Tentaculites Silur

e Lingulella (Brachiopoda: Inarticulata) f Callograptus (Hemichordata: Dendroidea) g Conotreta (Brachiopoda: Inarticulata)

Homalopteon, a nileid, is smooth and has much larger eyes than Asaphus or Ogygiocaris; the smooth outline to the carapace may have been an adaptation for easier movement through the mud on the sea floor. The trinucleids have long genal spines extending back from the sides of their heads; when the small posterior part of this animal rolled up, these spines still projected backwards and would have assisted the animal to remain stable even in soft mud. Different species of trinucleids can be distinguished by variations in the ornament round the front of the head.

A few articulate brachiopods occurred in deeper water environments during the Lower Ordovician, but the deeper brachiopods are mostly inarticulates like Lingulella and Conotreta, which are both small compared with the trilobites. Their sparseness, as compared with the Cambrian Lingulella Community (1), and their size may reflect the small amount of food available in suspension near the sea floor in this deeper environment. Both genera had a functional pedicle, but Conotreta probably lived with the flat part of the larger pedicle valve resting on the sea floor.

Most Ordovician graptolites were pelagic but some, like Callo-graptus, were attached to the sea floor. Callograptus resembles its relative Dictyonema flabelliforme (Community 3) in many details, including the large number of branches; its attached mode of life was similar to that of the earlier Middle Cambrian graptolites from which Dictyonema flabelliforme and the later pelagic graptolites were probably derived. The animals which inhabited the numerous small cups on this colony were probably suspension feeders.

It is unlikely that there were many active carnivores in the Ordovician, but scavenging worms, cephalopods and other animals were perhaps present, though only the cephalopods are preserved as fossils. Scavengers, like predators, are normally considerably less abundant than the animals on which they feed. The commonest fossil scavengers in the Lower Ordovician were nautiloids: some of these were straight orthocones but others had a gently curved shell. All the nautiloids have gas chambers which may have been partly full of liquid so that, even when they had a thick heavy shell, the overall density of the animal was probably close to that of sea-water.

Fig. 7 Triarthrus Community a Triarthrus (Arthropoda: Trilobita) b Hypermecaspis (Arthropoda: Trilobita)

Hypermecaspis

7 Triarthrus Community

In muddy environments, where the water near the sea floor was stagnant and poorly oxygenated, the fauna was sometimes restricted to just two trilobite genera, Triarthrus and Hypermecaspis. Both are related to the Cambrian Olenus (Community4) and have numerous segments. Since each segment carried a gill branch (for breathing) an increase in gill branches may have increased the oxygen intake.

In Wales and other areas with Ordovician marine deposits isolated from the main oceans, stagnant water seems to have been locally quite common. There are many extensive sedimentary formations in which only Triarthrus and Hypermecaspis occur as fossils.

8 Carbonate Community

In the Lower Ordovician, carbonates are much more abundant in North America than in England and Wales; they are present in Nevada, New York, western Newfoundland, Greenland and Spitzbergen. In Britain, carbonates similar to those of North America are represented by the Durness Group of north-west Scotland which was probably part of the North American continent at this time, as its faunas are similar. It is probable that most of North America lay in low latitudes during the Early Ordovician. Some carbonates are present around the Baltic Shield; these may also have been near the equator during this time, but their faunas are more similar to the rest of northern Europe. The differences were thus due to isolation, and not to climate. The Baltic area, like

England and Wales, appears to have been separated from North America by a wide Iapetus Ocean.

Similar genera of stromatolites, gastropods, trilobites, brachio-pods and nautiloids, occur in many Lower Ordovician limestones of Scotland and North America. The stromatolites are the commonest fossils in many of the Durness Group carbonates; they are layered dome-shaped structures built up by miscroscopic algae and bacteria. Today they are restricted to very shallow water and it is probable that after the appearance of grazing invertebrates the maximum depth for fossil stromatolites was about 45m (Walter, 1972). Often associated with the stromatolites are large genera of gastropods like Maclurites and Ceratopoea which probably grazed on them.

The sediment surrounding the stromatolites may consist of algal debris and pebbles of calcareous mud which contain fragmented trilobites and brachiopods. The most common trilobites are Hystricurus and Bathyurellus. The low profile of both genera

Fig. 8 Carbonate Community a Apheorthis (Brachiopoda: Orthida)

b Finckelnburgia (Brachiopoda: Orthida)

c Maclurites (Mollusca: Archaeogastropoda)

d operculum of Ceratopoea (Mollusca: Archaeogastropoda)

e Piloceras (Mollusca: Endoceratoidea)

f Tritarthiceras (Mollusca: Nautiloidea)

g Hystricurus (Arthropoda: Trilobita)

h Bathyurellus (Arthropoda: Trilobita)

Maclurite Cambrian

suggests that they were bottom dwellers; they could have moved over soft sediment supported on their long genal spines. Like most trilobites they were probably deposit feeders. Brachiopods are less common than the trilobites. Apheorthis and Finckelnburgia are two orthid brachiopods which occur as single species clumps between the stromatolites.

The nautiloids and other primitive cephalopods reached their maximum development in the Early Ordovician. Some of the gently curved forms, like Piloceras, and some more markedly

Dalmanitina Life Position
10 cm

coiled forms, like Tritarthiceras, may have grazed on the sea floor or may possibly have been buoyant enough to swim.

The Durness Group seldom has more than six or seven different species at any one locality. This is a much lower diversity than equivalent carbonates in Nevada, where sponges and other animal groups occur. But even in Nevada, the Early Ordovician carbonates have a lower diversity than most later carbonate environments. Corals and bryozoans, which became abundant in many later carbonates, were very rare indeed during the Early Ordovician.

9 Graptolite Assemblage

Pelagic graptolites were abundant throughout the Ordovician and we show as an example a Lower Ordovician reconstruction. Graptolites were colonial hemichordates which formed a protein cup around each individual. These cups are normally arranged linearly along branches. Each colony is attached to a float, either directly or by a thread (the nema).

There are two main types of graptolite: the Dendroidea (e.g. Dictyonema) which usually have two types of cup and always have an inner cylindrical canal system (the stolon), and the Graptoloidea which have only one type of cup and never have a stolon preserved fossil. It is the stolon that is the main indication of the graptolites' zoological affinities; a similar structure is present in certain other hemichordates, which ate primitive relatives of the vertebrates. The hard parts of a graptolite have been built up in two layers; the outer layer appears to have formed subsequently to the inner layer, suggesting that it was deposited by some outer covering of soft tissue and that the skeleton was thus internal. An internal skeleton is another indication of affinities between hemichordates and grap-tolites.

Our reconstruction shows two graptolite genera, Goniograptus and Cryptograptus, covered by a thin film of soft tissue. Both genera belong to the Graptoloidea, the order in which all the cups are the same size and in which no inner stolon is seen.

Like many early Ordovician genera, Goniograptus had numerous branches, while Cryptograptus had only two. A thin protein thread extends upwards from between the branches of Cryptograp-tus, which, in our reconstruction, is shown at the centre of a spiral ribbon of soft tissue which acted as a float.

Diverse graptolites are abundant in all marine Ordovician deposits where the rate of sedimentation was low. Most species only persisted for a relatively short time, though there were some exceptions. Since most graptolites were free-floating they have a wide distribution. Graptolites are thus the most useful Ordovician fossils for stratigraphical correlation.

10 Dinorthis flabellulum Community Middle Ordovician

Many genera in shallow marine environments (equivalent to those of the Orthambonites-cr'moid Community (5) of the Lower Ordovician) became more restricted in their environmental distributions during the later Ordovician. It thus becomes possible to distinguish two shallow water benthic communities: one which was probably restricted to coastal areas not far below the low tide level, and the

Graptoloidea

Fig. 9 Graptolite Assemblage a Goniograptus (Hemichordata: Graptoloidea) b Cryptograptus (Hemichordata: Graptoloidea)

other which occurred in slightly deeper conditions (perhaps in up to 15m of water); in addition to depth, the type of substrate could also be important.

The Dinorthis flabellulum Community is one of these near-shore communities. It is common in coarse sandy sediments which display a wide variety of sedimentary structures indicative of very shallow water; in particular it can be found close to the base of transgressive sedimentary sequences (for example the base of the Caradoc of Shropshire).

Only two articulate brachiopods {Dinorthis and Harknessella) were at all common in shallow water high energy marine environments. They were both orthides occupying similar niches, although they seldom occur together in quantity. Some orthides are punctate, like Harknessella, with numerous very small pores extending through the shell at right angles to the surface; others, like Dinor-this, are impunctate (with no pores). In living punctate brachiopods (like Terebratula) there is no obvious function for these puncta-tions; nevertheless it is clear that their development was significant in many brachiopod groups and they are of importance in the recognition and classification of many brachiopod orders and families. Dinorthis and Harknessella are distinguished by various internal features and also by the stronger ribs in Dinorthis. They both had large pedicle openings and were probably anchored vertically on the sea floor.

The inarticulate brachiopods were represented by the lingulid Palaeoglossa. The lingulid family developed the ability to burrow during the Ordovician and can sometimes be found in its vertical life position. By contracting its pedicle a lingulid could obtain the protection of its burrow, and by extension of the pedicle the shell margin could be raised above the sea floor for feeding. This family is unique among brachiopods in having a burrow; this could be an adaptation for environments with periodic sedimentation. Lingula itself is the only modern brachiopod capable of surviving temporary emersion and fluctuations in salinity.

Bryozoans are colonial organisms which often secrete a calcareous skeleton for support and protection. This is made up of numerous small cups, each containing an individual; it may take the form of an encrustation (often on a brachiopod shell) or the skeleton may rise above the surface (rather like some small coral) in a dome or with cylindrical branches. The individuals are suspension feeders; they have alophophore rather like brachiopods. Bryozoans first appear as fossils in the Lower Ordovician and by the Caradoc they were locally abundant in shelf environments. Some, such as Hallopora, were sometimes the dominant constituents of the Dinorthis Community.

By the Middle Ordovician, early representatives of most of the major modern bivalve groups had developed. These included the

Terebratula

Fig. 1 0 Dinorthis flabellulum Community a Plectonotus (Mollusca: Monoplacophora)

b Hallopora (Bryozoa: Ectoprocta)

c Ambonychia (Mollusca: Bivalvia: Pterioida)

d Dinorthis flabellulum (Brachiopoda: Orthida)

e Palaeoglossa (Brachiopoda: Inarticulata)

f Harknessella (Brachiopoda: Orthida)

g Modiolopsis (Mollusca: Bivalvia: Modiomorphoida)

shallow infaunal deposit-feeding nuculoids which were more common in fine-grained clastic sediments. Byssate bivalves (e.g. Modio-lopsis, Lyrodesma and Ambonychia) appeared in the Middle Ordovician: as they were immobile they must have been filter feeders. Ambonychia has fine ribs and Modiolopsis is smooth. Other groups present include the shallow, burrowing actinodonts and the deeper-burrowing early anomalodesmatids: these were probably also filter feeders. None of them had well developed siphons, so they either lived on the surface of the sediment or could only burrow to a small extent, since, having no spihon, they would need to have the posterior part of their shells extending above the sediment. Detritus-feeding monoplacophorans are sometimes also present (such as Plectonotus).

11 Sowerbyella-Dalmanella Community

In Middle Ordovician shallow water environments with finer elastics Dinorthis flabellulum became rarer and several other articulate brachiopods occur (including other species of Dinorthis). These were dominated numerically by two genera: Soiverbyella and Dalmanella. Dalmanella is a punctate orthide like Harknessella. Soiverbyella belongs to a very different group of brachiopods (the strophomenides) which have curved valves with a relatively small space between them for the soft parts of the animal and the mantle cavity. The inner layers of the strophomenide shell contain calcareous spicules (known as pseudopunctations) which are best seen on weathered specimens. Soiverbyella had no functional pedicle and lived resting on its convex valve. This position was unstable in fast currents; most fossil Soiverbyella are found convex side upwards, having been turned over after death. If they were flipped over while alive it is likely that quick muscle contraction could have righted the shell (Cocks, 1970).

Other, but much less abundant, brachiopods include the strongly ribbed biconvex Oxoplecia and the large strophomenide Kjae-rina. Crinoids such as Balacrinus were very abundant at many localities. Both the crinoid stems and the large strophomenide may have provided attachment areas for the pedicles of Dalmanella.

Bryozoa in this community are less robust than those in the Dinorthis flabellulum Community: they are abundant in places and can include six or seven different species.

Several different genera of scavenging gastropods are present. The commonest Archaeogastropod family is the Pleurotomariidae, which includes Lophospira. This family all have a notch on the outer part of their apertures which allowed a symmetrical current to flow over their two gills (many later gastropods have no sinus and only one gill). From the Upper Palaeozoic onwards many

Fig. 11 Sowerbyella-Dalmanella Community a Sowerbyella (Brachiopoda: Strophomenida)

b Dalmanella (Brachiopoda: Orthida)

c Kjaerina (Brachiopoda: Strophomenida)

d Oxoplecia (Brachiopoda: Orthida)

e Balacrinus (Echinodermata: Crinozoa)

f Lophospira (Mollusca: Archaeogastropoda)

Brachiopoda

gastropods were predators, drilling through the shells of their prey by means of their radulae. No Lower Palaeozoic shells are known which have been drilled by gastropod radulae and it is probable that most of these Ordovician genera were scavengers.

Trilobites were never very common in Middle Ordovician shallow water environments but a few forms are occasionally present.

12 Diverse brachiopod Community

In the middle to deep shelf areas there was a large increase in the diversity of the bottom-dwelling faunas, especially among brachio-pods, during the Middle Ordovician. The articulate orthides and strophomenides are the two dominant orders. The large strophomenides are represented by five or six genera; most of them lay free on the sea floor on the gently curved convex valve (in some genera this was the pedicle valve, in others the brachial valve). One member of this group was Leptaena, in which the part of the shell nearest the umbo has strong concentric ribs which helped to keep it stable; the distal parts of the shell were bent sharply upwards to allow the water intake to be clear of the sea floor.

The orthides include both punctate forms (like Dalmanella and Onniella) and impunctate forms (like Platystrophia). At most localities several genera of each type are usually present.

The inarticulate brachiopods are less common than the articulate forms but they are sometimes quite diverse; some collections may contain as many as fifteen articulate and five inarticulate genera.

Although the trilobites continued to diversify until near the end of the Ordovician, this expansion in genera is not reflected by much increase in the number of individuals. Except in the deepest communities, trilobites are always outnumbered by the brachio-pods. The trilobites show a variety of different forms. They include the large smooth Brongniartella, Calyptaulax which has large eyes, and Platylichas which has a crenulated head and tail.

The scavengers include bellerophontid monoplacophorans and orthocones.

In the Middle Ordovician, ostracodes became abundant for the first time. They are small crustaceans (often only one or two mm long) with a bivalve shell, which swim near the sea floor. They occur in most marine sediments but are more noticeable in clays and other fine grained sediments. Crinoids, bivalves and bryozoans are usually less common in the deeper parts of the shelf, in contrast with their abundance in the shallow water environments.

The straight calcareous cones of Tentaculites were common in this environment. Their zoological affinities and their mode of life are both uncertain, but it is likely that Tentaculites is related to

Fig. 12 Diverse Brachiopod Community a strophomenid (Brachiopoda

Strophomenida) b Leptaena (Brachiopoda:

Strophomenida) c Onniella (Brachiopoda:

Orthida) d Platystrophia (Brachiopoda:

Orthida) e Brongniartella (Arthropoda:

Trilobita) f Calyptaulax (Arthropoda:

Trilobita) g Playtlichas (Arthropoda:

Trilobita) h bellerophontid (Mollusca:

(Monoplacophora) i orthoceratid (Mollusca:

Nautiloidea) j Tentaculites (Cricoconarida)

Tentaculites Reconstruction

the molluscs and, as it is most common in shallow water deposits, it is likely to have been epifaunal rather than free-swimming or floating. It occurs mostly in siltstones and sandstones; it is thus unlikely to have been a burrowing deposit feeder. Our reconstruction, showing it resting partially buried, takes account of the possible function of the strong circular ribs which may have helped to stabilize it in the sediment.

This community occurs in Wales and England, but in Scotland (notably the Girvan district) different genera are present in similar environments. These faunal differences occur or) either side of the Iapetus Ocean (see Fig. d). This old ocean was wide enough during most of the Ordovician to prevent the pelagic spat of trilobites and brachiopods from migrating across, so although some species got across in the Late Ordovician, for most of this period the bottom-dwelling faunas were quite distinct on either side.

13 Trilobite-Onniella Community

Some quiet water or deeper-shelf environments of the Middle Ordovician include many diverse types of trilobites and a few species of brachiopods. The reconstruction shown is based on the (type) Caradoc sections of the Onny River in Shropshire. The two most common trilobites are the trinucleid Onnia and the calymenid Onnicalymene. The trinucleids had a large convex head with long genal spines which supported the whole animal. The thorax and tail were small in comparison with the head and appear to have been suspended above the surface of the sediment. Appendages below the thorax probably assisted in food collection. Onnicaly-mene is typical of the calymenid family in having strongly grooved thoracic segments which enabled it to roll up into a ball (in the same way as a modern woodlouse); it is sometimes found fossil in this attitude. The calymenids are one of the Ordovician trilobite groups which survived into the Silurian.

Although most trilobite communities have a high diversity of genera, only three other forms are illustrated here. Chasmops has large eyes which are compound, like all arthropod eyes, but the individual cells are unusually large so that they can be seen easily by means of a hand lens on well preserved specimens. These eyes are raised in semicircular arcs which gave Chasmops excellent all-round vision. Remopleurides was a swimming trilobite with long narrow eyes; this suggests that though it had good vision laterally it could not see forwards or backwards like Chasmops. Lonchodo-mas had extremely long genal spines and also a long rostral spine in front of the head. These spines served to support the trilobites above the fine grained sediment; many other trilobites with genal spines are found in this community, which normally occurs in mud-stones and siltstones.

The most characteristic brachiopod in this community is the orthide Onniella (see Fig. 12). Another brachiopod, which is found locally in large numbers (but not shown here) is the small (4mm) strophomenide Chonetoidea; it has a functional pedicle and may either have been attached to algae on the sea floor or have been epiplanktonic, attached to floating material.

Bivalves and gastropods, usually with thin shells, occur sometimes in the trilobite-Onra'e//a Community.

Trilobita Ecology
Fig. 13 Trilobite-Onniella Community a Onnia (Arthropoda: Trilobita) b Onnicalymene (Arthropoda: Trilobita) c Chasmops (Arthropoda: Trilobita) d Remopleurides (Arthropoda: Trilobita) e Lonchodomas (Arthropoda: Trilobita)

Upper Ordovician 14 Restricted Hirnantia Community

During the late Ashgill many assemblages have a very restricted fauna which often consist of only two dominant genera, with perhaps one or two other less common brachiopods together with a few crinoid and bryozoan remains.

Fig. 14 Restricted Hirnantia Community a Hirnantia (Brachiopoda: Orthida) b Eostropheodonta (Brachiopoda: Strophomenida) c bryozoan (Bryozoa: Ectoprocta)

Fig. 14 Restricted Hirnantia Community a Hirnantia (Brachiopoda: Orthida) b Eostropheodonta (Brachiopoda: Strophomenida) c bryozoan (Bryozoa: Ectoprocta)

Algi Graptus

Hirnantia is a large biconvex dalmanellid with weak ribs and distinctive muscle scars. The other brachiopod often occurring in large numbers in this community is Eostropheodonta, a large flat strophomenide. Eostropheodonta is an early member of a family which becomes common in the Silurian. In addition to the two teeth in the pedicle valve, this family, the stropheodontids, developed denticulations along their straight hinges: these assisted in holding the two valves together when the shell was open for feeding.

The crinoids are usually fragmented and may constitute a fairly large proportion of the sediment. Many areas of the sea floor which were receiving little clastic sediment were probably covered by large numbers of these attached animals. This restricted community also occurs in algal limestones; the algae grow in concentric layers around a nucleus and form subspherical objects 2 or 3mm in diameter (much like large ooliths) and denote growth under shallow clear water; however, not all Palaeozoic crinoidal and algal limestones have a restricted fauna. Trilobites are very rare or absent from this community; this may be due to the substrates having been unsuitable for these sediment feeders.

Brachiopods Orthida

Fig. 15 Diverse Hirnantia Community a Hirnantia (Brachiopoda: Orthida) b Eostropheodonta (Brachiopoda: Strophomenida) c Dalmanella (Brachiopoda: Orthida) d Phillipsinella (Arthropoda: Trilobita)

e Dalmanitina (Arthropoda: Trilobita)

f Brongniartella (Arthropoda: Trilobita)

g Plectothyrella (Brachiopoda: Rhynchonellida)

h Leptaena (Brachiopoda: Strophomenida)

i Tentaculites (Cricoconarida)

Fig. 15 Diverse Hirnantia Community a Hirnantia (Brachiopoda: Orthida) b Eostropheodonta (Brachiopoda: Strophomenida) c Dalmanella (Brachiopoda: Orthida) d Phillipsinella (Arthropoda: Trilobita)

e Dalmanitina (Arthropoda: Trilobita)

f Brongniartella (Arthropoda: Trilobita)

g Plectothyrella (Brachiopoda: Rhynchonellida)

h Leptaena (Brachiopoda: Strophomenida)

i Tentaculites (Cricoconarida)

15 Diverse Hirnantia Community

Hirnantia and Eostropheodonta also occur in beds with a large number of different brachiopods and other benthic animals. Amongst the other brachiopods are Plectothyrella (a ribbed rhyn-chonellide), Leptaena, Dalmanella and its relative Kinnella. As many as eighteen different brachiopod genera are present in some beds. Trilobites may also be important; as many as six or seven different forms can occur, including Phillipsinella, Dalmanitina and Brongniartella, although the trilobites are heavily outnumbered by the brachiopods. There are also a few bivalves, gastropods, ostra-codes and tentaculitids.

This community is very widespread around the world, particularly at the very end of the Ordovician Period. It has been recorded in Wales, Scotland, the Lake District and Ireland, the northern Appalachians, north Africa, Scandinavia, central Europe and Burma. It would appear that few Ashgill oceans were wide enough to prevent the migrations of the faunas. Wales and Scotland, for example, were separated by the Iapetus Ocean at this time (see Fig. d) and Bohemia and north Africa were separated from Wales by the Rheic Ocean. All modern brachiopods and modern Crustacea have pelagic larval stages when the young can drift with ocean currents for periods ranging from three to fifty days. It is probable that the young of these Ordovician brachiopods and trilobites went through the same pelagic larval stages, so that only wide oceans with few oceanic islands could act as barriers to migration.

In parts of north Africa, muddy siltstones containing examples of the diverse Hirnantia Community are found interbedded with glacial deposits. At this time there was an ice sheet covering much of Africa and South America. Thus the community could tolerate cold conditions in high latitudes, but its wide distribution suggests that it was not confined to polar regions.

16 Christiania-Sampo Community

The Ashgill benthic communities, like those of all ages in open marine flat-bottom conditions, show an increase in diversity with depth. The Christiania-Sampo Community characteristically contains fifteen to thirty different brachiopod genera (including Dinorthis and Rafinesquina, and ten to twenty other forms of bottom-dwelling organisms, chiefly trilobites, bivalves, gastropods, crinoids and bryozoans as well as rarer tentaculitids, sponges and other groups. Because of the great diversity in this community it is not easy to designate characteristic genera but the strophomenides Christiania and Sampo occur (often in quantities of five per cent or less) in most collections.

Christiania is a small (1cm) smooth brachiopod which probably rested loose on its convex pedicle valve in a similar manner to the Jurassic bivalve Gryphaea. In the Caradoc, Christiania is absent in England and Wales, but it spread across the Iapetus Ocean from North America and Scotland during the early Ashgill and is found consistently in the deeper water Ashgill shelf environments of Wales.

The trilobites are very varied (only Tretaspis and Brongniar-tella are shown) and can sometimes make up twenty per cent of the total fauna. The trilobites, being deposit feeders, were much more dependent on the type of substrate than were the suspensionfeeding brachiopods and thus the trilobite genera present in this community vary much from one locality to another, depending on the local substrate.

Fig. 16 Christiania-Sampo Community a Sampo (Brachiopoda: Strophomenida)

b Christiania (Brachiopoda: Strophomenida)

c Dinorthis (Brachiopoda: Orthida)

d Tretaspis (Arthropoda: Trilobita)

e Rafnesquina (Brachiopoda: Strophomenida)

f bellerephontid (Mollusca: Monoplacophora)

g Brongniartella (Arthropoda: Trilobita)

h crinoid (Echinodermata: Crinozoa)

Monoplacophora

17 Tretaspis Community

This community is dominated by trilobites. Up to twenty genera may be present at any one locality, but they are often distributed sporadically throughout the rock and large numbers are not easy to collect. The commonest genera are Tretaspis, Lonchodomas, Cybeloides and Remopleurides.

Tretaspis is a trinucleid with small eyes (many of its close relatives appear to have been blind) and long genal spines. Lonchodomas has a rostral spine projecting forwards from its head. These spines may have been either for protection or to prevent the animal sinking into the muddy sediment; since there were few large predators in the Ashgill the latter explanation seems more likely. In his reconstruction of this community, BergstrSm (1973) shows Cybeloides completely covered by sediment apart from its eyes. These were elevated above its dorsal surface, perhaps to project through the sediment as Bergstrbm thought, or perhaps just to allow the animal good all-round vision. Remopleurides had large semicircular eyes; it clearly had all-round vision, as the many cells (which make up all arthropod eyes) face in all directions.

In addition to the trilobites, there were some other groups occurring less commonly in the Tretaspis Community; these include two ostracode genera, an asterozoan, and tubes which may have been made by burrowing worms. Some brachiopods are present but they are not abundant; they include the orthacean Skenidioides, three different plectambonitaceans, and a small sub-spherical spire-bearer. All of these brachiopods are closely related to genera which occur in the Silurian Clorinda Community; they represent the earliest of the deeper water brachiopod faunas. Prior to the Caradoc, brachiopods are largely confined to shallower water environments.

Spire Bearers Brachiopods Silurian

Fig. 17 Tretaspis Community a Tretaspis (Arthropoda: Trilobita)

b Lonchodomas (Arthropoda: Trilobita)

c Remopleurides (Arthropoda: Trilobita)

d Cybeloides (Arthropoda: Trilobita)

e asterozoan (Echinodermata: Asterozoa)

f Skenidioides (Brachiopoda: Orthida)

Fig. d. The distribution of the late Llandovery marine benthic communities of Wales and the Welsh Borderland (after Ziegler, 1965; Ziegler et al. 1968a). The fact that these communities were parallel to the shore (on the east and south) and to the shelf margin (on the north-west) suggests that they are related to the depth of water. Dotted lines indicate the boundaries between communities.

Mollusc Evolution
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