Late Ordovician (Ashgill) ostracodes from the Drummuck Group, Craighead Inlier, Girvan district, SW Scotland

Scottish Journal of Geology

doi: 10.1144/sjg350100151999, v.35; p15-24.Scottish Journal of Geology

JAMES D. FLOYD, MARK WILLIAMS and ADRIAN W. A. RUSHTON Craighead Inlier, Girvan district, SW ScotlandLate Ordovician (Ashgill) ostracodes from the Drummuck Group,

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Late Ordovician (Ashgill) ostracodes from the Drummuck Group, Craighead Inlier, Girvan district, SW Scotland

JAMES D. FLOYD1, MARK WILLIAMS2 and ADRIAN W. A. RUSHTON3

1 British Geological Survey, Murchison House, Edinburgh, EH9 3LA 2 British Geological Survey, Keyworth, Nottingham, NG12 5GG

3 Department of Palaeontology, Natural History Museum, London, SW7 5BD

Synopsis

The Lady Burn and South Threave formations of the Drummuck Group (Ashgill Series, anceps Biozone) of the Craighead Inlier, southwest Scotland yield low diversity marine ostra-code faunas characterised by the binodicope Kinnekullea comma (Jones 1879), palaeocopes including Euprimites limus sp. nov. and a tetradellid (Vittellal sp.), and podocopes. Kinnekullea comma is re-described. It may have widespread correlative value as it occurs in the Por-trane Limestone of Ireland and is similar to, and possibly conspecific with, Kinnekullea species from the Ashgill of the Baltic States. The occurrence of typical Baltic ostracodes in the Drummuck Group supports the suggestion that by the late Ordovician the palaeoconti-nents of Laurentia and Baltica were in close geographical proximity.

Introduction

The study of British Ordovician ostracodes has recently undergone something of a renaissance with Llanvirn and Caradoc faunas being documented extensively from the Lake District, Welsh Borderland and Wales (Siveter 1978; Jones 1986, 1987 and references therein). Scottish Ordovician ostracodes have received less attention, but are known to occur in the Arenig, Llanvirn, Caradoc and Ashgill (Jones 1879,1893a; Siveter 1978; Siveter and Curry in Curry et al. 1984; and information of authors). Despite recent advances, the British Ashgill ostracode faunas remain largely neglected, though they occur widely and may have correlative value within Britain and with the Baltic region (see review in Jones 1987, pp. 108-109). The dearth of knowledge concerning the British faunas contrasts sharply with the detailed studies of the Ashgill ostracode faunas from the Baltic region (e.g. Schallreuter 1987; Meidla 1996).

Late Ordovician ostracodes from the Girvan district of southwest Scotland were documented by Jones (1879, 1893a) who described some 17 taxa from the late Caradoc-early Ashgill Whitehouse Group. Jones (1879, 1893a) also described a few specimens from the Starfish Beds, South Threave Formation, part of the Ashgill Drummuck Group of the Craighead Inlier (see Harper 1982; Figs. 1-3). Although the palaeontology of the Drummuck Group has since been intensively studied (see review in Harper 1982, p. 253), the ostracodes have been neglected. New collections from the Craighead Inlier contain numerous ostracodes from the South Drummuck Burn exposures of the Lady Burn Formation of the Drummuck Group (Figs. 1, 2). These include Kinnekullea comma (Jones, 1879), Euprimites limus, sp. nov. and other palaeocope and

podocope ostracodes. The aim of this paper is to describe these ostracodes and to indicate their biostratigraphical, palaeoecological and palaeogeographical significance.

Stratigraphical setting

The Craighead Inlier lies some 2 km NE of Girvan and consists of an anticlinal structure which plunges gently north-eastwards (Fig. 1). It is truncated along its southeastern margin by the Kerse Loch Fault and is uncon-formably overlain on the northwest by late Silurian to early Devonian sandstones and conglomerates of Old Red Sandstone facies. A considerable thickness of Ordovician and Silurian marine strata is preserved within the inlier, with the oldest rocks probably the pillow lavas exposed in the core of the anticline at Craighead Quarry. These volcanic rocks are unconformably overlain by the middle Caradoc Craighead Limestone and are assumed to correlate with similar lavas within the Arenig Ballantrae Complex to the south. The inlier is important since it contains parts of the Lower Palaeozoic succession not seen elsewhere in the Girvan district, particularly the youngest Ordovician (Ashgill) rocks of the Drummuck Group (Cautleyan-Rawtheyan) and High Mains Formation (Hirnantian).

The Drummuck Group (c. 350 m thick) is restricted to the Craighead Inlier and is largely a mudstone succession with subordinate sandstone and conglomerate units. The stratigraphy was revised by Harper (1982) who formalized the succession into four formations, the lower two (Auldthorns and Quarrel Hill formations) forming the Lower Drummuck Group, of Cautleyan age, and the upper two (Lady Burn and South Threave formations) forming the Upper Drummuck Group, of Rawtheyan age (Figs. 2, 3).

Scottish Journal of Geology 35, (1), 15-24,1999



16 JAMES D. FLOYD, MARK WILLIAMS AND ADRIAN W. A. RUSHTON

FIG. 1. Geographical location and geological map of the Craighead Inlier.

The new collections of ostracodes described here were' obtained from friable, brown-weathering, unbedded, nodular, silty mudstones near the base of the Lady Burn Formation (Fig. 3). These mudstones dip gently northwards and form almost a strike section in the bed of a small watercourse, here named the South Drummuck Burn, situated about 650 m SW of Drummuck farm (Fig. 2). Fragmentary

FIG. 2. Location of ostracode-bearing localities 1, 1A and 2 within the lower part of the Lady Burn Formation, South Drummuck Burn. Shelly fossils were also recovered from the Quarrel Hill Formation at Locality 3.

graptolites, trilobites, bivalves and brachiopods also occur in the weathered mudstones. The pervasive red-brown colour of the mudstones hereabouts may be largely a consequence of proximity to the unconformity with the overlying 'Old Red Sandstone' facies conglomerates.

Superficially similar, but fresh, hard and bedded, grey silty mudstones yielding sparse brachiopods, are exposed some 300 m upstream in a small wood adjacent to the road (Fig. 2, locality 3). These mudstones, together with grey thinly-bedded sandstones exposed in a grassy bank a few metres to the south, are taken to be the top of the underlying Quarrel Hill Formation (Figs. 2, 3). Allocation of the new exposures in South Drummuck Burn (Localities 1,1A and 2) to the Lady Burn Formation implies a slight (southwards) adjustment to the geological lines of Harper (1982, fig. 3).

Material

Collections of ostracodes from the Drummuck Group comprise those from the Starfish Beds (South Threave Formation) at Threave Glen, including the ostracodes documented by Jones (1879), and deposited at the Natural History Museum, London (NHM PM In20008, PM In20040, PM In20041, PM 1979.28.75). A few ostracodes from the Starfish Beds occur on a rock slab collected for echinoderms and trilobites housed at the British Geological Survey, Key worth (BGS Zs2821). The more extensive collections of the present authors, from four localities in the Lady Burn and Quarrel Hill formations in South Drummuck Burn (Figs. 1, 2), comprise several tens of



LATE ORDOVICIAN OSTRACODES FROM GIRVAN 17

System

Silurian

Ordovician

Series/Stage

Llandovery

Ashgill

Hirnantian

Rawtheyan

Cautleyan

Group

(no group designated)

Drummuck Group

Upper

Lower

Whitehouse Group

Formation thickness

Mulloch Hill Conglomerate (MLCG)

High Mains Formation (HMS)

South Threave Formation (STHV)

Lady Burn Formation (LBM)

Quarre l Hill Formation (QHF)

Waterfall Member

Cliff Member

~k 'Starfish Beds'

Farden Member

"k Localities 1,1 A,2

Locality 3

Auldthorns Formation (AUT)

30 m

60 m

140 m

115 m

35m

Shalloch Formation (SLH)

FIG. 3. Lithostratigraphy and chronostratigraphy of the Drummuck Group and contiguous units (modified from Harper 1982, fig. 2). Distribution of ostracodes is indicated by the asterisk.

ostracodes together with trilobites, brachiopods, bivalves and graptolites (BGS 16E876-950, 16E953-1023, 16E1283-1295). Figured and cited specimens from this collection are housed at the British Geological Survey's Key-worth office while other material is at the Edinburgh office. A few specimens collected during the 1960s from South Drummuck Burn (Floyd Collection) are housed in the Hunterian Museum, Glasgow (GLAHM 103065).

Ostracode preservation

All of the ostracode specimens occur as 'crack-out' material on mudstone slabs. In the laboratory individual specimens were developed from the rock by means of a Burgess vibrotool. Those from the Lady Burn Formation are predominantly decalcified moulds and are coated with a bright yellow ochre, which provides a good visible contrast against the dull brown mudstone. As the mudstones of the Lady Burn Formation are friable, the rock requires hardening (with polyvinyl acetate) before the fragile external moulds of the ostracodes can be cast (for methodology see Siveter 1982). For casting we used pre-vulcan-ized natural rubber latex (manufactured by Alec Tiranti Ltd, London). Despite their fragility some of the external moulds have preserved fine ornament (Fig. 4b, e). In some instances, ostracodes from the Starfish Beds of the South Threave Formation preserve remnants of shell material.

Mode of occurrence and palaeoecology of the ostracode fauna

The ostracode fauna of the Lady Burn Formation is of low diversity, comprising six species and other

indeterminate ostracodes. The most abundant species is the binodicope Kinnekullea comma, but palaeocopes such as Euprimites limus and Vittellal sp. and at least one podocope species are also present (see Fig. 4b-i). The Starfish Beds of the South Threave Formation yield K. comma, a podocope species (see Fig. 4a, j , k) and possible palaeocopes (BGS Zs2821, not figured). In the Lady Burn Formation the ostracodes occur mainly as disarticulated but largely complete individual valves. Larger juveniles of K. comma, Vittellal sp. and podocopes co-occur with adults. The associated trilobites, brachiopods, bivalves and graptolites are mostly fragmentary. The mode of occurrence in the Starfish Beds appears to be similar except that in some instances specimens occur in small clusters (Fig. 4j, k).

The co-occurrence of binodicope, palaeocope and podocope ostracodes, if representing a life assemblage, is typical of Ordovician and Silurian fully marine benthic environments (see Copeland 1982; Siveter 1984; Williams and Siveter 1996) and is consistent with the unstable continental slope setting suggested by Harper (1982) for deposition of the Drummuck Group. The low diversity of the ostracode fauna is also consistent with instability in the environment, while the abundance of binodicopes probably reflects the clastic nature of the host lithology (see Vannier et al. 1989).

Stratigraphical significance

The Lady Burn Formation has long been attributed to the anceps graptolite Biozone (Williams et al. 1972; Williams 1987). A new constraint is afforded by a specimen of Paraorthograptus pacificus (Ruedemann 1947)







collected from the Cliff Member of the overlying South Threave Formation (see Fig. 5). This species is indicative of the pacificus subzone, the upper subzone of the anceps Biozone (Williams 1982).

The ostracode Kinnekullea comma, which occurs in both the Lady Burn and South Threave formations, appears to have correlative potential as it is also present in material from the late Ordovician Portrane Limestone of County Dublin, Ireland (see Orr 1987, p. 175). In addition, there is an internal mould (BGS NIG261) of a possible abraded K. comma in BGS collections from the late Ordovician (anceps Biozone) Killey Bridge Formation of Pomeroy, Northern Ireland (for stratigraphy see Mitchell 1977; Tunnicliff 1982) and the species may also be present at a similar stratigraphical horizon in northern England (unpub. information of Mike Orchard).

According to Orr (1987), the Portrane Limestone fauna, which includes more than 100 ostracode species/subspecies, shows greatest similarity to the ostracode fauna of the upper half of the late Ordovician Pirgu Stage of Estonia (see Meidla 1996, fig. 2). Indeed, of the many Baltoscandian Kinnekullea species K intermedia, which is from the late Ordovician Pirgu and Porkuni stages of Lithuania, Latvia and Estonia, appears most similar to K. comma and may prove to be conspecific. Thus, K. comma might be a useful marker fossil for the anceps Biozone in non-graptolite-bearing sequences in Britain, Ireland and possibly the Baltic region (Fig. 6). Records of K. comma in the Ashgill of North Wales appear erroneous (see Bassler and Kellett 1934, p. 190) and may represent a misreading of Salter and Etheridge's (1881, see p. 409) appendix of 'The fossils of North Wales'.

Of the other ostracodes of the Lady Burn and South Threave Formations, none appears conspecific with those described elsewhere from the Ashgill of Britain. However, this may partly reflect the current state of knowledge. Thus, the ostracodes of the Whitehouse Group of the Girvan district (see Jones 1879, 1893a), which apparently were derived from the Mill Formation of complanatus Biozone age (Ingham 1992, p. 404), mostly comprise poorly preserved moulds (NHM collections) whose affinities are unclear. Ashgill ostracodes have also been documented in northern England from Westmoreland and Cross Fell by Jones (1893b). This material clearly needs modern revision, though the whereabouts of some of the figured specimens is uncertain (see Siveter 1978, pp.

45-46). None of the species T. R. Jones figured appear to resemble those of the Drummuck Group.

C. R. Jones (1987, p. 109) records several ostracode species from the Dent Group of the Ashgill Series in the English Lake District including a species of Euprimites. He also reports extensive ostracode faunas from the Ashgill of South Wales indicating that there are several taxa conspecific with Baltic species. As all of this material remains to be formally described its relationship to the fauna of the Drummuck Group cannot presently be determined.

Palaeogeographical considerations

During the Ordovician, the Girvan district was situated on the margin of the Laurentian palaeocontinent (see Bergstrom 1990; Rushton et al. 1996 and references therein). Nevertheless, the faunas of the Lady Burn and South Threave formations include ostracode taxa which are common, at least at the generic level, to those of the Baltic region and of those parts of Ireland and southern Britain which, during the Ordovician, were parts of palaeocontinental Avalonia. These include Kinnekullea and Euprimites, and possibly Vittella. The ostracode fauna of the Drummuck Group thus supports the suggestion that by the late Ordovician, the palaeocontinents of Laurentia, Baltica and Avalonia were in relatively close geographical proximity (Schallreuter and Siveter 1985; Pickering et al. 1988).

Systematic palaeontology

The terminology for the shell of Ordovician ostracodes used herein is summarized by Vannier et al. (1989). Although many ostracodes were recovered from the Drummuck Group, most are too poorly preserved to be determined. Nonetheless, seven species can be distinguished (Fig. 4a-i). The most common and best preserved of these, Kinnekullea comma and Euprimites limus sp. nov., are formally described below. The five other species, recorded in open nomenclature, are illustrated with accompanying notes in Figure 4. Podocope sp. 2 was referred to as 'somewhat like C. aldensis' by Jones (1879, p. 220), who questionably recorded the occurrence of that species at Threave Glen (p. 16). A revision of Cytheropsis aldensis McCoy (and allied taxa; see Jones 1879) is needed before this referral can be properly assessed, but is beyond

FIG. 4. Ostracodes from the Lady Burn Formation, localities 1 (h, i), la (b, d, g) and 2 (c, e, f) and Starfish Beds of the South Threave Formation (a, j , k). a-i are scanning electron micro-graphs; j and k are light photographs of counterpart rock slabs which have been coated with ammonium chloride sublimate to enhance photography, b, c, e, h, g and i are latex casts of external moulds. Magnifications are: a, c, d, e, x 40; f, g, h, i X 50; j , k X 11; b, approx. X 165. (a) NHM PM In20041, podocope sp. 2, carapace, right valve, lateral view, (b) BGS 16E998, Vittellal sp.(?), close-up of reticulate ornament on incomplete right valve, (c) BGS 16E900 palaeocope sp. A, right valve, lateral view, (d) BGS 16E1005, Euprimites limus sp. nov., tecnomorphic right valve, lateral view, (e) BGS 16E895, Euprimites limus sp. nov., holotype, heteromorphic right valve, lateral view (stereo-pair). Note the well-preserved ornament of reticulation and granulation, and presence of possible marginal spines mid-posteriorly. (f) BGS 16E902, podocope sp. 1, vafve, lateral view (stereo-pair), (g) BGS 16E1016, palaecope sp. B, left valve, lateral view (stereo-pair). Poorly preserved reticulate ornament is visible postero-ventral of the adductorial sulcus. Note short anterodorsal spine(?). (h) BGS 16E919 Vittellal sp., juvenile right valve, lateral view (stereo-pair). The ventrally narrowing sigmoidal sulcus and strongly developed ridge-like posteroventral lobe are typical for Vittella Schallreuter, 1964. (i) BGS 16E913, Kinnekullea comma (Jones, 1879), left valve, lateral view (stereo-pair), (j, k) part (NHM PM In20008) and counterpart (NHM PM In20040) rock slabs with specimens of K. comma and podocope sp. 2. In order to orientate the counterparts, one of the carapaces of podocope sp. 2 is arrowed.




/ / >

lmm

FIG. 5. Paraorthograptus pacificus (Ruedemann, 1947) (BGS RX6339). Magnification X 15. From the Cliff Member of the South Threave Formation, Lady Burn cliff section.

the scope of the present paper. C. aldensis is derived from a much older Llanvirn horizon at Aldons (McCoy 1851). Jones (1879, pi. 15, fig. 8a, b) also referred to a specimen of 'Beyrichia kloedeni McCoy' from the Ordovician at Threave Glen. As this specimen (NHM PM Inl9996) is a beyrichiacean tecnomorph it must have been derived from a Silurian horizon.

Class OSTRACODA Latreille Order BEYRICHIOCOPA Pokorny, 1954

Suborder BINODICOPA Schallreuter, 1972 Genus Kinnekullea Henningsmoen, 1948

Type-species. Kinnekullea waerni Henningsmoen, 1948, by original designation. From the upper Ordovician Black Tretaspis Shale, Kinnekulle, Vestergotland, Sweden.

Kinnekullea comma (Jones, 1879) (Figs 4i, 7a-f)

1879 Beyrichia comma, Jones (sp. nov.), p. 219, pi. 15, fig. 9a-c.

1881 B. comma, Jones; Salter and Etheridge (in Ramsay), p. 409.

1886 B. comma, Jones; Jones and Holl, p. 362. 1892 Beyrichia comma, Jones; Smith, p. 158. 1893 Beyrichia (Primitial) comma, Jones; Jones, pp. 301,

305. 1899 Beyrichia comma (Jones); Peach and Home, p. 690. 1934 Beyrichia'} (Bollial) comma Jones; Bassler and

Kellett, p. 190.

Type specimen. The register at the Natural History Museum, London states that rock slab NHM PM In20008,

from the Starfish Beds, contains the 'syntypes' of K comma, including the specimens figured by Jones (1879, pi. 15, fig. 9a-c). However, none of the specimens on this slab, or on its counterpart (NHM PM In20040) can be matched exactly with Jones' figured specimens (see Fig. 4j, k). Furthermore, from his plate explanation it is uncertain whether Jones (1879) figured one or two specimens of K comma. His initial explanation refers to a 'bivalved carapace, imperfect at the edges' and indeed a carapace is illustrated in his figure 9b and 9c. However, he also refers to 'a left valve', as opposed to the left valve of the aforementioned carapace (fig. 9a). As the figured material has not been identified we prefer not to designate a lectotype.

Material and measurements. More than 20 specimens from the Starfish Beds, NHM PM In20008, PM In20040 (part and counterpart), PM 1979.28.75: 12 specimens (including counterparts) from the Lady Burn Formation, BGS 16E897/16E898, 16E903/16E904, 16E911/16E914, 16E912, 16E913, 16E914, 16E915, 16E916, 16E934/16E935,16E962,16E963,16E984/16E985. Possibly conspecific is 16E940/16E941. Specimens from the Starfish Beds are 0.95-1.20 mm (NMH PM 1979.28.75) and 1.00-1.10 mm long (NHM PM In20008, In20040). Those from the Lady Burn Formation are 0.75-1.00 mm in length, perhaps representing two moult stages (see Fig. 7b-f).

Diagnosis. Species of Kinnekullea having its arcuate lobe terminated in a short, posterodorsally projecting narrow spine.

Description. Valves postplete to subamplete. Dorsum straight, remainder of lateral valve outline convex. No adventral or marginal structures. Well-developed, subcir-cular preadductorial lobe connects ventrally to 'comma'-shaped arcuate lobe. Arcuate lobe terminated posteriorly in a short, posterodorsally projecting narrow spine. Valves smooth.

Discussion. The lobal morphology of K. comma varies greatly (Fig. 7a-f). Mostly this appears to be the product of preservation. Thus, in eroded specimens the arcuate lobe appears to be discrete from the preadductorial lobe and is much less extensive posteriorly.

Kinnekullea includes several species of quite disparate lobal morphology (Henningsmoen 1948; Gailite 1970, 1975; Schallreuter 1971, 1986; Vannier 1986: see also Meidla 1996). The type-species, K. waerni is characterized by its arcuate lobe which connects anterodorsally to a node or spine. Well-preserved specimens of K. comma differ from K. waerni in having the arcuate lobe extending more posterodorsally where it terminates in a short spine. Therefore, in its lobal morphology, K. comma appears very similar to K. martinssoni Gailite, 1970 and K. intermedia Gailite, 1975, although in those species the arcuate lobe terminates in a spine that projects slightly from the posterior margin. Nevertheless, in view of the variation shown by K comma it is possible that they are conspecific.




Stratigraphy Series and

Stages

ASH

GIL

L

Hirnantian

Rawtheyan

Cautleyan

Pusgillian

Graptolite Biozones

and extraordinarius

pacificus

complexus

complanatus

linearis

Girvan (Harper 1982)

§^« High Mains Formation

Upper Drummuck 1

Group 1

Lower Drummuck

Group

Upper Whitehouse

Group

Pomeroy (Mitchell 1977; Tunnicliff 1982)

Timaskea Formation

Killey B r i d g e t ?

Formation

? Junction

Beds ?

Portrane (Mitchell 1977,

Orr 1987)

Portrane Limestone^

Dark Grey Shales

Estonia (Meidla 1996)

Porkuni Stage

Pirgu Stage

Vormsi Stage

FIG. 6. Stratigraphic distribution of Kinnekullea comma (icon) in the late Ordovician of Girvan, Portrane and Pomeroy, and of K. intermedia in the Pirgu and Porkuni stages of Estonia.

K. thorslundi Henningsmoen, 1948 is larger (up to 1.7 mm long) and has a less posteriorly extensive arcuate lobe. The arcuate lobe of K. pedigera (Ulrich, 1890) terminates mid-posteriorly. In its lobal morphology K. comma differs most markedly from K. hesslandi Henningsmoen, 1948, K. herrigi Schallreuter, 1971 and K. morzadeci Vannier, 1986 which appear to have a discrete posterodorsal node. K. hofsteni Henningsmoen, 1948 appears to lack the arcuate lobe and may not be congeneric.

Occurrence. Late Ordovician, Ashgill Series (anceps Biozone). Lady Burn Formation, Drummuck Group, South Drummuck Burn, localities 1, 1A and 2, and the Starfish Beds, South Threave Formation, Threave Glen. Also from the late Ordovician, Portrane Limestone, Ireland (Orr 1987). Possibly from the late Ordovician of northern England, Northern Ireland (Pomeroy) and the Baltic States (see above).

Suborder PALAEOCOPA Henningsmoen, 1953 Genus Euprimites Hessland, 1949

Type-species. Euprimites reticulogranulatus Hessland, 1949 emend. Jaanusson, 1957. From the lower Ordovician of the Siljan district, Dalecarlia, Sweden.

Euprimites {Euprimites) limus sp. nov. Fig. 4d, e

Derivation of name. Latin, limus (noun), an apron or girdle; referring to the broad velum of the tecnomorph.

Holotype. External mould of a heteromorphic right valve, BGS 16E895 (Fig. 4e).

Diagnosis. Species of Euprimites with tecnomorph having a broad flange-like velum, which, in lateral view, has a rounded ventral margin.

Material and measurements. Five well-preserved valves (including counterparts) from the Lady Burn Formation: BGS 16E895,16E1000/1001,16E1004/1005,16E1008/1009, 16E1290/1291. All specimens appear to be adults and range 1.4-1.5 mm in length, being of typical size for a Euprimites species.

Description. Valves weakly preplete. Dorsal plica absent. Adductorial sulcus well-developed and deep, slightly constricted adjacent weak preadductorial node; zygal arch absent. Tecnomorph with a narrow flange-like velum, which, in lateral view, has a rounded ventral margin: velum extends from the mid-anterior to the mid-posterior region. In the heteromorph the velum is anteriorly and antero-ventrally more convex and broad where it forms the dolon. Ornament comprises fine reticulation and granulation.

Discussion. Absence of a distinct plica or z^gal arch requires the assignment of E. limus to the subgenus Euprimites {Euprimites) Hessland, 1949 (see Schallreuter 1993).

E. limus most closely resembles the middle Ordovician Baltic Euprimites locknensis (Thorslund, 1940) but can be




FIG. 7. (a)-(f) variation in lobation for Kinnekullea comma (Jones, 1879): all specimens from the Lady Burn Formation at South Drum-muck Burn locality 1, except (a) which is from the Starfish Beds at Threave Glen. Lateral valve outlines have been simplified. Dotted lines indicate uncertainty about the margin of the lobes. Based on camera lucida drawings of: (a) NHM In PM 20008, right valve of specimen on slab; (b) BGS 16E915, partly exfoUated mould of left valve; (c) BGS 16E911, latex cast of right valve; (d) BGS 16E913, latex cast of left valve; (e) BGS 16E916, partly exfoUated mould of left valve; (f) BGS 16E912, partly exfoUated mould of juvenile right valve. Key to lettering: pi, preadductorial lobe; al, arcuate lobe; sp, spine.

distinguished from that, the type- and other Euprimites (Euprimites) species by the shape and breadth of its tecnomorphic velum (cf. Hessland 1949; Sarv 1959; Schall-reuter 1973, 1993; Schallreuter and Kruta 1988; Meidla 1996).

The marginal structures of E. limus are mostly not preserved though some specimens appear to show a few spines (see Fig. 4e).

Occurrence. Late Ordovician, Ashgill Series (anceps Biozone). Lady Burn Formation, Drum muck Group, South Drummuck Burn, localities 1A and 2.

Acknowledgements

M. W. thanks the Geology Department of Leicester University for facilities provided through a Honorary Visiting Fellowship. Steve Tunnicliff and Mark Dean (BGS), Giles Miller (NHM) and Keith Ingham (GLAHM) faciUtated the loan of material and access to relevant archives. Andrew McMillan (BGS Edinburgh) collected the specimen of P. pacificus from the South Threave Formation. We are grateful to Roger Schallreuter (Hamburg) for information regarding Baltoscandian Euprimites species, Mike Orchard (Geological Survey of Canada) for unpublished information concerning late Ordovician ostracodes in northern England, Steve Tunnicliff for information about late Ordovician ostracodes from Pomeroy and David Siveter (Leicester) for discussion. Giles Miller and Alan Owen (Glasgow) gave constructive reviews of the manuscript. This paper is published with the permission of the Director, British Geological Survey (NERC).




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MS. accepted for publication 10th July 1998



Late Ordovician (Ashgill) ostracodes from the Drummuck Group, Craighead Inlier, Girvan district, SW Scotland

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