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The Only Fossils Which Have Been Foundwides VOL. 53, 1965 N. A. S. SYMPOSIUM: A. G. FISCHER 1205 (Discussion of Dr. Davidson's paper) DR. R. B. HARGRAVES (Princeton University): I would like to enter an appeal on behalf of the opposition, comprising most of the geologists who have worked in the Witwatersrand. First, I would say that Professor Davidson provides us with two extreme alter- natives; namely, either the gold and uraninite is detrital or it must be hydrother- mally produced. I think it is fair to say that the fundamental fact about these gold and uranium deposits is the incredibly detailed fidelity with which they follow sedimentary guides. It seems to me the rocks in which these deposits occur are gravels. Gravels today are known to contain much of the gold in this country. I think it makes it easier if one assumes that at least some,if not all, of the gold and uraninite was already present in these gravel deposits. Then, if the gold and if the uranium could only be syn- genetic under a reducing atmosphere, I do not think that there is any valid alterna- tive to that hypothesis. DR. DAVIDSON: I can only say, sir, that there is no known geological environment from which detrital gold, uraninite, and pyrite devoid of other heavy minerals can be obtained as alluvial deposits. If Dr. Hargraves can suggest how such deposits could form syngenetically in the Dominion Reef, the southeast of Finland, Blind River, and so forth-if he can suggest any geological environments from which such associations can be derived syngenetically, then he will have made a very great step forward. DR. CLOUD: I am sure we could have a very lively discussion on these matters, but we are short of time. FOSSILS, EARLY LIFE, AND ATMOSPHERIC HISTORY BY ALFRED G. FISCHER DEPARTMENT OF GEOLOGY, PRINCETON UNIVERSITY The Fossil Record.-From the paleontological standpoint, earth history is sharply divided into two parts-the vast Pre-Cambrian, with a sparse fossil record, and the Phanerozoic, beginning with the Cambrian period some 600 million years ago, with its rich documentation of life (Fig. 1). The only fossils which have been found widespread and abundant in the Pre- Cambrian are stromatolites: head-shaped or branched, laminated structures, generally composed of calcite or dolomite, but in some cases siliceous, and generally attributed to the Cyanophyta (also known as Myxophyta or blue-green "algae"). They appear to be present in most Pre-Cambrian limestones and include the oldest fossils known to date, from the African Bulawayan,' about 2.7 billion years old. Like most fossil stromatolites, those from the Bulawayan show no cellular detail; their interpretation as fossils rests on three lines of evidence: we know of no inorganic processes which form such structures; cyanophyte colonies produce structures of this type today;2, 14 and well-preserved stromatolites of various ages have yielded microscopic cellular detail which confirms their organic origin. Thq Downloaded by guest on September 23, 2021 1206 EVOLUTION OF THE EARTH'S ATMOSPHERE PROC. N. A. S. oldest of these cell and filament-retaining stromatolites studied to date -are from the Pre-Cambrian Gunflint chert, about 1.9 billion years old.3' 4 In addition to algae or cyanophyte structures they seem to'contain remains of-bacteria and per- haps aquatic fungi. The presence of life in the Pre-Cambrian is also attested by the widespread oc- currence of large organic molecules, and microscopic work is turning up filaments, globules, etc., which are of organic origin but of uncertain affinities. But we are only beginning to learn about these. The animal record is almost nonexistent. While a fair number of rock struc- tures have been hopefully interpreted as animal fossils of Pre-Cambrian age, the majority of these are either clearly of inorganic origin, or are dubious as to origin or age., To very skeptical people, such as Cloud,4 the existence of Pre-Cambrian animal fossils has yet to be established, while Seilacher6 accepts some bottom mark- ings in late Pre-Cambrian rocks as evidence of animals. The Ediacara fauna de- serves special consideration. In several parts of the world, sandstones occurring be- low Lower Cambrian rocks with normal Lower Cambrian faunas have yielded im- prints of soft-bodied organisms. The best assemblage stems from Ediacara in Australia,7' 8 and contains pennatulid and medusoid coelenterates, annelids, a possible echinoderm, and problematica. Elements of this fauna occur in Africa, Europe, and North America. A Lowermost Cambrian age cannot be entirely ruled out, but a latest Pre-Cambrian age seems more likely. The base of the Cambrian period is marked in marine sediments round the world by the appearance of abundant animal life. In detrital sediments worm burrows generally appear first, while in carbonate sequences the problematic archaeocyathids pioneer. Brachipods, trilobites, and other arthropods, sponges, mollusks, and echinoderms tend to appear somewhat later. All of the animal phyla likely to leave a fossil record, and nearly all of the classes thus endowed, appeared by mid-Ordo- vician time, i.e., over an interval of about 120 million years (Fig. 1). The Berkner-Marshall Theory.-This sudden appearance of the diverse animal stocks has been the most vexing riddle in paleontology, and a voluminous literature has accumulated around it.4--' 8-10 It has always been tempting to look to some important environmental change, which might or might not have been accompanied by an evolutionary outburst. But neither acid oceans, nor alkaline oceans, nor cosmic events provided plausible and coherent models which fit into earth history as a whole. This brings us to the Berkner-Marshall hypothesis. Some geologists and some biochemists have long suspected that the earth's atmosphere has evolved through time." In recent years it has become almost certain that the early atmosphere lacked free oxygen,12 and that the land surface and the upper layers of the water masses were subjected to rather intense ultraviolet radiation. Life originated un- der these conditions and played a large role in subsequent changes: photosynthe- tic plants freed oxygen, which accumulated in the atmosphere; and this, in turn, provided the ozone shield which today screens the earth's surface from most of the UV radiation received.13 Berkner and Marshall6 suggest that the beginning of Cambrian time marks an important threshold in atmospheric evolution: the attainment of about 1 per cent of present oxygen pressure (Pasteur point), at which level oxygen respiration becomes profitable to organisms and permits animal life Downloaded by guest on September 23, 2021 VOL. 53, 1965 N. A. S. SYMPOSIUM: A. G. FISCHER 1207 .Di - ,P ear paint 111 ~ M11L - -I |~~~~AH L.. A DECO T-PS- - - I MpI I DECOMP0O5ER 5 OriI I _-j---0;2P-tI7.0.-75I0.0 FIG. 1. The fossil record, plotted on a model of the evolving atmosphere. DI, Dinoflagellates (incl. Hystrichospheres); CY, Cyanophyta (blue-green algae); BA, bacteria; FU, fungi; CHL, Chlorophyta; TR, Tracheophyta, (the higher plants) - RH, Rhodophyta (redt algae); CO, Coc- colithophorida- DIA, diatoms; ARC, Archaeocyathida PO, Porifera; PRO, Protozoa, COE, Coelenterata- BRY, Bryozoa; BRA, Brachiopoda; CON,-Conodonta (affinities unknown); AN, Annelida; ARTH, Arhropoda; MO, Mollusca; ECH, Ekchinodermata; H. H~emichordata (incl.. Graptolites); V, Vertebrata. Downloaded by guest on September 23, 2021 1208 EVOLUTION OF THE EARTH'S ATMOSPHERE PROC. N. A. S. as we know it. They further suggest that the colonization of the continents, which occurred later in Silurian and Devonian times, marked a stage when atmospheric oxygen pressure had risen to 10 per cent of present level, and had thereby reduced UV radiation at the land surface to tolerable levels. In Figure 1, the fossil record is shown superimposed on an evolutionary model of the atmosphere following the general lines (but not the details) of the Berkner-Marshall theory. The question at hand is whether the fossil record supports the concept of atmos- pheric evolution in general, and the model proposed by Berkner and Marshall. We may look for evidence of radiation intensity; the evidence is not definite, but it is favorable. We shall also examine Berkner and Marshall's views on animal origins and the history of continental colonization, and shall formulate alternative views. UV Radiation.-According to Berkner and Marshall, dangerous quantities of UV radiation penetrated the waters to a depth of about 10 meters during most of the Pre-Cambrian, but decreased to a few centimeters in latest Pre-Cambrian to earliest Cambrian time, and continued to decrease, so that tolerable subaerial levels were attained in the Silurian. This is, of course, not a one-sided matter; not all organisms are equally tolerant of this and other forms of radiation." Tolerance may involve the structure of the protoplasm, as well as the presence of shielding devices. To most of us, a day at the beach occasionally demonstrates the inadequacy of our own shielding against present-day UV radiation levels. In any case, we might expect the earlier organ- isms of the shallow waters to have been especially adapted to cope with UV radia- tion. Stromatolites.-The abundance of Pre-Cambrian stromatolites may be an ex- pression of this: P6r&s&7 has found that cyanophytes are more resistant to UV than are bacteria. Cyanophytes are particularly successful today in such radia- tion-exposed environments as tropical tidal flats. And cyanophyte colonies of various kinds cover themselves with mineral particles-either by entrapping and binding extraneous grains, or by secreting calcium carbonate or silica. It is this tendency which provides us with a fossil record of stromatolites, in the form of stromatolite heads, and of horizontally laminated or minutely crinkled calcareous "algal mats" and algal mat limestones.'8 I suggest that this mineralization serves the purpose of shading the cells and the colony as a whole.
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