Volcanoes

Fig. g. Geography of North America and northern Europe during the Devonian (modified after House, 1967). The large land area is 'the Old Red Sandstone Continent' which formed after the closure of the Iapetus Ocean in the Late Silurian or Early Devonian. The Old Red Sandstone sediments were laid down in rivers and lakes, and yield freshwater fish and early land plants.

were responsible for the construction of bioherms and other carbonate accumulations in many warm shallow-water environments during the Devonian, which provided new niches for other benthic marine organisms, notably brachiopods, trilobites, molluscs and ostracodes. The role of algae in Devonian bioherms was variable. Encrusting red algae (solenoporans) are conspicuous in some Australian and Canadian reefs, but are absent in German and Belgian reefs. Filamentous blue-green algae (such as Girvanella and Sphaerododium) are geographically more widespread.

The Devonian system is divided into seven stages named after localities in Belgium and Germany where good marine sequences occur. They are:

Upper Devonian

Famennian

Middle Devonian

Lower Devonian

Frasnian

Givetian

Eifelian

Emsian

Siegenian

Gedinnian

These stages can be recognized by graptolites (two zones at the base of the Gedinnian) ammonoids (goniatitids and clymeniids), pelagic ostracodes and conodonts. However, each of these groups was restricted to a certain habitat and so is only found in certain sediments. The thin-shelled pelagic ostracodes are only preserved in fine-grained sediments. The ammonites, like Nautilus today, avoided turbulent environments, and are most common in pelagic carbonates or offshore muds. Conodonts occur in distinct assemblages related to reef distributions (off-reef, reef, and back-reef communities seem to be distinct). Some benthic fossils are also of use in stratigraphic correlation, notably the spiriferides.

In river and lake environments fish are the principal means of correlation, though the significance of plants and spores is important because of their usefulness in correlation between marine and non-marine environments.

The illustrated environments have been based on marine and freshwater Devonian faunas in the British Isles. Although including marine sequences in the nominate area for the system in Devon in south-west England, the British Isles are perhaps best known for the Old Red Sandstone facies, which represent the first major development of Phanerozoic continental sedimentation. The marine facies, exposed mainly in south-west England, Devon and adjacent Cornwall, were much disturbed during the Hercynian orogeny and are still relatively poorly understood compared with the marine Devonian elsewhere in Europe, and in North America and Australia.

31 Lower Devonian Freshwater Communities

The Devonian was the first period in which there were extensive non-marine faunas and floras. The fluvial, lacustrine and estuarine environments supported the first major freshwater faunas, which may have initially invaded these environments during the Silurian.

The most common fish are ostracoderms (Pteraspis, Anglaspis, Poraspis) which probably looked something like large armoured tadpoles. Lacking lateral fins they had little more manoeuverabil-

Archoceras

Fig. 31 Lower Devonian Freshwater Community a Turonia (Vertebrata: Agnatha)

b Pteraspis (Vertebrate: Agnatha)

c eurypterid (Arthropoda: Chelicerata: Merostomata)

d Cephalaspis (Vertebrata: Agnatha)

e acanthodian spine (Vertebrata: Osteichthyes)

f plant debris (Plant)

g fish scales (Vertebrata: Agnatha)

Fig. 31 Lower Devonian Freshwater Community a Turonia (Vertebrata: Agnatha)

b Pteraspis (Vertebrate: Agnatha)

c eurypterid (Arthropoda: Chelicerata: Merostomata)

d Cephalaspis (Vertebrata: Agnatha)

e acanthodian spine (Vertebrata: Osteichthyes)

f plant debris (Plant)

g fish scales (Vertebrata: Agnatha)

ity, unless they were able to utilize their exhalent respiratory currents. Their mode of life is thought to have been benthic, mud or weed being sucked into their jawless mouths, though no marks have been found in the muds to support such an interpretation. Alternatively they might have fed mouth upward on floating vegetation and detritus like tadpoles. The remains are always of adult animals and it is likely that the larval stages were spent in coastal marine waters which would facilitate their migration. On metamorphosis, the fish might have migrated to the rivers and acquired their characteristic dermal plating.

Cephalaspis is another armoured fish with fixed lateral fins which probably gave slightly better stability. This fish is characterized by large sensory areas on the headshield and high-placed eyes. Our reconstruction shows a Turonia in the water above, its body covered with minute denticles. It is depicted as dead, drifting downstream, slightly buoyed by gases from its decomposition. The skeleton would soon rupture and the denticles scatter and be incorporated into the sand and mud. Acanthodian spines can also be found in this environment. The acanthodians are small spiny fish which, though they had jaws, were generally edentulous. Many were microphagous, swimming in deeper v/ater and probably not competing with the agnathids.

Associated with the fish are fragments of eurypterid arthropods, which were scavengers and predators. Individual segments of the carapace can be recognized by the scale-like ornament, and trackways of these animals, generally preserved on the soles of fine sandstones, are known associated with overbank and lake sediments towards the tops of the cycles. A good deal of plant debris can also be found, but no plants have been found in their position of growth.

The Devonian of Wales and the Welsh Borderland is dom-inantly fluviatile though surprisingly few channels or channel associations are known compared with modern analogues. Sedimentation tends to occur in a cyclical manner with relatively coarse cross-bedded sandstones and conglomerates at the base of each cycle representing channel and lag deposits laid down under fairly high velocities. These pass upwards into muds, silts and often rippled sandstones representing quieter sedimentation on the flood plain or in pools and lakes. Soils with calcareous nodules (caliche), known locally as cornstones, are often present at the tops of the cycles indicating tropical soils formed in alternating wet and dry seasons. The section illustrated is constructed from the base of a cycle in the section of the Lower Old Red Sandstone (Dittonian) at Lydney, Gloucestershire, England. No determinations have been made of the plants at this locality, but rocks of about the same age in Wales and the Welsh Borderland yield simple vascular as well as non-vascular plants.

Fish are most common in the conglomerates associated with the bed load but the remains are generally disarticulated and fragmentary. Complete specimens were probably preserved by sudden overwhelming during floods. The more complete carcases at the top of the figure are shown resting on a rippled surface which would probably be reworked later, the debris being transported elsewhere downstream.

Thus the figure illustrates an assemblage of dead individuals swept together by flood waters, but exactly where the fish lived remains uncertain.

32 Lower Devonian Swamp Community

During the Lower Devonian land plants, which had first appeared in the Late Silurian, became common. Although the rootless and leafless psilophytes are seldom well preserved, at the world-famous locality of Rhynie in Aberdeenshire, Scotland the plants were silicified, sometimes in the position in which they grew on a delta marsh at the edge of a small lake, in a volcanic and fumarolic area. They are preserved as chalcedony, having been petrified by the siliceous waters often before much decay had taken place. An indication of how early the silification took place is the good preservation of cell structures. The plants were sometimes petrified in an upright growth position but more frequently delta streams cut through the vegetation and subsequently deposited their sand and plant load, much of the latter partly decomposed.

Rhynie is important not only because of the good preservation of these plants, but because the flora contains some of the earliest vascular forms. Three genera of vascular plants are known, Rhynia being the most common. Its simple, leafless, upright stems arose from rhizomes and the stems branched dichotomously, some terminating with an ovoid sporangium; the stem itself had a narrow woody conducting centre for water surrounded by a broad zone of soft tissue and an outer cuticle broken by stomata. There is much variation in the state of preservation and sometimes only the woody centre and cuticle are preserved. There are many indications of how decay was accomplished since preservation of fungal hyphae and reproductive structures are clear. Horneophyton is a rather similar plant.

Asteroxylon, a forerunner of the giant Carboniferous lycopods bore short leaves. The stem is much like that of Rhynia, but the central conducting zone (xylem) is star-shaped.

Fig. 32 Lower Devonian Swamp Community a Rhynia major (Pteiidophyta: Psilophytes)

b Asteroxylon (Pteridophyta: Lycopods)

c Lepidocaris (Crustacea: ? Branchiopoda)

d Rhyniella (Arthropoda: ? Hexapoda)

e Protocarus (Chelicerata: Arachnida)

f Palaeocharinoides (Chelicerata: Arachnida)

Arthropoda

The Rhynie chert is also remarkable in that evidence of the earliest plant-animal associations have been preserved. There are a number of minute arthropods in the chert (preserved analogously to insects in Tertiary amber). The mite Protocarus crani, with a body length of 0.5mm or less, had strongly cutinized and pointed chelicerae suggesting that it may have pierced plant or animal tissue to imbibe liquid food. Stem injury is not uncommon, some of which could have been due to damage by arthropods. In one instance a group of arachnids have been located in sporangia where they may have been feeding on the spores. The possible spider Plaeocteniza and the spider-like Palaeocharinoides, both less than 3.0mm in length, were small predators. Lepidocaris is a small (3.0mm long) elongated branchiopod shrimp,probably plankton-eating. It must have been preyed on by larger forms of which no evidence has yet been found. Rhyniella is a primitive collem-bolan (silverfish, which used to be considered as a primitive wingless insect). It is 1—2mm long and was possibly a scavenger, though with its well-developed incisor areas it could have pierced plant cuticle. Like so many rich fossil localities, Rhynie shows strong preservational bias, in particular, the larger faunal elements are absent.

There is no actual exposure at Rhynie and the fossils have been, in part, obtained from loose blocks collected from fields and walls. A trench was dug early this century when the significance of the plant-bearing cherts was first realized.

33 Middle Devonian Lacustrine Community

Lacustrine environments are frequently found in the Devonian, especially in and adjacent to the Calcdonides. However, the fauna indicates that there were connections with modern Russia and Canada, so we can conclude that sedimentation did not always take place in completely landlocked basins. One of the lakes encompassed Caithness and the Orkney and Shetland Islands. Water circulation in the lakes was at times incomplete so that the bottom waters became stagnant, and hence anaerobic, providing good preservation potential for the fauna living and dying in the surface waters.

The fauna is quite diverse and includes fish which occupied a variety of ecological niches. A shoal of small acanthodians (spiny sharks) is shown feeding on plankton in the surface waters. The large crossopterygian Osteolepis with lobe-like fins and a powerful tail was probably a scavenger, but neither it nor the predator Cheirolepis, a primitive actinopterygian fish, would have been able

Devonian Actinopterygii
Fig. 33 Middle Devonian Lacustrine Community a Diplacanthus (Vertebrata: Osteichthyes: Acanthodii) b Cheirolepis (Vertebrata: Osteichthyes: Actinopterygii) c Osteolepis (Vertebrata: Osteichthyes: Crossopterygii)

to penetrate to the foul substrate. Bottom-dwelling armoured placoderms such as Coccosteus are not figured. They, and the lung fish Dipterus, must have lived closer to the shore.

Overall, the fauna shows great advances over that of the Lower Old Red Sandstone. The fish had developed powerful jaws, a variety of fins and large eyes; all necessary for effective predation.

The Achanarras Fish Beds of Caithness and the Sandwick Fish Beds of the Orkney Islands are a famous and well-known horizon of Middle Devonian age. The sediments are laminated impure limestones and comparisons may be made with sediments now being deposited on the floors of modern deep lakes. The fauna is well preserved though compressed. The bottom waters would have been considerably deeper than the waters actually lived in, but this distance is reduced in the figure.

34 Middle Devonian Muddy Shelf Community

The marine muddy shelf environment is one that shows relatively little change during Phanerozoic times, frequently being dominated by protobranch bivalves. The environment illustrated has a diverse fauna, which may reflect more permanent substrates.

Most of the faunas were suspension feeders. Brachiopods are common, particularly spiriferides. Some are winged, and have large spiral lophophore supports, and seem well adapted for life in water with relatively low plankton levels. Others are more globose such as athyrids and atrypids; these sometimes developed spines or frills to aid stabilization. Globular rhynchonellids are more typical of turbulent environments.

Also conspicuous here are small chonetids with rake-like cardinal spines aiding stability. Small turbinate or cylindrical solitary rugose corals and colonies of the tabulate Heliolites are quite common. The latter is one of the few Palaeozoic corals that developed intrapolypoidal skeletal integration. Crinoids must have been rooted in the mud or attached to shells. Scavengers are represented by large-eyed phacopid trilobites which were quite often buried in the mud and are preserved without disarticulation.

The substrate illustrated is very much the result of a reasoned guess. Clastic sedimentation decreased in south Devon during the Middle Devonian. Organic remains comprise a greater part of the sedimentary record than hitherto, but muds were still important and sedimentation was often interrupted by volcanic lavas and tuffs.

Few vital substrates have been preserved. The figure has been based on observations made along the south side of Hope's Nose, Torquay, in south-west England. Patches of shells in a good state of preservation are probably in or close to their life position.

Fig. 34 Middle Devonian Muddy Shelf Community a crinoids (Echinodermata: Crinozoa)

Mesophyllum (Coelenterata: Anthozoa: Rugosa) Fenestella (Bryozoa: Ectoprocta) Phacops (Arthropoda: Trilobita) Spirifer (Brachiopoda: Articulata: Spiriferida) Athyris (Brachippoda: Articulata: Spiriferida) g Kayseria (Brachiopoda: Articulata: Spiriferida) h Chonetes (Brachiopoda: Articulata: Strophomenida)

Heliolites (Coelenterata: Anthozoa: Tabulata)

Thamnopora Environment

Fig. 35 Middle Devonian Off-reef Community a nautiloid (Mollusca: Cephalopoda: Nautiloidea)

b Thamnopora (Coelenterata: Anthozoa: Tabulata)

c Syringopora polyps (Coelenterata: Anthozoa: Tabulata)

d coral (Coelenterata: Anthozoa: Tabulata)

e Athyris (Brachiopoda: Articulata: Spiriferida)

f Syringopora (Coelenterata: Anthozoa: Tabulata)

g stromatoporoid (Porifera: Stromatoporoid)

Fig. 35 Middle Devonian Off-reef Community a nautiloid (Mollusca: Cephalopoda: Nautiloidea)

b Thamnopora (Coelenterata: Anthozoa: Tabulata)

c Syringopora polyps (Coelenterata: Anthozoa: Tabulata)

d coral (Coelenterata: Anthozoa: Tabulata)

e Athyris (Brachiopoda: Articulata: Spiriferida)

f Syringopora (Coelenterata: Anthozoa: Tabulata)

g stromatoporoid (Porifera: Stromatoporoid)

Bioherm Stromatoporoid Tabulate

35 Middle Devonian Off-reef Community

The marine shelf faunas of the Devonian are similar in aspect to those of the Silurian, whether in clastic or in calcareous facies. In the latter the epifauna is dominated by spiriferoid and atrypoid brachiopods and the fauna is in general more diverse than in the Silurian. This diversity is exemplified particularly by the trilobites, many of which have highly spinose and extravagantly sculptured skeletons.

The environment illustrated is that of a shallow shelf with a rich assemblage of corals and brachiopods. It was not a reef, for only a few beds show the organisms in their position of growth. The bedding surfaces represent the result of periodic events when the substrate was subjected to considerable turbulence halting growth, levelling off the beds and often depositing a layer of mud or coarser sediment that later had to be recolonized. In fact, most of the beds result from storm action, with broken masses of corals and disarticulated shells jumbled in a muddy matrix.

Stromatoporoids are plentiful, particularly massive forms. Exactly what stromatoporoids were is still not certain. The cellular skeleton did not house polyps (like corals), but they may have been the supports for hydrozoans or possibly algal tissue. Popular opinion at present is that it was the tissue of sclerosponges and that the star-shaped canals are some manifestation of the sponge exhalent system. Stromatoporoids often encrust rugose corals and are here depicted growing in association with the tabulate coral Syringopora. Shrub-like Syringopora also occur without the strom-atoporoid association, exhibiting a growth form suitable for living in an environment subject to mud influxes whilst still maintaining intracolony contacts. Between the low stromatoporoid mounds are abundant but fragmentary Thamnopora (also a tabulate coral). These probably lived between the massive growths as illustrated. There are occasional solitary corals embedded in the calciluite and a variety of brachiopods of which the smooth-shelled athyrids are the most conspicuous. Crinoids and bryozoans were also important, but seldom survive in position of growth. All these animals were filter and tentacle feeders. Trilobites probably scavenged but their skeletons were easily disarticulated. Nautiloid cephalo-pods of various but mainly carnivorous feeding habits must have lived in the waters above. The water was almost certainly too deep for substantial algal growth.

The substrate figured has been based on some 20 metres of Middle Devonian (Givetian) at Triangle Point, at the west end of Meadfoot beach, Torquay, south Devon, England. The rich assemblage lived on a shallow shelf almost free of terrigenous influence. Nearby, in the slightly younger rocks at Lummaton, organic growth was sufficient for bioherms to form.

Karoo Landscapes Diagrams

Fig. 36 Upper Devonian Coastal Deltaic C ommunity Diplocraterion (Trace fossil: ? crustacean) Teichichnus (Trace fossil: ?annelid)

Echinocaris (Arthropoda: Crustacea: Malacostraca)

Fig. 36 Upper Devonian Coastal Deltaic C ommunity Diplocraterion (Trace fossil: ? crustacean) Teichichnus (Trace fossil: ?annelid)

Echinocaris (Arthropoda: Crustacea: Malacostraca)

Coastal environments are not well represented in the geological record. The fauna is specialized and of low diversity.

Fig. 36 is a reconstruction from the Upper Devonian Baggy Formation of north Devon, England. The layered thin sands and muds were probably deposited just off a distributary and were subject to repeated scour by storms and floods. Burrowing offered protection for the fauna against turbulence and, probably more importantly, against changes in temperature and salinity. The burrows show both downward and, more commonly, upward migration suggesting that the animal responsible liked to maintain a constant depth below the substrate, yoyo-ing in response to the changing substrate level. Many modern animals make U-shaped burrows such as the amphipod Corophium on muddy tidal flats, but these are much smaller burrows, less permanent and easily renewed. It is, of course, by no means certain that the animal responsible for the trace fossil Diplocraterion was an arthropod;

various types of worms also make U-burrows. No hard parts have been preserved with the burrows, but associated with Lingula in the bay and coastal lagoon facies is the small phyllocarid arthropod Echinocaris preserved as phosphatic internal moulds. Diplocrater-ion is also present. Some of the phyllocarids may have burrowed but it is not very likely that the lobed and granular carapace of Echinocaris would have much aided burrowing. Small ovoid faecal pellets are distributed in the sediment around and within the burrows. A few other burrows are present, the most conspicuous being Teichichnus which is likely to have been made by a sedimentfeeding polychaete worm. The steep wall-like burrow system is rather similar to that made by the living Nereis.

Scouring by currents has usually removed all traces of the fauna, apart from the burrows. We know little about the other benthic fauna nor what the nekton was; a few fish scales have been reported but are inadequate reasons for any informed guess to be made in the reconstruction.

The north Devon area in the Devonian was seldom far from the southern shoreline of the Old Red Sandstone continent, which moved across the area several times during the period. Oscillations were particularly marked towards the end of the period, just preceding the main transgression of the basal Carboniferous, when the terrestrial input via deltas and estuaries was only just compensated by subsidence.

The Upper Devonian Baggy Formation (from which Fig. 36 is constructed) is a suite of shallow water sediments deposited in a variety ol nearshore environments but always under some degree of influence from a delta or distributary bringing in freshwater, sediment and plant debris. An offshore, benthic fauna, consisting of an assemblage similar to that in Fig. 37 occurs at scattered horizons, and thick-shelled mussels (Dolabra) are associated with the shallower sandstones. Bay environments are represented by muddy sediments where Lingula is often conspicuous, associated with bands of small bellerophontid gastropods swept together by wave action.

37 Upper Devonian Clastic Shelf Community

Devonian offshore shelf environments show little change in faunal or sedimentological aspects from those of the preceding Silurian. The sandy or muddy shelves were still colonized by a diverse fauna of brachiopods, bivalves, trilobites and echinoderms. The only major evolutionary innovation was the productid brachiopod in the Middle and, more typically, the Upper Devonian. However, the

Upper Devonian

Fig. 37 Upper Devonian Clastic Shelf Community a Pterinopecten (Mollusca: Bivalvia: Pterioida) b Ptychopteria (Mollusca: Bivalvia: Pterioida)

c Actinoceras (Mollusca: Cephalopoda: Actinoceratoidea)

d crinoids (Echinodermata: Crinozoa)

e Cyrtospirifer (Brachiopoda: Articulata: Spiriferida)

f Mesoplica (Brachiopoda: Articulata: Strophomenida)

g Phacops (Arthropoda: Trilobita)

h Productella (Brachiopoda: Articulata: Strophomenida)

Fig. 37 Upper Devonian Clastic Shelf Community a Pterinopecten (Mollusca: Bivalvia: Pterioida) b Ptychopteria (Mollusca: Bivalvia: Pterioida)

c Actinoceras (Mollusca: Cephalopoda: Actinoceratoidea)

d crinoids (Echinodermata: Crinozoa)

e Cyrtospirifer (Brachiopoda: Articulata: Spiriferida)

f Mesoplica (Brachiopoda: Articulata: Strophomenida)

g Phacops (Arthropoda: Trilobita)

h Productella (Brachiopoda: Articulata: Strophomenida)

i Chonetes (Brachiopoda: Articulata:

Strophomenida) j Athyris (Brachiopoda: Articulata: Spiriferida) k Camarotoechia (Brachiopoda: Articulata:

Rhynchonellida) 1 Palaeaster (Echinodermata: Asterozoa) m Schellwienella (Brachiopoda: Articulata:

Strophomenida) n Fenestella (Bryozoa: Ectoprocta)

Upper Devonian environment illustrated is almost the last representative of the facies so typical of the Ordovocian, Silurian and Devonian. The post-Palaeozoic rise in infaunal bivalves and echin-oderms and the decrease in epifaunal brachiopods meant a change in the overall aspect of the benthic faunas.

The depositional environment represented in Fig. 37 by the Pilton Formation of southern England varies a good deal in detail. Generally, turbulence was too frequent for any colonized substrates to be preserved. The general composition of the Pilton fauna is similar to Silurian examples, but of course the taxa present are quite distinct. Only a few taxa are illustrated. In the Upper Devonian, productellid and productid brachiopods are conspicuous. The productellids (Whidbornella and Hamlingella) are extremely spinose — the latter may even be described as hairy. The spines on the convex valve were for anchorage, while those on the upper brachial valve, would have protected the animal from settlement of pelagic larvae. The productid Mesoplica probably used its fewer and stouter spines as an anchoring device. It is quite likely that the productel-lids and productids colonized somewhat different substrates but no evidence for this is available. None have been found in life position.

Chonetes and the schellwienellid brachiopods probably lived flat on the substrate as adults, on their convex brachial valve, when they lacked a functional pedicle; Chonetes was perhaps kept stable by its cardinal spines. There are a number of winged spiri-ferides. The large Cyrtospirifer verneuili is conspicuous throughout the Devonian succession. It often developed a broad and flat cardinal area which must have aided its stability. In the Carboniferous part of the formation it is replaced by Spirifer tornacensis. There are a number of more globose brachiopods including athyrids and rhynchonellids, probably adapted to sandier substrates, though some may have been attached to crinoids. The water would have been too deep and muddy for algal growth.

Echinoderms are quite abundant. Elements of crinoids occur in almost every sandstone, but may well have been transported some distance. Starfish, which probably preyed on bivalves, also occur. Their not infrequent complete preservation in thick shelly sandstones testifies to the rapidity of their interment.

The bivalves are mainly epifaunal types, probably most attached by byssal threads to other shells and sand. The bryozoans were also suspension feeders. They formed delicate colonies easily fragmented by turbulence. Fenestella and stick bryozoans were-prob-ably attached to shell fragments.

There is only one species of trilobite in the Devonian part of the Pilton Formation, Phacops accipitrinus, which often reaches over 5cm in length. It probably scavenged and preyed on small soft-bodied elements. It is an interesting species, being the last phacopid, and has a broad geographical distribution across Europe, North Africa and into Asia. In the Carboniferous portion there are a few proetids. There is little evidence of substantial bioturbation by a soft-bodied infauna.

The Pilton Formation of north Devon in south-west England is some 700m thick and straddles the Devonian-Carboniferous boundary; there was ample time for considerable evolution and faunal change to have taken place. The section illustrated is mainly modelled on the shore section at Downend, Croyde, where the faunas are all transported. The actual substrate shown is modelled on similar facies in slightly older strata in New York State described by Bowen et al.(1974), where faunas were occasionally preserved in life position.

38 Late Devonian Basinal Environment

The pelagic animals in the Devonian were quite different from those of the Ordovician and Silurian. Goniatitic cephalopods (and in the Upper Devonian the clymenids) had replaced the graptolites which had become extinct during the Lower Devonian. The cono-donts were probably more abundant and there must have been considerable numbers of thin-shelled planktonic ostracodes.

Fig. 38 illustrates an Upper Devonian basinal environment exemplified by Saltern Cove in south Devon. Saltern Cove, during the Upper Devonian, was at least 100km south of the shoreline and only mud reached the area from the Old Red Sandstone continent to the north. But the bottom was by no means level. The topography of submarine ridges (often volcanic), slopes and hollows favoured gravitational slumping so that the Saltern Cove area, which was on a slope, received slumped muds, disrupted blocks of limy sediment, tuffs, conglomerate (often with coralli-ferous blocks) and agglomerates. The bottom was probably often rocky. There are no examples of pelagic limestones at Saltern Cove, but an example is present near Chudleigh, a locality to the north, where the limestones (often nodular) may be packed with the remains of pelagic animals, particularly ammonoids and cono-donts, indicating very slow sedimentation. The ammonoids at Saltern Cove are small, but this is because it is only the inner juvenile whorls that have been preserved as haematitic internal moulds.

Ostracodes are quite common but are always of pelagic type with rather thin shells bearing a pattern of fine 'fingerprint' lines. The occasional small bivalve Buchiola may occur with both valves together convex side down, suggesting that after death the open valves sank to the substrate where they rested undisturbed. Buchiola was probably attached to weed during life, together with soft-

bodied organisms. Small blind trilobites (Trimerocephalus), which scavenged and burrowed for worms are also present. Blindness in trilobites is typical of the deep-water Devonian and Carboniferous environments and contrasts with the large eyes of shallow-water species of the same group. (See Figs. 34, 37).

Observers at the locality will probably be able to find scattered conodonts, millimetre-sized complex tooth-like structures, which can most easily be picked out from small greenish reduction centres, where red ferric iron has been reduced by organic matter to green ferrous iron. Conodonts are composed of calcium phosphate and are likely to have affinities with the chordates. But the nature of the organisms that housed them is still unknown. There are several current hypotheses: the one suggested in the figure is that they were part of the mouth structure (teeth supports) for a widespread, but mainly pelagic, hagfish-like predator and scavenger.

The ammonoids, conodonts and ostracodes show considerable stratigraphic variation and are important zone fossils.

Styliolina

Fig. 38 Late Devonian Basinal Environment a ostracodes (Arthropoda: Crustacea: Ostracoda)

b Buchiola (Mollusca: Bivalvia: Cryptodonta)

c conodont animal (Conodontophora)

d Archoceras (Mollusca: Cephalopoda: Ammonoidea)

e Tornoceras (Mollusca: Cephalopoda: Ammonoidea)

f Trimerocephalus (Arthropoda: Trilobita)

g Styliolina (Mollusca: Cricoconarida)

Fig. 38 Late Devonian Basinal Environment a ostracodes (Arthropoda: Crustacea: Ostracoda)

b Buchiola (Mollusca: Bivalvia: Cryptodonta)

c conodont animal (Conodontophora)

d Archoceras (Mollusca: Cephalopoda: Ammonoidea)

e Tornoceras (Mollusca: Cephalopoda: Ammonoidea)

f Trimerocephalus (Arthropoda: Trilobita)

g Styliolina (Mollusca: Cricoconarida)

Vertical section through the sediment at Saltern Cove.

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    Is rhynia a living fossil?
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