108 Palaeocene Marine Sand Community

The depositional environment of this community was an offshore shelf sea with a water depth of probably not more than 50m and a substrate of fine glauconitic sand. This sand community was dominated by infaunal bivalves; various levels within the sediment were occupied by suspension feeding bivalves, while large, but fairly sluggish, Arctica morrisi was abundant near the surface of the sand; and the patchily distributed small Corbula regulbiensis was buried just beneath it. Dosiniopsis, with larger siphons, lived lower down in the substrate, and the sedentary Panopea intermedia with large, fused siphons, occurred in the deepest levels. In some sands Corbula was the dominant bivalve, often in small aggregations or 'nests'. Gastropods were much less common, but included the detritus-feeding Aporrhais triangulata, the semi-infaunal Euspira which preyed mainly upon bivalves, and the whelk Siphonalia, which may have fed on polychaete worms.

The oldest Cenozoic rocks exposed in Britain are fine glaucon-itic sands with thin clay bands (Thanet Beds); they are widespread in the London Basin and extend as far as Norfolk, and, although largely unfossiliferous, they do contain communities like the one described here.

Marine Infauna Soft Sediment Communities

Fig. 108 Palaeocene Marine Sand Community a Arctica morrisi (Mollusca: Bivalvia: Veneroida)

b Panopea intermedia (Mollusca: Bivalvia: Myoida)

c Corbula regulbiensis (Mollusca: Bivalvia: Myoida)

d Dosiniopsis bellovacina (Mollusca: Bivalvia: Veneroida)

e Siphonalia subnodulosa (Mollusca: Gastropoda: Neogastropoda)

f Euspira bassae (Mollusca: Gastropoda: Mesogastropoda)

g Aporrhais triangulata (Mollusca: Gastropoda: Mesogastropoda)

109 Estuarine Sand Community

The community illustrated is typical of early Eocene low salinity, intertidal or shallow sublittoral, estuarine areas with muddy sand substrates. A characteristic feature of this type of environment is the rather low diversity and the high abundance of animals. The dominant element in this community was the shallow-burrowing suspension-feeding bivalve Corbicula, which occurred in huge numbers. Also common was the oyster Ostrea bellovacina. Gastropods were abundant, and the common forms belong to the superfamily Cerithiacea. They included Brotia melanoides, Melanopsis antidiluviana and Tympanotonos funatus; these species were characterized by large populations which were extremely variable morphologically. Forms related to these genera abound today in subtropical and tropical estuarine mud flats, where they occur in densities as high as 2—3,000 per square metre, and feed upon organic detritus, diatoms and filamentous algae from the surface layer of sediment. Fish remains, particularly of sharks and rays, can be abundant in this deposit. Comparable communities, though with different genera, are found today in subtropical and tropical estuaries in south-east Asia and West Africa, environments characterized by fluctuating salinities, soft substrates and high productivity.

The type of community illustrated is recurrent throughout the British Cenozoic deposits, and examples can be found in many European formations. This reconstruction is from the Blackheath Beds which with the Oldhaven Beds, form a sequence of largely sandy sediments overlying the Woolwich and Reading Beds in south-east England.

110 Eocene Marine Mud Community

Although there are some sands at the base, most of the London Clay in the London Basin is formed from a mud sediment deposited in a marine basin open to the north and east, with a water depth of about 150m. The fauna of the Lower London Clay has a fairly low diversity and fossils are often extremely scarce, but some levels show a more abundant fauna; this community is based upon such a horizon at Aveley, Essex (Kirby, 1974).

The dominant species were the deposit-feeding Nucula bovoer-banki, the siphonate suspension feeder Nuculana amygdaloides, and two species of the lucinoid Thyasira which were suspension feeders occupying semi-permanent burrows and connected to the

Fig. 109 Eustarine Sand Community a Corbicula cuneiformis (Mollusca: Bivalvia: Veneroida) b Corbicula tellinoides (Mollusca: Bivalvia: Veneroida) c Ostrea bellovacina (Mollusca: Bivalvia: Pterioida) d Brotia melanoides (Mollusca: Gastropoda: Mesogastropoda) e Melanopsis antidiluviana (Mollusca: Gastropoda: Mesogastropoda) f Theodoxus (Mollusca: Gastropoda: Archaeogastropoda) g Tympanotonus funatus (Mollusca: Gastropoda: Mesogastropoda)

Gastropods Live Neoegene

surface by an inhalent tube. The scaphopod Laevidentalium, which probably fed mainly upon benthic foraminifera, was also common. Most of the gastropods were semi-infaunal species, which spent much of their time ploughing through the sediment just below the surface. Species of the family Turridae were particularly common; many of them were equipped with highly developed dart-like radula teeth and a toxic venom with which they immobilized their prey. The Turridae feed today mainly upon poly-chaetes, which are swallowed whole. The gastropod Euspira probably fed largely on other molluscs, boring holes in their shells; Ficus and Galeodea probably ate echinoderms and Streptolathyrus probably bivalves. Solariaxis has living relatives which live in associ-tion with, and feed upon, various soft-bodied coelenterates such as sea anemones. The tubes secreted by the polychaete Ditrupa are common throughout the London Clay; although the genus is still extant, very little is known about its living habits, but it was probably a surface-living deposit feeder.

Sunken drifted wood is abundant in the London Clay, and this material, including the palm Nipa, is famous as a diverse tropical flora (Davis and Elliott, 1958). The pieces of drifted wood often contain the tubes and valves of the wood-boring bivalve Teredo, the ship worm. Pteriid bivalves and the crinoid Balano-crinus subbasaltiformis were probably also attached to this wood.

London Clay sediments contain large numbers of otoliths from teleost fishes as well as the teeth of sharks and rays. Benthic invertebrates are the major part of the food for many fish; the behaviour and the number of molluscs, polychaetes and echino-derms would have been controlled largely by predatory fish.

Assemblages similar to this one, containing abundant Thyasira and nuculid bivalves, are known today from deep, very quiet, basinal situations on the continental shelf, floored with black mud, such as the deep basins in the northern North Sea and the deeper parts of Scottish sea lochs.

Fig. 110 Eocene Marine Mud Community a Odontaspis (Vertebrata:

Chondichthyes) b Synodus (Vertebrata:

Osteichthyes) c Dirematichthys (Vertebrata

Osteichthyes) d Nucula bowerbanki (Mollus

Bivalvia: Palaeotaxodonta) e Nuculana amygdaloides (Mollusca: Bivalvia: Palaeotaxodonta) f Pteria papyracea (Mollusca:

Bivalvia: Pterioida) g Thyasira angulata (Mollusca

Bivalvia: Veneroida) h Thyasira goodhalli (Mollusca: Bivalvia: Veneroida) i Laevidentalium nitens

(Mollusca: Scaphopoda) j Cimomia imperialis (Mollus Cephalopoda: Nautiloidea) k Turricula teretrium (Mollus Gastropoda: Neogastropod; 1 Streptolathyrus cymatodis (Mollusca: Gastropoda: Neogastropoda) m Fisus (Mollusca: Gastropod

Mesogastropoda) n Galeodea gallica (Mollusca: Gastropoda: Mesogastropod: o Solariaxis gallica

(Mollusca: Gastropoda: Mesogastropoda) p Euspira glaucinoides (Mollusca: Gastropoda: Mesogastropoda) q Ditrupa plana (Annelida) r Teredo (Mollusca: Bivalvia: Myoida)

Gastropod MorphologyNautiloidea Fossils

Fig. Ill Eocene Shallow Marine Clay Communities a Nuculana amygdaloides (Mollusca: Bivalvia: Palaeotaxodonta)

b Pitar sulcatarius (Mollusca: Bivalvia: Veneroida)

c Arctica morrisi (Mollusca: Bivalvia: Veneroida)

d Pinna affinis (Mollusca: Bivalvia: Pterioida)

e Ostrea ovicina (Mollusca: Bivalvia: Pterioida)

f Aporrhais sowerbyi (Mollusca: Gastropoda: Mesogastropoda) g Orthochetus elongatus (Mollusca: Gastropoda: Mesogastropoda)

111 Eocene Shallow Marine Clay Communities

The general character of the Eocene upper London Clay communities is illustrated by this Aporrhais-dominated community. It shows a much more diverse and more numerous benthic fauna than the lower London Clay, with a much shallower water aspect. Amongst the epifaunal species there was a wide variety of gastropods, the most abundant of which was Aporrhais sowerbyi. This had a thick and heavy shell with a broad apertural lip which extended to become digitate processes. Recent species of Aporrhais often live partially buried; the broad lip and process separate the inhalent and exhalent respiratory currents. It probably fed upon diatoms and algal detritus. Turritella sulcifera, also common, was an infaunal suspension feeder: detritus particles were collected from the gills and trapped in a mucous string which was then pulled into the mouth. Tibia and Orthochetus were also vegetarian, while Crenaturricula and Pleurotoma probably ate polychaetes. The polychaete Ditrupa is again abundant.

The bivalves include Ostrea ovicina and a large number of infauna, mostly suspension feeders, like the byssate Pinna, which, although sedentary, can withdraw deep into its shell and rapidly repair any damage to the shell's exposed posterior. Other common bivalves include the sluggish, shallow-burrowing Arctica and the more mobile, siphonate Nuculana and Pitar.

Teleost fish and rays were abundant, the latter feeding mainly upon infaunal bivalves.

Similar communities to this may be found living today in shallow shelf environments on muddy sands at depths of 10—50m in warm temperate to subtropical areas.

The London Clay of the Hampshire Basin and the higher parts of the succession in the London Basin represent much shallower water conditions than the previous community. There is great variation in the fossil assemblages from bed to bed and this probably represents a mosaic of bottom communities which developed in a shallow shelf sea 20—50m deep. The community illustrated is one of the many diverse assemblages encountered at Shinfield (James et al. 1974).

h Turritella sulcifera (Mollusca: Gastropoda: Mesogastropoda)

i Ditrupa plana (Annelida)

j Tibia lucida (Mollusca: Gastropoda: Mesogastropoda)

k Ficus londini (Mollusca: Gastropoda: Mesogastropoda)

1 Pleurotoma (Mollusca: Gastropoda: Neogastropoda)

m Streptolathyrus cymatodis (Mollusca: Gastropoda: Neogastropoda)

n ray (Vertebrata: Chondrichthyes)

Gastropods Anatomy

Fig. ll2 Eocene Sublittoral Sandy Clay Community Pinna affinis (Mollusca: Bivalvia: Pterioida) Arctica planata (Mollusca: Bivalvia: Veneroida) Pitar sulcatarius (Mollusca: Bivalvia: Veneroida) Panopea intermedia (Mollusca: Bivalvia: Myoida) Thracia oblata (Mollusca: Bivalvia: Anomalodesmata) Ostrea tabulata (Mollusca: Bivalvia: Pterioida) Ditrupa (Annelida)

Hoploparia (Arthropoda: Crustacea: Malacostraca) i Graphularia (Coelenterata: Anthozoa: Octocorallia) j Acroscalpellum (Arthropoda: Crustacea: Cirripedia) k Holocentrus (Vertebrata: Osteichthyes) l Caesio (Vertebrata: Osteichthyes) m Apogon (Vertebrata: Osteichthyes)

112 Eocene Sublittoral Sandy Clay Community

Conspicuous elements here are groups of the infaunal, sedentary bivalve, Pinna affinis, which today may be abundant in sheltered, stable substrates, particularly those colonized by marine grasses. The long-siphoned, deep-burrowing bivalves Panopea and Thracia are also common, with shallow burrowers represented by Arctica and Pitar. No deposit-feeding bivalves occur in this community.

Epifaunal animals are the oyster, the large barnacle Acroscal-pellum, the worm Ditrupa, the lobster Hoploparia, and the erect, stick-like gorgonacean Graphularia.

This third London Clay community is found in the Hampshire Basin at Bognor.

113 Eocene Glauconite Sandy Clay Community

The community inhabited a nearshore, shallow water shelf environment with a water depth of between 10—30m. Like most other Cenozoic communities it was dominated numerically by shallow-burrowing, suspension-feeding bivalves; and important elements were the large thick-shelled Venericardia planicosta, Glycymeris pulvinata, and Crassatella, none of which had siphons. Slightly deeper siphonate burrowers were Callista laevigata and Macrosolen. Other common infaunal animals were the scaphopod Dentalium and the ciliary-feeding gastropods Mesalia and Turritella. Euspira probably preyed upon shallow-burrowing bivalves. Amongst the epifaunal elements in the community were the ciliary-feeding, limpet-like Calyptraea, which probably lived in groups attached to empty shells. Area biangula, a suspension-feeding bivalve, was byssally attached, probably again to empty shells. The large gastropod Athleta probably preyed mainly upon bivalves, pulling the valves apart with its large extensible foot. The highly ornamented gastropod Pterynotus fed by means of holes drilled through the shells of epifaunal and shallow-burrowing bivalves. Hemipleuro-toma probably ate infaunal polychaetes. The herbivorous gastropod Campanile cornucopiae reached lengths of 50cm, and was one of the largest gastropods ever to live. The solitary ahermatypic coral, Turbinolia, probably lived partially embedded in the sediment.

Abundant teeth of sharks and rays, teleost fish otoliths and occasional cuttlebones, indicate a diverse nektonic fauna, largely dependent upon the benthic invertebrates for food.

The Bracklesham Beds, exposed in the Hampshire Basin and the Isle of Wight, consist of two main facies: one of glauconitic sandy clays with a marine fauna, and the other of laminated clays,

Fig. 113 Eocene Glauconite Sandy Clay Community Glycymeris pulvinata (Mollusca: Bivalvia: Arcoida) Area biangula (Mollusca: Bivalvia: Arcoida) Crassatella sowerbyi (Mollusca: Bivalvia: Veneroida) Venericardia planicosta (Mollusca: Bivalvia: Veneroida) Callista laevigata (Mollusca: Bivalvia: Veneroida) Corbula gallica (Mollusca: Bivalvia: Myoida) Dentalium (Mollusca: Scaphopoda)

Turritella conoides (Mollusca: Gastropoda: Mesogastropoda) Mesalia (Mollusca: Gastropoda: Mesogastropoda) Calyptraea aperta (Mollusca: Gastropoda: Mesogastropoda) Athleta spinosa (Mollusca: Gastropoda: Neogastropoda) Hemipleurotoma (Mollusca: Gastropoda: Neogastropoda) Macrosolen hollowaysii (Mollusca: Bivalvia: Veneroida) Nemocardium (Mollusca: Bivalvia: Veneroida) Campanile conucopiae (Mollusca: Gastropoda: Mesogastropoda) Euspira (Mollusca: Gastropoda: Mesogastropoda) Pterynotus (Mollusca: Gastropoda: Neogastropoda) Turbinolia (Coelenterata: Anthozoa: Scleractinia) Sepia (Mollusca: Cephalopoda: Coleoidea — cuttlefish) Squatina (Vertebrata: Chondrichthyes)


sands and lignite beds of deltaic and marsh origin (Fisher, 1862). Palaeogeographic reconstructions show that at this time the Isle of Wight and the Hampshire areas were at the edge of a shallow shelf sea into which a river system, flowing from the west and northwest, deposited deltaic sediments. During Bracklesham times an embayment formed in the shoreline, with shallow clear warm water, and perhaps sea-grass beds. Later, the embayment silted up, and marshland probably developed all around. The foraminiferal evidence suggests that sea water temperatures were around 18° C and salinities normal or very slightly reduced.

114 Eocene Shelf Silt and Sand Community

Bivalves were very common here, in particular the shallow-burrowing suspension feeders such as the large, thick-shelled Venericardia planicostata, Crassatella sulcata, Glycymeris deletus and the small siphonate Corbula cuspidata. Deposit feeders were, with the exception of Nucula, infrequent. The most abundant epifaunal bivalve was Chama squamosa; although young individuals were cemented to pieces of shell debris, the adults may often have been free-living. Although not illustrated, oysters (Ostrea plicata), were also common. Other suspension-feeding animals were the gastropod Turritella and the ahermatypic solitary coral Turbinolia. Gastropods were particularly abundant, and very diverse. Apart from Turritella, the vast majority were predators; particularly important were Athleta and Volutocorbis. Comparable forms living today have a large foot which envelops bivalve prey and pulls the valves apart. Natica is a well known predator, feeding upon other molluscs by drilling holes. Other carnivorous gastropods, including Conus and Bathytoma, were particularly common and diverse; by means of a highly developed dart-like radula they injected a toxic salivary secretion into the (usually polychaete) prey which is then swallowed whole.

The nektonic fauna was diverse, with sharks and rays being particularly common.

Assemblages of organisms in the Barton Beds are extremely diverse, and a large number of species, especially of mollusca, have been recorded (Burton, 1933). The communities are all characteristic of nearshore to offshore shelf habitats developed on silt and sandy substrates; the water depth throughout most of the beds was probably not more than about 50m, and foraminiferal evidence suggests a sea temperature of about 16—18° C; however, the mol-luscan evidence points to a rather higher temperature. Bed by bed studies have shown quite a large variation in the faunal composition of the communities, but the assemblage illustrated here from the well known Chama bed shows the general character of the communities.

Mesalia Fossils

Fig. 114 Eocene Shelf Silt and Sand Community a Turbinolia (Coelenterata: Anthozoa: Scleractinia)

b Athleta spinosus (Mollusca: Gastropoda: Neogastropoda) c Volutocorbis scabricula (Mollusca: Gastropoda: Neogastropoda)

d Pollia labiata (Mollusca: Gastropoda: Neogastropoda)

e Turritella imbricataria (Mollusca: Gastropoda: Mesogastropoda)

f Bittium semigranosum (Mollusca: Gastropoda: Mesogastropoda)

g Conus scrabicula (Mollusca: Gastropoda: Neogastropoda)

h Chama squamosa (Mollusca: Bivalvia: Veneroida)

i Crassatella sulcata (Mollusca: Bivalvia: Veneroida)

j Venericardia planicosta (Mollusca: Bivalvia: Veneroida)

k Nucula similis (Mollusca: Bivalvia: Palaeotaxodonta)

1 Glycymeris deletus (Mollusca: Bivalvia: Arcoida) m Corbula cuspidata (Mollusca: Bivalvia: Myoida)

n Natica ambulacrum (Mollusca: Gastropoda: Mesogastropoda)

o Bathytoma turbida (Mollusca: Gastropoda: Neogastropoda)

Teredo Fossil

115 Palaeogene Brackish Water Community

The community illustrated was of low diversity, but had large populations of a few species. The substrate surface was dominated by elongate high-spired gastropods like Batillaria and Potámides. These fed upon the surface layer of the sediment, extracting food

Sediment Epifauna

Fig. 115 Palaeogene Brackish Water Community a Batillaria ventricosa (Mollusca: Gastropoda: Mesogastropoda)

b Batillaria coricava (Mollusca: Gastropoda: Mesogastropoda)

c Potamides vagus (Mollusca: Gastropoda: Mesogastropoda)

d Globularia grossa (Mollusca: Gastropoda: Mesogastropoda)

e Theodoxus concavd (Mollusca: Gastropoda: Archaeogastropoda)

f Ostrea plicata (Mollusca: Bivalvia: Pterioida)

g Corbicula obovata (Mollusca: Bivalvia: Veneroida)

h sturgeon — Acipenser (Vertebrata: Osteichthyes)

i polychaete burrows (Annelida)

Fig. 115 Palaeogene Brackish Water Community a Batillaria ventricosa (Mollusca: Gastropoda: Mesogastropoda)

b Batillaria coricava (Mollusca: Gastropoda: Mesogastropoda)

c Potamides vagus (Mollusca: Gastropoda: Mesogastropoda)

d Globularia grossa (Mollusca: Gastropoda: Mesogastropoda)

e Theodoxus concavd (Mollusca: Gastropoda: Archaeogastropoda)

f Ostrea plicata (Mollusca: Bivalvia: Pterioida)

g Corbicula obovata (Mollusca: Bivalvia: Veneroida)

h sturgeon — Acipenser (Vertebrata: Osteichthyes)

i polychaete burrows (Annelida)

material, probably diatoms and blue-green algae, from the surface algal mat. Like similar gastropods in modern communities, they exhibited an extremely variable morphology which is thought to be associated with the unpredictability of the estuarine environment; if a traumatic change in physical conditions occurred, at least a part of a very variable population would have a chance of survival. This theory assumes that morphological variation reflects variation in physiological tolerance.

Other gastropods included Theodoxus, an algal grazer, and Globularia, a predator on infaunal bivalves. Oysters are common at some horizons, and are typical today of quiet estuarine conditions. The shallow-burrowing, suspension-feeding bivalve Corbicula obovata dominates the infauna of the community.

Like its more modern relative found in the Caspian Sea, the sturgeon Acipenser probably ate mainly polychaete worms, small crustacea and molluscs.

Brackish water communities are common throughout the Headon, Osborne and Hamstead Beds of southern England, where they are interspersed with deltaic, flood plain, lacustrine and freshwater environments.

116 Palaeogene Freshwater Community

In some freshwater lake environments, the sediment is largely composed of the remains of the alga Chara and of freshwater snails. The lake bottom was colonized by extensive growths of this branching alga, and the tiny, spirally ornamented, reproductive bodies or nucules are easily recognizable in the sediments. Chara is the largest of the freshwater green algae, and forms erect branching growths, often in large, monospecific stands, in calcium-rich waters. The whole plant becomes encrusted with calcium carbonate, which, on the death of the plant, makes a considerable contribution to sediment formation.

The dominant elements in the fauna are freshwater snails, mainly the forms Galba and Planorbina which fed upon algal films on freshwater plants. The smaller Viviparus was a ciliary-feeder. The shallow-burrowing, suspension-feeding bivalve Corbicula obtusa is the only abundant component of the infauna.

Vegetation, insects and invertebrates from the surrounding land indicate subtropical to tropical temperatures. Local abundance of land snails indicates emergence from the water in some places.

The Bembridge limestone of the Isle of Wight is a freshwater deposit averaging about 3m in thickness which occurs within a sequence of brackish estuarine, flood plain and lacustrine sediments. The marine communities of the Osborne and Upper Headon Beds are generally similar to the Headon Beds Community illustrated (H5), with abundant species characteristic of reduced salinity environments.

Fig. 116 Palaeogene Freshwater Community a Galba longiscata (Mollusca: Gastropoda: Pulmonata)

b Planorbina discus (Mollusca: Gastropoda: Pulmonata)

c Viviparus minutus (Mollusca: Gastropoda: Mesogastropoda)

d Corbicula obtusa (Mollusca: Bivalvia: Veneroida)

e Chara (Algae)

f turtle (Vertebrata: Reptilia: Anapsida)

g freshwater fish (Vertebrata: Osteichthyes)

Was this article helpful?

0 0


  • ettore
    Are PTERYNOTUS carnivore or herbivore?
    9 years ago
  • Layton Martin
    Why ornamented gastropod?
    9 years ago

Post a comment