[Lecture Notes in Earth Sciences] Sedimentary and Evolutionary Cycles Volume 1 || Oyster beds:...
Transcript of [Lecture Notes in Earth Sciences] Sedimentary and Evolutionary Cycles Volume 1 || Oyster beds:...
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.
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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
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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}.
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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).
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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.
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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
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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 -
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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-
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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.
REFERENCES
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