Low Diversity Temporarily Stable Calcarenite Community

This sediment consisted of finely broken shell debris which was the result of a combination of predation and mechanical erosion. It is likely that originally the sediment surface was rippled and provided a fairly mobile substrate. Thus, the fauna which inhabited it consisted of fairly specialized animals all adapted to withstand the somewhat rigorous environmental stresses. Many of the colonizing animals took refuge within the sediment and, consequently, many of the original sedimentary structures have been totally obliterated by their burrowing activities.

Irregular Echinoids Reconstructions

Fig. 77 Low Diversity Temporarily Stable Calcarenite Community

The dominant genera were the streamlined shallow burrowers Eocallista and Protocardia; they were relatively mobile bivalves and were capable of ploughing their way through the sediment, with pauses from time to time in places where there was a good supply of suspended food material. The Trigonia-like bivalve Vaughonia probably lived in a similar fashion.

Also prominent within this sediment was the mobile burrowing coral Chomatoseris which, unlike many corals, did not produce an extensive skeletal frame, but merely grew up from a disc of carbonate that may have aided its burrowing activities. These carnivores could move either up or down in response to deposition or erosion on the sea floor.

The epifauna was very restricted; the only epifaunal bivalve was the rather rare suspension-feeding Parallelodon that was probably byssally fixed to fragments of shell debris. This bivalve was probably capable of exhuming itself if covered by deposited sediment. The only other epifaunal organisms in this community were high-spired nerineid gastropods that grazed over the surface of the sediment. Their main food source in this environment probably consisted of the bacterial linings around the sediment grains, although some modern nerineids are capable of suspension feeding by a ciliary process and are sedentary within shallow burrows.

This fauna is present in the Middle Jurassic of the English Midlands.

Fig. 77 Low Diversity Temporarily Stable Calcarenite Community

Parallelodon (Mollusca: Bivalvia: Arcoida) Chomatoseris (Coelenterata: Anthozoa: Scleractinia) Eocallista (Mollusca: Bivalvia: Veneroida) Vaughonia (Mollusca: Bivalvia: Trigonioida) Protocardia (Mollusca: Bivalvia: Veneroida) nerineid gastropod (Mollusca: Gastropoda: PMesogastropoda)

78 Diverse Calcarenite Community

As in the previous environment, the sediment was composed of sand grade shell debris which was likely to have been rippled at the time of deposition, but here too the action of burrowers has led to the total obliteration of the original cross-bedding. There is often a gradation between the last community and this type, with the development of this diverse fauna having been allowed by the reduction in both turbulence and sedimentation rates. The sediment may also show a concomitant increase in the content of mud.

As the sediment became more stable, so the diversity of the epifauna showed a marked increase, and the more sensitive in-faunal suspension feeders (Pleuromya and Pholadomya) also made an appearance. These deep-burrowing bivalves with their long siphons would have been excluded previously by high rates of deposition. Also within the sediment were the shallow-burrowing and more mobile suspension feeders Astarte and Trigoma while at much greater depth (a metre or more, and thus too far down to be included correctly in our diagram) were crustaceans producing Thalassinoides burrows. Sometimes, some of these crustaceans can be found within their burrows (for instance Glyphaea as depicted in our diagram). Occasional irregular echinoids (Nucleolites) were deposit feeding within the sediment.

With lower sediment mobility, and probably decreased turbidity, brachiopods appeared. These consisted of both rhynchonellids and terebratulids. The rhynchonellids occurred in clusters or'nests' composed of a single species while the terebratulids lived either in clusters or as isolated individuals. Ornithella, which may have had a relatively long pedicle, and Cererithyris, with its nearly planar commissure between the valves, are both characteristic of these environments.

Epifaunal bivalves included the byssally attached Meleagrinella which was anchored to shell particles on the sea-floor. Its flattened shape was ideal for it to withstand strong currents as it filtered the sea water for its food. The smooth and fairly flattened pectinid Entolium may have been free-living. The filter-feeding oyster Lopha with its thick and strongly ribbed shell was attached to shell particles and other individuals of its own kind by a carbonate cement.

The sea water above the bottom was of sufficiently normal marine character to support belemnites, nautiloids and ammonites living as scavengers and predators.

In England the Inferior Oolite and Great Oolite Groups of Dorset, the Midlands and Yorkshire contain limestones showing this community (for instance, the Cornbrash).

Ornithella

Fig. 78 Diverse Calcarenite Community a Clydoniceras (Mollusca:

Cephalopoda: Ammonoidea) b Cenoceras (Mollusca:

Cephalopoda: Nautiloidea) c Ornithella (Brachiopoda: Articulata: Terebratulida) d Meleagrinella (Mollusca: Bivalvia: Pterioida)

Lopha (Mollusca: Bivalvia: Pterioida — oyster) rhynchonellids (Brachiopoda: Articulata: Rhynchonellida) Cererithyris (Brachiopoda: Articulata: Terebratulida) Entolium (Mollusca: Bivalvia: Pterioida — pectinid) Nucleolites (Echinodermata: Echinozoa — sea urchin) Trigonia (Mollusca: Bivalvia: Trigonioida) Pleuromya (Mollusca: Bivalvia: Anomalodesmata) Pholadomya (Mollusca: Bivalvia: Anomalodesmata) Astarte (Mollusca: Bivalvia: Veneroida) Thalassinoides (trace-fossil) with burrowing Glyphaea (Crustacea) inside e m n

79 Shelly Lime Mud Community

The sediment in between the shells of this community consists of mud grade calcium carbonate (micrite), and at the time of deposition the substrate is likely to have had a pasty consistency. In view of the richness of the epifauna, the bottom was unlikely to have been very soft and the assemblage of sediment and fauna represents a shallow, but quiet water, bay association comparable to modern equivalents from Florida and the Bahamas.

Much of the shell material has been infested by microscopic-boring algae and fungi; it is likely that the water was well lit and that the sediment surface supported a large quantity of lowly plant life which is not now preserved. The delicate stromatolic structures produced by some algal growths would have been totally obliterated by the burrowing activities of the infauna.

The epifauna consisted of grazers, suspension feeders and carnivores, while the infauna was dominated by suspension feeders. On the surface, the nerineid Fibula grazed or ingested surface detritus to digest the bacteria lining the sediment, while the mobile regular echinoid Acrosalenia scavenged or preyed upon soft-bodied animals that lived near the sediment surface. The teeth marks from echinoids sometimes scar shell material but the echinoids themselves are normally only represented as fossils by isolated spines and plates. These stable surfaces, covered by water with a relatively low turbidity, supported clumps of terebratulid brachiopods and in our diagram we have depicted Epithyris. Bromley and Surlyk (1973) have shown that brachiopod pedicles may etch a circle of dots a millimetre or more in diameter on other shells, each dot etched by a separate strand of the pedicle. These attachment scars may be commonly found on shell fragments and other epithyrids in this facies.

Epifaunal bivalve suspension feeders were represented by the byssally attached or free-living Camptonectes and Pseudolimea, while the streamlined Costigervillia probably swung freely, possibly attached to anchored gorgonians that have left no fossil record. The oysters (Liostrea) were attached by a carbonate cement to shell and other hard debris upon the substrate.

Within the sediment, helping to destroy the internal structure, were a variety of invertebrates. Modiolus were sometimes common, living semi-infaunally with a byssal attachment to buried shell debris, while fat sluggish burrowers (Anisocardia and Sphaeriola) filter fed through short siphons. Clusters of terebellid worms extended their filamentous tentacles into the water, and lined their tubes below this crown on tentacles with shell and fish-scale debris. Sometimes their tubes (trace fossils) were removed and redeposited parallel to the current.

Crustaceans also produced complex Thalassinoides burrow

Thalassinoides

Fig. 79 Shelly Lime Mud Community a Epithyris (Brachiopoda: Articulata: Terebratulida)

b Camptonectes (Mollusca: Bivalvia: Pterioida — pectinid)

c Liostrea (Mollusca: Bivalvia: Pterioida — oyster)

d Pseudolimea (Mollusca: Bivalvia: Pterioida)

e Costigervillia (Mollusca: Bivalvia: Pterioida)

f Acrosalenia (Echinodermata: Echinozoa)

g Modiolus (Mollusca: Bivalvia: Mytiloida)

h Fibula (Mollusca: Gastropoda: Archaeogastropoda)

i Thalassinoides (trace-fossil) with burrowing Glyphaea (Crustacea)

j Anisocardia (Mollusca: Bivalvia: Veneroida)

k terebellid worms (Annelida)

1 gorgonian (Coelenterata: Octocorallia) hypothetical in this reconstruction

Fig. 79 Shelly Lime Mud Community a Epithyris (Brachiopoda: Articulata: Terebratulida)

b Camptonectes (Mollusca: Bivalvia: Pterioida — pectinid)

c Liostrea (Mollusca: Bivalvia: Pterioida — oyster)

d Pseudolimea (Mollusca: Bivalvia: Pterioida)

e Costigervillia (Mollusca: Bivalvia: Pterioida)

f Acrosalenia (Echinodermata: Echinozoa)

g Modiolus (Mollusca: Bivalvia: Mytiloida)

h Fibula (Mollusca: Gastropoda: Archaeogastropoda)

i Thalassinoides (trace-fossil) with burrowing Glyphaea (Crustacea)

j Anisocardia (Mollusca: Bivalvia: Veneroida)

k terebellid worms (Annelida)

1 gorgonian (Coelenterata: Octocorallia) hypothetical in this reconstruction systems within the sediment, sometimes descending to considerable depths (more than lm) and may well have caused the original sediment surface to have been irregularly marked by mounds of excavated material (not shown); the burrows were often partially filled with faecal pellets called Favreina.

This fauna exhibits the same characteristics as modern assemblages of bay or lagoonal environments in most respects, except for the greatly decreased diversity of modern brachiopod faunas. Restricted circulation probably prevented the more stenohaline ammonites and belemnites from making an appearance while these sediments were being deposited at various times over the Midlands area in England.

80 Muddy Lime Sand Community

Many of the Middle Jurassic limestones contain carbonate mud (micrite) and there are many gradations between those that contain pure micrite and those that contain detrital clay. The cleaner and less muddy limestones also show many gradations both laterally and vertically into less well sorted muddy sediments, often within one quarry exposure.

Much of the micrite of these Middle Jurassic limestones is assumed to have been formed in the same way that the majority of modern micrite is produced, namely by the degradation of algal material, probably red and green algae with a very low preservation potential.

The environment probably consisted of an area covered mainly by mud deposits into which lime sand was periodically introduced. The sandier layers promoted a more stable substrate condition which probably accounts for the increased infaunal diversity by comparison with the Shelly Lime Mud Community described earlier.

The epifauna was similar to that of the latter community (Fig. 79) containing cemented oysters and byssally attached Campton-ectes. It also included Gervillella and Isognomon, again byssally attached to shell fragments, and living as fairly closely attached suspension feeders. No clusters of terebratulids are found, but instead there are isolated brachiopods which were the only forms able to live there. It is probable that the slightly increased rates of sand accumulation were enough to inhibit the development of brachiopod clusters.

Gastropods such asAlaria grazed over the surface, again feeding upon presumed algal films or upon bacterial slimes around deposited grains. Within the sediment, the diversity of infaunal suspension feeders was quite high. Deep-burrowing Homomya

Cererithyris

b Camptonectes (Mollusca: Bivalvia: Pterioida — pectinid)

c Epithyris (Brachiopoda: Articulata: Terebratulida)

d Isognomon (Mollusca: Bivalvia: Pterioida)

e Gervillella (Mollusca: Bivalvia: Pterioida)

f Homomya (Mollusca: Bivalvia: Anomalodesmata)

g Trigonia (Mollusca: Bivalvia: Trigonioida)

h Pleuromya (Mollusca: Bivalvia: Anomalodesmata)

i Pseudotrapezium (Mollusca: Bivalvia: Veneroida)

j Pholadomya (Mollusca: Bivalvia: Anomalodesmata)

k Alaria (Mollusca: Gastropoda: Mesogastropoda)

1 serpulids (Annelida)

appeared (we have had to show it at shallow depths) associated with Pleuromya while the shallow-burrowing and very mobile trigonids and Pseudotrapezium took advantage of the slightly coarser and more stable substrate.

This assemblage occurs in many horizons of the Middle Jurassic in the English Midlands; however towards the west and south, where salinity was more normal, some ammonites and nautiloids are also present.

Rhynchonella

b Liostrea (Mollusca: Bivalvia: Pterioida — oyster)

c serpulids (Annelida)

d bryozoan (Bryozoa: Ectoprocta)

e Exogyra (Mollusca: Bivalvia: Pterioida — oyster)

f Lithophaga (Mollusca: Bivalvia: Mytiloida)

g Radulopecten (Mollusca: Bivalgia: Pterioida — pectinid) h Rhynchonella (Brachiopoda: Articulata: Rhynchonellida) i Dictyothyris (Brachiopoda: Articulata: Terebratulida)

j ornithellids (Brachiopoda: Articulata: Terebratulida) k Avonothyris (Brachiopoda: Articulata: Terebratulida)

1 tubular worm (Annelida) m Oxytoma (Mollusca: Bivalvia: Pterioida)

n Liostrea (Mollusca: Bivalvia: Pterioida — oyster)

81 Clear Water, Firm Substrate Communities

Stable and firm substrates developed in a variety of ways and at many times in the shallow marine and marginal marine areas of central and southern England. These substrates offered attachment sites for a broad spectrum of cementing, encrusting and boring organisms that were otherwise restricted in their distribution to

Boring Mollusc

5cm.

the surfaces of scattered skeletal debris in other environments. Indeed, the recognition of fossil borings and a diverse array of associated encrusting organisms is usually a strong indication that the sea bottom was firm and the waters above clear of large amounts of detrital materials at the time of deposition.

Slow rates of deposition allowed consolidation of the substrate, and sometimes the interstitial precipitation of a carbonate cement led to the formation of hardgrounds which, after differential erosion of uncemented sediment, provided irregular and even undercut surfaces for colonization. In these cases, the faunas inhabiting the lower surfaces were frequently different from those inhabiting the more exposed upper surfaces (Palmer and Fursich, 1974). Unfortunately, many of the characteristic genera were extremely small organisms that are too tiny to be represented on our diagram.

The exposed upper surface epifauna included foraminifera like Nubeculinella (not shown) that inhabited both hardened substrate and shell debris, and certain types of serpulid worms. These, like many of the organisms, lived as suspension feeders. Cemented oysters (Liostrea, Exogrya and Lopha) were also present, while other suspension-feeding bivalves such as Radulopecten and Pseudolimea were attached by byssal threads. Surfaces and shell debris were often encrusted by various types of bryozoans and these, like the serpulids and foraminiferida frequently grew upon living shell material.

Terebratulides and rhynchonellid brachiopods flourished in the clear waters and often formed 'nests' comprising hundreds of individuals. This community includes thecideidine brachiopods, which lived on the undersurfaces of overhangs and within cavities, but are too small to be shown.

The largest and most spectacular organisms that attached themselves to these hardened surfaces were crinoids, and even where the crinoid stalks have been removed by later erosion, the hold fasts of these creatures may still be preserved, themselves encrusted by bryozoans and foraminiferida.

Beneath overhangs and within fissures in the sediment, a nestling fauna became established, often containing different serpulids, sponges and bryozoa and these organisms either preferred the dark or tolerated less turbulent conditions than the organisms on upper surfaces.

The infauna too was markedly different from that found upon softer bottoms. If the substrate was not actually hard, but was merely stiff, normal burrowing infaunas were developed, but when it became cemented only specialized boring organisms thrived. Of these, the most characteristic was the bivalve Lithophaga which bored into both upper and lower surfaces. They produced 'crypt' shaped cavities, particularly in carbonate substrates, and fed as suspension feeders. Other borers included worm-like organisms which produced straight and inclined 'Trypanites' borings and which in our figure are drawn as worms with filamentous tentacles.

When the Middle Jurassic rests directly on older carbonates (for instance the Carboniferous Limestone in Somerset) the fauna may be identical to that of a hardground. However, if the older rocks are not calcareous, as in Normandy (where the Middle Jurassic rests on Lower Palaeozoic sandstones and shales), only the encrusting components of this hardground community may be present.

There are all gradations between a hardground community and Diverse Calcarenite Community (Fig. 78). A typical intermediate assemblage occurs in the Boueti Bed of Dorset, where the substrate of shell debris became firm enough for the attachment of abundant brachiopods (notably Goniorhynchia boueti, Avonothyris, Ornith-ella and Dictyothyris). Most of these brachiopods were encrusted by serpulids and bryozoans and byssally-fixed bivalves also flourished.

Figs. 81a and 81b show a flourishing hardground assemblage which was inundated by a clay influx. Clay influxes were the result either of increased rates of terrigenous deposition or of the sudden arrival of airborne tuffs from distant volcanoes.

82 Lime Mud Coral Community

In environments where salinities were normal or only slightly hypersaline, patch coral communities developed. The local colono-nization and proliferation of corals probably required relatively clear water with not too much suspended detritus. Some modern corals are able to cope with modest and temporary incursions of sediment into their habitats but continuous and copious amounts of turbid water normally cause their eventual demise.

Middle Jurassic coral faunas in central England are of low diversity, rarely containing more than three genera of colonial corals. Isastrea was the commonest genus in either its rounded 'Isastrea' form or in the branching form that has often been termed 'Thamnasteria'. The colonies of corals, like those of their

Coral Polyp Diagram

Fig. 82 Lime Mud Coral Community a rounded Isastrea (Coelenterata: Anthozoa: Scleractinia)

b branching Isastrea (Coelenterata: Anthozoa: Scleractinia)

c Eonavicula (Mollusca: Bivalvia: Arcoida)

d Plagiostoma (Mollusca: Bivalvia: Pterioida)

e Lithophaga (Mollusca: Bivalvia: Mytiloida)

f Pleurotomaria (Mollusca: Gastropoda: Archaeogastropoda)

g Thalassinoides (trace-fossil) made by Glyphaea (Arthopoda: Crustacea)

h Ornithella (Brachiopoda: Articulata: Terebratulida)

i serpulid (Annelida)

j rhynchonellids (Brachiopoda: Articulata: Rhynchonellida)

k Entolium (Mollusca: Bivalvia: Pterioida — pectinid)

Fig. 82 Lime Mud Coral Community a rounded Isastrea (Coelenterata: Anthozoa: Scleractinia)

b branching Isastrea (Coelenterata: Anthozoa: Scleractinia)

c Eonavicula (Mollusca: Bivalvia: Arcoida)

d Plagiostoma (Mollusca: Bivalvia: Pterioida)

e Lithophaga (Mollusca: Bivalvia: Mytiloida)

f Pleurotomaria (Mollusca: Gastropoda: Archaeogastropoda)

g Thalassinoides (trace-fossil) made by Glyphaea (Arthopoda: Crustacea)

h Ornithella (Brachiopoda: Articulata: Terebratulida)

i serpulid (Annelida)

j rhynchonellids (Brachiopoda: Articulata: Rhynchonellida)

k Entolium (Mollusca: Bivalvia: Pterioida — pectinid)

modern relatives, were made up of a number of carnivorous polyps. In between the coral fronds, 'nestling' in shell debris, were byssate arcid bivalves such as Eonavicula, which, like other arcids, probably had fairly short siphons and lived as a suspension feeder in the protected niches between the corals. There were also free-living or byssally attached limids (Plagiostoma) nesting beneath coral heads.

The coral skeletons themselves provided firm substrates in which Lithophaga bored, and in our sketcA we have shown the nearer specimen cut away to reveal the living lithophages boring into the dead under-surfaces of the living coral heads.

The sediment consisted partly of shell debris derived from the degradation of local coral and other shell material. The rest was composed of fine grained micrite taken out of suspension by the molluscs and corals and deposited initially as pellets that are no longer preserved. The sediment thus had a muddy surface, and in places a Shelly Lime Mud Community (Fig. 79) became established (not shown here). Grazing gastropods, like Pleurotomaria, lived upon the surface, while within the soft sediment crustaceans such as the shrimp Glyphaea produced Thalassinoides burrows and Favreina faecal pellets.

In the coral rubble, too small to be shown in the picture, were encrusting organisms typical of hardgrounds. These included small calcareous sponges, serpulids, thecideidine brachiopods and bry-ozoans.

Coral communities with lime mud or clay occur in the Middle Jurassic of the Cotswold Hills and Oxfordshire.

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