A reassessment of facies and faunas in the type Llandeilo (Ordovician), Wales

Palaeogeography, Palaeoclimatology, Palaeoecology, 34 (1981): 285--314

285

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

A REASSESSMENT OF FACIES AND FAUNAS IN THE TYPE LLANDEILO (ORDOVICIAN), WALES

CHRIS J. WILCOX and MARTIN G. LOCKLEY

Institute of Geological Sciences, 5 Princes Gate, London SW 7 (Great Britain) Department of Geology, University of Colorado at Denver, 1100 14th Street, Denver Colo. 80202 (U.S.A.) (Received June 30, 1980; revised version accepted February 20, 1981)

ABSTRACT Wilcox, C. J. and Lockley, M. G., 1981. A reassessment of facies and faunas in the type Llandeilo (Ordovician), Wales. Palaeogeogr., Palaeoclimatol., Palaeoecol., 34: 285--314. Detailed analysis of the lithological and faunal successions of the type Llandeilo forms the basis of an evaluation of the sequence of depositional environments and a quantitative description of the entombed faunas, comprising mainly brachiopods, trilobites, and ostracods. Recorded distribution patterns confirm the validity of the existing trilobite (trinucleid) biostratigraphic zones but emphasize the importance of benthic bivalved taxa which are particularly abundant in the Lower Llandeilo. Groups of assemblages comprising localized faunules or associations, despite exhibiting evidence of transportation, are readily identified as representative of more widespread contemporary palaeocommunities recently described from Wales and the Welsh Borderland. The relationship between these palaeocommunities and the transgressive Llandeilo sequence indicates a shoreface to open-shelf zonation of ecosystems controlled primarily by substrate conditions rather than depth. INTRODUCTION

Since Williams (1948, 1949, 1953) concluded his studies of the type Llandeilo, the area has received scant attention. His trinucleid zonal scheme and stratigraphical classification (1953) have remained essentially unchanged and only recently have the brachiopod faunas been reviewed (Lockley and Williams, 1981). Other recent work on the Llandeilo Series of South Wales, both in the type area (Wilcox, 1979) and elsewhere (Addison, 1974) has remained largely unpublished although some of the latter author's more important biostratigraphic conclusions have been incorporated into appropriate summary publications (Williams et al., 1972; Bassett et al., 1974). This paper, therefore, aims to outline the more recent, novel observations presented by one of us (C. J. W., 1979); these deal largely with the quantitative distribution of the fauna and resultant interpretations. We emphasize the importance of benthos in the type Llandeilo, showing that dominant brachiopod species 0031--0182/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

286

can be considered to represent residues of recently described ancestral Palaeocommunities (Williams et al., 1981). A complete faunal list for the type area, which is included as an appendix, indicates that the brachiopods outnumber the trilobites by a 3:2 ratio. All material other than the few specimens figured by Lockley and Williams (1981), which are housed in the British Museum (Nat. Hist.), have been deposited in the National Museum of Wales.

Methods In accordance with the type of sampling schemes employed by Williams et al. (1981) for the underlying, Llanvirn, Ffairfach Group, and Hurst (1979) and Lockley (1980) for younger Caradoc successions, the bulk "census" sampling methods used in this study permit direct quantitative comparisons between the distribution of faunas in rocks belonging to three successive middle Ordovician series. In all, 395 census samples representing 1.35 metric tonnes of rock were collected from 19 key sections (Fig.l) and a rock-crushing machine was used to extract all contained fossils for identification. Full details of the methods and the stratigraphical position and derived fauna from each sample is given in Wilcox (1979). Numerous additional, oriented samples were collected in order to prepare cut-slabs for the purposes of examining sedimentary structures and 14 were figured (Wilcox, 1979) as representative of particular sedimentary facies. STRATIGRAPHY

The Llandeilo Series type succession sensu Williams (1953) consists of a number of litho-, bio- and chronostratigraphical subdivisions which only partially satisfy the conventions of modern usage (e.g., Holland et al., 1978) but are acceptable in their historical context. As the changes between successive Llandeilo facies and faunas are mainly transitional, strict redefinition of the litho- and biostratigraphy would prove difficult, unrewarding, and beyond the scope of this paper. However, we recognize that terms like "Sandy flags" and "Sowerbyella Limestones" (Williams, 1953) represent an outdated and incongruous mixture of lithoand biostratigraphical terminology and therefore attach no formal significance to them. However, as a prelude to considering some minor stratigraphical amendments, it is necessary to review the existing knowledge and terminology so as to clarify some unexplained stratigraphical "revisions" presented in the Palaeontological Association guidebook (Basset et al., 1974). Williams (1953) subdivided some 850 m of Llandeilo "formation" into Lower (237 m), Middle (427 m), and Upper (183 m) "subformations", largely on the basis of biostratigraphical evidence, and derived the corresponding "tripartite division of the Series into Lower, Middle, and Upper"

287 chronostratigraphical stages (Williams et al., 1972, p. 33). Bassett et al. {1974, Fig.l), in referring to these respective units as the Lower, Middle, and Upper Llandeilo Flags (with an increased thickness estimate of 520 m for the middle unit), implied that the strata-equivalents of these biostratigraphically derived units have at least the status of formations which is not inappropriate since Williams (1953) considered the units lithologically distinguishable. Since Lockley and Williams (1981) have already formalized stratigraphical terminology for the underlying Upper Llanvirn Ffairfach Group by referring to its five subdivisions as formations, it would be inappropriate to regard the Llandeilo Flags (sensu Williams, 1953) as anything less than a Group consisting of three major units. Such an arrangement avoids unnecessary interference with existing nomenclature and conveniently allows the thirteen stratigraphical subdivisions proposed by Williams to be regarded, at least informally, as members. It also retains a familiar stratigraphic framework with which we can compare our conclusions. Our re-evaluation of Llandeilo stratigraphy is based primarily upon detailed measurement of the type section where "the most complete exposures occur" (Williams, 1953, p. 188) and other key sections (Fig.l); our measurements indicate that the total thickness of strata in the type section is about 716 m (or 15% less than previously estimated). About 63% of this succession is exposed though neither the contact with the underlying Ffairfach Group or overlying Dicranograptus Shales is seen. However, in contrast to the inferences of Williams (1953, fig.5 and pl.9) and Bassett et al. (1974, fig.4), which suggest that Wenlock strata overstep on to the Upper Llandeilo in the type section, the discovery of Marrolithus sp. in distinctive black shale outcrops 70 m south of the Cennen railway bridge (Grid ref. SN 622200; Wilcox, 1979) leads us to conclude that at least part of the Dicranograptus Shales is represented here and that the Wenlock overstep is correspondingly less pronounced. Measurement of the type section suggests that respectively 199, 278, and 239 m of strata should be considered equivalent to the Lower, Middle, and Upper Llandeilo Stages (Fig.l). Although the disparity in thickness between present and previous estimates for Lower Llandeilo strata results solely from differences in measurement, the disparity for strata equivalent to the Middle and Upper Stages is the result of the discovery of the Upper Llandeilo zonal trinucleid Marrolithus favus moderatus Williams lower in the succession than previously known. With respect to this latter amendment, it is interesting to note that Bassett et al. (1974; fig.4) redrew the Middle--Upper Llandeilo Stage boundary lower in the succession than on Williams' original map (1953: fig.5), therefore inexplicably, partially anticipating our revision. However, their placing of the Lower--Middle Llandeilo Stage boundary higher in the succession, again without explanation, is not supported by our observations which are entirely in accord with Williams' Original cartography (see Wilcox, 1979, fig.2, p. 11). Generally, both our studies and those of Addison (1974) have confirmed the validity of the trinucleid zonation proposed by Williams (1948, p. 87;

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1953, p. 190) and the minor revisions we propose (Fig.2) result only from our interpretation of the taxonomic status of some of the zonal forms. Fortunately also, our observations require no taxonomic revisions and serve only to simplify the zonal scheme. Addison's three main conclusions (pp. 6 9 - 110) were: (a) that the subspecies Marrolith us in flatus maturus is specifically distinct from M. inflatus, (b) that all Marrolithoides species all show a high degree o f morphological overlap and might, therefore, be a single species group; and (c) that Marrolithus favus Williams and the subspecies M: favus moderatus are identical. We agree: ( a ) t h a t M. inflatus maturus should be elevated to M. maturus, a distinct zonal species. We also agree, (b) that Marrolithoides species and subspecies tend to exhibit considerable morphological overlap but consider that whilst M. simplex cannot be effectively differentiated from the subspecies M. simplex elevata, which should be suppressed as a synonym of the nominate species, it can be differentiated from M. anomalis. Such conclusions contribute to a simplification of the Middle

289 Llandeilo zonal scheme (Fig.2). Finally, (c) although we feel unable to fully support Addison's (p. 72) contention that M. favus moderatus should be suppressed as a synonym of M. favus, we do not consider that the distribution of the two forms is indicative of separate Upper Llandeilo zones (Fig.2). Since Wilcox (1979, p. 56) observed that Williams' thirteen lithostratigraphical subdivisions of the Llandeilo Flags (Fig.l) are "hard to separate in the field on purely lithological grounds", we feel justified in not formally regarding them as members and emphasize that for this and for historical reasons, we should continue to regard biostratigraphical criteria as the most effective means of providing a detailed subdivision of the Llandeilo Series. SEDIMENTOLOGY

Introduction and methods Detailed logging of sedimentary successions through 1125 m of strata corresponding to sections shown in Fig.1 indicates that the Llandeilo Flags represent a fining-upwards sequence which can only effectively be differentiated into four lithofacies (Fig.3). Although a simplified lithological key is used to depict the sedimentary succession in sections illustrated herein (Figs.4 and 5), full details of successive local facies changes, typical sedimentary structures, shell beds and bed thicknesses are given in Wilcox (1979). We have used the descriptive terminology of Ingram (1954) to categorize bedded units, referring respectively to very thickly, thickly, medium, thinly, very thinly bedded and laminated for units 100, 30--100, 10--30, 3--10, 1--3 and 1 cm. In addition, we have used orientated, cut and polished slabs to examine sedimentary structures and thin sections to confirm grain size. Orientational structures are rarely sufficiently pronounced to derive directional measurements and, therefore, no attempt has been made to accumulate such data.

Regional setting During the Ordovician the Llandeilo area was situated in the southeastern part of the Welsh Basin (Williams, 1976) where emergent volcanic islands periodically erupted, providing fresh sources of sediment. There is little doubt that the Llandeilo succession in the type area represents a transgressive sequence following the cessation of Upper Llanvirn vulcanism immediately to the northwest (Williams, 1953). Although the localized nature of exposures militates against palaeoenvironmental reconstructions, it is evident from the volcanic quiescence (Mitchell, 1957), progressive decline in siliciclastics and corresponding increase in carbonate accumulation, that stable conditions with a low sediment supply prevailed in a predominantly shallow-water, level-bottomed marine environment. Johnson (1978, p. 233), who provided what Walker (1979, p. 87) described as "the best overall review of ancient and modern siliciclastic seas", warned

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291 that "classification of shallow marine lithofacies in terms of the dominant physical process has to be approached with caution". Having also encountered difficulties in unequivocally explaining Llandeilo sedimentary facies in terms of tidal or wave domination, we have confined our interpretations to the assessment of implied energy conditions and simply interpreted facies in terms of their probable location in an onshore to offshore transect.

Coarse sandstone facies This lithofacies, the most distinctive and localized encountered in the Llandeilo succession, is reminiscent of parts of the underlying Ffairfach sequence (e.g., the Rhyolitic Conglomerates of the type section; Williams et al., 1981). It is best seen where the Lower Llandeilo "Basal Sands" and "Calcareous Sandstones" (sensu Williams, 1953) are exposed between Maes y fallen and Dynevor Park (Fig.3). Typically, the facies consists of mainly medium to thickly bedded units {5--50 cm) of coarse sandstone and fine granular conglomerate which exhibits well-developed parallel or lowangle cross-lamination and accumulations of shell and shale clast debris (cf. Williams, 1953). These units alternate with thinly to very thinly bedded (1--10 cm) laminated sandstones and siltstones which are often slightly bioturbated. Whilst the coarser facies generally indicates a moderately high-energy environment where erosion resulted in the reworking of sediment and skeletal remains, the finer grained bioturbated sandstones indicate quieter phases permitting temporary faunal colonization. According to Reineck and Singh (1975), analogous modern sandstone facies are associated primarily with marine shoreface environments exposed to wave action. There are also numerous examples of ancient intertidal sandstone facies with which the Llandeilo facies compares (cf. Masters, 1967; Campbell, 1971; Narayan, 1971; Pryor, 1971). The sedimentary structures described may be indicative of beach and shallow subtidal coastal sand bodies and certainly compare with those listed for ancient tidal environments (Klein, 1970; Wilcox, 1979; fig.4.4). Since the structures also compare with those of the shoreline association of sublittoral sheet sandstones (Goldring, 1966; Goldring and Bridges, 1973) which is subtidal in origin, we do not wish to restrict our interpretation to one particular inferred environmental category, so we suggest that the coarse-sandstone facies may represent low tidal sand-flat or subtidal bodies (cf. Facies 1 of Klein, 1970).

Sandstone--siltstone facies This facies is a finer-grained lateral equivalent of the coarse-grained sandstone facies and in many respects is very similar; it constitutes the remainder of the Lower Llandeilo Flags. Typical sedimentary structures include predominantly parallel or low-angle cross-lamination, small-scale erosional

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293 features, shell lags, shale-chip conglomerates and some bioturbation. Undulating bedding structures of low troughs and swells with wave lengths up to 2 m and amplitudes up to 30 cm are conspicuous, particularly in the Sowerbyella and Corineorthis Beds (Wilcox, 1979, figs.2.17--18), and are particularly associated with well-developed shell beds. The size of these structures closely accords with dimensions quoted by Walker (1979, pp. 81--85) for hummocky cross-stratification features. However, sectioning of these undulating bedded units reveals truncated wave-ripple lamination structures similar to those described by Campbell (1966, 1971) from shallow-water supra-wave base and shoreface sandstone deposits. Similar truncated structures and shell lags were described by Goldring and Bridges (1973) in sublittoral sheet sandstones and by Reineck and Singh (1975) in North Sea shoreface deposits below low water mark. Shale clast conglomerates are found in inter- to subtidal sequences (Mazzullo, 1978; Reineck and Singh, 1975) and may indicate erosion of an emergent, mud-cracked surface. In any event, our environmental interpretation of this facies is much the same as for the coarser facies.

Siltstone--mudstone facies This facies, which is typical of much of the Middle and Upper Llandeilo Flags, is characterized throughout by thin- to medium-bedded, interlayered mudstone and siltstone units consisting of irregular, alternating muddy and silty layers rarely exceeding 1 cm in thickness (cf. rhythmites of Reineck and Singh, 1975). Flaser, lenticular and wavy bedding structures which predominate in this interlayered facies (Reineck and Wunderlich, 1968) are attributable to the rhythmic fluctuation between slack and turbulent water phases in the current regime. Indeed, such bedding, which can be tidally controlled, may originate in the inter- or subtidal zones above storm wave base, and is frequently referred to as tidal bedding (op. cit., p. 108). The relative proportions of mud, silt, and sand reflect load-carrying capacities of currents and can be environmental indicators; e.g., Reineck and Singh (1975) subdivided the intertidal zone into mudflat (near the high-water mark) and mixed and sandflat nearer the more active seaward side of the fiat. Respective typical bedding types along this environmental gradient are: (1)lenticular bedding, where mudstones are interlayered with thin, partially coalesced silt layers resulting from small sand ripple migration; (2) wavy bedding, where rippled layers form continuous horizons; and (3) flaser bedding, where the muddy fraction is preserved only as thin discontinuous drapes (cf. the sandstone--siltstone facies). Middle and Upper Llandeilo sediments are predominantly silty muds with a corresponding preponderance of lenticular and wavy bedding. The absence of larger-scale structures, common in modern and ancient tidal-flat deposits (e.g., Mazzullo, 1978; Selley, 1978) probably results from the fineness of the sediment and/or a lower-energy depositional environment.

294 The "M. anomalis Limestones" (Fig.l), which consist of thin- to mediumbedded, hard, calcareous siltstones and mudstones with ubiquitous interlayered bedding, represent as coarse a development of this facies as is known in the Middle and Upper Llandeilo. As the faunas also exhibit greater affinities with those of the Lower Llandeilo, we infer that these deposits may have originated in a higher-energy, inter- to subtidal environment.

"Distal" siltstone--mudstone facies The sediments of this facies are predominantly mudstones interlayered with fine-grained siltstones. The diffuse alternation of muddy and silty layers or laminae (sensu Ingram, 1954) is measurable in millimetres with thicker silty intercalations (up to 5 cm) occurring only rarely. Such rhythmically interlayered deposits which exhibit few structures indicative of lateral sediment transport (cf. the lenticular and wavy bedding of the other facies) probably originated through the settling of material forced into suspension by storm activity (Hayes, 1967; Reineck and Singh, 1972). Although occasional silt lenses may be attributable to slight lateral sediment transport or periodic post-depositional scouring, even the most pronounced silt/sand intercalations may be storm sand layers (sensu Reineck and Singh, 1972) which settled from sediment-charged, storm-agitated waters. This evidence, therefore, suggests that this facies accumulated in deeper offshore waters below normal wave base and beyond the sphere of normal tide or wave influence. This facies probably represents a transition towards the "Black Shale Facies" (Fig.3) associated with the "Dicranograptus Shales" which, since they exhibit no silt other than rare laminae (<1 mm) and no current-produced sedimentary structures, must have accumulated either in a sediment-starved haven or more probably in deeper offshore waters below storm wave base and far from the sediment source.

Interpretation of Llandeilo sedimentary facies Although the Llandeilo sedimentary succession generally becomes finer upwards and mainly represents an inter- to subtidal or shoreface to shallowshelf sequence deposited above storm wave base, this pattern need not necessarily represent a progressively deepening environment of deposition. Local fluctuations in the progressive fining-upwards sequence (e.g., the "M. anomalis Limestones") as well as uncertainty about the feasibility of subdividing ancient inter- to subtidal sequences (Reineck and Singh, 1975; De Raaf and Boersma, 1971) militate against drawing unequivocal conclusions concerning the environment of deposition of these related facies. However, differences in the character of faunal assemblages from one facies to the next, and in particular the increased proportion of articulated trilobite remains in the siltstone--mudstone facies (Wilcox, 1979), at least support

295 the conclusion that the Lower to Middle and Upper Llandeilo sequence represents a high- to low-energy environmental gradient even if a corresponding shallow to deeper water transition cannot be confidently deduced. The coarser Lower Llandeilo facies are characterized by what are inferred to be disturbed fossil assemblages which were probably associated with a turbulent, low-intertidal sandflat or subtidal, shoreface sheet-sandstone environment. The closest analogues to the Middle and Upper Llandeilo siltstone--mudstone facies is probably the coastal sand environment of the North Sea Biisum region where a typical shoreface sandy b o d y is absent; instead, mainly finegrained, tidally bedded, silty sediments form an intertidal to transition zone sequence between intertidal sediments and shelf muds. Such an interpretation helps account for the lack of larger-scale sedimentary structures associated with coarser-grained coastal sequences and again emphasizes that different facies may not be related to pronounced depth gradients, but, as in this case, may reflect such factors as decreased sediment supply. LLANDEILO FAUNAS

In troduc tion Since Williams (1948, 1949 respectively) outlined the t a x o n o m y of the trilobites and brachiopods of the type series, only the work of Addison (1974), Wilcox (1979), and Lockley and Williams (1981} have added to our knowledge of the t a x o n o m y and stratigraphical distribution of these faunas. The Mollusca and other phyla (e.g., Bryozoa and Echinodermata) remained unstudied and even n o w knowledge of their t a x o n o m y and stratigraphical distribution is limited. Pertinent palaeoecological notes by Williams (1953) remain valid in the light of our census observations and our conclusions suggest that the variable and patchy distribution of benthic forms may result from significant reworking in a predominantly shallow, fluctuating marine environment. The distribution of large trilobites (Basilicus, Flexicalymene and Ogygiocarella) is strongly facie~controlled and the pattern of mutual exclusion noted between the latter genus (species) and the former two (Williams, 1953) can also be detected on a local scale (Fig.6). The Lower Llandeilo is rich in benthic skeletal remains and most of our detailed observations pertain to these "assemblages" (sensu Williams et al., 1981) which comprise recurrent Dalmanella and Sowerbyella dominated faunules comparable with those described by Williams et al. (1981) from the preceding Ffairfach Group. We present the entire logged distribution of Brachiopoda for the Lower Llandeilo (Fig.4) and a similar summary of the more ratified Middle to Upper Llandeilo data (Fig.5); both diagrams illustrate the facies/fauna relationships discussed herein. These data, together with information on the stratigraphical distribution and abundance of zonal trinucleids and other Trilobita (Figs.3

296 and 6), form the basis for our subdivision of the faunal succession. The skeletal remains of various Crinoidea, Cystoidea, Bryozoa, Mollusca and Miscellanea are considered too fragmentary to permit the satisfactory assessment of numbers of individuals (Wilcox, 1979), and their presence is, therefore, denoted only by adding one to the totals for each sample. Since such arbitrary counting methods for these groups largely conform with those employed by Lockley (1980) and Williams et al. (1981) and differ only in the relative underestimation of bryozoan proportions, direct comparisons with both older and younger faunas are considerably facilitated. Estimates of the proportional representation (%) of taxa (species) in each assemblage (Figs.4 and 5) are based on total figures recorded by one of us (C. J. W., 1979). Although assemblage totals vary from only a few to several hundred individuals, it has been shown (Lockley, 1980) that direct proportional comparisons will not generate undue bias.

Lower Llandeilo faunal associations The quantitative outline of Lower Llandeilo faunas presented in Table I is based on a synthesis of data from all sections except Ponbren Araeth (Fig.l) where the fauna is relatively sparse (low density) though moderately diverse (Table II). On the basis of our assessment of the relative abundance of the Brachiopoda and Arthropoda (Trilobita and Ostracoda) from the Lower Llandeilo, we have identified eleven interrelated faunal associations (Table I, cf. Fig.4) in addition to the basal Tissintia assemblage. There are at least two cycles in the faunal succession with the Brachiopoda declining from 84.2% to 32.1% between the "Basal Sands" and "Sandy Flags" whilst the Trilobita and Ostracoda respectively increase from 4% to greater than 20% and 37%; similarly, in the second cycle from the brachiopod
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5.9(I-9)

12.3(9-14)

9.6(3-17)

7.0(I-15)

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10.9(1-23-8)

3.5(0.2-6.7)

14(4-5-29.4)

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TABLE I Quantitative biostratigraphical parameters for the lower Llandeilo Flags in the type and adjacent sections (drawn to scale); ns, Ns, ]~ and d respectively (in Tables I--IV) refer to number of sections, number of samples mean diversity and mean density

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Corineorthis

-- ?

basal M. ~ l e x Beds. (M.L.) M.inflatus matures Limestones

STRATIGP0~PHICALUNIT after (Williams 1955)

D~] m~ella Assoc.

Sowerbyella Flags

ASSOC. of the lower

~ASSOC. of the upper Sower~ella ~ags

~AS80C. of the M.matuz~s Limestones and associated beds

BIOSTRATIGRAPHICKL 8UBDMSION (This study)

11

45.9

59.6

51.5

5o.5

22.4

14.1

45.1

24,4

% Trilo.

% Brach.

21.0

19.0

2.5

50.3

Ost.

%

8.1 (3-11)

7.7 (2-12)

(1-11)

4.6

(}-16)

7.3

~ (range)

6.1 (1-15.2)

9-4 (2-22.6)

1.9 (0.7-8.4)

7.8 (2-17.2)

(range)

Quantitative biostratigraphical parameters for the Lower Llandeilo Flags in the Pontbren Araeth section (drawn to scale)

TABLE II

CO

(D

299 The local profusion (50%) of Macrocoelia llandeiloensis in the "Basal Sands" at Maes y fallen is noteworthy since the species, although persistent in the Ffairfach Group (Williams et al., 1981), is poorly represented in the Llandeilo. In succeeding low-density assemblages from the "Calcareous Sandstones" the decline of Sowerbyella is marked by an increase in Dalmanella and Horderleyella (H. cf. convexa) which characterize the association. In both these basal sandstone units, Flexicalymene cambrensis and Basilicus tyrannus are common whilst Ogygiocarella debuchii is absent. The transition from the "Calcareous Sandstones" to the succeeding "Lloydolithus lloydi Flags" is marked by a decline in the Brachiopoda and an increase in the Trilobita (Lloydolithus and Ogygiocarella) and Ostracoda. The assemblages are distinctive, diverse (Table I), contain rare elements like Schizotreta cf. transversa, and are probably best regarded as representative of the first of a number of transition phases characterized by a mixed benthic fauna. Both the most ubiquitous elements, D. parva (44.1%) and S. antiqua (35.5%), are represented by almost equal proportions. By contrast, "L. lloydi Flags" exposed 10 km northeast of Llandeilo at Coed-Shon quarry (Fig.l) yield a low-diversity fauna dominated by trilobites (73%), consisting predominantly of L. lloydi topotypes (42.1%) and O. debuchii (26.3%). The brachiopods (27%) are all inarticulates with Palaeoglossa attenuata (Sowerby) dominating Pseudolingula granulata (Phillips) by a 2:1 ratio. The composition of this fauna undoubtedly resembles patterns observed in the sparse assemblages from similar Upper Llandeilo facies. Assemblages from the upper part of the "L. lloydi Flags" are particularly characterized by the presence of Oxoplecia cf. nantensis and the zonal trilobite Marrolithus inflatus. The former brachiopod species only occurs sporadically elsewhere but averages 16.8% here and occurs in as many samples as D. parva (27.5%). Inarticulate brachiopods are well represented in this mixed association and S. antiqua occurs only sporadically; the only known specimen of Parastrophinella parva from the type Llandeilo and rare Glyptorthis cf. viriosa are also recorded. The "Limestones and Shales" and the succeeding "Sandy Flags" both contain mixed faunas (phases 2 and 3) yielding relatively large proportions of trilobite and ostracod remains. D. parva dominates in the lower unit (Fig.6) whilst S. antiqua dominates in the succeeding "Sandy Flags" (Phase 3). Variation in the composition of successive assemblages is evidently faciesrelated with a decline in the proportion of brachiopods in the upper, "Sandy Flags" unit corresponding to an increase in the proportion of the Ostracoda. In the succeeding "Sowerbyella Flags" the Brachiopoda, and in particular S. antiqua and D. parva, again become dominant, comprising an average of 79.7% of the total fauna and 89.8% of the total Brachiopoda in the two .sections studied. The trilobites and ostracods together only comprise 15.3% of the total fauna as compared with over 50% in the underlying "Sandy

300 DYNEVOR PARK [DP)

DP west

west

1

2 3 7

eost

34

9

9 OF ~DA

la

i.i;,

C

i

CENNEN (C)

I

}::i:"oe°t~a~

central I

40

20

.,xed

267

36 7 9

.....

h

(C)

(C)

i

1 2 4 6 789

25 i

i

i

I L)I]I o

• _

Mixed Assoc

I" I~

DAMANELLAAsl

~!:"

SOWERBYELI. ( Phase

;'--~o

I 2 3 4 5 6 7 8 9

INARTICULATA D~Imanella Horderleyella Tissintla Corineorthil Oxoplecia Sowerbyella Macrocoella Rostrlcellula

30 m

o

, ~

Fig.4. Proportions of the main taxonomic groups and the brachiopod species represented in the Lower Llandeilo Flags. Named genera refer unequivocally to species mentioned in text. Diversity and density profiles are also shown. Key to lithological succession given in Fig.5.

Flags". However, it is interesting to note that Flexicalymene reappears with the ubiquitous Basilicus whilst Ogygiocarella is again absent. Sowerbyella and Dalmanella occur in roughly equable proportions, respectively comprising 49.5% and 40.3% of the Brachiopoda in the two sections; however, although SowerbyeUa is dominant in the more complete western section (55--36%) it

301 is actually subordinate to Dalmanella (40.9--46.9%) in the eastern section. Although it is reasonable to refer to a third phase of S. antiqua domination in the basal part of the "Sowerbyella Flags" (Table I, Fig.6), the importance of D. parva must be emphasized. In the middle part of the "Sowerbyella Flags", a transition from horizontally to irregularly bedded facies (Wilcox, 1979) is accompanied by an equally abrupt faunal change (Fig.4 and Table I). The Trilobita increase from an average of 7.1% to 24.7% whilst the Brachiopoda decline from 79.7% to 44%. D. parva persists, but S. antiqua disappears from the succession for a stratigraphical interval of 22 m, being partially replaced by sporadic influxes of bivalve-dominated assemblages (coquinas) consisting mainly of nuculid remains (cf. Similodonta) and a few larger forms like Cyrtodonta. The dominance of these bivalves in the three successive assemblages (i.e., 72.8%, 47.5%, and 93.4%) means that they actually constitute an average of over 11% of the total fauna in the middle part of the "Sowerbyella Flags" (Table I). The assemblages represent the first substantial record of Mollusca in the type Llandeilo; they have also yielded gastropods (Sinuites) and brachiopods (Mcewanella) unknown elsewhere in the succession. Schizocrania also occurs in an assemblage immediately above the second coquina where irregular bedding gives way to horizontal strata. In the uppermost part of the "Sowerbyella Flags" the predominance of D. parva accounts for the proportional increase in the brachiopods (66.5%) and the corresponding decline of the trilobites to 7.8%. Ostracods are numerous at several horizons and maintain a fairly constant average abundance throughout the "Sowerbyella Flags" (Table I). The uppermost two units of the Lower Llandeilo, the "Corineorthis Beds" and the "Marrolithus inflatus maturus Limestones", are characterized by two contrasting sedimentary facies consisting of lenticular, calcareous sandstones with shell beds overlain by very thinly bedded, almost unfossiliferous, laminated sandstones. The fauna of the "Corineorthis Beds" is variable with the Brachiopoda (58.3%) dominating, particularly in the shell beds, although the Trilobita are also well represented (26.6%). The brachiopods, Horderleyella (35.7%) and Dalmanella (23.2%) are both more abundant than Corineorthis (10.7%) and Sowerbyella (8.9%), whilst the Trilobita are dominated by Flexicalymene (50%) and Basilicus (43.8%). In the succeeding "M. inflatus maturus Limestones" the fauna is so sparse as to make proportional estimates very generalized and the paucity of brachiopods means that they actually become subordinate to the trilobites for the first time in the Llandeilo succession.

The Pontbren Araeth section This section is considered separately from those in the immediate Llandeilo area since, being geographically more removed, it exhibits a different faunal succession. The oldest exposed beds, the "Sandy Flags", yield a fauna

302 similar to that found around Llandeilo (Tables I and II) except that D. parva is more dominant. In the succeeding "Sowerbyella Flags" differences between the two areas become progressively more pronounced. In the lower part of this unit (Table I) the high proportion of Brachiopoda and average diversity values are reminiscent of other sections although the average faunal density is lower and the proportion of trilobites and ostracods is greater. It is also interesting that D. parva dominates S. antiqua by a 3:2 ratio (45.2% to 29.0% of the Brachiopoda). The decline in density is most pronounced in the upper part of the "Sowerbyella Flags" (Table I) where the Bivalvia and D. parva dominated assemblages of the Llandeilo area are absent and the Trilobita (45.1%) and Inarticulata (34.2%) occur persistently. In the upper part of the Lower Llandeilo succession it is hard to differentiate between the "Corineorthis Beds" and the "M. inflatus maturus Limestones" as Corineorthis occurs only rarely and the fauna consists of almost equal proportions of brachiopods, trilobites and ostracods (Table II). D. parva is the dominant brachiopod (40%) with Horderleyella (23.3%) and Tissintia immatura (11.7%) also well represented; M. inflatus maturus and Basilicus are the dominant trilobites comprising 50% of the group (Wilcox, 1979). The two youngest assemblages of this mixed association yielded Marrolithoirles simplex and are therefore indicative of the transition to the Middle Llandeilo. The younger assemblages also yielded Paterula (Lockley and Williams, 1981). Horderleyella increases from an average of 11.6% and 10.7% respectively in the two older units to 20% and 23.3% in the younger units and is therefore represented by greater proportions at Pontbren than in the type and adjacent sections.

Middle Llandeilo faunal associations In addition to the incomplete Middle Llandeilo section studied at Pontbren (Table III) which consists mainly of strata representative of the older "M. simplex Beds" (lower subzone) and "M. anomalis Limestones" (Fig.l), the equally incomplete type and adjacent sections of the Llandeilo area consist mainly of beds representative of the upper part of the latter unit and the succeeding "M. simplex elevata Beds" (sensu Williams, 1953) which we refer to as the upper M. simplex subzone (Fig.2). The Lower to Middle Llandeilo transition is characterized by a marked decline in articulate brachiopods and ostracods, and a corresponding increase in trilobites and inarticulate brachiopods (Fig.5). In the lower M. simplex subzone at Pontbren, all the brachiopods (13.0%) are inarticulates (Pseudolingula), whilst the trilobites (78.3%) consist predominantly of M. simplex (77.8%) with Ogygiocarella (16.7%); ostracods are absent. In the succeeding lower part of the "M. anomalis Limestones" the sparse fauna (Table III) is mainly represented by Ogygiocarella (80%). In the Pontbren quarry (SN 659237), the locality cited by Williams (1948, p. 83) as the type locality for M. anomalis, six census samples (Table III)

M. simplex Beds

_M. anomaliE Limestones

_

0

50

=

Upper

M. Simplex elevata Beds.

/

Lower M. simplex subzone

~Lowerf M. anomalis Lst. with. diverse and depleted f~lules

fUpperi M. anomalis Lst. with Tissintia

subzone

M. simplex

Depleted transition fauna

~

~

GAP

BIOSTRATIGRAPHICALSUBDIVISION (This study)

Poorly fossili~ro~ ~age

S~ATIGRAPHICAL U N I T (after Willisms 1955)

I

2

2

2

ns

33

5

61IlO 4

15

18

13.o

20.0

43.0

155 49.5

59.51

43"5I) 25.1)30.9

13.3~i~ 22.2) F'vo

1~I18

151 31 46

% Brach.

NS

78.3

80.0

18. 9

43.8

73,2)

32.3

65,6

28.8

5.6

O.4/

21

7.4

1.2

/

~:;/81.1

5o I

Os±.

Trilo.

Quantitative biostratigraphical parameters for the Middle Llandeilo Flags (drawn to scale)

TABLE III

2.4(1-4)

1.3(1-2)

8.7(4-12)

5.3(I-13)

3.3(I-16)

4.3(2-8)

4.3(2-8)

2.4(I-52

2.3(1-4)

range

1.6(0.5-2.5)

17.1(3.6-44)

1.5(0.3-6.9)

1.8(0.3-8.7)

0,6(0.4-0o9)

range

O

304

I , .

.

.

.

.

.

.

.

.

.

I i

J--

LITHOLOGICAL SUCCESSION ]

J

Mudstone

]

Siltstone

Calc. siltstone Calc. sandstone

DYNEVOR PARK ]

\

Sandstone

] Impure limestone -- [ ] Shell beds _~ irregular bedding ~ SAMPLEPOINTS

124

CENNEN

!

.

.

.

.

.

9 i

30

m

Z

I

2

4

ii!

0 ,

?

PONTBREN

I

4 2

i, 2

2

Fig.5. Proportions of brachiopod species represented in the Middle and Upper Llandeilo Flags (1, 2, 4 and 7 as in Fig.4). from a 1-m interval of strata yielded diverse assemblages reminiscent of Dalmanella.dominated Lower Llandeilo faunules. D. parva (46.3%) and Tissintia immatura (19.5%) dominate a fauna which includes Horderleyella, Glyptorthis, Oxoplecia, Macrocoelia, Rostricellula lingulids and the trilobites Basilicus and M. anomalis. In the succeeding upper part of the "M. anomalis Limestones" exposed in the Llandeilo area (Fig.l), the brachiopods (55%) remain dominant over the trilobites (32.3%). However, both Tissintia (25.8%) and the lingulids

305 (24.1%) dominate D. parva (17.7%) and brachiopod diversity declines progressively (Fig.6);Sowerbyella is known in one assemblage only (Fig.5). In the succeeding upper M. simplex subzone the trilobites again dominate (65.6%) the brachiopods (30.9%), which are mostly inarticulates (65%). Tissin tia persists (23.3--25.5% in two sections) but D. parva declines (2--5%). M. simplex is by far the most dominant trilobite comprising at least 53.6% of that group in addition to a further 16% of trinucleid remains. The change from the upper M. simplex subzone to "poorly fossiliferous flags" (sensu Williams, 1953) is characterized by the disappearance of articulate brachiopods and ostracods and an increase in trilobites to 81.1%. These beds represent the crucial transition zone between Middle and Upper Llandeilo (Fig.l). In the type section, M. simplex persists into this transition zone but, above a 15-m gap in exposure (Fig.l), is all but absent and is replaced by the Upper Llandeilo zonal forms M. favus and M. favus moderatus.

Upper Llandeilo faunal associations Although the fauna of the Upper Llandeilo is sparse and dominated by trilobites (44.5--80% in six faunules; Table IV) analysis of 56 samples from the type section allows us to recognize three associations and identify Tissintia immatura as an important element in the younger two. Our biostratigraphical subdivisions differ from those proposed by Williams (1948) mainly because we record relatively abundant M. favus moderatus lower in the succession and find that, higher in the succession, this subspecies does not become subordinate to M. favus which occurs only sporadically throughout. The oldest of the three Upper Llandeilo associations is dominated by the Trilobita (80%) with Basilicus, Ogygiocarella, M. favus moderatus, and M. favus respectively comprising 31.3%, 20.8%, 18.8%, and 6.3% of the class. The brachiopods consist mainly of inarticulates. Contemporary assemblages from a small section exposed in Dynevor Park (SN 619224; Wilcox, 1979) reveal similar proportions of the main groups and these same trilobite taxa. In the succeeding fine calcareous sandstones of the type section, both Basilicus and M. favus moderatus are absent; the Trilobita (59.3%) are dominated by Ogygiocarella (62.5%) (Fig.6), whilst the Brachiopoda (40.7%) consist of almost equal proportions of Tissintia immatura (44.5%) and inarticulates (55.5%). Contemporary assemblages from more massive limestones in Dynevor Park (SN 615228; Wilcox, 1979) yielded a few M. favus and M. favus moderatus specimens, and confirm the dominance of Ogygiocarella (53.8%). The uppermost part of the type succession yields a fauna similar to the preceding association except for its reduced proportion of trilobites (44.5%) Ogygiocarella declines to only 25% of the fauna and is absent in contemporary beds in Crug dingle (SN 623228; Wilcox, 1979). The proportions of Brachiopoda like Tissintia (33.3%) and the Inarticulata (60%) resemble distributions

0

elevata.

M. Simplex vat.

with occasional

poorly fossili£erous

.

30 m

moderatus

M. favus vat.

with

flag~ and limestones

M. f a v u s

with

flags and limestones

S~a~ZCm~CAL m~ (after Williams 1955)

Assoc.

M. favtts

Basilicus -

Assoc.

-Tissintia

-

moderatus

O~ygiocarella

Assoc.

Tissintia. - Schizocrania

BI0S~aTZ~m~ICAL s~mDrws10~ (This study)

2

2

2

ns

29

3

15

8

18.5

27.5

40.7

40

55.5

5o.7

8

12

% Brach.

NS

Quantitative biostratigraphical parameters for the Upper Llandeilo Flags (drawn to scale)

TABLE IV

80

59.1

59.3

52

44-5

55.6

%

-

13.6

-

8

-

Ost.

2.1(1-4)

6(4-8)

1.7 (1-3)

2.6 (1-5)

2.2 (I-4)

0-3)

1.5

range

-

1.9 (1o4-2o4)

t-5 (o.e-5.2)

(o.3-~ .0)

0.6

rang~

O

307

LLANDEILO

~Llanviril ',, ,"

LOWER

, ~ I

.

i

I

I

i

I

i

i

i

i

i

Lo

Caradoc

UPPER

MIDDLE i

i

,

faunules

Range of

_i

_i

0

PROPORTION O F MAIN GROUPS .<

0

!

I N A R TICULATA Dalmanello

parva

Horderleyella -'~VOxo.

~

' ~- ~ ',

Corineorthis

~ ~

',

"'

Rostricellulo

T

J

Sowerbyella

--

T-

--

-

Merrolithoides

:

~

I

~

_

__

Basilicus

tyr~nnus

Ogygiocarella

debuehfi

=, Flexicalymene

!~"o , ',~,' o . _ _ o -'-o,,, ,,p-o". o ~ o ,,4 ",, , ,

simplex

M. anomalis IVIgrrolithus favus M . favus m o d e r a t u s

L~

i

i I :

Qntiqua

!

,, mr-

sp.

immotura

L--~loydofithus Iloydi I Merrofithus inflatus ~ M. m a t u r u s ',

~ __

Tissintio

O .......

~:---m

,,

"

O.........

O .......

combrensis

v

~

DIVERSITY

H

DENSITY

H

Fig.6. The distribution of main groups and d o m i n a n t b r a c h i o p o d and trilobite based on the m e a n p r o p o r t i o n s r e c o r d e d in successive faunules. Diversity and profiles are also based on m e a n values. O S T . = Ostracoda; Biv. = Bivalvia; O x o . p l e c i a . N u m b e r e d phases of D, p a r v a and S. a n t i q u a d o m i n a t i o n also shown; see details.

species density = Oxo-

t e x t for

in the preceding association, except that the latter group includes Schizocrania which comprises 75% of the meagre brachiopod fauna in the predominantly shaly Crug section. CONCLUSIONS

Our analysis of the distribution of Llandeilo faunas clearly confirms the effectiveness of Williams' trinucleid zonal scheme (1948, 1953); it also highlights the importance of bivalved benthic elements, particularly in the Lower Llandeilo, and clearly shows their occurrence to be facies-related as in the case with trilobite distributions. For example, the ostracods and articulate brachiopods,like the trilobite Flexicalymene and three of the zonal trinucleids,

308 are confined almost exclusively to the Lower Llandeilo, sandstone--siltstone facies (Figs.3 and 6). With respect to the Ostracoda, Copeland (1977, p. 16) observed that Palaeozoic beyrichiids were "benthonic, shallow water facies dwellers" and Warshauer and Smosna (1977) concluded that subtidal environments below the breaker zone represented the optimum habitat. Such observations clearly support our environmental interpretation of the Lower Llandeilo facies. As in the underlying Ffairfach Group (Williams et al., 1981), the brachiopods D. parva and S. antiqua dominate the Lower Llandeilo Flags. The dominance of the former species is reflected by the recognition of five successive pre-Middle Llandeilo phases as compared with only three for S. antiqua. Although both species are considered representative of distinctive palaeocommunities and are almost mutually exclusive in the Ffairfach Group, a considerable degree of mixing is evident in the Lower Llandeilo. Proportions of these species in successive assemblages and faunules are often similar and may indicate that both the palaeocommunities evolved to accommodate a more equable distribution of these two species. However, such an interpretation is not necessarily applicable since pronounced variation in the Ffairfach sedimentary succession appears to have exercised a strong facies control over the fauna, accounting for more dramatic change-overs and reduced intergradation. In contrast, the gradational facies changes encountered in the Lower Llandeilo succession are reflected by more subtle faunal changes and a number of transitional, mixed faunules. Remnants of the Horderleyella palaeocommunity (sensu Williams et al., 1981) which persist in the Lower Llandeilo demonstrate a limited success comparable to that exhibited by Oxoplecia. In view of Williams' observation (1974, p. 124) that "the distinction between Oxoplecia and Bicuspina is finely drawn" the Lower Llandeilo occurrence of the former genus may be considered an early expression of the type of association referred to as the Bicus. pina "set" (Williams, 1976) from contemporary deposits of the Shelve area. Interpretation of Lower Llandeilo successions as shallow, shoreface facies poses the question of whether skeletal remains represent residues of inter- to subtidal shoreface palaeocommunities, transported assemblages of uncertain origin or intermediate residual categories. Resolution of this question is aided by our understanding of the facies/fauna relationships demonstrated for the Ffairfach Group (Williams et al., 1981), where the highest shell articulation ratios are recorded in assemblages from fine calcareous siltstones whilst coarser facies generally exhibit very low ratios. Since the proportions of taxa in assemblages with lower articulation ratios do not differ significantly from those apparently indicative of in situ benthos and are further represented by similar proportions of opposing valves, both the dominant Dalmanella and Sowerbyella palaeocommunities apparently remain recognizable after undergoing some postmortem disturbance. However, "abnormal" size frequency distributions and disparity in pedicle/ brachial valve ratios from assemblages in the lower part of the Sowerbyella

309 Flags (mean P/B ratio 0.75, range 0.50--1.25) could be interpreted as evidence that the valves were "transported" or "derived from different populations" (Wilcox, 1979, p. 89). Supporting evidence for such inferred submarine transport (and erosion) comes from assessing such assemblage parameters in the context of their relationship to the sedimentary succession. The Dynevor Park sequence (Fig.4) of regularly bedded calcareous siltstones contains roughly equal proportions of Dalmanella and Sowerbyella and passes up into variably composed shell beds where Sowerbyella dominates Dalmanella by a 9:1 ratio. Since Sowerbyella lacked a functional adult pedicle and was correspondingly mobile in response to current activity, the succession from these shell beds into irregularly bedded strata (cf. erosional bedding sensu Selley, 1978) containing shell accumulations of infaunal bivalves and other rare species, probably implies an increase in current velocities resulting first in shell transportation and then in an erosional sequence. The disappearance of Sowerbyella in the irregularly bedded facies again implies a high-energy flow regime capable of removing all "surface" shell accumulations and some underlying sediment. Paradoxically, although transported assemblages might be considered of limited value in the interpretation of palaeocommunities, in this case they provide a useful insight into the palaeoenvironment by illustrating Sowerbyella's mobility potential at sub-erosional current velocities and therefore implying a semi-mobile palaeocommunity (cf. Williams et al., 1981); they also point to the existence of a hitherto unknown Similodonta dominated infauna. Undulating bedding like that associated with the middle part of the "Sowerbyella Flags" also occurs in the "Corineorthis Beds" succession (Fig.4) and locally in the "M. anomalis Limestones" of the Middle Llandeilo (Fig.5). At both horizons pronounced faunal changes occur. The sudden influx of Corineorthis pustula Williams in the trilobite-rich shell beds of the older facies cannot be attributed directly to changing colonization patterns and may, therefore, be attributable to transportation. Similarly, the unique Middle Llandeilo occurrence of Sowerbyella antiqua (Fig.5) is coincident with a gradation from undulating to regularly bedded strata and may therefore imply waning current velocities conducive to the settling of these shells. The Middle and Upper Llandeilo siltstone--mudstone facies (Fig.3) appears to have represented the preferred substrate for inarticulates like Pseudolinqula granulata (Phillips) and Palaeoglassa attenuata (Sowerby), and the articulate species Tissintia immatura Williams (Figs.5 and 6). The facies preference of the Llandeilo lingulids accords with that noted for modern species (Paine, 1970) which generally inhabit silty substrates in shallow to moderate water depths. Similarly, Williams et al. (1981) have shown that opportunistic Tissintia species also thrived on silty substrates, presumably in a similar shelf setting. Species of the dominant genera, Pseudolingula, Palaeoglossa, and Tissintia are all .typical of recently defined palaeocommunities from coeval deposits in Wales (Williams et al., i981) and more particularly the

310 Welsh Borderland (Williams, 1974, 1976) where all three forms occur in the contemporary Meadowtown Beds of the Shelve area. Ogygiocarella debuchii is particularly abundant in the Middle and Upper Llandeilo and appears to have progressively replaced the widespread species Basilicus tyrannus and the more restricted species Flexicalymene cambrensis. Although the six marrolithinid species and one subspecies recognized in this study all exhibit well-defined stratigraphical and hence environmental restriction, we are uncertain of the ecological controls and can only infer that the four characteristic (zonal) Middle and Upper Llandeilo taxa evolved in a quieter, offshore environment than that inhabited by the three Lower Llandeilo species. The occurrence of Schizocrania in the Upper Llandeilo distal siltstone-mudstone facies is closely reminiscent of the facies relationships noted elsewhere (Lockley and Williams, 1981; Williams et al., 1981). Although lengthy' discussion of the distribution of Llandeilo palaeocommunities is outside the scope of this paper, sufficient information is available to infer the palaeoenvironmental setting occupied by the dominant species (Fig.7). This summary diagram clearly shows the succession of palaeocommunities observed in a fining-upwards sequence representative of a shoreface to offshore shelf transect.

• Typical palaeocommunity I

Environment

1

f

HW

DALMANELLA INTERTIDAL LW HORDERLEYELLA HIGH SUBTIDAL & SOWERBYELLA

PSEUDOLINGULA SCHIZOCRANIA

-

WB

TRANSITION ZONE -_Siltst.-_-

SWB

OPEN SHELL~__~ /~Black .//

shale - facies

/

///

£-2~-2+ Sandst. ::i::t 2~-±-±+2-~:&: i i::::: :: :: 2- sandst:~.sill.st.:2:£ -~-±-±-ifaciesi:::iii:

LOW SUBTIDAL

TlSSlNTIA

. . . . . -Distal--

_mudst._-_facies_-

-siltst.--mudst. - . . . . . -facies~- . . . . .

- : + +:÷: 2 : : : - : : . : ~-T-T-~T÷T: :£~I :::::: i: _~÷-÷~÷Tii!: ! i! I i£il i --

Caradoc

Upper

,

.'

"."

','.

Lower

Middle I

~'m'---',

I

Fig.7. Inferred Llandeilo palaeocommunities. Arrow represents time transgression through Llandeilo stages (Lower, Middle and Upper) to Caradoc.

311 APPENDIX The faunal list given below includes taxa which occur only sporadically and have been hitherto u n k n o w n or unrecorded in the type Llandeilo. This list is therefore prefaced by the following explanatory notes which elucidate the significance of a few known mugeum specimens and clarify minor taxonomic ambiguities.

Brachiopoda Prior to this study, species representative of the genera Glyptorthis, Mcewanella, Oxoplecia, Parastrophinella and Schizotreta had not been recorded in the type Llandeilo, although references by Williams to Cliftonia undoubtedly referred to Oxoplecia, the most common of above five taxa (A. Williams, pers. comm.). Although Lockley and Williams (1981) have figured a specimen of Parastrophinella cf. musculosa Williams, known from the Llandeilo area, the species remains a tentative addition to the list as its horizon of origin remains u n k n o w n and only congeneric rather than conspecific material has been recovered (op. cit.). Species denoted by an asterisk (*) were not recovered during the course of this study and the reader is referred to Williams (1953) and Lockley and Williams ( 1981 ) for available information.

Trilob ita Although both Basilicus tyrannus (Murchison, 1839) and B. peltastes (Salter, 1866) have been recorded from the type Llandeilo (Williams, 1953; Wilcox, 1979), Fortey (in press) considers the latter species to be a junior synonym of B. tyrannus and we accordingly refer all Basilicus material to this taxon. Williams (1953, pp. 191--192) recorded Acidaspis sp., Cybele sp. and Hornalonotus sp. from the Lower Llandeilo. Although we recognize none of these taxa, we have recorded Atractopyge sp. and an odontopleurid resembling Primaspis (cf. Sedgwick Mus. specimen No. SMA 45029).

Mollusca Hitherto unrecorded molluscan genera from the Llandeilo include the bivalves Cyrtodonta and Similodonta and amongst the gastropods Sinuites and a planospiral form (cf. S M A 45027). These genera have also been identified amongst material from the Geological Survey M u s e u m which originates from horizons and localities closely associated with those referred to in the text. Further investigation of these and other contemporary Mollusca are being conducted (Morris and Lockley, in prep.).

Other taxa Although we have not examined the remaining listed taxa in sufficient detail to comment on the validity of the chosen assignments we feel they should give the reader an indication of the composition of the entire fauna.

List o f species recorded in the type Llandeilo Brachiopoda

Palaeoglossa cf. attenuata (Sowerby) Pseudolingula granulata (Phillips) emend. Lockley and Williams (1981) Paterula cf. bohemica Darrande Schizocrania ef. salopiensis Williams

Schizotreta cf. transversa Williams Schmidtities micula (M'Coy)* Glyptorthis cf. viriosa Williams Corineorthis pustula Williams Mcewanella cf. berwynensis MacGregor Dalmanella parva Williams Horderleyella cf. convexa Williams

312 Tissintia immatura (Williams) Tissintia plana Williams emend. Lockley and Williams (1981) Oxoplecia cf. nantensis Sowerbyella antiqua Jones Macrocoelia llandeiloensis (Davidson) Parastrophinella cf. parva MacGregor Parastrophinella cf. musculosa Williams* Rostricellula triangularis Williams

Bryozoa Morphotype Morphotype Morphotype Morphotype

1, 2, 3, 4,

cf. Prasopora cylindrical ramose form flat ramose form "stick-like" ramose form

Crinoidea Not generically assigned

Trilobita Cystoidea Basilicus tyrannus (Murchison) Ogygiocarella debuchii (Brongniart) Flexicalymene cambrensis (Salter) Marrolithus in flatus Williams M. maturus Williams M. favus (Salter) M. favus moderatus Williams Marrolithoides simplex Williams M. anomalis Williams Lloydolithus lloydi (Murchison) Atractopyge sp. Odontopleurid indet.

Not generically assigned Porifera Hyalostelia fasicula M'Coy Gastropoda Sinuites sp. Planospiral form Bivalvia

Ostracoda Primitia sp. Tallinnella sp.

Similodonta sp. Cyrtodonta sp. Graptoloidea Diplograptus sp.* (see Williams, 1953) Corynoides sp.* (see Wilcox, 1979)

*Asterisks denote taxa not recovered during the course of this study. ACKNOWLEDGEMENTS

Both authors wish to thank Dr. A. Williams for his'advice and encouragement and thank N.E.R.C. for the sponsorship which made the project feasible. We are also indebted to Glasgow University Geology Department for the provision of facilities and to Dr. B. J. Bluck for reading the sedimentology section. Chris Wilcox wishes to thank the Director of the Institute of Geological Sciences for permission to publish. REFERENCES Addison, R., 1974. The Biostratigraphy of the Llandeilo Facies of South Wales. Thesis, Queen's University, Belfast (unpublished). Bassett, D. A., Ingham, J. K. and Wright, A. D., (Editors), 1974. Field Excursion Guide to Type and Classical Sections in Britain. In: Ordovician System Symposium, Birmingham, 1974. The Palaeontol. Assoc., London.

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