'The Pr e cambnan-Camb rian boundary most plants, move around their habitat seeking food resources scattered throughout the environment.
The traditional distinction between plants and animals has had difficulties ever since the invention of the microscope and the discovery of unicellular organisms or microbes. Cyanobacteria, also called blue-green algae, are common microbes and are among the simplest and toughest forms of life. They are familiar today as the green scum that forms in pet watering dishes. Their green color indicates the presence of chlorophyll, a pigment that captures the energy of sunlight and allows the cyanobacteria to create food in the process called photosynthesis. Since these microbes are feeding themselves with the help of sunlight, their feeding strategy is called photoautotrophy — literally, feeding oneself with light. The difficulty that microbes pose for the classical distinction between plants and animals is this—many microbes are both autotrophic (feed themselves with sunlight or simple, energy-rich molecules) and het-erotrophic (gain food from other living things). Few familiar, large multicellular plants and animals are able to do this, but for many microbes it is commonplace.
Biologists now recognize five major categories or kingdoms of living things based on biochemistry and cell structure (as opposed to just two kingdoms based on color and capacity for movement). The five newer groups are: monerans, protists, fungi, plants, and animals (Margulis and Schwartz 1982). Monerans are simple unicellular organisms such as bacteria and cyanobacteria. Protists are larger, more complex, mostly unicellular organisms such as the amoeba. Fungi are unusual unicellular and acellular organisms ("acellular" refers to the fact that the bodies of large fungi are generally not subdivided into discrete cells). Fungi are primarily terrestrial (living on land) and mostly heterotrophic. Plants are multicellular organisms that (with a few exceptions) use photosynthesis to create their food. Plants also have rigid cell walls for support, an asset considering the stationary lifestyles of most plants.
Animals are multicellular heterotrophs dependent on other organisms for food; animals with complex organ systems are called meta-zoans. Most familiar types of animals are metazoans. Metazoans do not have rigid cell walls because these could interfere with a common animal characteristic —motility or mobility. As will be discussed later, multicellular animals can participate in autotrophy only with the aid of internal, symbiotic microbes. It is interesting to note that in terms of biomass (total amount of living matter), moner-
ans and protists in the first two kingdoms vastly outweigh the biomasses of the other three multicellular kingdoms taken together.
Animals, then, are multicellular organisms, often motile and without rigid cell walls, which require other organisms (living or dead) as food sources or providers. Alas, this is not a complete solution to the problems with the traditional definition of animals; some modern fungi could fit this definition. This classification problem is even more vexing for paleontologists than for biologists. There are many ambiguous fossils which have been interpreted as animals but which may in fact have belonged to some other kingdom of organisms.
Fortunately for the science of biostratigraphy, it does not matter what kingdom any particular fossil belongs to. For the stratigrapher trying to determine the age of a particular stratum, the most important factors are: (a) that the fossil discovered in the layer of interest be reasonably common, and (b) that it be restricted to a particular segment of geologic time.
Animal fossils allowed one of the most important scientific breakthroughs of the last century, the development of biostratigra-phy and the "modern" stratigraphic research program. All groups of organisms inhabit the earth for finite periods of time, and once extinct, particular groups never reappear. The tragic irreversibility of extinction has a positive side for geologists practicing stratigraphy. Stratigraphers attempt to date rocks using a simple principle; when rocks form layers, the oldest rocks are on the bottom of the pile and the youngest are at the top. This principle of superposition is a powerful tool for determining the relative ages of adjacent rock layers, but it has limited usefulness, particularly when one tries to compare the ages of completely dissimilar stacks of sedimentary rocks (called sedimentary sequences) in different parts of the world. In the 1830s, early stratigraphers recognized that certain animal and plant fossils were restricted to specific intervals of earth history. Rocks that contained similar groups of fossils had similar ages, and a global sequence of sedimentary rock ages could be made by the comparison throughout the world of fossil biotas unique to a particular time. This global sequence is the familiar geologic time scale (figure 1.1). The major subdivisions of this scale (periods) have been in international use for over 100 years.
The oldest widely accepted period in the geologic time scale is the Cambrian Period. Until the 1930s, the Cambrian Period contained the oldest unambiguous evidence for life (Wilson 1931; Vidal
1984). From the completion of the subdivision of the geologic time scale into periods (1879) until the 1930s, the base of the Cambrian was where the fossil record ended. Rocks older than the Cambrian are still unceremoniously lumped together as the Precambrian. The Precambrian encompasses more than five-sixths of geologic time, and it seems neglectful to call this huge block of time by describing what it isn't — "pre-Cambrian." Some geologists have proposed dividing the geologic time scale into the Phanerozoic ("age of visible life") and PrePhanerozoic to describe the presence and absence of large animal fossils. But the base of the Phanerozoic unfortunately does not correspond to the base of the Cambrian (conventionally recognized by the appearance of shelly fossils), and the term Pre-Phanerozoic has only caught on among a few specialists in Pre-Phanerozoic paleontology. So the name Precambrian has stuck, and it is testimony both to the importance of fossils for unravelling earth history and to our relative ignorance of old rocks lacking visible fossils. Not surprisingly, the major division in the geologic time scale is often called the Precambrian-Cambrian boundary. Before 1930, this boundary was commonly understood to mean "fossils above, no fossils below." Our understanding of the Precambrian-Cambrian boundary has become more elaborate, although—as discussed earlier—some of the central unanswered questions concerning this boundary are still unresolved.
Before the concept of geological periods gained wide acceptance, the oldest fossils known were referred to as "Primordial." Many fossils that were first described as primordial would be placed today in the Cambrian Period. An early find of American "Primordial" fossils was made in 1843 by Asa Fitch in deformed strata in western Rensselaer County, New York, and described a year later by the early American geologist and obstetrician Ebenezer Emmons. Emmons was born in the Massachusetts section of the Berkshire-Taconic ranges, not far from our home institution of Mount Holyoke College. These fossils were the trilobites Elliptocephala asaphoides (figure 1.2) and Atops trilineatus from shales east of the Hudson River (Emmons 1847). Emmons believed that he had located the "Primordial fauna"—the opening chapter of life history on earth.
Emmons' views were met with disdain by emminent geologists of his day. Prominent geologists such as Sir Charles Lyell, widely acclaimed as the greatest geologist of his century, were unwilling to accept the antiquity of Emmons' fossils (Lyell 1845, 1849). In addition to disputing Emmons' claims for the great age of his fossils,
Lyell and others also denigrated Emmons' interpretation of the geologic structure of this part of eastern New York state (Schneer 1969). Emmons soon became aware of the hostility to his ideas, and he later remarked that "I do not know that I am indebted to any one for favors, or for suggestions. Indeed, nothing very flattering has ever been said, or published, respecting the views I have maintained on this subject" (Emmons 1856:vii).
Emmons' bitterness was well justified. In a particularly sordid episode in the history of geology, the greatest geologists of Emmons' day (including Lyell and the Harvard biologist/geologist, Louis Agas-siz) successfully twisted the dispute over Emmons' scientific ideas into a personal attack in court. The primary objective of this legal action was to discredit Emmons and to destroy his personal reputation (Schneer 1978; Johnson 1982).
Emmons turned out to be correct about the age of the fossils, which makes it especially ironic that he suffered character assassination for presenting his scientific views. Emmons had, in essence, correctly answered one of the major stratigraphic problems of his day. He had shown that there was indeed a substantial interval of strata with metazoan fossils occurring below the oldest (Silurian Period) fossiliferous interval yet reported (Emmons 1847:49). He had described some of the oldest fossils known at the time of their discovery. They came from a stratigraphic level or horizon which we now know to be quite close to the base of the Cambrian. Despite their age, there is nothing particularly special about these fossils.
Elliptocephala and Atops are fully formed trilobites, not primitive ancestral forms. They are not unlike animal fossils that occur in much younger strata.
Charles Darwin was greatly distraught by the "explosion" of animal fossils at the Precambrian-Cambrian boundary. It posed a serious threat to his theory of evolution, which became generally available with the publication of On the Origin of Species in 1869. Darwin was strongly influenced by Lyell's (1830) vision of gradual, cyclic change throughout geologic time. Darwin's view of evolution was one of gradual change, in which one species slowly transformed into another in the fullness of geologic time. Darwin recognized that the the Precambrian-Cambrian boundary did not accord well with his gradualistic views. No animals were present before the Cambrian; then diverse, complex, fully formed groups of animals appeared after the boundary. In the sixth and last edition of his famous book, Darwin (1872:313) stated that "the case [for the abrupt appearance of Cambrian fossils] at present must remain inexplicable . . . and may be truly urged as a valid argument against the views [on evolution] here entertained."
Creationists still occasionally offer the Precambrian-Cambrian boundary problem as a fatal flaw for evolutionary theory (Gordon 1987). This is no longer a valid approach, however, because true animal fossils (soon to be discussed) are now known from sedimentary rocks which are much older that the base of (or lowest level of) the Cambrian. Creationists continue to trot out a once-respectable hypothesis (the sudden creation of Cambrian life) that was shown to be false by the middle part of this century.
Late nineteenth- and early twentieth-century paleontologists and geologists were unaware of these Precambrian animal fossils, so they were forced to come up with other explanations for the sudden appearance of Cambrian animals. Darwin, true to his preferences for gradual evolutionary change, reasoned that there was a gap in our knowledge of Cambrian ancestors, possibly due to a gap in the sedimentary record. Charles D. Walcott, discoverer of many important Cambrian fossils, followed Darwin's suggestion and formally postulated that the sudden appearance of fossils was due to a break in the recorded history of life. According to Walcott (1910), the earliest stages of animal evolution were not recorded as fossils because no sedimentary rocks were deposited and preserved during this time. He called this gap the Lipalian (from the Greek word for "lost") interval. A period of time not represented by sedimentary rock is often called a hiatus or unconformity by geologists, and the Lipalian interval was viewed as a worldwide, unbridgeable gap in the geological record. The Lipalian interval supported Darwin's gradualistic views, because it meant that the abrupt appearance of Cambrian animals was more apparent than real. Unfortunately for gradualism, this did not prove to be a viable solution to the Precambrian-Cam-brian boundary problem. We now know of many sedimentary sequences that span the Precambrian-Cambrian boundary, but which lack the profound gap predicted by the Lipalian interval hypothesis.
The study of Precambrian paleontology began in 1858, when a collector for the Geological Survey of Canada found some curious specimens in very ancient metamorphic Precambrian rocks. These specimens were made of thin, alternating concentric layers of the calcium carbonate mineral calcite and the silicate mineral serpentine. Sir William Logan, director of the Canadian Geological Survey, thought that these banded specimens might be fossils. He was able to find better specimens near Ottowa in 1864. Logan brought them to J. William Dawson, principal of McGill University and the leading Canadian paleontologist. Dawson found what he thought were biologic structures in the calcite. Dawson identified these specimens as the skeletons of giant protists called foraminifera. Foraminifera can be very large by protistan standards, but these specimens were hundreds of times larger than any previously known foraminifer. Dawson named these objects "Eozoon canadense," literally, the dawn animal of Canada (O'Brien 1970, 1971). Not everyone accepted the biologic origin of "Eozoon," and Logan's pioneering study sparked a half century of controversy over whether there were any fossils at all in the Precambrian. Darwin was a staunch supporter of the biologic origin of "Eozoon" and he wrote that he had no doubt regarding its organic nature. This is not too surprising, considering the problems that Darwin's principal theory encountered if the Precambrian was lifeless. It was embarassing for the entire paleontological profession for there not to be any Precambrian life —all that strata with no demonstrable fossils! Dawson defended "Eozoon" to the end with what Gould (1980) calls "some of the most acerbic comments ever written by a scientist." Responding to Mobius (1879), a German critic of "Eozoon," Dawson (1879) remarked that Mobius did not have adequate geological background to evaluate the fossil properly, and charged that Mobius unfairly created a misleading view of "Eozoon" (O'Brien 1970). In spite of Dawson's deeply held desires, "Eozoon" proved to be inorganic, a product of metamorphosis found only in rocks that have been altered by heat and pressure to the point that all fossils will have been destroyed (Hofmann 1982).
Unusual layered structures in sedimentary rocks, somewhat reminiscent of "Eozoon," were discovered by John Steele (1825) in Lester Park, New York State (in rocks now known to be Upper Cambrian in age). These structures, because of their size, shape, and internal layering, are very reminiscent of large cabbages which have been sliced in half. Several sedimentary horizons at Lester Park have abundant examples of these vertically stacked, hemispherical structures (figure 1.3). Steele did not recognize these structures as fossils, but the renowned New York paleontologist James Hall (1883) called these structures Cryptozoon proliferum (literally, "prolific hidden animal"). We now call these structures stromatolites. Stromatolites (figures 1.3 and 1.4) are curious fossils. They occur as regularly layered mounds of sediment (usually limestone or dolomite) whose layers curve upward and away from the substratum to form domes, cones, or branching columns. Stromatolites are both organic and sedimentary, having been built by the trapping and binding of sediment particles by communities of tangled threadlike monerans (mostly cyanobacteria). The tangled filaments, growing together, form a feltlike mat.
The formation of a stromatolite can be compared to the mound of dirt found within a clump of grass surrounded by barren, windswept earth. Not only does the grass clump prevent erosion of soil from immediately underneath it, it can actually trap windblown sediment because the velocity of the wind slows when it tries to blow through the clustered grass blades. When the wind velocity drops to a certain point, the soil particles that the wind had carried in suspension begin to fall, eventually adding to the mound of soil. Stromatolites form in a somewhat analogous fashion. When bottom currents sweep across the feltlike mesh of cyanobacterial filaments, the current slows and drops tiny, suspended sedimentary particles. With slow but steady growth, stromatolites add layer upon layer of trapped sediment particles to build the colony upwards and above the sea floor. The felty mat prevents erosion of the sediments stored underneath. A fossilized stromatolite records in its layers the growth and expansion of a moneran colony, and cannot be thought of as a fossil of a single organism, let alone an animal. The fossil history of stromatolites, however, has an important link to early animal evolution which will be discussed in chapter 8.
Field work by C. D. Walcott in the Grand Canyon (1899) and the
Belt Supergroup of Montana (1914) led to his recognition of the first Precambrian stromatolites. Walcott found microfossils in stromatolites from the Belt Supergroup which he correctly compared to modern photosynthesizing monerans. Walcott also described many supposed Precambrian animals. His description of Precambrian animals was largely in error, but as we shall see, he was by no means the last to make mistakes in the interpretation of supposed Precambrian animal fossils. Walcott's work established the importance of the Precambrian-Cambrian boundary as a discontinuity in the history of life, which was a big advance over the earlier ideas linking the origin of life to the "Primordial [read Cambrian] faunas." After Walcott's
death in 1927, Precambrian paleontology languished for a quarter of a century.
A renaissance in Precambrian paleontology began in the 1950s — discoveries and techniques developed then still guide current research activities. In 1954, S. A. Tyler and E. S. Barghoorn reported moneran microfossils in Precambrian cherts from near the Canadian shore of Lake Superior (Tyler and Barghoorn 1954). Chert, a micro-crystalline form of silica, can beautifully preserve the delicate, microscopic structure of sea-floor monerans. Many Precambrian microbial fossil localities are now known from cherts, and some are from silicified stromatolites, confirming Walcott's inference about the organisms responsible for stromatolite formation.
The first conclusive proof of Precambrian animals came just before the 1950s with the discovery of organisms now referred to as the Ediacaran fauna.
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