pcbstromatbiostrat

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Pr e c am br ian Re s e ar c h , 5 (1977) 143--17 3 143 Elsevier Scientific Publishing Comp any - - Printed in The Netherlands

B I O S T R A T IG R A P H I C U S E F U L N E S S O F S T R O M A T O L IT I C P R E C A M B R I A NM I C R O B I O T A S: A P R E L I M I N A R Y A N A L Y S I S

J. WILLIAM SCHOPFD e par tm e n t o f G e o logy and I n s t i t u t e o f G e ophy s i c s and P lane ta r y Phy s i c s , U n iv e r s i t y o fCalifornia, L os An geles , Calif . (U.S.A .)(Received and accepted February 8, 1977)

ABSTRACTSchopf, J.W., 1977. Biostratigraphic usefulness of stroma toliti c Precambria n microbiotas:a prelimi nary analysis. Precambrian Res., 5: 143--173.

Diverse, cellularly preserved microbial commun iti es are now k now n from st romato liticsediment s of at least tw enty-e ight Precambrian formations. These fossiliferous deposits,principally cherts and cherty porti ons of carbon ate units, range in age from Early Protero-zoic (Transvaal Dolomite, ca. 2250 Ma old) to V endian (Chichkan For mati on, ca. 650 Maold) and inclu de units from Australia, India, Canada, South Africa, Greenland, the UnitedStates and the Soviet Union. More than three-quarters of these microbi otas have been dis-covered since 1970. Although few, therefore, have as yet been studied in detail, virtuallyall of the assemblages are kno wn to be dom inat ed by prok aryoti c (bacterial and blue-greenalgal) microorganisms and to cont ain three major categories of microfossils: spheroidalunicells, cylindrical tube-like sheaths, and cellular trichomi c filaments. Analyses of datanow available (inclu ding measur ement s of more than 7800 fossil unicells) indicate thateach of these three types of microfossils exhibit ed a gradual, but marked, increase in meandiameter and size range during the Proterozoic and that taxonomic diversity apparentlyalso increased, especially beginning abo ut 1400 Ma ago. Thus, it now seems evident that(i) the microbial comp one nts of Proterozoic stromat oliti c assemblages have varied system-atically as a func ti on of geologic age and tha t (ii) such commu nit ies are both more a bun da ntand more widespread than had previously been recognized. These observations augur wellfor the futur e use of such assemblages in Precambrian biostratigraphy. At present, however,data are sufficie nt to warran t the provisional establishmen t of onl y a few microfossil-basedsubdivisions of the Proterozoic. Such zones, necessarily relatively long-ranging, are heretentatively defined; it is of interest to note that boundaries between certain of these micro-fossil-based subdivisions appear to coincide, at least approxi matel y, wi th previously sug-gested stromato lite-ba sed boundaries. To some ex tent, therefore, results of this study seemconsistent with, and may be supportive of, the concept of stromatolite-based biostratigraphy.At the same time, however, the study seems to indicate that stromatolites of markedlydiffering age, whe ther of similar or of dissimilar morphology, were proba bly formed bydistinc tly differing microbiotas. Data are as yet in suffici ent to indicate whether differingtypes of coetaneous, stratigraphically useful, stromatoli tes were formed by differing micro-bial communities and to what ext ent the " evol utio n" of stromatolite morphology was aresult of t he biologic evolu tion of stro matol ite-b uildi ng microorganisms. There is thus con-tinued need for investigation of the potentia l biostratigraphic usefulness of stromatoliticmicrobiot as and, especially, for more effective integratio n of results of such studies with


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t h o s e a v ai la b le f r o m s t u d i e s o f s t r o m a t o l i t e s w i t h o u t p r e s e r v e d m i c r o b i o t a s a n d f r o ms t u d i e s o f t h e a c r i ta r c h s p r e s e r v e d i n P r o t e r o z o i c s h al e s.

I N T R O D U C T I O NDuring the past two decades -- and especially within the past several years --there has been a marked increase of interest throughou t the world in thepaleobiology of the Precambrian. There is now a greater number of activeworkers in the field, and a greater influx of fresh data and new ideas, thanever before. As a result of this recent activity, questions regarding the existenceof Precambrian life, questions that were popular and pressing little more thana decade ago, are no longer raised. The question now is not whether evidence

of Precambrian life exists, but rather, what does it say? What does it tell us ofthe course of early evolutionary advance and of the interactions between thedeveloping biosphere and the evolving early environment? Although the fieldis still in its formative stages, considerable progress has been made towardsolution of such problems, progress that has been amply chronicled in recent,comprehensive, review articles (e.g., Glaessner, 1962, 1966; Cloud, 1968, 1974;Schopf, 1970, 1975b; Barghoorn, 1971). Moreover, in recent years numerousspecialists have discussed in detail available evidence bearing on specificaspects of early biologic history: current concepts regarding the origin of lifehave been summarized by Ponnamperuma and Gabel (1968), Kenyon andSteinman (1969) and by Miller and Orgel (1974); the earliest (Archean) fossilrecord has been reviewed by Schopf (1975a, 1976 a, b; data relating tothe mid-Precambrian development of an oxygenic atmosphere have beensummarized by Garrels et al. (1973), Holland (1973) and by Cloud (1972,1973); physiological aspects of early microbial evolution have been treatedby Margulis (1970) and by Broda (1975); the phylogeny of blue-green algae,as reflected in the known early fossil record, has been discussed by Schopf(1974a) and by Cloud et al. (1975); organic geochemical studies of Pre-cambrian sediments have been summarized by McKirdy (1974) and by Kven-volden (1974); the nature and distribution of known Precambrian stromatoliteshave been reviewed by Hofmann (1973), Semikhatov (1974) and by Walter(1976); data relating to the time of origin of the eukaryotic (nucleated) celltype have been summarized by Schopf and Oehler (1976); and the earliestknown records (from the upper Precambrian) of megascopic, multicellularorganisms have been reviewed by Glaessner (1971), Schopf et al. (1973a),Stanley (1976), and by Walter et al. (1976).P R E C A M B R I A N B I O S T R A T I G R A P H Y

From the above summary it is evident that during the past decade thestudy of Precambrian life has grown to encompass a broad spectrum of inter-related subjects. Among such topics, however, is one area of particular import


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145that has remained relatively unexplored, especially on the part of non-Sovietscientists--namely, the possible biostratigraphic usefulness of the early fossilrecord. The first impor tant discovery of Precambrian microorganisms wasmade in the early 1950s in algal laminated (stromatolitic) cherts of the nowfamous Gunflint Iron-Formation of Ontario, Canada (Tyler and Barghoom,1954). Spurred by this discovery, most workers in the United States and inother western countries have concentrated chiefly on cherty stromatoliticfacies of this typ e and on the Precambrian microfossils there contained. Thethree-dimensional preservation of microorganisms in such cherts is often ofexcellent quality. Moreover, since the cherts are readily studied in petro-graphic thin sections, it is possible to demonst rate clearly the indigenousnature (and hence the Precambrian age) of the detected microfossils. Never-theless, studies of this type are tedious and time-consuming and early investi-gations seemed to indicate that such microbiotas were of rare occurrence;prior to 1970, only a handfu l of such assemblages had been discovered ofwhich only two had received detailed atten tion (Barghoom and Tyler, 1965;Schopf, 1968). In addition, and probably of at least equal importance, it hadbecome evident by the late 1960s tha t many Precambrian microfossils were ofsimilar, and in some cases of virtually identical morphology to Phanerozoicand mo de m microorganisms (Schopf, 1967; Schopf and Blacic, 1971). Re-cognition o f this striking degree of morphological evolutionary conservatismled to the ten tative conclusion that Precambrian microfossils would be oflimited use for biostratigraphic correlation (Schopf and Barghoom, 1969).Thus, by 1970 it seemed apparen t that Precambrian biostratigraphy, at leastas based on stromatolite-building microorganisms, might prove infeasible;efforts of most western workers were directed principally toward decipheringthe evolutionary implications, rather than the possible stratigraphic utility, ofthe Precambrian biota.During this same period, studies in the Soviet Union advanced in two ratherdifferent directions. First, several Soviet scientisis, most notably B.V Timo-feev and his colleagues in Leningrad, began comprehensive surveys of micro-fossils extracted by palynological techniques from Precambrian shales of theRussian and Siberian Platforms. In such studies, the microfossils are freedfrom the rocks and concentrated in acid-resistant organic residues. Thus, al-though info rmat ion is lost regarding the original distribution of microfossilswithin the sedimen t (and although such macerations are notoriously suscepti-ble to contaminat ion during preparation), t he carbonaceous microfossils areconcentrated and can be studied relatively rapidly. Based on studies of shalesfrom more than one hundred Precambrian formations, Timofeev (1973) hasshown tha t assemblages of de tec ted microfossils (mostly regarded as acritarchs)vary systematically as a funct ion o f geologic age. At about t he same time,B.M. Keller and his colleagues in Moscow began studies of a dif feren t type,designed to determine whether the temporal distribution of stromatolites ofvarious forms could be used effectively to zone the Precambrian. Workingprincipally in the Russian and Siberian Platforms and using a "purely empirical"


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approach--an approach intended to determine whether, regardless of for whatreason(s), stromatoli tes change with time--these and other Soviet workersamassed data showing that at least some forms of relatively complex, columnarstromatolites seem to exhibit limited range zones (e.g., Semikhatov, 1974). Inthis manner, and although recognizing that stromatolite morphology can beinfluenced markedly by solely physical factors of the environment (Logan,1961; Monty, 1967, 1972; Hofmann, 1969, 1973; Horodyski, 1975), theSoviet studies indicated that such stromatolites might prove useful both forintra- and inter-basinal correlation. In the 1960s, stromatolites thus becamean increasingly accepted tool in the Soviet Union for subdivision and cor-relation o f Proterozoic strata.Thus, by the beginning of the 1970s, three separate approaches--each large-ly untested and each with its peculiar set of apparent strengths and probableweaknesses--appeared to hold some promise for biostratigraphic subdivisionof the Precambrian:(i) S t u d i e s o f s tr o m a t o l i te - f o r m i n g m i c r o b io t a s : assemblages known to bewell-preserved and biologically diverse but thought to be of rare occurrenceand to exhibit little evolutionary change through time.(ii) S tu d ie s o f m ic ro fo ss i l s ex t ra c ted f ro m sh a le s : microfossils presumed tobe of worldwide distribution but little studied outside the Soviet Union and,because of relatively poor preservation and the possibility of contamination ,widely regarded as being of uncertain biological affinities and(or) geologicage.(iii) S t u d i e s o f a s s em b l ag e s o f s t r o m a t o l i t e s : organosedimentary structuresregarded as useful in the Soviet Union but relatively little studied, and of un-certain applicability, elsewhere.More recen t studies have expanded upon this earlier base; as is indicated inTable I, a listing of some of the currently apparent strengths and weaknessesof these potential biostratigraphic tools, each of the three approaches continuesto hold promise. For example, although the biologic factors presumed to haveresulted in the "evolu tion" of stromatolite morphology as yet remain largelyundefined, recent attempts to apply the Soviet stromatolite zonation to com-parably-aged strata of Australia (Walter, 1972; Preiss, 1972, 1973), Nor thAfrica (Bertrand-Sarfati and Raaben, 1970) and North America (Cloud andSemikhatov, 1969; Hofmann, 1969) have met with mixed, but generally en-couraging, results. Moreover, recen t discoveries of cellularly preserved micro-biotas in relatively complex, columnar stromatolites, the type regarded asbiostratigraphically useful, have provided new reason to believe tha t thepresumed biologic underpinning of stromatolite-based biostratigraphy willultimately prove decipherable (Schopf and Sovietov, 1976; Schopf et al., 1977).Similarly, studies of Proterozoic shales have continued to yield promi-sing results. During the past year, for example, the known range zones oflarge, spiny acritarchs (Timofeev et al., 1976) and of flask-shaped, chitinozoar~like protists (Bloeser et al., 1977) have both been extended well into thePrecambrian and it now seems rather well established that diverse assemblages


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TABLEI

AttributesofpotentialbasesforProterozoicbiostratigraphy*

Potentialbasisofzonation

Apparentstrengths

Apparentweaknesses

IMicrofossilsin

AUncompressed,commonlywellpreserved.

stromatoliticcherts

BOfdiversemorphology(include

planktonicandbenthonicforms).

CNotsubjecttocontaminationduring

study(thinsections).

DProvideevidenceofgrowthhabitand

ecologicorganizationneededfor

III,B',below.

A'Occurinrelativelyuncommon(chert)

facies.

B'Notusefulinfieldgeology.

C'Taxonomysomewhatconfused(dueto

post-mortemalteration).

D'Requirerelativelytime-consuming

study.

IIMicrofossilsinshales

AOccurinwidespread,commonfacies.

BOfmoderatelydiversemorphology

(chieflyplanktonicforms).

CCanbestudiedrelativelyrapidly.

DMatrixcanbedatedradiometrically

(whole-rocktechnique).

A'Notusefulinfieldgeology.

B'Commonlycompressed,notwell

preserved.

C'Taxonomyconfused(duetopost-

mortemanddiageneticalteration).

D'Subjecttocontaminationduring

study(macerations).

IIICarbonatestromatolites

AOccurinwidespread,commonfacies.

BUsefulinfieldgeologyforboth

stratigraphicandenvironmental

interpretations.

CCommonlycanberecognizedinsomewhat

metamorphosedterrains.

DModernanaloguesprovidepartialbasisfor

interpretation.

A'Taxonomysomewhatconfused(duein

parttotheinfluenceofphysical

environmentalfactors).

B'Basesof"evolutionary"changeare

undefined.

C'Stratiformanddomalformsgenerally

notapplicable.

D'Identificationofforms("species")

istime-consuming(serialsections).

*ExcludingmegascopicbodyandtracefossilsfromtheuppermostProterozoicandnon-oncoliticmicrophytoUtes.


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o f d i s ti n c t i v e m i c r o f o s s il s o c c u r i n s h al e s d a t i n g b a c k t o a t l e a s t t h e M i d d leR i p h e a n ( T i m o f e e v , 1 9 7 3 ) . M o r e o v e r , in r e c e n t y e a r s , p a l e o b i o l o g i s t s i nw e s t e r n c o u n t r i e s h a v e f o l l o w e d t h e S o v i e t l e ad a n d h a v e b e c o m e i n c re a s in g l ya c t iv e in s t u d i e s o f P r e c a m b r i a n s h a le s (A l l is o n a n d M o o r m a n , 1 9 7 3 ; M o o r -m a n , 1 9 7 4 ; A l l is o n , 1 9 7 5 ; J a v o r a n d M o u n t j o y , 1 9 7 6 ; B l o e s e r e t al. , 1 9 7 7 ) .

P e r h a p s th e m o s t n o t a b l e a d v a n c e s h av e b e e n m a d e , h o w e v e r , in s tu d i e s o fP r e c a m b r i a n , s t r o m a t o l i t e - f o r m i n g m i c r o b i o t a s ; i t is n o w e v i d e n t t h a t s u c h a s-s e m b l a g es a re b o t h m o r e a b u n d a n t a n d m o r e w i d e s p r e a d t h a n h a d p r e v i o u s l yb e e n s u s p e c t e d . A s is s u m m a r i z e d i n T a b l e I I, t w e n t y - e i g h t s u c h m i c r o b i o t a s ,t e m p o r a l l y s p a n n i n g v i r tu a l ly t h e e n t ir e P r o t e r o z o i c , a r e n o w k n o w n f r o mA u s t r a li a , C a n a d a , I n d i a , S o u t h A f r i ca , G r e e n l a n d , t h e U n i t e d S t a t e s a n d t h eS o v i e t U n i o n . O f t h e se , a b o u t 2 0 % w e r e f ir s t r e p o r t e d i n 1 9 7 6 a n d m o r e t h a nt h r e e -q u a r t e r s h a v e b e e n d i s c o v e r e d s i nc e 1 9 7 0 ; o n l y t w o o f th e s e a s s e m b l a g esw e r e k n o w n a d e c a d e a g o . T h is p r o g r e s s is p a r t i c u l a r l y e n c o u r a g i n g ; s t r o m a t o -l it es c o m p r i s e t h e m o s t a b u n d a n t m e g a s c o p i c e v i d e n c e o f b io l o g i c a c t iv i t yn o w r e c o g n i z e d in t h e P r e c a m b r i a n . A s s u c h , t h e y r e p r e s e n t a to o l o f g re a tp o t e n t i a l u s e f u l n e s s t o t h e s t r a t i g r a p h e r a n d f i el d g e o lo g i s t ( T a b l e I ). T h eg e n e r al a p p l i c a b i l it y o f t h i s t o o l , h o w e v e r , w i ll p r o b a b l y r e m a i n i n q u e s t i o nu n l es s th e b i o lo g i c b a s e s o f s t ro m a t o l i t e " e v o l u t i o n " c a n b e fi r m l y d e m o n -s t r a te d . A n d , a s is n o t e d a b o v e , c e l lu l a r ly p r e s e rv e d s t r o m a t o l i t e - b u i l d i n gc o m m u n i t i e s p r o v i d e p o t e n t i a l f o r u l ti m a t e r e s o l u t i o n o f t h is q u e s t i o n . U n -f o r t u n a t e l y , h o w e v e r , o n l y a f e w m i c r o b i o t a s h a v e a s y e t b e e n d i s c o v e r e d inc o l u m n a r s t r o m a t o l i t e s o f t h e t y p e u s e d f o r s t r o m a t o l i t e - b a s e d c o r r e l a t i o n( T a b le I I) . F o r th i s r e a so n , i t c a n n o t y e t b e d e t e r m i n e d w h e t h e r c o m p l e xc o e t a n e o u s s t r o m a t o l i t e s o f d if f e ri n g t y p e w e r e f o r m e d b y d i f f e ri n g m i c ro b i a lc o m m u n i t ie s o r w h e t h e r a s in g u la r c o m m u n i t y m i g h t ha v e f o r m e d d i f fe r e n ts t r o m a t o l i t e s i n d i f f e r e n t e n v i r o n m e n t s . N e v e r t h e le s s , t h e d a t a t h a t a r e a va ila -b l e m a y b e s u f f i c i e n t t o s u g g e s t g e n e r a ll y t h e n a t u r e a n d d e g r e e o f c h a n g ee x h i b i t e d b y s u c h m i c r o b i a l c o m m u n i t i e s d u r i n g t h e P r o t e r o z o i c a n d p e r h a p st o i n d i c a t e w h e t h e r m a j o r c h a n g e s a t t h e m i c r o b i a l le v e l a r e t e m p o r a l l y c o r-r e la t iv e w i t h ( a n d t h u s p e r h a p s c a u s a l l y r e l a t e d t o } t h o s e c h a n g e s i n t h e s t r o-m a t o l i t e s t h a t h a v e f o r m e d t h e b a si s f o r s tr o m a t o l i t e b i o s t ra t ig r a p h y . M o r e -o v e r , i t s e e m s p o s s i b l e , a n d i s p e r h a p s l i k e l y , t h a t t h e m i c r o b i a l c o m p o n e n t so f s u c h c o m m u n i t i e s c o u l d p r o v e t o b e b i o s t ra t ig r a p h i c al ly u s e f u l b y t h e m -s elv es , w i t h o u t r e g ar d t o t h e m o r p h o l o g y o f t h e s t r o m a t o l i t e s i n w h i c h t h e yo c c u r . T h e f o l l o w i n g d i s cu s s i o n is t h u s i n t e n d e d t o s u m m a r i z e a n d c o n s i d e rt h o s e b i o t i c as p e c ts o f P r o t e r o z o i c , s t r o m a t o l i te - b u i l d i n g c o m m u n i t i e s t h a ts e e m e s p e c ia l ly r e le v a n t t o p r o b l e m s o f P r e c a m b r i a n b i o s t ra t ig r a p h y . A t t h eo u t s e t i s s h o u l d b e s t r e e s e d t h a t t h is a n a l y s is is i n c o m p l e t e a n d n e c e s s a r i lys i m p l is t ic ; d e s p i t e t h e p r o g r e s s o f r e c e n t y e a r s ( a n d a l t h o u g h e f f o r t h as b e e nm a d e t o c o n s i d e r h e r e all i n f o r m a t i o n n o w a v a il ab le , b o t h p u b l i s h e d a n d u n -p u b l i s h e d ) , r e l e v a n t d a t a r e m a i n r e l a ti v e l y fe w . T h u s , t h e f o l l o w i n g s y n t h e s i ss h o u l d b e r e g a r d e d a s p r e l i m i n a r y o n l y , a n a n a ly s i s th a t w i ll b e s u b j e c t t o re -v is io n a n d r e f i n e m e n t a s n e w d a t a c o n t i n u e t o b e c o m e a v ai la b le .


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TABLEII

Microbiotuknownfromalgallaminated(Stromatolitic)Precambriansediments

ApproximateAge

GeologicUnit

Locality

FossiliferotmStromatolite.(s)References

(X10'years)

570>675

PortfjeldFm.

southernPearyLand,

(Eocambrian)

Greenland

570>675

ChichkanFn~

southernKazakstan,

(KaroyGrp.)

USSR

675>800

OlkhinFm.

Irkutskregion,Siberia,

USSR

600>1000

KwaguntFm.

GrandCanyon,Arizona,

(ChuarGrp.;WalcottMem.)USA

600>1000

GalerosFm(ChuarGrp.;

GrandCanyon,Arizona,

CarbonCanyonMere.)

USA

675>800

MinyarFn~(KaratauGrp.)

Ufaregion,Bashkiria

USSR

740>870

AuburnDolomite

SouthAustralia

BurraGrp.)

740>870

MyrtleSpringsFm.

SouthAustralia

(BurraGrp.)

740>870

SkillogalceDolomite

SouthAustralia

(BurraGrp.)

beddedcherts(Stratifera?)

andsmalloncolites

Conophytongaubitza

smalloncolites

smalloncolites

cf.Stratifera

cf.Stratifera

Pedersen,1970

SchopfandSovietov,1976

Schopfetal.,1977

Schopf,FordandBreed,1973b;

Bloeseretal.,1977

Schopf,1975b;Bloeseretal.,

1977

Schopfetal.,1977

unknown(disruptedalgal

mats)

columnaranddomai

columnar(e.g.Baicalia)

anddomal

Schopf,1975b;Schopfetal.,

inprep.

Schopf,1975b;Schopfetal.,

inprep.

SchopfandFairchild,1973;Knolletal.,

1975;Fairchild,inprep.;

SchopfandFairchild,inprep.

790>1080

BitterSpringsFn~

centralAustralia

cf.Stratifera

Sehopf,1968;SchopfandBlacic,

~,

(Love'sCreekMem.)

1971;Schopf,1974b

~" .o


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o

TABLEII(cont.)

ApproximateAge

GeologicUnit

(X10'years)

740>870

BlythDolomite

(RiverWakefieldGrp.)

Locality

SouthAustralia

800>950

ShorikhaFro.

Turukhanskregion,

Siberia,USSR

950>1100

ValukhtinFro.

LakeBaicalregion,

Siberia,USSR

1050>1200

SukhotungusinFro.

Turukhanskregion,

Siberia,USSR

1175>1225

DismalLakesGrp.

GreatBearLakeregion,

arcticCanada

1200>1400

BeckSpringDolomite

southeasternCalifornia,

(PahrumpGrp.)

USA

1340>1430

VempalleFro.

south-centralAndhra

(CuddapahGrp.)

Pradesh,India

1390>1575

BalbiriniDolomite

northeasternNorthern

(McArthurGrp.)

Territory,Australia

1390>1575

AmeliaDolomite

northeasternNorthern

(McArthurGrp.)

Territory,Australia

1390>1575

BungleBungleDolomite

northernWesternAustralia

FossiliferousStromatolite(s)

References

unknown(disruptedalgal

mats)

cf.Stratifera

Baicaliahirta

cf.Stratifera

cf.Stratifera

cf.Stratifera

anddomal

domal

cf.Stratifera

unknown(microfossilsin

drillingcores)

cf.Stratifera(?)

anddomal(?)

Schopf,1975b;Schopfand

Fairchild,inprep.Schopf

etal.,inprep.

Schopfetal.,1977

Schopfetal.,1977

Schopfetal.,1977

Schopf,1975b;Donaldsonand

Delaney,1975;Horodyskietal.,

inprep.

Licari,1971

Schopf,1975b;SchopfandPrasad,

inprep.

D.Z.Oehler,pets.comm.,1975

Muir,1974

Diver,1974


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1400>1540

1700>2200

1800>2500

1800>2500

1800>2500

1700>2200

1800>2500

2220>2300

ParadiseCreekFro.

(Mt~IsaGrp.)

DuckCreekDolomite

(WylooGrp.)

KasegalikFrvL

(BelcherGrp.)

McLearyFm.

(Belche~Grp.)

GunflintIronFm.

(AnimikieGrp.)

FrereFm.

PokegamaQuartzite

(AnimikieGrp.)

TransvaalDolomite

(OlifantsRiverGrp.)

northwesternQueensland,

Australia

centralWesternAustralia

southernHudsonBay,

Canada

southernHudsonBay,

Canada

southernOntario,Canada

centralWesternAustralia

northeasternMinnesota,

USA

easternTransvaal,South

Africa

Conophyton,Eucapsiphora,

andCoilenia

cf.8tratifera(?)

el.8tratifera

anddomal

cf.Stratifera

anddomal

Stratiferabiwabikensis,

Osag/a,anddomaland

columnar

ef.8tratiferaandsmall

oncolitas

unknown(microfoasiisin

reentrantsintoerosion

surface)

columnar

Licarietal.,1969;

LicariandCloud,1972

KnollandBarghoorn,1975

HofmannandJackson,1969;

Hofmann,1974;Hofmann,1976

Hofmann,1974;Hofmann,1976

BarghoornandTyler,1965;Cloud,

1965;Hof~mnn,1969;Darby,1974;

AkiyamaandImoto,1975

Walter,1975;Walteretal.,1976

CloudandLicari,1972

Nagy,1974


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STROMATOLITIC MICROBIOTASL i k e m o d e r n m a t - b u i l d in g m i c r o b i a l c o m m u n i t i e s , t h o s e o f t h e P re c a m b r i a n

w e r e d o m i n a t e d b y p r o k a r y o t i c ( n o n - n u c l ea t ed ) , p h o t o a u t o t r o p h i c , m i cr o -o r g an i sm s . I n b o t h m o d e r n a n d f o ss il s t r o m a t o l i t ic c o m m u n i t i e s , t w o m o r -p h o l o g i c c a te g o r ie s o f m i c r o o r g a n i s m s p r e d o m i n a t e : filaments, u s u a l l y t h em o r e a b u n d a n t o f t h e t w o a n d t h e p r in c ip a l m a t - f o rm i n g c o m p o n e n t s o f t h ea s s e m b l a g e ; a n d spheroids, u n i c e l lu l a r m i c r o o r g a n i s m s t h a t o c c u r b o t h s i ng l y( c o m m o n l y r e p r e s e n t i n g p l a n k t o n t h a t h a v e s e t t l e d o n t o t h e m a t s u r fa c e ) a n di n c o lo n i es ( f or m s w h i c h c o m m o n l y c o n t r i b u t e t o m a t f o r m a t i o n ) .T h e m i c r o o r g a n i sm s i n t h e s e c o m m u n i t i e s a re o f e x t r e m e l y s i m p l e m o r -p h o l o g y . T h e f i la m e n t s g e n e r al ly a re u n i s e ri a te a n d u n b r a n c h e d , c o m p o s e d o fa l i n e a r s e r ie s o f u n d i f f e r e n t i a t e d d i s c - s h a p e d t o e l o n g a t e c e l ls (i n b l u e - g r e e na lg ae , t h e p h o t o s y n t h e t i c " t r i c h o m e " ) t h a t c o m m o n l y a re en c l o s ed b y a h y a -l in e , o f t e n la m e l l a t e d , m u c i l a g e n o u s c y l i n d e r ( th e e x t r a c e l lu l a r " s h e a t h " ) .F i l a m e n t d i a m e t e r s t y p i c a l ly r an g e f r o m le ss t h a n 1 t o a b o u t 2 5 p m ; g r o w t ho c c u r s b y c e l lu l a r f i ss i o n a n d a l t h o u g h g r o w t h i s i n d e t e r m i n a t e ( f i l a m e n t l e n g t hn o t b e i n g a g e n e ti c a ll y d e f i n e d t ra i t) , b r e a k a g e a n d c o n s e q u e n t r e p r o d u c t i o nt e n d t o l im i t f i la m e n t s t o le s s t h a n s e v e ra l h u n d r e d m i c r o m e t e r s i n l e n g th .D i f f e r e n t i a t e d i n t e r c a la r y c e ll s ( e. g ., th e e n l a r g e d h e t e r o c y s t s a n d a k i n e t e s o fn o s t o c a c e a n c y a n o p h y t e s ) a n d t e rm i n a l c el ls ( e. g. , t e rm i n a l h e t e r o c y s t s a n dt h e t a p e r e d t e r m i n a l " h a i r s " o f r iv u l ar ia c e an s ) o c c u r i n s o m e s u c h p r o k a r y o t e s ;t h e s e c e ll s, h o w e v e r , a r e o f r ar e o c c u r r e n c e r e l a ti v e t o u n d i f f e r e n t i a t e d c e ll sa n d i n f os s il f i l a m e n t s t h e y a r e d i f f i c u l t t o d i s ti n g u i s h f r o m c e ll s t h a t h a v e b e -c o m e e n la rg e d , t a p e r e d o r o t h e r w i s e m o d i f i e d a s a r e s u lt o f p o s t - m o r t e m d e -g r a d a t i o n ( A w r a m i k e t a l. , 1 9 7 2 ) . I n m o s t e n s h e a t h e d t a x a , t h e f il a m e n t s a r ec o m p o s e d o f a s in g le tr i c h o m i c s t ra n d e n c o m p a s s e d b y a c lo s e ly a d p r e s s e ds h e a t h ; s h e a t h d i a m e t e r s a r e t h u s g e n e r a l ly o n l y s l ig h t l y l a rg e r th a n t h o s e o ft h e e n c lo s e d tr i c h o m e s a n d t h e s h e a th s ( e v e n w h e n e m p t y ) a re c o m m o n l y o fa r e l a ti v e l y c o n s t a n t , c y l i n d ri c a l s h a p e . I n s o m e f o r m s , h o w e v e r , s e v e r a l t ri -c h o m e s c a n o c c u r w i t h i n a s in g le s h e a t h ; i n th e s e f o r m s , s h e a t h s a r e o f c o r -r e s p o n d i n g l y g r e a t e r d i a m e t e r a n d o f m o ~ e ir r eg u l a r o u t l in e . F i n a l ly , t h e t ri -c h o m e s o f s u c h o rg a n is m s c o m m o n l y e x h i b i t t h e c a p a b i l i ty o f m o v e m e n t ,g e n e r a ll y i n r e s p o n s e t o l i g h t o r o t h e r s t i m u l u s , b y a p r o c e s s k n o w n a s " g l id i n gm o t i l i t y " ; t h e i m m o b i l e s h e a th s a r e t h u s le f t b e h in d , o f t e n o c c u r r in g i n ad e n s e l y i n t e r w o v e n , m a t - li k e fa b r i c. T h u s , t h e f i la m e n t s o f s t r o m a t o l i t i c c o m -m u n i t ie s , a l t h o u g h m i c r o s c o p i c in s iz e a n d s i m p l e i n m o r p h o l o g y , e x h i b i tn u m e r o u s f e a t u r e s o f d i a g n o s t i c v a lu e t h a t a r e p r e s e r v a b l e b y p e r m i n e r a l i z a t i o na n d , h e n c e , a r e o f p o t e n t i a l b i o s t r a t i g r a p h i c i n t e re s t . A t p r e s e n t , h o w e v e r , d a t af r o m P r o t e r o z o i c : a s s e m b l a g e s a r e i n s u f f i c i e n t t o e v a l u a t e t h e d e v e l o p m e n t o fm o r e t h a n t h e m o s t r u d i m e n t a r y o f t h e s e p a r a m e t e r s , n a m e l y , c h a n g e s i n t h ed i a m e t e rs o f s h e a t h s a n d t r i c h o m e s o v e r t im e .

T h e sp h e r o i d a l m i c r o o r g a n i s m s o f s t r o m a t o l i t i c b i o c o e n o s e s , c o m m o n l ys h e a t h - e n c l o s e d a n d l es s t h a n 1 t o a b o u t 3 0 ~ m i n d i a m e t e r , o c c u r s i n g ly ( F i g . l ,A C , E H , M a n d N ) o r i n c o l o n i e s o f i r re g u l a r ( F i g . l , D , K , L , O ) o r r e g u l a r


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form (e.g., flat plates, quartets, rosettes, cubes, etc.; Fig.l, I and J). In fossil-ized material, the original cell wall surface texture of such spheroids is com-monly altered (Fig.l, B, C, M, N) and is thus of limited taxonomic value. Incontrast, colony for m is a preservable diagnostic featuer, the evolution ofwhich may eventually prove useful for biostratigraphic purposes. At present,however, data f rom the Proterozoic are sufficient only to evaluate the biostra-tigraphic potential of t he evolution in cell size of such spheroids over time.The following discussion will therefore deal primarily with the (i) spheroidalunicells, (ii) cylindrical sheaths and (iii) cellular filaments now known fromProterozoic microbiotas and, specifically, with size trends suggested by availa-ble data for these th ree categories of microfossils. The data will thus be treatedprincipally from a morphologic, rather than a taxonomic or phylogenetic,point of view. Clearly, this is an artificial and less than satisfactory approachsince it serves to group together for analysis organisms of disparate biologicalaffinities. The Proterozoic unicells, for example, almost certain ly represent awide range of spheroidal bacteria, blue-green algae and early eukaryotes andmay include reproductive cells of various origins and(or) spheroidal, ext inct,microorganisms only distantly related to modern biologic groups. Similarly,the sheaths and filaments seem likely to be of rather disparate biologic origin.Unfortunately, however, the data now available do not permit realisticassessment of size trends within well-defined taxo nomic categories-- the dataare simply too few, and understanding of early evolution too limited, toattempt such a feat. Thus, at best, this study can be expected only to revealbroad morphologic trends, trends tha t seem likely to reflec t principallypatterns of early prokaryotic diversification.Spheroidal unicells

In Fig.1 are shown examples of solitary and colonial uniceUs from stroma-tolitic deposits of Early Proterozoic (Fig.l, A--C), Late Riphean (Fig.l, D-L)and Vendian age (Fig.l, M--O). Available size data for more than 7800 suchcells, measured in petrographic thin sections of sediments from 20 Proterozoicformations, are plot ted in Yig.2. As is there shown, unicells repor ted f romsediments older than abou t 1400 Ma are of small dimensions (1--35 ~m indiameter with an average diameter of about 5 ~m); most fall in a modal sizegrouping between about 2 and 7 ~m. In contrast, assemblages younger thanabout 1400 Ma contain relatively large unicells (up to 80 ~m in diameter);cells in these microbiotas have an average diamete r of about 13 um and mostfall in a modal size grouping between about 6 and 18 ~m. A similar increase incharacteristic cell size, also beginning at about 1400 Ma ago, has been ob-served in fossil unicells extracted from Proterozoic shales (Timofeev, 1973).This size increase thus appears to be evidenced in geographically widespreadunits and in two quite d ifferent facies; judging from data now available, itappears to represent a promising basis for biostratigraphic subdivision.Although the cause of this rather marked increase in cell size is somewhat


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F i g .1 . O p t i c a l p h o t o m i c r o g r a p h s s h o w i n g o r g a n i c a l ly p r e s e r v e d , p e r m i n e r a l i z e d , s p h e r o i d a lm i c r o f o s s i ls i n p e t r o g r a p h i c t h in s e c t i o n s o f s t r o m a t o l i t i c c h e r t f r o m t h e E a r l y P r o t e r o z o i cG u n f l i n t I r o n - F o r m a t i o n ( A - - C , B a r g h oo r n a n d T y l e r , 1 9 65 ) ; f r o m t h e L a t e R i p h e a n B l y t hD o l o m i t e ( D , S c h o p f , 1 9 7 5 b ) , S k i l l o g a le e D o l o m i t e ( E - - H , F a i r c h i l d , in p r e p . ; I , J , S c h o p fa n d F a i r c h i l d , 1 9 7 3 ) a n d B i t t e r S p r in g s F o r m a t i o n ( K , L , S c h o p f , 1 9 6 8 ) ; a n d f r o m t h eV e n d i a n C h i c h k a n F o r m a t i o n ( M - - O , S c h o p f a n d S o v i e to v , 19 7 6 ). P a r ts E th r o u g h H m a ys h o w s t a g e s in c e l l d i v i s i o n o f a s in g l e t a x o n ( F a i r c h i l d , i n p r e p . ) ; p a r t s M a n d N s h o w s u r -f i c ia l v i e w s a n d m e d i a l o p t i c a l s e c t i o n s o f s i n g l e s p e c i m e n s .


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S P H E R O I D A L A L G A L M I C R O F O S S I L SS O L I T A R Y A N D C O L O N I A L U N I C E L L S M E A S U R E D I N P E TR O G R AP H IC T H I N S E C T I O N S

O F A L G A L L A M I N A T E D ( S T R O M A T O L I T I C ) P R EC A M B RI AN S E D I M E N T S

G E O L O G I C U N I Tb CHICHKAN FM. , U S S .R .CHUAR GRP (2 FMS.), U.S.A.AUBURN DOLOMITE , AUSTMYRTLE SPRINGS FM. , AUSI~B ITTER SPRINGS FM. , AUBT.) SK iLLOGALEE DOLOMITE , AUSTI BLYTH DOLOMITE , AUST.01SMAL LAKES GRP. , CANADA' BECK SPRING DOLOMITE , U .SA .VEMPALLE FM . , INDIAI McARTHUR GRP (3 FMS. ) AUST.PARADISE CREEK FM., AUSTBELCHER GNP ( 2 F M S . ) , CANADAGUNFLINT Fe " FM , CANADAF R E R E F M , A U B TTRANSVAAL DOLOMITE , AFRICA

C E L L D I A M E T E R ( N , m )~ o 4 ~ ~ o ~ o i~o ~

I --=:xu= A , I N .~ "~ N=ZlSOg,t lFMS......~ : - : " = : . ! i ' ~ I R A , ~ . , - = , . , .

" 2 2 % 1 5 F ~; ",~ I , B Y .> zoo,.,0.6% > 55 /=mN " 3 0 6 0 , B FMS.R A N G E I -S S / ~ nM E A N " B p m

. - " i n : ~ . . > , s . . . . . . i0 . 4 % > z q , ,m IO % > 5 S p m I ' , , = ~ - ' RAI~iGEmi i i [ i i0 20 40 60 80 t00 t20M I C R O N S

A G E(x ~0=%>lSpm %>20/de %>5,~u .m YEARS|z ~ o ~ 2 5 ~ 0 r ~ ; ~ ; , - - 0

- 5 0 ~im ~ I L

r - F _ , - 1 0 0 0I I I - - I ~

- 2 5 0 (o 255o75o L~5o]~ o I 2 $ 4~ I NT ~ I N T ~ KI NT

~ r

~R

Fig.2. Size ranges, me an cell sizes and mod al size groupings ("m ode ") of simple, spheroidalmicrofossils kno wn from the twent y Proterozoic formations indicated. Data are taken fromthe references listed in Table II.

14- M O D E R N S P H E R O I D A L A L G A E

~.Xt / Classff icahon of Desikachory, lgS 9

t-Zul0fl~ 4 1 1 I G R E E N A L G A E ( E U K A R Y O T E S )I o f P r e s c o t t , 1 9 5 I

o ~ o ~ ,~ 3 '0 4 '0 ~ o 6 0 1 2 o 3 5 0C E L L D I A M E T E R ( F m )Fig. 3. Size distr ibut ions of mo dern, spheroidal, pro karyot ic and eu kary otic uni cellula ralgae. Comparison of the t axo nom y of Desikachary (1959) with that of Drouet and Daily(1956), a nd the t axo nom y of Prescott (1951) with that of Lindau and Melchior (1930),indicates that these patter ns of size distri buti on are not artifacts of classification.

u n c e r t a i n , s e v e ra l li n e s o f e v i d e n c e s u g g e s t t h a t i t m a y r e f le c t t h e a d d i t i o n o fe u k a r y o t i c u n i c e l l s t o a p r e v i o u s l y e n t i r e ly p r o k a r y o t i c b i o t a ( S c h o p f a n dO e h le r , 1 9 7 6 ) . A s i s s h o w n i n F i g .3 , m o d e m t a x a o f s p h e r o id a l c y a n o p h y t e sr an g e i n d i a m e t e r f r o m l e ss th a n 1 t o a b o u t 5 5 p m , b u t ar e c o m m o n l y a b o u t4 p m i n d i a m e t e r a n d ra r el y h a v e a m a x i m u m c e ll s iz e g re a te r th a n 1 5 p m( a b o u t 1 0 % a re g re a te r th a n 1 5 # m a n d 6 % a re gr e at e r t h a n 2 0 ~ m ) . S i m i la r l y ,


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a s is i l l u s tr a te d i n F ig .4 , m o d e m t a x a o f s p h e r o i d a l p h o t o s y n t h e t i c b a c t e r iaa r e o f s m a ll d i m e n s i o n s , c o m m o n l y b e i n g b e t w e e n a b o u t 2 an d 5 g m in dia -m e t e r . T h u s , i n c el l s iz e, m o d e m s p h e r o i d a l p r o k a r y o t e s a r e s i m i la r t o th eu n i c e ll s m e a s u r e d i n f o s s il s t r o m a t o l i t i c a s s e m b l a g e s o l d e r t h a n a b o u t 1 4 0 0M a . S u c h is n o t t r u e , h o w e v e r , o f m a n y o f th e P r o t e r o z o i c u n ic e ll s y o u n g e rt h a n a b o u t 1 4 0 0 M a ; m o s t o f t h e y o u n g e r a s se m b l a g es c o n t a i n c e lls in th e 6 0t o 8 0 p m s iz e r a n g e ( l ar g er t h a n d e s c r i b e d p r o k a r y o t e s ) a n d v i r t u a l l y al l h a v em o d a l si ze g ro u p i n g s a t s u b s t a n t i a l l y l a rg e r d i a m e t e r s t h a n t h o s e o f t h e o l d e rf o r m a t i o n s ( F i g .2 ) . I n s iz e , h o w e v e r , t h e s e l ar g er u n i c el ls a r e c o m p a r a b l e t om o d e m t a x a o f s p h e r o id a l , e u k a r y o t i c , c h l o r o p h y t e s w h i c h ra n g e f r o m 1 t oa b o u t 3 5 0 ~ m in d i a m e t e r ( F i g .3 ; a b o u t 5 0 % a re g re a t er t h a n 1 5 ~ m a n d 3 5 %a r e g r e a t e r t h a n 2 0 p m ) . T h u s , i t s e e m s r e a s o n a b l e t o p o s t u l a t e t h a t t h e i n c r e as ei n c e ll s iz e e x h i b i t e d b y s t r o m a t o l i t i c u n i c el ls a t a b o u t 1 4 0 0 M a a g o m a y r e -f l e c t t h e o r ig i n, o r p e r h a p s t h e e a r l y d i v e r s i f ic a t i o n o f u n i c e ll u l a r e u k a r y o t i cm i c r o o r g a n i s m s . A d d i t i o n a l d a t a c o n s i s t e n t w i t h t h is i n t e r p r e t a t i o n a r e s u m -m a r i z e d i n F i g . 5 . A s is t h e r e i l l u s t r a te d , u n i c e l l u l a r m i c r o f o s s i l s e x t r a c t e df r o m P r o t e r o z o i c s h a l es a re r e p o r t e d t o e x h i b i t a n a p p r o x i m a t e l y f iv e - fo l d in -c r ea s e in t a x o n o m i c d i v e r s i t y b e t w e e n a b o u t 1 4 0 0 a n d 1 1 0 0 M a a g o ( T i m o f e e v ,

40-

.30-

X


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1973); this rather abrupt increase in diversity might well reflect a relativelyrapid phase of diversification early in the evolution of primitive eukaryotes.Other evidence supportive of such an interpretation has recently been summa-rized by Schopf and Oehler (1976).In sum, data now available appear to indicate that a t abou t 1400 Ma agothere was a rather marked increase in the average cell size, size range andrange of modal size groupings exhibited by unicellular microorganisms oc-curring in stromatolit ic deposits (Fig.2). Several lines of evidence suggest thatthis increase may reflect the addition of eukaryoti c algae to a previously pro-karyot ic biota. Regardless of cause, however, the size increase appears to re-present a promising biostratigraphic marker; as such, it is of interest to notetha t the increase appears to coincide, at least approximately, with the stro-matolite-based boundary be tween the Early and Middle Riphean (Fig.2).Cylindrical sheaths

In Fig.6 are shown tubular, cylindrical sheaths preserved in stromatoliticmicrobiotas of Early Proterozoic (Fig.6, A), Late Riphean (Fig.6, J) andVendian age (Fig.6, K); for comparison, in Fig.6, M, is shown a modem,sheath-enclosed, oscil latoriacean blue-green alga. In Fig.7 are summarized sizedata report ed fo r sheaths from 14 Proterozoic assemblages; the sheaths exhibita gradual, but substantial, increase in max imum diameter from abou t 10 ~min the Early Proterozoic to about 30 ~m in the Vendian.Three stratigraphic subdivisions based on sheath size ranges are tentativelysuggested in Fig.7; although arbitrarily delimited, the subdivisions could beof phylogenetic significance. As is illustrated in Fig.4, mo de m taxa of sheathedbacteria are enclosed by sheaths with diameters up to about 9 ~m; these bac-terial sheaths thus exhibit a size range approximately coincident with that ofreported fossil sheaths 1500 Ma in age and older. Since the oldest assured'fossil oscillatoriaceans are also of about this age (Diver, 1974; Plumb andSweet, 1974), it seems conceivable tha t sheaths older than 1500 Ma co ul db eentirely of bacterial origin. It is similarly possible, however, that oscillato-~

riaceans may have been extant much earlier; many mo de m members of thefamily have sheaths in the 1 to 10 ~m size range (Fig.8) [and sheaths some-what larger than 10 ~m have apparently been de tected, but are as ye t unre-ported, from Early Proterozoic cherts (S.M. Awramik, personal communi-cation, July, 1976)].As for the younger two subdivisions indicated in Fig.7, the size ranges ex-hibited by mo de m microorganisms suggest that sheaths younger than about1500 Ma are almost certainly not solely of bacterial origin. Indeed, there ismuch evidence (Schopf, 1974a; Schopf and Sovietov, 1976) indicating that,like most sheaths in mode m algal mat communities, the majo rity o f theseyounger fossil sheaths were derived from oscillatoriacean blue-green algae.Comparison of the range of sheath diameters exhibited by modem oscillato-riaceans (Fig. 8) with those characterizing the younge r two subdivisions of the


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Proterozoic (Fig.7) seems to indicate, as has been suggested on o ther grounds(Schopf, 1974a), that the OsciUatoriaceae was a well established and highlydiverse group during the late Precambrian.In summary, extracellular sheaths now known from stromatolitic Protero-zoic microbiotas exhibit a gradual, approximately three-fold increase in maxi-mum diameter over time. Although sheaths older than about 1500 Ma couldbe entirely of bacterial origin, they may also have been derived from ancientcyanophytes. Younger sheaths appear to be principally of oscillatoriaceanorigin and the evidence indicates tha t members o f the Oscillatoriaceae havebeen important, and in many cases predominant, components of algal bio-coenoses from the Middle Riphean to the present.

Cellular filamentsIn Fig.6 are shown representative cellular filaments from s tromatoli tic as-semblages of Early Proterozoic (Fig.6, B--D), Middle Riphean (Fig.6, E),Late Riphean (Fig.6, F--I) and Vendian age (Fig.6, L). In Fig.9 are summa-rized available size data for such filaments reported from 18 Proterozoic as-semblages. As is true also of the other charts summarizing the size distributionsof microfossils (Figs.2 and 7), the size ranges indicated by continuous lines inFig.9 are based on known fossil microorganisms; they are therefore presumablyminimum ranges only. It is probable that larger microfossils, not yet detec ted

and(or) repor ted, occur in many o f these assemblages. For the cellular fila-ments, this supposition seems supported by the occurrence in some assemblagesof large empty sheaths which seem almost certain to have initially enclosedtrichomes broader than those yet r eported from the deposits. As is shown inFig.10, in modern monot richomic oscillatoriaceans there is a rather regular re-lationship between trichome breadth and the diameter of the enclosing sheath.Thus, if it is assumed that the fossil sheaths similarly contained only singletrichomes (an assumption apparently justified by the regular, cylindrical out-lines of such sheaths; e.g., Fig.6, K), and if it is assumed that the size relation-ship between sheaths and enclosed trichomes in Proterozoic prokaryotes wassimilar to that exhibited by their modem analogues (an assumption consistentwith the fossil evidence available), then the observed occurrence of broad

Fig.6. Optical photomicrographs showing organically preserved, permineralized, cellularfilaments (B--I, L) a n d s h e a t h s (A, J, K) in petrographic thin sections of s t r o m a t o l i t i c c h e r tfrom the Early Proterozoic Gunflint Iron-Formation (A, B, D, Barghoorn and Tyler, 1965;C, Schopf, unpublished); from the Middle Riphean Dismal Lakes Group (E, Schopf, 1975b);from the Late Riphean Skillogalee Dolomite (F, Fairchild, in prep.) and B i t t e r S p r i n g sF o r m a t i o n (G, I, Schopf a n d B l a c ic , 1971 ; H, J, Schopf, 1968 ); and from t h e V e n d i a nChichkan Formation (K, Schopf et al., 1977; L. Schopf and Sovietov, 1976). For com-parison, t h e m o d e r n o s c i ll a t o r ia c e a n Lyngbya majuscula Harvey ex Gomont i s s h o w n inpart M. (Schopf unpublished) Parts E--I, K and L are photomontages; part K shows a sur-ficial view and a medial optical section of a s i n g l e s p e c i m e n .


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Fig. 11. Size distributions of trichome diameters of modern, cylindrical, gliding bacteria(Buchanan and Gibbons, 1974), and of modern nostocacean and oscillatoriacean blue-greenalgae (Desikachary, 1959).y e t s u b s t a n t ia l , i n c r e a s e in a v e ra g e s iz e a n d m a x i m u m d i a m e t e r d u r i n g t h e P r o -t e r o z o i c . L i k e t h e s iz e d a t a f o r s h e a t h s d i s c u s s e d a b o v e , t h e d a t a f o r c e l lu l a rf i l a m e n t s h a v e b e e n u s e d t e n t a t i v e l y a n d r a t h e r a r b i t ra r i l y t o s u b d i v i d e t h eP r o t e r o z o i c i n t o t h r e e m a j o r z o n e s ( F i g .9 ) . T h e e a r l ie s t o f t h e s e z o n e s , in -c l u d i n g a s se m b l a g e s a b o u t 1 5 0 0 M a i n a g e a n d o l d e r , is c h a r a c t e r i z e d b y v e r yn a r r o w f i l a m e n ts , c o m m o n l y o n l y 1 t o 2 . 5 p i n d i a m e t e r ( F i g .6 , B a n d C ). T h eb i o l o g i c a l a ff i n it ie s o f t h e s e f i l a m e n t s a r e n o t w e l l e s t a b l i s h e d ; a s i s s h o w n i nF i g . l l , t h e y a r e s i m i la r i n d i a m e t e r t o m o d e r n g l id i n g b a c t e r i a ( e s p e c i a l l y t ot h o s e t a x a n o t o b v i o u s l y d e r i v e d f r o m b l u e - g re e n a lg al p r e c u r s o r s ) a n d t on a r r o w s p e c i e s o f t h e O s c i l l a to r i a c ea e . T h e o c c u r r e n c e o f e n la r g e d c e l l- li k es t r u c t u r e s i n t e r p r e t e d a s h e t e r o c y s t s a n d a k i n e t e s i n c e r t a in o f t h e s e f o s s ilf i la m e n t s h as le d s o m e w o r k e r s t o r e ga r d t h e m a s n o s t o c a c e a n c y a n o p h y t e s( L i ca r i a n d C l o u d , 1 9 6 8 ; C l o u d a n d L i c a ri , 1 9 7 2 ) ; h o w e v e r , i t s e e m s p o s s i b l et h a t t h e p u t a t i v e h e t e r o c y s t s a n d a k i n e t e s m i g h t a c t u a l l y b e p r e s e r v a ti o n a la r t i fa c t s ( S c h o p f , 1 9 7 5 b , p p . 2 2 6 - - 2 2 7 ) a n d i t is e v i d e n t th a t i n g e n e r a l t h e s ef i la m e n t s a r e o f s u b s t a n t ia l ly s m a l l e r d i a m e t e r t h a n m o s t m o d e r n m e m b e r s o ft h is g r o u p ( F i g .1 1 ) . J u d g i n g f r o m f i l a m e n t d i a m e t e r s , i t s e e m s p r o b a b l e t h a ti f n o s t o c a c e a n s w e r e e x t a n t p r i o r t o 1 5 0 0 M a ag o , th e f a m i ly w a s s u b s t a n t ia l lyl es s d i v er s e, a n d w a s c h a r a c t e r i z e d b y m u c h m o r e n a r r o w t a x a , t h a n is t h eN o s t o c a c e a e o f t h e m o d e r n f lo r a.


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T h e r e m a i n i n g t w o , y o u n g e r , s u b d i v i si o n s o f t h e P r o t e r o z o i c a r e c h a r a ct e r-i z e d b y c e l l u l ar f il a m e n t s o f i n c r e a si n g l y b r o a d e r d i a m e t e r ( F i g . 9 ). C e r t a in o ft h e s e b r o a d e r f il a m e n t s , s u c h a s t h o s e r e p o r t e d f r o m t h e p r o b a b l y U p p e rR i p h e a n ( W . V . P r ei ss , p e t s . c o m m . , 1 9 7 5 ) S k i l lo g a l e e D o l o m i t e o f A u s t r a li a( F ig . 6, F ; F a ir c hi ld , i n p r e p .) , m a y b e o f e u k a r y o t i c ( c h l o r o p h y c e a n , p o s s i b l yu l o t r i c h a c e a n ) a f f i n it ie s . O t h e r s , h o w e v e r , s u c h a s m a n y o f t h e f i l a m e n t s d e -t e c t e d i n t h e U p p e r R i p h e a n O l k h i n F o r m a t i o n ( S c h o p f e t a l. , 1 9 7 7 ) a n di n t h e V e n d i a n C h i c h k a n F o r m a t i o n ( F ig . 6, K a n d L ; S c h o p f a n d S o v i e t o v ,1 9 7 6 ) o f t h e S o v i e t U n i o n s e e m c e r t a in t o b e o f o s c i l l a to r i a c e a n a f f i n i ti e s .I n d e e d , t h e l ar g e s iz e o f m a n y o f t h e o s c i l la t o ri a c e an t r i c h o m e s d e t e c t e d i nt h e s e t w o f o r m a t i o n s , 2 0 t o n e a r l y 3 0 ~ m i n d i a m e t e r ( F i g . 9 ) a n d l ar g er t h a na ll b u t a f e w s p e c ie s o f e x t a n t m e m b e r s o f t h e f a m i l y ( F i g . l l ) , s u g g es ts th a tt h e f a m i l y w a s p r o b a b l y a b o u t a s d iv e rs e in t h e l a t e P r e c a m b r i a n a s is t h eO s c i U a t o ri a ce a e o f th e m o d e r n f lo r a.

I n s u m m a r y , l i k e t h e c y l i n d r i c a l fo s s i l s h e a t h s d i s c u s s e d a b o v e , t h e c e l lu l a rf i la m e n t s k n o w n f r o m P r e c a m b r i a n s t r o m a t o l i t i c b i o t a s e x h i b i t a n e v i d e n t in -c r e a s e i n s iz e r a n g e a n d i n a v e ra g e d i a m e t e r d u r i n g t h e P r o t e r o z o i c . F u r t h e rs t u d i e s w i ll b e r e q u i r e d t o i n d i c a t e w h e t h e r t h i s s iz e i n c r e a s e o c c u r s in ag r ad u a l, c o n t i n u o u s f a s h io n , o r in a m o r e a b r u p t , s t ep - l i k e m a n n e r , a n d t od e t e r m i n e t h e m o s t u s e f u l p l a c e m e n t o f b o u n d a r i e s b e t w e e n s iz e r a ng e z o n e s.A t p r e s e n t , h o w e v e r , l i k e t h e d a t a f o r t h e f o s s il s h e a t h s , t h e s i z e t r e n d s f o r t h ec e l l u l a r f i l a m e n t s h a v e b e e n u s e d t e n t a t i v e l y t o p r o p o s e a t r i p a r t i t e s u b d i v i s i o no f t h e P r o t e r o z o i c . T h e e a r l i e st s u b d iv i s io n , i n c lu d i n g k n o w n m i c r o b i o t a s1 5 0 0 M a i n a g e a n d o l d e r , is c h a r a c t e r i z e d b y v e r y n a r r o w f i l a m e n t s t h a ta p p e a r t o b e o f b a c t e r i a l a n d p o s s i b l y c y a n o p h y t i c ( v iz ., o s c i l l a t o r ia c e a n ) a f-f in i ti es . I n c o n t r a s t , t h e y o u n g e r s u b d i v i s i o n s a r e c h a r a c t e r i z e d b y l a rg e rf i la m e n t s ; a l t h o u g h p o s s i b l y in c l u d in g a ls o f i l a m e n t o u s e u k a r y o t e s , t h e a s-s e m b l a g e s o f t h i s ag e a r e d o m i n a t e d b y m a t - b u i l d i n g b l u e - g r e e n a lg a e.P o t e n t i a l i n d e x f o s s i l s

I n F i g .1 2 a r e s u m m a r i z e d t h e k n o w n g e o l o gi c d i s t r ib u t i o n s o f th e v a ri o u sm o r p h o l o g i c a l c a t e g o r i e s a n d s u b c a t e g o r i e s d i s c u s s e d a b o v e , t o g e t h e r w i t h al is ti n g o f s e v e n P r o t e r o z o i c t a x a t h a t a t p r e s e n t a p p e a r t o h o l d p r o m i s e a sp o t e n t i a l i n d e x f o ss il s f o r v a r i o u s p o r t i o n s o f t h e P r o t e r o z o i c . T h e s e t a x a ,s e l e c t e d b e c a u s e o f th e i r d i s t i n c ti v e m o r p h o l o g i e s , t h e i r r e la t i v e ly l im i t e dk n o w n r a n g e z o n e s , a n d t h e ir a p p a r e n t l y w i d e s p r e a d d i s t r ib u t i o n ( al l b u t A r -c h a e o r e s t i s h a v i n g b e e n r e p o r t e d f r o m t w o o r m o r e m i c r o b i o t a s ) , a r e i l l u s t r a t e di n F ig . 1 3 . F u r t h e r s t u d i e s w i ll b e r e q u i r e d t o d e f i n e b e t t e r t h e d i s t r i b u t i o n ( inb o t h s p a c e a n d t i m e ) o f t h e s e f o r m s a n d t h u s t o d e t e r m i n e w h e t h e r t h e y w i llu l t i m a t e l y p r o v e u s e f u l f o r b i o s tr a t ig r a p h i c p u r p o s e s . A s is s u m m a r i z e d i nF i g .1 2 , i t n o w s e e m s e v i d e n t t h a t s t r o m a t o l i t ic m i c r o b i a l c o m m u n i t i e s e x -h i b i t e d a m a r k e d i n c r e a s e i n d i v e r s i t y d u r i n g t h e P r o t e r o z o i c , e s p e c i a l l y b e -g in n in g a b o u t 1 4 0 0 + 1 0 0 M a a g o .


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R e f e r e n c e s f , N o g y . 1 9 7 4 , 2 , L i c o r i , P h D t h e s ~ s , ~ 9 7 4 , 3 , L m o f e e v , G e r m a n 8 . M i k h o y l o v o , 4 9 7 6 ; 4 , T i n n o f e e v, 1 9 7 3 , 5 , H o f m o n n , 4 9 7 4 ; 6 , H o f m o n n , m p r e s s , 7, b c o r h C l o u d 8$ m d h , I g 6 g , 8 , O e hle r , O e h le r & M u i r , t g 7 5 ; 9 , Horody s k i , S c hopf 8 = Dona lds on , in p r e p , tO , S c h o p f & S O v l e to v , 4 9 7 6 , H , D i v e r , 1 9 7 4 , f2 , B o r g h o o r n 8 T y $ e r , 4 9 6 5 , f3 , S c h o p f& P r o s o d , i n p r e p , f4, S c h o p f , F o i , c h i l d B P r e i s s , i n p r e p , I S , S c h o p f , D o t n i k , K r y l o v , M e n B e l s o n , N o z o r o v , N y b e l g , S o v l e f o v 8 Y o k s h m , i n p r e s s , f B, F a i r c h i l d .in p r e p , i 7 , H o f m o n p

J a c k s o n, 4 9 6 9 , f ~ W a l t e r , 49 7 5 , / 9 , K n o l l & B ( ] r gh o o r n, t 9 7 5 , 2 0 , K h n e , M S c t h e s i s , 1 9 7 5 ; 2 f , S c h o p l ~ F a i r c h i l d , , n p r e p , 2 2 , F o r d ~ B r e e d , 4 9 7 3

F i g. 1 2 . C h a r t s u m m a r i z i n g t h e k n o w n g e o l o g i c d i s t r i b u t i o n s o f t h e v a r io u s m o r p h o l o g i c a lc a t e g o r ie s o f P r o t e r o z o i c m i c r o f o s s i l s d is c u s s e d i n t h e t e x t .

C O N C L U S I O N ST h e d a ta p r e s e n t e d h e r e a m p l y d e m o n s t r a t e t h a t t h e m i c r o b i a l c o m p o n e n t s

o f P r o t e r o z o i c s t r o m a t o l i t ic b i o c o e n o s e s h a v e v ar ie d sy s t e m a t i c a l l y a s af u n c t i o n o f g e o l o g i c a g e . T h is c o n c l u s i o n , t o g e t h e r w i t h t h e r e a l iz a t io n t h a ts u ch c o m m u n i t i e s a re a p p r ec ia b ly m o r e w i d e s p r ea d a n d m o r e c o m m o n t h anh a d p r e v i o u s l y b e e n s u s p e c t e d , i n d i c a t e s t h a t s u c h a s s e m b l a g e s a re o f p o t e n t i a lb i o s t r a t i g r a p h i c u s e f u l n e s s . A t p r e s e n t , h o w e v e r , d a t a a r e t o o f e w a n d t h eb i o l o g i c a f f i n i t i e s o f P r e c a m b r i a n m i c r o f o s s i l s t o o p o o r l y u n d e r s t o o d , t op e r m i t m o r e t h a n a p r e l i m i n a r y e v a l u a t i o n o f t h is p o t e n t i a l ; t h e p r e s e n t s t u d yi s t h u s c o n c e r n e d m o r e w i t h m a j o r m o r p h o l o g i c c h a n g e s i n t h e b i o t i c c o m -p o n e n t s o f s t r o m a t o l it i c c o m m u n i t i e s t h a n w i t h t h ei r d e t a il e d p h y l o g e n e t i cr e l a t io n s h i p s . N e v e r t h e l e s s , t h i s a p p r o a c h h a s r e v e a l ed s i z e t r e n d s i n a l l o f t h et h r e e p ri n c ip a l t y p e s o f e a r ly m i c r o f o s s il s n o w k n o w n , t r en d s t h a t h a v e p e r-m i t t e d t e n t a t iv e s u b d i v i si o n o f t h e P r o t e r o z o i c i n t o s ev e ra l m a j o r r a n ge z o n e s .T h e b a s e s o f t h e s e m i c r o f o s s i l - b a s e d s u b d i v i s i o n s a n d t h e i r d u r a t i o n s a r es u m m a r i z e d i n F i g . 1 4 , i n w h i c h t h e y a r e c o m p a r e d w i t h s u b d i v i s i o n s p r e v i o u s -l y e st a b l i s h e d o n t h e b a s is o f t h e d i s t r i b u t i o n o f fo s s i l s t r o m a t o l i t e s . S o m e


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F i g. 13 . O p t i c a l p h o t o g r a p h s ( H , L , M ) a n d p h o t o m i c r o g r a p h s ( A - - G , I - - K ) s h o w i n g p o t e n t i a li n d e x f o s si ls n o w k n o w n f r o m P r o t e r o z o i c s t r a t a ( se e F ig . 1 2 f o r k n o w n r a n g e z o n e s) .Figured foss i l s inc lude Eosphaera ( A , B a r g h o o r n a n d T y l e r , 1 9 6 5 ) ; Archaeorestis (B, C,B a r g h o o r n a n d T y l e r , 1 9 6 5 ) ; Kakabekia ( D , B a r g h o o r n a n d T y l e r , 1 9 6 5 ) ; Eoastrion (E, F ,B a r g h o o r n a n d T y l e r , 1 9 6 5 ) ; Palaeosiphonella (G, Fa i r ch i ld , i n p rep . ) ; " P o ly be ssu rus "(H- - J , S cho pf and Fa i r ch i ld , i n p rep . ; K , Schop f , 197 5b) ; a nd Beltanelloides, cf. Chuaria-Kildinella (L , M , Soko lov , 1972) .


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F i g .1 4 . C h a r t s u m m a r i z i n g s i z e d a t a f o r t h e t h r e e m a j o r c a t e g o r i e s o f P r o t e r o z o i c m i c ro -f os si ls ( c el l ul a r f i l a m e n t s, s h e at h s , a n d s p h e r o i d a l u n ic e ll s ) a n d c o m p a r i n g t e n t a ti v e l y s u g -g e s t e d m i c r o f os s i l - b a s e d s u b d i v i s i o ns o f t h e P ro t e r o z o i c w i t h s u b d i v i s i o n s b a s e d o n t h ed i s t ri b u ti o n o f c o l u m n a r s t r om a t ol i t es .

c o r r e s p o n d e n c e b e t w e e n t h e s e t w o s e t s o f s u b d iv i s io n s m i g h t b e e x p e c t e d s in c eo n e s e t i s b a s e d o n s t r o m a t o l i t e m o r p h o l o g y a n d t h e s e c o n d i s b a s e d o n t h ef o s s i l m i c r o o r g a n i s m s t h a t o c c u r w i t h i n s u c h s t r u c t u r e s a n d t h a t w e r e r e -s p o n s i b l e f o r t h e ir f o r m a t i o n . A t t h e s a m e t i m e , h o w e v e r , s u c h c o r r e s p o n d e n c em i g h t b e e x p e c t e d t o b e r a t h er i n e x a c t s i n c e ( i ) o n l y a s c o r e o r s o o f m i c r o -b i o t a s h a v e b e e n i n v e s t i g a te d h e r e ; (i i) t h e m a j o r i ty o f t h e s e m i c r o b i o t a s h a v eo n l y r e c e n t l y b e e n d i s c o v e r e d a n d ar e n o t y e t f u l l y c h a r a c t e ri z ed ; ( ii i) t h er a d i o m e t r i c a g e s o f t h e s e a s s em b l a g e s a r e k n o w n o n l y v e r y a p p r o x i m a t e l y( t a b le I I ); ( iv ) m o s t o f th e s e m i c r o b i o t a s o c c u r i n s t r o m a t o l i t e s o f a t y p e n o tg e n e r a l l y r e g a r d e d as u s e f u l fo r s t r o m a t o l i t e - b a s e d b i o s t r a t i g r a p h y ( T a b l e I I) ;a n d ( v) o n l y t h e m o s t r u d i m e n t a r y o f p o t e n t i a l m o r p h o l o g i c p a r a m e t e rs - -m i c r o f o s s i l d i a m e t e r - - h a s b e e n a n a l y z e d h e r e . N e v e r t h e l e s s , i t i s i n t e r e s ti n g ,a n d p e r h a p s s ig n i f ic a n t , t h a t t h e t w o s e ts o f s u b d i v i s i o n s d o a p p e a r t o c o i n c i d e ,a p p r o x i m a t e l y , a t t h e E a r ly t o M i d d le R i p h e a n t r a n s i t i o n a n d , a p p a r e n t ly ,n e a r t h e b o u n d a r y b e t w e e n t h e L a t e R i p h e a n a n d V e n d i a n . T o t h is e x te n t ,t h e n , t h is p r e l im i n a r y a s s e s s m e n t o f e v o l u t i o n a r y c h a n g e s i n P r o t e r o z o i c m i c ro -b i o t a s p r o v i d e s e v i d e n c e c o n s i s t e n t w i t h , a n d a p p a r e n t l y i n s u p p o r t o f , st ro -m a t o l i te - b a s e d b i o s tr a t ig r a p h y . I t s h o u l d a l s o b e n o t e d , h o w e v e r , t h a t n o s u c hc o r r e s p o n d e n c e is e v i d e n t a t b o u n d a r i e s b e t w e e n t h e E a r ly P r o t e r o z o i c a n dt h e E a r l y R i p h e a n , a n d b e t w e e n t h e M i d d l e a n d L a t e R i p h e a n .

A l t h o u g h r e s u l ts p r e s e n t e d h e r e t e n d t o s u g g e s t t h a t th e r e m a y b e a c o r-r e la t io n b e t w e e n t h e e v o l u t io n o f s t r o m a t o l it e - b u i l d i n g c o m m u n i t i e s a n dc h a n g e s i n t h e g r o ss m o r p h o l o g y o f s t r o m a t o l i t e s , t h e y a l so s u g g e s t t h a t s tr o -m a t o l i te s o f s i m i la r ( o r d i s si m i la r ) m o r p h o l o g y b u t o f m a r k e d l y d i f f e r in g a g ew e r e p r o b a b l y b u i l t b y d i f f e r e n t m i c r o b i o t a s . F o r e x a m p l e , a l t h o u g h t h e


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167range zone of fossil C o n o p h y t o n extends from the pre-Riphean into the Ven-dian (Semikhatov, 1974), Early Riphean forms were apparent ly built bynarrow (< 3 um diameter) filamentous microorganisms (Licari and Cloud,1972) whereas those of the Vendian contain broad (10- to 30-pm diameter)Lyngbya- l i ke oscillatoriaceans (Schopf and Sovietov, 1976). That C o n o p h y t o nwas built by different communities at different times seems further evidencedby changes in some aspects of the microstructure (viz., thickness of laminae)of these stromatolites during the Proterozoic (Schopf and Sovietov, 1976).Thus, the distinctive conical organization of such forms is apparently due moreto the physiological characteristics of the formative microorganisms than totheir specific morphology (Water et al., 1976; Schopf and Sovietov, 1976). Itseems probable that stromatol ites of o ther groups having especially long rangezones (e.g., Jacutophyton, Colonnella, Kussiel la, Tungussia, Strati fera, etc.)were also formed by dist inctly differing microbiotas at di ffe rent times; datasummarized here seem to indicate that in addition to C o n o p h y t o n this true atleast of many strat iform, spheroidal (i.e., oncolitic) and domal stromatolites.This emphasizes the need for form level identifications of stromatolites beforeattempting to use them for biostratigraphy (W.V. Preiss, pets. comm., 1976).It is as yet unknown whether representatives of a particular stromato liteform ("species") were always built by the same microbial assemblage andwhether differing stromatolites of a given age were characteristically producedby di ffe rent microbiotas. With regard to the former of these questions, de-tailed studies of f our specimens of Con ophy t on gaub it za from the Vendian ofSouth Kazakstan have revealed tha t each contained the same microbial com-ponents and that the components were similar both in abundance and in theirpatterns of distribut ion within each of the specimens (Schopf and Sovietov,1976). However, since all of the s tudied specimens were collected from thesame horizon within a small geographic area (apparently less than 2 km in later-al extent), these similarities do no t necessarily indicate tha t C. gaubitza wasbuilt always by this specific microbial community. Moreover, the data do notexclude the possibility that the C. gau bitza microbiota may have formed otherstromatolites in other environments; although this specific community has notbeen found elsewhere, microfossils of very similar morphology, occurring asmonospecific assemblages or in association with microorganisms not detectedin C. gaubitza, occur in other stromatolites of comparable age (Pedersen, 1970;Schopf, 1975b; Schopf et al., 1977; Bloeser et al., 1977). Similarly, thefew data now available do n ot indicate whether coexisting stromatolites ofdiffering morphology were typically produced by dissimilar communities.Bloeser et al. (1977) have observed major differences in the biotic com-position of microbiotas preserved in oncolites and those occurring in approxi-mately coetaneous stratiform stromatolites of the Late Riphean Chuar Group;Awramik (pets. comm., 1976) has detected what he regards as significantdifferences in the relative abundance of the microbial componen ts of (other-wise similar) microbiotas preserved in morphologically differing stromatolites


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o f t h e E a r l y P r o t e r o z o i c G u n f l i n t I r o n - F o r m a t i o n ; L i c ar i a n d C l o u d { 1 9 7 2 ),h o w e v e r , h av e i n d ic a t e d t h a t t h e r e a r e o n ly m i n o r d i f f e re n c e s b e t w e e n t h em i c r o b i o t a s d e t e c t e d i n d i f fe r i n g s t r o m a t o l i t e s o f t h e E a r l y R i p h e a n P a r a di s eC r e e k F o r m a t i o n ; a n d F a i r c h i l d ( in p r e p . ) h a s o b s e r v e d li t tl e o r n o e v i d e n td i f fe r e n c e b e t w e e n m i c r o b i o ta s p r e s e n t in c o l u m n a r a n d n o n - c o l u m n a r s tr o-m a t o l i t e s o f t h e p r o b a b l y L a t e R i p h e a n S k i ll o g al e e D o l o m i t e .

F i n a ll y , a l t h o u g h i t s e e m s r e a s o n a b l e t o c o n c l u d e f r o m t h is s t u d y t h a ts t r o m a t o l i t i c m i c r o b i o t a s p r o v i d e p r o m i s i n g n e w m e a n s f o r b io s t r a ti g r a p h i cs u b d i v is i o n o f t h e P r e c a m b r i a n , m u c h r e m a i n s t o b e l e a rn e d b e f o r e t h is t o o lc a n b e e x p l o i t e d e f f e c t i v e ly . A s i d e fr o m m o r e o b v i o u s n e e d s - - s u c h a s t h e d is -c o v e r y a n d c h a r a c t e r i z a t i o n o f a d d i t i o n a l s t r o m a t o l i t i c c o m m u n i t i e s ( a nd ,i n d e e d , t h e m o r e d e t a i l e d c h a r a c t e r i z a t io n o f t h o s e a s s e m b l a g e s c u r r e n t l yk n o w n ) - - t h e r e is s p e c ia l n e e d a t p r e s e n t t o i n t e g r a te m o r e e f f e c t i v e l y r e su l tsf r o m s t u d i e s o f s t r o m a t o l i t e - b u i l d i n g m i c r o o r g a n i s m s w i t h t h o s e o b t a i n e df r o m s t u d i e s o f t h e s t r o m a t o l i t e s t h e m s e l v e s a n d t h o s e a v a i la b l e f r o m s t u d ie so f m i c r o f o s s i l s in s h a le s . S e v e r a l o f t h e o u t s t a n d i n g p r o b l e m s i n s t r o m a t o l i t e -b a s e d b i o s t r a t i g r a p h y , a n d e s p e c i a l ly t h e l o n g s o u g h t c a u s a l e x p l a n a t i o n o fs t r o m a t o l i t e " e v o l u t i o n , " c a n p r o b a b l y b e s o lv e d o n l y b y c o o r d i n a t e d s t u d yb o t h o f i n d iv i d u a l s t r o m a t o l i t e s a n d o f t h e i r c o n t a i n e d m i c r o b i a l c o m m u n i t i e s .M o s t s t r o m a t o l i t e s , h o w e v e r , a r e c a l c a r e o u s r a t h e r t h a n c h e r t y i n c o m p o s i t i o na n d c o n s e q u e n t l y d o n o t c o n t a i n m i c r o f o s si l s; t h e r e i s t h u s p r e s si n g n e e d t oc h a r a c t e r iz e m o r e f u l ly th e m i c r o s t r u c t u r e o f c a l c a r e o u s s t r o m a t o l i t e s a n d t od e f i n e t h e d e g r e e t o w h i c h d i f f e r e n c e s i n m i c r o s t r u c t u r e r e f l e c t d i f f e r e n c e si n t h e o r ig i na l b i o t ic c o m p o n e n t s . A s H o f m a n n ( 1 9 7 4 ) h a s s h o w n , i n v es ti -g a t i o n o f c a r b o n a t e s t r o m a t o l i t e s c o n t a i n i n g i n t e r d i g i t a te d m i c r o f o s s i l i fe r o u sc h e r t p r o v i d e s p r o m i s i n g m e a n s t o a t t a c k t h is p r o b l e m . I n t e g r a t i o n o f re s u l tso f s t u d ie s o f t h e m i c r o f o s s il s in c h e r t s w i t h t h o s e o f m i c r o f o s s i l i f e ro u s s h a le sis l ik e l y t o p r o v e m o r e d i f fi c u l t . B e c a u s e t h e m o d e o f p r e s e r v a t i o n o f s t ro m a -t o l i t ic m i c r o b i o t a s { p e r m i n e r a li z a t i o n ) m a r k e d l y d i f f e rs f r o m t h a t o f s h al em i c r o f o s s i l s ( p r e s e r v e d a s o r g a n i c c o m p r e s s i o n s ) , r e s u l t s o f st u d i e s o f t h e t w of a c ie s a r e d i f f i c u l t t o i n t e r r e l a t e . I n d e e d , a l t h o u g h s h a l e s a n d c h e r t s o f t h es a m e a g e h a v e b e e n s h o w n t o c o n t a i n s o m e s im i l ar p l a n k t o n i c t a x a ( B l o e s e re t a l., 1 9 7 7 ) , i t is c u r r e n t p r a c t i c e t o t r e a t s u c h m i c r o f o s s i l s t a x o n o m i c a l l yi n w h o l l y d i f f e r e n t m a n n e r s - - t h e c o m p r e s s i o n m i c r o f o s s il s o f sh a le s a re u s ua l -l y r e f e r re d t o t h e A c r i ta r c h a , a n d a r e t h u s r e g a rd e d a s b e i n g o f u n k n o w n b io -l o g ic a l a f f in i t i e s, w h e r e a s t h e t h r e e - d i m e n s i o n a l l y p r e s e r v e d m i c r o f o s s i l s o fc h e r t s a r e u s u a l l y c o m p a r e d w i t h l i vi ng t a x a a n d a r e t r e a t e d u n d e r a b i o l o g i c a ls y s t e m o f c l a ss if ic a t io n . T h e r e c a n b e n o d o u b t t h a t s o r t in g o u t t h e c o m m o n -a l it ie s a n d d i f f e r e n c e s b e t w e e n c o e v a l as s e m b l a g e s f r o m t h e t w o f a ci e s w i ll b et i m e - c o n s u m i n g a n d d i f f i c u l t . I n t h e m e a n t i m e , h o w e v e r , i t is p a r t i c u l a rl y i m -p o r t a n t t h a t s t a t is t ic a l l y s i g ni fi c a nt m e a s u r e m e n t s d e s c r i b i n g t h e m o r p h o l o g yo f t h e m i c r o f o s s i l s o c c u r r i n g i n s u c h a s s e m b l a g e s b e a c c u m u l a t e d ; i n t h i sm a n n e r , a n d r eg a rd le s s o f w h a t n o m e n c l a t o r ia l o r t a x o n o m i c s y s t e m s m a y b eu s e d o r w h a t b i o l o g i c a l a f f in i t i e s m a y b e i n f e r r e d , i t s h o u l d p r o v e p o s s i b l e t oc o m p a r e b e t t e r , m o r p h o l o g i c a l l y , f os si ls d e t e c t e d i n t h e t w o f a c ie s .


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I n su m , t h e s t u d y o f P r e c a m b r i a n b i o s t r a t i g r a p h y is in i t s f o r m a t i v e s ta g es .T h r e e s e p a r a t e a p p r o a c h e s - - s t u d i e s o f s t r o m a t o l i t e s , o f s t r o m a t o l i t i c m i c ro -b i o t a s a n d o f s h a le m i c r o f l o r a s - - e a c h w i t h i ts o w n s e t o f a p p a r e n t s t r e n g t h sa n d w e a k n e s s e s , s e e m a p p l i c a b le t o t h e p r o b l e m . A t p r e s e n t , t h e r e i s p r e ss i n gn e e d f o r f u r t h e r r e s ea r c h l e a d in g t o b e t t e r i n t e g r a t i o n o f th e s e p o t e n t i a l t o o ls ;t o g e t h e r t h e y h o l d p r o m i s e f o r e f f e c t i v e b i o s t r a t i g r ap h i c z o n a t i o n o f th e P re -c a m b r i a n .A C K N O W L E D G E M E N T S

T h i s s t u d y w a s i n i t i a t e d i n M a r c h , 1 9 7 5 , w h i l e I w a s v i s i t in g t h e S o v i e tU n i o n a s a n E x c h a n g e S c i e n t i s t s p o n s o r e d b y t h e A c a d e m i e s o f S c ie n c e s o ft h e U S S R a n d t h e U S A . I a m g r a t e fu l f o r t h e s u p p o r t p r o v i d e d b y t h e s e or -g a n i z a t i o n s a n d f o r t h e h e l p f u l d i sc u s s i o n re g a r d i n g P r o t e r o z o i c b i o s t r a t i g r a p h yt h a t t h is t r ip e n a b l e d m e t o h a v e w i t h m a n y S o v i e t s p e ci a l is t s , e s p e c ia l l yI .N . K r y l o v , M . A . S e m i k h a t o v , B . S . S o k o l o v , a n d B . V . T i m o f e e v . I a l so a p -p r e c i a t e t h e g e n e r o u s c o o p e r a t i o n o f S .M . A w r a m i k , B . B l o e s e r , T . R . F a i r c h i l d ,H . J . H o f m a n n , R . J . H o r o d y s k i , C . V . M e n d e l s o n , M . D . M u i r , A . V . N y b e r g ,D . Z . O e h l e r , J . H . O e h l e r a n d M . R . W a l t e r, e a c h o f w h o m p r o v i d e d m e w i t hu n p u b l i s h e d d a t a r e l e v a n t t o t h e p r e s e n t p a p e r . T h e f ig u re s w e re d r a f t e d b yJ . G u e n t h e r . S t u d i e s a t U C L A w e r e s u p p o r t e d b y N S F G r a n t B M S 7 3 - - 6 8 1 2 A 0 1( S y s t e m a t i c B i o l o g y P ro g r a m ) a n d b y N A S A G r a n t N G R 0 5 - - 0 0 7 - - 4 0 7 .R E F E R E N C E SA ki yam a , M. and I m o t o , N . , 1 9 7 5 . S c a n n in g e l e c t r o n m i c r o s c o p ic e x a m i n a t i o n o f s o m emicrofoss i l s f ro m the Gu nf l in t cher t . E ar th Sci . , J . Assoc. fo r Geologica l Co l labora t ioni n J apan , 29 : 2 80- -281 .Al li son, C .W. , 1975. Pr imi t ive foss il f l a tw orm f ro m A laska: new evidenc e bear ing on an-ces t ry o f the M et azoa . G eo l ogy , 3: 64 9- -652 .A lli son , C .W. and M oorman , M .A ., 1973 . M i c rob i o t a f rom t he l a t e P ro t e roz o i c T i nd i r G roup ,Alaska. Geo logy, 1 : 65- -68 .A w rami k , S . M . , G o l u b i c , S . a n d B a r g h o o r n , E . S . , 1972 . Blue-green a lga l ce l l deg radat ionand i t s impl ica t ion for the foss i l record . Geol . Soc . Am. , Abs t r . Progr . , 4(7) : 438.Bargh oorn, E.S. , 1971. T he oldes t fossi ls . Sc i. Am. , 224: 30- -42.Barghoorn, E.S. and Tyler , S .A. , 1965. Microorganisms f rom the Gunf l in t cher t . Sc ience ,1 4 7 : 5 6 3 - - 5 7 7 .B er t r and-Sa r f a t i, J. and R aaben , M. E., 1970 . C om par i son des ensembl es s t r oma t o l i t i quesdu Pr~ca mb r ien sup~r ieur du S ahara occid enta l de l 'Oura l . Bull . Soc . g~ol . de F rance ,(7) , XII : 364- -371.Bloeser , B ., Schop f , J .W. , H orod yski , R . J . and Breed, W.J., 19 77; Chi t ino zoan sf rom t he l a t e P r ecambr i an C huar G roup o f t he G rand C a nyon , A r i zona . Sc i ence , 195 :6 7 6 - - 6 7 9 .Broda, E. , 1975 . T he Ev olut io n of the B ioenerget ic Processes . Pergam on, New Yo rk, N.Y.211 p p .B uchanan , R . E . and G i bbons , N . E . (Ed i t o r s ), 1974 . B e rgey ' s Manua l o f D e t e rmi na t i veBacter io logy . Wi ll iams and Wi lkins , Bal t imore . Md, 1 268 p p .Cloud, P .E. , J r . , 1965. Signi f icance of the G unf l in t (Preca m br ian) microf lora . Science ,148 : 27 - -35 .


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