The Taphonomy of Plant Macrofossils

Chapter 7 THE TAPHONOMY OF PLANT MACROFOSSILS David R. Greenwood INTRODUCTION Taphonomyis definedas the studyof the transitionof organicremains from the living organismto fossil assemblages(Efremov, 1940).As such, plant taphonomyincorporates the processesof the initial abscissionof plant parts, their transport (by air and/or water) to a place of eventual deposition, entrapmentand eventualburial, and subsequentlithification. Within these processesa number of factors can be identified which influenceboth the characterof the eventualassemblage and its taxonomiccomposition. These factorsproduce a taxonomicmix within the assemblagewhich is a subsetof the taxonomiccomposition of the originalsource plant communityor communities. The organographiccharacter of the assemblagemay also be biased (Collinson,1983). Spicer (1989, p. 99) defineda plant fossilassemblage as'an accumulationof plant parts, derivedfrom one or severalindividuals, that is entombedwithin a volume of sedimentthat is laid down in essentiallythe sameconditions'. His definitionwill be usedhere. Plant macrofossilassemblages are fundamentallydifferent from most animal assemblagesin that they are almostentirely composedof disarticulated parts.This resultsin part from the continualproduction throughout a plant's life cycle of generally temporary modular plant organs-leaves, stems, flowersand fruits-which are shed from the plant when their usefulnessis complete,through traumatic loss, or for the dispersalof propagules.Each of theseparts is givena separatename until connectionwith other partscan be demonstrated(brgan taxa; for example,Stigmaria and Lepidophylloidesare the root systemand leaves,respectively, of the sameCarboniferous lycopod: Thomas and Spicer, 1987;Thomas, 1990). A further factor is that plant 142 David R. Greenwood communities,in contrast to most animal communities,are composedof organismsthat remain in one place throughout their whole life cycle. In common with animal remains, the behaviour of the disarticulatedplant organsas sedimentaryparticles varies, as doestheir preservationpotential. Considerationof the above factors in the interpretationof plant fossil assemblageshas only occurredcomparatively recently. However, early work by Chaney(L924; Chaney and Sanborn,1933) pioneered the idea of using modernleaf assemblagesto interpretTertiary localities.Over the lastdecade a number of seminalworks have appearedon plant taphonomy(Hickey, 1980;Spicer, 1980; 1981; Ferguson, 1985; Scheihing and Pfefferkorn,1984; Gastaldo,1986; 1988;Burnham, 1989).Spicer (1930; 1989)and Hickey (1980)stressed the need to considerplant macrofossilassemblages within their stratigraphicand sedimentologicalcontext. In a seriesof experiments Spicer(1981) and Ferguson(1985) have investigatedthe behaviourof plant parts,mainly leaves,as sedimentaryparticles to determinethe factorswhich control their dispersal.Other plant taphonomistshave concentratedon the influenceof the-character of the standingvegetation and sedimentaryen- vironments on the resulting plant macrofossilassemblages (Drake and Burrows, 1980;Scheihing and Pfefferkorn,1984; Gastaldo, 1986; 1988; Hill andGibson, 1986; Taggert, 1988; Burnham, 1989). Thesestudies have contributedto a changein how plant assemblagesare sampledand interpreted.This chapteris not intendedas a review of plant taphonomy,as thorough reviewshave appearedrecently (Gastaldo,1988; Spicer,1989). Rather, this accountemphasizes the influenceof vegetationon plant taphonomy,although necessarily some major pointsare reviewed. The natureof theplant macrofossilrecord Most plant macrofossilassemblages are in fact fossilizedlitter (Figures7.1, 7.2).Plant macrofossilsconstitute any plant organor organpart whichis large enough to be recognizablewithout microscopy. Thus, plant macrofossils includeleaves and other foliar organs(such as stipulesand raches),flowers, fruit and other reproductiveorgans (for example,cones, sporangia, etc.), stemsand stem fragments(including woody axes),and root systems.These organsare produced in considerablydifferent proportions, with, for example, mosttrees producing only a singletrunk throughouttheir life, but thousands of leavesin any givenyear. Although strictlynot macrofossils,the cuticleof fossilleaves (as dispersed cuticle) and its associatedfungal microflora are also studied by palaeobotanists,providing both biostratigraphicand palaeo- climaticinformation (Kovach and Dilcher, L984;Lange, 1976;L978; Wolfe andUpchurch, 1987). Somepalaeobotanists have stressedthe varyingpreservation potential of plant organs(Gastaldo, 1988; Spicer, 1989).Heavily lignified plant tissue, suchas woody stems and somefruits, decaysfar slowerthan, for example,the delicateperianth and stamensof most flowers,and so has a higher preservationpotential than the latter. The chemicallyresistent cuticle of leavesalso Taphonomyof plant macrofossils 1tli| hasa very high preservationpotential and may persistextremely well in many different sedimenttypes. However, in contrastto many animals,most plant organshave a similarpreservation potential. In many animalshardparts such as shellsor bones have high fossilizationpotential, whereasthe softparts (visceralmass) have a very low preservationpotential and are only preserved in fossil-Lagerstiitten(Seilacher, L990). However, the relative preservation potentialof relatedplant organsvaries enormously between different taxa, institutingan importantbias in the plant fossilrecord. In general,most plant organshave a poor preservationpotential compared to vertebratebones or shellyfossils, and so havea very limited potentialto be reworkedfrom older sedimentsinto much youngersediments. Thus, time- averaging-the emplacementof fossilsfrom older stratigraphicposition to a youngerpart of the sequence-isfar lesscommon in plant macrofossilassem- blagesthan animalassemblages. Fossilized wood, either as unalteredlogs or as petrifications,and (more so) pollen and spores,are highly resistantto decay,and may experiencesignificant reworking (see,for example,Kemp, L972).More rarely,blocks of sedimentmay be reworked,carrying portions of an older macrofloraintact into youngersediment (see, for example,Hill and Macphail,1983). Plant fossilsare preservedprimarily within clastic sediments(including volcanoclastics):as compressionswith much of the organicmatter preserved (but flattened);as impressions with the organicmatter mostlylost, leavingan imprint or stainon the rock; asfusain through the conversionof the cell wall into charcoal by burning; and as permineralizationsand petrifications (Schopf,1975; Scott, 1990).Compression fossils vary from essentiallymum- mified remainspreserving considerable anatomical detail to preservingonly the cuticularenvelope of leaves.The sedimentsmay becomelithified, resulting in the chemicaltransformation of the plant remainsinto petrificationswith little or none of the original organic material retained. This often occurs through the secondarydeposition of silica, carbonateor pyrite, preserving internal or externalmoulds of cells,tissues or whole organs(Schopf, 1975; Scott,1990). Examples range from silicifiedtransported leaf litter (Petersand Christophel, t978; Greenwoodet al., 1990)to in situ plant communities (Knoll, 1985).Silicified plant assemblagesoften preservea high degreeof internalanatomical detail (see,for example,Peters and Christophel,1978). The cell wall of plantsis primarily celluloseand generallyonly preservesin acidicanoxic conditions. These conditions are most commonlvfound at the bottom of lakes,(generally abandoned) stream channels, and in swampsor river deltas. The plant macrofossilrecord is therefore random and strongly biasedtowards plant communitiesrepresentative of high rainfall environ- ments, or environmentsassociated with water coursesand other water bodies.Important plant macrofossilassemblages are also found in volcano- clasticdeposits (see, for example,Burnham and Spicer,1986). These include many importantNorth AmericanPalaeogene floras (Wing, 1987).Individual plant fossil beds consistprimarily of disarticulatedintermixed individual organsof many different taxa from the local vegetation.Only rarely are whole plantsor whole plant communitiespreserved in a form which approxi- matesthe originalplant or vegetation. David R. Greenwood Figure7.1 The forest-floorlitter of ComplexNotophyll Vine Forestfrom north- eastOueensland (equivalent to ParatropicalRainforest of Wolfe,1979). Taphonomy of plant macrofossils 145 Figure7.2 The oxidized leaf-matfrom Golden Grove (Middle Eocene,South Australia). 1rt6 David R. Greenwood The traditional approachin palaeobotanyhas been to view plant macro- 'flora' fossillocalities as a single and primarily as depositoriesof specimens. Thefossils were viewed as examples of extinctplant speciesand so, therefore, as evidenceof morphologicalevolution (phylogeneticpalaeobotany). The representationof particulartaxa or lineagesof taxawas then usedas evidence foi the presenceof particular analogousmodern plant commy{ties and/or climates.This approachignores the reality that most individuallocalities are composedof a numberof separatedepositional structures, each representing discieteevents, separatedin time and/or space.Even within apparently uniform assemblagessuch as seen in low-rank coals (lignites), individual lithotypes often contain separate macrofloras, reflecting differences in edaphicconditions and ecologicalprocesses such as succession. In a review of North American Eocenemacrofloras, Wing (1987' p. 75I) 'actually statedthat manyfloras consistof the summedfloral

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