GEOLOGICAL TIME

All of geological time – the total history of the Earth – is divided into two great Eons, the Precambrian and the Phanerozoic (Figure 2.1). The Precambrian Eon is the older and much the longer of the two, extending from when the planet formed, about 4500 million years (Ma: mega anna) ago, to the appearance of fossils of hard-shelled invertebrate animals such as trilobites and various kinds of shell-bearing molluscs at about 550 Ma ago. The eon is composed of two Eras, the older Archean Era (from the Greek archaios, ancient) that spans the time from 4500 to 2500 Ma ago; and the younger Proterozoic Era (the era of earlier life, from the Greek proteros, earlier, and zoe, life) that extends from 2500 Ma ago to the close of the Precambrian.

The younger and shorter eon is the Phanerozoic (the eon of evident life, from the Greek phaneros, evident or visible, and zoe). The Phanerozoic Eon encompasses the most recent roughly 550 Ma of Earth history and is divided into three eras (from oldest to youngest, the Palaeozoic, Mesozoic, and Cenozoic Eras; Figure 2.1), each subdivided into shorter segments known as geologic periods. The oldest such period of the Palaeozoic Era (and, consequently, of the Phanerozoic Eon), spanning the time from about 550 to roughly 500 Ma ago and named after Cambria, the Roman name for Wales where rocks of this age were first formally described, is known as the Cambrian Period.

Tracing life's roots: progress and problems

Precambrian paleobiology: a new field of science

In 1859, when Charles Darwin unveiled his monumental monograph, The Origin of Species, major chapters in the history of life had already been deciphered. The familiar progressions from seaweeds to land plants, from marine invertebrates to higher mammals, provided Darwin a fossil-based foundation for his grand thesis. This evolutionary sequence from water to land is the history of Phanerozoic life, dating from 543 million years (Ma) ago and the appearance of calcareous algae and shelled invertebrates that marks the beginning of the Cambrian Period of geologic time. But what happened earlier? What were the forerunners of these early algae and primitive invertebrates? In short, how did evolution proceed during the Precambrian Eon, the pre- Phanerozoic seven-eighths of Earth history?

To these questions Darwin had no answers – indeed, until a scant three decades ago, Precambrian biologic history was terra incognita. But Darwin did know that if answers were not forthcoming, his theory was in jeopardy:

The 41 contributors to this monograph are members of the Precambrian Paleobiology Research Group (PPRG), a multidisciplinary, international consortium of physical and biological scientists actively investigating the Precambrian history of the earth, and of life. Initially organized more than a decade ago, the PPRG includes geologists and biologists of diverse backgrounds and expertise: sedimentologists, petrologists, geochemists, and atmospheric scientists, microbiologists, phycologists, molecular phylogenists, and paleontologists. In 1979–80–supported by NASA and by funds provided by the National Science Board's 1977 Alan T. Waterman Award to J. W. Schopf–the group worked together at UCLA for 14 months on problems centered on the paleobiology of the Archean, the earlier half of Precambrian time encompassing the initial 40% of the history of the planet (viz., extending from 4500 to 2500 Ma ago). This first PPRG project culminated with publication in 1983 of Earth's Earliest Biosphere (J. W. Schopf, ed.; Princeton University Press), a monographic work concerned with the origin and evolution of the Archean biota and the development of the Archean environment.

Stemming from what was viewed by the group as the success of this initial venture–a remarkable educational experience that both broadened and deepened the perspectives of the two dozen participants–in the mid-1980s the group elected to undertake a follow-on study, a sequel to be devoted to the paleobiology of the Proterozoic, the latter half of Precambrian time extending from the close of the Archean, 2500 Ma ago, to the first appearance in the geologic record of skeletonized invertebrate metazoans, about 550 Ma ago, that marks the beginning of the richly fossiliferous (and paleobiologically well-documented) Phanerozoic Eon of earth history.

Although micropaleontological study of the Proterozoic was already underway nearly a century ago (Walcott 1899), significant progress has been of surprisingly recent vintage. Indeed, of the 2800 occurrences of authentic microfossils now known from sediments of Proterozoic age (Table 22.3), more than 85% have been reported during the past two decades (Section 5.2). Reported chiefly from shales (62% of occurrences) and stromatolitic cherts (38%), planktonic (1580 occurrences in 248 formations) and benthic taxa (1219 occurrences in 208 formations) are now well known from units worldwide (Sections 5.4, 5.5; Chapter 22).

Unfortunately, however, meaningful interpretation of these newly available data is difficult, largely because of the lithology-related differing systems of classification and taxonomy of such microfossils that are currently in use. In particular, the majority of microfossils preserved as essentially two-dimensional compressions in Proterozoic shales, extracted from their surrounding matrix by palynological techniques and studied in acid-resistant residues, have been classified as “acritarchs”– organic-walled microfossils, commonly spheroidal, of uncertain systematic position–and have therefore been grouped into morphological categories by use of an “artificial” (non-biologic based) system of classification. In contrast, because of their three-dimensional preservation and morphological comparability to extant bacteria, cyanobacteria, and eukaryotic microalgae, permineralized microfossils detected in Proterozoic cherts and studied in petrographic thin sections have generally been grouped into the same categories as those used for classification of living microorganisms.

Introduction

On the following plates are illustrated microfossils from 48 geologic units (representing about 10% of the units listed in Table 22.3). Although coverage is by no means exhaustive, the fossils illustrated are representative of the diversity and level of organization exhibited by particularly well-preserved Proterozoic assemblages. In selecting the fossils illustrated, emphasis has been placed on inclusion of holotype specimens, particularly holotypes of taxa described from the Soviet Union, relatively few of which have been previously illustrated in the non-Russian language literature (and none of which has been previously illustrated using interference contrast optics). As summarized below, emphasis has also been placed on illustration of assemblages of Middle and Late Proterozoic age; this atlas therefore complements, but does not duplicate, illustrations of Early Proterozoic communities included in Hofmann and Schopf (1983). The atlas contains more than 300 new illustrations, including photomicrographs of specimens not previously figured from the Dismal Lakes Group, River Wakefield Subgroup, Skillogalee Dolomite, the Gunflint Iron Formation, and the Gaoyuzhuang, Galeros, Kwagunt, and Myrtle Springs Formations.

Acknowledgments

Soviet type specimens were provided for study by E. Aseeva, T. N. Hermann, T. Jankauskas, N. Mikhailova, V. Sergeev, and A. Weiss; photomicrography of these specimens was carried out by C. V. Mendelson, J. W. Schopf, and G. Vidal; this work, conducted at the Palaeontological Institute in Moscow, was facilitated through the courtesy of M. A. Fedonkin and B. S. Sokolov. Studies of new material from the Gaoyuzhuang Formation were carried out in collaboration with R. Mikawa (University of Chicago); studies of new specimens from the River Wakefield Subgroup and the Skillogalee Dolomite were carried out by or in collaboration with T. R. Fairchild (Universidade de Sao Paulo, Brazil).

Abiotic. Pertaining to substances or objects that are of nonbiologic origin; used especially in reference to organic matter produced via chemical reactions in the absence of living systems.

Acetic acid. An organic compound, the two-carbon carboxylic acid CH3COOH.

Acetogenesis. Formation of acetic acid (e.g., from alcohol by bacterial fermentation).

Acetyl-coenzyme A (acetyl-co A). An organic complex involved in enzymatic acetyl transfer reactions, CH3COSCoA.

Acritarch. A “phytoplankton-like” microfossil of unknown or uncertain biologic relationships, unicellular or apparently unicellular, organic-walled, commonly spheroidal, smooth (e.g., sphaeromorphs) or spiny (e.g., acanthomorphs); “Acritarcha” is an artificial group composed of such microfossils.

Actinolite. A mineral of the amphibole group, Ca2(Mg,Fe)5Si8O22(OH)2.

Adenosine triphosphate (ATP). An organic compound, adenosine 5′-triphosphoric acid, C10H16N5O13P3, a coenzyme involved in the transfer of phosphate bond energy.

Adiabatic. In thermodynamics, pertaining to a relationship of pressure and volume when a gas or fluid is compressed or expanded without either giving or receiving heat.

Aerobe. An organism able to live in the presence of free (uncombined) molecular oxygen; aerobes are termed “obligate” if they can exist and grow only in the presence of O2.

Aerotaxis. The locomotory movement of an organism in response to an environmental gradient of molecular oxygen, either toward a higher oxygen concentration (“positive aerotaxis”) or away from a higher oxygen concentration (“negative aerotaxis”).

Akinete. Specialized reproductive cells (resting spores) occurring singly or in uniseriate groups, commonly adjacent to heterocysts, in various types of (predominantly filamentous) cyanobacteria.

Investigation of a large number of samples during this project led to development of the following processing routine.

Curation of samples was performed at the University of California, Los Angeles. As appropriate, subsamples were subsequently distributed to PPRG members for analyses to be carried out at their home institutions. A flow chart outlining the various procedures involved is shown in Figure 15.2 and is summarized below.

The initial curation for every incoming rock sample consisted of assigning a PPRG Sample Number (e.g., “1001”). Pertinent geological information was compiled and entered into the databases “Inventory,” “Site,” and “Strat” (see Chapter 21).

For paleontological and mineralogical studies, petrographic thin sections were prepared of each sample: for microfossil studies, a 150µi-thick “paleo”-section (“1001-1-A”); and for petrographic studies, either a standard 30 µm-thick section for non-carbonates (“1001-1-B”) or a 5 to 15 µm-thick section for carbonates (“1001-1-C”). In addition, large-area thin sections (150 µm-thick, “1001-1-STROM”) were prepared of selected stromatolitic samples.

Sample processing for geochemical and/or palynological studies was initiated by discarding any weathered surface or secondarily emplaced vein material and generating a mass of clean interior rock chips ≤ 1 cm in diameter (“1001-1-RC”). Chipping of small samples was performed using a geologic hammer; larger samples were chipped with a jawbone (i.e., “chipmunk”) crusher. In order to remove any organic contaminants, the chips were etched in a 20% HF-10% HC1 solution, then rinsed with large volumes of distilled water and dried in a drying oven at 75° C.