The biostratigraphic potential ofNerineacean Gastropodscase studies from the Middle Jurassic of England and the Upper Jurassic of France

THE BIOSTRATIGRAPHIC POTENTIAL OF NERINEACEAN GASTROPODS CASE STUDIES FROM M1DDI,E JURASSIC OF ENGLAND AND THE UPPER

JURASSIC OF FRANCE MICHAEL JOHN B A R K E R BARKER M.J. 1994. The biostratigraphic potential of Nerineacean Gastropods - case studies from the Middle Jurassic of England and the Upper Jurassic of France. [Le potentiel biostratigraphique des N~rin~es - ~tude de cas du Jurassique moyen de l'Angleterre et du Jurassique sup~rieur de la France]. GEOBIOS, M.S. 17 : 93-101. ABSTRACT Nerineid gastropods are abundant in the White Limestone Formation (M.- U, Bathonian) of Gloucestershire and Oxfordshire (England). Multivariate analysis of the populations has demonstrated an evolving lineage in the genus Aphanopyxis ; from A. excavata in the Shipton Member through A. langrunensis at the base of the succeeding Ardley Member to A. ardleyensis at the top. They have good biostratigraphical value, appear to be relatively independent of facies and their distribution is in agreement with the very much rarer ammonites. Species discrimination is based upon the ontogeny of the whorl cross-section, apical angle of the shell, whorl width/height ratios and external ornament. Other genera (especially Eunerinea, NerineUa and Bactroptyxis) are also useful for correlation. The Upper Kimmeridgian - Portlandian carbonate sediments of the Bienne Valley (nr. St Claude, Jura) also contain an abundant and highly diverse nerineid fauna. Characteristic Portlandian species include Polyptyxis calypso, Acrostylus trinodosa and abundant Ptygmatis sp. (bruntrutana/ pseudo-bruntrutana/ carpathica complex). The Upper Kimmeridgian (Couches des Prapont) can be divided into a lower sequence characterised by JoUyanella danusensis and Aptyxiella retrogressa and an upper sequence with a rich fauna (12 genera) including Iteria cabanetiana, Bactroptyxis spp., Cryptoplocus depressus and Cossmannea spp. ; all associated with high energy grainstones and patch reefs. Nerineid gastropods are common faunal constituents of Jurassic-Cretaceous carbonate sequences and in the absence of other proven biostratigraphic indices, they are potentially useful with a discriminatory value ranging from the level of the stage through to the zone. KEY-WORDS : GASTROPODA, NERINEACEA, STRATIGRAPHIC DISTRIBUTION, MIDDLE JURASSIC, ENGLAND, UPPER JURASSIC, JURA, FRANCE. RESUMI~ Les N~rin~es foisonnent dans la Formation White Limestone (Bathonien moyen ~ sup~rieur) de la r~gion du Gloucestershire et de l'Oxfordshire (Angleterre). Une ~tude multivari~e des populations a r~v~l~ une lign~e ~volurive dans le genre Aphanoptyxis, de A. excavata dans le Shipton Member ~ travers A. langrunensis ~ la base du Ardley Member jusqu'~ A. ardleyensis au sommet. Ces esp~ces ont une valeur biostratigraphique fiable, et semblent ~tre relativement ind~pendantes des facies. Leur distribution coincide avec la presence, extr~mement rare, d'ammonites. L'identification des esp~ces se fait ~ partir de l'ontog~nie selon une coupe transversale de la spire, l'angle apical de la coquille, les mesures largeur/hauteur du tour, et l'aspect externe. D'autres genres (notamment Eunerinea, Nerinella et Bactroptyxis) sont ~galement utites pour les correlations. Les s~diments ~a~'bonat~s du Kimm~ridgien - "Portlandien" de la Vall~e de la Bienne (prOs de St Claude, Jura) contiennent, eux aussi, une faune de N~rin~es abondante et tr~s vari~e. Les esp~ces portlandiennes particuli~res comprennent Polyptyxis calypso, Acrostylus trinodosa et un tr~s grand nombre de Pygmatis sp. (complexe bruntrutana/pseudo-bruntrutana/carpathica). Le Kimm~ridgien sup~rieur (Couches de Prapont) peut ~tre divis~ en une s~quence inf~rieure caract~ris~e par Jollyanella danusensis et Aptyxiella retrogressa et en une s~quence sup~rieure comportant une faune riche (12 genres) qui comprend Iteria cabanetiana, Bactroptyxis spp., Cryptoplocus depressus et Cossmannea spp., associ~e ~ des grainstones d~pos~s dans des environnements de haute ~nergie avec de petits r~cifs. Les N~rin~es sont des constituants communs de la faune des s~quences carbonat~es jurassiques-cretac~es et, en l'absence d'autres indices biostratigraphiques, elles sont potentiellement utiles, leur valeur discriminatoire variant du niveau de l'~tage ~ la zone. MOTS-CL]~S : GASTROPODES, NERIN]~ES, RI~PARTITION STRATIGRAPHIQUE, JURASSIQUE MOYEN, ANGLETERRE, JURASSIQUE SUPERIEUR, JURA, FRANCE.

94 INTRODUCTION Nerineacean gastropods are frequently encountered in Jurassic-Cretaceous carbonate successions of shallow water origin. They are highly distinctive molluscs usually recognised by the presence of internal spiral lamellae (called "folds") within the internal cavity of the shell. However, the unifying features of the superfamily are the possession of a juxta-sutural selenizone, a rudimentary siphonal canal and a presumed heterostrophic protoconch (although this latter feature has been confirmed in only a few genera). The internal folds within the nerineid shell are not universally present. Some genera do not possess folds (eg Aphanoptyxis, Aptyxiella or Endiaplocus ) whereas others have such complex internal deposition of secondary aragonite that the volume available for soft tissue within the shell can be reduced by over 50% (eg Bactroptyxis or Ptygmatis) -see Barker (1990) for details. The superfamily comprises more than 90 published genera ranging in age from the Hettangian species Nerinella grossouvrei COSSMANN from Vend6e, France (Cossmann 1896) through to the Maastrichtian Plesioptygmatis burckhardti (BOSE) and Nerinella sp recorded from the Caribbean region by Knipscheer (1938) and Sohl (1987) respectively. The problems of using benthic organisms in biostratigraphy are well known and the utility of various groups in the Jurassic of the British Isles has been summarised by Torrens (1980). In general, benthic organisms have been ignored even though they are plentiful in the fossil record, because of the widely held view that they would not spread rapidly enough through a depositional basin to have near-isochronous distribution boundaries. The speed of migration and geographical range of a gastropod species is largely a function of dispersal ability and habitat availability. Many studies have demonstrated the remarkable powers of dispersal in gastropods by means of telepanic larvae stages in both Recent and fossil forms (Hanson 1980 ; Scheltma & Williams 1983; Vermeij 1987). Consequently habitat suitability and availability are probably the major controlling factors in the distribution of nerineacean gastropods in Mesozoic shallow water carbonates. In the absence of suitable planktic or nektic faunas for biostratigraphy, benthic faunas have been utilised, most notably in relatively small depositional basins of brackish or freshwater facies where endemic radiations have taken place (Papp et al. 1985 ; Williamson 1983). However, benthic faunas can be used in marine habitats and Kauffman, for example, has shown that in spite of

their sessile mode of life, inoceramid bivalves have a wide distribution at the species/subspecies level with range boundaries almost isochronous with those of coeval ammonites or planktic foraminfera (Kau_ffman 1970). Nerineid gastropods frequently occur in assemblages in which they are both taxonomically diverse and numerically abundant. These assemblages are often laterally persistent and some degree of facies independence is exhibited. Transportation and reworking are also easily detected. Together, these features suggest that the Nerineacea have a high biostratigraphic potential. The stratigraphic distribution of nerineid gastropods has been discussed by several authors, most notably Cossmann (1898) and Pchelintsev (1965) but the stratigraphic data given are usually very generalised appendages to taxonomic studies. The first use of nerineid gastropods in detailed stratigraphy was by Arkell (1931) to establish a zonal scheme for the Bathonian White Limestone Formation of England. This was later refined and extended by Barker (1976). The examples discussed below represent the varying levels of discriminatory value to be found in nerineids : namely the distribution and changes across the Kimmeridgian - Portlandian boundary in the Bienne valley, St Claude, Jura, France and the detailed zonation of the M.-U.Bathonian White Limestone Formation of England. T H E U. JURASSIC (KIM-POR) OF THE BIENNE VALLEY Situated NE of St Claude (Jura, France), the Bienne valley is steep sided and heavily wooded. Exposures are a mixture of natural outcrops and roadside cuttings ; their occurrence and correlation are shown in Fig. 1. A wide range of carbonate lithologies are present with peloidal grainstones and packstones comprising approximately 80% of the succession. These frequently exhibit signs of high energy depositional regimes (e.g. herring-bone and large-scale cross stratification). Other notable components include micrites, tidal l~miuites (Tidalites de Vouglans) and occasional small bioherms. The sediments and their micropalaeontology have been comprehensively reviewed by Bernier (1984). Historically, the Bienne valley and the locality of Valfin were the subject of pioneer researches notably by Bourgeat (1883, 1886, 1888) and Loriol and Bourgeat (1886-1888). The abundance and local range biozones for the nerinacean gastropod species occurring in the Bienne valley are shown in Table 1. The oldest distinctive nerineid association is the Jollyanella

95 Lithostratigraphic terminology after Bernier 1984

Mnr~7

\\ \

\

\

me river

\ ~'~

\

F composite section 1 ] of mainvelley toNE \ ~

Beds with abundant C.macrogonius

~'~

Tidalites

~'~

I A. trinodose I S.salinensisPtg:bruntrutana"

TIDALITES DE VOUGLANS E]lanche

Tidalltes

............. " COUCHES DU CHAIL LY ~ ~ --~'~PORTLANDIAN

Tidallte

~

"

"

cailloux noirs ~-erosion

KIMMERIDGIAN

..== C.depressus CALCAIRES A "•'~. STROM ATOPOROIDES/" DE MATAFE LON /" COUCHES /./ DE • PRAPONT ~" SUPERIEURES "-_T~--- C.depressus

ian" lian idgian

/./

S

hm

ctions

D437

Road section

I

..'~____ C. depressus

./

.... .... B. clio Nov.gen.. nov, sp.

.s

% "x

......... 222; INFERIEURES

St C l a u d e 0.5 k m

Polyptyxis calypso

~

Jollyanella denusensis

- --

Figure 1 Location of exposures and their correlation in the Bienne Valley, near St Claude, France. Localisati,~n des couches exposdes et leur corrglation dans la Vall~e de la Bienne, pros de S t Claude, France. -

danusensis assemblage in the Couches de Prapont inf~rieures (?=Calcaires oolithiques de Corveissiat see Bernier 1984)• The genus Jollyanella has a very distinctive complex fold configuration and the occurrence of J. danusensis (d'ORB) in these U p p e r Kjmmeridgian strata is the last known species of this genus. Earlier species of this genus are recorded as Lewinskia sp• by Wieczorek (1979) from the Upper Oxfordian to Lower Kirnmeridgian of Poland and the Crimea. However, the genus Lewinskia WIECZOREK, 1979 is clearly a junior objective synonym of Jollyanella ALIEV, 1968• Of the associated nerineid species, the rare Aptyxiella retrogressa (ETALLON) is also restricted to this horizon. -

Above this, towards the base of the overlying Couches de Prapont sup~rieures, an horizon characterised by a b u n d a n t Bactroptyxis clio (d'ORB) and Auroraella bernardiana (d'ORB) occurs. To-

gether with the rare Nerinella chantrei (LORIOL), a new genus and species (a tall, high spired nerineid with a small labral fold - fold diagnosis 0.0.1.0.) also occurs uniquely in this assemblage• The remainder of the Couches de Prapont sup~rieures exhibit three rather diffuse gastropod horizons with the upper horizon immediately below the Portlandian boundary containing prolific Cryptoplocus depressus (VOLTZ), Cossmannea spp. and Sculpturea spp. (see Table 1 for details). The Kimmeridgian - "Portlandian" boundary in the Bienne valley is both lithologically and palaeontologically distinctive and signifies a Late Kimmeridgian regression. An irregular nodular horizon with black peloids and intraclasts (cailloux noirs) and derived Nerineacea, corals and Thalassinoides occurs at the boundary. As discussed by Bernier (1984), pedogenic processes in marginal littoral environments similar to those

96 Nedneld gM~opod. .,

IOmrn~ldgfan

5,, rT(l 1,5o Couches de Prapont Inf

Po~m'*clian

Couches de Prapont sup ,I

I US rnacr

I

Co~hes de Chilly

"ndaJitesde Vo~glanl

I

I

I

i

I

i

onh.m

ffhurmlnn) d'(~b)

I

I

vo~)

L

I

EunMJo~l lip, I

Table i - Stratigraphic distribution and abundance of Nerineacea (Gastropoda) in t h e Upper Kimmeridgian - Port l a n d i a n of the Bienne Valley, near St Claude, France. Distribution " stratigraphique et abondance de Ndrindes (Gastdropodes) dans le Kimmdridgien - Portlandien de la Vallde de la Bienne, pros de S t Claude, France.

i (d'O,b) Cn~toplocu~ d ~ r u l U s

(vol~)

,

III

(d. ~,)

J

I

(d'O.b)

I

Nov.

g*n., nov. sp.

Aurorasltabsmatdiana (d'Ob)

,

,

l

i

i I

,

,

Joll~fcnelta danusensls (d'Orb) Apty~ells *'e':ogressa Coumannee dosvoldyl (d~!

I

I

Contain

m m ~ , c . ~ (d'Orb)

IJ

,

,

,

,

I

,I

Coesmsnn, es turba~0c

(de Lot) I

I

Nedr~la ©tle~ol

(de LoO

i

IJmbones dllatats

(¢Od~)

....

|

8¢ulotur*a eCuIDta

(Ill.n) Scu,p~,.,r.ab~,,od~a IL=~,,~,'O

I

i

,

I

!

I

I,

,

I

|

|1

8¢ulotunm ¢oelokdchs

(Cox) Auroral* Iort*U (C.=amann)

i

!

t

.Aumr~seilll marlae

(d'Orb)

found today in the Florida Keys produce exactly analogous deposits i.e. black limestone fragments, laminated crusts with mollusc and coral debris. The fluctuating environmental conditions in the Florida Keys include frequent salinity changes, periodic subaerial exposure, solution of CaCo3 and high levels of microbial activity. If these were the conditions during the terminal Kimmeridgian, then the subsequent marine transgression in the Portlandian would rework any such marginal littoral deposits as an irregular organic-rich nodular horizon and produce a boundary between the Kimmeridgian and the Portlandian that is to some unknown extent diachronous. Certainly in some places, e.g. sous Roche Blanche, a very slight angular unconformity is apparent.

Above the boundary, the Couches du Chailley were deposited as grainstones and packstones (frequently dolomitised or micritised) and with abundant bioturbation (Thalassinoides). They are intercalated with tidal laminite cycles which become more abundant as the Couches du Chailley pass into the Tidalites de Vouglans. Amongst the grainstones, one fine-grained pink variety contains the small and highly distinctive Polyptyxis calypso (d'ORB.) together with Eunerinea sp. This forms another correlatable horizon in the Bienne valley. The Tidalites de Vouglans were deposited in fluctuating environments from subtidal to supratidal (Bernier 1984). The subtidal sediments are finegrained peloidal grainstones - wackestones,

97 highly bioturbated with Thalassinoides and incipiently to pervasively dolomitised. They are frequently rich in nerineid gastropods, particularly those of the Ptygmatis carpathica/bruntrutana/pseudobruntrutana group, together with Acrostylus trinodosa (VOLTZ) and Salinea salinensis (d'ORB.). Also the ubiquitous Kimmeridgian Cryptoplocus depressus is now largely replaced by the much more phaneromphalous Conoplocus macrogonius (THURMANN). These form a highly distinctive Portlandian nerineid assemblage. Detailed logging and plotting of local range biozones has demonstrated a succession of correlatable nerineid gastropod horizons in the Upper Kimmeridgian - Portlandian sediments of the Bienne valley. Unfortunately the absence of a detailed ammonite zonation in these complex fluctuating carbonate environments prevents independent assessment of their true biostratigraphic worth. THE BATHONIAN WHITE LIMESTONE OF ENGLAND The White Limestone Formation (Middle - Upper Bathonian) outcrops in Gloucestershire and Oxfordshire, Central England (Fig. 2) and together with the Forest Marble Formation, forms the top of the Great Oolite Group. In general terms, this region comprised a prograding carbonate ramp during the Bathonian. A high-energy shallow water shoal of grainstones and packstones (the Minchinhampton facies) trended NW - SE and back shoal conditions, which shallowed onto the lowlying topography of the London - Brabant Massif, continued to the NE. The White Limestone Formation was deposited in the back shoal environment (Palmer & Jenkyns 1975 ; Barker 1976 ; -

~

~

~:

¢

N

Sumbler 1984 ; Sellwood et al. 1989). The prevailing palaeogeography had concomitant effects upon the sediments that were deposited not only in the White Limestone Formation but also in the underlying Hampen Marly Formation and the overlying Forest Marble Formation. Finegrained low-energy lagoonal carbonates with features indicating subaerial exposure occur around the margins of the London-Brabant Massif (Palmer & Jenkyns 1975) and give way to coarsergrained more open marine lithologies southwestwards. Terrigenous clastic detritus derived from the London-Brabant Massif also decreases southwestwards. The White Limestone is extensively cb~nnelled, - by coarse grainstones in the southwest and clay filled channels (the product of meandering rivers cut into the tidal mudflats) in the northeast. Vertical changes in lithology are just as frequent with the development of prominent hardgrounds reflecting periods of slow or non-deposition and shallowing-upwards cyclicity mirroring changes in water depth. Despite such variations the sequence is well bedded throughout, in marked contrast to the Upper Jurassic limestones of the Bienne valley. The White Limestone Formation has a long history of research dating from Hull (1857) but no adequate stratigraphic progress was made until Arkell (1931) suggested that "...the gastropod zones have been found to be distributed more consistently...". Most recently, following the works of Barker 1976 (mainly palaeontology), Palmer 1974, 1979 (mainly lithology) and Sumbler 1984 (stratigraphy) a broad consensus has been reached. The consistent and reliable internal correlation and subdivision of the White Limestone

NORTHAMPTON ~

/

b®''+ .,:=,-

. ~

2_-._

i.'.:.'.:.'." "

~ o

D ",",on',n

~

C'~RENCESTER

f

~

ns l i e Barker 19; 6

, 10 krn ,

Figure 2 - Location of exposures in the White Limestone Formation of central England. Localisation des couches expos~es dans la Formation White Limestone de la rggion centrale de l'Angleterre.

98 Formation which now exists has been achieved through the use of nerineid gastropods. The gastropods are overwhelmingly concentrated into definite horizons and in order to elucidate both their taxonomy and their stratigraphic distribution, biometric analyses of the gastropod populations were undertaken. The choice of measurable shell parameters was constrained by the fact that most nerineid specimens cannot satisfactorily be removed from their matrix without some loss of the shell and ornament. Furthermore, since nerineids continue to grow t h r o u ghout their life, the aperture lacks terminal growth features defining an adult stage so that measurements could not relate to any physiologically meaningful point in the shell development such as a definable adult stage or a post - protoconch growth stage. The biometric parameters were obtained from polished axial sections or from photographs with the axis of the gastropod parallel to the photographic plate. Since gastropod ontogeny is in part recorded by the successive whorl width and height measurements of the shell, regression analyses of these two variables (by Reduced Major Axis) will reveal their growth. Growth was found to be isometric in those Bathonian Nerineacea studied - with the notable exception of the coeloconoid Fibuloptyxis witchelli (Cox 8~ ARKELL). Multivariate analysis using stepwise discrimination indicated that of the 5 shell parameters measured, the apical angle was the best discriminator between the s~mples followed equally by the whorl width/height ratio and the sutural angle (see Barker 1976 for details). Plots of apical angle vs. whorl width/height ratios show that within the genus Aphanoptyxis, populations from the lower two members of the White Limestone Formation (the Shipton and Ardley Members) exhibit morphological trends which are correlated with their stratigraphic position. A. excavata (Barker MS) is found abundantly in the uppermost bed of the Shipton Member. It has larger apical angles and wider whorls than stratigraphically higher species of the same genus and there is no morphological overlap with other stratigraphically higher samples (see Fig. 3). In the central part of the area, the top of the A. excavata Bed is also a pronounced hard ground. At the base of the overlying Ardley member, the distinctive Roach Bed occurs (Arkell 1931) and can be recognised throughout much of the area by its nerineid gastropod fauna - Nerinella acicula (d'ARCHIAC) and Eunerinea arduenensis (BUVIGNIER). Above this the peloidal limestone is dominated by A. langrunensis (d'ORB.) together with

15

20

"" "- A.langrunensis ~

]

~= =, ~ 25 o

ellipses defined by *-1S about each M

I

I

2.5 2.0 WHORL W I D T H / H E I G H T RATIOS

Figure 3 - Morphological variation in Aphanoptyxis spp. from the White Limestone Formation of central England. Variation morphologique chez Aphanoptyxis spp. de la Formation White Limestone de la rdgion centrale de l'Angleterre.

N. pseudocylindrica (d'ORB.). Between 4.5 m and 6 m above the base of the Ardley Member, the A. ardleyensis Bed forms a distinctive marker horizon with Aphanoptyxis ardleyensis ARKELL having the smallest apical angle and highest whorls of any Aphanoptyxis species. Other morphological features also change in a systematic manner, namely the spiral cords on the base of A. excavata are less pronounced in A. langrunensis and are absent in A. ardleyensis. Spiral ornament on the sides of the whorl are normally absent in A.excavata, variably developed in A. langrunensis and readily apparent in A. ardleyensis. Within this short evolving lineage, A. ardleyensis and A. lan-

99 FOSS CROSS

--~--

SW

NE

. . . . . . . . . . . . . . . . . . . . . . . ~ ~ ~ _ _

. . . . . . .

~ .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ARDLEY

FOREST MARBLE FM.

BI-don

aspidoides S. irnpHcata

Member ~ o I,-

' :'~ ~"~

I ;;..

- .

A.langrunensis

Aphanoptyxls

hodsoni A.

•

.

d ~ - ' - -S~e- - ' - ' - ~ ' ~

e x c a v a t a_a

excavata

Ardley Member Shipton Member

"; "T"

~-~ - - - ~-""-----" . . . .

~ ~ - - - - - ; "

morrisi - ~ ~ " ~ - - - - - - - - - - - - ' - ~ - ' - - - - -

,..........

subcontractus

Hardground

~

Malls

I:

)

~

0

70kin

~

MARLY

FM.

Emergence Wlokestones detritus

OT

Length of section Ipp.oxlmately Adapted (rom $umbler 1984

~

...........

--;-----" . . . . . .

.......................

( Forlst Math,e} O--Ammonite {T:Tulite$

found insitu : M = Morrisicerls

: P :

ptoceritld)

F i g u r e 4 - S c h e m a t i c cross-section t h r o u g h t h e W h i t e L i m e s t o n e F o r m a t i o n of c e n t r a l E n g l a n d s h o w i n g m a j o r g a s t r o p o d horizons. Coupe transversale schdmatique de la Formation White Limestone de la rdgion centrale de l'Angleterre, rdvdlant les niveaux

principaux ~t gastdropodes.

grunensis show some morphological overlap but the complete morphological separation ofA. excavata from A. langrunensis is probably due to the period of non-deposition represented by the hardground at the top of the Shipton Member and consequently no intermediate forms are found. Correlation of the upper horizons of the White Limestone Formation has proven notoriously difficult. The present author concurs with the correlations of Sumbler (1984) but believes that the boundary between the Ardley Member and the Bladon Member remains problematic. As proposed by Arkell, the Ardley Beds were "defined upwards by a limestone near the top crowded with the gastropod Aphanoptyxis ardleyensis " a,ld the Bladon Beds " ...are characterised by beds of white sublithographic or marly limestone packed with the subzonal gastropod..." (i.e. Aphanoptyx~s bladonensis) (ARKELL 1947). This usage has been adopted by most subsequent workers (Barker 1976 ; Torrens 1980) and in this context, the lithostatigraphic revisions of Sumbler (1984) seem unnecessary. He places the boundary between the Ardley and Bladon Members immediately above the Aphanoptyxis bladonensis Bed which thus becomes the upper bed of the Ardley member. According to Sumbler 1984, the Bladon Member comprises the Fimbriata - Waltoni Bed

and the Upper Epithyris Bed. Since Aphanoptyxis bladonensis is also recorded sporadically from the Upper Epithyris Bed, it seems preferable to retain all occurrences of A. bladonensis within the Bladon Member. As defined here, the Bladon Member includes all strata from the basal beds above the A. ardleyensis Bed to the true Forest Marble (i.e. the A. bladonensis, the Fimbriata Waltoni and the Upper Epithyris Beds). Based on four species of the genus Aphanoptyxis, an internally consistent stratigraphy has now b e e n established for the White Limestone Formation. Other nerineid species augment the scheme e.g. Nerinella pseudocylindrica and N. acicula which are found only in the lower horizons of the Ardley Member. Southwestwards where higher energy grainstones become more dominant, the nerineid faunas change and the Aphanoptyxis dominated faunas give w a y to Bactroptyxis implicata (d'ORB.), Melanioptyxis altararis (COSSMANN), Cossmannea bathonica (RIGAUX 8c SAUVAGE) and Fibuloptyxis witchelli (Cox & ARKELL). Although less stratigraphically precise, these latter gastropods occur in horizons which interdigitate with the Aphanoptyxis occurrences and permit correlation of the White Limestone Formation over a distance of some 70Kin (details are given in Barker 1976 and a summary in Fig. 4).

100 The s t a n d a r d am m oni t e zonation of the Bathonian in B r itain and N o r t h e r n Europe have been discussed by several authors notably Torrens (1969 ; 1974 ; 1980 - and references therein). The correlation o f the White Limestone Formation achieved by utilising the nerineid gastropod horizons can be compared with t he s t a n d a r d because rare ammonites occur in the White Limestone and t h e y are of some stratigraphic significance. As defined here, t h e White Limestone For m at i on ranges with certainty from t he Subcontractus Zone to the Hodsoni Zone and possibly into t he Aspidoides Zone. T he Shipton Member spans the Subcontractus and Morrisi Zones since Tulites mustelus and T. glabrus have been found at t he base of th e White Limestone For m at i on immediately above th e H a m p e n Marly For m at i on and Morrisiceras morrisi, M. cf. morrisi and M. comma h a v e been found in t h e top 1.5 m of t he Shipton Member, including the A. excavata Bed (see Fig. 4). These ~mmonite occurrences dem on st rat e t h a t th e Shipton M em be r is of t h e same age t h r o u g h o u t its outcrop. The overlying Ardley Member occasionally yields large proceritids and those collected in situ include Procerites quercinus in th e s o u thw es t and Procerites sp. from the e x tr eme n o r t h e a s t of the outcrop suggesting t h a t most of this m e m b e r lies within t he Hodsoni Zone. It is t e m p t i n g to speculate t h a t the h a r d g r o u n d at t h e top of the A. ardleyensis Bed (i.e. th e b o u n d a r y between the Ardley and Bladon Members as defined here) also represents the b o u n d a r y b e t w e e n the Hodsoni a nd Aspidoides Zones. However because of the absence of ammonites in th e top of t he Ardley Member and t he whole of the Bladon Member, a more precise age cannot be given for t he uppermost par t of the White Limestone Formation. However, the correlation of the White Limestone For m at i on provided by nerineid gastropod horizons is in agreem e n t with t h e biostratigraphical zonation obi/ained from t h e k n ow n ammonites. CONCLUSIONS The detailed s tu dy of nerineacean faunas using range zones and evolving lineages (determined t h r o u g h population biostatistics) can provide useful stratigraphic information in carbonate successions t h a t lack conventional biostratigraphic indices. In the B a t h o n i a n White Limestone Formation of England, reference to a conventional biost r a t ig r ap h ic zonation was possible and demonst r a t e d t h a t t h e distribution of nerineid gastropods was isochronous with t h a t of the known ammonites. In spite of t h e i r widespread distribution, morphological diversity and abundance, t he taxon o m y of these gastropods is still in a relatively

primitive state. Identifications must be based upon both the external and the i n t e r n a l morphologies. Because the internal folds are absent at the aperture of the shell only specimens t h a t show the fully developed fold configuration in t rue axial section can safely be used in identification and biometrics (Barker 1976 ; 1990).

Acknowledgements - I wish to thank the following : Mike Taylor and Dave Mackertich for their enthusiasm and company ; Robert le Pennec for Gallic hospitality and local knowledge ; Mme Sirven for assistance with the collections at the Universit6 Claude-Bernard, Lyon and Hugh Torrens for discussions on biostratigraphy and his incomparable bibliographic knowledge. Greg Power and Roger Pulley administered computational first-aid when necessary.

REFERENCES ARKELLW.J. 1931 - The Upper Great Oolite, Bradford Beds and Forest Marble of South Oxfordshire and the succession of Gastropod Faunas in the Great Oolite. Q. JL geol. Soc. Lond., 87 : 563-629. ARKELLW.J. 1947 - The Geology of Oxford. Clarendon Press, Oxford : 267 p. ALIEV K.A. 1968 - A new genus of the family Nerineidae. Dok. Akad. Nauk Azerb. S.S.R., 24 : 41-44 (in Russian). BARKERM.J. 1976 - A stratigraphical, palaeoecological and biometrical study of some English Bathonian Gastropoda (especially Nerineacea). Ph.D. Thesis. University of Keele (unpublished). BARKER M.J. 1990 - The Palaeobiology of Nerineacean Gastropods. Hist. Biol., 3 : 249-264. BERNIER P. 1984 - Les Formations Carbonat6es du Kimm6ridgien et du Portlandien dans le Jura M6ridional. Docum. Lab. G~ol. Lyon, 92, 1 : 1-443 ; 2 : 445-803. BOURGEATE. 1883 - Note sur la position vraie du Corallien de Valfin dans le Jura. Ann. Soc. Sc. Bruxelles, 7~ ann6e : 389-401. BOURGEATE. 1886 - Considerations sommaires sur les formations corallig~nes du Jura M6ridional. M4m. Soc. d'Emulation : 3-21. BOURGEATE. 1888 Recherches sur les formations corallig~nes du Jura M6ridional. Saw et Lefort, Paris: 181 p. COSSMAN M. 1896 - Essais de Pal~oconchologie Compar6e. Liv. 2, Paris : 179 p. COSSMAN M. 1898 - Contribution ~ la pal~ontologie Fran~aise des terrains Jurassique: Gasteropodes: N6rin6es. Mdm. Soc. gdol. Fr. (Pal.), 19 : 1-179. HANSON T.A. 1980 - Influence of larval dispersal and geographic distribution on species longevity in neogastropods. Paleobiol., 6 : 193-297. HULL E. 1857 - The Geology of the Country around Cheltenham. Mem. geol. Surv. G.B. : 104 p. KAUFFMANE.G. 1970 - Population Systematics, Radiometrics and Zonation - a new Biostratigraphy. Proc. North American Paleontological Convention : 612666. -

101

KNIPSCHEER H. 1938 - On C r e t a c e o u s N e r i n e a s from Cuba. Proc. K. ned. Akad. Wet., 41 : 673-676. LORIOL P. de & BOURGEAT E. 1886-1888 - E t u d e s s u r les Mollusques des Couches Corallig~nes de Valfin (Jura). Mere. Soc. Pal. Suisse : 13-15. PALMER T.J. 1974 - Some palaeoecological studies in the Middle a n d U p p e r B a t h o n i a n of central E n g l a n d a n d n o r t h e r n France. D. Phil. Thesis. U n i v e r s i t y of Oxford (unpublished). PALMER T.J. 1979 - T h e H a m p e n M a r l y a n d W h i t e L i m e s t o n e formations : F l o r i d a t y p e c a r b o n a t e lagoons in t h e J u r a s s i c of c e n t r a l England. Palaeontology, 2 2 : 189-228. PALMER T.J. & JENKYNS H.C. 1975 - A c a r b o n a t e i s l a n d b a r r i e r from t h e G r e a t Oolite (Middle J u r a s sic) of c e n t r a l E n g l a n d . Sedimentology, 22 : 125-135. PAPP A., JAMBOR ~ . & STEININGER F.F. 1985 - Chron o s t r a t i g r a p h i e u n d N e o s t r a t o t y p e n , Miozan der Z e n t r a l e n P a r a t e t h y s , Bild VI, M6 P a n n o n i e n (Slavonien u n d Serbien). A k a d e m i a i Klado, B u d a p e s t : 283 p. PCHELINTSEV V.F. 1965 - Mesozoic M u r c h i s o n i a t a from the s t r a t a of t h e C r i m e a n m o u n t a i n s . Science Publishing House, Moscow : 215 p. (in Russian). SCHELTMA R.S. • WILLIAMS I.P. 1983 - Long-distance d i s p e r s a l of p l a n k t o n i c l a r v a e a n d the biogeography a n d evolution of some P o l y n e s i a n a n d W e s t e r n Pacific molluscs. Bull. Mar. Sci., 33 : 545-565. SELLWOOD S.W., SHEPHERD T.J., EVANS M.R. & JAMES B: 1989 - Origin of l a t e cements in oolitic reservoir facies : a fluid inclusion and isotopic s t u d y (Mid-Jurassic, s o u t h e r n England). Sed. Geol., 61 : 223-237.

SOHL N.F. 1987 - Cretaceous gastropods: c o n t r a s t s bet w e e n T e t h y s a n d the T e m p e r a t e Provinces. J. Pa-

leont., 61 : 1085-1111. SUMBLER M.G. 1984 - The s t r a t i g r a p h y of t h e Bathon i a n W h i t e Limestone and F o r e s t M a r b l e F o r m a tions of Oxfordshire. Proc. geol. Ass. Lond., 95 : 5164. TORRENS H.S. 1969 - The s t r a t i g r a p h i c a l d i s t r i b u t i o n of B a t h o n i a n a m m o n i t e s in central England. Geol. Mag., 106 : 63-76. TORRENS H.S. 1974 - S t a n d a r d Zones of t h e B a t h o n i a n . Mem. Bur. Rech. geol. mini~res, 75 : 581-604. TORRENS H.S. 1980 - J u r a s s i c S t r a t i g r a p h y in Practice. In COPE J.C.W. et al. : A correlation of J u r a s s i c Rocks in t h e British Isles. Geol. Soc. Lond., Special Report 14 : 2-16. VERMEIJ G.J. 1987 - The d i s p e r s a l b a r r i e r in t h e tropical Pacific: implications for m o l l u s c a n speciation a n d extinction. Evolution, 41 : 1046-1058. WIECZOREK J. 1979 - U p p e r J u r a s s i c g a s t r o p o d s from t h e Holy Cross M o u n t a i n s (Poland). Acta Palaeont. Polonica, 24 : 299-350. WILLIAMSON P.G. 1983 - S p e c i a t i o n in molluscs from T u r k a n a Basin. Nature, 304 : 661-663.

M.J. B A R K E R Department of Geology University of Portsmouth Burnaby Building, Burnaby Road Portsmouth PO1 3QL, England