Y

Limits of extremophiles? Sedimentary rocks or not? Taphonomic modification?

A hollow tube is not evidence of life

Nature of branching, size changes, directionality, fractal? Could this indicate Eukarya. How could a prokaryote do this? Taphonomic difficulties, geochemical corollary - is it a nucleus or a black blob?

Population dynamics: Prokarya large populations of similar forms. Eukarya more diversity and potentially recognisable developmental systems

Single celled Eukarya can be as small as Prokarya and as large as 30-40 cm, overlapping with many multicelled Eukarya. Constraints on surface area to volume ratios of prokaryotes may help to distinguish them from abiotic artefacts This is suggestive of Prokarya, but is plausible for many abiogenic mechanism e.g. the growth of crystallites

Do prokaryotes and eukaryotes do this differently?

that are yet to be fully deciphered (see review by Grotzinger and Knoll, 1999). Some first order controls on their abundance have been identified such as the changes in seawater carbonate super-saturation through time (e.g. Riding and Liang, 2005). Stromatolite macro-morphology has also been shown not to be uniquely biogenic by both numerical modelling studies (e.g. Grotzinger and Rothman, 1996) and recent experimental studies (e.g. McLoughlin et al., 2008). Branched, dendritic stromatolite morphologies can be produced by diffusion limited aggregation processes and more condensed, stratiform to domal forms can be modelled by interface growth equations such as the Kardar-Parisi-Zhang equation (see review by Grotzinger and Knoll, 1999). However, there are some workers who argue that coniform stromatolite morphologies in particular, may be uniquely biogenic, as it is thought that these forms require diffusion up the flank of the cones. This upward diffusion is contended to only take place within a microbial mat (Jogi and Runnegar, 2005). In light of these discussions, the rejection of an "abiogenic null hypothesis" for an individual stromatolite occurrence requires careful testing of the relationships between stromatolite morphology, distribution and depositional environment, in addition to examining any micro-textures and geochemical signatures that may be preserved. Numerous lists of criteria have been proposed for testing the biogenicity of candidate stromatolites, and these are discussed extensively elsewhere (see Hofmann, 2000; Buick et al., 1981). We summarise these criteria in Fig. 2, a flow diagram that can be used to assess the biogenicity of any given stromatolite. Further techniques are emerging that may help investigate stromatolite biogenicity such as high resolution elemental and isotopic analysis at a nanometre scale using NanoSIMS. This gives the opportunity to investigate putative microbial fabrics at a scale never previously obtainable (e.g. Wacey et al., 2008).

Some of the oldest putative stromatolites are described from the 3.4 Ga Strelley Pool Chert of West Australia. These coniform structures, first reported by Lowe (1980), were initially interpreted as biogenic structures, but this interpretation was later rescinded in favour of an abiogenic evaporitic precipitation (Lowe, 1994). The Strelley Pool Chert stromatolites are discussed further elsewhere in this volume (see Wacey et al.) including those from the remarkable Trendall locality, which is notable for the large coniform stromatolites. The structures exhibit a diverse range of coniform and rare columnar morphologies, with significant variation in size, plus examples of putative branching (see Hofmann et al., 1999). A biological origin for these structures has been advanced based largely upon morphological and sedimentological arguments plus rare earth element data suggestive of a shallow marine setting (Van Kranendonk et al., 2003; Hofmann et al., 1999). In a recent study, Allwood et al. (2006) present a shallow-water carbonate platform depositional model for the Strelley Pool Chert to support a phototrophic origin for these stromatolites. Our own field work, undertaken across the whole of the outcrop belt, however, leads us to contest this interpretation. In the West Strelley belt, for example, small unbranched coniform stromatolites are common and do not show strongly depth controlled changes in morphology or distribution (McLoughlin et al., in preparation). We also find a close interrelationship between coniform stromatolites and crystal fan arrays, upon which they can be

Marsupial Lion Flow Chart

Figure 2. (overleaf) Flow chart summarising the hierarchical series of questions that has been proposed to assess the biogenicity of candidate stromatolites. (Key: Yellow rectangle = decision regarding geologic context; green rectangle = decision regarding biogenic morphology and processing; blue rectangle = decision regarding geochemical evidence for biological processing; pink oval = answer; white diamond = variable; white cloud = additional consideration.)

Figure 2. (overleaf) Flow chart summarising the hierarchical series of questions that has been proposed to assess the biogenicity of candidate stromatolites. (Key: Yellow rectangle = decision regarding geologic context; green rectangle = decision regarding biogenic morphology and processing; blue rectangle = decision regarding geochemical evidence for biological processing; pink oval = answer; white diamond = variable; white cloud = additional consideration.)

seen to nucleate, suggesting a strong physio-chemical component to their growth. In the absence of compelling micro-textural and geochemical evidence for microbial mat remains in these stromatolites, we argue that questions remain regarding the biogenicity of at least the simplest, unbranched coniform stromatolites of the Strelley Pool Chert (McLoughlin et al., in preparation). In other words the most probable abiotic null hypothesis has not been rejected in all cases and so demands further testing.

What can confidently be deciphered from the morphology of laminated stromatolites? Firstly, the morphology and distribution of stromatolite build-ups or 'bioherms' can be used as a palaeo-current indicator (Hoffmann, 1967) given that their axes of accretion often inclines towards the sediment source and 'bio-herms' may show elongation parallel to the current direction. It has also long been appreciated that a predictable sequence of stromatolite morphologies occurs with changing water depths, as first recognised in Proterozoic carbonate shelves of Northern Canada, and this can be utilised as a relative palaeo-depth proxy (e.g. Hofmann, 1976). In the rare cases where exceptional preservation permits, efforts have also been made to correlate changes in stromatolite morphology with the preserved microfossil communities that they contain. For example, it has been argued that the microstructure and microfossil biota preserved within an outcrop of three vertically intergrading stromatolite morphologies from the Gunflint Chert of Ontario Canada, can account for the gross changes in morphology (Awramik and Semikhatov, 1979). The challenge here, however, is to demonstrate a one to one causal relationship. Namely that the microfossil are actively involved in accreting the stromatolite structure rather than just being passively entombed and are the dominate control on the resultant stromatolite morphologies. This requires the elimination of a multitude of physical and chemical factors that also operate in the sedimentary environment the stromatolite accreted. For instance, changes in current velocity andor seawater carbonate super-saturation may also account for some of the changes seen in stromatolite morphology. In the Bitter Springs Formation, for instance, it has been demonstrated that cyclic changes in stromatolite morphology, in particular the onset of branching, can be attributed to changes in the sedimentary environment rather than the microbial community (Southgate, 1989). In Mesoproterozoic stromatolites from China, it has also been shown that changes in sedimentation rate have a strong control on the community composition, density and orientation of frame building microfossils within the stromatolites (Seong-Joo and Golubic, 1999). In summary, we agree with previous authors that stromatolites hold the potential to act both as "evolutional mileposts" and "environmental dipsticks" if only we can decipher the morphological expression of the physical, chemical and biological controls upon their growth. Like attempting to decipher the poem Jabberwocky, we need to understand the alphabet and grammatical rules that together make up stromatolite morphologies. This enigmatic code will only be cracked by continued numerical and experimental modeling combined with field and petrographic mapping of fossil stromatolites.

4. The Ediacara Biota and the Origin of Animals

The fossils from the most recently defined geological interval of Precambrian time (Knoll et al., 2004), the Ediacaran, are perhaps amongst the most controversial of all. They sit at the critical point just before the Cambrian explosion and as such hold clues to understanding the mechanisms and processes that triggered arguably, the greatest of all evolutionary events. If only it could be decided what the fossils of the Ediacaran rocks actually represented© The morphology of many of the forms preserved in the Ediacara biota is reassuringly familiar. These impression fossils have been variously argued to represent the remains of: jelly fish, sea pens (a soft coral group), worms, molluscs, soft bodied sea urchins, and arthropods (see review by Antcliffe and Brasier, 2008). The early classifications of the biota were based upon simple morphological comparisons (e.g. Glaessner, 1984; Jenkins, 1992), and these have not withstood the test of time. Slowly, the pantheon of Ediacaran animals is crumbling. The interpretation of the Ediacara biota is a good example of a Kuhnian paradigm shift that may be nearing completion. It was in a series of classic papers that this paradigm was first challenged on grounds that the forms attributed to so many different groups actually shared a similar fundamental construction that could not be matched in any modern group. This uniquely Ediacaran group was termed the Vendobionta (Seilacher, 1984, 1989; Buss and Seilacher, 1994). Since then it has emerged that some forms previously thought to be jelly fish are actually attachment disks for other organisms, now termed Aspidella terranovica (Gehling et al., 2000), while others have been reconsidered, correctly in our view, as microbial communities (Grazhdankin and Gerdes, 2007). The idea that sea pens are represented in the Ediacaran biota has also recently been challenged on developmental grounds, see Antcliffe and Brasier (2007, 2008). The possibility that some of the 'lesser' forms such as some of the disks may be abiogenic in origin has not yet been exhaustively tested (see discussion earlier regarding Kullingia and Jensen et al., 2002) though, as noted above, some have recently been relegated to microbial (Grazhdankin and Gerdes, 2007). Seilacher et al. (2003) proposed that many of the forms usually attributed to the Ediacara biota may actually be giant single celled Eukaryotes similar to the foraminiferid group the Xenophyophorea. This is the first time that a non multi-celled model has been postulated for the Ediacara biota (with the exception of the early paper by Ford, in 1958, in which Charnia was compared to an alga). Whether Seilacher et al. (2003) are correct or not is still open to discussion; nonetheless, their hypothesis is the first explicit testing of the eukaryote null hypothesis and as such is a significant contribution to discussion within the field (again see Antcliffe and Brasier, 2008 for a wider discussion of affinities). If the advocates of an animal affinity for the Ediacara biota are to sustain these arguments, then the hypothesis of single celled Eukarya origin must be continually tested and refuted in each instance. Otherwise, in our view this is the null hypothesis that we must revert to (presuming of course that the abiotic null hypothesis has already been rejected).

So, what of the other forms still hanging on to that crumbling temple of ancient animal taxonomy? Perhaps the two most important forms that have not yet been discussed here are the "Precambrian worm" Dickinsonia (e.g. Glaessner, 1984; Gehling, 1991; Gehling et al., 2005) and also the "Precambrian mollusc" Kimberella (Fedonkin and Waggoner, 1997; Runnegar, 1992; Gehling, 1991). We note that Seilacher et al. (2003) gave exemption to Kimberella in their xenophyophore hypothesis.

This is interesting as many singled celled organisms achieve startling size and complexity, the giant foraminifera not the least. Perhaps Kimberella is unrelated to the other Ediacaran forms discussed by Seilacher et al. (2003) but that does not mean that it should be exempt from similar scrutiny. At the current time of writing, there has not yet been a challenge to the 'Kimberella is a mollusc' hypothesis, and there has been little consideration of a non-metazoan affinity. Yet, it should be remembered that 20 years ago its affinities to the cubozoa (a jellyfish group) seemed assured (Glaessner, 1984).

The interpretation of Dickinsonia is perhaps the most controversial of all the Ediacaran organisms. Dickinsonia has been translated variously as a 'dipleurozoan' cnidarian (Harrington and Moore, 1956), an annelid (Wade, 1972; Runnegar, 1982; Glaessner, 1984), a platyhelminth (Palij et al., 1979; Fedonkin, 1981), in its own unique phylum (Fedonkin, 1985), a member of the uniquely Ediacaran Vendobionta (Seilacher, 1985, 1989), or even perhaps a subdivision of the Vendobionta the 'Dickinsoniomorphs' of Brasier and Antcliffe (2004). Other workers have been less specific and likened 'the grade of its organisation,' to groups such as 'cnidarian-like' (Valentine, 1992), 'annelidian' (Conway Morris, 1979), and 'xenophyophore-like' or some other giant single celled Eukarya (Seilacher et al., 2003), or as a member of other potentially mystifying groupings of the Ediacaran such as the 'Dipleurozoa' of Dzik and Ivantsov (2002) or the 'Protarticulata' of (Fedonkin, 2003), or a member of the 'Petalonamae' of Pflug (1970, 1972). Recall here the "Jubjub bird" and the "frumious Bandersnatch", not to mention the "borogoves" and the "mome raths," who feature alongside the mysterious Jabberwocky (Carroll, 1871). It makes one wonder what scientists would make of the jabberwocky. Even though the jabberwocky is pure invention, it could hardly attract a wider range of possible affinities than Dickinsonia. Such a profusion of taxonomy results when there is no agreed methodology to test between many hypotheses. It is possible to narrow the list of hypotheses, and many of these ideas have great merit, but as the profusion of possible affinities continues, scientist seem more and more resigned to the idea that we will never understand what Dickinsonia is. As we have said a number of times already in this paper, it is essential to work with multiple hypotheses and test them against each other. But how are we to perform such a test? What criteria can be used? One of the authors (JBA) has argued elsewhere that one possible test that may help to distinguish between these many hypotheses is the consideration of the growth and development of Ediacaran organisms (see Brasier and Antcliffe, 2004; Antcliffe and Brasier, 2007, 2008), and we reiterate the rationale behind this approach once again here.

The standard method for establishing phylogenentic relationships is, of course, cladistics. Like all methods, however, the answers arising from cladistic analysis are only as good as the quality of input (see Felsenstein, 1978; Maddison, 1991; Page and Holmes, 2001 for approaches and problems). That said, we would argue that cladistic analysis provides the only sure way to approach the problem of affinities for the Ediacara biota. A major challenge within any cladistic study is to be sure that the features being considered are homologies and not anything else. That is to say, these diagnostic features should be shared as a result of common ancestry (e.g., five digits in reptiles and mammals) rather than any other reason, such as evolutionary convergence (e.g., wings in birds and bats). Curiously, no work known to us has sought to test and establish homologies (using the standard criteria outlined below) between members of the Ediacara biota and any other group of organisms, living or fossil. Hence, no phylogenetic hypothesis, given above, has yet gained support from a cladistic analysis. We recommend that such an analysis should follow the criteria for the establishment of homologies formalised by Patterson (1982). For a particular feature to be considered a homology between two groups, it ideally needs to satisfy the following three criteria. The more of these three it can satisfy, the more confidence there is in any suggested homology. Firstly, the feature should appear and develop in similar ways in the two forms being compared. (Examples of this are the ways that certain embryos share a similar pattern of cell division or the similar way in which the vertebral column develops in mammals, here called 'the ontogenetic criterion'.) Secondly, the derivation of the features should be traceable back through the fossil record - for example, the way in which the hand of a human, a bat and a whale merge into one common structure in their last common ancestor. Hopefully, we can trace the differentiation of such structures through the course of the fossil record (here called 'the phylogenetic criterion'). Thirdly, homologies should have further, associated, homologies that also meet the first two criteria. Hence, a named carpel bone in the 'hand' of a human, bat and whale is surrounded by further carpel bones that also can be satisfied as homologous (here called 'the association criterion'). Could such an analysis help us understand the nature of the ancient and enigmatic Ediacara biota? Problems certainly emerge with the second criterion (phylogenetic) whenever we are interested in the first fossils of a particular group, because we cannot trace the differentiation of features when there are no preceding fossils. Moreover, the third criterion is only useful if we have already satisfied one of the first two criteria. When used alone, we cannot avoid circular reasoning. For the Ediacara biota, therefore, we must place our belief in the analysis of development as a methodology that is not just informative for the analysis of homology but also comprises the homology itself. In this way, the study of development may be our only direct key to unlocking the phylogeny and evolution of the Ediacara biota. It should be noted that this emphasis upon development is not to be confused with the discredited 'recapitulation' hypothesis of Haeckel (1879), whose biogenetic law made the mistake of comparing the embryo and development of one organism with the adult of another. What we would advocate is comparison of the embryo and of development in both organisms. Nor, indeed, does it have anything to do with the modern theory of 'heterochrony' (see McNamara, 1990). Both ideas are concerned with understanding how ontogenetic states actually change throughout phylogeny. In the case of 'recapitulation', there follows an inductive extrapolation of such changes backward in time to reveal a hypothetical phylogeny. What we advocate above is something quite different: an understanding of the similarities within the developmental programs of two or more organisms. Development deserves to be highly weighted within any phylogenetic analysis and particularly when dealing with enigmatic fossils and the origin of any major group. The Ediacara biota satisfies both of these criteria, so the need to understand their development can hardly be overstated. This first attempt has now been made to understand the growth and development of some members of the Ediacara biota (Antcliffe and Brasier, 2007, 2008) and has helped to narrow this list of possible affinities for these fossils.

5. Concluding Remarks: "And, has thou slain the Jabberwock?"

Those who investigate the Precambrian fossil record for the rise of life and the major animal groups seek answers to very similar questions: what are the criteria for recognising putative biological structures? How can these criteria be best measured or observed? What new technologies or techniques may aid our progress? Palaeontologists and astrobiologists must also deal with very similar challenges and ask: how uniformitarian can we be about a planet, be it the early Earth or another planet that is truly alien to us?

From the origin of life to the emergence of animals and everything in between, the earliest fossil record is riddled with pitfalls that may lead the unwary palaeontologist to misinterpret their data. We hope to have explained that all structures should initially be considered to be abiogenic until this hypothesis can no longer be sustained. There is always the possibility that a structure is abiogenic because abiogenesis must have existed long before (perhaps 8 or 9 billion years before) processes that we recognise today as biogenic could have developed. Furthermore, abiogenic processes have continued to operate throughout the whole of earth history and the morphospace of abiogenesis remains poorly mapped.

When Precambrian palaeontologists are faced with usual fossil morphologies, there will always be the temptation to erect another Jabberwocky: a mythical beast with uncertain affinities that will reside in our consciousness regardless of how secure the science behind this interpretation may be. So how are such Jabberwock(ies) slain? We must strive to always adopt a critical approach to the enigmatic morphologies we are faced with, and then it will become harder to build new Jabberwockies. But what of the Jabberwockies that already exist? Is there any hope that we may ever really be able to confidently state the affinities of creatures like Dickinsonia for instance? Or to establish whether a particular stromatolite is of biogenic origin or not? We believe that there is considerable reason to be optimistic. New methodologies and techniques are continually presenting themselves, some of which are reviewed above. Finally, we draw some consolation from the fact that enigmatic fossils were not created by Lewis Carol or anyone else. The fossil record is unlike the Jabberwocky, which was written with deliberate obfuscation, so we will never know the express intent of the author, save that to create debate and stimulate a philological frenzy. There is no miscreant author of the fossil record that deliberately set out to confuse us. The fossil record is the product of processes that are understandable in terms of the natural world and given time and the appropriate methodology, there is no reason why they cannot be deciphered.

6. Acknowledgements

Both authors acknowledge the guidance of their doctoral advisor Professor M.D. Brasier who encouraged the development of many of the ideas presented in this chapter. This work has also greatly benefited from discussion with L. Battison and S. Moorbath. We gratefully thank K. Grey of the Geological Survey of Western Australia and also the Darwin Centre at the Natural History Museum London for access to their collections. Also to Jane Ellingsen who kindly drafted Fig. 1. This contribution was made possible by NERC research grants to J. Antcliffe and N. McLoughlin and a Norwegian Research Council post-doc to N. McLoughlin.

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Biodata of Helga Stan-Lotter, author (with her co-authors) of "Microorganisms in the Ancient Terrestrial Subsurface - And in Outer Space?"

Professor Dr. Helga Stan-Lotter is currently at the Department of Microbiology of the University of Salzburg, Austria. She obtained her Ph.D. in 1976 from the Technical University of Munich, Germany. She was a postdoc at the University of Calgary (Canada) and a research associate at the University of British Columbia, Canada. She held a US National Research Council Fellowship at NASA Ames Research Center in Moffett Field, California. Her scientific interests are extremophilic microorganisms and astrobiology.

E-mail: [email protected]

Helga Slan-Lotter
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