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Chapter 86. Ostracoda across the Albian/Cenomanian (Cretaceous) boundary in Cambridgeshire and western Suffolk, eastern England
TEXT-FIG.l S a m p l e localities and geological sketch map
Ostracoda Across the AlbianlCenomanian in Eastern England 1231
Foraminifera1 assemblages from the Upper Gault and Cambridge Greensand of Barrington
and Arlesey were examined by Hart (1973a), Primarily on the basis of the lack of planktonic
foraminifera in the 250-500 micron sieve fraction, Hart argued that the highest Gault at Barrington and Arlesey is of the C . auritus Subzone, confirming Casey’s (1965) reinterpretation of Fearnside’s (1904) material. This age determination was later modified by Carter and Hart (1977, p. 75)
who placed the top of the Gault in the area within the A . substuderi Subzone (i.e. the M . rostratum
Subzone of Owen, 1976). These same authors assigned the top of the Gault to benthonic foraminifera zone 6i on figure 18 (p. 72, 73), but did not define the zone (they refer only to zones 6 and 6a
in the text). However, Hart’s (1973b) concept of benthonic foraminifera zone 6i would place
it in the upper part of the C. auritus Subzone. The cause of this apparent anomaly appears to be due
to the incomplete succession at Folkestone, the section on which Hart relied heavily for his benthonic foraminifera zonal scheme. The first occurrence of certain index species (e.g. Globigerinelloides bentonensis) is at a higher stratigraphical level at that locality compared to other sequences in
southern England (e.g. Glyndebourne Borehole) as a result of the non-sequence below Bed XII.
Hart (1973a) recovered a mixed late Albian and Lower and early Middle Cenomanian foraminifera1
assemblage from the Cambridge Greensand at Barrington. The faunas were not sufficiently diagnostic to provide a restricted age determination for the deposit, but he suggested that the absence of
benthonic foraminifera zone 7, which is restricted to the Glauconitic Marl in southern England,
and the reduced thickness of zone 8, compared to the Lower Cenomanian Chalk of Kent, indicated
that the earliest part of the Neostlingoceras carcitanense ammonite Subzone was missing. The
upper boundary of the Cambridge Greensand was placed within benthonic foraminifera zone 8
by Hart on the basis of the similarity of planktonic to benthonic ratios of the Lower Chalk at
Dover and Barrington.
The Ostracoda from the Cambridge Greensand of Swaffham were first examined in detail by
Chapman (1 898), who expanded on and revised the species lists which had been published previously (Sollas, 1872; Vine, 1889; Jones and Hinde, 1890). Kaye (1964) re-examined as many of
Chapman’s original specimens as were available and, together with additional material from
Barrington, systematically described the ostracod assemblage.
Until now there has been no attempt at biostratigraphical analyses using Ostracoda, but investigations into the Albian (Wilkinson and Morter, 1981; unpublished data) and Cenomanian
(Weaver, 1982) faunas of southern and eastern England prove distributions which are used as the
basis for the present discussion. The significant biostratigraphical events of the interval are illustrated in Text-fig. 2.
Grantchester Cutting (National Grid Reference TL 4200 5626)
Within a drainage trench at this locality, 1.7 m of Gault were seen to be overlain by 0.25 m of
Cambridge Greensand which graded up into the Chalk Marl. The claylgreensand junction is
an irregular erosion surface and pockets of glauconitic siltstone had accumulated in the hollows
The Gault was sampled approximately 0.6 m below the base of the Cambridge Greensand at
which level 22 species and subspecies of ostracod were recovered including Cythereis (Rehacythereis) luermannae hannoverana Bertram and Kemper, Eucythere (Phodeucythere) trigonalis (Jones
and Hinde), Neocythere (Neocythere) vanveeni Mertens and Planileberis scrobicularis Weaver. The
assemblage is typically late Albian and the association of Cythereis (Rehacythereis) luermannae
1232 I. P. WILKINSON
hannoverana together with Planileberis scrobicularis and the absence of Cythereis (Cythereis)
folkestonensis Kaye is indicative of the upper part of the C. auritus and the M . rostratum subzones,
i.e. the upper part of the hannoverana ostracod Zone (Bertram and Kemper, 1971; Wilkinson and
The basal glauconitic siltstone of the Cambridge Greensand contains 11 species in common
with the Gault, but 10 further forms are present. Amongst these are Cythereis (Cythereis) humilis
humilis Weaver, Cythereis (Rehacythereis)paranuda Weaver, Cytherelloidea globosa Kaye, Bythoceratina umbonata glabra Weaver and Planileberis foveata Weaver. Cythereis (Rehacythereis) bemerodensis was found in small numbers. Bertram and Kemper (1971) used this latter form as an
index for the ostracod zone, equating it with the upper part of the M . rostratum and M . perinflatum
subzones in Germany, but later on (Kemper, 1984) this was modified to (?)M. perinflaturn and N .
carsitanense subzones. In southern England, it has been found only in the M . perinflatum Subzone
(at Borrow Pit, near Godstone, Surrey; N.G.R. TO3408 5315) and Cenomanian (Weaver, 1982),
but it is not present in the Gault of East Anglia (Wilkinson and Morter, 1981). Eucythere (Phodeucythere) trigonalis becomes extinct within the topmost Albian, but other forms which first develop
in the S. dispar Zone and cross the AlbianICenomanian boundary to become extinct within the
lower part of the N . carcitenense Subzone, were recovered. The extinction level of Platycythereis
chapmani Kaye and Neocythere (Physocythere) steghausi (Mertens), for example, is in the lower
part of benthonic foraminifera zone 8 and that for Phthanoloxoconcha icknieldensis (Weaver) is
within the lower part of zone 9.
The ostracod fauna from the overlying greensand with phosphatic nodules and the chalky marl
is similar to that of the basal siltstone. The presence of Bythoceratina (Bythoceratina) umbonata
magna Weaver, Bythoceratina (Bythoceratina)bonnemai (Kaye), Cytherelloidea globosa and Planileberis foveata Weaver is characteristic of the basal Cenomanian in eastern England. Although
Planileberis foveata has been recorded from the uppermost Stoliczkaia dispar Zone of Kent, that
horizon is absent in East Anglia and the species is unknown below the Cambridge Greensand.
Grantchester footbridge (N.G.R. TL 4255 5506)
Overlying 1.22 m of Gault in the footbridge foundations excavation near Trumpington, 0.18 m
of glauconitic silt with phosphatic nodules was present (Text-fig. 3.2). A sample of Gault was
taken above a phosphatic nodule band 1.2 m below the contact with the Cambridge Greensand.
The ostracod assemblage is essentially identical to that found in the Gault of the Grantchester
Cutting and the presence of Cythereis (Rehacythereis) Iuermannae hannoverana and Planileberis
scrobiculata indicates the upper part of the C. hannoverana ostracod Zone and the upper part of the
C. auritus and M. rostratum ammonite subzones. This sample also contains the coccolith Eiyelithus turrisezffeli S.S. which extends from Gault Bed 16 (upper C. auritus subzone) up into the Cenomanian(A.W. Medd, pers. comm.). The ostracod assemblage recovered from the phosphatic nodule
bed that overlies the Gault is also similar to that of the Cambridge Greensand of the GrantChester cutting. Cythereis (Cythereis) humilis humilis, Cythereis (Rehacythereis)paranuda. Cytherelloidea globosa, Planileberis foveata and Bythoceratina umbonata glabra form a characteristic assemblage.
Overlying the Cambridge Greensand is 0.23 m of disturbed sandy clay containing a mixed
fauna of Albian and Albo-Cenomanian forms together with rare species restricted to the early
Cenomanian. The most likely explanation for the presence of this sandy clay is that it is Gault
thrown up during the last century by “c~prolite~’
diggers (A. A. Morter, pers. comm.). This would
account for the disturbed nature of the deposit and the intense reworking of the ostracod fauna.
Ostracoda Across the AIbianlCenomanian in Eastern England 1233
? E L
Qault Bed no's (eastern England)
Plat ella ichieldensis
Nemoceratino (P.J trlcwpidata
Bythoceratim (C.J rnarginata
Bythoceratina (B.J wnbonata rnogna
Bythoceratina (6.) umbonata g l a h
Bythocemtina (6.) bonnemai
Cythereia (R.J bemercdensis
Cythereia (R.) paranuda
Cythereis (C.J hwnilis hwnilis
Cythereis (C.J folkestonensis
locythereia fortinodis reticulota
Eucythere (P.J trigonalia
Dolocytheridea (PS baaquetiana
Neocythere (P.J steghawi
Cythereia IR.J Iuermannae hannowmma
Qeun bed no'.
Benthonlc foramlnlfera zonee
2-The stratigraphical distribution of some of the more important species of Albian and Cenomanian
Hauxton Interchange (N.G.R. TL 4363 5348)
Within the cutting dug during the construction of the interchange with the A10, 0.25-0.30 m
of Cambridge Greensand was exposed. It overlies pale grey Gault and grades upward into Chalk
Marl. The Gault was not examined at this locality, but an assemblage of 25 species of Ostracoda
was recovered from the Cambridge Greensand (Text-fig. 3.3). Planileberis scrobicularis, P. foveata,
Cytherelloidea globosa and Cythereis (Cythereis) humilis humilis are present, but in this locality
species of Bythoceratina were not recorded. The age of the Cambridge Greensand at this site is no
younger than the lower part of the N . carcitanense Subzone (lower part of benthonic foraminifera
1234 I. P. WILKINSON
zone 8) as Platycythereis chapmani, P. gaultina, Neocythere (Physocythere) steghausi and
Phthanoloxoconcha icknieldensis were all recovered.
Mildenhall Borehole No. 6 (N.G.R. TL 6928 7307)
The borehole penetrated the Cambridge Greensand between 51.78 and 52.50 m before entering
the Gault, which is 20.04 m thick at this locality. The Cambridge Greensand is thicker here compared to the other localities examined and could be subdivided into a series of beds bounded by
erosion levels (as indicated in Text-fig. 3.4).
The faunas from the upper part of the Gault and lower part of the Cambridge Greensand
(below the erosion surface at 52.12 m) are essentially identical. Cythereis (Rehacythereis) Zuermannae
hannoverana, Eucythere (Phodeucythere) trigonalis and Zsocythereisjissicostis are present together
with Planileberis scrobicularis, but Cythereis (Rehacythereis) bemerodensis is not present. This
fauna is indicative of the higher part of the hannoverana ostracod Zone, i.e. stratigraphically no
younger than the M . rostratum Subzone of the S. dispar Zone. Above the erosion surface at 52.12 my
Cythereis (Rehacythereis) bemerodensis Bertram and Kemper was found associated with several
species occuring for the first time, notably Platella icknieldensis Weaver and numerous members
of Bythocytheridae-Bythoceratina (Bythoceratina) umbonata umbonata (Williamson), B. (B.) umbonata glabra Weaver, B. (B.) umbonata magna Weaver, B. (B.) umbonatoides Kaye, B. (Cuneoceratina) herrigi Weaver, B. (C.) marginata Weaver, ‘Monoceratina’ longispina (Bosquet) and
Nemoceratina (Pariceratina) tricuspidata (Jones and Hinde). This association is very characteristic
of the basal Cenomanian.
It would appear, therefore, that in the Mildenhall Borehole 6 , where the Cambridge Greensand
is more fully developed, the lower part may be of M . rostratum Subzone age (Upper Albian). It is
interesting to note that Neohibolites praeultimus Spath and common oysters are present at the
base of the Cambridge Greensand at this locality (Morter and Wood, 1983), as they are in the
base of Gault Bed 19 (of Gallois and Morter, 1982) in the Gayton Borehole (N.G.R. TF7280,
1974) (A. A. Morter, personal communication). The presence of ostracod species such as Platycythereis gaultina and Neocythere (Physocythere) steghausi suggests that the highest sample examined is no younger than the middle part of benthonic foraminifera zone 8 (middle N . carcitanense
Subzone). It seems likely that where the deposit is not fully developed, it is the earliest part which
is cut out.
The Cythereis (Rehacythereis) luermannae hannoverana Ostracod Zone (Upper Albian) of
Bertram and Kemper (1971) was defined for eastern England by Wilkinson and Morter (1981).
It can be subdivided into two subzones, the earlier one being recognised by the presence of Cythereis
(Cythereis) folkestonensis when associated with the zone fossil (equivalent to the lower part of
the C. auritus ammonite Subzone) and the base of the higher subzone being defined by the first
occurrence of Planileberis scrobicularis (equating with the upper part of the C. auritus and M.
rostratum subzones) (see Text-fig. 2).
The succeeding ostracod zone was recognised in the upper part of the M . rostratum and the M .
perinflatum ammonite subzones in Germany by Bertram and Kemper (1 971). The exact stratigraphical position of the base of the bemerodensis ostracod Zone, in terms of the ammonite zonal scheme
is, however, uncertain and, as indicated by Morter and Wood (1983), may be approximately
equivalent to the M . rostratumfM.
perinflatum boundary. This conclusion was accepted by Kemper (1984) who redefined the zone as earliest Cenomanian and questionably highest Albian(M. perin-
Protocythere lineata driata
N. fP.) steghawi
N. (N.) yonweni
Nemoceratim (P.) trtcwpidata
N. (C.J denticulata
lsocythereis fortinadis reticulata
Eucythere (P.) trlSJOMliS
Dolacylheridea (P.) bosquetiana
C. h y e i
c. O M t O
Cytherella cf. trmcata
Cyihereis (R.) pomnuia
C. (R.) luermannae h a m o w m a
c. (R.) bemerodemis
C. (C.) thoerenensis
c. (C.)off reticulata
c. (C.) hwnilis humilis
c. (CJ himuta
Bythoceratina (C.) wnhatoides
B. (C.) hewigi
B. (C.) marglnata
B. (B.) umbonata umbonata
B. (B.) umbonata magna
B. (B.J wnbOnata g l a h
B. (B.J bonnemai
1236 I. P. WILKINSON
flaturn Subzone). Wilkinson and Morter (1981) and Morter and Wood (1983) noted that the highest
Gault (Bed 19) of the Gayton Borhole contained forms transitional between C . (R.) luermannae
hannoverana and C . (R.) bemerodensis, but these were not found elsewhere in eastern England and
the interval cannot be placed in the bemerodensis ostracod Zone, although it must be very high
within the hannoverana ostracod Zone. Although the zone is recognisable in the M . perinjlatum
Subzone of Godstone, Surrey (unpublished data), it has not been recognised in the Gault of eastern England, the stratigraphically oldest record being from the Cambridge Greensand.
In the Cenomanian, the association of numerous species of Bythoceratina together with Cytherelloidea globosa can be used to define an ostracod zone equivalent to the N . carcitanense ammonite
Subzone and benthonic foraminifera zone 8 of Carter and Hart (1977). In the lower part of the
ostracod zone (lower part of the N . carcitanense Subzone), Platycythereis gaultina, P . chapman i,
Dolocythereidea (Puracytheridea) bosquetiana and Neocythere (Physocythere) steghausi, all of
which have their origins in the Albian, become extinct.
The problem in assessing the stratigraphical position of the Cambridge Greensand results from
the reworking of faunas from the top of the Gault and, particularly in the case of the Ostracoda,
the indifferent preservation. Just below the Cambridge Greensand in Cambridgeshire and western
Suffolk, the Gault is of the C . (R.) luermannae hannoverana ostracod Zone, i.e. the upper part of the
C. auritus and H . rostratum subzones. Species characteristic of this zone are present in the lower
part of the Cambridge Greensand in the Mildenhall Borehole No. 6 , but in the upper part of the
sequence and in the samples from the M11 Motorway sections, the Ostracoda are indicative of the
early and mid part of the N . carcitanense ammonite Subzone (basal part of benthonic foraminifera
zone 8). It may be that the entire ostracod assemblage in the lower part of the Mildenhall Borehole
No. 6 is re-worked into deposits of Cenomanian age, but if this was the case one would expect to
find a mixed assemblage. The lack of Cenomanian species can only be explained by invoking a situation where the Albian assemblages were reworked in an interval barren of an indigenous ostracod
The base of the Lower Chalk at Grantchester Cutting is also of mid N . carcitanense age on the
basis of the ostracod assemblage, and the 0. donzei ostracod zone was not recognised.
PLATE1-Bythocytheridae and Cytherellidae from the Cenomanian of eastern England. Left valve : lateral views
except where stated. Fig. 1. Monocerutinu longispinu (Bosquet, 1854), MPK 4981, Mildenhall Borehole no. 6,
Cambridge Greensand, depth 51.85 m, length 600 pm, height 270 pm. Fig. 2. Bythocerurinu (Bythocerutinu)
umbonatoides (Kaye, 1964), MPK 4982, Mildenhall Borehole no. 6, Cambridge Greensand, depth 51.85 m,
length 1000 pm, heigh 480 pm. Fig. 3. Bythocerutina (B.) umbonutu (Williamson, 1847), MPK 4983, Mildenhall
Borehole no. 6, Cambridge Greensand, depth 51.85 m, length 820 pm, height 370 pm. Fig. 4. Bythocerutina
(B.) umbonutu mugnu Weaver, 1982, MPK 4984, Mildenhall Borehole no. 6, Cambridge Greensand, depth
51.94 m, length 860 pm, height 420 pm. Fig. 5. Bythoceratinu (B.) bonnemui (Kaye, 1964), MPK 4985,
Grantchester Cutting, Chalk Marl, 0.3 m above base of Cambridge Greensand, length 700 pm, height 320 pm,
Fig. 6. Bythocerutinu (Cuneocerutinu) herrigi Weaver, 1982, MPK 4986, Grantchester Cutting, Chalk Marl,
0.3 m above base of Cambridge Greensand, length 710 pm, height 350 pm. Fig. 7. PuteNacythere bicostatu n.
sp., Right valve, lateral view, MPK 4987, Mildenhall Borehole no. 6, Cambridge Greensand, depth 52.44 m,
length 520 pm, height 230 um. Fig. 8. Nemocerutina (Puricerutinu) tricuspidutu (Jones and Hinde, 1890).
MPK 4988, Mildenhall Borehole no. 6, Cambridge Greensand, depth 51.85 m, length 670 pm, height 280 pm.
Fig. 9. CytherelIoideu kuyei Weaver, 1982, Right valve, lateral view, MPK 4989, Grantchester Cutting, Chalk
Marl, 0.3 m above base of Cambridge Greensand, length 550 pm, height 330 pm. Fig. 10. Cytherelloideu
strictu (Jones and Hinde, 18901, Right valve, lateral view, MPK4990, Grantchester Cutting, Chalk Marl,
0.3 m above base of Cambridge Greensand, length 690 pm height 390 pm, Figs. 11, 12. Cytherelloideu globosu
Kaye, 1964, Grantchester cutting, Chalk Marl, 0.3 m above base of Cambridge Greensand. 11, Right valve,
lateral view, MPK4991, length 580 pm, height 340 pm; 12, Dorsal view; MPK4992, length 610 pm width
310 pm. Fig. 13. Plutella icknieldensis Weaver, 1982, MPK4993, Mildenhall Borehole no. 6, Cambridge
Greensand, depth 52.0 m, length 710 pm, height 360 p m .
1238 I. P. WILKINSON
During much of the Late Albian in eastern England, deposition of the Gault and Hunstanton
Chalk took place in quiet, shallow, but gradually deepening marine conditions. The regressive
regime, which began in the highest ( M . dispar) zone, led to erosion and the highest subzones are
wholly or partly removed (Gallois and Morter, 1982). Accumulation of the Greensand facies
began in the M . rostratum subzone and continued intermittently into the Cenomanian. The rapid
transgression which took place in the Early Cenomanian resulted in the Cambridge Greensand
facies rapidly passing up into the Chalk Marl.
Water temperature also increased with the transgressive regimes, the warmer water entering the
area from the south when the effect of the London-Ardennes Massif was reduced and the AngloParis Basin was widened (Owen, 1979). It increased slowly through the Upper Albian, but it was
not until after the ‘Mid-Cenomanian Break’ that major temperature increases took place. Other
parameters which affected the community (e.g. food supply, light, oxygen levels etc.), the physiological stress of Sanders (1969) and Slobodkin and Sanders (1969) and the spatial variability of
Valentine (1971, 1973) were such that the community diversity remained high throughout much
of the period in question.
The ostracod population responded to the variations in the environment in several ways. Depth
related parameters (e.g. thermocline, oxygen minima, hydrodynamism or light penetration and
related plant life) appear to be important in the case of Platycythereis, Dolocytheridea and Neocythere (Centrocythere) which are considered to be shallow water indicators (Bassoullet and Damotte,
1969; Rosenfeld and Raab, 1974; Babinot and Colin, 1983). Species of Platycythereis (P. gaultina
and P . chapmani) are frequently found in the S . dispar Zone of the Gault in eastern and southern
England, but become rather rare in the basal Cenomanian and extinct in the middle part of the
h’. carcitanense Subzone. Dolocytheridea (P.) bosquetiana is commonly found in the Middle Albian,
is somewhat rarer in the Upper Albian and becomes extinct towards the top of the N. carcitanense
Subzone. Finally, the subgenus Neocythere (Centrocythere) occurs in large numbers in the Albian,
but becomes rare towards the top of the stage, and disappears from the British record in the early
part of the M . saxbii Subzone (Weaver, 1981, 1982).
Although rare specimens of Bythoceratina (B.) umbonata umbonata, ‘Monoceratina’ longispina
and Nemoceratina (P.) tricuspidata are found scattered throughout the Gault, Bythocytheridae,
and particularly Bythoceratina, become more diverse in the early Cenomanian. Of the 13 species
PLATE2-Trachyleberididae from the Albian and Cenomanian of eastern England. Left valve; lateral views except
where stated. Fig. 1. Cythereis (Rehucythereis) luermannae hunnoveranu Bertram and Kemper, 1971, MPK4994,
Grantchester Cutting, Gault, 0.61 m below base of Cambridge Greensand, length 1080 pm, height 590 pm.
Fig, 2. Cythereis (R.) bemerodensis Bertram and Kemper, 1971, MPK4995, Grantchester Cutting, Chalk Marl,
0.3 m above base of Cambridge Greensand, length 480 pm, height 620 pm. Fig. 3. Cythereis (Cythereis) humilis
humilis Weaver, 1982, MPK 4996, Grantchester Cutting, basal Cambridge Greensand, length 580 pm, height
350 pm. Fig. 4. Cythereis (R.)paranuda Weaver, 1982, MPK 4997, Grantchester Cutting, basal Cambridge
Greensand, length 840 pm, height 490um. Fig. 5. Planileberis scrobicularis Weaver, 1982, MPK 4998,
Grantchester Cutting, Gault, 0.61 m below base of Cambridge Greensand, length 1040 pm, height 600 pm.
Fig. 6. Planileberisfoveata Weaver, 1982, MPK4499, Grantchester Footbridge, Cambridge Greensand, length
700 pm, height 400 pm. Fig. 7. Zsocythereis fissicostis fissicostis Triebel 1940, MPK5000, Grantchester
Footbridge, Gault, 1.22 m below base of Cambridge Greensand, length 530 pm, height 280 pm. Fig. 8.
Zsocythereisfortinodis reticulata Griindel, 1964, MPK5001, Mildenhall Borehole no. 6, Cambridge Greensand,
depth 52.25 m, length 630 pm, height 280 pm. Fig. 9. Platycythereis gaultina (Jones, 1849), MPK5002,
Mildenhall Borehole no. 6, Cambridge Greensand, depth 52.44 m, length 640 pm, height 280pm. Fig. 10.
Plutycythereis chumpuni Kaye, 1964, MPK 5003, Grantchester Cutting, basal Cambridge Greensand, length
1000 pm, height 500 pm.
1240 I. P. WILKINSON
and subspecies recorded in the Cenomanian (Weaver, 1982), ten were recovered from the Cambridge
Greensand and Chalk Marl, examined by the present author, and seven are members of the genus
Bythoceratina. This latter genus is associated with present day deeper water (middle and outer
shelf down to bathyal regions). Off southeastern USA, for example, it has an upper depth limit
of 220 m (but extends down to 1034 m) where temperature oscilates a few degrees on either side
of 10"C (but goes as low as 5" C in the bathyal areas) (Cronin 1983a, b). Their presence in small
numbers in the basal Cenomanian of eastern England probably indicates that they were close to
their upper depth limit; they become more common higher in the Upper Cretaceous where water
depths are greater.
Although Neale (1964) considered salinity a primary factor in controlling ostracod distribution,
he regarded hydrogen ion concentration in seas of open circulation almost certainly insignificant,
but did not consider water chemistry further. Peypouquet (1979, 1980) and Peypouquet et ul.
(1980), however, showed that the ionic relationship of Mg/Ca or that of other ions could affect the
ostracod carapace and it seems likely that the ionic balance can considerably affect the population
also. There was certainly a change from argillaceous Gault to carbonate Chalk Marl, but this does
not necessarily mean that sea bed conditions were saturated with CaCO, (see Hancock, 1976, for a
summary). The effect of carbonate sedimentation on the ostracod population has, however, not
been fully studied. Babinot and Colin (1983) showed the effect of the ionic balance on the ornament
of Plutycythereis (their Chuprnanicythereis).This effect is also seen in Britain where the circular to
sub-ovate punctations of Albian P . gaultina become distinctly elongate in the Cenomanian chalk
(see Weaver, 1982, P1. 14, figs. 16, 18). Specimens from the Cambridge Greensand are intermediate
between the two, but closer to the Gault form. The genera's preference for the middle and inner
areas of the carbonate platform where there is an input of argillaceous material and organic matter
(Babinot and Colin, 1983) is a reflection of food supply and, to some extent, water chemistry. It is
interesting to note that the only record of a British Cenomanian species of Dolocytheridea,
above the mid part of the N . carcitanense Subzone, is near the top of the M . dixoni Zone (Weaver,
1982) where there is a reduction in calcimetry (Destombes and Shephard-Thorne, 1971;Weaver;
It would appear, therefore, that the important changes in the structure of the ostracod population were not entirely due to evolutionary trends, but also a response to environmental parameters.
The faunal changes do not coincide with lithological boundaries as might be expected if the forms
were facies controlled, but there is a gradual change beginning in the latest Albian and continuing
in the earliest Cenomanian. It appears to be depth related, but water chemistry probably also played
All specimens are housed with the British Geological Survey, Keyworth, Nottinghamshire,
Griindel and Kozur, 1971
Type species.-Monoceratina williuinsi Stephenson, 1946.
BICOSTATA n. sp.
(PI. 1, fig. 7)