ScienceWeek

PALEONTOLOGY: ON EDIACARANS

The following points are made by Simon Conway Morris (Current Biology 2005 15:R8):

1) Ediacarans are a highly distinctive assemblage of fossil organisms, apparently all marine. Most typical are frondose, discoidal, and segmented forms. Their overall taxonomic diversity is modest, but there is considerable disparity and they include one rather bizarre group with prominent three-fold body symmetry. A striking feature of many forms is their modular construction and their sometimes fractal growth pattern. Ediacarans appear in the latest Precambrian, towards the end of the Neoproterozoic, ranging between ca. 570-550 million years ago, but some of them even trickle into the Cambrian.

2) It is not clear what the Ediacarans are exactly, and much speculation exists concerning these life forms. The majority view regards them as metazoans, but the assignments to known animal groups can be a difficult procedure. Some of the fronds are probably sea-pens, but the jellyfish-like discs are more likely to be the holdfasts of fronds. Through some eyes the Ediacaran sea-floor was also host to molluscs and arthropods, but none of these interpretations seems acceptable.

3) There are some pretty radical proposals. Bizarre, mattress-like forms, the so-called vendobionts, are probably giant syncitial protistans. Maybe the Ediacarans were a parallel attempt at achieving a metazoan grade of organization, but where they fit into eukaryote phylogeny is largely guesswork. Even fungi and lichens have been suggested as possible groups.

4) One careful study of Ediacarans from Newfoundland demonstrated both a community structure and an ecological succession. This in turn was used to support the idea of Ediacarans as Metazoa. An alternative is that Ediacarans built "para-communities".

5) Ediacarans lack hard skeletons, yet the fossil preservation of their soft-parts has almost no counterpart to the classic examples of soft body fossils, such as those found in the Burgess Shale. One important factor was the presence of abundant microbial mats that covered the seafloor. These acted as "death-masks", trapping the Ediacarans and facilitating very fast fossilization. But that cannot be the entire story. At least the vendobionts may have had a more robust composition than once thought.[1-5]

References (abridged):

1. Clapham, M.E., Narbonne, G.M. and Gehling, J.G. (2003). Paleoecology of the oldest known animal communities: Ediacaran assemblages at Mistaken Point, Newfoundland. Paleobiol. 29, 527-544

2. Conway Morris, S. (1993). Ediacaran-like fossils in Cambrian Burgess Shale-type faunas of North America. Palaeont. 36, 593-635

3. Gehling, J.G. (1999). Microbial mats in terminal Proterozoic siliciclastics: Ediacaran death masks. Palaios 14, 40-57

4. Grazhdankin, D. (2004). Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution. Paleobiol. 30, 203-221

5. Narbonne, G.M. (2004). Modular construction of early Ediacaran complex life forms. Science 305, 1141-1144

Current Biology http://www.current-biology.com

--------------------------------

Related Material:

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

--------------------------------

Related Material:

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

ScienceWeek http://scienceweek.com