Ostracods as palaeoenvironmental indicators in the Lower Carboniferous Yoredale Series of northern England

The ostracod fauna and vertical changes in its composition were examined in a shale sequence of the 5-Yard Limestone cyclothem (Yoredale Series) at a locality in Bishopdale, N. Yorkshire. The ostracod tax a are mainly confined to three superfamilies: Kirkbyacea, Healdiacea and Bairdiacea. The co-occurrence of several relatively short-ranging species such as Kirkbya quadrata, Cribroconcha insculpta and Bairdiolites elevatus suggests a Late Brigantian age. Four new species are described: Cornigella posteroextensa, ? Eriella minima, Rectobairdia bavarica and Roundyella binoda. Fluctuations in influx of terrigenous mud and water turbulence, related to delta growth, appear to have been the main environmental parameters that controlled the ostracod distribution and abundance along a nearshore-offshore gradient. Three different ostracod assemblages are recognized. A Roundyella-Cribroconcha assemblage, dominated by kirkbyacean and healdiacean ostracods, represents a quiet, nearshore environment with a fairly high depositional rate of terrigenous mud. A Bairdia assemblage, higher in the section, is dominated by bairdiacean ostracods that lived in a more turbulent, relatively offshore environment with less input of terrigenous sediment. A third, intermediate assemblage consists of almost equal amounts of bairdiacean and kirkbyacean ostracods, and reflects a transition from the Bairdia to the Roundyella-Cribroconcha assemblage.

Masurel disarticulated, are autochtonous. Ostracods and other bioclasts often show some degree of pyritisation, which indicates that deposition of mud took place under quiet, reducing conditions. Poor oxygenation of the water and a fairly high influx and depositional rate of clay may have been important limiting factors preventing most organisms from colonising the sea tloor. Shales that exhibit arelatively high abundance and diversity of ostracods and other bioclasts are largely confined to horizons below the 75 cm sample, in and around the three main nodule beds, and above the third nodule bed ( fig. 3, 4). These probably reflect periods of decreased sedimentation rate linked with a temporarily increased water turbulence. Most of the ostracods that occur here do not show signs of post-mortem transport, and probably formed part of autochtonous populations. Their shells are usually articulated and various growth stages are found. Several specimens of spinose forms such as Corrzigella tuherculospinosa and Crihroconchu insculpta have been found with their spines still intact. The associated, stenohaline macrofauna which occurs at most of these fossiliferous shale horizons is dominated by benthic suspension feeders, most of which are probably indigenous. This, together with local concentrations of fossil debris, implies deposition in shallow, well-oxygenated water of normal marine salinity (Masurel, 1987).
The nodules that occur at 3 distinct, relatively fossiliferous shale horizons have been described by the present author (Masurel, 1987) and are interpreted as being algal in origin, since laminated structures were occasionally found in their Fe-rich matrix (dolomicrosparite). Furthermore, the matrix shows minor bioturbation phenomena and vague relicts of brachiopods, bryozoa, foraminifera and ostracods in cracks subsequently filled by calcite. The formation and diagenetic history of these nodules is probably analogous to that of septarian concretions, which are not uncommon in clayey sediments (Vandenberghe & Laga, 1986;Astin & Scotman, 1988). The nodule beds represent carbonate-rich sedimentation horizons, resulting from periods of reduced clay sedimentation. During these periods conditions apparently were suitable for growth of algae, which may have formed algal mats that were suitable for attachment by suspension feeders such as brachiopods and bryozoa. Nodules were formed as result of remobilisation and redistribution of carbonate at an early diagenetic stage, the final shape being determined by the original thickness of the carbonate-rich sedimentation horizon and the amount of carbonate available in the horizon (Vandenberghe & Laga, 1986  A. Roundycllu hinodu dominates the ostracod fauna (400, I075 cm), and is associated with only a scarcity of other taxa including Crihrocmchu cuneyensis. Roundvc,llu hinodu and Crihrocmcha cuneyensis together dominate the ostracod fauna, which exhibits a variable diversity of other species (500-550 cm).
C . Crihroconcha caneyensis dominates, and is associated with a relatively high abundance and diversity of other species (560 cm).
as macrofauna (Masurel, 1987): table ID) shows a relatively high peak at these horizons. Amphissites and Kirkhyu probably needed well oxygenated water for growth of their fairly large and elaborate carapaces. Where the assemblage reappears at 1075 cm, just above the thin limestone, it is characterised by a low density and diversity (figs. 3.4). A new, relatively common component is Crihr.oc.onc,hupPrplr~-u, which was already fairly common in a transitional assemblage detected somewhat lower in the section (920 cm, see below).

Bairdia assemblage
This assemblage occurs in coarsely laminated, calcareous shales in and above the highest nodule bed, between 845 and 920 cni ( fig. 2 ) .
Buirdiu is the most characteristic genus of the assemblage, associated with other bairdiaceans including Rectohuirdiu spp.,Buir.diut~~pri.s spp., and, less commonly, Buirdio-1ifc.s elrvutus. Poor preservatilon of most of the ostracods in the calcareous shales made identification of species very dif-ficult, but it is eliminated that there are probably more than 10.
Bairdiaceans are by far the most abundant ostracods in samples derived from 845 cm. The only accessory component of the assemblage is Cr-ihr-oc.oncha pcrplexu, which is not found at lower horizons. The abundance 0fGiguntopr.oductids shows a peak slightly higher in the sequence, between 870 and 898 cm. Gigantoproductus was probably an opportunist that took advantage of a reduced mud sedimentation rate, an increased water turbulence supplying sufficient food and oxygen, and a lack of competition by other large benthic suspension feeders (Masurel, 1987, p. 232-233, fig. 8).
It appears that the bairdiacean ostracods had already become established before the explosive colonisation by this large brachiopod.
Buirdiocypr-is joins the assemblage at 870 cm, and remains a fairly common component.
Assemblages dominated by bairdiacean ostracods are indicative of offshore marine environments. Late Palaeo-
The fact that bairdiaceans do not reach significant numbers in the lower part of the sequence would suggest that conditions remained hostile to them. Only Rair~diolitrs elc\wtirs appears to have been a fairly adaptable species, an inference drawn from its fairly common occurrence (in different growth stages) in a sample from 25 cm ( fig. 3).
A common occurrence of healdiid ostracods is often assumed to be indicative of relatively nearshore, muddy conditions (e.g. Bless, 1983 andMelnyk, 1985). It is therefore suggested that the main reason for the change in the ostracod fauna and for the extinction of Gi,qiintopr.odu(,tu.s may have been a slight increase of mud influx and sedimentation rate, related to a slowly retreating shoreline. Giguntoproditctus, which probably had taken advantage of a temporarily reduced sedimentation rate, appears to have become locally extinct very quickly (Masurel, 1987, fig. 8), even before the appearance of significant amounts ofhealdiaceans in the ostracod fauna. Opportunists are characterised by relatively unstable populations, and often become extinct quickly at the onset of unfavourable conditions (Levinton, 1970).

CONCLUSIONS AND DISCUSSION Depositional environment
The main part of the shale sequence, characterised by a scarcity of ostracods, was deposited in quiet shallow water of the prodelta area. Poor oxygenation of the water and a high influx of terrigenous mud probably inhibited colonisation by ostracods and other benthic organisms. Nevertheless, fairly diverse ostracod assemblages, sometimes containing more than 15 species, have been recognised at several different shale horizons. These are usually associated with adiversity of other microfossils ( fig. 3), and an autochtonous macrofauna consisting mainly of suspension feeders, including stenohalinerugosecorals (Masurel, 1987). The fossiliferous shale horizons thus reflect episodes of temporarily increased water circulation and reduced influx and deposition of mud under normal marine salinity conditions.
Assemblage I consists mainly of kirkbyacean and healdiacean ostracods ( Fig. 3: 1 YO. 550 cm). These must have had some tolerance to the influx of clay under very quiet conditions. The assemblage is dominated by either one or both of its end members, the ratio between which forms a basis for further division into 3 sub-assemblages ( 1 A, I B, 1 C). Roundyella hiriodu was probably the most adaptable species, dominating a very impoverished ostracod fauna (sub-assemblage 1A at 400 cm, where other, often pyritised bioclasts only sporadically occur (Fig. 3,4). At other shale horizons. where both end members or only CiYhroc.onc~hu i~~rievcnsi.s dominate (sub-assemblages 1 B and IC), a fairly high diversity of ostracod species is usually found.
The most important and consistent components of assemblage I , besides its end members, are Scrohicwla lato, Aniphissitcs ur-ci. Kirkhvu yuudratu. Heulrliaiiellu c.f:

~I u i~~~i i~u l~i d t~s
and Ci~ihrocwichu insculptu. Assemblage 2 is dominated by bairdiacean ostracods (Fig. 3: 845 cm), occurring in calcareous shales. It probably occupied a more offshore, open marineenvironment that was less influenced by the delta. Micro-and macrofauna contain evidence of a considerably reduced sedimentation rate of mud in well oxygenated, nutrient-rich, shallow water.
Assemblage 3 is transitional between assemblages 1 and 2, consisting of almost equal numbers of bairdiacean and healdiacean ostracods, associated with considerably fewer kirkbyaceans ( Fig. 3: 920 cm). This assemblage reflects depositional conditions which are intermediate between that of the relatively nearshore, clastic-dominated environment of the kirkbyaceans and healdiaceans (assemblage I ) , and the more offshore. carbonate-dominated environment of the bairdiaceans (assemblage 2).
The ostracod assemblagec and related palaeo-environmental conditions described above are comparable with those recognised by Devery (1987). in the Bangor Limestone Formation (Chesterian, Mississippian) of Alabama. He described ostracods from intercalations of argillaceous limestones and calcareous shales, which were probably deposited in a middle shelf-carbonate environment where delta progradation played an important role. Salinity percentages probably remained within normal marine levels.
Devery related his assemblages to a nearshore-offshore gradient, and concluded that fluctuations in the influx of terrigenous sediment and small variations in salinity were probably the main environmental parameters that controlled the ostracod distribution along the nearshore-offshore coenocline.
The Transitional assemblage (3) is comparable with subgroup2 of the Bair-diagolc~~ndensis assemblage of Devery, which contains a glyptopleurid-bairdiacean fauna and represents a1 transitional phase between a nearshore and an offshore assemblage.

Biostratigraphy and palaeogeography
The ostracold fauna of the 5-Yard Limestone cyclothem contains several short range species that have been recorded from the Cawdor Limestones of the Derbyshire massif, England (Robinson, 1959), and are diagnostic for the upper Brigantian stage, e.g. Crihroconcha insculpta, C. perplexa, Kir-kbya quadrata and Bairdiolites elevatus. The stratigraphic range of two species that were known from the Pendleian to Arnsbergian of Great Britain (Robinson, 1978), Coryellina reiticosa and Bairdia heedei, is here extended.
The fauna shows affinity to Lower Carboniferous ostra-cod faunas from Belgium, Germany, France, Russia and North Africa, described by several authors, e.g. Bushmina (1968Bushmina ( , 1970Bushmina ( , 1975, , , Bless & Massa (1982) and Crasquin (1984). Several forms that are conspecific or closely related to Russian species, e.g. Shivaella armstrongiana, Bairdiocypris filmikhaensis and Bairdiacypris sp. aff. rohusta, appear to occur later in western Europe than in Russia. This phenomenon was already noted by Bless ( 1 983), and can be explained by considering palaeogeographic and palaeo-oceanic reconstructions of the Hercynian Ocean and adjacent areas during Visean times (Ziegler et al., 1979 andDewey, 1985). A westward equatorial ocean current fed by the Uralian Ocean would have flowed along the northern margin of the Hercynian Ocean, whereas a weaker counter equatorial current would have flowed along the southern edge. extensive development of epicontinental shelves in the northern Hercynian Ocean further would have facilitated migration of ostracod larvae (Dewey, 1985). Evidence for westward migration can also be derived from the distribution of other faunal groups, such as conodonts. Higgins (198 1 ) noted that the range of the cosmopolitan conodont species Paragnathodus comvzutufus is not identical in Eurasia and the Midcontinent of North-America. In Europe it appears at the base of the Visean, but in the Midcontinent its first appearance is in Masurel the late Visean. The ostracod assemblages described in this paper are equivalent in age and partly conspecific to faunas from the Chesterian Kinkaid and Bangor Limestone formations of the Midcontinental United States (Cooper, 1942(Cooper, , 1947Devery, 1987), and the upper units of the Codroy and Windsor Groups of Maritime Canada (Dewey, 1983(Dewey, ,1985 and Dewey & Fihraeus, 1 9 8 7~ all representing series of transgressive and regressive episodes. The chronostratigraphical relationship is based on several sources, e.g. Mamet (1970). Conil et al. (1976) and George et al. (1976). The Midcontinental faunas contain several forms that are conspecific to those described herein, such as Crihroconchu cuneyensis, Cornigellu tuh~.ri,ulo.spinosu and Tetrusacwlus mirahilis. The assemblages from Maritime Canada are often dominated by paraparchitacean ostracods, and show considerably less similarities to both the Midcontinental and the faunas described herein. This can be explained by a restricted circulation of sea water and prevailing hypersaline conditions in the Maritime Basin, which was a shallow embayment of the Hercynian Ocean. Dewey ( 1 985) considered the low affinity between the Midcontinental and eastern Canadian ostracod faunas to be a function of the Appalachian barrier and circulation of ocean currents away from the Midcontinent in northward direction. Data obtained in this study on the other hand provide evidence of a faunal connection between northern England and the Midcontinent during the late Visean, thus implying a less isolated position of the latter.

SYSTEMATIC PALAEONTOLOGY
The material described in this paper is housed in the collec- Material. 4 carapaces, two of which are completely pyritised.
Description. The most prominent node is located in front of the dorsal spine, five others border the free margin at some distance, including a weakly developed node behind the dorsal spine. Surface regularly reticulated, spines and nodes almost smooth with scattered pits.
Remarks. Corxigella tuhercxlospinosu may show some variation in the number of nodes. The specimens described herein closely resemble those figured by Jones & Kirkby ( 1 886) and Devery ( 1987)

Masurel
Material. Two complete carapaces. Type locality. See Fig. 2. It corresponds to locality 55 of Moore (1958) and section 5 of Masurel (1987: fig. 1) in Bishopdale, northern England. Description. The two specimens of this new species distinguish themselves from C . tuherc~ulospinosu (Jones & Kirkby, 1886) by a more elongated shape, due to a slightly extended posterior end. The dorsal spines are less pronounced and the reticulation pattern is variable, reticulum being relatively smaller in the posterior part, along the margins and on the nodes. The slightly swollen anterior is separated from the flattened posterior half of the carapace by a more or less distinct, narrow sulcus in front of the dorsal spine.
Remarks. The new species shows similarities in shape, presence of a median sulcus, and delicacy of the dorsal spines with C. tuberculospinosu var. B from the Upper Mississippian Bangor Limestone formation of U.S.A., described by Devery (1987). It is distinguished from the latter by the variation in reticulation on different parts of the carapace. Furthermore it lacks the posterodorsal tubercle that marks the surface of all specimens of C. tuherc.ulospinosa var. B

Explanation of Plate 2
Figs. 1-2 Coronakirkhya cornuta (Robinson, 1959). Diagnosis. Carapace oblong, rounded in lateral view, elongate subrectangular in dorsal view. Greatest length just above midline, hinge line straight. Surface reticulate, with a subcentral oval pit only observable on well preserved specimens. Three strong concentric carinae enclose an inner, more delicate V-shaped ridge, all attached to the marginal dorsal carina. Material. About SO carapaces and 5 valves. Dimensions: See Fig. 5 . Remarks. Robinson (1978) noted that a study of the variation in the pattern of the carinae, particularly the inner one (which may be U-, V-or J-shaped), might justify further subdivision. Occurrence. Holkerian  Diagnosis. Carapace semicircular in lateral view, maximum height posterior of midline. DM straight, VM and ends rounded, cardinal angles obtuse. Two hollow spines rise from the cardinal angles, pointing slightly backwards. A central lobe is more or less strongly developed, carrying a kirkbyan pit. The inner of the two marginal carinae is somewhat irregular to the outline. Surface finely reticulate. Material. 2 carapaces with damaged spines. Remarks. The specimens described in this paper closely resemble ?Kirkbya c'ornuta illustrated by Robinson (1 978, PI. 7, figs. la,b). Robinson assigned his specimens with some doubt to the genus Kirkbya, because of the absence of a distinct central lobe. Nevertheless, it is suggested here that the central lobe, which is a diagnostic feature of Coronakirkbya, may sometimes be obscured by abrasion or may be less well developed, as illustrated by the specimens described in this paper. Other species of Coronakirkbya also often show some variation in the prominence of the central lobe, e.g. C. krejcigraji from the Upper Carboniferous of N. Spain (Becker, 1978: p. 57-58, PI. 4, figs. 19-22).

Remarks.
This new species has similarities in shape and surface ornament to R . reticulosa (Jones & Kirkby, 1886) and R. simplicissima (Knight, 1928

Explanation of Plate 4
Figs. 1-4 Crihroconchu caneyensis (Harlton, 1927). Diagnosis. Carapace obliquely subovate in lateral outline. 2. DM straight, VM arched, ends rounded, posterior broader. Valves almost equal, RV larger. Surface regularly reticulate, median sulcus anterior of mid-length. Material. 1 carapace. Description. A deep median sulcus marks the finely reticulate surface, with an obscure node behind and a more clearly Material. 6 poorly preserved carapaces. Diagnosis: Carapace subcircular in lateral view, greatest width above mid-height. Hinge short, straight and depressed below DM. Surface ornamented by very faint concentric striae, more or less parallel to the free margin.
developed lobe in front of the sulcus. Small spines are scattered in the posteroventral area, and there is a larger posteroventral spine (broken off in the material examined). Remarks. Robinson ( 1978) described this species from the Pendleian of Great Britain, and illustrated it with a specimen with a slightly weaker median sulcus. Occurrence. C. reticosa is recorded from the Pendleian to lower-most Arnsbergian of Great Britain (Robinson, 1978 Harlton, 1934Family Healdiidae Harlton, 1933 1 Ct~ihroc~onc~hu cmryensis (Harlton, 1927) (

Remarks. The assignment of this species to the genus
Crihrm.oncha is based on the presence of a pit-field anterior of the posterior ridge (Sylvester-Bradley in Moore, 1961, p. 36 1 ). It is probably conspecific with Hruldia sp. ex gr. H . c~crneyensis recorded from the Upper Mississipian of northern Arkansas (Sohn, 1977). The latter only differs in that the posterior ridge is straight which may be a geographic variation. Occurrence. So far only recorded from the Upper Mississippian of U.S.A., this is the first record from Great Britain. Robinson, 1959 ( Remarks. The lenticular and transversely stretched meshes of the reticulation pattern, which is sometimes visible in the posterior area, are similar to those illustrated by Gramm 1982, PI. 2, fig. 7, P1.3, fig. 4). This author suggested that in a related species ("Healdia cornuta" Posner, 195 1) the posteroventral spine, which is usually found on right valves only, may be a larval adaption that is sometimes retained into the adult stage. In this study, the presence of a posteroventral spine is confined to early instars and males (tecnomorphs), which are distinguished from females (heteromorphs) by a less symmetrically rounded, generally more elongated shape ( fig. 11). The tecnomorphs illustrated in Plate 5 , figs. 3-5, resemble Healdianella darwinuloides Posner (Gramm. personal communication). The subcircular adductor muscle scar is sometimes faintly visible just to the postero-ventral of centre. Occurrence. Healdianellu danvinuloides has been recorded from Visean strata of U.S.S.R. (Posner, 1951;Gramm, 1982). Species with affinities have been described from the Visean of France (Crasquin, 1987, PI. 1 I , figs. 9a,b) and Mississipian of U.S.A. (Dewey, 1987, PI. 6, figs. 9-1 I).

Diagnosis.
Carapace subtriangular in lateral view, DM of right valve gently angular, LV overlapping. Maximum height just anterior to mid-length. VM gently concave, ends rounded, anterior slightly narrower and pointed in dorsal view. Surface smooth. Material. 9 carapaces and 1 1 single valves or fragments. Remarks. B. fomikhuensis has not been recorded from British strata before. The specimens that are illustrated in this paper closely resemble those described from Lower Tournaisian to Upper Visean strata of France (Crasquin, 1984(Crasquin, , 1987, and from the Tournaisian of Russia  Occurrence. Summarised by Dewey ( 1987): Newfoundland, Great Britain, U.S.S.R., U.S.A. The range of this species, known from Chadian to Asbian strata of Great Britain (Robinson, 1978), is now extended to the Upper Brigantian stage. Dewey ( 1987) also suggested a possible extension of the species range to the Upper Chesterian of the Midcontinent of U.S.A.
?Class Ostracoda Latreille, I806 Genus Cryptophyllus Levinson, I95 I Cryptophyllus sp. (PI. 8, fig. 2.) Diagnosis. Carapace umbonate, greatest width at about midheight, greatest length ventral. Hinge line short, depressed below the dorsal order. Ventral, anterior and posterior borders rounded, ventral broader. Antero-and posterodorsal borders straight or slightly concave. Material. 9 carapaces, generally poorly preserved. Remarks. A variable number of delicate concentric ridges (7 or more) is detected on the surface of some of the specimens examined.