Biological Science

Pre-Palaeozoic sunbathers

The Garden of Ediacara

Idiosyncratically erudite and replete with self-congratulatory dicta, The Garden of Ediacara amuses, intrigues, cajoles and invites the reader to share the joys of palaeontology - in the field, the laboratory and the library. When it does not infuriate by hyperbolic overstatement and wild speculation, it amuses by provocation. Yes, come and learn about past life 600 million years ago when the earth was very different. Live among ancient soft-bodied beings, and make real for yourself a time "before life even got its bones. Take your ticket and get on board." But be sure to carry with you a great load of scepticism.

The book transports us to time's middle kingdom when the animals originated. By 541 million years ago, at the base of the Cambrian period, ie the opening of our Phanerozoic eon, shelly animal fossils, sclerites, trilobites and brachiopods abound. Geologically speaking, just before that, at the very end of the Palaeozoic eon, on nearly all the beaches of the pre-Pangean world thrived the enigmatic heroes of this story: the Ediacaran biota or, for short, the Ediacarans. The term by which they are often indexed, "Ediacaran fauna", argues Mark McMenamin, is incorrect. McMenamin not only, in his terms, definitively shows the vexing palaeontological problem of what the Ediacarans are and how they developed, but, by recombination and redefinition, he renames and promotes new Proterozoic time rock divisions. He recognises the Sinian era from about 680 mya to 600 mya and the Lipalian era, 600 mya until the base of the Cambrian at 541 mya.

Even more indulgently, he credits the Urantia Brotherhood's literature (The Urantia Book) with the anticipation in the 1930s of a subsequent scientific discovery. I feel that McMenamin's arguments that Ediacarans represent a great evolutionary diversification towards cephalisation (forming heads), intelligence and powerful coexistence independent of and parallel to the evolution of animals would carry far greater weight if he had kept his epiphanous Urantia insights to himself.

The really compelling aspect of the book is the point made in Dorion Sagan's preface: "Here then, we have a work with all the allure of a medieval bestiary, with the difference that the creatures herein derive their mystery not so much from a distant, make-believe place as from a long-elapsed time." For the fossils are fascinating: the three-armed Tribachidium, the spindle-shaped ribbed Pteridinium crowding a palaeo-Namibian beach, the garlic-clove-like Inaria, the composite-fronded Rangea, the pentaradial Arkarua, the nipple-shaped Cyclomedusa and the petal-like Mawsonites all stretch the imagination. The cast of characters proliferates: Spriggina from South Australia and Russia does look as if it had a head, and so does Bomakellia from the White Sea region of Russia. Whereas "cloudinids", with their ridged stacked-cup appearance and their calcareous shelly composition, "are without question the first animal-style shelly organisms to occur abundantly in the fossil record", the other late Proterozoic large fossils (and here McMenamin is convincing) are not animals. They have no features to assure us of their development as sperm-egg fusion-formed diploids from the blastula, the animal embryo. The Ediacarans can not be shoehorned into any phyla with extant relatives.

Furthermore, no one can any longer claim there are so few examples that the absence of well-preserved fossils precludes interpretation. No, the Ediacarans thrived. They used to live on the beaches of ancient Rodinia and their members today are found in Russia, China, Mexico, Nevada, Nova Scotia, England and many other places. Furthermore, they were most likely dwellers in sand who were buried by sand and whose remains were filled with sand.

They were, I suspect, psammophils: beach lovers, sand dwellers. After sedentary lives on stretches of beach-front worldwide, covered with sand or in some cases volcanic ash, they probably died peacefully at home. As McMenamin urges, the diversity of the Ediacarans may well have been generated by differential growth of "founding cells" in cell lineages. It is indeed likely, as he writes, that at least some Ediacarans spent their time sunbathing because their quilted textures harboured lively active photosynthesisers (eg purple bacteria, cyanobacteria, green or other algae). And it is possible, as he insists, that in anoxic, dark sulphur-rich zones, the place of oxygenic photosynthesis was taken up by purple photosynthetic-sulphide-oxidising bacteria, ie chemosynthesis, not photosynthesis, prevailed locally.

Others probably did not depend on either photosynthetic or chemosynthetic symbionts. Rather, organic detritus perhaps fed these lazy sand dwellers as dissolved carbon com£ seeped into their flat, extended bodies. McMenamin's lively and imaginative scenarios may contain germs of truth.

In any case, our author makes strong arguments that Ediacarans thrived on marine sandy flats widely distributed, and that, after dominating the 100 million years before the Phanerozoic opened, these large beings extinguished and were entirely gone by the early Palaeozoic era. The appreciation of the extent of their diversity and originality impresses: they display symmetries exceeding the radial pentameral and the bilateral of today's animals. No animal like any of them is alive today. For the most part, Ediacarans were neither predatory nor preyed upon. All of this seems to me well documented. In summary, in spite of the nonsense and error, neovitalist sloppiness and personal indulgence, this panoramic view of a lost world is exciting and fun. The reader travels to German southwest Africa where he suffers Gibeon meteorite showers in Windhoek, cold shivers and bleak desolation in the 130 million-year-old deserts of the Nama plateau. He collects fossils and horned toads in Sonora, Mexico, and images of fossil volcanic ash on the Avalon peninsula of southeastern Newfoundland. This is, indeed, a travelogue through time and space when organisms abounded, yet none was either animal or plant.

Lynn Margulis is a professor in the department of geosciences, University of Massachusetts, Amherst, United States.

The Garden of Ediacara: Discovering the First Complex Life

Author - Mark A. S. McMenamin
ISBN - 0 231 10558 4
Publisher - Columbia University Press
Price - £23.95
Pages - 295

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