Paleontology - How it works

Thinking in Terms of Geologic Time

The term geologic time refers to the great sweep of Earth's history, a timescale that dwarfs the span of human existence. The essays Historical Geology and Geologic Time offer several comparisons to emphasize the proportions involved and to illustrate the very short period during which human life has existed on this planet.

As one example shows, if all of geologic time were compressed into a single year, the first Homo sapiens would have appeared on the scene at about 8:00 p.m. on December 31. Human civilization, which dates back about 5,500 years (a millennium before the building of Egypt's great pyramids) would have emerged within the last minute of the year.

In another example, geologic time is compared to the distance from Los Angeles to New York City. On this scale, the period of time in which humans have existed on the planet would be equivalent to the distance from New York's Central Park to the Empire State Building, or less than 2 mi. (3.2 km). The history of human civilization, on the other hand, would be less than 16 ft. (4.9 m) long.

THE "ABYSS OF TIME."

Needless to say, the scope of geologic time compared with the units with which we are accustomed to measuring our lives (or even the history of our civilization) is more than a little intimidating. This fact perhaps was best expressed by the Scottish geologist John Playfair (1748-1819), friend and countryman of the "father of geology," James Hutton (1726-1797). At a time when many people were content to believe that the Earth had been around no more than 6,000 years (see Historical Geology), Hutton suggested that to undergo the complex processes that had shaped its landforms, the planet had to be much, much older. Commenting on Hutton's discoveries, Playfair said, "The mind seemed to grow giddy by looking so far into the abyss of time."

Carbon: The Meaning of "Life"

A discussion of life on Earth requires us to go deep into this "abyss," though not nearly as far back as the planet's origins. It does appear that life on Earth existed at a very early point, but in this context "life" refers merely to molecules of carbon-based matter capable of replicating themselves. Knowledge of these very early forms is extremely limited.

Carbon appears in all living things, in things that were once living, and in materials produced by living things (for example, sap, blood, and urine). Hence, the term organic, which once meant only living matter, refers to almost all types of material containing carbon. The only carbon-containing materials that are not considered organic are oxides, such as carbon dioxide and carbon monoxide, and carbonates, a class of minerals that is extremely abundant on Earth.

Precambrian Time

We will return to the subject of carbon, which plays a role in one technique for dating relatively recent items or phenomena. For the present, however, let us set our bearings for a discussion of the Phanerozoic eon, the fourth and last of the major divisions of geologic time. Though extremely primitive life-forms existed before the Phanerozoic eon, the vast majority of species have evolved since it began, and consequently paleontological work is concerned primarily with the Phanerozoic eon.

The divisions of geologic time are not arranged in terms of strict mathematical relationships of the type to which we are accustomed, for example, ten years in a decade, ten decades in a century, and so on. Instead, each era consists of two or more periods, each period consists of two or more epochs, and so on. The first 4,000 million years or so of Earth's existence (abbreviated as 4,000 Ma, or 4 Ga) are known as Precambrian time. In discussing this period of time, the vast majority of the planet's history, it is seldom necessary to speak of geologic time divisions smaller than the largest unit, the eon. Precambrian time consisted of three eons, the Hadean or Priscoan, Archaean, and Proterozoic.

THE FIRST THREE EONS.

The Hadean (sometimes called the Priscoan and dating to about 4,560 Ma to 4,000 Ma ago) saw the formation of the planet and the beginnings of the oceans and an early form of atmosphere that consisted primarily of carbon dioxide. It was during this eon that the carbon-based matter referred earlier made its appearance, perhaps by means of the meteorites that bombarded the planet during that long-ago time.

In the Archaean eon (about 4,000 Ma to 2,500 Ma ago) the first clear evidence of life appeared in the form of microorganisms. These were prokaryotes, or cells without a nucleus, which eventually were followed by eukaryotes, or cells with a nucleus. Many of the prerequisites for life as we know it were established during this time, though our present oxygen-containing atmosphere still lay far in the future.

Longest of the four eons was the Proterozoic eon (about 2,500 Ma to 545 Ma). This phase saw the beginnings of very basic forms of plant life, while oxygen in the atmosphere assumed about 4% of its present levels. Animal life, meanwhile, still consisted primarily of eukaryotes.

The Phanerozoic Eon

The majority of paleontologic history has taken place during the Phanerozoic eon. In the course of this essay, we discuss its eras and periods (the second-and third-longest spans of geologic time, respectively) as they relate to life on Earth. The three Phanerozoic eras are as follows:

Eras of the Phanerozoic Eon

• Paleozoic (about 545-248.2 Ma)
• Mesozoic (about 248.2-65 Ma)
• Cenozoic (about 65 Ma-present)

Within these eras are the following periods:

Periods of the Paleozoic Era

• Cambrian (about 545-495 Ma)
• Ordovician (about 495-443 Ma)
• Silurian (about 443-417 Ma)
• Devonian (about 417-354 Ma)
• Carboniferous (about 354-290 Ma)
• Permian (about 290-248.2 Ma)

Periods of the Mesozoic Era

• Triassic (about 248.2-205.7 Ma)
• Jurassic (about 205.7-142 Ma)
• Cretaceous (about 142-65 Ma)

Periods of the Cenozoic Era

• Palaeogene (about 65-23.8 Ma)
• Neogene (about 23.8-1.8 Ma)
• Quaternary (about 1.8 Ma to the present)

The Carboniferous period of the Paleozoic era usually is divided into two subperiods, the Mississippian (about 354 to 323 Ma) and the Pennsylvanian (about 323-290 Ma). In addition, the Palaeogene and Neogene periods of the Cenozoic era often are lumped together as a subera called the Tertiary. By substituting that name for those of the two periods, it is possible to use a time-honored mnemonic device by which geology students have memorized the names of the 11 Phanerozoic periods: "Camels Ordinarily Sit Down Carefully; Perhaps Their Joints Creak Tremendously Quietly."

EPOCHS OF THE CENOZOIC ERA.

An epoch is the fourth-largest division of geologic time and is, for the most part, the smallest one with which we will be concerned. (There are two smaller categories, the age and the chron.) Listed here are the epochs of the Cenozoic era from the most distant to the Holocene, in which we are now living. Their names are derived from Greek words whose meanings are provided:

Epochs of the Cenozoic Era

• Paleocene (about 65-54.8 Ma): "early dawn of the recent"
• Eocene (about 54.8-33.7 Ma): "dawn of the recent"
• Oligocene (about 33.7-23.8 Ma): "slightly recent"
• Miocene (about 23.8-5.3 Ma): "less recent"
• Pliocene (about 5.3-1.8 Ma): "more recent"
• Pleistocene (about 1.8-0.01 Ma): "most recent"
• Holocene (about 0.01 Ma to present): "wholly recent"

A Brief Overview of Paleontologic History

The title "A Brief Overview of Paleontologic History" is almost a contradiction in terms, since virtually nothing about the history of Earth has been brief. Moreover, the history of life on Earth is so filled with detail and complexity that it could fill many books, as indeed it has. Owing to that complexity, anything approaching an exhaustive treatment of the subject would burden the reader with so much technical terminology that it would obscure the larger overview of paleontology and the materials of the paleontologist's work. Therefore, only the most cursory of treatments is possible, or indeed warranted, in the present context. For additional detail, the reader is invited to consult other texts, including

those listed in the suggested reading section at the end of this essay.

As with many another process, the evolution of organisms was exceedingly slow in the beginning (and here the comparative term slow refers even to the standards of geologic time), but it sped up considerably over the course of Earth's history. This is not to suggest that the development of life-forms has been a steady process; on the contrary, it has been punctuated by mass extinctions, discussed at the conclusion of this essay. Nonetheless, it is correct to say that during the first 80%-90% of Earth's history, the few existing life-forms underwent an extremely slow process of change.

PRECAMBRIAN AND PALEOZOIC LIFE-FORMS.

Life existed in Precambrian time, as noted, but over the course of those four billion years, it evolved only to the level of single-cell microorganisms. Samples of these organisms have been found in the fossil record, but the fossilized history of life on Earth really began in earnest only with the Cambrian period at the beginning of the Paleozoic era and the Phanerozoic eon. The early Cambrian period saw an explosion of invertebrate (without an internal skeleton) marine forms, which dominated from about 545 Ma-417 Ma ago. By about 420 Ma-410 Ma, life had appeared on land, in the form of algae and primitive insects.

The beginning of the Devonian period (approximately 417 Ma) saw the appearance of the first vertebrates (animals with an internal skeleton), which were jawless fish. Plant life on land consisted of ferns and mosses. By the late Devonian (about 360 Ma), fish had evolved jaws, and amphibians had appeared on land. Reptiles emerged between about 320 Ma and 300 Ma, in the Pennsylvanian subperiod of the Carboniferous. In the last period of the Paleozoic era, the Permian (about 290-248.2 Ma), reptiles became the dominant land creatures.

MESOZOIC AND CENOZOIC LIFE-FORMS.

The next era, the Mesozoic (about 248.2-65 Ma), belonged to a particularly impressive form of reptile, known as the terrible lizard: the dinosaur. These creatures are divided into groups based on the shape of their hips, which were either lizardlike or birdlike. Though the lizardlike Saurischia emerged first, they lived alongside the birdlike Ornithischia throughout the late Triassic, Jurassic, and Cretaceous periods. Ornithischia were all herbivores, or plant eaters, whereas Saurischia included both herbivores and carnivores, or meat eaters. Naturally, the most fierce of the dinosaurs were carnivores, a group that included the largest carnivore ever to walk the earth, Tyrannosaurus.

Though dinosaurs receive the most attention, the Mesozoic world was alive with varied forms, including flying reptiles and birds. (In fact, dinosaurs may have been related to birds, and, in the opinion of some paleontologists, they may have been warm-blooded, like birds and mammals, rather than cold-blooded, like other reptiles.) Botanical life included grasses, flowering plants, and trees of both the deciduous (leaf-shedding) and coniferous (cone-bearing) varieties.

A violent event, discussed in the context of mass extinction later in this essay, brought an end to the Mesozoic era. This cleared the way for the emergence of mammalian forms at the beginning of the Cenozoic, though it still would be a long time before anything approaching an ape, let alone a human, appeared on the scene. The earliest hominid, or humanlike creature, dates back to about four million years ago, in the Pliocene epoch of the Neogene period.

Historical Geology and Paleontology

One of the two principal divisions of geology (along with physical geology) is historical geology, the study of Earth's physical history. Other subdisciplines of historical geology are stratigraphy, the study of rock layers, or strata, beneath Earth's surface; geochronology, the study of Earth's age and the dating of specific formations in terms of geologic time; and sedimentology, the study and interpretation of sediments, including sedimentary processes and formations.

Paleontology, the investigation of life-forms from the distant past (primarily through the study of fossilized plants and animals), is another subdiscipline of historical geology. Though it is rooted in the physical sciences, it obviously crosses boundaries into the biological or life sciences as well. Related or subordinate fields include paleozoology, which focuses on the study of prehistoric animal life; paleobotany, the study of past plant life; and paleoecology, the study of the relationship between prehistoric plants and animals and their environments.

Classifying Plants and Animals

Given the close relationship between paleontology and the biological sciences, it is necessary to discuss briefly the taxonomic system applied in biology, botany, zoology, and related fields. Taxonomy is an area of biology devoted to the identification, classification, and naming of organisms. Devised in the eighteenth century by the Swedish botanist Carolus Linnaeus (1707-1778) and improved in succeeding years by many others, the taxonomic system revolutionized biology.

Linnaeus's taxonomy provided a framework for classifying known species not simply by superficial similarities but also by systemic characteristics. For example, worms and snakes have something in common on a surface level, because they are both without appendages and move by writhing on the ground. A worm is an invertebrate, however, whereas a snake is a vertebrate. The Linnaean system therefore would classify them in widely separated categories: they are not siblings or even first cousins but more like fourth cousins.

Moreover, the system created by Linnaeus gave scientists a means for classifying and thereby potentially understanding much about the history and characteristics of species as yet undiscovered. Thus, it would prove of immeasurable significance to the English naturalist Charles Darwin (1809-1882) in formulating his theory of evolution. As Darwin showed, the varieties of different organisms have increased over time, as those organisms developed characteristics that made them more adaptable to their environments. Plants and animals that failed to adapt simply became extinct, though failure to adapt is only one of several causes for extinction, as we shall see.

A BRIEF OVERVIEW.

The Linnaean system uses binomial nomenclature, or a two-part naming scheme (in Latin), to identify each separate type of organism. If a man is named John Smith, then "Smith" identifies his family, while John identifies him singularly. Likewise each variety of organism is identified by genus, equivalent to Smith, and species, analogous to John. In the Linnaean system, there are eight levels of classification, which, from most general to most specific, are kingdom, phylum, subphylum, class, order, family, genus, and species.

These levels can be illustrated by identifying a species near and dear to all of us: Homo sapiens, commonly known as humans. We belong to the animal kingdom (Animalia), the Chordata (i.e., possessing some form of central nervous system) phylum, and the Vertebrata subphylum, indicating the existence of a backbone. Within the mammal (Mammalia) class we are part of the primate (Primata) order, along with apes. Humans are distinguished further as members of the hominid (Hominoidea), or "human-like" family; the genus Homo ("man"); and the species sapiens ("wise").

Dating Materials From the Past

In studying the past, paleontologists and other earth scientists working in the field of historical geology rely on a variety of dating techniques. "Dating," in a scientific context, usually refers to any effort directed toward finding the age of a particular item or phenomenon. It may be relative, devoted to finding an item's age in relation to that of other items; or absolute, involving the determination of age in actual years or millions of years.

Among the methods of relative dating are stratigraphic dating, discussed in the essay Stratigraphy, as well as seriation, faunal dating, and pollen dating. Seriation entails analyzing the abundance of a particular item and assigning relative dates based on that abundance. Faunal dating is the use of bones from animals (fauna) to determine age, and pollen dating, or palynology, analyzes pollen deposits.

FAUNAL DATING AND PALYNOLOGY.

The concept of faunal dating emerged from early work by the English engineer and geologist William Smith (1769-1839), widely credited as the "father of stratigraphy." In particular, Smith established an important division of stratigraphy, known as biostratigraphy, that is closely tied to paleontology. While excavating land for a set of canals near London, he discovered that any given stratum, or rock layer, contains the same types of fossils, and therefore strata in two different areas can be correlated.

Smith stated this in what became known as the law of faunal succession: all samples of any given fossil species were deposited on Earth, regardless of location, at more or less the same time. As a result, if a geologist finds a stratum in one area that contains a particular fossil and another in a distant area containing the same fossil, it is possible to conclude that the strata are the same.

Pollen dating, or palynology, is based on the fact that seed-bearing plants release large numbers of pollen grains each year. As a result, pollen spreads over the surrounding area, and in many cases pollen from the distant past has been preserved. This has occurred primarily in lake beds, peat bogs, and, occasionally, in areas with cool or acidic soil. By observing the species of pollen deposited in an area, scientists are able to develop a sort of "pollen calendar," which provides information about such details as changes in climate.

DENDROCHRONOLOGY.

Scientists use relative dating when they must, but they would prefer to determine dates in an absolute sense wherever possible. Most methods of absolute dating rely on processes that are not immediately comprehensible to the average person, but there is one exception: dendrochronology, or the dating of tree rings. As almost everyone knows, trees produce one growth ring per year. There is nothing magical about this, since a year is not an abstract unit of time; rather, it is based on Earth's revolution around the Sun, during which time the planet undergoes changes in orientation that result in the four seasons, which, in turn, affect the tree's growth.

Though dendrochronology makes use of a principle familiar to most people, the work of the dendrochronologist requires detailed, often complex study. Just as the layers of rock beneath Earth's surface reveal information about past geologic events (a matter discussed in the essay Stratigraphy), tree rings can tell us much about environmental changes. Thin rings, for instance, suggest climatic anomalies and may provide clues about cataclysmic events that were understood only vaguely by the ancient humans who experienced them. (An example of this is the apparent cataclysm of A.D. 535, which is discussed Earth Systems.)

AMINO-ACID RACIMIZATION.

Dendrochronology is useful only for studying the relatively recent past, up to about 10,000 years—a span equivalent to the Holocene epoch, which began with the end of the last ice age. To investigate more distant phases of Earth's history, it is necessary to use forms of radiometric dating, which we will discuss shortly. The principles of radiometric dating, however, are illustrated by another method, amino-acid racimization.

With the exception of some microbes, living organisms incorporate only one of two forms of amino acids, known as L-forms. Once the organism dies, the L-amino acids gradually convert to D-amino acids. In the 1960s, scientists discovered that by comparing the ratios between the L-and D-forms, it was possible to date organisms that were several thousand years old. Unfortunately, it has since come to light that because of the many factors affecting the rate of amino-acid conversion, this method is less reliable than once was believed. Moisture, temperature, and pH (the relative acidity and alkalinity of a substance) all play a part, and because these factors vary so widely, amino-acid racimization no longer is used commonly.

Nonetheless, the basic principle behind amino-acid racimization plays a part in other, more reliable forms of absolute dating. Many of them are based on the fact that over time, a particular substance converts to another, mirror substance. By comparing the ratios between them, it is possible to arrive at some estimate of the amount of time that has elapsed since the organism died.

RADIOCARBON DATING.

The most significant method of absolute dating available to scientists today is radiometric dating, which is explained in detail in the essay Geologic Time. Each chemical element is distinguished by the number of protons (positively charged particles) in its atomic nucleus, but atoms of a particular element may have differing numbers of neutrons, or neutrally charged particles, in their nuclei. Such atoms are referred to as isotopes.

Certain isotopes are stable, whereas others are radioactive, meaning that they are likely to eject particles from the nucleus over time. The amount of time it takes for half the isotopes in a sample to stabilize is called its half-life. By analyzing the quantity of radioactive isotopes in a given sample that have converted to stable isotopes, it is possible to determine the age of the sample. In other situations, it is necessary to compare ratios of unstable "parent" isotopes to even more unstable "daughter" isotopes produced by the parent.

As we noted earlier, carbon is present in all living things, and thus an important means of dating available to paleontologists uses a radioactive form of carbon. All atoms of carbon have six protons, and the most stable and abundant carbon isotope is carbon-12, so designated because it has six neutrons. On the other hand, carbon-14, with eight neutrons, is unstable.

When an organism is alive, it incorporates a certain ratio of carbon-12 in proportion to the (very small) amount of carbon-14 that it receives from the atmosphere. Once the organism dies, however, it stops incorporating new carbon, and the ratio between carbon-12 and carbon-14 begins to change as the carbon-14 decays to form nitrogen-14. Therefore, a scientist can use the ratios of carbon-12, carbon-14, and nitrogen-14 to estimate the age of an organic sample. This method is known as radiocarbon dating.

Carbon-14, or radiocarbon, has a half-life of 5,730 years, meaning that it is useful for analyzing only fairly recent samples. Nonetheless, it takes much longer than 5,730 years for the other half of the radiocarbon isotopes in a given sample to stabilize, and for this reason radiocarbon dating can be used with considerable accuracy for 30,000-40,000 years. Sophisticated instrumentation can extend this range even further, up to 70,000 years.

THE LIMITS OF ABSOLUTE DATING.

While 70,000 years, or 0.07 Ma, may be a long time in human terms, from the standpoint of the earth scientist, 0.07 Ma is only yesterday—the latter part of the last epoch, the Pleistocene. Other forms of radiometric dating, such as potassium-argon dating and uranium-series dating, can be used to measure truly long spans of times, in the billions of years. These methods are discussed in Geologic Time.

Potassium-argon dating and uranium-series dating can be useful to the paleontologist, inasmuch as they aid in determining the age of the geologic samples in which the remains of life-forms are found. Nothing is simple, however, when it comes to dating specimens from the distant past. After all, geologists working in the realm of stratigraphy face numerous challenges in judging the age of samples, even with these sophisticated forms of radiometric dating. (For more on this subject, see Stratigraphy.)

In fact, the work of a paleontologist is much like that of the stratigrapher. The success of either type of scientist relies more on detailed and painstaking detective work than it does on sophisticated technology. Both must analyze layered

samples from the past to form a picture of the chronology in which the samples evolved, and oftentimes the apparent evidence can be deceptive. The principal difference between stratigraphic and paleontologic study relates to the materials: rocks in the first instance and fossils in the second.