Cardiola Interrupta Shell Fossil

Stricklandia

Table III The Silurian ecogroups and chief animal communities and their evolution with time

Table III The Silurian ecogroups and chief animal communities and their evolution with time low. A few species change from one habitat to another with time, but the great majority of Silurian brachiopod lineages remain in the same habitat throughout the period.

The succession of communities in one habitat (with component genera changing through time) must be distinguished from a single community (where the component genera are always similar). We use the term COMMUNITY for an association of genera which live in a single habitat and occur in roughly similar proportions, and we use the word ECOGROUP for successive communities in a similar habitat. Within a single community there are often local variations in the presence and proportion of genera. For example the Eocoelia Community may sometimes include an ASSEMBLAGE (a group of organisms) rich in rhynchonellids, and at other times rhynchonellids may be less common. Whenever a fossil collection is made from one site it is an assemblage that is collected. Similar assemblages are grouped into communities, and communities occupying similar habitats at different times can be classified together into ecogroups (Table III).

The Silurian Beds of Britain vary greatly from place to place (Table IV) and a large number of formations (each with a local name) are present. For details of these and their stratigraphical thickness in each area see Cocks et al. (1971) and Ziegler et al. (1974), which are papers dealing with the correlation of the whole of the British Silurian and some other countries.

Time Division

Wales

Welsh Borderland

Scotland and Lake District

Pridoli and Ludlow

Turbidites in Lower Ludlow, then no record in North Wales. Regression in SE Wales from Dicoelosia to Lingula Communities, then freshwater (Old Red Sandstone).

Progressive shallowing from Dicoelosia to Lingula Communities, then freshwater (Old Red Sandstone).

Graptolite shales in Lake District becoming shallow at top. No record in Southern Uplands. Freshwater or very shallow marine (Stonehaven) in Midland Valley.

Wenlock

Turbidites and graptolitic shales in north, irregularly shallower to south.

Deeper water at first, becoming shallower with Wenlock Limestone at top.

Mostly turbidites in Southern Uplands and Lake District. Non-marine in Midland Valley.

Upper Llandovery

Marine basin continues, with thick turbidities at Aberystwyth. Shallower to south and SE with Pentamerus to Clorinda Communities.

Marine transgression south-eastwards, with progressive deepening from Lingula to graptolitic Communities.

Mostly thick turbidites in Southern Uplands, and graptolitic shales in Lake District. Regression from Clorinda to Lingula Communities in east Midland valley.

Lower and Middle Llandovery

Marine basin with deep water in North Wales, shallower in Pembrokeshire.

Land

Deepwatcr deposits across Southern Uplands and Lake District. Marine transgression in Girvan, Lingula through deeper brachiopod communities to graptolites.

Table IV The history of the chief Silurian areas of Britain.

Table IV The history of the chief Silurian areas of Britain.

THE LINGULA-BIVALVE ECOGROUP (18)

In the marine areas closest to the coast, and also in lagoons and estuaries where the salinity was slightly reduced, the commonest fossils are bivalves, gastropods and Lingula. This association is seen throughout the Silurian. Although there were few major changes with time, the composition of individual assemblages varies greatly; in particular gastropods and ostracodes are sometimes very common, and sometimes absent. The ecogroup is stable through time, and only includes one community, the Lingula Community.

It is important to realize that Lingula can occurAit any depth. It is not found only in this ecogroup. Individuals which were present in deeper environments can often be distinguished by their smaller size, and they are seldom very common.

18 Lingula Community

Where the salinity was normal, Lingula occurred with more dominant articulate brachiopods (rhynchonellids and strophomenids), many of which are also present in the Eocoelia-Salopina ecogroup. In other assemblages Lingula is accompanied by burrowing and epifaunal bivalves. If the water was slightly less saline than the open sea, then the number of species became reduced, the fauna became very restricted and in extreme cases may consist of Lingula alone, as in the Skomer Volcanic Group of Pembrokeshire, or of a single species of bivalve.

Even in normal salinity, the Lingula Community is not diverse, but its constituents represent a wide range of adaptive types. As in the other Silurian marine communities suspension feeders are dominant, but here they include as many bivalves as brachiopods. The suspension feeders are dominated by bivalves (including the bys-sally attached Pteronitella and Modiolopsis) and brachiopods; the latter include the burrowing Lingula, and Stegerhynchus which was attached to other shells or to seaweed by its pedicle. Other suspension feeders include stick bryozoans and cornulitids. Stegerhynchus, like some other rhynchonellids throughout the Upper Palaeozoic and Mesozoic, was euryhaline, probably more so than any other articulate brachiopod.

Deposit feeders include burrowing bivalves like Palaeoneilo and Lyrodesma. Some muddier beds containing this community lack fine bedding because of disturbance by burrowers (bioturba-tion); it is not known which animals made these burrows, but arthropods or some group of worms seem the most likely. The gastropods present were probably either grazers or scavengers, since Silurian shells seldom show signs of attack by gastropods g. 18 Lingula Community Pteronitella (Mollusca: Bivalvia: Pterioidea) Lingula (Brachiopoda: Inarticulata) Stegerhynchus (Brachiopoda: Rhynchonellida) Cornulites (Cricoconarida) seaweed (Algae)

Palaeoneilo (Mollusca: Bivalvia: nuculoid)

g. 18 Lingula Community Pteronitella (Mollusca: Bivalvia: Pterioidea) Lingula (Brachiopoda: Inarticulata) Stegerhynchus (Brachiopoda: Rhynchonellida) Cornulites (Cricoconarida) seaweed (Algae)

Palaeoneilo (Mollusca: Bivalvia: nuculoid)

Bivalvia Fossils

with rasping radulae and the only known predators were cephalo-pods and starfish. Abundant grazing gastropods in some localities suggest the presence of enough seaweed to support the population. Seaweed may also be linked with the local abundance of Stegerhynchus in some areas. This brachiopod has a large pedicle opening, suggesting that it was attached throughout its life, and the seaweeds probably provided the best anchorage.

The Lingula Community has been recorded from Llandovery and from Downtonian beds in the Welsh Borderland.

THE EOCOELIA-SALOPINA ECOGROUP (19-21)

In the Upper Llandovery the Eocoelia Community occurs to the seaward side of the Lingula Community, and many brachiopods, especially rhynchonellids and Eostropheodonta, are common to both communities. Similarly on the seaward edge of the Eocoelia Community, the Eocoelia and Pentamerus Community constituents may also be found together. In the Lower and Middle Llandovery the Eocoelia-Salopina ecogroup is represented by the Cryptothy-rella Community, and during the late Silurian by the Salopina Community.

Brachiopoda Living Forms

Fig. 19 Cryptothyrella Community a Cryptothyrella (Brachiopoda: Spiriferida)

b dalmanellid (Brachiopoda: Orthida)

c Zygospiraella (Brachiopoda: Spiriferida)

d streptelasmatid coral (Coelenterata: Rugosa)

e Liospira (Mollusca: Archaeogastropoda)

f Loxonema (Mollusca: Mesogastropoda) g Eostropheodonta (Brachiopoda: Strophomenida)

19 Cryptothyrella Community

The Ordovician-Silurian boundary was marked by a glacial period at least as severe as that of the Pleistocene, which, together with the resultant lowering of sea level, appears to have caused the extinction of many of the Ordovician marine benthos. At the very base of the Silurian, marine faunas are both scarcer and less diverse than later; and the Lower and Middle Llandovery saw a steady increase in the number of different animal types, as well as a gradual evolution of their ecological distribution into the markedly depth related communities of the Upper Llandovery and later. The depth range occupied by the Eocoelia Community, and perhaps also the shallower part of the Pentamerus Community in the Upper Llan-dovery is taken up by the Cryptothyrella Community in the Lower and Middle Llandovery. Four brachiopod genera are dominant, Cryptothyrella (a smooth atrypoid), Zygospiraella (a ribbed atry-poid externally similar to Eocoelia) and two different dalmanel-lids. Also present are the strophomenid Eostropheodonta, Tenta-culites and stick bryozoans. Most of the habitats appear to be directly comparable to those of the Eocoelia Community, and many of the brachiopods in the Cryptothyrella Community are closely similar in external shape to other brachiopods present in the Eocoelia Community. The Cryptothyrella Community occurs in Lower Llandovery beds of the Girvan area, Ayrshire.

20 Eocoelia Community

This community is dominated by Eocoelia (whose range is late Middle Llandovery to Lower Wenlock), with the rhynchonellid Ferganella and dalmanellids the two commonest associated forms. Eostropheodonta or Leptostrophia, bivalves, gastropods, and Ten-taculites are common in places. The rhynchonellids and the bivalves are especially common in the shallower part of the community depth range. Only one or two species of trilobites are common in the Eocoelia Community. In contrast to the suspension-feeding brachiopods, the trilobites and many of the molluscs are related to particular substrates. Those molluscs which are characteristic of particular sediments may also have been deposit feeders like the trilobites.

Although the Eocoelia Community can be found in any clastic sediment, even fine muds, it is most often found in sands. It is of interest that the two most abundant forms in this community, Eocoelia and Stegerhynchus, are both strongly ribbed. Most brach-iopods only need to open their valves by 1 or 2mm to feed; the

Tentaculites

Fig. 20 Eocoelia Community a crinoid (Echinodermata: Crinozoa) b seaweed (Algae) c Eostropheodonta (Brachiopoda:

Strophomenida) d Stegerhynchus (Brachiopoda:

Rhynchonellida) e Salopina (Brachiopoda: Orthida) f Tentaculites (Cricoconarida) g Pteronitella (Mollusca:

Bivalvia: Pterioidea) h Poleumita (Mollusca:

Archaeogastropoda) i Eocoelia (Brachiopoda:

Rhynchonellida) j Palaeoneilo (Mollusca: Bivalvia: nuculoid)

irregular opening provided by ribs acted as a sieve which excluded sand grains from the interior when the two valves were open (Rudwick, 1964).

The majority of collections from this community contain only four or five common species (more than three per cent), while some assemblages are almost completely dominated (over 90 per cent) by Eocoelia itself. Both today and in the Silurian an increase in species diversity is often related to an increase in water depth. We do not know of any Eocoelia Community in which salinity changes can be proved to be the cause of low diversity.

The Eocoelia Community occurs in many shallow water sands and shales in the Welsh Borderland and in the Girvan area.

21 Salopina Community

The Upper Silurian Salopina Community occupies the same habitat as the Upper Llandovery Eocoelia Community. The small finely ribbed dalmanellid Salopina is rare in the Llandovery and becomes more abundant as Eocoelia disappears during the Wenlock. Rhyn-chonellids and bivalves are still important constituents of the community. A species which occurs locally in huge numbers is Micro-sphaeridiorhynchus nucula; this can be distinguished from most other Silurian rhynchonellids by its small number of ribs. In the Ludlow the brachiopod Protochonetes also becomes an important element in the Salopina Community. Protochonetes has small spines sticking outwards from its hinge line and these may have served as an anchor and counterweight, so that the curved feeding edge away from the spines would be held stable above the substrate. The practical advantage of this arrangement would have been great, since brachiopods can be overwhelmed and killed by quantities of sediment. Protochonetes occurs with Microsphaeridiorhynchus nucula and little other fauna in some beds just below the transition from marine (Ludlow) to non-marine (Downtonian) deposits in the

Mollusca Resimleri

Fig. 21 Salopina Community a crinoid (Echinodermata: Crinozoa) seaweed (Algae)

Pteronitella (Mollusca: Bivalvia: Pterioidea Poleumita (Mollusca: Archaeogastropoda)

Microsphaeridiorhynchus (Brachiopoda) Rhynchonellida) Salopina (Brachiopoda: Orthida)

Protochonetes ludloviensis (Brachiopoda: Strophomenida) h Palaeoneilo (Mollusca: Bivalvia: nuculoid)

Welsh Borderland. This distribution suggests that Protochonetes may have resembled the rhynchonellids in being curyhaline.

The Salopina Community is normal in sandstones or siltstones, but it can occur in clays. Though most of the genera present are suspension-feeding brachiopods and bivalves, bioturbation (from burrowing bivalves, worms or trilobites) is present in places. The Salopina Community occurs in the late Ludlow beds of the Welsh Borderland.

THE PENTAMERUS-SPHAERIRHYNCHIA ECOGROUP (22-24)

Pentamerus is unknown from the Lower Llandovery and no community representing the Pentamerus-Sphaerirhynchia Ecogroup can be recognized clearly at this time in Britain. In the Middle and Upper Llandovery Pentamerus becomes very common and dominates the Pentamerus Community, as does its descendant Pentamer-oides. Pentameroides disappears at the end of the Llandovery, and its habitat occupied by the Sphaerirhynchia Community in the late Silurian. Locally in the Ludlow the large pentameride Kirkidium occurs in great numbers and can be assigned to this ecogroup. The Wenlock Sphaerirhynchia Community is chiefly developed in limestones, but in the Ludlow it occurs in both limestones and clastic sediments. In Wenlock clastic sediments the ribbed spiriferid Homoeospira is present; it gives its name to the Homoeospira Community, but elastics containing this community are rare in Britain (the water was mostly too deep). We include the Reef Assemblage with this ecogroup since most Wenlock Limestone reefs are associated with brachiopods of the Sphaerirhynchia Community.

22 Pentamerus Community

This community was dominated by the large brachiopod Pentamerus (up to lOcms long), or its descendant Pentameroides. Although to seaward of the Eocoelia Community, the habitat of the Pentamerus Community was still fairly shallow water; many of the sands and shelly limestones show evidence of turbulence, such as cross-bedding and scour marks. Shells and shell debris are often much more abundant here than in land-derived sediment, so that limestones are produced.

The shell of Pentamerus is very thick at the umbones (posterior) and very thin at the other end (anterior). It rested on its posterior end (the umbones) and was much more stable than other shells of even thickness. In adults the pedicle became atrophied, and the pedicle opening blocked by shell material. In some beds with abundant Pentamerus in this upright growth position the shells are touching, and occasionally have interfered with one another's growth.

Pentamerus is so named after the five chambers inside the shell separated by plates which help to strengthen it. There is a Y-shaped plate in the larger (pedicle) valve, and two plates are present in the smaller (brachial) valve; these plates extend for half or more of the length of the shell. A section through both valves near the umbones is shown in the inset to Fig. 22. Halfway through the Upper Llandovery Pentameroides developed from Pentamerus. In Pentamerus the two plates in the brachial valve first diverge and then continue parallel to each other at some distance apart; in Pentameroides the two plates also diverge at first, but then converge again at about one-third of the way down the valve to become a single plate.

Both the Pentamerus Community and the slightly deeper Stricklandia Community contain Atrypa, solitary conical corals,and also compound corals, particularly Favosites and the chain coral Halysites. Eocoelia is also present in the Pentamerus Community, but in very much smaller numbers than in the Eocoelia Community. Eocoelia (like all brachiopods except spire bearers and penta-merids) is progressively smaller in size in the deeper water communities. This decrease in size can be linked with a decrease in food supply with depth. It is perhaps significant that the exceptions to this rule include the spire-bearing brachiopods, in which a much more efficient filter-feeding system existed. The Pentamerus Community occurs in the Llandovery of the Welsh Borderland and Carmarthenshire.

a Pentamerus: lateral view and horizontal cross-section a Pentamerus: lateral view and horizontal cross-section

Pentamerus Brachial Valve

Fig. 22 Pentamerus Community a Pentamerus (Brachiopoda: Pentamerida)

b Halysites (Coelenterata: Tabulata)

c streptelasmatid (Coelenterata: Rugosa)

d Atrypa (Brachiopoda: Spiriferida)

e Hallopora (Bryozoa: Kctoprocta)

f Eocoelia (Brachiopoda: Rhynchonellida)

Fig. 22 Pentamerus Community a Pentamerus (Brachiopoda: Pentamerida)

b Halysites (Coelenterata: Tabulata)

c streptelasmatid (Coelenterata: Rugosa)

d Atrypa (Brachiopoda: Spiriferida)

e Hallopora (Bryozoa: Kctoprocta)

f Eocoelia (Brachiopoda: Rhynchonellida)

Mode Life Brachiopods

23 Sphaerirhynchia Community

Sphaerirhynchia is a globular rhynchonellid which is very rare in the Llandovery, but which becomes common, especially in limestones, in the Wenlock after Pentameroides becomes extinct. In the Ludlow of Britain the Sphaerirhynchia Community is equally common in limestones or clastic sediments. The Sphaerirhynchia specimens in the Wenlock Limestone are smaller and have a smoother valve margin than those in the Ludlow. The larger size and the more strongly deflected valve margins of the Ludlow Sphaerirhynchia may be adaptations to calmer water. Both these features helped the brachiopod to separate the water sucked in from that which was expelled,, having been extracted from it. This separation was not needed in the more turbulent water of the Wenlock Limestone environment.

The most abundant general in the Sphaerirhynchia Community are often Atrypa and Howellella, but these spire-bearing brachio-pods are not confined to this one community. Other common brachiopods include strophomenides,spiriferides and Protochonetes ludloviensis. Bivalves, gastropods and trilobites can occur, but are not very common. The distribution of corals and bryozoans is patchy.

The Sphaerirhynchia Community occurs in the upper parts of the Wenlock limestone of the Welsh Borderland.

Fig. 23 Sphaerirhynchia Community a Leptostrophia (Brachiopoda: Strophomenida) b Streptelasmatid (Coelenterata: Rugosa) c Atrypa (Brachiopoda: Spiriferida) d Sphaerirhynchia wilsoni (Brachiopoda:

Rhynchonellida) e Liospira (Mollusca: Archaeogastropoda) f Protochonetes ludloviensis Brachiopoda:

Strophomenida) g Howellella (Brachiopoda: Spiriferida) h Eospirifer (Brachiopoda: Spiriferida) i Loxonema (Mollusca: Mesogastropoda)

BrachiopodaTabulata Silurian

Fig. 24 Silurian Reef Assemblage

Heliolites (Coelenterata: Anthozoa: Tabulata) Favosites (Coelenterata: Anthozoa: Tabulata) Halysites (Coelenterata: Anthozoa: Tabulata) Hallopora (Bryozoa: Ectoprocta) streptelasmatid (Coelenterata: Anthozoa: Rugosa)

Atrypa (Brachiopoda: Spiriferida) crinoid (Echinodermata: Crinozoa)

Leptaena (Brachiopoda: Strophomenida)

Dalmanites (Arthropoda: Trilobita)

orthocone (Mollusca: Cephalopoda: Nautiloidca)

Wenlock Limestone Fauna

24 Reef Assemblage

A reef limestone (or bioherm) is a rock composed of remains of organisms which are still in their life position. None of the debris of a true reef has been transported and therefore a reef limestone can be recognized by the absence of bedding. Reef limestones are, of course, subject to attack by wave action, and fragments of eroded reef can be laid as bedded sediments in the surrounding sea.

Reef limestones are only common in the Silurian of Britain during the late Wenlock Series, and the following description is based on the assemblage in the Much Wenlock Limestone of Shropshire (Scoffin, 1971).

The frame builders were dominantly tabulate corals (Heliolites, Favosites, Halysites) and large, domed stromatoporoids (twenty or more genera have been recognized). The frame is bound together by calcareous algae and bryozoans, with subsidiary solitary rugose corals and flat stromatoporoids. Crinoids are also abundant.

In addition to the frame builders and binders, the Wenlock reef assemblages contain very varied shelly faunas which never occur in large numbers. These minor elements of the reef fauna include numerous trilobite species, brachiopods and molluscs. Most of these flourished adjacent to the reefs, rather than within them. Clusters of Atrypa are sometimes found in hollows within the reef.

THE STRICKLANDIA-ISORTHIS ECOGROUP (25-6)

The smooth pentamerid Stricklandia dominates the Stricklandia Community from the Lower Llandovery until high in the Upper Llandovery, when it evolves into the ribbed Costistrieklandia. In the Lower Wenlock Cos tis trie klandia gradually decreases in abundance and it is extinct by the Upper Wenlock. As Costistrick-landia declines, the finely ribbed Eospirifer becomes more common, but by the Upper Wenlock the characteristic brachiopods of the ecogroup are the dalmanellid Isorthis clivosa and associated stropheodontid brachiopods. Isorthis continues in this habitat through the Ludlow, but in Wales and the Welsh Borderland there is no record of this ecogroup in the latter part of the Ludlow, for the water in Britain and much of North America was too shallow.

25 Stricklandia Community

In the Llandovery there are all gradations between the Pentamerus, Community and the deeper water Stricklandia Community. In borderline cases the assignment of an assemblage to one or other community depends simply upon whether Stricklandia or Pentamerus is more abundant. The sediments in which the Stricklandia Community occurs are usually fine sands, silts or clays. Though small scale cross-bedding may be present there is seldom any indication of severe turbulence or strong bottom currents. Like Pentamerus, Stricklandia rested on its beak, without a functional pedicle in adult life. Some beds are crowded with vertical individuals in their growth position. In addition to Stricklandia itself, the Stricklandia Community often contains the spire-bearing brachio-pods Atrypa and Eospirifer, and other genera (including corals)

Fig. 25 Stricklandia Community a crinoid (Echinodermata: Crinozoa b Costistricklandia (Brachiopoda: Pentamerida)

c Atrypa (Brachiopoda: Spiriferida)

d Clorinda (Brachiopoda: Pentamerida)

e Salopina (Brachiopoda: Strophomenida)

f Pholidostrophia (Brachiopoda: Strophomenida)

g Leptaena (Brachiopoda: Strophomenida)

h Eospirifer (Brachiopoda: Spiriferida)

i Eocoelia (Brachiopoda: Rhynchonellida)

j Whitfeldella (Brachiopoda: Spiriferida)

k Loxonema (Mollusca: Mesogastropoda)

1 Euomphalopterus (Mollusca: Archaeogastropoda)

Bioturbation

which are typical of the shallower Pentamerus Community. The community may also contain Clorinda, Leangella and other brach-iopods and straight nautiloids which were more typical of deeper water. Crinoids are common at some localities, but have a very sporadic distribution; solitary conical corals and compound corals may be present.

All the brachiopods, bryozoans, crinoids and corals were suspension feeders; these make up the vast bulk (perhaps 98 per cent) of the animals found fossil in this community. The gastropods and trilobites were deposit feeders and may also have been scavengers. It is most probable that little organic matter was left to decay on the sea floor, because there is hardly any indication of it in the sediments, nor is the sea floor likely to have been stagnant, as this would not have suited bottom dwellers. The only fossil carnivores are echinoderms and nautiloids, but it is very likely that there were some soft-bodied carnivores, including worms, in the Silurian.

The Stricklandia Community occurs in the Upper Llandovery beds of Ayrshire, Wales and the Welsh Borderland, and is also very common in eastern North America.

26 Isorthis clivosa Community

In the Lower Wenlock, Costistricklandia gradually disappears and Eospirifer becomes more abundant in the Stricklandia-Isorthis Ecogroup. The diversity in this community continues to be greater than in those of shallower water habitats, so that no one genus is dominant. In the Upper Wenlock, the distribution of Eospirifer becomes more sporadic and the commonest brachiopods are the two dalmanellids Dalejina and Isorthis and the atrypoid Atrypa. In addition, there are numerous strophomenide genera, including Protochonetes minimus, Leptostrophia and Leptaena. Most strophomenides lacked a functional pedicle, and Protochonetes developed spines which acted as anchors; Strophonella and Leptaena both developed curved shells so that the opening between the valves (for feeding and respiration) was raised above the sea floor. In Leptaena this takes the form of a sharp bend in the shell. Proto-chonetes minimus is very much smaller than P. ludloviensis, which is most common in the shallow water Salopina Community (21), but may also occur in the Sphaerirhynchia Community (23).

Dalejina and Atrypa were often more abundant than Isorthis clivosa in this community, but they also occur in greater numbers in the shallower Sphaerirhynchia Community and in the deeper Dicoelosia Community. Isorthis clivosa was more restricted in its distribution and the community is therefore named after it, but other species of Isorthis were abundant in shallower environments.

Bivalves and gastropods are occasionally present in the Isorthis Community, but they are seldom as abundant as in shallower water Silurian Communities.

In this community the proportion of genera varies from one locality to another. This is a characteristic feature of all the deeper water Silurian Communities. It reflects the sporadic distribution of bottom-dwelling marine animals (benthos) on the sea floor at all times (from the Cambrian to the present day). Many brachiopod genera occur quite randomly on the sea floor, but articulated

Fig. 26 Isorthis clivosa Community a Isorthis clivosa (Brachiopoda: Orthida)

b Dalejina (Brachiopoda: Orthida)

c Protochonetes minimus (Brachiopoda: Strophomenida)

d Leptostrophia (Brachiopoda: Strophomenida)

e Mesopholidostrophia (Brachiopoda: Strophomenida)

f Leptaena (Brachiopoda: Strophomenida g Atrypa (Brachiopoda: Spiriferida)

h Acaste (Arthropoda: Trilobita)

Living Brachiopods

specimens of some genera are found in clusters. The clustering is particularly noticeable in smaller brachiopods with a relatively large pedicle opening. The pedicle was functional throughout the life of these animals which probably attached themselves to shell fragments or to shells of living animals. The clusters, which are usually all of one species, originate when a group of brachiopods anchored themselves on living members of their own species. Isorthis, Dalejina and Atrypa all had functional pedicles; although they usually lived as isolated individuals, they are occasionally found in clusters.

The Isorthis clivosa Community occurs in the Wenlock and Ludlow beds of Wales and the Welsh Borderland.

THE CLORINDA-DICOELOSIA ECOGROUP (27-29)

This ecogroup consists of the Silurian shelly benthos living in the deepest water habitats. In the Llandovery Series, this ecogroup is represented by the Clorinda Community, although locally Eoplec-odonta and Dicoelosia were more common than Clorinda. In the Wenlock and Ludlow, Dicoelosia became generally more common while Clorinda was rare, and the ecogroup, at least in shales and sandstones (clastic sediments), is represented by the Dicoelosia Community. In limestones, Dicoelosia is rare, and Eoplectondonta is again the characteristic genus. In addition to the Clorinda and Dicoelosia Communities this ecogroup includes the Visbyella trewerna Community which consists of a few small brachiopods living near the maximum depth limits of bottom dwellers.

27 Clorinda Community

Assemblages from this community commonly contain over twenty genera of brachiopods (and sometimes as many as 35). In addition, there is a large diversity of other animal types present, but never a high density.

Trilobites are more abundant in the deep water habitats of this community than in any other Silurian habitat, but they are never as common as they were in the Cambrian and Ordovician. Crinoids, carpoids, ostracodes, starfish and bryozoans are also present. Bivalves and gastropods are rarer and less diverse than in shallower water assemblages. In addition to these bottom dwellers, there is a fauna of swimmers and floaters (nekton and plankton). These include both straight and coiled nautiloids, and sporadic graptolites.

The brachiopods are thinly spread over the bedding planes. Like modern deep water benthos, they do not occur in clusters and never reach the densities seen in the shallower communities. Although the Clorinda Community is known from coarse sandstones, the majority of occurrences are in fine silts or muds. This reflects the weakness of the currents, which were not fast enough to transport coarse sand, in the deep water environments.

The commonest brachiopods are the pentamerid Clorinda, the strophomenides Eoplectodonta, Coolinia and Mesopholidostrophia, the dalmanellids Resserella and Skenidioides and the spire bearers Glassia, Atrypa and Eospirifer: Dicoelosia is present, but less common.

As in modern times the deep sea environment contains considerably more species than nearer the shore, though it has lower densities. The Clorinda Community normally contains five or six times as many species as the Lingula Community. But towards the extreme depth limits of the bottom-dwelling brachiopods there is

Fig. 27 Clorinda Community a Clorinda (Brachiopoda: Pentamerida)

b Cyrtia (Brachiopoda: Spiriferida)

c Eoplectodonta (Brachiopoda: Strophomenida)

d Atrypa (Brachiopoda: Spiriferida)

e Glassia (Brachiopoda: Spirferida)

f Skenidioides (Brachiopoda: Orthida)

g Calymene (Arthropoda: Trilobita)

Strophonella Life Mode

a reduction in diversity. Five or six species characteristic of the Clorinda Community were able to live in water deeper than the remainder. These species are known as the Marginal Clorinda Community and occupy the deepest habitats of any shelly fauna in the Lower Silurian.

The Clorinda Community occurs in the Upper Llandovery of Wales and the Welsh Borderland, the Lower and Middle Llandovery of Scotland, and comparable assemblages in North America, Asia and Australia.

28 Dicoelosia Community

The Dicoelosia Community occurs in the Upper Silurian (Wenlock and Ludlow Series). It contains many of the same genera seen in the Lower Silurian Clorinda Community, the main differences between the two being in the relative abundance of Dicoelosia and Clorinda.

The pentamerid Clorinda became rare in the Wenlock and Ludlow, while the dalmanellid Dicoelosia became abundant in clastic sediments. Dicoelosia is a small brachiopod (about lAcm long) with bilobed shell (inset to Fig. 28). Brachiopods are filter feeders which admit water to the shell, circulate it internally to extract oxygen and food particles, and then eject it. For small shells living in quiet water, it becomes important that the same water is not circulated repeatedly and many brachiopods develop characteristics which prevent this from happening. The deep groove in Dicoelosia was probably developed to separate the water going in (at the top of the two lobes) from the water coming out (in the groove).

Many brachiopods in the Dicoelosia Community are small and widely scattered. This may be a reflection of the reduced supply of plankton for food in the deep habitat of this community (though at least acritarchs, a common part of the plankton, are more abundant in deeper waters). For example, all three strophomenides (Mesopholidostrophia, Leangella and Protochonetes minimus) common in the community are small compared with their closest relatives living inshore.

Other common brachiopods include the dalmanellids (Dalejina, Isorthis clivosa, Resserella) and the spire-bearers Eospirifer and Cyrtia and Glassia.

Bivalves, gastropods and trilobites form a low proportion of the groups present, although they are often diverse. In Upper Silurian limestones, Dicoelosia became rare. The commonest brach-iopods in the limestones deposited at these great depths include Cyrtia, Eoplectodonta, Atrypa and Resserella.

The Dicoelosia Community occurs in the Wenlock and early Ludlow beds of Wales and the Welsh Borderland.

Resserella Life

Fig. 28 Dicoelosia Community a Dicoelosia (Brachiopoda: Orthida) b Nucleospira (Brachiopoda: Spiriferida) c Resserella (Brachiopoda: Orthida) d Atrypa (Brachiopoda: Spiriferida) e Cyrtia (Brachiopoda: Spiriferida) f Eoplectodonta (Brachiopoda: Strophomenida) g Mesopholidostrophia (Brachiopoda: Strophomenida) h Glassia (Brachiopoda: Spiriferida) i Skenidioides (Brachiopoda: Orthida)

Halysites Coelenterata
Fig. 29 Visbyella trewerna Community a Dalmanites (Arthropoda: Trilobita)
Dicoelosia Community

29 Visbyella trewerna Community

This community is a restricted variety of the Dicoelosia Community, but with many elements of the latter absent. It is dominated by Visbyella trewerna, similar very small dalmanellids, and the small spined strophomenide, Protochonetes minimus. It also includes three brachiopod species less common in the Dicoelosia Community: "Clorinda" dormitzeri, Mesounia and Bracteolep-taena. Trilobites, especially Dalmanites, are characteristic. Dalman-ites is unusual in having a long tail spine, and large lenses in its compound eyes, which were raised above the level of its head-shield. So, like the Llandovery Marginal Clorinda Community, the Visbyella trewerna Community contains species more adapted to these deep conditions.

Today there are three depth related features seen in level bottom communities: diversity increases and size and abundance decrease with depth. Comparison between modern suspension feeders and Upper Silurian brachiopods with regard to these changes (Hancock et al. 1974) suggests that the Visbyella trewerna Community may have extended to depths greater than the average shelf. The decrease in diversity may also be related to oxygen deficiency in the deeper waters.

Graptolites and other pelagic forms occur preserved with the Visbyella trewerna Community. Among the latter, straight nauti-loids such as Michelinoceras, and Cardiola interrupta are common. Cardiola is a bivalve not closely related to any modern form. Its mode of life is conjectural, but, as its ecological distribution resembles that of graptolites (commoner in very deep water deposits) it may have lived near the surface of the sea, perhaps attached to drifting algae.

The Visbyella trewerna Community occurs in deep water Upper Silurian sediments in North Wales.

GRAPTOLITIC ECOGROUPS (30)

Animals and plants floating on the surface of the ocean and those living at intermediate depths are not considered as components of the benthic ecogroups and communities. Although their remains sink to the sea flo'or on death, there was no important relationship between the benthos and these nektonic and pelagic forms.

30 Silurian graptolite Assemblage

The dominant Silurian animals in this category are the graptolites, of which there is a large variety. In the Lower Silurian, biserial diplograptids (which are common in the Ordovician) were present together with monograptids (which appeared in the Silurian). Most of the monograptids have a single branch, made up of connected thecae (each of which contained a separate animal), but the cyrtograptids show side branches coming off the convex sides of the main branch. The cyrtograptids occur mainly in the Wenlock. Other groups of Silurian graptolites are the retiolitids (which have a skeleton in the form of a network of connecting rods, and are thus not easy to find in the field) and the dendroids (which include forms ranging from the Cambrian to the Carboniferous).

Graptolite Cluster Colony

Fig. 30 Silurian graptolite Assemblage a Pristiograptus (Hemichordata: Graptoloidea) b Cyrtograptus (Hemichordata: Graptoloidea)

Besides the dendroids, monograptids alone survive through the uppermost Silurian and into the Lower Devonian. Our reconstruction shows a Wenlock Cyrtograptus and the monograptidPristiograptus. This monograptid has several single-branched colonies, each connected by a protein thread (the nema) to a central float of soft tissue.

It has been suggested that different graptolites lived at different water depths. Graptolite faunas are most diverse in shales without benthic faunas. But in shallower water with benthic brachiopods, the number of graptolite genera decreases progressively towards the shallow water habitats. For example, cyrtograp-tids and monograptids with lobed thecae are confined to areas with a Clorinda Community or to deeper waters; biserial diplograp-tids occur only in water deeper than the Pentamerus Community, while only retiolitids, Monograptus priodon and Monograptus tunicis (and their close allies) are present in water as shallow as for the Eocoelia Community (Berry and Boucot, 1972).

Only a very limited number of animals floated (or were suspended) with the graptolites; these include straight nautiloids, the bivalve Cardiola and the horny brachiopod Orbiculoidea. Many of the varied deep water brachiopod communities occur in black shales, rich in carbonaceous matter. It is probable that the carbon is derived from floating seaweed to which the Cardiola, the Orbiculoidea, and perhaps also some graptolites, were attached.

Graptolite communities occur in deep water shales in southern Scotland, the Lake District and Wales.

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Responses

  • pirkko waltari
    Are bivalves and bottomdwellers carnivores?
    7 years ago
  • Jan-Erik
    Where were resserella fosssils found?
    5 years ago
  • furuta luwam
    Where are resserella fossils found?
    5 years ago
  • SINIKKA VIRRANKOSKI
    How old is a pentamerus fossil?
    4 years ago

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