a variety of other genera that were predominantly grazers. Many of these gastropods were pulmonate, lacking gills but breathing air through a mantle modified into a simple lung. This is a common feature of modern pond snails. Pulmonates included the smooth planispiral Planorbis. The smooth-shelled ostracodes complete the list of freshwater invertebrates which are preserved as fossils.

Fossils of the vegetation upon which the gastropods and other soft-bodied grazers fed include fragments of the calcareous algal Chara, and presumably other forms of aquatic vegetation were present but apart from spores these have not been preserved. Diverse spores and pollen assemblages may be found in these sediments after the matrix has been dissolved with strong acids.

Many of the freshwater beds contain vertebrate remains. Typically aquatic forms include a variety of crocodiles [Goniopholis and many others) some of which were small genera a mere 450mm in length (e.g. Nannosuchus). Larger vegetarians were represented by turtles (Tertosternum) and dinosaurs (Iguanodon ). Some beds were traversed by the footprints of these large reptiles.

A large fauna of ganoid fish has been recorded and these presumably formed the stable diet of the crocodile population.

In addition, insects are often represented by fragments in a remarkable state of preservation. These include the wings and wing-cases of beetles, cockroaches, grasshoppers, dragonflies and various other types. The insects were the staple diet of the shrewlike mammals Amblotherium and Triconodon.

The environments suggest a low-lying swampland probably similar in some respects to the modern Everglades in Florida, with a wealth of freshwater habitats at or very close to sea level; hurricanes and storms sometimes temporarily spread sea-water many

Fig. 90 Freshwater Lagoon Communities a Iguanodon (Vertebrata: Reptilia: Archosaur — dinosaur)

b Amblotherium (Vertebrata: Mammalia — panthothere)

c Goniopholis (Vertebrata: Reptilia: Archosauria — crocodile)

d Equisetites (Pteridophyta: Catamites — horsetails)

e cycads (Gymnospermae)

f surface covered with blue-green algae partially sun-dried (stromatolitic)

Fig. 90 Freshwater Lagoon Communities a Iguanodon (Vertebrata: Reptilia: Archosaur — dinosaur)

b Amblotherium (Vertebrata: Mammalia — panthothere)

c Goniopholis (Vertebrata: Reptilia: Archosauria — crocodile)

d Equisetites (Pteridophyta: Catamites — horsetails)

e cycads (Gymnospermae)

f surface covered with blue-green algae partially sun-dried (stromatolitic)

kilometres over the marsh. In the freshwater regions of the Everglades the dofhinant floras are grasses, algae, cypresses and occasional 'hammocks' of dense mahogany forest. Of this flora, only the algae and conifers were represented in the Jurassic.

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