Mcmenamin FM

The Garden of Ediacara • Frontispiece: The Nama Group, Aus, Namibia, August 9, 1993. From left to right, A. Seilacher, E. Seilacher, P. Seilacher, M. McMenamin, H. Luginsland, and F. Pflüger. Photograph by C. K. Brain. The Garden of Ediacara • Discovering the First Complex Life Mark A. S. McMenamin C Columbia University Press New York C Columbia University Press Publishers Since 1893 New York Chichester, West Sussex Copyright © 1998 Columbia University Press All rights reserved Library of Congress Cataloging-in-Publication Data McMenamin, Mark A. The garden of Ediacara : discovering the first complex life / Mark A. S. McMenamin. p. cm. Includes bibliographical references and index. ISBN 0-231-10558-4 (cloth) — ISBN 0–231–10559–2 (pbk.) 1. Paleontology—Precambrian. 2. Fossils. I. Title. QE724.M364 1998 560'.171—dc21 97-38073 Casebound editions of Columbia University Press books are printed on permanent and durable acid-free paper. Printed in the United States of America c 10 9 8 7 6 5 4 3 2 1 p 10 9 8 7 6 5 4 3 2 1 For Gene Foley Desert Rat par excellence and to the memory of Professor Gonzalo Vidal This page intentionally left blank Contents Foreword • ix Preface • xiii Acknowledgments • xv 1. Mystery Fossil 1 2. The Sand Menagerie 11 3. Vermiforma 47 4. The Nama Group 61 5. Back to the Garden 121 6. Cloudina 157 7. Ophrydium 167 8. Reunite Rodinia! 173 9. The Mexican Find: Sonora 1995 189 10. The Lost World 213 11. A Family Tree 225 12. Awareness of Ediacara 239 13. Revenge of the Mole Rats 255 Epilogue: Parallel Evolution • 279 Appendix • 283 Index • 285 This page intentionally left blank Foreword Dorion Sagan Virtually as soon as earth’s crust cools enough to be hospitable to life, we find evidence of life on its surface. But we are latecomers, and just as we must be familiar with the beginning of a mystery novel to understand its end, we must scrutinize the often ignored early phase of evolution. Mark McMenamin’s allusively named Garden of Ediacara hones in on some of the key events and players in life’s early phase—a time for the biosphere that, like the first three years of a human life, is not only for- mative and revealing but essential to understanding the full sweep of a living existence. Da Vinci found shells on mountains that suggested a long geological past. Hutton and, later, Darwin extended such thinking, drawing forth a temporal expanse wide enough to explain modern anomalies and complexities. But when early commentators surveyed the fossil history of life on earth, they were not overly impressed with life’s earliest phase. It almost seemed as if nothing was going on. Until the “Cambrian explosion”—the widespread appearance of fossil forms, including the famous horseshoe-crab-like trilobites, during the Cambrian geological period—it seemed as if life had barely started. Now you don’t see them, now you do: Like the goddess Minerva bursting forth fully formed from the head of Zeus, the sudden appearance of hard-backed animals in the fossil record had about it the lingering aura of myth or celestial-fostered miracle. Whence come animals from evolutionary chaos? For geologist Preston Cloud, one of the first of the modern paleobiol- ogists, the appearance of animal life corresponded to a global atmospheric increase in free oxygen. This theory, repeated in textbooks, may be an anthropomorphic fairy tale, a kind of industrial fiction. Fire-starting oxygen, the gas of choice, spurs the biosphere to produce complex life forms, paving the way for air-breathing mammals. But there is probably no x • Foreword causal relationship between oxygen increase and animal life. The Cambrian explosion was 540 million years ago, whereas according to the rock record of oxygen-rich uranium and iron ores, atmospheric oxygen began to build up far earlier, some 1800 million years ago. Nothing is so destructive of a beautiful theory as an ugly fact. Although classical evolutionists pictured a gradual evolution of animal life from soft-bodied to hard-bodied forms, the shelled creatures of the Cambrian stick out like a sore thumb. There does seem to be a sudden- ness about them, one not explicable on the basis of gradual evolution. Today we understand that the Cambrian fauna were preceded by a strange and motley collection of often symmetrical soft-bodied forms. These are the Ediacarans, eponymous subheroes of the Australian outcrop where the first such fossils were found. The Ediacarans’ global “garden,” more than a cryptic play on Eden’s idyllic and instantaneous fertility, refers to their largely vegetative existence. With the playful attitude of a true sci- entific explorer, Mark McMenamin treks the planet and mines the litera- ture, some of it itself almost fossiliferous, in an exposition of medusoids, ring stones, concentrically fretted, radially flaring, and other enigmatic trace and body fossils left by the soft-bodied pre-Cambrian forms. Who were these beings? Were they animals? Our ancestors? More likely, we find, they were our cousins. Although superficially similar to jellyfish, the Ediacaran medusoids probably never swam: They are preserved concave side up, like bowls rather than like swim- mers. They may have been so quickly replaced all over the world not as a result of evolution-igniting oxygen, but because evolution had gotten to the point where predators with eyes and a murderous appetite for Ediacaran sushi had come into their own. In McMenamin’s persuasive reading, the earliest animals (or animal-like life, for they may have been blastula-less colonial microbes called protoctists—predating predation) were languid, limpid vegetarians. Harmless antecedents to Tennyson’s bloody nature tooth and claw, they were eyeless representatives of a vic- timless Edenlike world. This was a green and serene world where there was no reason for calcified coverings, for carapaces or spiky armor because the victimizing element of Animalia had not yet evolved. The Ediacarans, on this view, were translucent beings with photosynthetic inclusions, soaking up the sun and living off the excess of their living internal gardens. Today creatures such as the snail Placobranchus, the giant clam Tridacna, and the seaweed-looking worm Convoluta roscoffensis, whose mouth is closed throughout its adult life, have gone back to the simpler “Edenic” lifestyle of autotrophic sunbathing. Ryan Foreword • xi Drum has even made the semiserious suggestion that this would be a good thing for junkies: Inject them with algae, select for ever more pal- lid and translucent demeanor, and in time we might have societally harmless anthropoids nutrifying themselves in languor at a delicious if safe remove from the normal frenetic hustle of urban animal life. The photosynthetic cells might even migrate to the germ cells of these veg- etablarians, permitting true speciation of Homo photosyntheticus from Homo sapiens ancestors. It would be a mistake to think such “planimals” only freaks of sym- biosis or figments of science fiction. All plants and algae on the planet are composed of eukaryotic cells whose existence has been brought to us by primeval mergers with cyanobacteria, green prokaryotes that were eaten (living salad) by larger cells. Blessed with a permanent case of indi- gestion, these larger cells benefitted from the metabolic independence of the preplants now dwelling Jonah-like inside them. McMenamin’s Garden of Ediacara hypothesis explains the widespread soft-bodied beings, many flat and fronded, as early testimony to the power of pho- tosymbiosis. By turns starfishy and Star Treky, branched and sand dol- laresque—switching, like some fossil equivalent to a Necker cube, from twisting worm to growing stem in the paleontologist’s imagination— these creatures may well have been neither plant nor animal. On the television program The X Files, FBI agents Sculley and Mulder find “a cell that is not a plant cell or an animal cell. And it’s dividing mitotically.” How creepy! How strange! The only problem with this would-be biological bizarrerie is that such uncanny cells are more prevalent on the surface of earth than either plant or animal cells. They’re in your house right now, and they live in your body. Bacteria, although they do not divide mitotically, are neither animals nor plants. And their symbio- genetic offspring, the protoctists, which do divide mitotically (and which are on your skin right now), were certainly among the ancestors to the Ediacarans. But the “metacellularity” of the Ediacaran organisms may not have led to any large extant forms of life familiar to us today. They were jellyfish-like and starfish-like, but neither truly medusoids nor echinoderms. Some secreted copious mucous layers as a means of gliding locomotion. True aliens from our own past, some of these fabu- lously real beings lost their innocence and symmetry. Concentrating sense organs at one end, some may have been, before animals proper, the first organisms to evolve heads. Which, as in the knowledge gained from the Tree of Good and Evil in the redolent Garden of Eden, may have spelled the beginning of the end. xii • Foreword For despite the scene drawn by some evolutionists of a planet of pure con- tingency, devoid of direction, certain patterns do have a way of cropping up again and again in evolution. Eusocial animals, for example, come not only in bee and ant but in naked mole rat flavors. Humans too, losing the inevitable link between sex and reproduction, may be moving toward eusociality. Like a tale told by a stutterer, evolution doubtless is circuitous and may well contain only the raw material, rather than the denouement, of meaning. But there are these patterns. Not just hominids but many species of mammal, for example, show an increase in body and brain size over evolutionary time.

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