42I

O Y S T E R B E D S : M O R P H O L O G I C R E S P O N S E

T O C H A N G I N G S U B S T R A T E C O N D I T I O N S

A. Seilacher

Ttibingen

B.A. Matyja & A. Wierzbowski

Warszawa

Abstract: Morphotypes and set t l ing behaviors of oysters in coarsening-upwards cycles of the Polish Upper Jurassic and at different localit ies of the South Australian Pliocene re f lec t temporal and spatial changes in substrate softness that the winnowed matr ix fails to record. Adaptational s t ra tegies are similar in the two cases except that the lack of comissural overlap did not allow the Jurassic Lopha to develop gryphid mud dwellers. It remains uncertain, however, whether we deal with ecophenotypie or evolution- ary phenomena.

Oysters have been the most successful group of all cemented bivalves. Part of

their morphological diversity is due to the fact that during the Mesozoic and Cenozoic,

they repeatedly evolved sidelines that left their original rocky substrates to become

secondary soft bottom dwellers (SEILACHER, 1984). These soft bottom oysters are

of particular interest to paleontologists, (I) because of their high fossilization potential

and (2) because of their more regular shapes, which faci l i ta te taxonomic classification

as well as functional interpretations. In terms of constructional morphology various

types of mudstickers and recliners (Fig. 1) can be distinguished and compared to simi-

lr.r "strategies" in other bivalves, but their adaptational significance is necessarily in-

ferred and needs to be tested against field evidence. The present study is such an

a t tempt . It also il lustrates the problem of distinguishing ecophenotypic from genetical ly controlled characters in fossil forms.

422

outriggered fan shaped

Arctostrea ~i:i~:!:::::::. :::::::::::::::::::::::: Lopha ,ii~ji:Ji;~::/:~J

SOFTB ~ iM ~ OT OYSTERS :~i',ii~:ii:~i'; : ~

boulder shaped

~' Crassostrea I I

Exogyra "lO

Q.

t - in Q.

(J

Gryphaea

Konbostrea

stick shaped

~ i ~ Platygena ~

q

spoon shaped

Saccostrea

I

coneshaped

423

r~

r-

3 I N 0

tD

3

g

I MAEOGOSZCZ

t r t <

c 3

, < " 0 (/1

0 o -g 3 ~

tl)

vl.. ~'q,. t "~. X ~ , ~ . - - ~ ' ~ . . { . ^ J

Tethys

f f .

. / . /

\

13 ° I c)

3 ~.

diversity

terrigenous

Fig. 2: The Lopha beds (black) of the Skorkow Lumachetle (U. Jut. , Poland) increase in thickness and faunal diversity towards the edge of the carbonate platform. The basal hard ground (black bar) serves as a datum line.

Fig. 1: The unusual morphogenetic plasticity of oysters, inherited from the original rock-encrusting habit, allowed them to adapt to soft substrates by a var ie ty of strategies. (From SEtLACHER 1984}.

424

I. UPPER JURASSIC OYSTER BEDS IN CENTRAL POLAND

A. Geologic se t t ing

The Upper Jurass ic of Poland differs from its equivalent in South Germany by

tha t a ca rbona te p la t form facies with mainly bioclast ic sed imenta t ion follows the

ear l ier spongy facies. S ta r t ing during the Middle Oxfordian in the no r th -eas t e rn par ts

of the Holy Cross Mountains, this p la t form prograded to the southwest during the La te

Oxfordian (Fig. 3} and the Kimmeridgian (KUTEK 1969). In sect ions the p la t form se-

d iments show a ver t i ca l a l t e rna t ion of cross-bedded ooli tes or coquinas with micr i t i c

l imestones and marls.

The oyster beds to be discussed are found within the Kimmeridgian par t . They

have a wide distr ibution, but become gradually replaced by ooli tes towards the advan-

cing outer edge of the p la t form (KUTEK 1968 and 1969}. Depending on whether

Lopha or Nanogyra predominates , one speaks of "Alectryonia" or "Exogyra" lumachelles.

In the Skorkow Lumachel tes a t the boundary of the hypselocyclum and divisum zones

(Lower Kimmeridgian; Figs. 3-5.) Lopha is found in the lower and Nanogyra in the upper

par t of the unit . In te rca la t ing l imestones and marly l imestones consist of fine bivalve

shell de t r i tus (biomier i te and biopelmicr i te) with varying amounts of onkoids and/or

ooids. A hardground at the base of the unit is used as a marker horizon for regional

cor re la t ion (KAZMIERCZAK & PSZCZOLKOWSKI 1968) and has been recognized in

an ident ical s t ra t ig raphic position on the nor theas te rn side of the Holy Cross Mountains.

(Wierzbica sect ion in Fig. 3 ). Since both areas have been connec ted before the La-

ramide uplif t of the Paleozoic core (KUTEK & GLAZEK 1972), it can be assumed tha t

the Skorkow Lumachel le originaIly covered an area of at least 1OO OO km 2.

A few addit ional Nanogyra beds are found above the Skorkow Lumachel le in

the uppermost par t of the Lower and the Upper Kimmeridgian. They can be indivi-

dually t raced over dis tances of several ki lometers , but are insuff ic ient ly f ingerpr in ted

to assess the i r real extensions.

As may be expected, there is a general decrease of ter r igenous mater ia l across

the p la t form in an offshore direct ion. In the same di rec t ion the faunal divers i ty in -

creases, wi th s tenohal ine forms such as echinoids, ammoni te s and brachiopods being

ra re or lacking in the east . One also observes an increase in thickness, but this may

be largely due to the fac t t ha t the ca rbona te edge had to cross the SE-NW trough

of the Polish-Danish aulacogene as it advanced to the west (KUTEK & GLAZEK 1972).

425

WlERZBICA Abundance of fossits

v ~ ~ v

v

- - m

I I I ~ • ~ ~ m m ~ L "

~ - ~ ~ 2 ~ ,

< < ~ < J

I I!1 f/

J I I ' , | i m m m m i _ _ a ~ g ~ , Z ~ L

. . . . ~ e T lll!!!l _

0 100%

Rate of carbonate mud sedimentation

i - - L L ~ mar[s

marry limestones timestones

• onkoids o ooids ~ bioctasts

,.,.,..,Ir~ } hard ground

, ,~,~ carbonate nodules /'~// Thatassinoides

~- cemented Nanogyra ,-, reclining Nanogyra

Lopha Trichites, Stegoconcha brachiopod

c~ Arcomyti[us Jsognomon Inoperna TrJgonia, Anisocardia, Myophore[La

• ~ P[euromya, Thracia tb Pholadomya

Fig. 3: Recurring faunal successions from infaunal to epibenthic to encrusting communities of bivalves in this section suggest cycles of decreasing substrate muddiness. Rather than being true ecological successions, however, the faunistic changes are probably caused by taphonomic feedback in shallowing-upwards cycles.

426

GRUSZCZYN Abundance of i~ l~l~i fossils A

0 100%

~° d~ a~ °"

Rate of carbonate Abundance of fossits Rate of carbonate mud sedimentation D mud sedimentation

12) =~1 ~'1~ t ~,~._L__

/ / - ~ o ~ o f

- - - !1 , , 9 ~ 1 . -~ -~ • I1~

3 - f o : ,

0 100%

Fig. 4: At a large scale (A), the Gruszczyn section shows simiIar cyctes as observed in Wierzbica, but cycle thicknesses are higher. At a smaller scale (B), however, the lower part of the section fails to show uniform trends, although taphonomic feedback should work at any scale.

B. Fossil associations (Fig, 5)

In addition to the lithologic alternation of coquinas with beds of l imestone, mar-

ly l imestone and marl, sections of the Skorkow Lumachelle show various kinds of dis-

continuity surfaces (firm and hard grounds) indicating periods of reduced net-sedimen-

tation and of erosion to already compacted or even cemented levels. The firm grounds

427

DECREASIN( RATES OF SEDIMENTATION & MUDDINESS _- ) burrowers byssate mud stickers cemented 1 cemented recliners- l & rec nets mud stickers

Deltoideum Cera t omya

encrusters

[ NanoEyra I Nan°gY ra Trichites

I BIVALVE COMMUNIT IES in ', SKORKOW REGRESSIVE CYCLE

Fig. 5: Sequential bivalve associat ions observed in the sec t ions (Figs. 2 4) re f lec t changing subs t ra t e condit ions r a the r than t rue geologic successions. Note the s imi lar i t ies of Deltoideum to Platygena , and of the recl ining morphotype of Nanogyra to Exogyra (Fig. t), which has lost the a l t e r na t i ve abi l i ty of Eanogyra to encrus t a l t e rna t i ve appropr ia te hard subs t ra tes . Compare also the b e t t e r resolvable assoc ia t - ions in the Oxfordian of wes te rn Europe (FORSICH 1977).

are c h a r a c t e r i z e d by c rus t acean burrows wi th preserved" sc ra tch marks ( G l o s s i f u n g i t e s ,

Spongeliomorpba), the hard grounds by borings and by encrus ta t ions , mainly of Nano-

gyra. In the Skorkow Lumaehe l le -par t of the sect ion (Fig. 3) only the basal hardground

is continuous. The o thers consis t of nodules tha t are equally enc rus ted . This means

erosion down to a level in which concre t ions had already formed diagenet ical ly , folio-

wed by enc rus ta t ion of the exposed pebbles and rebur ia l under the overlying marl . It

is also possible t ha t these hardgrounds were a c t i v a t e d repeatedly , but to co r robora te

this assumption i t would be necessary to find hiatus concre t ions (VOIGT 1968) wi th

mult iple encrus ta t ions .

In any case i t is c lea r t ha t during the deposit ion of the Skorkow Lumachel le

sed imenta t ion ra t e s and subs t r a t e condi t ions were subjec t to considerable f luctuat ion.

In response to these f luctuat ions, the benth ic fauna must have changed as well, but

since the fossils a re largely separa ted from the i r home sediments and mixed to various

degrees, it is now dif f icul t to ident i fy the original biota. Never the less , some major

associat ions of bivalves (Fig. 5) can be r a the r c lear ly distinguished. Since the cons t i -

428

tuen t bivalves happen to be also mineralogical ly d i f fe rent (the burrowers have aragoni-

tic, the oysters ca lc i t ic and the byssates composi te shells), the i r separa t ion might be

d iagenet ica l ly enhanced, but c r i t e r i a of funct ional morphology suggest tha t envi ronmen-

tal condit ions had a more impor tan t control (Fig. 5L

1. Associa t ion of burrowers

The chief advantage of bivalves over o ther soft bo t tom f i l te r feeders, such as

a r t i cu la t e brachiopods, is the i r abil i ty to burrow. Major requ i rements for the infaunal

mode of life were the t rans format ions of the foot into a hydraulic burrowing organ

and of the fused mant le margins into siphonal tubes (STANLEY 1972). Modif icat ions

of shell geomet ry and shell sculpture enhance the burrowing process and the s tabi l i -

za t ion within the sed iment (SEILACHER 1984) and allow to dist inguish deep and

shallow burrowers by morphological cr i ter ia .

In the Skorkow Lumachel le deep burrowers, such as Pholadomya, Pleuromya, Cera-

tomya , and Thracia,are more common than shallow burrowers (Trigonia, MyophoreII~

Anisocardia etc.) . This is because the deeply burrowed shells are less commonly re-

worked. In many cases they are still in l ife position. But also the i r sed imenta ry infill

becomes more easily cemen ted into molds tha t survive erosion even a f t e r the aragoni t ic

shell around them has been dissolved.

More shelly sediments , however, make burrowing increasingly expensive, par t icu-

larly for the deep burrowers, while they do favor an epibenthic mode of l ife (KIDWELL

& AIGNER, this volume). The epifaunal niche is occupied mainly by t e rebra tu l id bra-

chiopods and secondary sof t bo t tom dwellers, which are phylogenet ical ly derived from

sessile rock dwellers r a the r than from ac t ive burrowers. The inher i ted inabil i ty of

these bivalves to r ight themselves up, to crawl and to burrow was only in ra re cases

compensa ted by the evolution of new modes of locomotion (Pectinids, Limids). More

cha rac t e r i s t i c is passive s tabi l iza t ion by par t icu lar shapes and d i f fe ren t ia l weight ing

of the shells (SEILACHER 1984). While adapta t iona l s t r a teg ies are comparable in secon-

dary soft bo t tom dwellers derived from byssate and cemen ted stocks, the i r d i spara te

and sequent ia l occur rence in the studied and o the r sect ions suggests t ha t the two

groups should be t r e a t e d as d i f fe ren t associations.

2. Associat ion of byssate mudst ickers and recl iners

This group is represen ted (a) by the myt i l acean genera Arcomytilus, Stegoconcha

and Trichites, whose broad an te r io r res t ing surfaces and various degrees of shell th icke-

429

ning ident ify them as edgewise recl iners , and (b) by mul t iv incular forms such as I s o -

gnomon and C, ervi l l ia , in which shelI morphology alone is not as indicat ive for a

par t icu lar mode of life, so t ha t o ther c r i t e r i a , such as o r ien ted overgrowth, have to

be used in addit ion (SEILACHER 1954), As a whole, this group cha rac t e r i ze s soft bo t toms

in which sed imen ta t ion r a t e a t t he on togene t i c scale is reduced, so t ha t smother ing

is not a cons tan t menace and where the mud is shelly enough for byssal anchoring

at the sur face or within the sediment .

3. Lopha Associat ion

Like in the previous group f ixat ion is c r i t ica l only for the ear ly on togene t i c s tages,

while passive s tab i l i za t ion mechanisms take over as the animals grow larger . Neve r the -

less, byssal a t t a c h m e n t may be main ta ined as an addi t ional means of s tab i l iza t ion and

l imi ted mobil i ty throughout life. Cemen ted rec l iners however, comple te ly give up the i r

juveni le f ixat ion and the i r morphogene t ic program has to change dras t ica l ly conforming

to the new paradigm. It is probably for this reason tha t encrusting recl iners are less

t o l e r an t to mud sed imen ta t ion than the byssa te ones.

Cemen ted mudst ickers have an i n t e rmed ia t e posit ion in this respect , because they

need a ce r t a in degree of mud sed imenta t ion to become stabi l ized, as will be shown

in the case of Lopha.

4. Nanogyra Associat ion

The ecologic separa t ion of the oys ter IVanogyra from Lopha has probably to do

with i ts much smal ler size. In sof t bo t tom condit ions i t can grow into a rec l ine r si-

milar to Exogyra (Fig. 1); but being so small i t would be more sens i t ive to mud smo-

thering. On the o ther hand, Nanogyra may remain an enc rus te r throughout life, provided

there are adequate ly large hard subs t ra tes , such as pebbles or hardgrounds to grow

on. Accordingly we can dist inguish be tween associat ions of recl ining and of incrus t ing

Nanogyra in the s tudied sect ions.

C. Ecologic response to sed imenta ry cycles

The dis t r ibut ion of the associat ions in the sect ion is not random but follows a

sequent ia l p a t t e r n (Figs. 4-5). It is t empt ing to in t e rp re t such pa t t e rn s in t e rms of

the ecological successions observed in modern biota, par t icular ly in plant ecology. But

the t ime scale of t rue ecological successions is in the order of years, far beyond the

430

resolution of ordinary strat igraphic sequences. More probably we deal with pseudosuc-

cessions caused by a temporal gradient in sedimentational conditions plus an interdepen-

dence of successive associations via taphonomie feedback (KIDWELL & AIGNER, this

volume).

I. Regressive cycles expressed by faunistic pseudosuccessions {Fig. 5)

The repeated sequence in the studied sections from infaunal to epibenthic and

encrusting associations (Figs. 2 - 4) can be interpreted as a response to cycles of

decreasing mud sedimentation, provided that this decrease was e f f ec t ive at the scale

of individual l ife cycles and not simply the result of long-scale erosive events. In a

way we can compare these ecological cycles to the coarsening- or shallowing-upward

cycles of the sedimentologists. Nevertheless, faunistic replacement may have taken

place episodically af ter the previous community had been locally wiped out by smothe-

ring events.

2. Regressive cycles expressed by morphotypie trends (Fig. 6)

In addition to the ecological response at the faunistic level, a sequential change

of sett l ing behavior and growth form is observed within individual oyster beds.

This phenomenon is most clearly expressed in the thick Lopha bed of the Ma-

logoszez quarry (Fig. 2 ). In its lower part, one finds a large number of elongated

Fig. 6_.', The reworked and winnowed nature of fossil oyster beds is commonly revealed by functional morphologies and sett l ing behaviors that are in confl ict with observed burial positions and host rock granulometries. The transition of Lopha grega rea from slope-oriented a t tachment on other mud-sticking bivalves (a-b) to self-supported mud-sticking (c) and to cup-shaped growth on dead shells (d) probably ref lec ts decreasing rates of mud-sedimentation in accordance with the coarsening-upwards cycle of which the Skorkow Lumachelle forms the top. Convergent forms of Pliocene oysters (e-h) charac ter ize different localit ies within the S-Austral ian basin. In this form, how- ever, commissural overlap has also allowed the additional establishment of pseudo-commissures between adjacent mud-sticking individuals (g) and the transformation into heavygryphaea- l ike forms reclining on muddy bottoms (1-0) -- a mode of life that should by current opinion have become extinct by the end of the Cretaceous (LA BAR- BERA 1981; JABLONSKI & BOTTJER 1983). Note that actual path- ways probably went from shelly to increasingly muddy substrates. The sequence of the morphotypes in the coarsening-upwards cycle is probably midleading. More likely rock-dwelling oysters have entered unstable substrates as miniaturized enerusters on shelly bottoms and only then became adapted to muddier environments. In view of the morphogenetic plasticity of oysters it also remains an open question whether these adaptations are phenotypic or evolutionary in character .

431

MORPHOTYPIC TRENDS IN OYSTERS

-]

~ % ~ /"

u)

witl in-cycle (U.Jur., Poland)

ul o1

r= . m

L g

within- basin ( Plioc., S. Australia )

432

Lopha specimens attached to a very elongate species of Gervillia. In all studied spe-

cimens (about 20) the Lopha grew in the same direction as the host, as did successi-

ve generations of the same species that happened to settle on the gerv221ia-attached

founder. This and the fact that gervillia is still double-valved in specimens, in which

it is not only preserved as an attachment scar, proves (I) that unlike similar-shaped

pendant species of the L. Toareian (SEILACHER 1984, Fig. 7 ) this Gervillia was

a mudsticker, (2) that it became encrusted by Lopha in a slope-oriented fashion while

it was still sticking vertically in mud, and (3) that all specimens became subsequently

reworked to their present horizontal position in the lumachelle. Only in one specimen

the attached Lopha changed growth direction in a later stage, suggesting that it sur-

vived reworking and adjusted upward growth to the new orientation.

Higher up in the bed one increasingly finds Lopha with a similar elongate shape,

but not associated with Gervi22~a. Instead we observe a t t achment scars (probably on

dead, horizontal Lopha shells) that were too small to freely support the upright shell.

Still it had remained in this unstable position long enough for successive Lopha genera-

tions to grow on it with the same slope orientation. This means that given an adequa-

te, but not excessive, mud sedimentat ion, this Lopha species could i tself become a

mudsticker.

The third morphotype lacks the elongate outline and is cup-shaped instead. It

could easily be considered a di f ferent species, but more probably represents simply

an eeophenotypic response to an environment, in which many dead shells of previous

oyster generations covered the sea floor and remained exposed for a longer time. As

a consequence the encrusters could extend their cemented stage over most of the sub-

s t ra te shell before they grew up to form the cup with the f la t te r right valve as a

lid.

II. PLIOCENE OYSTER BEDS IN SOUTH AUSTRALIA

(Fig. 6)

It is interest ing to compare the Jurassic example with oyster beds in the Plio-

eene of South Australia, where we meet similar, as well as additional, morphotypes

not in the same bed, but at d i f ferent localit ies within the same basin.

Counterpar ts to the Gervillia-attached forms are found in coastal exposures

at Aldinga Beach, where they occur at cer ta in levels in a very fine and well sorted

sand without forming thick accumulations. They are a t tached to posterior ends of large

Pinna shells and their uniform slope orientat ion leaves no doubt that they incrusted

these byssate mudstickers while they were still in their vert ical life position (Fig. 6e).

Most of the Pinna-shells are now horizontal but double-valved (as are most of the

433

oysters) , suggest ing rapid exhumat ion and burial during a s torm event . Compared

to the o ther occur rences these oys ters a re re la t ive ly f la t an th inshel led (Fig. 6f).

O the r morphotypes are found a t Shell Hill near Black Hill to the Eas t of Ade-

laide, where they form monotypic accumula t ions t h a t get more than 5 m thick and

are quarr ied for l ime. Here the oysters form bunches with a ch a r ac t e r i s t i c re la t ionship

be tween the individuals {Fig. 6g-h). Not only do all member s of a bunch grow in

the same d i rec t ion {which was upward in life}. They also tend to grow along a joint

f ront , so t ha t the margins of ad jacent l e f t valves form a kind of commissure , which

does not coincide wi th the t rue commissures because the r ight valves are always a

l i t t l e smal ler than the le f t ones. This means tha t unlike in the associa t ion wi th Pinna

the individuals did not ove rcompe te each o ther but grew up synchronously, much

like the ca l ices in a coral head. Bunches of 2-4 oysters are the rule, while sol i tary

spec imens are conspicuously lacking. Thus we deal with gregarious mudst ickers t ha t

b e c a m e invar iably reworked by s torm, while the original host muds b e c a m e winnowed

away to deeper par t s of the basin.

A d i f fe rent s i tuat ion was found at a ce r t a in horizon within the oys ter beds in

a road cut a few hundred me te r s from Oyster Hill. Here the oysters have ex t r eme ly

large a t t a c h m e n t scars (not the con tac t s resul t ing from communal growth), from which

they grew up in a cup-like fashion, much like the similarly shaped rec l iners in the

Jurass ic example. This mode of growth (Fig. 6 i-k) allows the lef t valve to become

excessively thickened, thus providing the d i f fe ren t ia l weight ing tha t is found in many

recl iners . It should be also no ted t ha t ad jacent individuals on the same subs t r a t e

shell never show the communal pseudo-commissures t ha t are so typical for mud-s t icking

bunches.

In addit ion the PIiocene oysters have produced a Gryphaea,like var ian t tha t is lacking

in the Jurass ic mater ia l . In t e rms of morphogenesis this means tha t the a t t a c h m e n t

scar is re la t ive ly small and t ha t subsequent f ree growth of the lef t valve proceeds

in a spiral fashion. Funct ional ly i t is an approximat ion to the horn coral paradigm,

which provides not only a s tab le posit ion in sof t subs t ra tes , but also allows the passive

reburia l into this position a f t e r the shells have become displaced by s to rms or biolo-

gical ac t iv i ty (SEILACHER 1984, Fig. 12; see also BAYER e t al., this volum(e).

Gryphid morphotypes are cha r ac t e r i s t i c for two local i t ies in the South-Aust ra l ian

Basin, but geomet r ies are s ignif icant ly d i f fe ren t in the two. At Maslin Bay in a bed

several me te r s above the one with the P i n n a - a t t a c h e d oysters the gryphid forms have

an a lmost equidimensional out l ine (Fig. 6 l-m), while they are much nar rower in

an oyster bed a t Over land Corner (Murray Basin;Fig. 6 n-o). It should be noted t ha t the

two figured specimens a re r ep r e sen t a t i ve for samples of severa l dozen specimens

from the f irst local i ty and several hundreds from the second.

434

The absence of s imilar gryphid types in the Jurass ic Lopha and the i r subst i tu t ion

by Nanogyra (ZIEGLER 1969) is probably not coincidental . As expressed by the zigzag

commissure , Lopha lacked the precondit ioning for this type of growth, namely the

s l ight overlap of the lower (in this case the left) valve over the o ther along the

commissure . Radial ribs, where present in gryphid bivalves, are r e s t r i c t ed to the

more convex valve and do not re f lec t folding of the proper commissure .

III. CONCLUSION

In t radi t ional descr ip t ive paleontology, the discussed morphotypes would probably

have been t r e a t e d as d is t inc t taxonomic en t i t i e s a t t he subspecies or species level,

par t icu lar ly when they occur in s t r i c t s t r a t ig raph ic or geographic separa t ion. In view

of the e x t r e m e morphogenet ic p las t ic i ty of modern oysters , however, i t is more likely

t ha t we deal only with an ecophenotypic response to tempora l or regional f luctuat ions

in ra tes of sed imenta t ion and in bo t tom consistency. Never the less the modif ica t ion

of form was adapta t ional , because it followed pathways known from other , convergen t

l ineages. This means tha t such modif icat ions can be used to indica te env i ronmenta l

deta i ls which the l i thologic record does not reveal; but they tell us l i t t l e about the

modes and tempoes of t ruly evolut ionary processes.

Acknowledgements : The project or ig inated in a joint s tudent ' s excursion to the Holy Cross Mountains in summer 1983 as par t of a par tnership program be tween the univers i t ies of Warsaw and Ttibingen. Addit ional field work was supported by the SFB 53.

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Carter, R.M., 1968: Functional studies on the Cretaceous oyster Arctostrea.- Palaeontology,ll(3): 458-487.

FNrsich, F.T., 1977: Corallian/Upper/Jurassic marine benthic associations from Eng- land and Normandy.- Palaeontology, 20(2): 337-385.

Jablonski, D. & Bottjer, D., 1983: Soft-bottom epifaunal suspension-feeding assem- blages in the Late Cretaceous. Implications for the evolution of benthic paleo- communities. In: Tevesz & McCall (eds.): Biotic interactions in Recent and ~os- sil benthic communities.- Plenum Publishing Corp.: 747-812.

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