Lower Jurassic ostracods from Ilminster, Somerset, England

Pliensbachian and Toarcian ostracods are described from an interbedded marlstone/carbonate-mud •sequence in South-West England. Sediments of this age in other parts of Britain rarely offer a lithology favourable to the examination of calcareous microfossils. The fauna possesses similarities with other European sequences, particularly to the south. The presence of one species, Ektyphocythere dharennsourensis Boutakiout, Donze & Oulmalch, 1982 suggests possible links with North-West Africa at this time.


INTRODUCTION
Roadworks in connection with the A303 Ilminster By-pass ( Fig. 1) have revealed an alternating carbonate/argillite sequence which appears to span the uppermost Pliensbachian and most of the Toarcian. The sediments are generally fossiliferous and abundant ammonites have permitted the section to be accurately dated. The stratigraphical importance of this section becomes apparent when one looks at contemporary sediments on the nearby Dorset coastal section. There, the greater part of the Toarcian is represented by a condensed sequence, the Junction Bed (Cope et al. 1980).
The sediments, which apparently lack any anaerobic or dysaerobic conditions, have been the subject of an integrated lithological, micro/palaeontological study at University College London and the British Geological Survey. (Boomer et al. in press). An initial report on the ostracods of the section was given by the author in that work.
The only previously published works on British Toarcian ostracods are those of Bate & Coleman (1975) from the East Midlands, Lord (1974) from the Down Cliff Clay, Dorset, Ainsworth (1986) from the Fastnet Basin and Boomer (1991) from the Mochras Borehole. The present lithology contrasts with the more extensive argillaceous sequences in other parts of Britain, therefore, any faunal differences observed may reflect the facies dependance of certain taxa.

LITHOLOGY
The sediments encountered at Ilminster are contemporary with the Middle and Upper Lias Junction Bed (sensu Wilson et al., 1958). The presence of microfossil rich marl bands within this section permits the first detailed chronostratigraphical and micro-biostratigraphical study of this interval, in full, in Britain. A detailed lithological description together with the stratigraphical distribution of ammonites, calcareous nannofossils, foraminifera and palynological remains are described elsewhere (Boomer et al.). The sediments represent the Junction-Bed Formation, which is composed of two members, namely the Barrington Beds Member (equivalent to the Junction-Bed of Howarth in Cope et al., 1980) and the Marlstone Rock Bed Member (equivalent to the Marlstone Rock-Bed of Howarth in Cope et al. 1980).
Due to the unfavourable lithology, only one horizon within the Marlstone Rock Bed Member was suitable for study. This was the oldest sample studied (ILM-KP 1) and was assigned to the spinatum zone, hawskerense subzone based on ammonite evidence. The remaining sixteen samples, all Toarcian, were from the Barrington Beds Member and range in age from tenuicostatum zone to the thouarsense zone.
The samples investigated are listed below and a brief lithological description is given for each. The sequence discussed is a composite of two separate sections, see location map ( Fig.   1

Upper Pliensbachian
The lowest sample investigated is characterised by the occurrence of species belonging to the genera Ogmoconcha and Ogmoconchella including vallate forms, assigned to the genus Hermiella by Kristan-Tollmann, (1977) but considered by the present author to belong to Ogmoconcha Triebel, 1941. These have been referred to as Tethyan forms of the genus Ogmoconcha due to their occurrence in the sediments of southern Germany (Malz, 1975;Lord & Moorley, 1974), Portugal (Exton, 1979), Italy (Farinacci et al., 1979and North Africa (Boutakiout et al. 1982, Maupin 1977, Maupin & Vila 1976). These forms have now also been recorded, although in fewer numbers, from the Fastnet Basin (Ainsworth, 19861, Mochras Borehole, (Boomer, unpublished) and the Paris Basin (Bodergat & Donze, 1988). Other Ogmoconcha specimens from ILM-KP 1 possess carapaces with a bi-convex outline in dorsal view thus differentiating them from species attributed to the 'arnalthei' and 'contractula' groups (Michelsen, 1975) which possess flattened lateral surfaces in dorsal view. 0. conuexa Boomer, 1991 originally recorded from the spinatum and ..urn zones of the Mochras Borehole is one such tenuicosta species. The assemblage is similar to that recorded from the Ammonitico Rosso of the Appennines by Lord in Farinacci et al. 1979 and also to samples recovered from massive carbonate sediments at Djebel Zaghouan, Tunisia (Lord, pers. comm.).
The present section differs, however, in that unlike the aforementioned sections both cytheracean and metacopine ostracods occur together.
A comparison with the Mochras section reveals that the ostracod assemblage from sample ILM-KP 1 is similar to samples of tenuicostatum zone age rather than those of spinatum age. This may be due to the distinctly different lithologies. ILM-KP 1 includes the youngest record of both Cytherelloidea anningi Lord, 1974, although only one specimen = Very R a r e ( 0-

C o u n t s )
= P a r e (   was recovered, and a few poorly preserved specimens of Ektypkocytkere quadrata Boomer & Lord, 1988. The low abundance and poor preservation of these specimens may indicate reworking. Specimens of Ektypkocythere tenuicostati (Martin, 19601, previously recorded from the spinatum zone of Portugal (Exton, 1979) and Germany (Martin, 1960), arecommon while a few valves of E. ckampeauae (Bizon, 1960) are also present.

C o u n t s ) = F e w
Ektyphocytkeredharennsourensis (Boutakiout, Zone & Oumalch, 1982), originally described from Morocco, is a large species (? >lmm) with a distinctive outline in lateral view and strong sexual dimorphism. The species continues through the section to sample ILM-KP 5. Only in sample ILM-KP 3 are the male dimorphs present. ILM-KP 2 has the lowest occurrence of E. sermoisensis Apostolescu, 1959 the most abundant ostracod within the sequence and common in many European Toarcian sequences. The following sample (ILM-KP 3) marks the first appearance in the section of E. intrepida Bate &Coleman, 1975. Sample ILM-KP 5 marks a recovery in ostracod diversity within the section to nine species. This is particularly due to the introduction of four cytherurid species, Prucytkerura sp. (represented only by A-1 instars), Cytheropteron guiaskense Bate & Coleman, 1975, Rutlandella sp. and Prucytlzerum cf. P.

Lower Toarcian
The succeeding sample (ILM-KP 2) marks a distinct faunal change with the disappearance of the Metacopina and a reduction in diversity from nine to two species. A similar faunal change occurs in the lowermost part of the falciferum zone, Lower Toarcian of the Mochras Borehole.  Kozur, 1970 (Pl. 3. fig. 1) a species originally described from the Upper Anisian of Hungary. This species has not previously been recorded from Lower Jurassic sediments in Britain but does occur in the Middle Jurassic of the Cotswolds (Morris, 1983). Further specimens have been seen by the author in the collections of the Senckenberg Museum Frankfurt from theMiddle Jurassicsediments of Eastfield Quarry,SouthCave, Yorkshire. The species has also been recorded from the Middle Jurassic of France (Depeche, 1985, p. 136, P1.31, fig. 14) and the Lower Jurassic of Germany (Knitter & Reigraf, 1984, p. 69, P1. 3, fig. 5).
Four of the aforementioned species are only recorded within this sample (see Fig. 3). The succeeding sample ILM-9937 sees the introduction of two more cytherurid species Rutlandella transversiplicata Bate & Coleman, 1975 and Wellandia ornata Ainsworth, 1986 the former species being recorded throughout much of the succeeding sequence. ILM-9938 is dominated (50%) by K. sermoisensis with no new species introduced while the following ILM-9940 is marked by the introduction of seven species including Ektyphocythere sp. A and Cytheropteron supraliassicum Herrig, 1969 previously recorded from Upper Liassic sediments throughout Germany and now from the Toarcian of South-West England (this work) and the Mochras Borehole, Wales (Boomer, unpublished). Praeschulerideu pseudokinkelinella Bate & Coleman, 1975 is recorded from this and the succeeding sample, ILM-9939. Although this species possesses a similar lateral ornament to E. dharennsourensis it can be distinguished by its smaller size.

Middle Toarcian
Ten incoming taxa are recorded from samples ILM-9934 and ILM-9936. The lowest sample from the Middle Toarcian (ILM-9936) includes Cytherella praetoarcensis Boomer, 1991 and Cytheropteron byfieldensis Boomer & Bodergat M.S. (in press) a speciessimilar toC.alafastigatum Fischer, 1962butdistinguished by the coarse lateral punctation and the more robust nature of the carapace. This section is the type locality for E. anterocosta Boomer, 1988 which first occurs at the base of the fibdatum/ crassum subzone, Middle Toarcian, and continues into the Upper Toarcian. Sample ILM-9936 marks the first occurrence in the section of Kinkelinella costata Knitter, 1983. K. sermoisensis (Apostolescu) and K . costata (Knitter) are quite discrete species within this sequence. There does not appear to be any intermediate specimens between the former reticular species and the latter dominated by vertical ribbing. Furthermore, whereas E. sermoisensis is by far the most abundant species (46-70%) in all but one sample from ILM-9937 to ILM-9935 (8 samples, lowest falciferum zone to uppermost bifrons zone) the succeeding four samples are dominated (<55%) by K. costata with the former species constituting less than20% of the fauna.

Upper Toarcian
The thouarsense zone assemblages are composed of species recorded from contemporary sediments elsewhere in Europe. The faunas are more closely allied to those recorded from France and Germany rather than from Mochras (Boomer, 1991) or the Fastnet Basin (Ainsworth, 1986 et seq.) with the incoming species including Cytherella toarcensis Bizon, 1960, Cytherellu praecadomensis (Knitter & Reigraf, 1983 and Otocythere callosa Triebel & Klingler, 1959. Fig. 4 shows that the rate of faunal change during the Toarcian reflects a relatively stable community given that these assemblages were recovered from marl bands which intercalate with 'limestone' horizons. The marked increase in extinctions, and concomitant decrease in diversity at the top of the section, reflects a facies change. It should be noted that only one specimen, probably reworked, was recorded from the youngest sample. Apart from the extinction of the Metacopina in the lowest part of the sequence there would not appear to be any distinct changes in the Toarcian ostracod fauna of South West Britain.

FAUNAL CHANGE WITHIN THE SEQUENCE
The most abundant superfamily is always represented by the Cytheracea except for the lowermost sample (KPI) which is dominated by the Healdiacea (Metacopina). The Most Abundant Species graph would indicate that the lowest Toarcian saw not only the disappearance of the Metacopina but alsochanges in the stability of theenvironment ke. ranging from 2040%). Subsequently the most abundant species, always cytheracean, generally constitutes 50-60% of the assemblage. This final graph would indicate a higher dominance figure than would be expected in an open marine setting. The average value for the open marine Toarcian sequence in the Mochras section (Boomer, 1991) is 42%. This may reflect a stressed environment under which these alternating limestone and argillaceous sediments were deposited.
Explanation of Plate 1.
(SP) indicates the use of Stereo-Pairs.

COMPARISON WITH OTHER SECTIONS
Many of the species recorded by Bate & Coleman (1975) have been found in the present sections, the main exceptions being species attributed by those authors to the genera Procytherura, Camptocythere and Eucytherura. Direct biostratigraphical comparison between the present material and that from the East Midlands is not possible due to the absence, at Ilminster, of two of the zonal fossils designated by Bate & Coleman (i.e. Camptocythere toarciana Bate & Coleman and Kinkelinella persica Bate &Coleman). The ranges of the two remaining zone fossils described by those authors (E. intrepida and E. debilis) accord well with the present section. It should be noted that the ranges of these last two species, although apparently concurrent in both the East Midlands and Dorset, are much shorter than their total ranges recorded in the Mochras Borehole. This is true for many of the species recorded from these sites.
Although the Ilminster section has certain affinities with assemblages further to the south, there would appear to be no direct connection with either the Fastnet or Cardigan Bay Basins.

Subclass Ostracoda
The two species described below are smaller than Ogmoconchu inflata, however, they are not considered to be juveniles of that species since the marginal inflations seen in suites of vallate healdiids in the collection of A.R. Lord, including Ogmoconcha ambo, are distinctly developed as early as the A-3 instar.

Boomer
Remarks. A species of Ogmoconcha without the distinct marginal inflations of species such as 0. ambo and 0. inflata but with a definite ventro-lateral swelling on each valve, thus implying a relationship to the vallate forms. The lateral swellings give the carapace a sub-triangular outline in anterior view. In lateral view the carapace has a triangular oval outline with a convex ventral margin. Steeply rounded anterior and posterior margins converge at the greatest height just anterior of themid-1ength.This speciesis distinguished fromO. conuexa (Boomer, 1991) by the different outline in anterior view, the latter species having a strongly convex oval outline. A similar species was figured by Knitter & Riegraf (1983, P1. 4, fig. 3) from the Upper Pliensbachian of SW Germany. Although that material was assigned to 0. circumuallata Dreyer, 1965 the figured specimen is quite different from the holotype figured by Dreyer (1965, P1.1, fig. 4; P1.2, figs 1-4). Distribution. Present in the lowest sample from Ilminster with similar material recorded from contemporary levels in southern Europe.

Suborder Platycopina Sars, 1866 Family Cytherellidae Sars, 1866
Genus Cytherella Jones, 1850 Cytherella praecadomensis (Knitter & Riegraf, 1984) (Pl. Material. Mochras, 17 CP, 40 V; Ilminster 7 CP, 15 V. Remarks. The carapace is more robust than in Cytherella cadomensis (Bizon) which possesses distinct marginal inflations. The deep muscle sulcus is a very distinctive feature which exaggerates the marginal inflation. The females appear to possess only one posterior swelling and therefore this species, as with the descendant C. cadomensis (Bizon), is assigned to Cytherella. Cytherella? depressum Ainsworth, 1986 is considered to be a junior synonym of the present species. Distribution. Recorded in the thouarsense zone of Ilminster and mid-bifrons to uppermost variabilis zone at Mochras (Boomer, unpublished). Although only recently described, this species appears to be geographically widespread in the Middle and Upper Toarcian sediments across North-West Europe. 14-19.

The species bears similarities to Prueschulerideu pseudokinkelinella
Bate & Coleman. The latter species, however, is neither as large as, nor does it display the marked dimorphism seen in E. dhurennsourensis. Examination of the marginal zone reveals between twelve and fifteen simple straight marginal pore canals anteriorly and four to five posteriorly contrasting with the typical pattern in Ektyphocythere where the maximum numer anteriorly is about ten.
The nature of the dimorphism and distribution of anterior marginal pore canals is similar to that observed in Prueschuleridea, however, the present species appear to possess an antimerodont hinge typical for Ektyphocythere. The present material has been retained within the latter genus due to the poorly preserved nature of the hingement and the form of the external ornament. It is possible, however, that this species Description. An oval carapace in lateral outline similar to E. intrepidu Bate & Coleman, distinct dimorphism is apparent with the males somewhat more elongate and particularly notable for the poster-dorsal flange in the left valves which is a much stronger feature than in the females. Both anterior and posterior margins are rounded, the latter more narrowly so. In dorsal view the outline is moderately inflated with the maximum width just behind the mid-length. The greatest height is at theanterior cardinalangle, dorsal margin is straight in the right valve and slightly concave in the left valve due to the presence of the postero-dorsal flange referred to above. Ventral margin slightly sinuous. Left valve overlaps the right dorsally especially at the cardinal angles, also in the mid-ventral area. Weak eye swelling present below the anterior cardinal angle. Ornament typical for the genus but the present material is characterised by strong secondary cross ribbing producing a coarse reticulation in the mid-valve region. The inner lamellae is fused throughout, broad anteriorly moderate ventrally and posteriorly. Hinge is antimerodont, muscle scars indistinct but appear to represent a crescentic row of four adductor scars, the frontal scars are not clearly observed. Marginal pore canals not seen.
Remarks. A species of Ektyphocythere which bears a strong resemblance to E. intrepida Bate & Coleman. The presence of a postero-dorsal flange is the main difference between this species and E. intrepidu Bate & Coleman. It must be noted, however, that the two species have not been found in the same sample at Ilminster (see Fig. 3). The current author believes that one example of this species was figured in the type description of E. intrepida (op. cit. 1975; PI. 7, fig. 4). The species is distinguished by the stronger development of secondary cross-ribs than is seen in E. intrepida producing a more strongly reticulate pattern. Distribution. Uppermost falciferum and lower bifrons at Mochras, falciferum and bifrons zones of the East Midlands and falciferum to thouarsense zones, Ilminster.

CONCLUSIONS
The lowest sample examined (ILM-KP 1) although assessed as spinatum zone (Upper Pliensbachian) on the basis of ammonites, has an ostracod fauna similar to that of tenuicostatum zone assemblages (Lower Toarcian) from the Mochras Borehole. The succeeding samples, particularly in the falciferum and bifrons zones, bear a strong correlation with those described by Bate & Coleman (1975) from theLower and Middle Toarcian of the East Midlands. Stratigraphical ranges of certain species at the English sites are, however, much shorter than the total ranges recorded from the extensive sequence at Mochras. Furthermore the lowest sample contains an assemblage which may indicate a certain degree of reworking. Indeed, some samples higher up the section include badly dissolved specimens, also possibly indicative of erosion/ reworking (see P1.1, figs 1,4,7).
It is important to note that despite the lithological differences between Ilminster and the East Midlands section, the faunal similarities are quite strong. However, the present material would suggest that the Fastnet and Cardigan Bay Basins to the West are systems quite distinct from other parts of Britain as reflected in the facies differences. There would appear to be a strong palaeoceanographical linkwith parts of southern Europe and North Africa at certain times, particularly so in the lowest Toarcian. This may be due to changes in sea-level and/or the inception of new pathways bringing Tethyan forms further North. Records of vallate forms of Ogmoconckn in theuppermost Pliensbachian and lowest Toarcian of North West Europe would support such an idea. These species are considered to be essentially Tethyan in their distribution. Indeed during the Upper Triassic, ornamented Healdiids often dominated marine ostracod assemblages from Eastern Europe through theTethyan region into what is now South-East Asia. It should also be noted that specimens of Oligocythereis? inoclirnsensis from the Pliensbachian of the Mochras Borehole have been recorded from D.S.D.P. site 547 of northwestern Africa (Boomer, 1991 ). The section at Ilminster therefore details the evolution of ostracod faunas in Southern England during the Toarcian and implies an increased influence from Tethys in the earliest Toarcian.