The Ediacaran Diversification of Organic-walled...

ACTA

UNIVERSITATIS

UPSALIENSIS

UPPSALA

2008

Digital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 428

The Ediacaran Diversification ofOrganic-walled Microbiota

Ocean Life 600 Million Years Ago

SEBASTIAN WILLMAN

ISSN 1651-6214ISBN 978-91-554-7185-9urn:nbn:se:uu:diva-8684

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List of publications

I Willman S, Moczyd�owska M, Grey K (2006) Neoprotero-zoic (Ediacaran) diversification of acritarchs – A new record from the Murnaroo 1 drillcore, eastern Officer Basin, Austra-lia. Review of Palaeobotany and Palynology 139, 17–39.

II Willman S, Moczyd�owska M (2008) Ediacaran acritarch biota from the Giles 1 drillhole, Officer Basin, Australia, and its potential for biostratigraphic correlation. Precambrian Re-search 162, 498–530.

III Willman S (accepted) Acritarchs and their potential in Edia-caran biostratigraphy – Examples from the Officer Basin, Australia. Comunicações Geológicas, 14 pp.

IV Willman S, Moczyd�owska M (2007) Wall ultrastructure of an Ediacaran acritarch from the Officer Basin, Australia. Le-thaia 40, 111–123.

V Willman S (manuscript) Morphology and wall ultrastructure of leiosphaeric and acanthomorphic acritarchs from the Edia-caran of Australia. Submitted to Geobiology, 15 pp.

VI Grey K, Willman S (accepted) Taphonomy of Ediacaran acritarchs from Australia: significance for taxonomy and bio-stratigraphy. PALAIOS, 30 pp.

VII Hill AC, Haines PW, Grey K, Willman S (2007) New records of Ediacaran Acraman ejecta in drillholes from the Stuart Shelf and Officer Basin, South Australia. Meteoritics & Planetary Science 42, 1883–1891.

Additionally, the following paper was written during my studies at Uppsala University, but is not included in this thesis: VIII Willman S (2007) Testing the role of spines as predatory

defense. Journal of Shellfish Research 26, 261–266.

Reproduction of the published papers is made with kind permission from the copyright holders. Paper I and II © Elsevier Paper III © by the author Paper IV © Wiley-Blackwell Publishing Ltd. Paper V © by the author Paper VI © by the authors Paper VII © Meteoritical Society Paper VIII © National Shellfisheries Association/Journal of Shellfish Re-search Statement of authorship Paper I and II: S. Willman analysed the material and interpreted the results, shared writing. Paper III: S. Willman wrote the manuscript. Paper IV: S. Willman performed the experimental part and analysed the material, shared writing. Paper V: S. Willman performed the analysis and wrote the manu-script. Paper VI: S. Willman helped in interpretations, shared writing. Paper VII: S. Willman assisted in data collection and writing. Front cover: Tanarium pluriprotensum, an Ediacaran age acritarch from the Giles 1 drillcore, Officer Basin, South Australia.

Contents

List of publications ...................................................................................... vii�

1. Introduction ............................................................................................... 13�Aims and introduction to the studies ........................................................ 13�The Precambrian – life evolves, the environment changes ...................... 14�The Ediacaran Period ............................................................................... 16�What is an acritarch? ................................................................................ 18�

2. Ediacaran organic-walled microbiota from Australia ............................... 21�New records and the potential for biostratigraphy ................................... 22�Wall ultrastructure and biological affinities ............................................. 23�Life after death: the degradation of acritarchs and its consequences ....... 25�New records of the Acraman impact ejecta layer and its importance ...... 25�

3. Svensk sammanfattning ............................................................................ 26�Istiderna .................................................................................................... 26�Meteoritnedslaget ..................................................................................... 26�Min forskning ........................................................................................... 27�

4. Acknowledgements ................................................................................... 29�

5. References ................................................................................................. 31�

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1. Introduction

Aims and introduction to the studies Ediacaran fossils document a most remarkable palaeobiological and evolu-tionary event that includes the origin and diversification of a variety of mi-croorganisms and metazoans. Among these, organic-walled microbiota of unknown affinities form an integral part of the biosphere. What was life in the oceans like between 600 and 550 million years (Ma) ago? What kind of organisms populated coastal and marine environments? These are some questions that I address in this thesis. I studied organic-walled microfossils of Ediacaran age (~635-542 Ma) recovered from subsurface rock-successions in South Australia (Figure 1). Organic-walled microfossils, es-pecially those referred to as acritarchs, are common, widespread and mor-phologically distinguishable in these rocks and are therefore ideal objects to study when addressing the questions posed above. The thesis consists of three thematic parts: (1) documentation of the morphological disparity and taxonomic diversity of the microbiota, interpretation of its diversification through time with regards to environmental change, and applying this to biostratigraphy and correlation of rock successions on both a local and global scale (2) investigation of the biological affinities of selected acritarchs by examining their wall ultrastructure by use of transmission electron mi-croscopy, and to infer their relationship to modern biota (3) study effects of the post-mortem degradation of individual acritarchs on taxonomic interpre-tation and what further implications this has on stratigraphic correlation and subdivision. In addition, the thesis consists of a report on the discovery of the Acraman impact ejecta layer in two drillcores and shows how this marker level can be used to confidently correlate geological successions.

Acritarchs have been studied from sedimentary successions worldwide for more than a century. Although many studies have focused on Ediacaran assemblages, research is still in an early phase and in need of sub-stantial work. South Australia has proved to be a key area, alongside China, Siberia and Baltica, in studies of Ediacaran organic-walled microfossils. It is likely that the fossils from these areas will help to solve many purely palaeo-biological problems. The real challenge, however, lies in tying the diversifi-cation of the microbiota to environmental change, such as the rise of oxygen, to the subsequent emergence of metazoans and the explosion of diverse life-forms in the Cambrian (e.g. McFadden et al. 2008). From an applied per-spective, acritarchs appear to be important for biostratigraphic subdivision of

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the late Neoproterozoic. The increased interest in Neoproterozoic rocks as a potential source of fossil fuels underlines the need for firm biostratigraphic subdivision.

The Precambrian – life evolves, the environment changes The Precambrian1 spans almost 90 % of Earth’s history but, despite the long duration, we know surprisingly little about early life and the evolutionary history of Precambrian organisms. However, it was during this period that many key evolutionary traits first evolved. Especially important were the 1 The Precambrian is an informal but widely used term that encompasses the time prior to the Cambrian, i.e. before 542 Ma ago.

Figure 1. Regional overview of Australia showing the location and extent of the Officer Basin, and adjacent basins, including the positions of the drillcores studied.

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ability for organisms to produce (and later use) oxygen2 that changed the Earth's atmosphere from an anoxic to an oxic state, the evolution of eukaryo-tes, the invention of sexual reproduction and multicellularity. It is unclear how, when and where life first evolved but the earliest evidence comes in the form of biogeochemical fractionation diagnostic of life 3.8 billion years (Ga) ago (Mojzsis et al. 1996). The earliest organisms were probably single-celled prokaryotic organisms (from the Archaea and/or Bacteria domains) that used chemical pathways to obtain energy. Carbonaceous structures re-sembling such organisms from the 3.4-3.5 Ga old Apex Chert of the Warra-woona Group in Western Australia have been considered the oldest bona fide body fossils (Schopf & Packer 1987; Schopf 1993), although their au-thenticity has been questioned (e.g. Brasier et al. 2002). Nevertheless, sedi-mentary structures called stromatolites that are assumed to have a biological origin indicate the presence of cyanobacteria-like organisms since the early Archaean (e.g. the ~3.4 Ga old stromatolites from the Strelley Pool Chert in Australia; Allwood et al. 2006). Whether or not the Archaean stromatolites are biogenic or constructed through abiogenic processes is another ongoing debate, but all evidence suggests that life was present at least 3.5 Ga ago (Figure 2). Bacteria seem to have been the predominant forms of life for at least 2 Ga until acritarchs of assumed eukaryotic affinities first appeared in the rock record (ca 1850 Ma ago). In summary, although life evolved early, the Precambrian fossil record documents a biosphere that is populated by long-ranging (i.e. lasting virtually unchanged for hundreds of millions of years), mainly single-celled prokaryotes and eukaryotes that show very little morphological diversity.

2 This ability is called oxygenic photosynthesis and probably evolved more than 2.8 Ga ago (Des Marais 2000).

Figure 2. Schematic diagram showing the appearance of major groups; note that the early biosphere was quite complex, and that plants and animals appeared very late in earth history; appearances are approximate (modified from Des Marais 2000).

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The Neoproterozoic (1000-542 Ma ago) was a time of major

environmental change including the formation and rifting of supercontinents, chemical changes of the atmosphere and hydrosphere, and a culmination in global glaciations towards the end of the Era. At least two major glaciations, the older “Sturtian” and the younger “Marinoan”3, occurred in the Cryo-genian Period (~850-635 Ma) (e.g. Hoffman et al. 1998; Zhou et al. 2004). Each of these glaciations must have severely affected the biosphere.

The Ediacaran Period The Precambrian time interval is divided into eons, eras and periods strictly based on chronometric ages without connotation to the rocks themselves. The exception to this is the Ediacaran Period and System, which is the first formal stratigraphic unit established in the last 100 years4. The type-section of the Ediacaran System is in the Flinders Ranges in Australia. The lower boundary is placed at the base of the Nuccaleena Formation5 and the upper boundary at the onset of the Cambrian System at 542 Ma (Knoll et al. 2006b). Correlation of the Ediacaran System is mainly based on the correla-tion of glaciogenic sediments and the overlying cap-carbonates on the as-sumption that they are global in extent and synchronous. However, no abso-lute dating has been acquired from the type section in Australia and although the Ediacaran System is well defined lithologically, the timing is conse-quently less precise. Thus, the Ediacaran Period still awaits a great deal of geological and palaeobiological investigation. It is during this time that the first diversification of animal life6 is documented (e.g. Narbonne & Gehling 2003), so that the Ediacaran forms an essential prelude to the well-known “Cambrian explosion”7.

3 Many researchers now prefer to use the term Elatina instead of Marinoan glaciation because it specifies the type-glaciation in Australia, rather than a globally correlatable diamictite. 4 The Cryogenian Period and System is the next stratigraphic unit to be ratified. 5 The Nuccaleena Formation is a cap carbonate overlying the Elatina Formation, the Marinoan diamictite. 6 These problematic soft-bodied organisms are traditionally called the Ediacara fauna after the Ediacara Hills in Australia, one of the classical fossil localities. 7 The Cambrian explosion documents a rapid appearance of most major animal groups (phyla) in the early Cambrian. The evolution of hard-parts at this time facilitated their preservation.

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It is apparent that the Ediacaran biosphere is markedly differ-ent from the rest of the Precambrian. The Ediacaran biosphere became popu-lated, for the first time in Earth history, by large, morphologically complex micro-, and macroscopic organisms that show biogeographical partitioning and rapid evolutionary turnover (Butterfield 2007). Ediacaran acritarchs for example, appear, diversify and disappear within 50 million years (Figure 3) (Knoll 1994). This diversification is preserved and documented in rock suc-cessions, not only in Australia but worldwide, including China, Siberia, East European Platform, Svalbard, and India. Although this diversification is well known, no proper attempt at an embracive biostratigraphy has been made until recently (Moczyd�owska et al. 1993; Grey 2005; Zhou et al. 2007).

Figure 3. Taxonomic richness of acritarch assemblages through the Meso- and Neoprotero-zoic (bars) and morphological disparity through time (solid line). Solid triangles mark exten-sive ice-ages, (the small triangle not global in extent) and star shows approximate time of the Acraman impact. Note, bars are not to scale but the diagram is intended to show overall mor-phological trend (modified after Knoll et al. 2006a and Butterfield 2007).

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In addition to the climatic turbulence in the late Neoprotero-

zoic, the evolution of marine microbiota might have been influenced by ex-traterrestrial input, at least on a regional scale. At Lake Acraman in South Australia the remnants of a large bolide impact form an impact structure ca 90 km in diameter. The bolide struck Mesoproterozic basement rocks that extend between the Officer Basin and the Adelaide Rift Complex (e.g. Gostin et al. 1986; Williams 1986) and most estimates place the time of the impact at ca 580 Ma ago (e.g. Walter et al. 2000). Grey et al. (2003) noted a correspondence between a negative excursion in the carbon isotope curve, and the transition from a leiosphere-dominated acritarch assemblage below the ejecta layer to an acanthomorph-dominated acritarch assemblage above the ejecta layer. Consequently, they suggested that the impact may have triggered biotic turnover. Regardless of the impact’s effects on the biota, the associated ejecta layer provides a stratigraphic marker that can be used to reliably compare and correlate distribution of acritarchs between successions over wide areas.

Studying Ediacaran age micro- and macroorganisms helps to better understand the processes and conditions that gave rise to animal evo-lution. Thus, the palaeobiology of the Ediacaran Period is a fruitful object for future research.

What is an acritarch? The Acritarcha8 (Evitt 1963) is a group of acid-insoluble organic-walled microfossils that can not be assigned to any living organisms. Based solely on morphological similarities rather than true natural groupings, their taxon-omy is entirely phenetic. Based on ecology, morphology, size and excyst-ment structures many acritarchs are considered to be the resting cysts of marine phytoplankton, such as various groups of green algae (e.g. Colbath & Grenfell 1995; Strother 1996; Talyzina & Moczyd�owska 2000), but others have been assigned to red algae, fungi and metazoans (Butterfield 2000, 2005; Yin et al. 2007; respectively)9. Generally, most acritarchs are assumed to be eukaryotic and single-celled (e.g. Tappan 1980; Javaux et al. 2004). Pre-Ediacaran microbiota recovered from around the world seems to consist mainly of large populations of bacteria (mainly as colonial coccoids or filamentous forms) together with simple sphaeromorphic acri-

8 The term acritarch originates from the Greek words akritos (uncertain, mixed or confused) and arche (beginning or origin). 9 Acritarchs are treated under the International Code of Botanical Nomenclature despite the fact that the group may include organisms from other kingdoms.

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tarchs and occasionally large ornamented or spiny acritarchs. Despite the fact that these bacteria and acritarchs must have been important parts of the biosphere their simplicity make them difficult to properly identify, and con-sequently focus has been on more easily distinguishable forms. Trachyhys-trichosphaera is one such readily recognised form that is present already in the Mesoproterozoic. It is characterised by irregularly arranged processes that support a thin outer membrane. Tappania and Vandalosphaeridium are other well-known forms described from the Mesoproterozoic of Australia and China (Javaux et al. 2001; Yin & Gao 1996; respectively). Even though several complex acritarchs are present early in Earth history, there was little morphological change in the Mesoproterozoic. However a study by Huntley et al. (2006) revealed that many major morphological characters first ap-peared in the early Mesoproterozoic, before a morphological stasis lasting perhaps as long as 600 Ma set in. By the early Tonian (1000-850 Ma ago) acritarch assemblages begin to show considerable diversity shortly followed by serious depletion in both morphological disparity and taxonomic diver-sity. As already mentioned, the succeeding Ediacaran Period is characterised by a rapid radiation in acritarchs (Figure 4) and the appearance of animals may have altered the trophic structure, resulting in the complete disappear-ance of the ornamented acritarchs before the Cambrian (Peterson & Butterfield 2005). Acritarchs and many other microfossils are invaluable in bio-stratigraphic correlation and are used throughout the entire Phanerozoic. Acritarchs are most common and diverse in the Lower Palaeozoic and con-sequently of most importance in Lower and Middle Palaeozoic biostratigra-phy. The application of microfossils in biostratigraphic correlation was de-veloped by the petroleum industry in search for oil and gas and continues to be extended back in time.

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Figure 4. Representative acritarchs from the Ediacaran successions in South Australia, show-ing great morphological disparity. 1) Leiosphaeridia sp. showing simple morphology as represented by the Lj–Lc Assemblage Zone (as described in Grey 2005); Giles 1, 445.0 m. 2) Tanarium conoideum showing conical processes with wide bases; index-species for the Tc–Sr–Vl Assemblage Zone; Giles 1, 425.75 m. 3) Pennatosphaeridium chrysanthemoides bear-ing numerous densely crowded processes that divide into multi-stranded fibrils at the tips; Giles 1, 428.10 m. 4) Tanarium pluriprotensum displaying uniformly distributed, homomor-phic, tapering processes of fairly uniform length; Giles 1, 438.5 m.

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2. Ediacaran organic-walled microbiota from Australia

The organic-walled microfossils studied and described in this thesis derive from drillcores from the Officer Basin in South Australia. The Officer Basin is an intracratonic basin that covers a large part of Western and South Aus-tralia (Figure 1; Figure 5). In particular, focus is on the lower Ungoolya Group in which a diverse assemblage of microfossils is preserved (Grey 2005). In ascending stratigraphic order the Ungoolya Group consists of the Dey Dey Mudstone, the Karlaya Limestone and the Tanana Formation. Li-thologically, the Dey Dey Mudstone is divided into two units separated by a bed of dolomitic intraclasts; the lower unit is mainly red-brown and occasio-nally green-grey silty mudstone that was deposited in a fluvial environment and the upper unit is a laminated dolomitic or calcareous siltstone and mud-stone deposited in slightly deeper, prodelta and shelf environments (Zang 1995; Morton 1997). The Karlaya Limestone consists mainly of thin-bedded micritic limestone with dark grey silty mudstone layers and some limestone intraclasts, deposited on a subtidal shelf, probably below fair-weather wave base (Zang 1995). The Tanana Formation overlies the Karlaya Limestone and consists of micritic limestone with silty mudstone interbeds deposited in a prodelta and distal delta front to shelf settings (Morton 1997). The microfossils were collected from selected lithologies known to be suitable for palynomorph10 preservation. Such rocks are unoxi-dised mudstones, shales and carbonates. The material studied by use of transmitted light microscopy was processed by Laola Pty. Ltd, Perth Austral-ia using the method described by Grey (1999). The material studied by use of transmission- and scanning electron microscopy (TEM and SEM) was prepared following standard preparation protocols. All material is currently stored at Uppsala University while studies continue.

10 Palynomorph refers to a wide range of organic-walled microfossils, including acritarchs, dinoflagellates, spores, pollen, fungi, chitinozoans and even parts of metazoans.

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New records and the potential for biostratigraphy The first part of this thesis (Papers I-III) documents the acritarch diversity in the Murnaroo 1 and Giles 1 drillcores. The studied palynoflora comprises more than two dozen species of which two are described as new. Apart from documenting taxonomic diversity, the importance of specific taxa for bio-stratigraphy is discussed.

The common absence of the skeletonised fossils that are char-acteristic for the Phanerozoic11 causes difficulties in correlating Precambrian rocks. Different methods, such as chemostratigraphy, magnetostratigraphy, event stratigraphy and biostratigraphy, are used to correlate Ediacaran rocks. 11 The term Phanerozoic derives from the Greek and means visible life and refers to the sud-den appearance in the rock record of fossils of large size and with preservable hard-parts.

Figure 5. The Officer Basin, showing the locations of the drillholes mentioned in this study; Giles 1, Murnaroo 1, Munta 1, Observatory Hill 1 and Lake Maurice West 1 (mod-ified from Grey 2005).

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Although the soft bodied, bilaterian Ediacara fauna characterises much of the Ediacaran Period and its age is well constrained (e.g. Narbonne 2005) the distribution of the taxa is far too sparse to provide a precise correlation tool. Other palaeobiologically important fossils from the Ediacaran, such as eggs and embryos (e.g. Xiao & Knoll 2000), trace fossils and calcified macrofos-sils (e.g. Martin et al. 2000) are too infrequently found to offer meaningful correlations. Acritarchs, on the other hand, are important in Palaeozoic bio-stratigraphy (e.g. Moczyd�owska 1991) and will probably become valuable in Ediacaran biostratigraphy (Zang & Walter 1992; Moczyd�owska et al. 1993; Grey 2005; Moczyd�owska 2005).

In Papers I-III we discuss the usefulness, and potential for acritarch biostratigraphy from local and global perspectives. We show that thus far, a subdivision into five stratigraphically sequential assemblage zones is possible (following the work by Grey 2005). The zones are described as two palynofloras: the Ediacaran Leiosphere-dominated Palynoflora (ELP), defining one biozone, and the Ediacaran Complex Acanthomorph-dominated Palynoflora (ECAP), defining four succeeding biozones. Originally, this zonation scheme enabled the correlation between three tectonic units, the eastern Officer Basin, the Adelaide Rift Complex and the Amadeus Basin (Figure 5) (Grey 2005) and the information from the two new drillcores fur-ther validates this subdivision. In addition we compare Australian assem-blages to those from other continents and conclude that several taxa are shared between different palaeocontinents. These additional studies from the Officer Basin in Australia will aid in the correlation between different basins in Australia and enhance global correlation.

A most essential future step is to compare and evaluate se-lected acritarchs from different palaeocontinents in terms of taxonomic affin-ity and biostratigraphic potential.

Wall ultrastructure and biological affinities The second part of this thesis (Papers IV-V) addresses the biological affini-ties of selected acritarchs. Conventional use of transmitted light microscopy reveals mostly major morphological features, but gives no detail on small-scale wall ultrastructure. Transversal sections of acritarchs studied by use of transmission electron microscopy (TEM), on the other hand, can provide data on taxa-specific ultrastructures that is impossible to document other-wise. Talyzina & Moczyd�owska (2000) demonstrated the potential of TEM when they identified unornamented acritarchs (Leiosphaeridia spp.) as green algae, specifically related to the order Chlorococcales. This conclusion was based on the presence of a complex, layered wall ultrastructure with a diag-nostic trilaminar sheath structure known from certain modern genera. A study by Javaux et al. (2004) showed that morphologically simple, unorna-mented leiospheres were eukaryotic. TEM has been used to identify certain

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green algae and dinoflagellate cysts and therefore it should also be possible to find such characters in acritarchs. If such characters are absent, studies of wall ultrastructure can separate morphologically similar acritarchs from each other or perhaps identify acritarchs in different stages of their life cycle.

In the first case study (Paper IV) the acritarch taxon Gyalos-phaeridium pulchrum was examined using scanning- and transmission elec-tron microscopy in order to describe its wall ultrastructure. The study by use of TEM reveals a multilayered, complex wall ultrastructure, consistent with the eukaryotic clade of organisation. Although not fully conclusive with regard to the exact systematic position of Gyalosphaeridium pulchrum, the study suggests an algal origin (Figure 6).

Figure 6. Representative cross-sections through one acanthomorphic (1-2) and one leiosphae-ric (3-4) acritarch. 1) Schematic drawing of fig. 6.2 showing a four-layered wall ultrastructure (A-D) and internal void (E). 2) Cross-section of the vesicle wall of Gyalosphaeridium pul-chrum showing a four-layered wall ultrastructure (A-D) and internal void (E). 3) Leiosphaeri-dia sp., showing three distinct layers and a regular thinning of layer C. A is the external layer, B is the central (medial) layer and C the internal layer; specimen 12-A168. 4) Leiosphaeridia sp., showing a homogeneous wall ultrastructure, with a rounded internal void; specimen 3-A040.

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The second case study (Paper V) aims to discuss and compare

the wall ultrastructure of leiosphaeric and acanthomorphic acritarchs. It be-comes apparent that the wall ultrastructure of the specimens studied differs in terms of number of individual layers, their structure and thickness and that this may indicate either different biological affinities or different stages in their life cycles. In Paper IV the affinities of acritarchs to green algae and dinoflagellates are discussed, whereas Paper V deals with the possibility of some acritarchs being related to metazoans.

Life after death: the degradation of acritarchs and its consequences Before taxonomic evaluation can be made it is important to assess the pres-ervation and taphonomic history of the assemblage. Different stages of pres-ervation and degradation may influence the proper identification of taxa. Depending on the severity of degradation the result can be either an underes-timation or an overestimation of taxa present. Even though taphonomic deg-radation is experimentally relatively well known there has not been a contri-bution that graphically illustrates taphonomic degradation of acritarchs. This is undertaken in Paper VI. With focus on illustrated specimens we explain what happens to an acritarch as it is undergoes various types of degradation. We divide taphonomic processes into three broad categories: mechanical degradation, chemical degradation and biogenic degradation and show how such processes are reflected in morphological change.

New records of the Acraman impact ejecta layer and its importance In the final part of this thesis (Paper VII) we use palynological evidence, biomarker evidence and carbon isotope variations to predict the stratigraphic level of the Acraman impact ejecta layer (AIEL) in two drillcores, Munta 1 in the Officer Basin and SCYW-79 1a on the Stuart Shelf. The ejecta layer was identified close to the inferred position and the fact that it was possible to use such data to estimate the position of the AIEL indicates that there might be a causal connection between the impact and the evolving biota.

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3. Svensk sammanfattning

Den prekambriska tidsperioden omfattar nästan 90 % av jordens historia. Trots detta är det den period vi vet minst om. Den tidiga organismvärlden bestod till största delen av bakterier vilka för ungefär två miljarder år sedan fick sällskap i haven av mikroskopiskt små, encelliga organismer, så kallade acritarcher. Acritarchernas utseende är väldigt skiftande, några har formen av enkla och släta bollar medan andra kan vara komplexa taggiga fyrkanter. Det som idag finns bevarat som fossil är det yttre hölje som en gång för länge sedan var acritarchernas skydd. De flesta acritarcher anses representera något som liknade dagens grönalger men man vet inte med säkerhet vad dessa små saker var för något. De kan exempelvis representera ett yttre skydd för embryon av olika typer av djur. Acritarcher studeras för att förstå paleobiologiska frågor. Historiskt sett har de dock varit viktiga för att jämfö-ra åldern mellan olika bergarter över hela världen. Men kan vi inte använda dem till något mer spännande än så?

Istiderna Den tidsperiod som varade för ungefär 750 till 635 miljoner år sedan brukar kallas Kryogen. Nationalencyklopedin ger ordet betydelsen som alstrar mycket låga temperaturer och som namnet därmed antyder så var detta en period som saknar motstycke i jordens och livets historia. Istiderna avlöste varandra, den ena värre än den andra.

I den så kallade Snowball Earth-teorin påstår man att hela jorden var under miljontals år täckt av is, från pol till pol. Världshaven var bottenfrusna och flytande vatten fanns endast tillgängligt i närheten av var-ma källor, vulkaner eller i sprickor i isen. Kunde de enkla livsformer vi har funnit som fossil innan istiderna verkligen ha överlevt i sådana extrema mil-jöer?

Meteoritnedslaget För omkring 580 miljoner år sedan slog en stor meteorit ner i det grunda hav som då täckte södra Australien (Figur 1). Meteoritnedslaget var ett av de tio största som någonsin inträffat på jorden. Meteoriten var nästan 5 kilometer i

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diameter och slog ner med en sådan kraft att en 90 kilometer bred krater bildades på den plats som idag kallas Lake Acraman. Nedslaget var så kraf-tigt att ett världsomfattande stoftmoln förmörkade solen och förändrade livs-betingelserna för det myller av mikroskopiskt små organismer som var i behov av direkt solljus. I de vatten där meteoriten slog ner frodades små fotosyntetiserande acritarcher tillsammans med ett fåtal större organismer. Acritarcher är den dominerande gruppen som vi idag kan finna som fossil från denna tid. Långt under ytan i den australiska ödemarken ligger de fortfa-rande begravda, redo att berätta sin historia för oss.

Min forskning Genom att studera fossila acritarcher som lösts fram ur de borrkärnor som oljeprospekteringsföretag borrar upp från området så har man funnit ett spännande samband mellan meteoritnedslaget och en snabb utveckling av acritarcherna. Fossil från nivåer på båda sidor om meteoritnedslaget har givit oss de första preliminära resultaten i denna fängslande historia. Resultaten visar att acritarcherna förändras från att vara representerade av få, väldigt enkla arter utan några som helst besmyckningar före meteoritnedslaget till att representeras av många, stora och komplexa arter direkt efter nedslaget (Figur 3; Figur 4). För de organismer som överlevde nedslaget, om än i kraf-tigt reducerat antal, så gav de förändrade förutsättningarna möjligheter att utvecklas åt olika håll. Detta är ett fenomen som upprepar sig flera gånger senare i livets historia (jämför exempelvis dinosauriernas utdöende och den följande explosionen av däggdjur). Denna utveckling kan ha varit det första, och kanske avgörande, steget från encelliga organismer till den oerhörda mångfald av liv vi ser idag.

Förutom att studera utvecklingen av acritarcher så har jag även visat att man kan använda dem för att korrelera bergarter från olika kontinenter baserat på fossilinnehållet. Detta kan ha avgörande betydelse för till exempel företag som letar olja i bergarter av denna ålder. Dessutom har jag och mina medförfattare visat på hur nedbrytningen (så kallade tafono-miska processer) av organismerna påverkar vårt sett att identifiera dem.

Eftersom vi inte vet vad acritarcher har för släktskap till mo-derna organismer så har ett av målen med denna avhandling varit att försöka utröna dessa släktskap. Med hjälp av ett transmissionselektronmikroskop (en metod som gör det möjligt att ta bilder av väldigt små strukturer) har jag tittat på tvärsnitt av acritarchernas hölje (Figur 6) och jämfört resultaten med studier av nulevande organismer. Denna metod gör att man kan konstatera att de acritarcher jag studerat är eukaryota (det vill säga att de hade en cell-kärna) och att de förmodligen är besläktade med alger, men att de även kan representera tidiga djur.

Den sista studien beskriver det lager som spreds efter meteo-ritnedslaget vid Lake Acraman för cirka 580 miljoner år sedan. Lagret spreds

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över ett område med en radie av mer än 550 km. Med hjälp av acritarcher och mätbara skillnader i kolisotoper och olika biomarkörer (kemiska ämnen relaterade till levande ting) så kunde vi förutspå var detta lager borde finnas i två nya borrhål. Lagret hittades inom de förutspådda nivåerna och detta fynd gjorde att vi med stor precision kunde korrelera borrhålen till varandra och till andra borrhål i området.

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4. Acknowledgements

Is this really the only part of the thesis that you are going to read? All right then. First of all I thank my supervisor Małgorzata Moczydłowska-Vidal for her straightforward help and commitment. Kathleen Grey at the Geological Survey of Western Australia is a constant inspiration and support through email and I appreciate that enormously. The past and present palaeobiology staff including, in alphabetical order, Keith Bennett, Graham Budd, Lars Holmer, John Peel and Ulf Sturesson have all helped in various ways, in-cluding financial support, reading drafts of manuscripts and writing refer-ence letters. I thank Jan Ove Ebbestad for help with many practical issues. Joakim Samuelsson questioned my research (politely) and Michael Streng was being Michael Streng (which means finding obscure literature, fixing printers, etc. etc.), which is much appreciated. Martin Stockfors was most helpful during my first years and we spent perhaps too little time discussing acritarchs over a coffee and too much time discussing coffee.

It wouldn’t be work without someone to work with so there-fore I thank Chris Atkins, Uwe Balthasar, Allison Daley, Åsa Frisk, Anette Högström, Ralf Janssen, Cecilia Larsson, Anna Lindström, Gerard Malsher, Sandra Pettersson-Stolk, Christian Skovsted, Martin Stein, Jenny Sälgeback and Sofia Wennberg for making this place bearable.

I also want to show my appreciation to the Köping crew, Kalle, Micke, Nicklas, Sara and Peter, Tobbe, and their girlfriends for good times, those been and those yet to come.

I have been fortunate to attend courses, conferences and field trips in Australia, twice in China, India, Bahamas, twice in the USA, Wales, England, Portugal, Spain, Norway and different parts of Sweden during the past few years. This would not have been possible without funding from various sources. I would like to express sincere thanks to University of Washington, the Agouron Institute, The Royal Swedish Academy of Sci-ences, NorFA, the Palaeontological Association, Sederholms för utrikes resor, Ingegerd och Viking Olof Björks hedersledamotsstipendium, Geolo-giska sektionen, Rektors Wallenbergmedel, Vetenskapsrådet, XI Interna-tional Palynological Congress and IGCP 512 - Neoproterozoic Ice Ages. I would also like to thank Sadoon Morad for sending me to freezing Svalbard in the middle of summer.

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I have undoubtedly forgotten someone because this was, of course, written at a very late stage. My apologies if that is the case. Therese Huldtgren, du är bäst. Slutligen vill jag tacka mina föräldrar Ann-Christine och Jan och brorsan Christofer för att ni alltid stöttar och gör mer än vad man kan förvänta sig.

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5. References

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Huntley JW, Xiao S, Kowalewski M (2006) 1.3 Billion years of acritarch history: An empirical morphospace approach. Precambrian Research 144, 52–68. Javaux EJ, Knoll AH, Walter MR (2001) Morphological and ecological complexity in early eukaryotic ecosystems. Nature 412, 66–69. Javaux EJ, Knoll AH, Walter MR (2004) TEM evidence for eukaryotic diversity in mid-Proterozoic oceans. Geobiology 2, 121–132. Knoll AH (1994) Proterozoic and Early Cambrian protists: evidence for ac- celerating evolutionary tempo. Proceedings of the National Academy of Sciences of the United States of America 91, 6743–6750. Knoll AH, Javaux EJ, Hewitt D, Cohen P (2006a) Eukaryotic organisms in Proterozoic oceans. Philosophical Transactions of the Royal Society B 361, 1023–1038. Knoll AH, Walter MR, Narbonne GM, Christie-Blick N (2006b) The Edia- caran Period: a new addition to the geologic time scale. Lethaia 39, 13– 30. Martin MW, Grazhdankin DV, Bowring SA, Evans DAD, Fedonkin MA, Kirschvink JL (2000) Age of Neoproterozoic bilaterian body and trace fossils, White Sea, Russia: Implications for metazoan evolution. Science 288, 841–845. McFadden KA, Huang J, Chu X, Jiang G, Kaufman AJ, Zhou C, Yuan X, Xiao S (2008) Pulsed oxidation and biological evolution in the Ediacaran Doushantuo Formation. PNAS 105, 3197–3202. Moczyd�owska M (1991) Acritarch biostratigraphy of the Lower Cambrian and the Precambrian–Cambrian boundary in southeastern Poland. Fossils and Strata 29, 125 pp. Moczyd�owska M (2005) Taxonomic review of some Ediacaran acritarchs from the Siberian Platform. Precambrian Research 136, 283–307. Moczyd�owska, M, Vidal G, Rudavskaya VA (1993) Neoproterozoic (Ven- dian) phytoplankton from the Siberian Platform, Yakutia. Palaeontology 36, 495–521. Mojzsis SJ, Arrhenius G, McKeegan KD, Harrison TM, Nutman AP, Friend CRL (1996) Evidence for life on Earth before 3,800 million years ago. Nature 384, 55–59. Morton JGG (1997) Lithostratigraphy and environments of deposition. In: Morton JGG, Drexel JF (Eds.), Petroleum Geology of South Australia. Officer Basin. South Australia. Department of Mines and Energy Re- sources Report Book 3, 47–86. Narbonne GM (2005) The Ediacaran biota: Neoproterozoic origin of animals and their ecosystems. Annual Review of Earth and Planetary Sciences 33, 421–442. Narbonne GM, Gehling JG (2003) Life after Snowball: the oldest complex Ediacaran fossils. Geology 31, 27–30. Peterson KJ, Butterfield NJ (2005) Origin of the Eumetazoa: testing ecological predictions of molecular clocks against the Proterozoic fossil record.

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Acta Universitatis UpsaliensisDigital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 428

Editor: The Dean of the Faculty of Science and Technology

A doctoral dissertation from the Faculty of Science andTechnology, Uppsala University, is usually a summary of anumber of papers. A few copies of the complete dissertationare kept at major Swedish research libraries, while thesummary alone is distributed internationally through theseries Digital Comprehensive Summaries of UppsalaDissertations from the Faculty of Science and Technology.(Prior to January, 2005, the series was published under thetitle “Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology”.)

Distribution: publications.uu.seurn:nbn:se:uu:diva-8684

ACTA

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2008

AbstractList of publicationsContents1. IntroductionAims and introduction to the studiesThe Precambrian – life evolves, the environment changesThe Ediacaran PeriodWhat is an acritarch?

2. Ediacaran organic-walled microbiota from AustraliaNew records and the potential for biostratigraphyWall ultrastructure and biological affinitiesLife after death: the degradation of acritarchs and its consequencesNew records of the Acraman impact ejecta layer and its importance

3. Svensk sammanfattningIstidernaMeteoritnedslagetMin forskning

4. Acknowledgements5. ReferencesActa Universitatis Upsaliensis

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