Silurian marine communities west of Dingle, Ireland

79

Palaeogeography, Palaeoclimatology, Palaeoecology, 23(1978): 79--118 © Elsevier Scientific Publishing Company, Amsterdam--Printed in The Netherlands

S I L U R I A N M A R I N E C O M M U N I T I E S WEST O F D I N G L E , I R E L A N D

RODNEY WATKINS 1

Museum of Paleontology, University of California, Berkeley, Calif. 94720 (U.S.A.) (Accepted January 17, 1977)

ABSTRACT Watkins, R., 1978. Silurian marine communities west of Dingle, Ireland. Palaeogeogr., Palaeoclimatol., Palaeoecol., 23: 79--118. Coastal exposures of the Dunquin Group west of Dingle, Ireland, consist of shallow marine and volcanic deposits of Wenlock age. Stratigraphic sequences show a gradient from offshore silts to nearshore silts and sands, followed by nonmarine, lagoonal sediments and volcanics. Bioturbation decreases from the offshore to lagoonal ends of the gradient, and communities of shelled invertebrates are restricted to marine sediments. These communities are dominated by brachiopods, corals, and crinoids, and show the greatest species diversity and trophic complexity in offshore, slowly deposited silts. Nearshore sediments contain low-diversity, brachiopod-dominated assemblages in stormdeposited shell beds and large, isolated colonies of tabulate corals. The Dingle area was probably located along the eastern margin of the Proto-Atlantic Ocean in Wenlock times as a volcanic island area isolated from major shelves across Britain and Scandinavia. Derivatiol of Dunquin communities was from this shelf area, and at least one interval of geologically rapid, faunal invasion is apparent. INTRODUCTION The D u n q u i n G r o u p west of Dingle, Ireland is s h o w n in F i g . l , and consists o f i n t e r b e d d e d volcanics and terrigenous sediments of W e n l o c k and L u d l o w age (Holland, 1969). The W e n l o c k p o r t i o n of the D u n q u i n G r o u p , welle x p o s e d in an extensive series of low seacliffs and tidal o u t c r o p s , has been s e d i m e n t o l o g i c a l l y studied a n d q u a n t i t a t i v e l y sampled for fauna. T h e coastal exposures preserve several facies of shallow marine and n o n m a r i n e s e d i m e n t s w h i c h are t h o u g h t to have a c c u m u l a t e d in a relatively isolated, volcanic island area. T h e y c o n t a i n a b u n d a n t , shelled invertebrates s h o w i n g a n u m b e r of ecologic associations w h i c h c o r r e s p o n d to the p a t t e r n o f s e d i m e n t a r y environments. T h e e n v i r o n m e n t a l , trophic, and b i o g e o g r a p h i c analysis o f this Wenlock-age f a u n a is the p r i m a r y p u r p o s e of this study. Fossil collections are d e p o s i t e d in the University of California M u s e u m o f P a l e o n t o l o g y (UCMP), Berkeley. 1Present address: Department of Geological Sciences, University of Texas, Austin, Texas 78712 {U.S.A.).

80 Glengorriff Harbour

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Fig.1. Location of the Dingle area, County Kerry, Ireland, with general geology after Horne (1974). STRATIGRAPHY Previous work

The most comprehensive description of the Silurian rocks west of Dingle remains that of Gardiner and Reynolds (1902). Although the age assignments and nomenclature of these authors have been revised, their descriptive mapping and lithostratigraphy is highly accurate and has been used in this study, as discussed below. Holland (1969) redescribed these rocks as the Dunquin Group, and designated five formations (Fig.2), ranging from Wenlock to Ludlow in age. Horne (1974) described the overlying, nonmarine Dingle Group in detail, and also provided data on the upper contact and large-scale, structural features of the Dunquin Group. Holland (1969) also proposed the name Dunquin Group for Silurian rocks of the Anaascaul Inlier, east of Dingle, and the Blasket Islands, west of the area shown in Fig.2. These areas are not considered in the present study. Parkin (1974) has recently described the stratigraphy of the Dunquin Group on one of the Blasket Islands.

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Fig.2. Distribution of formations and sample localities in coastal exposures of the Dunquin Group west of Dingle: geology after Gardiner and Reynolds (1902), Holland (1969), and Horne (1974). Numerous faults within the Dunquin Group are not shown.

82

Silurian formations west of Dingle The stratigraphic succession and coastal outcrop of formations in the Dunquin Group west of Dingle are shown in Fig.2. The Ferriters Cove Formation is the oldest rock unit, and its base is not exposed. It crops out only in the vicinity of Ferriters Cove (Fig.2), and corresponds to units C(9) through C(17) of Gardiner and Reynolds (1902). The Ferriters Cove Formation consists of siltstone and fine sandstone with minor conglomerate and volcanic rocks, and has been considered as Wenlock in age by Holland (1969). The overlying Clogher Head Formation crops out along three areas of coast (Fig.2), and corresponds approximately to units A(4) through A(35), B(4) through B(15), C(5) through C(8), and D(4) through D(13) of Gardiner and Reynolds (1902). It consists primarily of volcanic rocks, with subordinate sediments. The Mill Cove Formation of Holland (1969) is a distinctive unit of red sandstone, siltstone, and ashes which everywhere overlie the Clogher Head Formation. The Mill Cove Formation, as shown in Fig.2, corresponds to units A(3), B(3), C(4), and D(3) of Gardiner and Reynolds (1902). The overlying Drom Point Formation (Fig.2) consists of siltstone, fine sandstone, and shell beds, with minor ashes. Its main coastal exposures correspond to units A(1) and A(2), B(1) and B(2), C(1) through C(3) and D(1) and D(2) of Gardiner and Reynolds (1902). Horne (1974) has also mapped two small areas of Drom Point Formation on the northwest side of the Dun An Oir fault (Fig.2). The Drom Point Formation is also known from inland exposures, not shown in Fig.2, from which Holland (1969) reported the brachiopod Rhipidium. Rhipidium, also known from the Dunquin Group on Great Blasket Island (Lamont, 1965), is considered an index for the upper part of the Wenlock by Berry and Boucot (1970). The Croaghmarhin Formation overlies the Drom Point Formation in inland exposures, and was referred to the Ludlow Series by Gardiner and Reynolds (1902) and Holland (1969). The Croaghmarhin consists mainly of calcareous siltstone, and is not treated in this study. Holland (1969) described a continuous, stratigraphic gradation between the Croaghmarhin Formation and the overlying Dingle Group, an extensive, nonmarine unit. In the coastal sections, the Dingle Group is in contact with the Drom Point Formation, and the Croaghmarhin Formation is not seen (Fig.2). These contacts have been discussed by Holland (1969) and Home (1974), and involve either faulting or an unconformity at the top of the Drom Point Formation.

Structure and stratigraphic columns Gardiner and Reynolds (1902) described four repeated sections of Silurian rocks along the coastline shown in Fig.2, which they designated with the letters A through D. Although wrong in their interpretation of stratigraphic relationships on the northwest side of the Dun An Oir fault, they were otherwise correct in their recognition of the four coastal sections (Shackleton,

83

1940; Horne, 1974). Horne (1974) named the two anticlines and syncline which repeat these sections, shown in Fig.2. Within the four coastal sections of the Dunquin Group, Gardiner and Reynolds {1902) recognized a detailed subdivision of lithologic units, whose relation to the modern formation names has been given above. The descriptive accuracy of this work is impressive, and the numbered units in sections A through D of Gardiner and Reynolds (1902) have been compiled as stratigraphic columns in Figs.3, 4, and 5. These columns provide the discipline in

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Fig.3. Generalized stratigraphic columns for the coastal exposures shown in Fig.2; lithologic symbols are explained in Fig.4. The letter designations of columns follow Gardiner and Reynolds (1902), f r o m w h o m thicknesses were compiled. Because of structural complexities, these thicknesses must be treated as approximate, as discussed in text.

this study for the stratigraphic relations of collecting localities, sedimentary facies, and benthic communities. They have not been remeasured, but many small, detailed stratigraphic sections were measured at particular places within the columns for sedimentologic analysis, as presented later. Faulting is very common throughout coastal outcrops of the Dunquin Group, as discussed by Holland (1969). While the stratigraphic thicknesses for individual, numbered lithologic units given by Gardiner and Reynolds (1902) are reasonable approximations, their entire, stratigraphic columns include many small and large-scale faults. The reader should not be mislead by the omission of these faults from the columns in Figs.3, 4, and 5, where the total, stratigraphic thicknesses must be treated as tentative.

84

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~ J M A J O R VOLCANIC INTERVALS, rhyolite, ashes, and greenstone.

Fig.4. Distribution of sedimentary facies and benthic communities within the stratigraphic columns shown in Fig.3, which should be referred to for names of formations.

SEDIMENTARY FACIES Facies 1

Facies 1 consists of calcareous siltstone with abundant disseminated shell material. It is restricted to the Ferriters Cove Formation, where it crops out in three areas along Ferriters Cove as two stratigraphic developments within the formation (Figs.4, 5), These occurrences are 25--35 m thick, and sediment texture and composition are very uniform throughout. Sedimentary structures were not observed, and a thick, blocky bedding is irregularly developed as a weathering feature. The siltstone is green to bluish grey when fresh, and in polished section shows a mottled pattern related to proportions of silt and clay-sized material. Shells and abundant shell fragments comprise 16% of sediment volume. These bioclasts are scattered fairly uniformly in sections studied in the laboratory, and supported by silt grains and clay. Most bioclasts are granule-size or smaller.

85 DIVERSITY

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Fig.5. Stratigraphic c o l u m n C, as located in Fig.2, with lithologic symbols as in Fig.4. Thicknesses, as compiled from Gardiner and Reynolds (1902), are approximate. The Diversity Index gives the number of species of brachiopods, molluscs, tentaculites, cornulites, trilobites, ostracods, and corals at a sample size of 50 individuals, following the rarefaction m e t h o d of Sanders.

The apparent lack of sedimentary structures, complex mottling, and grainsupported dissemination of shell material within Facies 1 is indication of intense, deformative bioturbation. Facies 1 corresponds to modern, bioturbated shelf sediments described as the "shelf mud facies" and "transition zone" by Reineck and Singh (1973). Such sedimei~ts accumulate by settling of suspended silt and clay under quiet-water conditions below wave base and away from shore. Sedimentation is slow enough for the continued activity of burrowers to prevent the preservation of discrete trace fossils.

86

Facies 2

Facies 2 consists of siltstone and fine sandstone with common shell lenses, laminated beds, and variable bioturbation. It is the most common, marine facies in the coastal exposures, and occurs within the Ferriters Cove Formation, subordinate parts of the Clogher Head Formation, and Drom Point Formation (Figs.3, 4). Bedding is apparent throughout the facies as contacts between the sedimentation units described below, and unweathered rock colour ranges through shades of blue, green, and olive green. The sediment is mostly siltstone to silty, fine-grained sandstone, always containing minor amounts of clay. Medium sand to granule-sized clasts are c o m m o n within shell lenses, and rarely occur as small lenticles within laminated siltstone and fine sandstone. Shell material comprises less than 1% of sediment volume outside of the discrete, shell lenses described below, and consists mainly of scattered, crinoid ossicles. Much of the siltstone and fine sandstone appears homogeneous in outcrop, and in polished section, shows a mottled texture similar to that described for Facies 1. Remnant areas of parallel lamination and low-angle cross lamination are present in these sediments. In the Drom Point Formation, homogeneousappearing sediment can also be traced into Chondrites lenses which overlie and disrupt well-laminated sediment. These relations suggest an original, laminated condition during deposition of Facies 2, with early disruption by bioturbation. The lower part of the rock slab illustrated in Fig.6B is an example of the homogeneous siltstones. Discrete burrows are present throughout Facies 2 as the trace fossil Chondrites, which has been figured by Simpson (1957, pl. XXIII, fig.2) and Gardiner and Reynolds (1902, fig.ll). Chondrites is only occasionally seen in the Ferriters Cove Formation and Clogher Head Formation, but is very abundant in Facies 2 sediment of the Drom Point Formation. Here it forms dense burrow systems within discrete lenses of siltstone and sandstone. The small-scale occurrence pattern of Chondrites lenses is shown in Fig.7. They are generally 20 cm or less thick, and pinch out within lateral distances of 50 cm. The dense, burrow systems either grade into homogeneous sediment, or extend, with decreasing density of burrows, as disruptions into the upper portions of underlying, laminated siltstone and sandstone (Fig.7). Other types of undeterminable trace fossils are present in Facies 2, including subvertical, cylindrical burrows about 5 mm in diameter. Well-laminated units of siltstone and fine sandstone occur throughout Facies 2, but are most c o m m o n in the Drom Point Formation, and are shown in Figs.6A and 7. Laminae are generally 2 mm or less thick, and defined by differences in grain size. In tl~e Drom Point Formation, the laminated units are developed as tabular beds or amalgamated beds (as defined by Goldring and Bridges, 1973), with erosional bases and burrowed tops. These relations are not clearly seen through much of Facies 2 in the Ferriters Cove Formation and Clogher Head Formation, probably because of more intense bioturbation.

87

Fig.6. Sediments of Facies 2 within the Drom Point Formation. A. Fine sandstone with low-angle cross-bedding, interrupted by two lenses of dense Chondrites above hammer. Length of hammer head is 17 cm; this outcrop is near locality D-7222 (Fig.2). B. Vertical section of a shell bed, showing sharp, basal contact upon bioturbated siltstone, locality D-7221. C. Latex cast of a surface within the same shell bed, showing dense crinoid ossic|es and the brachiopod Sphaerirhynchia wilsoni, UCMP 14022

L a m i n a e m a y be parallel and flat to wavy, or f o r m low-angle cross-sets of planar to t r o u g h f o r m (Fig.6A). S y m m e t r i c a l ripple marks are c o m m o n l y associated w i t h these units, with observed heights of 1--4 cm and wavelengths o f 1 8 - - 2 5 cm. Most bioclasts in Facies 2 o c c u r as discrete lenses of shells and shell fragments, w h o s e stratigraphic f r e q u e n c y of o c c u r r e n c e is s h o w n in Figs.7, 8, a n d 9. Shell material c o m p o s e s a b o u t 30% of the lenses b y v o l u m e , and is

88 m

m SILTSTONE 8. FINE SANDSTONE WITH DENSE C'~VONOA'/FLr$

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Fig.7. Detailed sections showing sedimentation units within Facies 2 of the Drom Point Formation. These sections can be located in Figs,2 and 3 with the "D" locality numbers. closely packed within a matrix of silt, sand, and occasional, intraformational pebbles (Fig.6B--C). The shell lenses usually have erosional bases and a sharp, textural distinction from surrounding sediment produced by the abundant bioclasts and larger, average sand size (Fig.6B). Although rarely forming tabular beds traceable across an outcrop, most shell lenses are thickest centrally and pinch o u t laterally. Twenty shell lenses scored around locality D-7217 in the Clogher Head Formation had a mean thickness of 6.4 cm (range of 1--25 cm) and a mean, apparent length of 52.8 cm (range of 8--200 cm). Facies 2 represents a shallow shelf environment with periodic, r a p i d sedimentation and c o m m o n transport of shell material. The laminated units within Facies 2 correspond to those discussed b y Goldring and Bridges (1973), who related them to a turbidity-current type of deposition on shallow, subtidal shelves. Tabular beds of laminated sand have been described from modern, bioturbated shelf sequences by Gadow and Reineck (1969) and Reineck and Singh (1972), and related to rapid sedimentation during storms. The shell lenses within Facies 2 also record periodic conditions of rapid emplacement and high depositional energy, as shown by their erosional bases and coarser texture than surrounding sediment. Similar shell concentrations have been described from Jurassic sediments by Brenner and Davies (1973), who interpreted them as storm deposits. Facies 3

Facies 3 consists of red siltstone and sandstone with subordinate ash beds. It comprises the Mill Cove Formation, and also occurs in the lower part of the

89 Ferriters Cove Formation (Figs.3, 4). The siltstone and sandstones form thick, tabular beds which have been illustrated by Gardiner and Reynolds (1902, fig.12). These beds are characterized internally by flat, parallel laminaUon, and rarely contain small, bedding-parallel lenticles of medium to coarse sandstone. Poorly preserved burrows filled with green silt or sand occur sporadically in the laminated beds, oriented at all angles to bedding. Intricately mottled areas of red and green sediment were observed to grade laterally from the discrete occurrences of burrows in the Mill Cove Formation north of Drom Point, indicating more intense bioturbation. Red ashes occur throughout Facies 3, forming tabular beds 20--200 cm thick. The ash beds have sharp, flat bases and include abundant, volcanic rock fragments from granule to 5-cm size. The clasts are usually dispersed throughout the ash bed, but sometimes show a crude grading toward larger sizes at the base of the bed, or occur in bedding-parallel lenticles suggesting the action of water currents. The tops of ash beds have been slightly reworked by currents, with lenticles of ash included in overlying siltstone and sandstone. Facies 3 does not contain marine fossils and occupies a stratigraphic position between shallow marine sediments of Facies 2 and extensive, volcanic intervals, as discussed later. On this basis it is considered to represent a nonmarine, lagoonal type of environment. A 10-m interval of conglomerates, unit C(10) of Gardiner and Reynolds (1902) in the lower part of the Ferriters Cove Formation, has been classified as Facies 3 for convenience, and is shown stratigraphically between localities D-7233 and D-7234 in Fig.5. These conglomerates consist of rounded pebbles to 15-cm size in a poorly sorted matrix of silt and sand, with two tabular interbeds of parallel-laminated red sandstone. They may represent grain-flow deposits into a marine environment, as they have relatively flat, parallel contacts and immediately underlie offshore marine sediments of Facies 1.

Facies 4

In the Clogher Head Formation, Facies 4 consists of brown siltstones at the transitions from sedimentary to volcanic units (Figs.3, 4, 8), and was described by Gardiner and Reynolds (1902) as "sandy slates". The siltstone shows flat, parallel lamination throughout, with no major bedding planes, and in contrast to other facies in the Dunquin Group, appears to completely lack bioturbation. Reworked, silty ash, with sorted bands of clasts, forms lenses several mm to 50 cm thick within the siltstone. Facies 4 also occurs at the contact of the Ferriters Cove Formation and Clogher Head Formation near locality D-7224 (Fig.9). Here it consists of green, clayey siltstone, also with flat, parallel lamination and no evidence of bioturbation. Silty ash, sorted by grain size into flat, parallel laminae, is also present. Facies 4 is interpreted below.

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Fig.8. T w o stratigraphic sections within the Clogher Head Formation showing the transition from marine sediments to the basal portions of major volcanic units. Stratigraphic and geographic location of these sections can be found in Figs.2 and 3 with the " D " locality numbers.

Stratigraphic analysis of environmental gradients The stratigraphic columns of Wenlock sediments and volcanics in the Dunquin Group (Figs.4, 5) may be used to reconstruct environmental gradients following Walther's law of facies. Major units of volcanic rocks have a ratio of 1:2.4 with the sedimentary facies, and occur within the Clougher Head Formation. Gardiner and Reynolds (1902) have described their component rhyolites and rhyolitic ashes, some of which are ignimbrites (W. S. McKerrow, personal communication, 1974). Lenses of red, parallel-laminated siltstone within the lower Clogher Head Formation near locality D-7224 indicate local, subaqueous

91

deposition of the volcanics. However, most of the rhyolites and massive ashes were probably deposited subareally, and are considered to represent the landward end of the environmental gradient in the Dunquin Group.

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Fig.9. Detailed section across ~he c o n t a c t of the Ferriters Cove F o r m a t i o n and Clogher Head F o r m a t i o n , w h i c h can be located in Fig.2 with reference to locality D-7224.

In those sections which have been studied in detail, the major, volcanic units in the Clogher Head Formation are stratigraphically separated from shallow marine sediments by nonmarine deposits of Facies 3 or Facies 4 (Figs.4, 5, 8, 9). Facies 3 and 4 were not favorable environments for bottom life, as indicated by the sporadic occurrence or absence of bioturbation, and they do not contain marine shells. They are considered to represent shallow lagoons between volcanic land areas and the open sea.

92 Stratigraphically adjacent and sometimes interbedded with the nonmarine deposits are siltstones and fine sandstones of Facies 2 (Figs.5, 8, 9), containing common bioturbation and a marine, shelly fauna. Because of this proximity, Facies 2 is considered to represent a shallow, nearshore environment. Its shell beds and sedimentary structures indicate agitated water, and suggest high sedimentation rates during storms. The offshore end of the environmental gradient is represented by calcareous siltstone of Facies 1, which stratigraphically bounds Facies 2 within the Ferriters Cove Formation (Figs.4, 5). The evidence presented for intense bioturbation in Facies 1 suggests a very slow sedimentation rate, which is in accord with its uniform, fine grain size and lack of current-produced, sedimentary structures. DEFINITION OF COMMUNITIES Four marine communities have been recognized in the Wenlock coastal exposures of the Dunquin Group. They are described below using the term "Association", and their stratigraphic and facies occurrence is shown in Figs.4 and 5. Quantitative composition of the associations is given in Tables I--IV, based on laboratory preparation of 26 samples of bulk rock collected from the locations shown in Fig.2. The samples were broken along fissile surfaces with a mechanical rock-splitter, and all observed macrofossils were counted. Individuals were scored directly for gastropods, tentaculites, cornulites, solitary corals, and cystoids. Tabulates were scored in Tables I--IV as number of colonies, and bryozoa as number of macroscopic, colony fragments. Brachiopods and bivalves were scored as the sum of articulated shells, maximum number of either type of opposing valve, and half the number of indeterminate, single valves. Trilobites were scored as number of pygidia or cranidia. Very rare fossils, such as a single conularid fragment, were scored simply as 1. Preparation of the samples was supplemented by field observations of fauna. The "Associations" were defined as similar assemblages of taxa occupying the discrete, stratigraphic intervals shown in Figs.4 and 5. The stratigraphic definition of ancient communities has been discussed by Watkins {1977), and is analogous to the definition of modern, marine communities along linear, environmental transects (DSrjes, 1972). In this method, it is the relation of a sample or bed of fossils to faunas stratigraphically above and below which determines its c o m m u n i t y assignment, and not its specific, quantitative composition. Further discussion is given below with reference to the Holcospirifer bigugosus Association. THE DOLERORTHIS R USTICA ASSOCIATION Occurrence and taphonomy

The D. rustica Association is restricted to Facies 1 of the Ferriters Cove Formation. Representative taxa are illustrated in Figs.10 and 11, and faunal

93

counts of four samples are given in Table I. Shells occur in fairly equitable density throughout the calcareous siltstone, and are oriented at all angles, though most are parallel to bedding. Measurements from four samples show a density of 1140 crinoid ossicles per 1000 em 2 of bedding surface, and 75 shells of other groups per 1000 cm 2 , exclusive of fragments. The dispersed occurrence of shells in a fine-grained matrix, m t h o u t associated, current-produced sedimentary structures, corresponds to the "disturbed neighbourhood assemblage" of Scott (1974). In such assemblages, the only significant movement of shells after death has been their mixing through the sediment during bioturbation. Many brachiopod valves show primary, non-compactional breakage (Fig.10K , M, O), and shell fragments below about 3 mm in size far outnumber identifiable fossils. The broken edges of fragments and shells are unabraded, as is the

Fig.10. Trilobites and brachiopods from the Dolerorthis rustica Association of the Ferriters Cove Formation, all x2. A. Lichid, latex cast of cranidium, UCMP 14021. B. Encrinurus sp., latex cast of incomplete pygidium, UCMP 14018. C. Proetid, internal mould of glabella, UCMP 14023. D. Acidaspis sp., internal mould of cephalon, UCMP 14019. E. Dalejina sp., latex cast of pedicle valve exterior, UCMP 14020. F. Dalejina sp., internal mould of brachial valve, UCMP 14026. G. Hedeina crispa (Linnaeus), internal mould pedicle valve, UCMP 14025. H. Dolerorthis rustica (J. de C. Sowerby), latex cast of brachial valve exterior, UCMP 10950. I. D. rustica, internal mould of brachial valve, UCMP 10937. J. D. rustica, internal mould of pedicle valve, UCMP 14075. K. Isorthis sp., internal mould of pedicle valve, UCMP 10948. L. Mesopholidostrophia sp., internal mould of pediele valve, UCMP 10947. M. Nucleospira pisum (J. de C. Sowerby), internal mould of brachial valve, UCMP 10935. N. Amphistrophia funiculata (M'Coy), internal mould of pedicle valve, UCMP 14029. O. Coolinia applanata (Salter), internal mould of brachial valve, UCMP 14028. P. A trypa reticularis (Linnaeus), internal mould of pedicle valve, UCMP 14027. Q. Gypidula sp., internal mould of pedicle valve, UCMP 14024.

94

external ornament of various taxa. A high proportion of broken shells and fragments also characterize the Elton Beds of the Welsh Borderland Ludlow, another fine-grained, quiet-water deposit (Watkins, 1977). This type of breakage is probably related to the effects on shells of long-term exposure and bioturbation, and not to transportation. Tabulate corals

Measurements in the field indicate that tabulate corals comprise 8% of sediment volume in Facies 1, making them much more important in biovolume within the D. rustica Association than is suggested in Table I. Tabulate colonies are distributed very widely and evenly through the sediment, and 18 colonies

F i g . l l . Gastropods, corals, crinoids, and bryozoa from the Dolerorthis rustica Association of the Ferriters Cove Formation, all x l . A. Euomphalopterus alatus (Hisinger), latex cast, UCMP 14023. B. Trochoid coral, internal mould, UCMP 10949. C. Syringoporid, latex cast, UCMP 14015. D. Oriostoma sp. latex cast, UCMP 14016. E. Favositid, latex cast, UCMP 14007. F. Articulated crinoid arm with pinnules, latex cast, UCMP 14008. G. Bryozoan, latex cast, UCMP 14009. H. Halysitid, latex cast, UCMP 14013.

95 were observed on 3.22 m s of bedding surface. This dispersal probably reflects their occurrence in life. Two species of syringoporid and halysitid ( F i g . l l C , H) had basal attachments to shell material, and grew upward as free, erect branches. Of 28 observed colonies, 61% were in probable, life position, and 39% were overturned. Nearly all tabulate colonies are below 15 cm in size. Measurements of 18 erect, syringoporid colonies are representative of those for the other corals. Mean breadth of these colonies is 13.7 cm (range of 8--42 cm), and mean height is 3.9 cm (range of 3--8 cm). In addition to the syringoporid and h a l y sitid, a heliolitid and two species of favositids were collected from the D. rustica Association. These have globular colony forms adapted for the "rolling" mode of life discussed by Abbott (1975).

Other fauna According to the methods used for scoring fossils, brachiopods comprise 35% of fauna in the D. rustica Association. They are the most diverse group within the community, and range in size from species a few mm in length to Protomegastrophia, which may exceed 6 cm. A wide range of brachiopod lifehabit groups are represented, and some characteristic species are shown in Fig.10. Species of Dolerorthis, Dale]ina, Gypidula, A trypa, and ~edeina predominate, and a complete list of brachiopod species is given in Table I. Bryozoa comprise 30% of fauna in the association, and occur through most sediment of Facies I as abundant, colony fragments below 1 cm in size. These fragments show an upright, branching mode of growth ( F i g . l l G ) , and a few, fairly complete branch systems have overall heights reaching 4 cm. Several species appear to be present, but the collected material consists only of decalcified, external moulds. A few globular bryozoan colonies, below 2 cm in breadth, have also been found (Table I). Solitary, trochoid corals comprise 16% of fauna. They are preserved only as moulds ( F i g . l i B ) , but appear to represent a single species. Forty-seven individuals measured from locality D-~230 have a mean, apertural diameter of 8.5 mm (observed range from 5 to 18 mm). The gastropods EuomphaIopterus and Oriostoma comprise 8% of fauna, and the individuals shown in F i g . l l A and D reach the maximum sizes for each taxon. Four species of trilobites are present (Fig.10), and comprise 3.7% of the sampled fauna. Very rare groups include bivalves, ostracods, a conularid, and a cornulitid, listed in Table I. Crinoids were also important members of the D. rustica Association, but except for one complete arm ( F i g . l l F ) , they have been found only as dissociated ossicles. The ossicles are derived from both crown and stem, and have a ratio of 15:1 with other, identifiable fossils, based on the surface area--density measurements. A few of these ossicles may represent non-crinoids, as articulated cystoids have been found at locality D-7234 ii~ the Ferriters Cove Formation.

96 TABLE I S p e c i e s c o m p o s i t i o n o f t h e Dolerorthis rustica A s s o c i a t i o n ( D ) a n d a s a m p l e n o t a s s i g n e d to an a s s o c i a t i o n (U), F e r r i t e r s Cove F o r m a t i o n D

D

D

D

U

t'~

¢q t'~

~,I t" ~

¢q t'~

¢q t~

--. -59 --

15 10

--

1 16 8

----

5

--

ANTHOZOA

F a v o s i t i d sp.1 Favositid sp.2 Halysitid Heliolitid Solitary rugose coral Syringoporid

4 -2 3 33 --

5 -. 1 31 --

.

2

.

BRACHIOPODA

Amphistrophia funiculata ( M ' C o y ) A trypa re ticu laris (Linnaeus) "Camarotoech& " sp. Coolinia applanata ( S a l t e r ) Craniops implicata (J. de C. S o w e r b y ) Dalejina sp. Dolerorthis rustica (J. de C. S o w e r b y ) Gypidula sp. Hedeina crispa ( L i n n a e u s ) Indeterminate rhynchonellides Isorthis sp. Mesopholidostrophia sp. Nucleospira pisum (J. de C. S o w e r b y ) Protomegastrophia sp. Rhynchotreta cuneata ( D a l m a n ) Salopina cf. conservatrix ( M c L e a r n )

5

2

I0

--

7

3

.

.

.

--

--

--

--

--

28

20

--

1

1 8

1 --

.

2

--

17

2

--

1 i

--

121 --

--

60

--

19

--

4

31

3

3

---

--

3

--

--

2

--

--

1

--

1

15

--

--

5

--

--

--

3 15

2

5 6

21

--

7

1

--

--

3

BRYOZOA Ramose forms Globular forms

14 - -

40 - -

1

204

37

- -

5

5

-10

6 6

-2

- -

2

- -

2

--

1

--

GASTROPODA

Euomphalopterus alatus ( H i s i n g e r ) Oriostoma sp. 2

11 21

Indeterminate gastropod

- -

8 7 - -

BIVALVIA

Actinopteria cf. sowerbyi ( M ' C o y ) Conocardium sp. I n d e t e r m i n a t e bivalves

1 --

1

--

1

97

TABLE I (continued) D

D

D

D

U

TRILOBITA

Acidaspis sp. Encrinurus sp.

m

Lichid

--

- -

3

Proetid

- -

5 --

6

7

1

F

1

i

--

1

4

--

I

2

--

1

- -

MINOR GROUPS Conularid

Cornulites sp.

m

.

_ _

.

.

Cystoid Ostracods

.

.

.

.

TOTALS

154

- -

.

.

.

150

.

.

1.

.

.

111

2

2

1

452

194

THE HOLCOSPIRIFER BIGUGOSA A S S O C I A T I O N

Occurrence and taphonorny The H. bigugosa Association is restricted to Facies 2 sediments, and occurs in the Ferriters Cove Formation and sedimentary intervals within the Clogher Head Formation (Figs.3, 4). Its faunal content is shown in Fig.12, and listed quantitatively in Tables II--III. The association characteristically occurs as lenses of concentrated shell material (Figs.8, 9) whose sedimentologic characteristics have been described under Facies 2. These lenses are not pure shells, being volumetrically dominated by silt and sand. They are distinguished, however, by a sharp discontinuity of shell density between the lens and the more extensive, surrounding areas of sediment with very few shells. The measured density, by surface area, of whole, identifiable shells within 7 lenses is given in Table IV. An average of 267 crinoid ossicles per 1000 cm 2 and 79 other types of shell per 1000 cm 2 was recorded. These densities are 26 times greater than those in siltstone and sandstone outside of shell lenses, where an average of 10.2 crinoid ossicles per 1 0 0 0 cm 2 and 3.3 other types of shell per 1 0 0 0 cm 2 was recorded. The very low-density shell occurrences in bioturbated sediment within Facies 2 can be considered as disturbed neighbourhood assemblages, as

38 discussed above. Field observations show that their fauna is qualitatively identical to that of the shell lenses, consisting of ~Iolcospirifer, "Camarotoechia Pteronitella, etc. However, far fewer specimens, and consequently, fewer species have been observed in the low-density areas. The shell lenses themselves are mechanically transported accumulations which are probably due to brief, high-energy effects of storms. These 4enses ~re the source of the samples listed in Tables II and III, upon which the :liscussion of faunal content in the H. bigugosa Association is based. In spite of shell transport, the samples are useful for community analysis for the following, empirical reasons: (1) they are qualitatively like the low-density, disturbed neighbourhood assemblages in species content; (2) faunal content from one lens to another, although variable, has an overall similarity, and contrasts with the stratigraphic intercalations of the D. rustica Association; (3) shell lens samples of the H. bigugosa Association have close, taxonomic parallels in other Silurian sediments as the "Salopina Communities" of Calef ~nd Hancock (1974) and Watkins and Boucot (1975). These relations indicate that the transported shells lived on b o t t o m s like Lhe sediment with which they are interbedded. The distance the shells have been moved is unknown, but may potentially be in the order of kilometres. Reineck and Singh (1973) discussed storm transport of the gastropod ~Iydrobia from North Sea tidal flats onto the offshore shelf. The gastropods arrive ~live and undamaged when deposited several kilometres from their living sites, indicating very rapid, turbidity-current transport. This type of transport may be involved with the H, bigugosa Association. It has the potential for moving ~hells to depositional sites from relatively restricted source areas, without significant sorting or abrasion.

Brachiopods and faunal variation between lenses Articulate brachiopods comprise 83% of the non-crinoid fauna sampled from shell lenses. Although seven species have been recorded, only Salopina of. conservatrix, Protochonetes ludloviensis, "Camarotoechia " sp., and Holcospirifer bigugosus are c o m m o n (Tables II, III). Bassett et al. (1976) have redescribed H. bigugosus, which represents a genus and species endemic to the Dingle area. The morphology and size range of these brachiopods is shown in Fig.12. Dominance relations of brachiopods vary markedly from one shell lens to another, as shown by numbers in Tables II and III. Field observations show several, stratigraphic patterns to this variation. In the upper part of the Clogher Head Formation at localities D-7240 and D-7241, observed shell lenses are dominated by "Carnarotoechia" throughout the section. In contrast, nearly pure lenses of either "Camarotoechia" or Holcospirifer alternate over short, stratigraphic distances in the Ferriters Cove Formation at localities D-7231 and D-7232. The upper 20 m of the Ferriters Cove Formation shows another pattern. Here, an interval in which shell lenses are consistently

99

Fig.12. Shells from the Holcospirifer bigugosus Association of the Ferriters Cove Formation (A, C, E, H, I, L, M) and Clogher Head Formation (B, D, F, G, J, K). Specimens A through D are ×1, and specimens E through M are ×2. A. Holcospirifer bigugosus (M'Coy), latex cast of two brachial exteriors, UCMP 14006. B. Pteronitella retroflexa (Wahlenberg), internal mould of left valve, UCMP 14014. C. Platyceratid, latex cast, UCMP 14017. D. Favositid, latex cast, UCMP 14011. E. Atrypa reticularis (Linnaeus), internal mould of brachial valve, UCMP 10939. F. Salopina cf. conservatrix (McLearn), internal mould of pedicle valve, UCMP 10938. G. Modiomorphid, internal mould of right valve, UCMP 14073. H. Protochonetes ludloviensis Muir-Wood, internal mould of brachial valve, UCMP 14012. I. Isorthis sp., internal mould of brachial valve, UCMP 14010. J. "Camarotoechia" sp., internal mould of brachial valve, UCMP 10823. K. "Camarotoechia" sp., internal mould of pedicle valve, UCMP 10833. L. Bryozoan, latex cast, UCMP 10824. M. Tentaculites sp., latex cast, UCMP 10825.

d o m i n a t e d by " C a m a r o t o e c h i a " is f o l l o w e d b y a series o f lenses with abundant P r o t o c h o n e t e s , t h e n lenses with several c o m m o n b r a c h i o p o d s (locality D-7226), and finally, lenses d o m i n a t e d by Holcospirifer at the t o p o f the f o r m a t i o n (localities D - 7 2 2 5 and D-7224). These patterns represent p h e n o m e n a related t o the source o f shells reaching the area of deposition. Stratigraphic changes in shell lens c o m p o s i t i o n m a y be explained by changes in the source areas c o n t r i b u t i n g the shells. Alternatively, the lenses in a local section m a y have been derived f r o m a single source area with m a r k e d , t e m p o r a l changes in p o p u l a t i o n s , m a k i n g shells o f d i f f e r e n t b r a c h i o p o d species available for s t o r m t r a n s p o r t at d i f f e r e n t times. The shell lenses w o u l d thus appear t o indicate, secondarily, a p a t t e r n o f geographic a n d / o r t e m p o r a l variation in m o n o s p e c i f i c d o m i n a n c e a m o n g a

100 TABLE

II

Species composition o f t h e Holcospirifer bigugosus A s s o c i a t i o n , e a c h s a m p l e is f r o m a l e n s o f t r a n s p o r t e d s h e l l s

Ferriters

-~ 5"q ¢q t"~

u~ ¢q 5"q t'--

¢D 5"q ¢q t'~

t'-5"q gq F--

7

--

4

--

--

11 106 1 7 2

1 9

-193

166 --

Cove Formation;

r-~ ~ej L'q ~

5"q ¢O 5",] ["-

u~ ~ ~ t"-

¢,D ¢0 ~'q F--

7

--

--

1 15

45 76

BRACHIOPODA

Atrypa reticularis ( L i n n a e u s ) "Camarotoechia" s p . Holcospirifer bigugosus ( M ' C o y ) Isorthis s p . Protochonetes ludloviensis M u i r - W o o d Salopina c f . conservatrix ( M c L e a r n )

2 36 1

4 --

2

.

16 22

-32

.

--

.

.

-1

3

.

1 32

---

15 24

---

6

--

4 2

BRYOZOA Ramose forms Globular forms

4 19

.

1

10 6 --

-. --

---

--

10 .

.

--

--

.

GASTROPODA

Oriostoma

sp. 1 Platyceratid Indeterminate gastropod

2 .

2 .

--

.

.

.

4

.

.

.

.

3

. ---

.

. 7 2

BIVALVIA Grammysid

Pteronitella retroflexa Indeterminate MINOR

(Wahlenberg)

bivalves

sp. Favositid Indeterminate nautiloid Indeterminate trilobite

--

6

. --

-. --

Ostracod

Tentaculites

set

this

sense,

the

type

of brachiopod the

common

species

times. It might

of

many

be argued

that

.

. 1

131

within

the

of the species

. 2

. 1

--

96

fossil

. --

.

brachiopods

of "opportunistic",

are characteristic

.

---

sp.

TOTALS

ern

1

---

.

. 1

--

---

12 1

--

--

GROUPS

Cornulites

limited

2

. .

. .

74

same,

.

.

.

.

1 1

. .

.

. 1

198

178

general by from

--

122

1

16

143

environment.

In

Association

Hallam

nearshore

environments

each

of single-species-dominated

type

. 3

.

H. bigugosus discussed

.

.

. .

. --

represent

(1972),

Palaeozoic

to

which mod-

assemblage

101 TABLE

III

S p e c i e s c o m p o s i t i o n o f t h e Holcospirifer bigugosus A s s o c i a t i o n , e a c h s a m p l e is f r o m a l e n s o f t r a n s p o r t e d s h e l l s

C"I

¢Xl

5 2

-.

Clogher Head Formation;

Cl

~

CXl

.

-

203 --

210 --

ANTHOZOA Favositid Syringoporid

1 .

.

BRACHIOPODA

Atrypa reticularis ( L i n n a e u s ) "Ca maro to echia" sp. Holcospirifer bigugosus ( M ' C o y ) Hyattidina canalis (J. d e C. S o w e r b y ) lsorthis s p . Protoehonetes ludloviensis M u i r - W o o d Salopina el. eonseruatrix ( M c L e a r n ) Indeterminate

2 66

127 1

--

-160

3 2

.

. .

--

9

-

3

brachiopod

1

.

.

.

.

.

.

.

.

.

.

121

--

--

193

--

--

.

.

.

.

BRYOZOA Ramose forms Globular forms

21

-

3

3

11

4

--

5

5

2

GASTROPODA

Murchisonia s p . Oriostoma sp. 1

--

19

--

2

1

Platyceratid

.

.

--

41

--

45

.

.

1 --

.

BIVALVIA

Actinopteria

cf.

sowerbyi

(M'Coy)

Modiomorphid

Pteronitella retroflexa Indeterminate MINOR

hivalves

(Wahlenberg)

2

--

5

--

20 1

6

--

--

--

6

6 1

--

-1

5 --

GROUPS

Cornulites

sp.

Cystoid Indeterminate Indeterminate

Tentaculites

TOTALS

4

--

--

.

1 nautiloid trilobite sp.

-.

6 . 1 .

.

. . -.

--

--

.

. 1

-1

--

--

9

4

--

118

191

505

309

230

should be named as a separate community, i.e., the "Carnarotoechia" Association, Protochonetes Association, etc. In Facies 2 of the Dunquin Group, this practice would mean recognizing almost randomly interbedded "communities"

102 within the same, s e d i m e n t a r y e n v i r o n m e n t , and c o n f u s e p o p u l a t i o n - l e v e l f l u c t u a t i o n s of a set o f o p p o r t u n i s t i c species with overall, s y n e c o l o g i c relations O t h e r fauna

G a s t r o p o d s c o m p r i s e 6% of the shell lens fauna, and are r e p r e s e n t e d b y Oriostorna, a p l a t y c e r a t i d (Fig.12C), and Murchisonia. Bivalves c o m p r i s e 4% o f f a u n a in shell lenses, a n d are r e p r e s e n t e d m a i n l y b y Pteronitella (Fig.12B) a n d Actinopteria, a l t h o u g h o t h e r species o c c u r (Tables II, III). Watkins ( 1 9 7 7 ) has discussed the o c c u r r e n c e o f bivalves in shell lenses o f t h e Leintw a r d i n e a n d Whitcliffe Beds of the Welsh B o r d e r l a n d L u d l o w , w h i c h share m a n y similar, s e d i m e n t a r y f e a t u r e s with Facies 2. In the L u d l o w e x a m p l e , TABLE IV Surface area densities of fossils, exclusive of unidentifiable shell fragments, in lenses of transported shells from Facies 2 (Fc = Ferriters Cove Formation; Ch = Clogher Head Formation; Dp = Drom Point Formation; H = Holcospirifer bigugosus Association; S = Sphaerirhynchia wilsoni Association)

Fc Fc Fc Fc Ch Ch Ch Dp Dp Dp Dp Dp

H H H H H H H S S S S S

Locality

Crinoid ossicles per 100 cm 2

Other fossils per 100 cm 2

Area examined (cm 2 )

D-7225 D-7226 D-7231 D-7232 D-7216 D-7217 D-7219 D-7220 D-7221 D-7222 D-7237 D-7238

19.9 11.1 40.7 90.9 23.0 1.4 0 4.3 308.3 44.7 47.8 328.2

6.6 1.6 6.5 5.2 7.9 6.2 21.3 108.5 25.0 80.9 54.4 41.0

241 244 155 77 126 146 155 47 36 47 46 39

bivalves are significantly less a b u n d a n t in shell beds t h a n in intervening, biot u r b a t e d siltstone because o f a l o w s u s c e p t i b i l i t y t o t u r b i d i t y c u r r e n t transp o r t relating t o their i n f a u n a l habit. L o w bivalve a b u n d a n c e in shell lenses o f the H. bigugosa Association m a y be a similar, t a p h o n o m i c artifact. O t h e r m i n o r groups in the association, listed b y s a m p l e in Tables I I a n d III, include b r y o z o a (Fig.12L), cornulitids, t e n t a c u l i t e s (Fig.12M), small t a b u l a t e s (Fig.12D), o s t r a c o d s , trilobites, o r t h o c o n i c nautiloids, a n d a cystoid. Crinoid ossicles have an average ratio o f 3.4:1 w i t h o t h e r i d e n t i f i a b l e fossils, b u t this varies greatly f r o m o n e lens to a n o t h e r , as i n d i c a t e d in T a b l e IV. Five w h o l e crinoid a r m s were f o u n d in the shell lens s a m p l e f r o m l o c a l i t y D-7217.

103 THE CORAL ASSOCIATION OF THE CLOGHER HEAD FORMATION Facies 2 sediments of the Clogher Head F o r m a t i o n in sections A and D c o n t a i n a species o f branching, t a b u l a t e coral which has n o t been f o u n d in o t h e r f o r m a t i o n s or s e d i m e n t a r y facies. In situ colonies of this coral were observed s o u t h o f Clogher Head near locality D-7217 (Fig.8) and 100 m west of locality D-7219. The species is illustrated in Fig.13. Branches are a b o u t

Fig.13. Tabulate coral colonies of Facies 2 sediments in the Clogher Head Formation. A. Vertical sections of in place tabulate colonies, about 100 m west along coast from locality D-7219 (Fig.2). Height of hammer is 50 cm. B. Distal branches of a colony fragment from locality D-7218, latex cast, UCMP 10826. C. Upper surface of the bed shown in A, showing horizontal cross-sections of two in place colonies. Length of hammer head is 17 cm. 1--2 c m in diameter, and f o r m an erect, b u s h y mass of hemispherical t o ellipsoidal form. Six observed colonies, including those s h o w n in Fig.13C, had a mean, overall b r e a d t h of 65 c m and range in b r e a d t h f r o m 30 to 96 cm. C o l o n y heights a p p e a r to be within the same range, b u t are n o t c o m p l e t e l y preserved (Fig.13A). The colonies o c c u r separately within the sediment, and have n o t been observed to f o r m coalescing or reef-like structures. In the o c c u r r e n c e s h o w n in F i g . 1 3 A and C, six separate colonies were c o u n t e d on a s a n d s t o n e surface o f 6 m 2.

104 The same species of tabulate coral occurs more c o m m o n l y in the Clogher Head Formation as beds of redeposited, broken branches. Hollow, decalcified moulds of coral branches comprise about 23% of bed volume. The branches are broken colony parts c o m m o n l y 5--20 cm across, with many of their long axes oriented parallel to bedding. Matrix enclosing the branches is siltstone to fine sandstone like that elsewhere in Facies 2. The beds generally have parallel bases and tops and are traceable across available outcrops, but some lense out over distances of 2--3 m , At locality D-7241, nine beds of broken corals have a mean thickness of 61 cm and a range of 5--170 cm thickness. Thickness and stratigraphic frequency of coral beds at locality D-7218 are shown in Fig.8. The tabular coral beds formed through the breakage of colonies like those described above, and subsequent transport and concentration. Although broken, the coral branches in the beds show sharp surface relief and no evidence of abrasion (Fig.13B). A storm-generated, turbidity-current event, like that discussed for shell lenses in Facies 2, is probably responsible for the beds of redeposited corals. This mechanism would explain the original breakage of colonies, the tabular nature of the beds, and the lack of abrasion of the broken branches. A bulk sample from a redeposited coral bed at locality D-7218 was prepared in the laboratory for associated fauna. Seventy-six single valves and three whole specimens of a leperditiid ostracod were found, reaching 10 mm in length. "Camarotoechia" sp., 5 mm or less in size, was represented by three pedicle valves, one brachial valve, fifteen undetermined, single valves, and thirteen whole shells. In addition, a single crinoid columnal was found. The ostracods and rhynchonellides may have lived near the bases or upon branches of the tabulate colonies. THE SPHAERIRHYNCHIA WILSONI ASSOCIATION Occurrence and taphonomy The S. wilsoni Association is restricted to Facies 2 of the Drom Point Formation. Representative shells are shown in Fig.14, and quantitative data for species in several samples are given in Table V. The pattern of shell occurrence and taphonomic implications for the S. wilsoni Association are identical to those discussed for the H. bigugosa Association. Most shell material in the Drom Point Formation occurs in discrete, erosively based lenses, shown in Fig.6B and C. Crinoid ossicles in these lenses have an average surface area density of 1467 per 1000 cm 2, and other, identifiable fossils, exclusive of fragments, have an average density of 620 per 1000 cm 2. These figures are based on measurements for five shell lenses given in Table IV. Shells are rare and widely scattered outside of the lenses, where a density of 6.9 crinoid ossicles and 3.4 other shells per 1000 cm 2 was observed.

105 The discussion of faunal c o n t e n t of the S. wilsoni Association is based solely on s a m p l e s f r o m the t r a n s p o r t e d lenses of shells. These lenses are sedim e n t o l o g i c a l l y identical to those o f the H. bigugosus Association, a n d have the same, p r o b a b l e s t o r m m o d e of genesis and t a p h o n o m i c i m p l i c a t i o n s discussed previously. The l o w - d e n s i t y , shelly f a u n a s c a t t e r e d t h r o u g h Facies 2 of the D r o m P o i n t F o r m a t i o n is qualitatively identical to t h a t in the shell lenses. Faunal c o n t e n t Articulate b r a c h i o p o d s c o m p r i s e 90% of the n o n - c r i n o i d f a u n a in the shell lenses, and twelve species have been r e c o r d e d (Table V). G e n e r a l l y t w o or m o r e species, a m o n g Salopina, P r o t o c h o n e t e s , Leptostrophia, Sphaerirhynchia, " C a m a r o t o e c h i a ' , and H y a t t i d i n a , d o m i n a t e the f a u n a of a n y single lens. D a t a given in Table V s h o w h o w the d o m i n a n t species vary f r o m o n e lens to a n o t h e r . This variation has the s a m e i m p l i c a t i o n s o f an " o p p o r t u n i s t i c " species p a t t e r n , as discussed for the II. bigugosus Association. M o r p h o l o g y and size range o f c o m m o n b r a c h i o p o d s o f the S. wilsoni A s s o c i a t i o n are s h o w n in Fig.14. O t h e r groups r e c o r d e d f r o m the association include b r y o z o a (1.7%), gastrop o d s (1.8%), bivalves (1.5%), c e p h a l o p o d s (0.2%), t e n t a c u l i t e s a n d c o r n u l i t i d s (1.2%), trilobites (2%), o s t r a c o d s (0.7%), and t a b u l a t e s (1.3%). T h e a b u n d a n c e

Fig.14. Shells from the Sphaerirhynchia wilsoni Association of the Drom Point Formation, all x 2. All brachiopods shown are internal moulds of the pedicle valve. A. Salopina cf. conservatrix (McLearn), UCMP 10827. B. Protochonetes ludloviensis Muir-Wood, UCMP 10828. C. "Camarotoechia" sp., UCMP 14079. D. Sphaerirhynchia wilsoni (J. Sowerby), UCMP 10829. E. A trypa reticularis (Linnaeus), UCMP 10830. F. Hyattidina canalis (J. de C. Sowerby), UCMP 10831. G. Dalmanites sp., internal mould of cranidium, UCMP 10832. H. Nuculoid bivalve, internal mould of left valve, UCMP 10835. I. Modiomorphid, internal mould of right valve, UCMP 10836. J. ?Kionoceras sp., internal mould of a body chamber fragment (note also an ostracod immediately to right of "J"), UCMP 10837. K, Indeterminate gastropod, internal mould, UCMP 10838. L. Bryozoan, latex cast, UCMP 10839. M. Howellella sp., UCMP 10840.

106 TABLE V Species composition of the Sphaerirhynchia wilsoni Association, Drom Point Formation; each sample is from a lens of transported shells ¢q Cq

Cq

¢,1

Cq

CO

¢e3

CO

Cq

¢q

Cq

Cq

Cq

ANTHOZOA Favositid

--

4

--

Syringoporid

--

2

--

--

_ _

m

--

--

_ _

m

--

1

BRACHIOPODA

A trypa re ticularis (Linnaeus) "Ca marotoechia " sp. Dalejina sp. Howellella sp, Hyattidina canalis (J. de C. Sowerby) Isorthis sp. Leptaena depressa (J. de C. Sowerby) Leptostrophia filosa (J, de C. Sowerby) Protochonetes ludlouiensis Muir-Wood Salopina cf. conservatrix (McLearn) Shagamella ludloviensis Boucot and Harper Sphaerirhynchia wilsoni (J. Sowerby)

9

--

2 --

1

--

11

--

--

3

--

--

41

17

6

1

.

2

6

--

26

.

-I

--

.

3

16

--

1

4

--

2

17

.

48

1

4

--

50

45

2

6

1

3

.

.

34

4

1

35

8

3

25

.

14

28

.

--

.

1

2

--

.

.

.

--

2

.

.

3 7

18

13

--

--

2

2

22

BRYOZOA Ramose

forms

.

.

.

.

Lamellose forms

GASTROPODA Bellerophontid Murchisonia sp, Oriostoma sp.1 Indeterminate gastropods

.

.

.

. .

. .

2

.

--

1 .

. 1

m

.

. --

__ 3

.

--

.

--

3

1

--

--

1

- -

- -

BIVALVIA

Actinopteria cf. sowerbyi (M'Coy) Modiomorphid Nuculoid Pteronitella retroflexa (Wahlenberg) Indeterminate bivalve

- -

m

- -

.

.

.

4

m

1

.

.

1

.

.

.

--

.

m

TRILOBITA

Calymene sp. Dalmanites sp. Proetid Indeterminate trilobites

2 . . - -

. .

. .

.

. - -

.

.

.

.

. 7

. 1

. - -

m

_

.

1 - -

1

- -

107 TABLE V (continued)

MINOR GROUPS Cornulites sp. ?Kionoceras sp. Ostracods Tentaculites sp.

-. . . . . . -----

---

1 4 5

TOTALS

66 140

22

181

51

-1 ..... --1 -86

60

of m a n y o f these groups m i g h t be greater in d i s t u r b e d n e i g h b o u r h o o d assemblages outside of the shell lenses, based on o b s e r v a t i o n s b y Watkins ( 1 9 7 7 ) on similar s e d i m e n t s a n d f a u n a in the L u d l o w o f the Welsh B o r d e r l a n d . A c o m p l e t e list o f n o n - b r a c h i o p o d s in the S. wilsoni Association is given in Table V, a n d several species are illustrated in Fig.14. Based on t h e d e n s i t y m e a s u r e m e n t s , crinoid ossicles have a ratio o f 2.4:1 with o t h e r fossils in the shell lenses, a l t h o u g h m a r k e d variation occurs f r o m one lense to a n o t h e r (Table IV). ENVIRONMENTAL RELATIONS OF THE ASSOCIATIONS General distribution p a t t e r n

The direct c o r r e s p o n d e n c e o f W e n l o c k c o m m u n i t i e s in the D u n q u i n G r o u p to the stratigraphic d i s t r i b u t i o n of s e d i m e n t a r y facies is s h o w n in Figs.4 a n d 5. As discussed earlier, the facies r e p r e s e n t an e n v i r o n m e n t a l g r a d i e n t f r o m b i o t u r b a t e d , o f f s h o r e silts t h r o u g h s t o r m - d e p o s i t e d , n e a r s h o r e silts a n d sands to n o n m a r i n e , lagoonal s e d i m e n t s . N o t o n l y are the w h o l e c o m m u n i t i e s restricted to particular facies along this gradient, b u t individual, i n v e r t e b r a t e groups s h o w a consistent, associated z o n a t i o n which is detailed below. T h e s e relations indicate an overall c o n t r o l of c o m m u n i t y d i s t r i b u t i o n b y b o t t o m t y p e a n d associated f a c t o r s o f s e d i m e n t a t i o n c o n d i t i o n s . This c o n c l u s i o n conflicts with t h a t o f Ziegler et al. ( 1 9 6 8 a , b) a n d Calef a n d H a n c o c k (1974), w h o s t a t e d t h a t w a t e r d e p t h exercised t h e m a j o r influence on Silurian c o m m u n i t y d i s t r i b u t i o n s in Wales a n d the Welsh Borderland. However, re-evaluation o f these studies b y Bridges (1975), H u r s t (1975a), L a w s o n (1975), a n d Watkins ( 1 9 7 7 ) have i n c o r p o r a t e d d a t a s h o w i n g t h a t f a c t o r s of b o t t o m t y p e , s e d i m e n t a t i o n , a n d regional, h y d r o g r a p h i c histo r y are b e t t e r i n t e r p r e t a t i o n s f o r the e n v i r o n m e n t a l relations of British Silurian c o m m u n i t i e s . These d a t a are in a c c o r d w i t h studies of m o d e r n b e n t h i c c o m m u n i t i e s , such as the detailed w o r k o f D b r j e s ( 1 9 7 2 ) and P a r k e r (1975).

108

Faunal trends from offshore to nearshore facies The offshore environment of bioturbated silts represented by Facies 1 contained a single b o t t o m community, the Dolerorthis rustica Association. Shell occurrence in Facies 1 indicates a distribution of populations throughout this environment. Facies 2 represents a nearshore, storm-influenced environment of silts and fine sands, occupied by the Holcospirifer bigugosus Association and large, tabulate coral colonies during deposition of the Ferriters Cove and Clogher Head Formations. During later deposition of the Drom Point Formation, the same environment was inhabited by the Sphaerirhynchia wilsoni Association because of biogeographic events discussed later. Shell occurrence in Facies 2 suggests a very patchy distribution of populations. The lenses of transported shells in sediments such as Facies 2 may record discrete, storm-related destruction of local populations, as discussed by Watkins (1977). Species diversity within Facies 1 is substantially higher than in Facies 2, as measured for shelled organisms exclusive of crinoids and bryozoa. This is shown both by a diversity index for single samples (Fig.5) and raw numbers of species recorded in each facies (Fig.15). The trend is also expressed by equitable, relative abundances within the D. rustica Association and single species dominance of samples from associations in Facies 2. Important changes within the higher taxa occur between the offshore and nearshore sediments. The ratio of crinoid ossicles to other fossils decreases from Facies I to Facies 2, as does the relative abundance of corals and bryozoa. An accompanying, diversity decline is shown by corals (Fig.15), and probably also by crinoids and bryozoa, although no precise data is available for these groups. Brachiopods and trilobites also descrease substantially in numbers of species from Facies I to Facies 2, while ostracods, tentaculites, cornulitids, gastropods, and bivalves show slight increases in diversity (Fig. 15). The diversity decline from the offshore to nearshore environment primarily affects the epifaunal suspension feeders, which suggests a relation to increased rate of sedimentation and amount of suspended sediment. This agrees with the sedimentary features of Facies 1 and 2 given earlier, and is also shown in the morphology of certain taxa. For example, brachiopods in Facies 2 include a high proportion of rhynchonellides and ribbed spiriferides, with zig-zag deflections of the commissure for minimizing sediment intake (Rudwick, 1964). Tabulates in Facies 1 are represented by diverse species below 10 cm in colony height. The single, abundant tabulate in Facies 2 of the Clogher Head Formation had colony heights of 30--90 cm, and was likely to be buried by rapid sedimentation. In summary, the faunal trends Facies 1 to Facies 2 include a decrease in bioturbation, an increased patchiness of populations, a decrease in species diversity, particularly among epifaunal suspension feeders, and an increased proportion of bivalves and brachiopods to crinoids, corals, and bryozoa. A unifying explanation for the trends appears to be a change from slow, quiet-

109

SOFT-BODIED BURROWERS

offshore

neorshore

mor/ne

morme

nonmar/ne

D~DODQDDDDD~DDDDDD~DDOQDDDDD~D

B/VALVEG

GASTROPODS

CORNUL/TES B TENTACUL ITES

I

OS TRA CODS

BRACH/OPODS

1

O[Species I0

TABULATES

TRILOBITES r E 7"R.~'CORAL S CONUL ~RIDS

Fig. 15. Environmental zonation of Wenlock invertebrates in the Dunquin Group, exclusive of bryozoa and crinoids. The histograms are based on data for the Dolerorthis rustica Association (Facies 1) and Holcospirifer bigugosa and tabulate coral Associations (Facies 2) of the Ferriters Cove Formation and Clogher Head Formation. Species diversity of the soft-bodied burrowing organisms is not known.

water sedimentation in Facies 1 to storm-related sedimentation in Facies 2, with periods of high energy and high sediment input.

The nonmarine fauna Facies 3 and 4 have been interpreted as nonmarine because of their lack of the shelly fauna discussed above and their stratigraphic relations with volcanics. Bioturbation in Facies 3 is evidence of a sparse c o m m u n i t y of softbodied burrowers. There is no evidence for a burrowing fauna in Facies 4. However, Platychisrna-like gastropods occur in ash beds associated with Facies 4 in one stratigraphic section (Fig.9). These gastropods may represent algal grazers within a nonmarine, lagoonal environment. BIOLOGIC RELATIONS OF THE ASSOCIATIONS

Trophic structures The offshore, D. rustica Association of Facies 1 had the most complex trophic structure of Wenlock communities in the Dunquin Group. Its shelled

110 fauna is dominated by sessile, epifaunal suspension feeders whose sizes. indicate a stratification of feeding levels. Crinoids would have c o m m o n l y reached feeding heights of 20 cm or more, based on measurements of complete individuals from the Wenlock Limestone of Dudley, England. A second, general feeding level, from about 3 to 10 cm above the bottom, was occupied by bryozoa and corals. The smaller bryozoa, brachiopods, solitary corals, and bivalves fed below this level, at or near the sediment bottom. Density and abundance data given earlier indicate equitable and probably intense utilization of each of these levels. Trilobites may have functioned within the c o m m u n i t y as scavengers, and bioturbation of th~ enclosing siltstone indicates an active infauna of deposit feeders. There is no preserved record of predators, unless they are represented among the gastropods present. Nearshore associations of Facies 2 contain all of the feeding types mentioned above, but show simpler, overall trophic structures. In the H. bigugosus and S. wilsoni Associations, the lowest level of suspension feeding appears to have been the most prominent part of the trophic structure, as indicated by dominance of articulate brachiopods. The evidence has been summarized earlier for localized, extreme dominance of these associations by 1 or 2 brachiopod species, further emphasizing the trophic simplicity. Higher levels of suspension feeding were less utilized relative to the D. rustica Association, with a particular decline in the intermediate level of bryozoa and corals. High-level, colonial suspension feeding is apparent in Facies 2 as the coral association of the Clogher Head Formation. The corals occur in different parts of stratigraphic sections from the H. bigugosus Association (Fig.8), indicating spatial distinction of the two communities. This represents a fragmentation of trophic activities relative to the D. rustica Association, where the sedimentary occurrence suggests a co-occurrence of suspension feeders of all levels.

Ecologic succession

Walker and Alberstadt (1975) summarized the application of concepts in ecologic succession to marine palaeoecology. There is no evidence within the associations described here for the "long-term succession" of these authors, in which a c o m m u n i t y within a particular environment changes progressively in composition through geologic time. A stratigraphic change in sedimentary facies within the Dunquin Group is invariably accompanied by a change in benthic community. Available data show that as long as the facies persists, its contained c o m m u n i t y maintains a relatively constant level of diversity and composition. "Short-term succession", as defined by Walker and Alberstadt (1975), was observed at locality D-7234, where the upper surface of an ash bed was col-

111

onized by halysitid corals, which in turn provided an initial substrate for the brachiopod-bryozoan assemblage listed in Table I(U), THE PALAEOGEOGRAPHIC SETTING OF THE DUNQUIN GROUP

A reconstruction of the major aspects of Wenlock palaeogeography of the British Isles and surrounding areas is given in Fig.16, based on McKerrow and

~-~corbonote plotform ~

~

~:;~--T~LT

~

0

p,

.

~ ~ ' " i " ~ .....

~

/~:~ "

~

@ ~

}~

~, ~TJ

: ..

~shallow shelf with ""3shelly communities F-qdeep shelf a bosin L-~lwith groptolitic fauno

Fig.16. Generalized palaeogeography of the North Atlantic area during Wenlock time, modified from McKerrow and Ziegler (1972).

Ziegler's (1972) consideration of lithofacies patterns and plate-tectonic theory. These authors summarized the Silurian evidence for a narrow, ProtoAtlantic ocean separating maritime Canada, eastern Newfoundland, and the southern British Isles from the remainder of North America, the north of Ireland, and Scotland. Four features of the Dunquin Group are significant in determining its relationship to this palaeogeography: (1) The Dunquin Group is located somewhere near the postulated suture between the areas formerly separated by the Proto-Atlantic ocean (McKerrow and Ziegler, 1972). (2) Wenlock brachiopods of the Dunquin Group are most closely related to those in the Welsh Borderland on the eastern side of the Proto-Atlantic, as discussed below. (3) The shelf-type, Wenlock sediments and fauna of the Dunquin Group are isolated from those in the Welsh Borderland by an extensive area of graptolitic shales and turbidites across the south of Ireland (Ziegier et al., 1974, Fig.6). (4) Sediments of the Dunquin Group are locally derived from volcanic

112

sources, and the lateral changes in thicknesses of volcanics and sediments are extreme. These relations, taken as a whole, suggest that Wenlock sediments of the Dunquin Group were deposited in limited areas around volcanic islands, and isolated by open seas from any major area of shallow shelf. This postulated island area around Dingle appears to have been situated along the eastern margin of the Proto-Atlantic, between deep shelf muds and turbidites to the east, and open ocean to the west, as shown in Fig.16. The entire, ProtoAtlantic area was situated within the North Silurian Realm of Boucot (1975), whose biota indicates a very widely distributed, warm climate. BIOGEOGRAPHIC RELATIONS OF THE DUNQUIN COMMUNITIES

Method o f analysis The isolated, geographic position inferred for the Dunquin Group suggests that its benthic communities must have been recruited, at some point, from outside areas. When considered at a local, ecologic level, regional differences in fauna within the North Silurian Realm are apparent which are significant to an analysis of this problem. Such analysis involves two considerations. If similar environments in two separated areas contain the same taxa, a biogeographic relation is indicated. If different taxa inhabit the same type of environment in different areas, a biogeographic distinction is implied. Comparison of communities from different environments will consistently show taxonomic differences in fauna, and are not a valid test of biogeographic relationships. The following analysis employs only genera of articulate brachiopods. T a x o n o m y of Silurian brachiopod species and most non-brachiopod groups is not well enough known for biogeographic comparisons throughout the area shown in Fig.16. The basis for comparison of brachiopod genera within Dunquin communities and those elsewhere will be a coefficient of association defined as: genera in common to both assemblages ~. genera in smaller assemblage Biogeographic relations o f the Dolerorthis rustica Association The D. rustica Association falls within a faunal category equivalent to "Benthic Assemblages 3 to 4 " of Boucot (1975), which consists of diverse, brachiopod faunas in quiet, offshore environments of mud to silt-grade bottoms. Wenlock communities of this t y p e in the Welsh Borderland include the Isorthis Community of the Wenlock Shale (Calef and Hancock, 1974) and the Isorthis clivosa Community of the Wenlock Limestone (Hurst, 1975a). In Nova Scotia, they include the Isorthis macadamensis and Meristina Communities of the French River Formation (Watkins and Boucot, 1975), and in Got-

113

land, the fauna of the Mulde Beds (Hurst, 1975b, table 2). Other, "Benthic Assemblage 3 to 4" communities are apparent in stratigraphic reports on the LaVille Formation of Quebec (Northrop, 1939: table 2), the Rochester Formation of Ontario (Bolton 1957, table 8), and the Joslin Member of the Herkimer Formation, New York (Zenger, 1971, table 2). Coefficients of association between the D. rustica Association and these communities, based on brachiopod genera, are shown in Fig.17. Highest ,numbers

L>27 ~Z

of

24 J J

brochi,

28 I.~Z .o

JV-

enero

17

31

42

24

W

.

~Z

~o

'," "

w t:~ID I a

ZW

O0 t~I,L

,.b, o.8

B

~[0 JI,L

C

D

~m

F

G

~z / /

/

]

G

H \

\

/

% 0.4

.~ ~z o

Fig.17. Similarity of t h e f i f t e e n b r a c h i o p o d genera in the Dolerorthis rustica A s s o c i a t i o n to o t h e r W e n l o c k b r a c h i o p o d faunas; sources of data given in text. The letters i n d i c a t e l o c a t i o n of t h e faunas u n d e r c o m p a r i s o n in Fig.16.

values of this coefficient are with British and Gotland faunas on the eastern side of the Proto-Atlantic. However, the very close, apparent relation between the D. rustica Association and Isorthis Community of the Wenloek Shale is probably an artifact relative to the other coefficients resulting from the larger number of genera recorded from the later community (Fig.17). Biogeographic relations of the Holcospirifer bigugosus Association The H. bigugosus Association represents "Benthic Assemblage 2" of Boucot (1975) and has parallels in other Wenlock communities which occur in nearshore, storm-influenced sediments. These communities include the Salopina Community of Wenlock clastics in Britain (Calef and Hancock, 1974) and the Salopina submedia -- "Camarotoechia" aff. planorugosa Community of the Doctors Brook Formation in Nova Scotia (Watkins and

114

Boucot, 1975). Equivalent communities can be recognized in faunal lists from the Lough Muck Formation of the Galway area, Ireland (Laird and McKerrow, 1970, p. 309), McKenzie Formation of Pennsylvania (Swartz, 1935, Fig.4), and Klinteberg Beds of Gotland (Hurst, 1975b, table 2, samples G610.1--2). Coefficients of association between the H. bigugosus Association and these communities are shown in Fig.18. The coefficients range from 0.4 to 0.7, 3

5

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30

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G

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llll 11

~0//

• Holoospir/fer b/gugosus ASSOC.

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Fig.18. Similarity o f the seven b r a c h i o p o d genera in the Holcospirifer bigugosus A s s o c i a t i o n and twelve brachiopod genera in the Sphaerirhynchia wilsoni A s s o c i a t i o n to other Wenlock b r a c h i o p o d faunas; sources o f data given in text. The letters indicate location o f the faunas under c o m p a r i s o n in Fig.16.

lower than those obtained with other Dunquin communities, and show no geographic trend with respect to east and west sides of the Proto-Atlantic. Also significant is the abundance in the association of Holcospirifer, a genus endemic to the Dingle area (Bassett et al., 1976). Biogeographic relations of the Sphaerirhynchia wilsoni Association

The S. wilsoni Association represents "Benthic Assemblages 2 to 3" of Boucot (1975). Wenlock communities of similar, general environment include the Homoeospira Community of Wenlock clastics in Britain (Calef and Hancock, 1974) and the Salopina submedia -- "Camarotoechia" aff. planorugosa Community of the Doctors Brook Formation in Nova Scotia (Watkins and Boucot, 1975). Communities of similar brachiopod content

115 but rather different, carbonate environments are present in the Hogklint Beds of Gotland (Hurst, 1975b, table 1, samples G5-5.1), and in the Wenlock Limestone of the Welsh Borderland (the Sphaerirhynchia wilsoni Community of Hurst, 1975a}. The S. wilsoni Association is compared with these communities in Fig.18, all of which are located on the eastern side of the Proto-Atlantic. Identical to very close matches of brachiopod genera occur with the British and Gotland communities.

THE HISTORY OF WENLOCKCOMMUNITIES IN THE DUNQUIN GROUP The earliest marine deposition in the Ferriters Cove Formation preserves a record of two b o t t o m communities in offshore and nearshore sediments, the Dolerorthis rustica Association and Holcospirifer bigugosus Association. The D. rustica Association appears to have been derived from shelf faunas of the Welsh Borderland or vicinity, and shares most of its brachiopod genera, and probably species as well, with the high-diversity, Isorthis Communities of this area. Its fauna of corals, bryozoa, and crinoids also have parallels within the British communities, and a similar, stratified trophic structure is apparent, for instance, in faunas of the Wenlock Limestone at Dudley. The D. rustica Association is present twice in the Ferriters Cove Formation, and shows no progressive, temporal modifications in either development. Higher-energy, nearshore sediments of the Ferriters Cove Formation contain the Holcospirifer bigugosus Association, which does not share the same, close affinity with shelf faunas of the Welsh Borderland. Local evolution within populations pre-dating the Ferriters Cove Formation is suggested by the presence in the oldest marine sediments in the formation of the endemic brachiopod Holcospirifer. Associated with Holcospirifer are several other genera characteristic of the nearshore "Salopina Communities" on both sides of the Proto-Atlantic. The H. bigugosus Association appears three times within the Ferriters Cove Formation with the development of its particular sedimentary facies. Although the association exhibits considerable variation in dominance among species, it shows no progressive or successional modifications during temporal persistence of the nearshore conditions. During the predominantly volcanic deposition of the Clogher Head Formation, marine conditions west of Dingle were restricted to temporary incursions of the nearshore environment. The H. bigugosus Association is found in each of these incursions and persisted in the area throughout this time, following the shifting facies pattern in distribution. It is unchanged from its content in the Ferriters Cove Formation, except for the appearance of an additional brachiopod, Hyattidina. The tabulate coral association of sediments in the Clogher Head Formation also represents a new invader from outside the Dingle area. Nonmarine conditions existed throughout the study area west of Dingle

116

during the deposition of the Mill Cove Formation. A return of the nearshore environment is represented by the overlying Drom Point Formation. Gardiner and Reynolds (1902) described an identical, green ash [units A(2), B(2), and C(3)] which occurs between redbeds of the Mill Cove Formation and marine sediments of the Drom Point Formation in several sections. This ash indicates that marine conditions resumed synchronously in the area. The Sphaerirhynchia wilsoni Association appears at the base of these marine sediments, and persists as the only observed shelly c o m m u n i t y throughout coastal exposures of the Drom Point Formation. Although the association shows stratigraphic variation in dominance among taxa, it shows no progressive changes in diversity or structure. The S. wilsoni Association has an identical suite of brachiopod genera, and probably species, with the Homoeospira and Sphaerirhynchia Communities of the Wenlock Shale and Limestone of the Welsh Borderland. Several taxa new to the Dingle area are contained within the S. wilsoni Association, including Shagarnella, Leptoo strophia, and Sphaerirhynchia. The appearance of the association represents an event of geologically rapid, faunal invasion at the level of an entire community. ACKNOWLEDGEMENTS

Six samples of bulk rock used in this project were provided by N. J. Hancock. W. S. McKerrow t o o k the field photographs shown in Figs.6 and 13, and F. J. Phillips provided camera equipment for laboratory photography. J. S. Peel and Donald Mikulic checked identifications of gastropods and trilobites, respectively.

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