PALEONTOLOGY

CONCEPT

Thanks to a certain 1993 blockbuster, most people know the name of at least one period in geologic history. Jurassic Park spurred widespread interest in dinosaurs and, despite its fantastic plot, encouraged popular admiration and respect for the work of paleontologists. Paleontology is the study of life-forms from the distant past, as revealed primarily through the record of fossils left on and in the earth. It is a complex and varied subdiscipline of historical geology that is tied closely to the biological sciences. As with other types of historical geology, the work of a paleontologist is similar to that of a detective investigating a case with few and deceptive clues. Reliable fossil samples are more rare, compared with the vast number of species that have lived on Earth, than one might imagine. Furthermore, several factors pose challenges for paleontologists attempting to interpret the fossil record. Nonetheless, paleontologic research has led to a growing understanding of how life emerged, how Earth has changed, and how vast animal populations became extinct over relatively short periods of time.

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."

THE HEAD OF A TYRANNOSAURUS REX ("KING OF THE TERRIBLE LIZARDS") FOUND IN MONTANA, DATING TO THE MESOZOIC PERIOD. (© Tom McHugh/Photo Researchers. Reproduced by permission.)
THE HEAD OF A TYRANNOSAURUS REX ("KING OF THE TERRIBLE LIZARDS") FOUND IN MONTANA, DATING TO THE MESOZOIC PERIOD. (
© Tom McHugh/Photo Researchers
. Reproduced by permission.)

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

A DINOSAUR IS EXCAVATED AT DINOSAUR NATIONAL MONUMENT IN COLORADO. (© James L. Amos/Photo Researchers. Reproduced by permission.)
A DINOSAUR IS EXCAVATED AT DINOSAUR NATIONAL MONUMENT IN COLORADO. (
© James L. Amos/Photo Researchers
. Reproduced by permission.)
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

STINGRAY FOSSIL, POSSIBLY DATING TO THE JURASSIC PERIOD. THIS IS A RARE FIND, BECAUSE STINGRAYS HAVE NO BONE, ONLY CARTILAGE, WHICH MAKES IT HARDER FOR THEM TO UNDERGO MINERALIZATION AND BE PRESERVED AS FOSSILS. (© Gary Retherford/Photo Researchers. Reproduced by permission.)
STINGRAY FOSSIL, POSSIBLY DATING TO THE JURASSIC PERIOD. THIS IS A RARE FIND, BECAUSE STINGRAYS HAVE NO BONE, ONLY CARTILAGE, WHICH MAKES IT HARDER FOR THEM TO UNDERGO MINERALIZATION AND BE PRESERVED AS FOSSILS. (
© Gary Retherford/Photo Researchers
. Reproduced by permission.)
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.

REAL-LIFE APPLICATIONS

FOSSILS AND FOSSILIZATION

The term fossil refers to the remains of any prehistoric life-form, especially those preserved in rock before the end of the last ice age. The process by which a once-living thing becomes a fossil is known as fossilization. Generally, fossilization refers to changes in the hard portions, including bones, teeth, shells, and so on. This series of changes, in which minerals are replaced by different minerals, is known as mineralization. Sometimes, soft parts may experience mineralization and thus be preserved as fossils. A deceased organism in the process of becoming a fossil is known as a subfossil.

The majority of fossils come from invertebrates, such as mussels, that possess hard parts. Generally speaking, the older and smaller the organism, the more likely it is to have experienced fossilization, though other factors (which we will discuss later) also play a part. One of the most important factors involves location: for the most part, the lower the altitude, the greater the likelihood that a region will contain fossils. The best place of all is in the ocean, particularly the ocean floor. Nonetheless, fossils have been found on every continent of Earth, and the great distances that sometimes separate samples of the same species have aided earth scientists of many fields in understanding the processes that shaped our planet.

FOSSILS AND GEOLOGIC HISTORY.

The earth beneath our feet is not standing still; rather, it is constantly moving, and over the great stretches of geologic time, the positions of the continents have shifted considerably. The details of these shifts are discussed in Plate Tectonics, an area of geologic study that explains much about the earth, from earthquakes and volcanoes to continental drift.

Paleontology has contributed to the study of plate tectonics by revealing apparent anomalies, such as fossilized dinosaur parts in Antarctica. No dinosaur could have lived on that forbidding continent, so there must be some other explanation: the continental plates themselves have moved. Long ago, the present continents were united in a "supercontinent" called Pangaea. When Pangaea split apart to form the present continents, the remains of various species were separated from one another and from the latitudes to which they were accustomed in life.

Fossilized remains of single-cell organisms have been found in rock samples as old as 3.5 Ga, and animal fossils have been located in rocks that date to the latter part of Precambrian time, as old as 1 Ga. Just as paleontologists have benefited from studies in chronostratigraphy and geochronometry, realms of stratigraphy concerned with the dating of rock samples, stratigraphers and other geologists have used fossil samples to date the rock strata in which they were found. Not all fossilized life-forms are equally suited to this purpose. Certain ones, known as index fossils or indicator species, have been associated strongly with particular intervals of geologic time. An example is the ammonoid, a mollusk that proliferated for about 350 Ma from the late Devonian to the early Cretaceous before experiencing mass extinction.

MAKING THE GRADE AS A FOSSIL.

Everything that is living eventually dies, but not nearly all living things will become fossils. And even if they do, there are numerous reasons why fossils might not be preserved in such a way as to provide meaningful evidence for a paleontologist many millions of years later. In the potential pool of candidates for fossilization, as we have noted, organisms without hard structural portions are unlikely to become fossilized. Fossilization of soft-bodied creatures sometimes occurs, however, as, for instance, at Burgess Shale in British Columbia, where environmental conditions made possible the preservation of a wide range of samples.

Furthermore, location is a powerful factor. Sedimentary rock, formed by compression and deposition (i.e., formation of deposits) on the part of other rock and mineral particles, provides the setting for many fossils. Best of all is sediment, such as sand or mud, that has not yet consolidated into harder sandstone, limestone, or other rocks. Organisms that die in upland locations are more likely to be disturbed either by wind or by scavengers, creatures that feed on the remains of living things. On the other hand, an organism at the bottom of an ocean is out of reach from most scavengers. Even at lesser depths, if the organism is in a calm, relatively scavenger-free marine environment, there is a good chance that it will be preserved.

Assuming that all the conditions are right and the dead organism is capable of undergoing fossilization, it will experience mineralization of one type or another. Living things already contain minerals, which is the reason why people take mineral supplements to augment the substances nature has placed in their bodies to preserve and extend life. In the mineralization of a fossil, the minerals in the organism's body may be replaced by other ones, or other minerals may be added to existing ones. It is also possible that both the hard and soft parts will dissolve and be replaced by a mineral cement that forms a mold that preserves the shape of the organism.

FINDING AND STUDYING FOSSILS.

Only about 30% of species are ever fossilized, a fact that scientists must take into account, because it could skew their reading of the paleontologic record. If a paleontologist judges the past only from the fossils that have been found in an area, it will result in a picture of a past environment that contained only certain species, when, in fact, others were present. Furthermore, there are many factors that contribute to the loss of fossils. For instance, if the area has been subjected to violent tectonic activity, it is likely that the sample will be destroyed partially or wholly.

The removal of a fossil from its home in the rock is a painstaking process akin to restoring a valuable piece of art. Before removing it, the paleontologist photographs the fossil and surrounding strata and records details about the environment. Only when these steps have been taken is the fossil removed. This is done with a rock saw, which is used to cut out carefully a large area surrounding the fossil. The sample is then jacketed, or wrapped in muslin with an additional layer of wet plaster, and taken to a laboratory for study.

Fossil research can reveal a great deal about the history of life on Earth, including the relationships between species or between species and their habitats. Studies of dinosaur bones have brought to light proteins that existed in the bodies of these long-gone creatures, while research on certain oxygen isotopes has aided attempts to discover whether dinosaurs were warm-blooded creatures. Thanks to advances in the understanding of DNA (deoxyribonucleic acid), which provides the genetic codes for all living things, it may be possible to make even more detailed studies in the future.

MASS EXTINCTION

The remains of dinosaurs, of course, have an importance aside from their significance to paleontology. The bodies of these giant lizards have been deposited in the earth, where over time they became coal, peat, petroleum, and other fossil fuels. The latter are discussed in Economic Geology, but the fact that the dinosaurs disappeared at all is of particular interest to paleontology. Why are there no dinosaurs roaming the earth today? The answer appears to be that they were wiped out in a dramatic event, perhaps brought about as the result of a meteorite impact.

Numerous species have become extinct, typically as a result of their inability to adapt to changes in their natural environment. More recently, some extinctions or endangerments of species have been attributed to human activities, including hunting and the disruption of natural habitats. For the most part, however, extinction is simply a part of Earth's history, a result of the fact that nature has a way of destroying organisms that do not adapt (the "survival of the fittest"). But there have been occasions in the course of the planet's past in which vast numbers of individuals and species perished at once. A natural catastrophe may destroy a large population of individuals within a locality, a phenomenon known as mass mortality. Or mass mortality may take place on a global scale, destroying many species, in which case it is known as mass extinction.

CAUSES OF MASS EXTINCTION.

The Bible depicts an example of near mass extinction, in the form of Noah's flood, and, indeed, several instances of mass extinction have resulted from sudden and dramatic changes in ocean levels. Others have been caused by tectonic events, most notably vast volcanic eruptions that filled the atmosphere with so much dust that they caused a violent change in temperature. Scientific speculation concerning other such extinctions has pointed to events in or from space—either the explosion of a star or the impact of a meteorite on Earth—as the cause of atmospheric changes and hence mass extinction.

Even though scientists have a reasonable idea of the immediate causes of mass extinction in some cases, their understanding of the ultimate or root causes is still limited. This fact was expressed by the University of Chicago paleobiologist David M. Raup, who wrote: "The disturbing reality is that for none of the thousands of well-documented extinctions in the geologic past do we have a solid explanation of why the extinction occurred."

EXAMPLES OF MASS EXTINCTION.

The five largest known mass extinctions occurred at intervals of 50 Ma to 100 Ma over a span of time from about 435 to 65 million years ago. Most occurred at the end of a period, which is no accident, since geologists have used mass extinction as a factor in determining the parameters of a specific period.

In the late Ordovician period, about 435 Ma ago, a drop in the ocean level wiped out one-fourth of all marine families. Similarly, changes in sea level, along with climate changes, appear to have caused the destruction of one-fifth of existing marine families during the late Devonian period (about 357 Ma ago). Worst of all was the "great dying," as the extinction at the end of the Permian period (about 250 Ma) is known. Perhaps caused by a volcanic eruption in Siberia, it eliminated a staggering 96% of all species over a period of about a million years.

During the late Triassic period, about 198 million years ago, another catastrophe eliminated a quarter of marine families. Paleontologists know this, as they know about other mass extinctions, by the inordinate numbers of fossilized samples found in rock strata dating to that period. This reliance on the fossil record is also reflected in the fact that the scope of early mass extinctions usually is expressed in terms of marine life. As we have seen, the ocean environment provides the most reliable fossil record. Creatures died on land as well, but the terrestrial record is simply less reliable or less complete.

Scientific disagreement over the late-Triassic mass extinction exemplifies the fact that our knowledge of these distant events is not firmly established, but rather is subject to much scientific conjecture and dispute. (This does not mean that just any old idea can compete on an equal footing: we are talking here about differences of opinion among highly trained specialists.) At any rate, some scientists refer to the late-Triassic mass extinction as being one of the less exciting or eventful mass extinctions. Of course, it is hard to see how a mass extinction could be unexciting or uneventful, but they mean this in comparative terms; on the other hand, some paleontologists maintain that the late-Triassic was among the most devastating.

As to the cause, some theorists point to a group of impact sites spread across Canada, the northern United States, and Ukraine, places that would have been more or less contiguous at the time of the mass extinction. Difficulties in analyzing the "signatures" left by the projectiles that made these impressions have prevented theorists from saying with any degree of certainty whether it was a comet or an asteroid that caused the impact. Others, in particular a team from the University of California at Berkeley led by geologist Paul R. Renne, cite a volcanic eruption as either the cause of the mass extinction, or at least a major abetting factor to an extinction already in progress. According to Renne and his team, basalt outcroppings scattered from New Jersey to Brazil to west Africa (again, areas that would have been contiguous then) suggest that a volcanic eruption of almost inconceivable magnitude occurred about 200 million years ago. Such an eruption would surely have destroyed vast quantities of living things.

The last and best known mass extinction occurred about 65 million years ago, marking the end of the Cretaceous period—and the end of the dinosaurs. As to what happened, paleontologists and other scientists have proposed a number of theories: a rapid climate change; the emergence of new poisonous botanical species, eaten by herbivorous dinosaurs, that resulted in the passing of toxins along the food web (see Ecosystems); an inability to compete successfully with the rapidly evolving mammals; and even an epidemic disease to which the dinosaurs possessed no immunity.

Interesting as many of these theories are, none has gained anything like the widespread acceptance achieved by another scenario. According to this highly credible theory, an asteroid hit Earth, hurtling vast quantities of debris into the atmosphere, blocking out the sunlight, and greatly lowering Earth's surface temperature. Around the world, geologists have found traces of iridium deposited at a layer equivalent to the boundary between the Cretaceous and Tertiary periods, the Tertiary being the beginning of the present Cenozoic era. This is significant, because iridium seldom appears on Earth's surface—but it is found in asteroids.

HUMANS AND MASS EXTINCTION.

There have been much more recent, if less dramatic, examples of mass extinction, including those caused by the most highly developed of all life-forms: humans. Among these examples are the well-documented (and very recent) mass extinctions brought on by destruction of tropical rainforests. Such activities are killing off a vast array of organisms: according to the highly respected Harvard biologist Edward O. Wilson, some 17,500 species are disappearing each year. But cases of mass extinction are not limited to modern times.

When prehistoric hunters (the ancestors of today's Native Americans) crossed the Bering land bridge from Siberia to Alaska some 12,000 years ago, they found an array of species unknown in the Americas today. These species included mammoths and mastodons; giant bears, beaver, and bison; and even saber-toothed tigers, camels, and lions. Perhaps most remarkable of all, it appears that prehistoric America was once home to a creature that would prove to be of enormous benefit to humans until the beginning of the automotive age: the horse. Horses did not reappear in the Americas until Europeans arrived to conquer those lands after A.D. 1500.

WERE DINOSAURS WARM-BLOODED?

One of the most significant scientific debates of the later twentieth and early twenty-first centuries, not only in paleontology but in the earth sciences or even science itself, is the question of whether or not the dinosaurs were warm-blooded. In other words, were they like modern reptiles, which must adjust their temperature by moving into the sunlight when they are cold, and into the shade when they are too hot? Or were they more like modern birds and mammals, whose bodies generate their own heat?

A warm-blooded animal always has a more or less constant body temperature, regardless of the temperature of its environment. This is due to the fact that it produces heat by the burning of food, as well as by physical activity, and stores that heat under a layer of fat just beneath the skin. Warm-blooded animals are also capable of cooling down their bodies by perspiring and panting. Birds and mammals are the only warm-blooded animals; all others are cold-blooded. A cold-blooded creature, on the other hand, lacks control over its body temperature and therefore is warm when its environment is warm, and cold when its environment is cold.

The difference between warm-and cold-blooded animals is partly one of metabolic rate, or the rate at which nutrients are broken down and converted into energy. Cold-blooded creatures have slow metabolic rates; think of a python that swallows a medium-sized mammal whole and takes several days to digest it. The dinosaur debate is therefore often framed as a question of whether the dinosaurs' bodies had a relatively high or relatively low metabolic rate.

THE DEBATE.

Until the 1960s, there was no debate: dinosaurs, whose existence had been known for about a century, were assumed to be big, dumb, slow, cold-blooded creatures. Then, in 1968, Robert T. Bakker—an undergraduate at Yale University, not a professor or a full-fledged paleontologist—revolutionized the world of paleontology with a paper called "The Superiority of Dinosaurs."

In his article, Bakker described dinosaurs as "fast, agile, energetic creatures" whose physiology was so advanced that even the biggest and heaviest of them could outrun a human. Just a year later, John H. Ostrom, a professor of paleontology who also happened to be at Yale, wrote that a recently identified species of theropod dinosaur must have been "an active and very agile predator."

Thus began the great dinosaur debate, which rages even today. Jurassic Park reinforced the Bakker-Ostrom position, portraying Velociraptor as a cunning, fast-moving predator with clear links to birds. And indeed there are many arguments for endothermy (warm-bloodedness) in dinosaurs—arguments that relate to everything from brain size to rate of growth to the latitudes at which dinosaur fossils have been located. On the other hand, there is plenty of evidence for ectothermy (cold-bloodedness), based on the dinosaurs' size, scaliness, the climate in the Mesozoic era, and so on.

To explore, compare, and judge these many arguments, the reader is encouraged to consult the "Were Dinosaurs Warm-Blooded?" Web site listed in the "Where to Learn More" section at the conclusion of this essay. However, a word of warning, as noted on that site: "The issue is a tangled, complex one. There are not just two sides to the issue; there are numerous competing hypotheses. If you're looking for a major controversy in science, look no further!"

WHERE TO LEARN MORE

Cadbury, Deborah. Terrible Lizard: The First Dinosaur Hunters and the Birth of a New Science. New York: Holt, 2001.

K-12: Paleontology: Dinos (Web site). <http://www.ceismc.gatech.edu/busyt/paleo.html>.

Morris, S. Conway. The Crucible of Creation: The Burgess Shale and the Rise of Animals. New York: Oxford University Press, 1998.

Munro, Margaret, and Karen Reczuch. The Story of Life on Earth. Toronto: Douglas and McIntyre, 2000.

Oceans of Kansas Paleontology (Web site). <http://www.oceansofkansas.com/>.

Paleontology and Fossils Resources—University of Arizona Library, Tucson (Web site). <http://www.library.arizona.edu/users/mount/paleont.html>.

Palmer, Douglas. Atlas of the Prehistoric World. Bethesda, MD: Discovery Communications, 1999.

Sanders, Robert. "New Evidence Links Mass Extinction with Massive Eruptions That Split Pangea Supercontinent and Created the Atlantic 200 Million Years Ago." University of California, Berkeley. (Web site). <http://www.berkeley.edu/news/media/releases/99legacy/4-22-1999b.html>.

Singer, Ronald. Encyclopedia of Paleontology. Chicago: Fitzroy Dearborn Publishers, 1999.

University of California, Berkeley Museum of Paleontology (Web site). <http://www.ucmp.berkeley.edu/>.

USGS (United States Geological Survey) Paleontology Home Page (Web site). <http://geology.er.usgs.gov/paleo/>.

"Were Dinosaurs Warm-Blooded?" (Web site). <http://pubs.usgs.gov/gip/dinosaurs/warmblood.html>.

KEY TERMS

ABSOLUTE AGE:

The absolute age of a geologic phenomenon is its age in Earthyears. Compare with relative age.

ATMOSPHERE:

In general, an atmosphere is a blanket of gases surrounding a planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%.

BIOSPHERE:

A combination of all living things on Earth—plants, mammals, birds, reptiles, amphibians, aquatic life, insects, viruses, single-cell organisms, and so on—as well as all formerly living things that have not yet decomposed.

BIOSTRATIGRAPHY:

An area of stratigraphy involving the study of fossilized plants and animals to establish dates for and correlations between stratigraphic layers.

CHRONOSTRATIGRAPHY:

A subdiscipline of stratigraphy devoted to studying the relative ages of rocks. Compare with geochronometry.

CONTINENTAL DRIFT:

The theory that the configuration of Earth's continents was once different than it is today; that some of the individual landmasses of today once were joined in other continental forms; and that these landmasses later separated and moved to their present locations.

CORRELATION:

A method of establishing age relationships between various rock strata. There are two basic types of correlation: physical correlation, which requires comparison of the physical characteristics of the strata, and fossil correlation, the comparison of fossil types.

DATING:

Any effort directed toward finding the age of a particular item or phenomenon. Methods of geologic dating are either relative (i.e., comparative and usually based on rock strata) or absolute. The latter, based on such methods as the study of radioactive isotopes, typically is given in terms of actual years or millions of years.

EON:

The longest phase of geologic time. Earth's history has consisted of four eons, the Hadean or Priscoan, Archaean, Proterozoic, and Phanerozoic. The next-smallest subdivision of geologic time is the era.

EPOCH:

The fourth-longest phase of geologic time, shorter than an era and longer than an age or a chron. The current epoch is the Holocene, which began about 0.01 Ma (10,000 years) ago.

ERA:

The second-longest phase of geologic time, after an eon. The current eon, the Phanerozoic, has had three eras, the Paleozoic, Mesozoic, and Cenozoic, which is the current era. The next-smallest subdivision of geologic time is the period.

FOSSIL:

The mineralized remains of any prehistoric life-form, especially those preserved in rock before the end of the lastice age.

FOSSILIZATION:

The process by which a once-living organism becomes a fossil. Generally, fossilization involves mineralization of the organism's hard portions, such as bones, teeth, and shells.

GA:

An abbreviation meaning "gigayears," or "billion years." The age of Earth is about 4.6 Ga.

GEOCHRONOMETRY:

An area of stratigraphy devoted to determining absolute dates and time intervals. Compare with chronostratigraphy.

GEOLOGIC TIME:

The vast stretch of time over which Earth's geologic development has occurred. This span (about 4.6 billion years) dwarfs the history of human existence, which is only about two million years. Much smaller still is the span of human civilization, only about 5,500 years.

HISTORICAL GEOLOGY:

The study of Earth's physical history. Historical geology is one of two principal branches of geology, the other being physical geology.

INVERTEBRATE:

An animal without an internal skeleton.

ISOTOPES:

Atoms that have an equal number of protons, and hence are of the same element, but differ in their number of neutrons. This results in a difference ofmass. An isotope may be either stable or radioactive.

LAW OF FAUNAL SUCCESSION:

The principle that all samples of any given fossil species were deposited on Earth, regardless of location, at more or less the same time. This makes it possible to correlate widely separated strata.

MA:

An abbreviation used by earth scientists, meaning "million years," or "megayears." When an event is designatedas, for instance, 160 Ma, it usually means 160 million years ago.

MASS EXTINCTION:

A phenomenon in which numerous species cease to exist at or around the same time, usually as the result of a natural calamity.

MINERALIZATION:

A series of changes experienced by a once-living organism during fossilization. In mineralization, minerals in the organism are either replaced or augmented by different minerals, or the hard portions of the organism dissolve completely.

ORGANIC:

At one time, chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals), and oxides, such as carbon dioxide.

PALEOBOTANY:

An area of paleontology involving the study of past plant life.

PALEOECOLOGY:

An area of paleontology devoted to studying the relationship between prehistoric plants and animals and their environments.

PALEONTOLOGY:

The study of life-forms from the distant past, primarily as revealed through the fossilized remains of plants and animals.

PALEOZOOLOGY:

An area of paleontology devoted to the study of prehistoric animal life.

PERIOD:

The third-longest phase of geologic time, after an era. The current eon, the Phanerozoic, has had 11 periods, and the current era, the Cenozoic, has consisted of three periods, of which the most recent is the Quaternary. The next-smallest subdivision of geologic time is the epoch.

PRECAMBRIAN TIME:

A term that refers to the first three of four eons in Earth's history, which lasted from about 4,560 to about 545 Ma ago.

RADIOACTIVITY:

A term describing a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of high-energy particles or radiation. Forms of particles or energy include alpha particles (positively charged helium nuclei), beta particles (either electrons or subatomic particles called positrons, or gamma rays, which occupy the highest energy level in the electromagnetic spectrum.

RADIOMETRIC DATING:

A method of absolute dating using ratios between "parent" isotopes and "daughter" isotopes, which are formed by the radioactive decay of parent isotopes. Radiometric dating also may involve ratios between radioactive isotopes and stable isotopes.

RELATIVE AGE:

The relative age of a geologic phenomenon is its age compared with other geologic phenomena, particularly the stratigraphic record of rock layers. Compare with absolute age.

SEDIMENT:

Material deposited at or near Earth's surface from a number of sources, most notably preexisting rock.

SEDIMENTARY ROCK:

Rock formed by compression and deposition (i.e., formation of deposits) on the part of other rock and mineral particles. Sedimentary rock is one of the three major types of rock, along with igneous and metamorphic.

SEDIMENTOLOGY:

The study and interpretation of sediments, including sedimentary processes and formations.

STRATA:

Layers, or beds, of rocks beneath Earth's surface. The singular form is stratum.

STRATIGRAPHY:

The study of rock layers, or strata, beneath Earth's surface.

VERTEBRATE:

An animal with an internal skeleton.

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