Reef Communities

The following account of Reef Communities (Nos. 44—48) ofmid-Vis'ean (Asbian) age has been contributed by Mr. D. Mundy.

Mid-Visean reef limestones are generally associated with a marginal situation between a stable shallow water 'shelf environment and a deeper subsiding'basin' environment, and are usually characterized by a marked contemporary topographic relief. This can take the form of discrete 'knolls' (as in the 'Craven reef belt') or a continuous 'apron' (as in north Derbyshire). In both cases the reef slopes now present represent palaeoslopes; during the later history of the reef these slopes could span a considerable depth range (122m near Castleton in Derbyshire, England).

The reef limestones are characteristically poorly bedded and the typical matrix between the fossils is a light grey calcilutite (the 'clotted' calcilutite of Schwarzacher, 1961). Bioclastic limestones of various grain sizes are also present. Fabrics once known as 'reef tufa' involving fibrous calcite (the 'radiaxial fibrous calcite' of Bathurst, 1959) are common, and have been interpreted as modified early diagenetic cements filling cavities.

The reef communities are well known for their diversity of taxa and abundance of individuals. However, the richly fossilifer-ous areas on the reef are often separated by limestones with a poor macrofossil content.Considered as a whole the fauna is dominantly benthic, with a great diversity of brachiopods, bivalves, gastropods and cephalopods,the occasional rostroconch and rare scaphophods. Bryozoans are common, particularly fenestrate, pinnate, and encrusting forms. Corals, including solitary and colonial rugose, tabulate and heterocorals are fairly well represented. Echinoderms, particularly crinoid remains, are abundant, but calices are rarely preserved; blastoids and echinoids are also present. Arthropods are common, particularly trilobites and ostracodes. Other less conspicuous components include sponges, annelids and rare fish remains. The microfauna is dominated by foraminiferida but conodonts are occasionally present. The flora includes stromatolitic, dasyclada-cean and other algae.

The relative percentages of shelly fauna in a large sample of shells from the Cracoe reefs of Yorkshire is:

The reef communities illustrated are based on large samples of

Articulate brachiopods

Bivalves

Cephalopods

Gastropods

Rostroconchia

Inarticulate brachiopods

Scaphopods

assemblages from the Cracoe reefs of Yorkshire, and are typical of other communities in reef limestones in other areas. They include an Algal Reef Community (No. 44) and several reef slope communities (reef slope is here used in the sense of the fore reef of Wolfenden, 1958). The term 'shallow water' is not defined in absolute depth. Such a community generally occupies the upper (topographic) portions of the reef and is associated with some evidence of algae. The deeper water communities occupy low levels on the palaeoslopes and are not associated with algae. It should be noted that many elements of the reef phase fauna are found at all depths.

44 Algal Reef Community

Autochthonous algal frameworks producing wall-like structures were recognized in the mid-Visean reefs of northern England by Wolfenden (1958). These frameworks represent the shallowest water deposits of the reef communities. They were probably subject to strong wave and current action. The community illustrated is based on an algal reef exposed on the top of Stebden Hill, Cracoe.

The primary frame-builder was an encrusting algal stromatolite (presumably a blue-green alga) which produced a finely laminated limestone clearly indicating its growth pattern; it normally has cavities within its structure. In section there is little evidence of the algae responsible apart from the occasional development of Aphralysia cells. The stromatolitic framework is often greatly modified by the assembled encrusting fauna, particularly the bry-ozoan Fistulipora and Tabulipora, the lithistid sponges Microspon-gia and Radiatospongia, and the tabulate corals Michelinia and Emmonsia. Groves of small solitary rugose corals grew on parts of the framework. These are generally enveloped in the stromatolite and it appears that the latter not only provided the corals with a firm substrate, but also supported them during growth. The tabulate coral Cladochonus grew in the same way. A conspicuous small solitary rugose coral is Cyathaxonia cornu, which is generally associated with deeper water muddy environments. The associated shelly fauna includes the brachiopods Leptagonia, Streptorhynchus, 'Reticularia'elliptica and Stenoscisma, as well as small rare produc-toids (not shown here, but including Stipulina, Rugicostella and Undaria). The bivalve Pachypteria, normally a rare genus, was attached, oyster-like, to the substrate. The large ostracode Ento-moconchus is common and is frequently found in large numbers in the 'internal sediment' of growth cavities. Colonies of fasiculate Lithostrotion are frequently associated with algal reef.

Coelenterata

Fig. 44 Algal Reef Community a Michelinia (Coelenterata: Anthozoa: Tabulata)

b Emmonsia parasitica (Coelenterata: Anthozoa: Tabulata)

c Cladochonus (Coelenterata: Anthozoa: Tabulata)

d Cyathaxonia (Coelenterata: Anthozoa: Rugosa)

e Fistulipora (Bryozoa: Ectoprocta)

f Fenestella (Bryozoa: Ectoprocta)

g lithistid sponge (Porifera: Demospongea)

h Pachypteria (Mollusca: Bivalvia: Pterioida)

i 'Reticularia' (Brachiopod: Articulata: Spiriferida)

j Stenoscisma (Brachiopod: Articulata: Rhynchonellida)

k Streptorhynchus (Brachiopod: Articulata: Strophomenida)

1 Leptagonia (Brachiopod: Articulata: Strophomenida)

m Entomoconchus (Arthropoda: Crustacea: Ostracoda)

Fig. 44 Algal Reef Community a Michelinia (Coelenterata: Anthozoa: Tabulata)

b Emmonsia parasitica (Coelenterata: Anthozoa: Tabulata)

c Cladochonus (Coelenterata: Anthozoa: Tabulata)

d Cyathaxonia (Coelenterata: Anthozoa: Rugosa)

e Fistulipora (Bryozoa: Ectoprocta)

f Fenestella (Bryozoa: Ectoprocta)

g lithistid sponge (Porifera: Demospongea)

h Pachypteria (Mollusca: Bivalvia: Pterioida)

i 'Reticularia' (Brachiopod: Articulata: Spiriferida)

j Stenoscisma (Brachiopod: Articulata: Rhynchonellida)

k Streptorhynchus (Brachiopod: Articulata: Strophomenida)

1 Leptagonia (Brachiopod: Articulata: Strophomenida)

m Entomoconchus (Arthropoda: Crustacea: Ostracoda)

Strophomenida) d Echinoconchus elegans (Brachiopoda: Articulata:

Strophomenida) e Acanthoplectamesoloba (Brachiopoda: Articulata:

Strophomenida) f Productina margaritaceus (Brachiopoda: Articulata

Strophomenida) g Alitaria (Brachiopoda: Articulata: Strophomenida) h Rhipidomella (Brachiopoda: Articulata: Orthida)

45 Upper Reef Slope Community

This community is probably typical of shallow water reef palaeo-slopes. It represents a slightly deeper water than the algal reef and occurs just below the latter on the reef slope. The matrix is a light grey calcilutite which has a clotted texture, and contains little recognizable organic detritus. Irregular patches of fibrous calcite are common. Algae, including Koninckopora, Girvanella, and oncolitic encrustation of shell fragments, give an indication of shallow water independent of topographical position. The shelly fauna is dominated by brachiopods, which include small and medium sized productoids and a variety of pedunculate genera. Small rhynchonellids such as Pleuropugnoides pleurodon and the tere-bratulid Dielasma hastatum are particularly common and are often found in localized clusters. The molluscan fauna includes the unusual semi-infaunal rostroconch Conocardium alaeforme, and local concentrations of gastropods; the latter could be indicative of the presence of non-preserved vegetation (Black, 1954). Occasionally large solitary rugose corals, particularly Caninia, occur, and the heterocoral Hexaphyllia is locally abundant (our tentative reconstruction shows them supported by conjectured vegetation). Bry-ozoans are largely of the 'stick' type with Penniretcpora and Rhombocladia being characteristic. Trilobite remains are common throughout, particularly the genus Cummingella.

This shallow water reef environment was probably well aerated and the sea turbulent, and it is possible that marine vegetation was very significant.

i Aulacophoria keyserlingiana (Brachiopoda:

Articulata: Orthida) j Spiriferellina insculpta (Brachiopoda: Articulata: Spiriferida)

k Dielasma (Brachiopoda: Articulata: Terebratulida) 1 Pleuropugnoides pleurodon (Brachiopoda:

Articulata: Rhynchonellida) m Conocardium (Mollusca: Rostroconchia) n Parallelodon (Mollusca: Bivalvia: Arcoida) o Straparollus (Mollusca: Gastropoda: Archaeo-

gastropoda) p Bellerophon (Mollusca: Monoplacophora) q Cummingella (Arthropoda: Trilobita) r Rhombocladia (Bryozoa: Ectoprocta) s Penniretepora (Bryozoa: Ectoprocta) t Koninckopora (Algae) u algal encrusted shells v seaweed (Algae)

46 Lower Reef Slope Mollusc Community

In deeper water reef environments there is often a great diversity of taxa, and communities are often very variable. Much of the fauna appear to have lived also in shallow water environments. There is no evidence in this community for the presence of algae.

Crinoid remains are more abundant here than in the shallower environments and are frequently associated with the coprophagous gastropod Platyceras. The rugose coral Amplexus coralloides flourished in the deeper water and occurred locally in dense groves. Amplexus is regarded by most authors as the coral most typical of the reef phase, but it is particularly abundant in deeper water reef communities. The presence of large delicate fronds of the bryozoans Fenestella and Polypora are indicative of the more tranquil conditions of this depth. The community illustrated shows molluscs, including a variety of bivalves and cephalopods, dominating the shelly fauna. Goniatites tend to be common in the deeper water environments, but elsewhere on the reef they are poorly represented, apart from localized concentrations (such as that described by Ford, 1965). The bivalves include shallow burrowers and epibyssate forms. It is noticeable that among the reef faunas infaunal detritus-feeding bivalves are markedly absent. Brachiopods are less abundant in this particular community, but can be more dominant elsewhere in the deeper water environments. Trilobites are slightly less well represented here than in shallow water environments, apart from Brachymetopus which is common. Large worm tubes of the genus Serpula are often present, and their exposed sections are frequently encrusted by the bryozoan Tabuli-pora.

The lithologies are variable, ranging from calcilutites to calcar-enites; coarser calcirudites composed of crinoid debris are also present. The cavity spaces between fenestellid fronds are often filled with successive layers of fibrous calcite.

Fig. 46 Lower Reef Slope Mollusc Community a Plicatifera plicatilia (Brachiopoda: Articulata:

Strophomenida) b Alitaria (Brachiopoda: Articulata: Strophomenida) c Pugnax pugnus (Brachiopoda: Articulata:

Rhynchonellida) d Tylothyris subconica (Brachiopoda: Articulata: Spiriferida)

e Fusella triangularis (Brachiopoda: Articulata: Spiriferida)

f Posidoniella vetusta (Mollusca: Bivalvia: Pterioida) g Aviculopecten (Mollusca: Bivalvia: Pterioida)

Spiriferida

h Streblopteria hemisphaericus (Mollusca: Bivalvia: Pterioida)

i Parallelodon reticulatus (Mollusca: Bivalvia: Arcoida)

j Schidozus (Mollusca: Bivalvia: Trigonioida) k Edmondia (Mollusca: Bivalvia: Anomalodesmata) 1 Platyceras (Mollusca: Gastropoda: Archaeogastropoda)

m Goniatites maximus (Mollusca: Cephalopoda:

Ammonoidea) n Bollandoceras (Mollusca: Cephalopoda:

Ammonoidea) o Amplexus coralloides (Coelenterata: Anthozoa:

Rugosa) p Serpula (Annelida) q Fistulipora (Bryozoa: Ectoprocta) r Brachymetopus (Arthropoda: Trilobita) s Polypora and Fenestella (Bryozoa: Ectoprocta) t crinoids (Echinodermata: Crinozoa)

47 Lower Reef Slope Brachiopod Community

In this deeper water reef community many features are similar to those of the previous community with which it merges. The lith-ology is a medium grey lime mud which probably was originally a fairly soft substrate. The dominant brachiopods include various productoids, spiriferoids, rhynchonellides and an orthide. The productoids are semi-infaunal forms supported in the soft sediment by spines extending from the convex pedicle valve. Some genera such asPlicatifera had a juvenile stage during which they clung to a host (such as a fenestrate bryozoan) by clasping spines (Brunton, 1966). The other brachiopods were largely epifaunal pedunculate forms. The large orthide Schizophoria resupinata and the rhyn-chonellide Pugnax acuminatus probably lived on the substrate, and the latter species may have been able to stand a slight amount of sediment covering by holding its high anterior margin clear of the sediment. The bivalves in this community are represented mainly by species of Leiopteria which often occur in large numbers. Other bivalves such as the epibyssate Parallelodon bistriatus are also present. Cephalopods are well represented, as they were in the previous community. Larger orthocone nautiloids are found, which have a tendency to come to rest facing down the slope. In addition the community includes crinoids, fenestrate bryozoans, trilobites and the coral Amplexus (not figured here).

The fine sediment is easily deposited in the empty shell cavities. At the time of deposition the surface of this sediment would have been horizontal; by comparing this surface with the adjacent beds the original palaeoslope can be determined.

Fig. 47 Lower Reef Slope Brachiopod Community a Acanthoplecta mesoloba (Brachiopoda: Articulata:

Strophomenida) b Plicatifera plicatilis (Brachiopoda: Articulata:

Strophomenida) c Krotovia spinulosa (Brachiopoda: Articulata:

Strophomenida) d Antiquatonia (Brachiopoda: Articulata:

Strophomenida) e Alitaria (Brachiopoda: Articulata: Strophomenida) f Echinoconchus punctatus (Brachiopoda: Articulata: Strophomenida)

1 Pugnax pugnus (Brachiopoda: Articulata:

Rhynchonellida)

m Schizodus (Mollusca: Bivalvia: Trigonioida)

g Overtonia (Brachiopoda: Articulata: Strophomenida) n Parallelodon (Mollusca: Bivalvia: Arcoida)

h Phricodothyris (Brachiopoda: Articulata: Spiriferida) o Leiopteria grandis (Mollusca: Bivalvia: Pterioida)

i Spirifer bisculcatus (Brachiopoda: Articulata: p Leiopteria lammosa (M°Husca: Bivalvia: pterioida)

Spiriferida) q orthocone (Mollusca: Cephalopoda: Nautiloidea)

j Martinia (Brachiopoda: Articulata: Spiriferida) r Fm&tefa (Bryozoa: Ectoprocta)

k Pugnax acuminatus (Brachiopoda: Articulata: s Brachymetopm (Arthropoda: Trilobita)

Rhynchonellida) t crinoids (Echinodermata: Crinozoa)

48 Productus-Buxtonia Community

This brachiopod-dominated reef slope community is characterized by the productoids Productus productus, Buxtonia and Echino-conchus subelegans, the davidsoniacean Schellwienella crenistria, the orthide Schizophoria connivens and the spiriferide Martinia, These six species are remarkable for their consistent association, and the community is probably one of the most well defined in the reef phase. Occasionally the productoid Striatifera striata occurs in this community. Bivalves are present but are much less numerous than the brachiopods; they include Leiopteria, Sulcatopinna and Myalina. The surface of the shells are often encrusted by the bryozoans Fistulipora and Tabulipora. Cyclus radialis, a crustacean, is common and appears to have replaced the trilobites in this environment. The semi-infauna and epifauna of this community tend to occur in shell beds with large numbers of individuals, the resulting rock often being a coquina with the spaces between the shells filled with fibrous calcite. Drifted assemblages contain a high percentage of epifaunal shells. It is probable that Cyclus lived between the shells in the original shell beds.

In northern England, this community is well developed on the middle and lower reef slopes of Pt age and is thus younger than the communities Nos. 44—47, which are of B2 age. It is however closely associated with encrusting algal stromatolites which suggests that the community probably lived in fairly shallow water; it may thus have developed low on the reef during periods when part of the reef was above water.

Gigantoproductus Illustration

c Echinoconchus (Brachiopoda: Articulata: Strophomenida)

d Schellwienella (Brachiopoda: Articulata: Strophomenida)

e Schizophoria (Brachiopoda: Articulata: Orthida)

f Martinia (Brachiopoda: Articulata: Spiriferida)

g Sulcatopinna (Mollusca: Bivalvia: Mytiloida)

h Leiopteria (Mollusca: Bivalvia: Pterioida)

i Myalina (Mollusca: Bivalvia: Mytiloida)

j Cyclus (Arthropoda: ?Crustacea: Cycloidea)

k Fistulipora (Bryozoa: Ectoprocta)

1 crinoid (Echinodermata: Crinoidea)

49 Coral-calcarenite Community

This community, mainly characterized by the presence of large and massive compound corals and an assortment of brachiopods, is developed in pale crinoidal bioclastic limestones. The species found in this clear water environment include the varied macrofauna illustrated, and also benthic foraminiferida (too small to be shown in the figure). The bulk of the rock is formed of broken shell fragments and shell debris, apparently for the most part transported into the area.

The particular community figured here is found in beds of Asbian (D!) age in the north of England. The large coral colonies can reach up to a metre across. Somewhat similar communities are found in most of the transgressive (deeper water) phases of cycles of deposition (Ramsbottom, 1973). One of the most characteristic species in such communities of mid-Visean age is the alga Koninck-opora inflata, which can be found in practically every thin section of these limestones, often more or less broken, and not commonly preserved complete. The large and massive compound coral colonies sometimes occur upside down, indicating a somewhat turbulent environment. These large colonies probably provided a number of niches in their shadows where small brachiopods, especially smooth spiriferides, could flourish. Trilobites with tumid form, which crawled around on the sea floor, possibly as scavengers, are not uncommon in this community.

Fig. 49 Coral-calcarenite Community a Lithostrotion arachnoideum (Coelenterata: Anthozoa: Rugosa)

b Lithostrotion portlocki (Coelenterata: Anthozoa: Rugosa)

c Paladin (Arthropoda: Trilobita)

d Koninckopora (Algae)

e Antiquatonia (Brachiopoda: Articulata: Strophomenida)

f Spirifer (Brachiopoda: Articulata: Spiriferida)

g Syringopora (Coelenterata: Anthozoa: Tabulata)

h Composita (Brachiopoda: Articulata: Spiriferida)

i Rugosochonetes (Brachiopoda: Articulata: Strophomenida)

j orthocone (Mollusca: Cephalopoda: Nautiloidea)

k Schellwienella (Brachiopoda: Articulata: Strophomenida)

1 Linoprotonia (Brachiopoda: Articulata: Strophomenida)

m Koninckophyllum (Coelenterata: Anthozoa: Rugosa)

n rhynchonellid (Brachiopoda: Articulata: Rhynchonellida)

Daviesiella

50 Brachiopod Calcarenite Community

This community occurs in dark relatively thin-bedded bioclastic limestones which differ from the paler limestones of similar type mainly in the amount of terrigenous matter present. This factor, however, is enough to cause considerable differences in the faunal associations between the clearer water pale limestones (community 49) and the darker limestones formed in more turbid waters. Large brachiopods, mainly with a costate shell ornamentation, such as Gigantoproductus, Semiplanus and Daviesiella, are characteristic, while the commonest of the smaller brachiopods are the spirifer-oids. The smaller productoids include many spinose forms such as Pus tula and Buxtonia. The corals are not only the simple forms like Dibunophyllum and 'Caninia', but also compound forms, both massive and fasciculate which may form large colonies more than half a metre across. Foraminiferida are abundant in these communities, but are too small to be shown in the figure. The large coral colonies may still sometimes be upside down, but the environment as a whole was less turbulent than that of the previous community (49).

Communities of this type are found in several transgressive phases of the major Dinantian sedimentary cycles (Ramsbottom, 1973) and are characteristic of many of the limestones of Brig-antian (D2 ) age in northern England and elsewhere.

Fig. 50 Brachiopod Calcarenite Community a Dibunophyllum (Coelenterata: Anthozoa: Rugosa)

b Eomarginifera (Brachiopoda: Articulata: Strophomenida) c Gigantoproductus giganteus (Brachiopoda: Articulata: Strophomenida)

d Semiplanus latissimus (Brachiopoda: Articulata: Strophomenida)

e Lonsdaleia (Coelenterata: Anthozoa: Rugosa)

f Aviculopecten (Mollusca: Bivalvia: Pterioida)

g small gastropod (Mollusca: Gastropoda)

h Composita (Brachiopoda: Articulata: Spiriferida)

i Lithostrotion junceum (Coelenterata: Anthozoa: Rugosa)

j Rugosochonetes (Brachiopoda: Articulata: Strophomenida)

k Schellwienella (Brachiopoda: Articulata: Strophomenida)

1 Buxtonia (Brachiopoda: Articulata: Strophomenida)

m orthocone (Mollusca: Cephalopoda: Nautiloidea)

n zaphrentoid (Coelenterata: Anthozoa: Rugosa)

o Chaetetes (PPorifera)

Chaetetes Features

51 Mud Community

These faunas occur in muds which are considered to have been deposited in slightly deeper water than the dark thin-bedded limestones (Community 50), but there are many elements in common. Usually, however, the large productoids such as Gigantoproductus are missing from the Mud Communities.

Small corals (such as Cyathoxonia and zaphrentoids), bryo-zoans (especially Fenestella and Penniretepora), crinoids, some echinoids (usually found dissociated), smooth spiriferides such as Martinia and Crurithyris, small chonetoids, a few productoids including Productus s.s., phillibolid trilobites (as opposed to the phillipsiid types more characteristic of shallower water shelf limestones), bivalve pectinoids and occasional goniatites comprise this community. Among the smaller fossils are ostracodes of many types, holothurians (only found as dissociated spicules), arenaceous and calcareous foraminifera, conodonts and occasional radiolaria. These groups are sometimes of exceptional diversity, as many as 50 or 60 or even more species being present.

Communities of this type are best developed in north-west Europe in high Dinantian beds, but are also found in Namurian shelf or near-coastal environments, and, rarely, in the Westphalian marine beds.

Fig. 51 Mud Community a Fenestella (Bryozoa: Ectoprocta)

b crinoid (Echinodermata: Crinoidea)

c Cyathoxonia (Coelenterata: Anthozoa: Rugosa)

d Rotiphyllum (Coelenterata: Anthozoa: Rugosa)

e orthocone (Mollusca: Cephalopoda: Nautiloidea)

f Sudeticeras (Mollusca: Cephalopoda: Ammonoidea)

g Productus (Brachiopoda: Articulata: Strophomenida)

h Crurithyris (Brachiopoda: Articulata: Spiriferida)

i holothurian (Echinodermata: Echinozoa)

j rhynchonellid (Brachiopoda: Articulata: Rhynchonellida)

k bryozoan (Bryozoa)

1 Rugosochonetes (Brachiopoda: Articulata: Strophomenida)

m Aviculopecten (Mollusca: Bivalvia: Pterioida)

n Pinna (Mollusca: Bivalvia: Mytiloida)

o Edmondia (Mollusca: Bivalvia: Anomalodesmata)

p Archegonus (Arthropoda: Trilobita)

q ostracodes (Arthropoda: Crustacea)

r Penniretepora (Bryozoa: Ectoprocta)

s gastropod (Mollusca: Gastropoda)

52 Namurian Goniatite Community

In the Namurian the basinal areas of relatively deep water, in which non-calcareous mudstones were being deposited, commonly contain no benthic fauna during the marine episodes. But there is an abundance of swimming forms, especially of goniatites and pectin-oids. Both these groups were free-swimming, though they may have come to rest on the bottom at times. Bivalves of pterioid form, such as Caneyella and more rarely Posidoniella, have the same distribution as the pectinoids and either had the same mode of life or were attached to plants of which there is now no trace.

The lack of benthos has been attributed to the lack of oxygen on the sea floor, though it was possibly due to the lack of a definitive division between the mud and the water, which would have made colonization of an unstable sea floor difficult (Holds-worth, 1966).

The community illustrated is of Marsdenian (R.2a) 'g''nd>' typical of many of the Namurian marine horizons of north-west Europe. The goniatites Reticuloceras and Hudsonoceras are the common forms though nautiloids, both orthocones and coiled forms, do occur rarely. Posidonia, Caneyella and the large pectin-oid Dunbarella, comprise the bivalves. Also present, though not shown in the figure, are radiolaria and conodonts (presumed to be derived from a free-swimming animal), and, rarely, fish, usually as fragments.

In some of the Namurian marine horizons there are large numbers of very small molluscan shells (spat), both gastropods and bivalves. They are of a size (< 1.0mm) at which the early planktonic shell sinks to the bottom and settles for its adult life, but in this case the animals died at once, presumably because of the unsuitable, possibly de-oxygenated, nature of the sea floor (Ramsbottom et al. 1962).

Goniatite communities of this type are also found in some of the Westphalian marine bands.

53 Namurian Basinal Benthic Community

An example of one of the rare benthic communities of the European Namurian basins is shown here. The goniatites and pectinoids occur as in the non-benthic Namurian faunas, but associated is a limited and restricted benthos which includes productoids, chonetoids, rare smooth spiriferides (not illustrated), crinoids and trilobites. Apart from the trilobites, which presumably acted as scavengers on the sea floor, the remaining benthic groups were probably all

Fig. 52 Namurian Goniatite Community a Hudsonoceras (Mollusca: Cephalopoda: Ammonoidea) b Reticuloceras (Mollusca: Cephalopoda: Ammonoidea) c Posidonia (Mollusca: Bivalvia: Pterioida) d Dunbarella (Mollusca: Bivalvia: Pterioida)

Fig. 52 Namurian Goniatite Community a Hudsonoceras (Mollusca: Cephalopoda: Ammonoidea) b Reticuloceras (Mollusca: Cephalopoda: Ammonoidea) c Posidonia (Mollusca: Bivalvia: Pterioida) d Dunbarella (Mollusca: Bivalvia: Pterioida)

Cephalopoda

filter feeders, and burrowing forms were not present. This fauna is widespread at one horizon (in E2D) in the Arnsbergian stage in north-west Europe. It presumably indicates either a shallowing of the water or the development of bottom currents so as to allow colonization of the sea floor by benthic animals. The mudstone containing this fauna is slightly calcareous.

Fig. 53 Namurian Basinal

Benthic Community a Dimorphoceras (Mollusca: Cephalopoda: Ammonoidea) Cravenceratoid.es (Mollusca: Cephalopoda: Ammonoidea) orthocone (Mollusca: Cephalopoda: Nautiloidea) crinoid (Echinodermata: Crinoidea) e Rugosochonetes (Brachio-poda: Articulata: Strophomenida) f Posidonia (Mollusca:

Bivalvia: Pterioida) g Paladin (Arthropoda:

Trilobita) h Productus hibernicus (Brachiopod: Articulata: Strophomenida)

Posidoniella Bivalve

Fig. 54 Westphalian Nonmarine Bivalve Community a Anthraconaia (Mollusca: Bivalvia: Pterioida) Carbonicola (Mollusca: Bivalvia: Unionida) Naiadites (Mollusca: Bivalvia: Pterioida) Spirorbis (Annelida: Serpulid)

Geisina (Arthropoda: Crustacea: Ostracoda)

Pelecypodichnus (Trace fossils of Bivalve)

Fig. 54 Westphalian Nonmarine Bivalve Community a Anthraconaia (Mollusca: Bivalvia: Pterioida) Carbonicola (Mollusca: Bivalvia: Unionida) Naiadites (Mollusca: Bivalvia: Pterioida) Spirorbis (Annelida: Serpulid)

Geisina (Arthropoda: Crustacea: Ostracoda)

Pelecypodichnus (Trace fossils of Bivalve)

54 Westphalian Non-marine Bivalve Community

Non-marine bivalves inhabited the shallow waters which spread over extensive areas after the flooding of the coal forests. There are no true marine fossils present, though some of the bivalve genera, particularly Naiadites and Curvirimula, were apparently able to tolerate salinity conditions which might have been brackish.

Naiadites appear to have been byssally attached, possibly to drifting vegetation, which would explain the wide distribution of this genus. The small adherent worm tubes of Spirorbis are commonly found attached to the shells of Naiadites, and the relationship appears to have been a commensal one, for the spirorbids are concentrated on the shell close to the inhalent siphonal current, which would have provided them with a source of food.

Carbonicola and the much rarer Anthraconaia were burrowers, and work by Eagar suggests that the variations in the shape of the shell in the different species indicate slightly differing habitats. The burrows of these bivalves, especially those of Anthraconaia, which made a deep vertical burrow, are found as the trace fossil called Pelecypodichnus. These are often aligned in one general direction, probably that of the current, and were oval or semi-oval depressions on the mud floor.

The fauna illustrated is thought to be typical of that found in the lower part of the Westphalian in north-west Europe. At this horizon a common benthic ostracode is Geisina arcuata, which is believed to have lived amongst the weeds presumed to have been either growing in the bottom muds or floating freely in shallow water.

Fig. 55 is described by Dr. R. Goldring. 55 Sea Level Lake Community

The Upper Carboniferous swamps and inland seas are often monotonously unfossiliferous except for plant fossils. The occasional marine transgressions are, of course, frequently marked by sediments rich in marine fossils: goniatites and productoid brachio-pods. The example shown is taken from the Bude Formation of the Upper Carboniferous of south-west England. It is a rather special situation where bedding surfaces have frequently preserved the impressions of limulids moving over the substrate. Some surfaces are densely tracked. Limulids are one of a very conservative group of animals. Their morphology, indicated by their tracks, shows little evolutionary change from the Devonian to the present. In the illustration the trackways show in detail the marks of the telson, the legs, especially the pushers, each with its hand-like extremity and, occasionally, traces of the genal spines. These tracks were probably made by Euproops, but as is so frequent in the fossil record, no specimen has been found associated with the tracks. The tracked horizons were unstable substrates and therefore not the most favourable for carapace fossilization. The traces vary in sharpness and in the amount of detail preserved. The pushing appendages penetrated slightly deeper than the anterior appendages. The sediment has not been much disturbed by the animals' pre-ambulations; they were certainly not digging extensively for food as do modern limulids living on sandy nearshore substrates. The individuals can be judged to have been of small size. There are also

Fig. 56 Westphalian Forest c tree fern (Pteridophyta: Fern)

a Stigmaria (Pteridophyta: Lycopod) d calamitid (Pteridophyta: Calamites)

b Lepidodendron (Pteridophyta: Lycopod) , dragonfly (Arthropoda: Hexapoda)

Fig. 56 Westphalian Forest c tree fern (Pteridophyta: Fern)

a Stigmaria (Pteridophyta: Lycopod) d calamitid (Pteridophyta: Calamites)

b Lepidodendron (Pteridophyta: Lycopod) , dragonfly (Arthropoda: Hexapoda)

some curious telson tracks in the form of a repetitive double curve (not figured). The curves are slightly out of phase and one curve is of rather greater amplitude than the others. The large curve follows and cuts the smaller and both must have been made by a unit which Dr. A. F. King envisaged as a pair of crabs mating with the male riding on the buckler of the female, just as in modern limulids. The tracked silts were not disturbed by any other organism, though in the black mudstones associated with the Bude Formation fish can be found in concretions, where the skeletons have resisted compaction. Two palaeoniscoids (primitive actinopter-igians) are figured; both were probably carnivorous with sharp conical teeth separated by wide gaps. The smaller is Cornuboniscus budensis and the larger Elonichthys aitkeni. A small acanthodian (spiny shark), Acanthodes wardi, and the coelocanthid Rhabdod-erma elegans also occur. The crustacean Crangopsis has also been found, and the black shales often contain the trace fossil Planolites.

56 Westphalian Forest

In the Coal Measures of Europe, which are mainly of Westphalian age, and in comparable beds in North America, vast areas (Fig. i) became intermittently covered with forest swamps of the type shown in this figure. The tallest trees, such as Lepidodendron (Sigillaria is the name for its stems, and Stigmaria for its roots), with their spreading branches formed the rather open canopy of the forest. Trees of rather lesser size included calamitids, especially Calamites itself, related to the horsetails of today and marked by prominent horizontal nodal lines. All these probably grew actually in or at the edge of shallow marshy pools. In among the trees grew the smaller tree-ferns. It is probable too that abundant representatives of the fungi, algae, mosses and liverworts also flourished in these forests, but they have left few fossil remains. The latter part of the Carboniferous saw the development of primitive amphibia and also of the early insects. One of these, a large dragonfly with a wing span of 50 or 60cm, is shown in the figure.

Fig. i. Outline of the geography of northern Europe during the Carboniferous (early Westphalian). Some islands may have existed in the deltaic area. The exact positions of the rivers flowing southwards is quite uncertain. The Rheic Ocean may have closed in the late Westphalian (McKerrow and Ziegler, 1972).

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  • Leon
    Which of the following corals were present in Mesozoic reef communities?
    2 years ago

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