ScienceWeek

PALEOBOTANY: ON FOSSILS OF THE FIRST LAND PLANTS

The following points are made by Paul Kenrick (Nature 2003 425:248):

1) The significance of microscopic spores entombed in rocks dating from about 440–470 million years ago has puzzled investigators of early life. Are these spores proof that plant life existed on land long before the time suggested by other forms of fossil evidence? And, if so, what sorts of plants do they represent? Wellman et al (1) present direct evidence of the life forms that produced these enigmatic spores, and their findings lend credence to the notion that minute plants existed on land 470 million years ago.

2) Spores are produced by land plants in prodigious quantities. These robust, decay-resistant particles can become incorporated into sediments, providing a record of floral change through geological time. Careful study of rocks dating from the Ordovician period, 443–489 million years ago, has revealed an unexpected diversity of spores that are much older than the fossilized remains of plants that could have produced them(2,3). These minute grains might represent evidence of the earliest land flora, but how can we be sure that these spores came from bona fide land plants, and what can they tell us about the nature of this early flora?

3) Answering these questions is difficult because the most ancient spores are rather odd. Instead of being dispersed as single grains, many are fused in pairs or in groups of four, and some are enclosed in an extra membrane(2,3). These so-called permanent diad and permanent tetrad configurations are unlike the spores of most living plant species, but they do bear some resemblance to the spores of certain present-day liverworts. Diad and tetrad spores have also been found in land-plant fossils dating from an early part of the Devonian period (400–417 million years ago)(4,5).

4) One school of thought holds that the tetrads and diads of the Ordovician period are evidence of land plants that are related to living bryophytes (liverworts, mosses and their kin)(2-5). Others contend, however, that the data linking these spores to bryophytes are too tenuous. The spore-producers might be close relatives of land plants, but that does not necessarily make them bryophytes. It is conceivable that in both ecological and physiological terms they were little more than aquatic algae. Direct evidence of the life forms that produced the Ordovician spores could settle this matter, but so far this has proved elusive.

5) Wellman et al(1) take us a step closer to resolving this controversy. They used standard spore-extraction methods to recover organic residues from Ordovician sediments collected from a borehole in Oman. When they passed the insoluble organic material through a series of sieves designed to trap plant fragments and spores of various sizes, the authors found many well-preserved spores -- but they also found something much more intriguing. Trapped in the sieve containing the largest pores were elongated, disc-shaped objects that, on closer inspection, proved to be clumps of spores packaged in a type of cuticle. These fossil fragments are exciting because they resemble the spore-bearing organs of later land plants. So Wellman and colleagues' trawl through organic residues had netted a catch of the tiny plants that produced the spores. Although far from complete, these specimens indicate that the Ordovician spores were indeed produced by land plants and not by algae.

References (abridged):

1. Wellman, C. H., Osterloff, P. L. & Mohiuddin, U. Nature 425, 282-285 (2003)

2. Edwards, D. & Wellman, C. H. in Plants Invade the Land: Evolutionary and Environmental Perspectives (eds Gensel, P. G. & Edwards, D.) 3-28 (Columbia Univ. Press, New York, 2001)

3. Gray, J. Phil. Trans. R. Soc. Lond. B 309, 167-195 (1985)

4. Edwards, D., Duckett, J. G. & Richardson, J. B. Nature 374, 635-636 (1995)

5. Wellman, C. H., Edwards, D. & Axe, L. Bot. J. Linn. Soc. 127, 117-147 (1998)

Nature http://www.nature.com/nature

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ATOMIC FORCE MICROSCOPY OF PRECAMBRIAN MICROSCOPIC FOSSILS

The following points are made by A. Kempe et al (Proc. Nat. Acad. Sci. 2002 99:9117):

1) Traditionally, understanding of the history of life has been based chiefly on studies of the morphology of fossils. Morphology, however, can provide only limited insight about the underlying biochemical and physiological capabilities of ancient organisms, a deficiency particularly detrimental to understanding the earliest, Precambrian, seven-eighths of life's history. Unlike the more recent, shorter, and much more familiar Phanerozoic fossil record, that of the Precambrian was dominated by diverse prokaryotic microorganisms having only a limited range of simple morphologies yet widely divergent metabolic capabilities. Because prokaryotic taxa having more or less identical morphologies can differ greatly in metabolic capability, and because the evolutionary development of these various capabilities had profound effects on the evolution of the Earth's oceans, atmosphere, and surficial environment (1), there is a fundamental need to develop new techniques that by correlating chemistry with morphology in individual Precambrian microscopic fossils can provide insight into their underlying biochemical makeup.

2) Significant progress toward answering this need has been made by using ion microprobe spectrometry to analyze the carbon isotopic composition of single Precambrian microfossils (2,3), an approach to understanding the chemistry of such fossils that has recently been extended to a molecular level by laser-Raman spectroscopic imagery of individual Precambrian fossil microbes (4,5). In principal, information about the structural makeup of such molecular components should be accessible by using atomic force microscopy (AFM), a technique used routinely in material science to elucidate the nm-scale structure of macromolecules such as DNA. The authors use AFM to image the fine structure of the cell walls of Precambrian microfossils, an approach to this problem that coupled with laser-Raman spectroscopy reveals the submicron-scale organization of their kerogenous components. This combination of AFM and laser-Raman spectroscopy provides means not only to elucidate the fine structure of individual microscopic fossils but to investigate the geochemical maturation of ancient organic matter, the mechanisms that underlie fossil preservation by permineralization, and, potentially, to determine whether carbonaceous microscopic fossil-like objects are true fossils rather than pseudofossil "look-alikes."

3) In summary: Atomic force microscopy (AFM) is a technique used routinely in material science to image substances at a submicron (including nm) scale. The authors apply this technique to analysis of the fine structure of organic-walled Precambrian fossils, microscopic sphaeromorph acritarchs (cysts of planktonic unicellular protists) permineralized in 650-million-year-old cherts of the Chichkan Formation of southern Kazakhstan. AFM images, backed by laser-Raman spectroscopic analysis of individual specimens, demonstrate that the walls of these petrified fossils are composed of stacked arrays of 200-nm-sized angular platelets of polycyclic aromatic kerogen. Together, AFM and laser-Raman spectroscopy provide means by which to elucidate the submicron-scale structure of individual microscopic fossils, investigate the geochemical maturation of ancient organic matter, and, potentially, distinguish true fossils from pseudofossils and probe the mechanisms of fossil preservation by silica permineralization.

References (abridged):

1. Schopf, J. W. (1999) Cradle of Life, The Discovery of Earth's Earliest Fossils (Princeton Univ. Press, Princeton).

2. House, C. H. , Schopf, J. W. , McKeegan, K. D. , Coath, C. D., Harrison, T. M. & Stetter, K. O. (2000) Geology 28, 707-710.

3. Ueno, Y. , Isozaki, Y. , Yuimoto, H. & Maruyama, S. (2001) Int. Geol. Rev. 43, 196-212.

4. Kudryavtsev, A. B. , Schopf, J. W. , Agresti, D. G. & Wdowiak, T. J. (2001) Proc. Natl. Acad. Sci. USA 98, 823-826.

5. Schopf, J. W. , Kudryavtsev, A. B. , Agresti, D. G. , Wdowiak, T. J. & Czaja, A. D. (2002) Nature (London) 416, 73-76.

Proc. Nat. Acad. Sci. http://www.pnas.org

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