ScienceWeek

EVOLUTIONARY BIOLOGY: ON THE EDIACARAN ENIGMA

The following points are made by M. Brasier and J. Antcliffe (Science 2004 305:1115):

1) Fossils and Egyptian hieroglyphs share daunting similarities: Both consist of arcane geometries, glyphs in rock that conceal deeper meanings from the rude enquirer, and are capable of false translation (1, 2). To read the fossil runes correctly, the paleontologist craves the stimulus of fresh fossil finds, channeled by insightful methodology to catalyze productive thought. The recent discovery of remarkably well-preserved, three-dimensional Ediacaran fossils in Newfoundland reported by Narbonne (3) may provide such a stimulus. The new fossil find raises the question of whether the study of the life history and growth plan of these fossil animals could provide a Rosetta stone for decoding Ediacaran animal evolution (4,5).

2) The Ediacara biota remains one of the greatest enigmas within evolutionary paleobiology. Discovered in 1946 by R. C. Sprigg in the Flinders Ranges of southern Australia, the Ediacara biota --which is 580 to 543 million years old (Ma) -- represents the most ancient complex organisms on Earth. Martin Glaessner (1984) provided the first insights into Ediacaran biology. He saw in the fossils of Ediacaran animals (so remarkably preserved in late Precambrian sandstones) the ancestors of Phanerozoic animal phyla. The Cambrian is the first period with abundant fossils and marks the start of the era of the Phanerozoic or "visible life" that continues through to the present. Before this came the vast interval of the Precambrian, which ranges back to the origins of the Earth about 4600 million years ago.

3) Paleontologists eagerly sought relationships between Ediacaran fossils and living seapens and worms, jellyfish and crabs. This "great ancestral" view has held sway for almost 40 years, but a growing number of paleontologists argue that Ediacaran creatures were not ancestral to Cambrian life at all. They suggest that members of the Ediacara biota were uniquely fashioned beasts that met their doom at the end of the Precambrian. Ediacaran animals -- each "quilted" like a mattress -- take many forms, resembling desiccated lichens, underwater fungi, enormous ferns, or giant deep-sea single-celled protists.

4) The authors believe the time has come to raise difficult questions about the methodology used to analyze Ediacaran fossils. Ediacaran forms once thought to be "jellyfish" by Glaessner (1984) have been reinterpreted as the attachment discs of fernlike fronds. And fronds once placed in discrete taxa now seem to be part of a much wider spectrum of intergrading forms (3). Could it be that other "Glaessnerian species" are not biological species in the sense of Mayr (2002) at all, but mere organs, different growth stages, or ecophenotypes of a single taxonomic unit?

5) The authors state their concern is that the current "Ediacaran species concept" is no longer tenable. It is based on a "typological" approach using type specimens rather than populations, and on an "analog" approach that compares fossil morphologies with modern organisms according to assumed similarities. But these similarities could well have evolved independently. This approach is therefore unsound for deciphering long-extinct groups and, unlike cladistics, is an insecure basis for classification. We need quantitative studies of fossil populations, with analysis of morphological gradients in the same geological successions and bedding planes, as well as detailed analyses of growth programs (morphospace), life history (ontogeny), and evolutionary history (phylogeny) (4-5). It is premature to put forth any evolutionary history for fossils whose diagnosis has been conceived without reference to a postulated growth program observed through successive stages of ontogeny. Without such reference, both the taxonomic pattern and the evolutionary processes responsible for it will remain obscure.

References (abridged):

1. J. W. Schopf, The Cradle of Life (Princeton Univ. Press, New York, 1999)

2. M. D. Basier et al., Nature 416, 76 (2002)

3. G. M. Narbonne, Science 305, 1141 (2004)

4. J. Antcliffe, M. D. Brasier, Workshop on the Rise and Fall of the Vendian (Ediacaran) Biota, International Commission of Stratigraphy, Prato, Italy, August 2004

5. J. Slack, thesis, University of Oxford (2003)

Science http://www.sciencemag.org

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EARTH SCIENCES: A NEW GEOLOGIC PERIOD

Notes by ScienceWeek:

The term "Ediacaran" refers to an assemblage (until recently the oldest) of soft-bodied marine animals, the assemblage first discovered in the Ediacara Hills in Australia. Although the recent discoveries of Ediacaran metazoans have extended the record of sponges and bilateral animals to 570 million years ago, the biological affinities of many Ediacaran organisms remains controversial.

The following points are made by A.H. Knoll et al (Science 2004 305:621):

1) The geologic time scale stands as a major achievement of 19th-century science, a coherent record of our planet's history fashioned from myriad details of individual rock outcroppings. The eras, periods, and finer divisions of the scale not only codify geologic time, they reflect our accumulated understanding of Earth's past -- or at least its more recent past. The Cambrian Period, with its fossil record of animal diversification, began only 543 million years ago (Ma), when Earth was already 4000 million years old. In the 19th century and for much of the 20th century, the beginning of the Cambrian (also the beginning of the Paleozoic era and the Phanerozoic eon) marked the most distant temporal reaches of Earth's tractable historical record.

2) The absence of skeletonized fossils that mark Phanerozoic time made Precambrian rocks difficult to correlate, and so the fine stratigraphic divisions of the younger record gave way to broad intervals that permitted only limited insight into foundational events of Earth history. In 1991, perhaps out of resignation, the International Union of Geological Sciences (IUGS) approved a division of Precambrian time into eons, eras, and periods defined strictly by chronometric age, without reference to events recorded in sedimentary rocks (1). The eras stuck, but the proposed period names are seldom used.

3) This tradition was swept aside in March 2004 with the approval by IUGS of an addition to the geologic time scale: the Ediacaran Period (2). This newly ratified period, which directly precedes the Cambrian, is the first Precambrian interval to be defined according to the principles that govern the Phanerozoic time scale. It is also the first stratigraphically defined new period of any sort to be added since 1891 when Williams divided the Carboniferous Period in two (Mississippian and Pennsylvanian).

4) The distinctive character of the Ediacaran interval (beginning 610 to 635 million years ago and ending 543 million years ago) has been recognized for decades, and numerous geologists --including Sokolov, Termier and Termier, and Cloud and Glaessner (2) -- have proposed formal definitions of this interval. Now, in accordance with international rules, the new period has been defined by an event recorded in a single section of rock outcropping termed the global stratotype section and point (GSSP). (The GSSP is the reference section that defines the "standard" for recognition of the base of the new period worldwide.) The initial GSSP of the Ediacaran Period lies at the base of a texturally and chemically distinctive carbonate layer that overlies glaciogenic rocks in an exposure along Enorama Creek in the Flinders Ranges, South Australia (2). The period's end coincides with the beginning of the Cambrian Period, which is defined by its own initial GSSP residing in Newfoundland, Canada.(3-5)

References (abridged):

1. K. A. Plumb, Episodes 14, 139 (1991)

2. A. H. Knoll et al., Lethaia, in press

3. G. M. Narbonne, GSA Today 8, 1 (1998)

4. K.-H. Hoffmann et al., Geology, in press

5. G. H. Barfod et al., Earth Planet. Sci. Lett. 201, 203 (2002)

Science http://www.sciencemag.org

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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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