Seilacher 84 Late Precambrian and Early Cambrian Metazoa: Preservational or Real Extinctions?

8/13/2019 Seilacher 84 Late Precambrian and Early Cambrian Metazoa: Preservational or Real Extinctions?

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Patterns of Change in Earth Evolution, eds. H.D. Holland and A.F. Trendall. pp. 159-168.Dahlem Konferenzen 1984. Berlin, Heidelbe rg, New York, Tokyo: Springer-Verlag.

Late Precambrian and Early CambrianMetazoa Preservational or Real Extinctions?

A. SeilacherGeologisches Institut der UniversiHit Tiibingen7400 TUbingen, F .R. Germany

Abstract The interpretation of Ediacara-type body fossils in termsof modern soft-bodied metazoans must be questioned. Their morphologyrather suggests foliate, non-locomotory quasi-autotrophs. Their modeof preservation, which has no counterpart in comparable post-Vendianrocks, remains problematical, since associated trace fossils attest tooxic conditions and the presence of worm-like heterotrophic burrowersin the same environment. It seems that Vendian biota mark not simplyanon-skeletai o:: wrt . r rnetazoaHc\lvlution, uu a Jb i incc epbout; i nthe history of life that was followed by a major extinction.

INTRODUCTION

Paleontologic research during the last decades has changed our viewsof early metazoan evolution tremendously. We recognize not only asudden appearance of a diversified Cambrian megafauna, but also a pretrilobite Tommotian) shelly fauna, which was preceded by a phase

Vendian-Ediacarian), in which larger, presumably metazoan organismswere present that had not yet developed mineralized skeletons. We canonly speculate about a still earlier phase of metazoan history. One could,for instance, interpret the presence of similar planktonic larvae indifferent phyla of modern marine organisms, not simply as a convergentlyevolved means of dispersal, but as an archaic heritage from a hypotheticalplanktonic and larviform stage of metazoan evolution. Metazoans ofthis stage would not have lef t a fossil record because of their microscopicsize and non-skeletal nature. The Vendian radiation might have expressedthe metazoan conquest of the benthic realm, in which size was no longer


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160 A. Seilacher

limiting. The resulting diversified extension of. ontogenetic historiescould therefore follow diverse pathways of adaptation to the new habitat:Wormlike infaunal organisms l e f t a trace fossil record and epifaunalorganisms lef t a record as body fossils. Cambrian radiations fol1owedthe introduction of mineralized skeletons that opened new possibilitiesof constructional design.

In this paper we do not want to emphasize the appearance, but ratherthe disappearance of these early metazoan faunas, and to discuss whetherthey are preservational in nature and hence reflect major changes inthe necrolytic and diagenetic regimes, or whether they are due to majorextinctions.

THE PROBLEM OF THE EDIACARAN FOSSILSSeveral decades ago the discovery of impressions of distinctive softbodied organisms in sandy deposits of late Precambrian age in SouthAustralia created a sensation. The Cambrian appearance of trilobitesand other Itshellyl organisms with mineralized skeletons no longer markedthe beginning of metazoan radiation but could be regarded as a secondmajor step in the evolution of multicellular animals. Since the discoveryof the Ediacara fauna, similar or even identical impressions have beenfound in similar stratigraphic positions in more than a dozen localit ies- particularly in Australia, South Africa, the Soviet Union, England, andNewfoundland (1, 2, 7). Without, a doubt we are dealing not with a local

. . prcscrvational 1t bonanza such as tht: Burgess Shales, but WIth tiledistinctive fauna of an Ediacarian period that preceded the Cambrian

that of i ts preservation. Most authors agree that Ediacara-type organismswere soft--bodied animals, similar to the jellyfishes that are so commonin most occurrenceso Such organisms have existed ever since.

Nevertheless, no remains comparable to the Ediacara soft-bodied fossils,a t least like their more characteristic members, have been found inyounger rocks. Individual lFossil-Lagerstatten l with an extraordinarylevel of preservation, including that of soft-bodied organisms, have formedrepeatedly since la te Precambrian time. These deposits usually consjstof very fine-grained sediments deposited in continuously or intermittentlystagnant basins, or of coarser deposits in which whole organisms becamesmothered. In contrast, Ediacara fossils are found in sandstones andsand/shale sequences whose sedimentary structures and t race fossil content


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Late Precambrian and Early Cambrian Metazoa 161

are characteristic of normal and well aerated shallow. marine environments(6). A question that is particularly relevant in the context of this workshopis this: nWhy did the Ediacaran mode of preservation become 'extinct 'with the beginning of the Cambrian?1I The recent discovery of non-skeletalmetazoans in the Precambrian of the Oleniok Uplift (Fedonkin andRozanov, Moscow, personal communication) will be of great interestin this connection, since they are preserved in thinly bedded dolomites,possibly of the lithographic type, i.e., in a facies with differentpreservational properties.

We must first test the reality of the l1extinction t l by answering thefollowing; questions:

1. Could the extinction be an artefact in the sense that we are dealingwith pseudo-fossils of inorganic origin? This is certainly true of manyso-called lTPrecambrian fossils. The supposed jellyfishes from theHakatai Shales of the Grand Canyon (3, 14) are surely nothing morethan compactional haloes around sand salt crystals. Another exampleof pseudo-fossils are the sinuous, wormlike ridges of llManchuriophycusT(3, 13), which are only shrinkage patterns of thin mud lenses depositedin ripple troughs. The complex forms of Ediacaran impressions clearlyexclude the possibility of a mechanical origin.

2. Do similar impressions occur in post-Vendian deposits, and have theyso fa-rr--esc-apedattention? -Most Ediacar 'anfossils a loe fahli uutdistinctive impressions on sole faces of sandstone beds; these are~ ~ ~ : . ' . ., r ' 1 ~ ~ 7 - - 1 - , ~ 4 - ~ - ,. o ~ - . ; - : :1-lr:--. : - : . . 7 ' l c t : " " ~ y ; . : ~ - : ~ + ' : ~ ~, - ~ ~ " , ~ ~ . " " " " " i ~ r :~ + 1 " . r 1 . c Y " l + C '~ f + ~ ~ r t t C ' : ;t t ~ v U ~ l l . U G U; : ) . l t i . : :> . .V. l u . e P ' . L \ . . . ~ C J .VcAi/ . . .J i .V.d ..... L v u . . . . . V w v ~ .~ J"" a .:i.JI. .... u ;.JI.... :o d - _+ ~ 1 ' C '' ':10 ~ ; : - " ' ; c ;-' ' f 'IJ-,h 0 t 1 ... - . ~ ~ , . . . " ' ~ ' h : C . r iC " ~ ' l I ~ O . " O- rn - i lQf ; Af ~ l i n h~ 0 1 . of ' p t " " o ~i n Y1'o.( 'tvc:. . . % . . f ~ ~ . . . J . . ~' \ O ' < : l \ , . . . . . ~ . . v " - , , c . . : . l - - . I - 1 . yu'- 'u .-_: t . - ' l '< . . . . , . . U ~ ~ . : I . . _a ......... . ~. . . . . . _ . . . . ' - ' ' U ~ _ ~ .............. _ _ .......... _

preservation. I t is therefore most unlikely that any similar impressionsoccur in post-Vendian deposits.

A full reevaluation of Ediacaran fossils is clearly needed; the followingsection of this paper is limited, however, to comments on publishedmaterial which are immediately relevant to the final discussion andsuggestions.

Ediacaran MedusoidsThe most common elements of the Ediacara fauna are round impressionswith radial and concentric structures that have been classified asmedusoids. Most published pictures are small and lighted in a variety


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162 A. Seilacher

of different directions so that is is difficult to judge critical details.We therefore owe a great deal to M Wade 18) for a critical analysisof preservational details. The mode of preservation of medusoids isconsistently different from that of other groups of Ediacaran fossils.Unfortunately, Wade's reconstructions of preservational processes arebased on the unproven assumption that these were really jellyfishes.

Stratinomy. Fossil-bearing sole faces record the change of pelitic tosandy sedimentation during an episodic rise in turbulence (commonlya storm event in whJch coarser material was deposited in a graded fashionfollowing an initial phase of mud erosion. Since medusoids are mainlyfound on these sole faces, they must have reached the bottom togetherwith benthic types of Ediacaran organisms. This is not what we might

expect from such light bodies. In the lithographic limestones (UpperJurassic of Pfalzpaint near Solnhofen - one of the few localities whereundoubted jellyfishes did become fossilized - they are invariably foundwithin the event-generated flinz beds and not at their base, where theheavier bodies of arthropods, echinoderms, squids, and fishes are located.If this is the case in a coccolithic mud, how much more should i t applyto sand

Postmortem deformation. Solnhofen specimens also show thecharacteristic alteration that occurs when a jellyfish shrinks by dehydrationwithin sediments. Radial and concentric wrinkles develop in the peripheralz v i l e ~ ~ o fthe Umbl"E:lla, - where ihe radial and circular nfu8cle fibers mustbe located in medusoid swimmers, while the central part of the impressionremains smooth or carries a regular number of radial furrows corresponding

in contrast, show strong concentric folds in the center and radial s t l ~ u t u r e sat the periphery.

Radial feeding burrows' Radial backfill burrows that reflect the probing

of wormlike s e i m e n ~feeders around a vertical burrow are among thestructures that are commonly mistaken for fossil jellyfishes. Medusina,Palaeosemaeostoma, Kirklandia, and Gyrophyllites (8) are familiarexamples from the Phanerozoic record. Brooksella from the latePrecambrian of the Grand Canyon may be another example. Amongthe Ediacara medusoids, Mawsonites spriggi (see (5), Figs. 1-2 and (1),cover picture is clearly such a burrow system. Like the CretaceousKirklandia, it shows several t iers of sharply separated radial lobes thatextend from the central shaft with a downward inclination. Seleniform


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backfill lamallae can also be seen in the lobes as well as m the verticalshaft of published Mawsonites specimens.

Actinian ?) burrows? Circular burrows, possibly made by actinians thatlived in sandy sediments, are another source of confusion. Paleozoicexamples (Bergaueria) are usually preserved as smooth and dome-shapedcasts, sometimes with a central depression; Mesozoic representations(Solicyclus) have regular radial grooves around the smooth central field.Without careful examination of the original material, i t is difficult toassign Ediacaran medusoids to this type of trace fossils; however, theseparation of specimens 9f Medusinites, Edicaria, and Cyclomedusa fromthe surrounding bedding surface by a sharp groove and the eccentricallyarranged annular furrows in some specimens (18) may indicate that theybelong to this kind of t race fossils.

Sandy sk l tons o f actinians? last possibility is related to a newinterpretation of Protolyella, a three-dimensional fossil of Cambrianto Ordovician age with a globular base and radial plus concentric ridgeson the truncated upper surface. In the past, i t has been interpreted asthe sandy filling of a medusoid stomach cavity. New material from theOrdovician of Jordan (16), however, shows that the concentric groovesof the upper surface continue inside the body as concentric hemisphericallaminae that cannot be explained by sedimentation within a cavity. Ratherthey reflect the laminar growth, possibly within the gastral cavity ofan actinian, of a heavy sandy skeleton that stabilized the animal on the

---.--., soft s U D s t r a f e ~ ~ - r h e - - 6 c c u f r e f i c e - - o r - - a - c e - f i t i a ldeI) ressioflatthe- base -of---many Protolyella casts suggests that the organisms were similar to those: esponsible for Bergaueria burrows.

three-dimensional character is also suggested y SOil le Ediacarallmedusoids. 1I In one specimen (18), Fig. 2) they are aligned in the troughsof oscillation ripples in the same manner as Protolyella in Cambrianand Ordovician sandstones. The animals may have accumulated on and

burrowed in the sand during the event and were then smothered by themuddy tail of the same tempestite (see (15), Fig. 5b for a similar behaviorin ophiuroids). The round basal disk of the Pennatula-like Charniodiscus1), Fig. 2A) could be a similar sandy weight belt.

Conclusion: None of the numerous IImedusoids in the Ediacaran faunacan be interpreted with certainty as a jellyfish. Rather we seem to bedealing with a h e t e r o g ~ n e o u sgroup of t race fossils and remains ofunidentified benthic organisms.


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Functional morphology. A feature that Ediacaran "pennatulids, 1chondrophores, and Dickinsonia have in common is their quilt-like

repetit ive structure which has been interpreted as expressing metamericsegmentation. t could, however, be simply a means to stiffen a hydraulicskeleton biomechanically, somewhat in the manner of an air mattress;i t would also have provided an internal compartmentalization thatfacili tated metabolic processes (17). The air mattress model does notapply to Spriggina, because some of the specimens are laterally bent,nor to Tribrachidium, Parvancorina, and Praecambridium, in all of whichthe boundaries between, radial sectors stick out as sharp ridges insteadof forming reentrant grooves. Nevertheless, these forms were also quiltedand frondTlike.

The frond-like shape of most Ediacaran fossils could be explained interms of metabolism, since i t maximizes the external surface. Thismay have been necessary for respiration a t a t ime when the atmosphereand seawater contained less oxygen than today. One might even speculatethat dissolved food could have been absorbed directly through the enlargedbody surface, since orifices and intestines have not been observed inEdiacaran fossils. Another possibility is an association with photosymbioticalgae, which would benefit from a frond-like shape and could have providedfood and oxygen directly to the host.

A mode of nutrition unfamilar in modern Metazoa is also indicated bythe non-locomotory chara-cter of - most-Ediacara.-type organisms.Morphological distinction between front and rear end - a universalattribute of mobile benthic organisms except regular echinoids - is found

' Y " ' ~ " " ;Y' i ~ ~ n ; n ~ ; 7 OQ ~ . r i G O ' P, ( l D n , - , . O Y l . j . . , . .. , , ) ,..,. t ~ - r v -~ ~ : > ' ~ , J..... '. - t ' i . . ~ 6 6 ~ " ' ; ' v . .'-:.. ''' --. . . J - . . . . \J ... ~ . _.... ~ - - . _ ' - ~ ~ : - e ~ ' ~ ; , : ~ ~ . : . ' U . ~ : . ~ g '~ ~ e s ~ e ~" ' - ~ ~-

ilmedusoids," were probably more or less sessile. e do nOi however,have indications of the presence of lophophores or other filtering devicesthat are the base for sedentary l ife in modern marine organismso

All these considerations are, of course, based on the assumption thatwe are really dealing with the Metazoa in the modern sense, and thisis not a t all certain.

Conclusion: Non-medusoid body fossils of the Ediacaran fauna -seemto be the remains of benthic organisms that are not referable to extantphyla. They had a flexible cuticle whose survival suggests that i t wascomposed of biomaterial indigestible for the contemporary microorganisms.


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66 A. Seilacher

EVOLUTION RY SCEN RIO

The problem that appeared to be mainly a matter of preservation atthe outset of this review has thus turned out to be of basic significancefor the theme of this Dahlem workshop. f one views the Ediacaran faunaas the initial stage of metazoan evolution, it could be incorporated intothe sigmoidal part of the Cambrian diversity explosion that was uniquein the sense that it took place in an ecological vacuum. In terms ofdiversity, this curve is still valid, because the number of species andthe degree of provincial differentiation was very low in the Vendi anbiota. But if the Ediacara-type fossils turn out to be basically differentfrom Metazoa in th modern sense and if their disappearance is not onlyan artefact of preservation, then the Cambrian radiation was precededby a m ~ o rextinction event in the benthic realm.

One could go a step further and ask whether a similar relationship mightnot exist between the shelly faunas of the Tommotian and the trilobitedominated stages of the Cambrian. After all, animals with a taxonomybased on that of modern organisms could have suppressed the basicconstructional and ecological differences of this earliest shelly fauna.The problem of the Tommotian biota also has preservational overtones,because the shelly fossils of this stage are all very small and phosphatized.But this time-specific mode of preservation continues throughout theremainder of the Cambrian. t became "extinct, however, in later periods,when suitable micromolluscs were still present but are significantly absentin the phosphatic residues studied by conodont workers 0. Walliser,personal communication) ..

An unusualiy high phosphorous content in Cambrian seas was probably

mineralized skeletons (9, 10), as well as o t the high percentage oforiginally phosphatic shells in Tommotian faunas.

We conclude that metazoan evolution was no smoother in i ts initial than

in its la ter phases. Extinctions caused by changes in the physicalenvironment preceded radiations that increased the overall diversityin a step-like mode. Since the levels of diversity were generally low,these early extinctions had l i t t le in the way of quantitative effects.Nevertheless, their consequences played a major role in the course ofmetazoan evolution.


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LR.te Precambrian and Early Cambrian Metazoa 167

SUGGESTIONS FOR FUTUR RESE RCH

This review of the l i terature has been admittedly cursory. We have cometo the conclusion that the nature of Ediacara-type body fossils of Vendianage may have been misinterpreted. Not only their affiliation with extant

phyla, but also their truly metazoan nature should remain open to question.More definite answers regarding their affiliations can be given only aftera very critical reexamination of the fossil material. Such a study shouldinclude associated t race fossils, which seem to be less different frompost-Vendian forms and that may represent the stocks from which marineepibenthic phyla radiated a t a later time. The study of t race fossilsand associated sedimentary structures will also provide better informationabout the depositional environments and the distribution of food insedimentsfduring those early times.

REFERENCES

1) Cloud, P., and Glaessner, M.F. 1982. The Ediacarian period andsystem: Metazoa inherit the earth. Science 217: 783-792.

2) Fedonkin, M.A. 1981. White sea biota of the Vendian. Precambriannon-skeletal fauna of the northern Russian platform. Trans. Akad.Nauk. SSR. 342: 1-100.

(3) Glaessner, M.P. 1969. Trace fossils from the Precambrian andbasal Cambrian. Lethaia 2: 369-393

.... - - ( 4 - r - l } I l r e s s n e r ; 1 V r . F : - ~ - 1 9 7 - g : -~ - A f i E f c f i i T I r r ( r w O r mfrom t he L a f e - P r e c a m b r i a j i ~. . . . . . . . -Lethaia 12: 121-124.

(5) Glaessner. M.F and V J a d e ~M. 1966. The late Precambrian fossils

Goldring, R. and Curnow C.N. 1967. The stratigraphy and fadesof the la te Precambrian at Ediacara, South Australia. J. Geol. Soc.Austral. 14: 195-214.

7) Jenkins, R.J.F. 1981. The concept of an Ediacaran Period andits stratigraphic significance in Australia. Tran. Roy. Soc. S. Austral.105: 179-194.

8) l\loore, R.C. 1956. Treatise on invertebrate paleontology. PartF. Coelenterata. Lawrence, KS: University of Kansas Press.

9) MUller, K.J. 1979. Phosphatocopine ostracodes with preservedappendages from the Upper Cambrian of Sweden. Lethaia 12: 1-


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10) Muller, K.J. 1982. Weichteile von Fossilien aus dem Erdaltertum.Naturwissenschaften 69: 249-254.

11) Pflug H.D. 1972. Systematik der jung-prakambrischen Petalonamae.Palaont.

Z. 46:56-67.

12) Pflug H.D. 1974. Feinstruktur und Ontogenie derjung-prakambrischen Petalo-Organismen. PaIaonet. Z. 48: 77-109.

13) Schindewolf O.H. 1956. Dber prakambrische Fossilien. InGeotektonishes Symposium zu Ehren von Hans Stille ed. F. Lotzepp. 45:9-480. Stut tgar t : F. Enke.

14) Seilacher A. 1956. Der Beginn des Kambriums als biologische Wende.N Jb. Geol. PaIaont. 103: 155-180.

15) Seilacher A. 1982. Distinctive features of sandy tempestites. InCyclic and event stratification eds. G. Einsele and A. Seilacher.Heidelberg: Springer-Verlag.

16) Seilacher A. 1982. Paleozoic sandstones in southern Jordan: t racefossils, depositional environments and biogeography. Contribution1st Jordanian Geological Congress Amman.

(17) Sernetz M ; Rufeger H.; and Kindt R. 1982. Interpretation ofthe reduction law of metabolism. Exp. BioI. Med. : 24.

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Seilacher 84 Late Precambrian and Early Cambrian Metazoa: Preservational or Real Extinctions?