paleontology

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Concept

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 subdiscipline of both the biological and the earth sciences, one that brings to bear the techniques of geologic study and several areas of biology, including botany and zoology. But the term paleontology, as used in the present context, also provides a convenient means of encompassing, in a single word, the study of the distant biological past. Such study, of course, is related intimately to the investigation of evolutionary processes and phenomena. In the present context, however, our concern is not so much with the theory and principles of evolution, discussed elsewhere in this book, but rather with a relatively short overview of biological history. This area of biological investigation calls upon such concepts as mass extinction and fossilization as well as the study of plant and animal forms that scientists know only from scientific reconstruction of the past rather than from direct experience. Chief among these life-forms are the dinosaurs, which dominated Earth for a period of more than 100 million years, ending about 65 million years ago. This span of time, impressive as it seems on the human scale, is minuscule compared with the entire history of life on Earth.

How It Works

Paleontology Among the Sciences

Paleontology is the investigation of life-forms from the distant past, primarily through the study of fossilized plants and animals. Most people are familiar with fossils, a term that probably calls to mind an image of a flat piece of rock with a shadowy imprint of a leaf or animal on it. In fact, a fossil is the preserved remains of a once living organism that has undergone a process known as mineralization, in which the organic materials in hard parts of the organism (for example, teeth or bones) are replaced by minerals, which are inorganic.

The difference between the organic and the inorganic, which we discuss later, is more than a difference between the living and nonliving, or even the living and formerly living. For now, though, it will suffice to say that all living and formerly living things, as well as their parts and products, are organic. The stress on formerly living things is important, since paleontology by definition encompasses plants, animals, and microscopic life-forms that lived a long time ago. (As for just how long "a long time ago" really is, that subject, too, is discussed later in this essay.)

Among the fields related, or subordinate, to paleontology are paleozoology, which focuses on the study of prehistoric animal life; paleobotany, the study of past plant life; and paleoecology, the study of the relationships between prehistoric plants and animals and their environments. Despite the emphasis on past life-forms, however, paleontologists also avail themselves of evidence gleaned from observation of animals living today. In this approach, paleontology shows its relationship to geology, which, along with biology, is one of its two "parents."

Geology and Paleontology

One of the key concepts in geology is the principle of uniformitarianism, which arose from the early history of the geologic sciences. At a time when the nascent science was embroiled in a debate over Earth's origins and the age of the planet, the Scottish geologist James Hutton (1726-1797) transcended this debate by focusing on the processes at work on Earth in the present day. (In terms of geologic time or even the evolution of species, the eighteenth century and twenty-first century might as well be a few seconds apart, so the expression present day applies to Hutton's time as much as to ours.) Rather than simply speculate as to how Earth had come into being, Hutton maintained that scientists could understand the processes that had formed it by studying the geologic phenomena they could see around them.

The reason this approach works is that natural laws do not change over time, nor do the processes that are at work within nature. On the other hand, particular processes may not be in operation at all times, nor can it be assumed that the rate at which the processes take place is the same. The two foregoing sentences encompass some of the observations made by the modern American paleontologist Stephen Jay Gould (1941-2002) with his formulation of the "four types of uniformity" in Ever Since Darwin: Reflections in Natural History (1977). It is certainly appropriate that paleontology and geology interface in this concept of uniform change, because both are concerned with piecing together the distant past from the materials available in the present.

In fact, from the standpoint of the earth sciences, paleontology belongs to a branch of geology known as historical geology, or the study of Earth's physical history. (This field of study is contrasted to the other principal branch of the discipline, physical geology, the study of the material components of Earth and of the forces that have shaped the planet.) 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.

Before Life

Later in this essay, we consider just what is meant by "geologic time," the grand sweep of several billion years during which Earth evolved from a cloud of gases to its present form. It may be surprising to discover the large proportion of that time during which life existed on Earth; it is equally amazing to learn what a small portion of the biological history of the planet involved anything that modern humans would recognize as "life."

For the moment, let us go even further back, to the very beginning—an almost inconceivably long time ago. Scientists believe that the universe began between 10 billion and 20 billion years ago, with an explosion nicknamed the "big bang," a cataclysm so powerful that it sent galaxies careening outward on a course they maintain even today. Among those galaxies moving outward from the universal point of origin was our Milky Way, which, about six billion years ago, began to develop a rotating cloud of cosmic gas somewhere between its center and its rim. That was the beginning of our solar system.

The Sun and Earth

The center of the cloud, where the greatest amount of gases gathered, was naturally the densest and most massive portion as well as the hottest. There, hydrogen—the lightest of all elements—experienced extraordinary amounts of compression owing to the density of the clouded gases around it, and underwent nuclear fusion, or the bonding of atomic nuclei.

This hot center became the Sun about five billion years ago, but there remained a vast nebula of gas surrounding it. As the fringes of this nebula began to cool, the gases condensed, forming solids around which particles began to accumulate. These were the future planets, including ours. The process of planetary formation took place over a span of about 500 million years. The planets contained various chemical elements, formed by nuclear fusion on the Sun—a fascinating aspect of life on Earth, since it means that the particles that make up the human body came from nuclear reactions on the stars.

The Early Atmosphere

The brand-new Earth possessed an atmosphere that consisted primarily of elemental hydrogen, nitrogen, carbon monoxide, and carbon dioxide. This "air" would have been unbreathable to all but a very small portion of the life-forms that exist on Earth today. Yet for the development of life, it was absolutely essential that no oxygen be present in Earth's atmosphere at that very early time.

The reason is that oxygen is an extremely reactive element, which is why it is involved in an array of chemical reactions (including combustion and rusting) known collectively as oxidation-reduction reactions. If oxygen had been present at that time, it likely would have reacted with other elements immediately, rather than permitting the formation of what eventually became organic materials.

Water

Water appeared as the result of meteorite bombardment from space, which took place in the first half-billion years of Earth's existence. It might seem strange to learn that water, a compound essential to life on Earth, came from the void of space—where, as far as we know, there are no other life-forms. But this is not as strange as it sounds: water itself is inorganic, and even today frozen water exists on several planets in our solar system.

In any case, water eventually accumulated on Earth's surface in quantities sufficient for its condensation, with the result that clouds formed and rain fell on Earth more or less continually for many millions of years. It was then that the beginnings of life made their appearance, in the form of self-replicating molecules of carbon-based matter.

Life Begins

One famous cliché of science-fiction movies is the use of the phrase "carbon-based life-forms" to describe humans. In fact, all living things contain carbon, and if we ever do find life, intelligent or otherwise, on other planets, chances are extremely high that it, too, will be "carbon-based." Carbon, in fact, is almost synonymous with life, and hence the word organic, in its scientific meaning, refers to all substances that contain carbon. The only exceptions are the elemental carbon in diamonds or graphite, the carbonate forms that make up many of Earth's rocks, and such oxides as carbon dioxide and monoxide, all of which are considered inorganic.

It may sound as though a huge portion of carbon's possible forms already have been eliminated from the list of organic substances, but, in fact, carbon is capable of forming an almost limitless array of compounds with other elements, particularly hydrogen. A class of molecule known as hydrocarbons, which are nothing but strings of carbon and hydrogen molecules bonded together, is the basis for literally millions of organic compounds, from petroleum to polymer plastics. It may sound odd to hear plastics referred to as organic, but this only highlights the difference between the popular and scientific meanings of that term.

Organic Materials

At one time, organic referred only to living things, things that were once living, and materials produced by living things (for example, sap, blood, and urine). As recently as the early nineteenth century, scientists believed that organic substances contained a supernatural "life force," but in 1828 the German chemist Friedrich Wöhler (1800-1882) made an amazing discovery.

By heating a sample of ammonium cyanate, a material from a nonliving source, Wöhler converted it to urea, a waste product in the urine of mammals. As he later observed, "without benefit of a kidney, a bladder, or a dog," he had turned an inorganic substance into an organic one. It was almost as though he had created life. Actually, what he had discovered was the distinction between organic and inorganic material, which results from the way in which the carbon chains are arranged.

This explanation of the difference between organic and inorganic is pivotal to understanding how the beginnings of life first formed on Earth. It is an almost inconceivably large step from the nonliving to the living but not nearly so much of a jump from the inorganic to the organic. In fact, it appears that what happened on Earth in its distant past was that organic (but not living) substances underwent chemical reactions with inorganic ones to produce the rudiments of life. The American chemist Stanley Miller (1930-) illustrated this with an experiment that involved a mixture of hydrogen, methane (CH₄), ammonia (NH₃), and water. Subjected to a discharge of electric sparks intended to simulate lightning in Earth's early atmosphere, the mixture eventually yielded amino acids, which are among the chief components of proteins.

Early Forms of Life

The course that early forms of life followed during the first 800 million years of Earth's existence was a lengthy one, and if a person could have glimpsed Earth at any interval of a few million years during this time, it might have seemed as though nothing at all was happening. In fact, however, life-forms were undergoing the most profound changes imaginable.

That span of 0.8 billion years saw a transition from elemental carbon to organic compounds and from organic compounds to organelles, which are discrete components of cells, and finally to cells themselves—the building blocks of life. The processes by which this happened were exceedingly complicated, and modern scientists have little to go on in forming their suppositions as to how these transitions came about. Among many key pieces of information missing from the picture, for instance, is the matter of how and when DNA first appeared in cells. (For more on DNA, see Cells.)

The first cells to form were known as prokaryotic cells, or cells without a nucleus. (These cells, too, are discussed in the essay Cells.) Prokaryotic cells may have been little more than sacs of DNA that were capable of self-replication—much like bacteria today, which are themselves prokaryotic. These early forms of bacteria, which dominated Earth for many millions of years, were apparently anaerobic and eventually split into three branches: archaebacteria, eubacteria, and eukaryotes. Out of the last group grew all other forms of life, including fungi, plants, and animals.

Geologic Time Marches on

By about 2.5 billion years ago, bacteria had begun to undergo a form of photosynthesis, as plants do today. As a result, oxygen started to accumulate in Earth's atmosphere, and this had two interesting implications for life on Earth. One of the results of oxygen formation was that formation of "new" cells—that is, spontaneously formed cells that did not come from already living matter—ceased altogether, because they were killed off by reactions with oxygen. Second, aerobic respiration thereafter became the dominant means for releasing energy among living organisms.

The real history of life-forms on Earth—the portion of that history about which paleontologists can learn a great deal from observation of fossils and other materials—dates from the beginning of the Cambrian period, about 550 million years ago. At this point, about 88% of Earth's history already had passed, yet the remaining 12% contains virtually all the really dramatic events in the formation of life on Earth. Later in this essay, we examine a few analogies that help put these time periods into perspective as well as the concept of geologic time itself.

Phases in Earth's History

In the course of discussing these topics, it is sometimes necessary to make reference to geologic time divisions—eons, eras, periods, and epochs. These are not units of a specific length in years, like a century or a millennium; instead, they are distinct phases in Earth's history that historical geologists (including paleontologists) have pieced together from fossils and other materials. Their names usually refer to locations where fossils relevant to that phase of geologic time were found: for example, the Jurassic period, whose name became a household word after the release of the 1993 blockbuster movie Jurassic Park, is named after the Jura Mountains of Switzerland and France.

The historical juncture mentioned in the paragraph before last—the beginning of the Cambrian period, some 550 million years ago, was also the end of the Proterozoic eon, the third of three eons in what is known as Precambrian time. The fourth and present eon is the Proterozoic, which has included three eras: Paleozoic (about 550-240 million years ago), Mesozoic (about 240-65 million years ago), and Cenozoic (about 65 million years ago to the present).

Cataclysms and Continental Drift

The divisions between these phases have not been drawn arbitrarily; rather, they are based on evidence suggesting that those points in Earth's history were marked by violent, cataclysmic events. Once life had come into the picture, these cataclysms brought about death on a vast scale, which we discuss later, in the context of mass extinctions. A number of phenomena caused mass extinctions at various points; most notable among them was the impact of meteorites. Also significant was continental drift, which, as its name implies, is the movement of the continents from distant origins.

Just as evolutionary theory informs much of our modern thinking about biology, the theory of plate tectonics holds a dominant position in geology and related earth sciences. Plate tectonics involves the movement of large segments in the crust and the upper mantle of Earth, and one of the outcomes of such movement is continental drift.

Even today, the continents we know are moving slowly away from or toward one another, but that movement is so slow that it would take millions of years for any change to be perceptible. At one time, however, the continents were distributed quite differently than they are today. For example, at the end of the Permian period and the beginning of the Triassic, when the dinosaurs began to appear on the scene, all of Earth's land-masses were joined in a single continent, Pangea, that stretched between the North and South Poles and was surrounded by a vast ocean.

Real-Life Applications

Our Place in the Grand Scheme of Things

All areas of historical geology—including paleontology—are concerned with geologic time, a term that refers to the great sweep of Earth's history. This is a timescale that dwarfs the span of human existence, and to study the history of life on Earth, a mental adjustment of monumental proportions is required. One must discard all notions used in studying the history of human civilization, including such concepts as modern, medieval, and ancient, which are essentially useless when discussing geologic time.

Human civilization has existed for about 5,500 years, the blink of an eye in geologic terms. Even the span of time since the first appearance of the genus Homo (to which humans, or Homo sapiens, belong), is minuscule: 2.5 million years compared with 4.6 billion years, or about 0.04% of the planet's history. A couple of analogies, one to a shorter span of time and another to a measurement of space, will help us understand the scope of time encompassed in the evolution of species.

Life Appears in June—so to Speak

Suppose that the entire history of Earth were likened to a single year of 365.25 days, starting with the formation of the planet from a cloud of dust and ending with the present. At what point in the year would the first aerobic life-forms appear? We are not talking about anything approaching a human or even a clam or a fern or a piece of algae—just a single-cell organism that depends on oxygen for energy.

One might guess that such life-forms appeared sometime in January, or at least by the end of winter in late March. In fact, the first aerobic single-cell life-forms arose around June 15—nearly halfway through a year. Even at about the time of Thanksgiving, the most complex organisms would be fish and a few early amphibians.

We tend to associate the dinosaurs with the early phases of Earth's history, but this only illustrates our distorted view of geologic time. In fact, the Jurassic period would be analogous to a period of about five days from December 15-20. By Christmas Day, all remaining dinosaurs would have been headed toward extinction, their dead bodies eventually forming the fossil fuels that power present-day civilization.

By this point we are within a few days of the year's end, and yet nothing remotely resembling a human has appeared. Our own genus would not come on the scene until around 8:00 P.M. on December 31. The New Year's Eve countdown would be nearing by the time human civilization began, at about 42 seconds before midnight. Christ's birth would have occurred at about 14 seconds before midnight, and the final 10-second countdown would begin about the time the Roman Empire fell. The life span of the average person would correspond to approximately half a second or less.

Another Analogy: Los Angeles to New York

To use another analogy, suppose we are driving from Los Angeles to New York City, and that this distance corresponds to Earth's history. Once we reach western Nebraska, with about 46% of the distance behind us, we would come to the beginning of the Proterozoic era and the origins of aerobic life-forms. One might wonder why "nothing" happened on all those long miles from L.A. through the deserts, mountains, and plains of the western United States, but as we have seen, a great deal happened: Earth formed from a cloud of gas, was pounded by meteors, and gradually became the home to oceans.

The end of the Proterozoic era (about 545 million years ago) would be at about 88% of the distance from Los Angeles to New York—some-where around Pittsburgh, Pennsylvania. By this point, the continental plates have formed, oxygen has entered the atmosphere, and soft-bodied organisms have appeared. We are a long way from Los Angeles, and yet almost the entire history of life on Earth, at least in terms of relatively complex organisms, lies ahead of us.

If we skip ahead by about 339 million years—a huge leap not only in time but also in biological development—we come to the point when the dinosaurs appeared. We are now 95% of the way from the beginning of Earth's history to the present, and if it is measured against the distance from Los Angeles to New York, we would be somewhere around Scranton, in eastern Pennsylvania. Another 89 mi. (142 km) from the Scranton area would put us at a point about 65 million years ago, or the time when the dinosaurs became extinct. We then would have less than 40 mi. (64 km) to drive to reach the period where genus Homo appeared, by which time we would be in the middle of Manhattan.

Compared with the distance from L.A. to New York, the span of time that Homo sapiens has existed would be much shorter than the drive from Central Park to the Empire State Building. The entire sweep of human civilization and history, from about a thousand years before the building of the pyramids to the beginning of the third millennium A.D., would be smaller than a city block. It is much smaller, in fact—close to the size of a modest storefront, or 15.54 ft. (5 m).

How Did We Get Here?

On the one hand, the history of relatively complex life-forms is relatively short compared with Earth's history; on the other hand, it is unbelievably long, when one considers the diversity of forms that have evolved in the past half-billion years. What follows is an extremely brief overview, touching on a few points in the development of life on Earth.

The discussion here is far from comprehensive, the purpose being not to provide a detailed overview of evolutionary processes but to illustrate the evolution of life-forms by a few examples. The reader is strongly encouraged to consult a textbook or other reliable information source for greater illumination of these particular topics. Along with this overview, we look at some of the more dramatic aspects of paleontologic study: dinosaurs and mass extinction.

The fossilized history of life on Earth really began in earnest only with the Cambrian period, which saw an explosion of invertebrate (without an internal skeleton) marine forms. These dominated from about 550 to about 435 million years ago. At the latter point, the boundary between the Ordovician and Silurian periods of the Paleozoic era, there occurred the first of five major mass extinctions, as the "supercontinent" of Gondwana crossed the South Pole, freezing most life-forms that were then alive. As always happened with these mass extinctions, some life-forms survived, and within a few million years life again was thriving.

Coming Out of the Water

One of the favorite subjects of modern cartoonists is the migration of creatures from the water to the land. For example, following the enactment of stiff airport security guidelines in the wake of the September 11 terrorist attacks, a New Yorker cartoon in late 2001 showed this evolution from fish to amphibian (a creature that can live both in the water and on the ground) and, ultimately, to the human being. Then, in the last evolutionary sequence, Homo sapiens is depicted in a business suit, going through an airport security scanner. Most cartoons based on this event in paleontologic history follow a common theme, which can be summed up thus: "We went through all that evolution just for this ?"

Certainly, the transition from water to solid ground was a massive step—one of almost inconceivable importance. This invasion of the land, which probably took place about 400 million years ago, perhaps started when fishes of the Devonian period (named after Devonshire in England) began breathing oxygen near the surface of shallow waters. In time they made their way onto the land, and as the number of species grew, their lobed fins developed in different ways for different groups of fish, at length becoming the many limbs and appendages specific to large groups of species, such as birds or mammals. This may seem a bit far-fetched, but even today catfish are known to pull themselves out of the water and onto land if a specific need to do so arises.

Why did the first fish leave the water? Perhaps because the water levels were receding, as they have done periodically in the course of Earth's history as a result of massive freezing of global water supplies—that is, ice ages. Whatever the reason, we can be sure of the fact that the fish were not in any way attempting to reach some "higher" state of development, nor was any force pushing them to "evolve." Therein lies one of the major misconceptions about evolution, even among many of its adherents. The very name evolution is somewhat unfortunate, because it suggests that life-forms are moving gradually toward a state of heightened development. Instead, they simply are adapting to circumstances that confront them; hence, the entire phenomenon might well be called by the much less dramatic-sounding name adaptation.

Mass Extinction

Let us now pause in our narrative to discuss examples of a phenomenon that has been mentioned several times already: mass extinction. One of the truly amazing things about the history of life on Earth is the way in which forces have seemingly conspired to wipe out virtually all living things, not just once but at least five major times, along with a number of more limited instances.

Over the course of Earth's history—even its very recent history—numerous species have become extinct, usually as a result of their inability to adapt to changes in their natural environment. In the extremely recent past (recent geologically, that is—since the end of the last ice age about 10,000 years ago), 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 organisms incapable of adaptation soon die out. This is the infamous "survival of the fittest," an aspect of evolutionary theory that many a social or political ideologue has misappropriated for the purpose of insisting that one particular ethnic or social group is naturally superior to another (see Evolution).

Why does mass extinction occur? As we noted earlier, one possible cause of mass extinction is a sudden and dramatic change in ocean levels. Other causes include volcanic eruptions or the effects of events or objects from space—the explosion of a star, perhaps, or the impact of a meteorite on Earth. Although scientists have a reasonable idea of the immediate causes of mass extinction in some cases, their understanding of the root causes still is limited. This fact was expressed by the University of Chicago paleobiologist David M. Raup, who wrote in Extinction: Bad Genes or Bad Luck? : "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."

The Great Five Mass Extinctions

The five largest known mass extinctions occurred at intervals of between 50 million and 100 million years over a span of time from about 435 million to 65 million years ago. The first of them we have mentioned as perhaps being associated with the first migrations of organisms to land. This was at the end of the Ordovician period, about 435 million years ago, when a drop in the ocean level wiped out one-fourth of all marine families. No wonder some of the fish adapted to life on land!

Changes in sea level along with climate changes also appear to have brought about the second great mass extinction, which we also have mentioned, near the end of the Devonian period, about 370 million years ago. But no biological cataclysm in Earth's history can compare to the mass extinction that attended the close of the Permian period some 240 million years ago—an event so traumatic to life on Earth that it has been dubbed the great dying.

Imagine if 96% of all people alive today were killed—a staggering and terrifying concept. This would be far more fatalities than the combined death toll of both world wars; the Black Death of 1347-1351 and the influenza epidemic of 1918-1920; the various Nazi, Stalinist, and Maoist acts of genocide; and all major earthquakes and other natural disasters in history. Now imagine that along with all those people, 96% of all other life-forms—plants and trees, birds and fishes, single-cell organisms and the like, which vastly outnumber the relatively small human population of Earth—all ceased to exist as well. Such was the almost incomprehensible scale of the so-called great dying, an event whose cause has not been determined, though it may have resulted from a volcanic eruption in Siberia.

The Age of the Dinosaurs

Now that we have mentioned only three of the five great mass extinctions, it is appropriate to discuss the creatures that died in the fourth and fifth: the dinosaurs, or "terrible lizards." Though they were the most well-known creatures of the Mesozoic era, which lasted from about 240-65 million years ago, dinosaurs were far from the only notable species on Earth in that phase of geologic history. In fact, it was during the first period of the Mesozoic era, the Triassic, that a group of creatures destined to make a greater impact on Earth—mammals—first appeared. During this time, botanical life included grasses, flowering plants, and trees of both the deciduous (leaf-shedding) and coniferous (cone-bearing) varieties.

Backtracking just a bit, at the beginning of the Devonian period, about 410 million years ago, the first vertebrates (animals with an internal skeleton) made their appearance in the form of jawless fishes. Plant life on land consisted of ferns and mosses. Then came the late-Devonian mass extinction we have mentioned, which removed many of the newly evolved invertebrates, including most fishes. By the end of the Devonian, about 360 million years ago, fish had evolved jaws, and life was thriving on land. Among these life-forms were reptiles in the animal kingdom, and gymnosperms, or plants that reproduce sexually by spreading exposed seeds—a primary example from our own time being pine trees.

Monsters of the Mesozoic

Although the existence of dinosaurs has been an established fact for as long as anyone alive can remember, we have had knowledge of them only since the mid-nineteenth century. It was then that early paleontologists began to piece together evidence from fossils, which revealed the record of a time when enormous reptiles had walked the earth. Not all of these dinosaurs were so large, however. Some were as small as chickens, while others—the sauropods, the largest terrestrial animals of all time—were the equivalent of half a dozen stories tall.

Some dinosaurs were awesomely fierce predators, while others were mild-mannered plant eaters. Dinosaurs fell into two 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 rex.

Most dinosaurs had long tails and necks and walked on four legs, though some were bipedal, meaning either that they had only two legs or that they used only their two rear legs for movement or locomotion. Again, the shape of the creature correlated to some degree with its habits: all four-legged dinosaurs were relatively gentle, slow-moving herbivores, whereas most of the bipedal ones were fast-moving predators. Their teeth also reflected their diet: not surprisingly, carnivores had much sharper cutting surfaces shaped like serrated knives, which made it easy to rip their prey into smaller pieces before swallowing the pieces whole.

As recently as the 1970s, scientists tended to believe that dinosaurs were cold-blooded creatures markedly lacking in brain power. This view is reflected in a number of popular-culture sources, such as the 1983 song by the Police, "Walking in Your Footsteps," which suggests that if humans were to destroy themselves by nuclear warfare, they would seem even more "dumb" than the dinosaurs. By contrast, the film Jurassic Park, made a decade later, reflects changes in paleontologists' attitudes toward dinosaurs, gained as a result of continued research. As it turns out, dinosaurs were far from dumb, and they may even have been warm-blooded—unlike most reptiles but like birds, to whom they may have been related.

Some Types of Dinosaur

Few creatures have ever captured the imagination of humans as much as Tyrannosaurus rex—a particularly impressive feat, given the fact that the last T. rex died tens of millions of years before humans ever came on the scene. With a name that means "absolute ruler lizard" in Greek, this terrifying creature reached a maximum length of 45 ft. (14 m) and may have weighed as much as 9 tons (8 metric tons).

T. rex must have made a truly awesome sight running through the forests of its Mesozoic world. Moving on its powerful hind legs with a gait that was at once swift and lumbering, it used its tail as a counterbalance and stabilizer. Once it had come abreast of its prey, T. rex probably attacked its victim with powerful head butts and then tore the animal apart with its massive jaws before ripping into it with some 60 dagger-shaped teeth as much as 6 in. (15 cm) long.

Of course, T. rex is notable precisely because it was so unusual among dinosaurs in size and, for the most part, power. Deinonychus, for example, was much smaller—only about 10 ft. (3 m) and 220 lb. (100 kg)—but these "running lizards" were still fearsome predators, noted for their sickle-like claws. The phrase "sickle-like claws" might call to mind another creature, this one made famous by Jurassic Park: Velociraptor, or "swift plunderer," a walking death machine that for all the terror it inspired in moviegoers (and, no doubt, in many a Mesozoic creature) attained a length of only about 6 ft. (2 m).

Among the many lessons about dinosaurs to be learned from Jurassic Park was the striking contrast between herbivores and carnivores, a fact emphasized by the scene in which the two children take refuge with a herd of sauropods. These gentle giants possessed four legs the size of large pillars and had extraordinarily long necks and tails. Most famous among the sauropods was the Apatosaurus, which achieved a length of 65 ft. (20 m) and a weight of 30 tons (27 metric tons). Apatosaurus 's appearance in the scene with the children was not the great creature's first time in the limelight of popular culture: known as Brontosaurus until the early 1990s, this dinosaur was a popular fixture of the Flintstones cartoon.

In addition to land-based dinosaurs, there were such creatures as the flying Triceratops, which may have been a link between dinosaurs and birds. (The name raptor, incidentally, usually refers not to a type of dinosaur, but to a bird of prey.) It should be noted that Triceratops and Velociraptor were creatures of the late Cretaceous period, during which about a hundred different dinosaur genera (plural of genus) flourished. This is an important point, because many books picture all manner of dinosaurs coexisting, when, in fact, various ones existed at different times over a period of about 180 million years. Also, it is worth noting that the major nonhuman "star" of Jurassic Park did not even live during the Jurassic period!

Later Mass Extinctions—and New Life

Today, of course, the dinosaurs are long gone—so distant in time, in fact, that the remains of many millions have become petroleum. This has happened in situations where specific conditions prevail: for example, the remains could not be allowed to rot, and decay had to be anaerobic and had to take place within certain types of rock. Yet oil reserves (for as long as they last) are not the only reason why the dinosaurs' disappearance was beneficial from the human standpoint. At the time when dinosaurs controlled the world, mammals were small and hid from predators such as T. rex. Without the mass extinction of the dinosaurs, this class of creatures might never have come to the forefront, and human beings might not have evolved.

Actually, two mass extinctions rocked the world of the dinosaurs. The first (fourth among the list of five major mass extinctions) happened at the end of the Triassic, the first period of the Mesozoic era. This was about 205 million years ago, when an asteroid may have hit Earth. Whatever the cause, the result was that creatures in the seas suffered major mass extinction. So, too, did those on the land, but many species of dinosaurs and mammals managed to survive the event, which marked the transition between the Triassic and Jurassic periods.

Theories Regarding the Dinosaurs' Demise

The dinosaurs continued to flourish for another 140 million years, and their descendants might still be walking the earth were it not for the last great mass extinction, which took place about 65 million years ago. What killed the dinosaurs? Paleontologists and other scientists have proposed several 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; 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.

The Asteroid Hits

It appears that the asteroid smashed into what is now the northern tip of Mexico's Yucatán peninsula. Today that area is home to a crater some 6 mi. (9.6 km) deep and 186 mi. (300 km) in diameter. Note, however, that 65 million years ago, the Yucatán was not exactly where it is now. It had begun to drift in the direction of its present location, but the map of the future continent of North America was quite different from what it is today and included large submerged areas. Among them was the region of impact, which only later rose to the surface as a result of tectonic activity.

The nearest large landmass at the time was a good distance away—equivalent to northern Louisiana. But it hardly mattered that there were no dinosaurs at the point of impact. Traveling at more than 100,000 MPH (160,000 km/h), which is almost four times as fast as the fastest spacecraft built by human beings, the asteroid probably brought about an explosion as intense as many thousands of hydrogen bombs. It may not have raised a mushroom cloud per se, but it probably produced an even more dramatic formation as it sent more than 48,800 cu. mi. (200,000 km³) of debris and gases into the atmosphere. This would be enough dust to cover the state of Mississippi to a depth of 1 mi. (1.6 km)

The sound of the impact must have been ear-shattering, even far away on what would one day become North America. Then came the tidal waves, some as tall as 394 ft. (120 m), with earthquakes soon to follow. But while these tidal waves undoubtedly caused massive localized death, or mass mortality, what really brought about the mass extinction of the dinosaurs was the aftermath of impact. All that dust in the atmosphere effectively blocked out the Sun's light, bringing about a dramatic cooling on Earth's surface. As a result, plants died, thus depriving herbivorous dinosaurs of an energy source. The herbivores died and then, in a domino effect brought about by the interdependence of components in the food web, the carnivores soon followed.

Mammals, Humans, and Mass Extinction

When that asteroid hit, it brought an end to the dinosaurs and their Mesozoic world, ushering in the Cenozoic era, which saw the rise of mammals. Some examples of the creatures that proliferated in the course of the past 65 million years are tiny prehistoric horses with four toes as well as a giant rhinoceros-like herbivore, both of which lived in the Eocene epoch about 40 million years ago.

As time went on, the range of species—not only animals and plants but also the much less complex organisms of the other three kingdoms (Protista, including protozoa and algae; Monera such as bacteria; and Fungi)—grew larger and larger. An ice age struck the planet about 1.65 million years ago, bringing about much smaller cases of mass extinction than the ones we have discussed. But other instances of mass extinction would occur at the hands of a creature that appeared about 2.5 million years ago: Homo sapiens. The ice age—which was only one of many and thus is referred to often as the "last ice age"—would prove a major turning point in the history of the human species. At the beginning, we were just embarking on the beginnings of the Stone Age, whereas at the end, just 10,000 years ago (or a few seconds ago, in geologic terms), we emerged from the cold, ready to take over the world.

Near the end of the last ice age, peoples from Siberia crossed a land bridge spanning what is today the Bering Strait between Russia and Alaska. Their descendants, of course, are the Native Americans, and native is a fitting term, since they arrived in the Americas about 12,000 years ago, or about 7,000 years before the Europeans' ancestors arrived in Europe. What they found, as they poured into the Americas, was a range of species quite different from those known today. There were mammoth and mastodons; giant bears, beaver, and bison; and even saber-toothed "tigers" (which were not directly related to modern-day tigers), camels, and lions. Prehistoric America was also home to horses, but these creatures and many others were wiped out by hunting. Horses did not reappear in the New World until Europeans brought them after A.D. 1500, when they would prove an indispensable aid in European efforts to conquer the Native Americans' lands.

Where to Learn More

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

Gould, Stephen Jay. Ever Since Darwin: Reflections in Natural History. New York: W. W. Norton, 1977.

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.

Raup, David M. Extinction: Bad Genes or Bad Luck? New York: W. W. Norton, 1991.

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

Starr, Cecie, and Ralph Taggart. Biology: The Unity and Diversity of Life. 7th ed. Belmont, CA: Wadsworth, 1995.

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/>.

The study of animal history as recorded by fossil remains. The fossil record includes a very diverse class of objects ranging from molds of microscopic bacteria in rocks more than 3 × 10⁹ years old to unaltered bones of fossil humans in ice-age gravel beds formed only a few thousand years ago. Quality of preservation ranges from the occasional occurrence of soft parts (skin and feathers, for example) to barely decipherable impressions made by shells in soft mud that later hardened to rock. See also Fossil; Micropaleontology.

The most common fossils are hard parts of various animal groups. Thus the fossil record is not an accurate account of the complete spectrum of ancient life but is biased in overrepresenting those forms with shells or skeletons. Fossilized worms are extremely rare, but it is not valid to make the supposition that worms were any less common in the geologic past than they are now. See also Ediacaran biota.

The data of paleontology consist not only of the parts of organisms but also of records of their activities: tracks, trails, and burrows. Even chemical compounds formed only by organisms can, if extracted from ancient rocks, be considered as part of the fossil record. Artifacts made by people, however, are not termed fossils, for these constitute the data of the related science of archeology, the study of human civilizations. See also Archeology; Paleobiochemistry.

Paleontology lies on the boundary between two disciplines, biology and geology. See also Biology; Geology.

Geological aspects

A major task of any historical science, such as geology, is to arrange events in a time sequence and to describe them as fully as possible.

Fossils only tell that a rock is older or younger than another; they do not give absolute age. The decay of radioactive minerals may provide an age in years, but this method is expensive and time-consuming, and cannot always be applied since most rocks lack suitable radioactive minerals. Correlation by fossils remains the standard method for comparing ages of events in different areas. See also Index fossil; Stratigraphy.

The physical appearance and climate of the Earth during a given period of the geologic past can be described from compilation and analysis of the data which is obtained through studies of the habitats of extant fauna, the geographic distribution of fossils, and the climatic preferences of ancient forms of life. See also Paleoclimatology; Paleoecology; Paleogeography.

Biological aspects

The most fundamental fact of paleontology is that organisms have changed throughout earth history and that each geological period has had its characteristic forms of life. An evolutionist has two major interests: first, to know how the process of evolution works; this is accomplished by studying the genetics and population structure of modern organisms; second, to reconstruct the events produced by this process, that is, to trace the history of life. Any modern animal group is merely a stage, frozen at one moment in time, of a dynamic, evolving lineage. Fossils give the only direct evidence of previous stages in these lineages. Horses and rhinoceroses, for example, are very different animals today, but the fossil history of both groups is traced to a single ancestral species that lived early in the Cenozoic Era. From such evidence, a tree of life can be constructed whereby the relationships among organisms can be understood. See also Animal evolution.

The study of prehistoric life and the fossilized remains of that life found in the rocks.

The study of ancient life forms, particularly as they are seen in fossils.

• Occupations and Job Titles

"Palaeontology" redirects here. For the scientific journal, see Palaeontology (journal).

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Paleontology or Palaeontology ( /ˌpælɪɒnˈtɒlədʒi/) is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments (their paleoecology). As a "historical science" it attempts to explain causes rather than conduct experiments to observe effects. Palaeontological observations have been documented as far back as the 5th century B.C.E. The science became established in the 18th century as a result of Georges Cuvier's work on comparative anatomy, and developed rapidly in the 19th century. The term itself originates from Greek: παλαιός (palaios) meaning "old, ancient," ὄν, ὀντ- (on, ont-) meaning "being, creature" and λόγος (logos) meaning "speech, thought, study."

Palaeontology lies on the border between biology and geology, and shares with archaeology a border that is difficult to define. It now uses techniques drawn from a wide range of sciences, including biochemistry, mathematics and engineering. Use of all these techniques has enabled palaeontologists to discover much of the evolutionary history of life, almost all the way back to when Earth became capable of supporting life, about 3,800 million years ago. As knowledge has increased, paleontology has developed specialized sub-divisions, some of which focus on different types of fossil organisms while others study ecology and environmental history, such as ancient climates.

Body fossils and trace fossils are the principal types of evidence about ancient life, and geochemical evidence has helped to decipher the evolution of life before there were organisms large enough to leave fossils. Estimating the dates of these remains is essential but difficult: sometimes adjacent rock layers allow radiometric dating, which provides absolute dates that are accurate to within 0.5%, but more often palaeontologists have to rely on relative dating by solving the "jigsaw puzzles" of biostratigraphy. Classifying ancient organisms is also difficult, as many do not fit well into the Linnean taxonomy that is commonly used for classifying living organisms, and palaeontologists more often use cladistics to draw up evolutionary "family trees". The final quarter of the 20th century saw the development of molecular phylogenetics, which investigates how closely organisms are related by measuring how similar the DNA is in their genomes. Molecular phylogenetics has also been used to estimate the dates when species diverged, but there is controversy about the reliability of the molecular clock on which such estimates depend.

Contents 1 Definition 1.1 A historical science 1.2 Related sciences 1.3 Subdivisions 2 Sources of evidence 2.1 Body fossils 2.2 Trace fossils 2.3 Geochemical observations 3 Classifying ancient organisms 4 Estimating the dates of organisms 5 Overview of the history of life 6 Mass extinctions 7 History of palaeontology 8 See also 9 Notes 10 References 11 External links

Definition

A paleontologist carefully chips rock from a column of dinosaur vertebrae.

A palaeontologist at work at John Day Fossil Beds National Monument

The preparation of the fossilized bones of Europasaurus holgeri

The simplest definition is "the study of ancient life".^[1] Paleontology seeks information about several aspects of past organisms: "their identity and origin, their environment and evolution, and what they can tell us about the Earth's organic and inorganic past".^[2]

A historical science

Palaeontology is one of the historical sciences, along with archaeology, geology, biology, astronomy, cosmology, philology and history itself.^[3] This means that it aims to describe phenomena of the past and reconstruct their causes.^[4] Hence it has three main elements: description of the phenomena; developing a general theory about the causes of various types of change; and applying those theories to specific facts.^[3]

When trying to explain past phenomena, palaeontologists and other historical scientists often construct a set of hypotheses about the causes and then look for a smoking gun, a piece of evidence that indicates that one hypotheses is a better explanation than others. Sometimes the smoking gun is discovered by a fortunate accident during other research. For example, the discovery by Luis Alvarez and Walter Alvarez of an iridium-rich layer at the Cretaceous–Tertiary boundary made asteroid impact and volcanism the most favored explanations for the Cretaceous–Paleogene extinction event.^[4]

The other main type of science is experimental science, which is often said to work by conducting experiments to disprove hypotheses about the workings and causes of natural phenomena – note that this approach cannot prove a hypothesis is correct, since some later experiment may disprove it. However, when confronted with totally unexpected phenomena, such as the first evidence for invisible radiation, experimental scientists often use the same approach as historical scientists: construct a set of hypotheses about the causes and then look for a "smoking gun".^[4]

Related sciences

Palaeontology lies on the boundary between biology and geology since palaeontology focuses on the record of past life but its main source of evidence is fossils, which are found in rocks.^[5] For historical reasons palaeontology is part of the geology departments of many universities, because in the 19th century and early 20th century geology departments found palaeontological evidence important for estimating the ages of rocks while biology departments showed little interest.^[6]

Palaeontology also has some overlap with archaeology, which primarily works with objects made by humans and with human remains, while palaeontologists are interested in the characteristics and evolution of humans as organisms. When dealing with evidence about humans, archaeologists and palaeontologists may work together – for example palaeontologists might identify animal or plant fossils around an archaeological site, to discover what the people who lived there ate; or they might analyze the climate at the time when the site was inhabited by humans.^[7]

Analyses using engineering techniques show that Tyrannosaurus had a devastating bite, but raise doubts about how fast it could move.

In addition palaeontology often uses techniques derived from other sciences, including biology, ecology, chemistry, physics and mathematics.^[1] For example geochemical signatures from rocks may help to discover when life first arose on Earth,^[8] and analyses of carbon isotope ratios may help to identify climate changes and even to explain major transitions such as the Permian–Triassic extinction event.^[9] A relatively recent discipline, molecular phylogenetics, often helps by using comparisons of different modern organisms' DNA and RNA to re-construct evolutionary "family trees"; it has also been used to estimate the dates of important evolutionary developments, although this approach is controversial because of doubts about the reliability of the "molecular clock".^[10] Techniques developed in engineering have been used to analyse how ancient organisms might have worked, for example how fast Tyrannosaurus could move and how powerful its bite was.^[11][12]

A combination of palaeontology, biology, and archaeology, paleoneurology is the study of endocranial casts (or endocasts) of species related to humans to learn about the evolution of human brains. ^[13]

Palaeontology even contributes to astrobiology, the investigation of possible life on other planets, by developing models of how life may have arisen and by providing techniques for detecting evidence of life.^[14]

Subdivisions

As knowledge has increased, Palaeontology has developed specialised subdivisons.^[15] Vertebrate paleontology concentrates on fossils of vertebrates, from the earliest fish to the immediate ancestors of modern mammals. Invertebrate paleontology deals with fossils of invertebrates such as molluscs, arthropods, annelid worms and echinoderms. Paleobotany focuses on the study of fossil plants, but traditionally includes the study of fossil algae and fungi. Palynology, the study of pollen and spores produced by land plants and protists, straddles the border between paleontology and botany, as it deals with both living and fossil organisms. Micropaleontology deals with all microscopic fossil organisms, regardless of the group to which they belong.^[16]

In the Carboniferous period, the continents were not in the same places as they are today, and there was extensive glaciation.

Instead of focusing on individual organisms, paleoecology examines the interactions between different organisms, such as their places in food chains, and the two-way interaction between organisms and their environment^[17] – for example the development of oxygenic photosynthesis by bacteria hugely increased the productivity and diversity of ecosystems,^[18] and also caused the oxygenation of the atmosphere, which in turn was a prerequisite for the evolution of the most complex eucaryotic cells, from which all multicellular organisms are built.^[19] Paleoclimatology, although sometimes treated as part of paleoecology,^[16] focuses more on the history of Earth's climate and the mechanisms that have changed it^[20] – which have sometimes included evolutionary developments, for example the rapid expansion of land plants in the Devonian period removed more carbon dioxide from the atmosphere, reducing the greenhouse effect and thus helping to cause an ice age in the Carboniferous period.^[21]

Biostratigraphy, the use of fossils to work out the chronological order in which rocks were formed, is useful to both paleontologists and geologists.^[22] Biogeography studies the spatial distribution of organisms, and is also linked to geology, which explains how Earth's geography has changed over time.^[23]

Sources of evidence

Body fossils

Main article: Fossils

This Marrella specimen illustrates how clear and detailed the fossils from the Burgess Shale lagerstätte are.

Fossils of organisms' bodies are usually the most informative type of evidence. The most common types are wood, bones, and shells.^[24] Fossilisation is a rare event, and most fossils are destroyed by erosion or metamorphism before they can be observed. Hence the fossil record is very incomplete, increasingly so further back in time. Despite this, it is often adequate to illustrate the broader patterns of life's history.^[25] There are also biases in the fossil record: different environments are more favorable to the preservation of different types of organism or parts of organisms.^[26] Further, only the parts of organisms that were already mineralised are usually preserved, such as the shells of molluscs. Since most animal species are soft-bodied, they decay before they can become fossilised. As a result, although there are 30-plus phyla of living animals, two-thirds have never been found as fossils.^[27]

Occasionally, unusual environments may preserve soft tissues. These lagerstätten allow palaeontologists to examine the internal anatomy of animals that in other sediments are represented only by shells, spines, claws, etc. – if they are preserved at all. However, even lagerstätten present an incomplete picture of life at the time. The majority of organisms living at the time are probably not represented because lagerstätten are restricted to a narrow range of environments, e.g. where soft-bodied organisms can be preserved very quickly by events such as mudslides; and the exceptional events that cause quick burial make it difficult to study the normal environments of the animals.^[28] The sparseness of the fossil record means that organisms are expected to exist long before and after they are found in the fossil record – this is known as the Signor-Lipps effect.^[29]

Trace fossils

Cambrian trace fossils including Rusophycus, made by a trilobite

Main article: Trace fossil

Trace fossils consist mainly of tracks and burrows, but also include coprolites (fossil feces) and marks left by feeding.^[24][30] Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily-fossilized hard parts, and they reflect organisms' behaviours. Also many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them.^[31] Whilst exact assignment of trace fossils to their makers is generally impossible, traces may for example provide the earliest physical evidence of the appearance of moderately complex animals (comparable to earthworms).^[30]

Geochemical observations

Main article: Geochemistry

Geochemical observations may help to deduce the global level of biological activity, or the affinity of a certain fossil. For example geochemical features of rocks may reveal when life first arose on Earth,^[8] and may provide evidence of the presence of eucaryotic cells, the type from which all multicellular organisms are built.^[32] Analyses of carbon isotope ratios may help to explain major transitions such as the Permian–Triassic extinction event.^[9]

Classifying ancient organisms

Main articles: Biological classification , Cladistics , Phylogenetic nomenclature , and Evolutionary taxonomy

Tetrapods	Amphibians Amniotes Synapsids Extinct Synapsids     Mammals Reptiles Extinct reptiles Lizards and snakes Archosaurs  ?  Extinct Archosaurs Crocodilians Dinosaurs  ?  Extinct Dinosaurs  ?  Birds

Simple example cladogram
    Warm-bloodedness evolved somewhere in the
synapsid–mammal transition.
 ?  Warm-bloodedness must also have evolved at one of
these points – an example of convergent evolution.^[33]

Levels in the Linnean taxonomy

Naming groups of organisms in a way that is clear and widely agreed is important, as some disputes in palaeontology have been based just on misunderstandings over names.^[34] Linnean taxonomy is commonly used for classifying living organisms, but runs into difficulties when dealing with newly-discovered organisms that are significantly different from known ones. For example: it is hard to decide at what level to place a new higher-level grouping, e.g. genus or family or order; this is important since the Linnean rules for naming groups are tied to their levels, and hence if a group is moved to a different level it must be renamed.^[35]

Palaeontologists generally use approaches based on cladistics, a technique for working out the evolutionary "family tree" of a set of organisms.^[34] It works by the logic that, if groups B and C have more similarities to each other than either has to group A, then B and C are more closely related to each other than either is to A. Characters that are compared may be anatomical, such as the presence of a notochord, or molecular, by comparing sequences of DNA or proteins. The result of a successful analysis is a hierarchy of clades – groups that share a common ancestor. Ideally the "family tree" has only two branches leading from each node ("junction"), but sometimes there is too little information to achieve this and palaeontologists have to make do with junctions that have several branches. The cladistic technique is sometimes fallible, as some features, such as wings or camera eyes, evolved more than once, convergently – this must be taken into account in analyses.^[33]

Evolutionary developmental biology, commonly abbreviated to "Evo Devo", also helps paleontologists to produce "family trees". For example the embryological development of some modern brachiopods suggests that brachiopods may be descendants of the halkieriids, which became extinct in the Cambrian period.^[36]

Estimating the dates of organisms

Main article: Geochronology

Cenozoic

Mesozoic

Paleozoic

Proterozoic

Quater-
nary

Tertiary

Creta-
ceous

Jurassic

Triassic

Permian

Missis-
sippian

Pennsyl-
vanian

Devo-
nian

Silurian

Ordo-
vician

Camb-
rian

Pecten gibbus

Calyptraphorus
velatus

Scaphites
hippocrepis

Perisphinctes
tiziani

Trophites
subbullatus

Leptodus
americanus

Cactocrinus
multibrachiatus

Dictyoclostus
americanus

Mucrospinifer
mucronatus

Cystiphyllum
niagarense

Bathyurus extans

Paradoxides pinus

Neptunea tabulata

Venericardia
planicosta

Inoceramus
labiatus

Nerinea trinodosa

Monotis
subcircularis

Parafusilina
bosei

Lophophyllidium
proliferum

Prolecanites gurleyi

Palmatolepus
unicornis

Hexamocaras hertzeri

Tetragraptus fructicosus

Billingsella corrugata

Common index fossils used to date rocks in North-East USA

Paleontology seeks to map out how living things have changed through time. A substantial hurdle to this aim is the difficulty of working out how old fossils are. Beds that preserve fossils typically lack the radioactive elements needed for radiometric dating. This technique is our only means of giving rocks greater than about 50 million years old an absolute age, and can be accurate to within 0.5% or better.^[37] Although radiometric dating requires very careful laboratory work, its basic principle is simple: the rates at which various radioactive elements decay are known, and so the ratio of the radioactive element to the element into which it decays shows how long ago the radioactive element was incorporated into the rock. Radioactive elements are common only in rocks with a volcanic origin, and so the only fossil-bearing rocks that can be dated radiometrically are a few volcanic ash layers.^[37]

Consequently, palaeontologists must usually rely on stratigraphy to date fossils. Stratigraphy is the science of deciphering the "layer-cake" that is the sedimentary record, and has been compared to a jigsaw puzzle.^[38] Rocks normally form relatively horizontal layers, with each layer younger than the one underneath it. If a fossil is found between two layers whose ages are known, the fossil's age must lie between the two known ages.^[39] Because rock sequences are not continuous, but may be broken up by faults or periods of erosion, it is very difficult to match up rock beds that are not directly next to one another. However, fossils of species that survived for a relatively short time can be used to link up isolated rocks: this technique is called biostratigraphy. For instance, the conodont Eoplacognathus pseudoplanus has a short range in the Middle Ordovician period.^[40] If rocks of unknown age are found to have traces of E. pseudoplanus, they must have a mid-Ordovician age. Such index fossils must be distinctive, be globally distributed and have a short time range to be useful. However, misleading results are produced if the index fossils turn out to have longer fossil ranges than first thought.^[41] Stratigraphy and biostratigraphy can in general provide only relative dating (A was before B), which is often sufficient for studying evolution. However, this is difficult for some time periods, because of the problems involved in matching up rocks of the same age across different continents.^[42]

Family-tree relationships may also help to narrow down the date when lineages first appeared. For instance, if fossils of B or C date to X million years ago and the calculated "family tree" says A was an ancestor of B and C, then A must have evolved more than X million years ago.

It is also possible to estimate how long ago two living clades diverged – i.e. approximately how long ago their last common ancestor must have lived  – by assuming that DNA mutations accumulate at a constant rate. These "molecular clocks", however, are fallible, and provide only a very approximate timing: for example, they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved,^[43] and estimates produced by different techniques may vary by a factor of two.^[10]

Overview of the history of life

Main article: Evolutionary history of life

Further information: Timeline of evolutionary history of life

The evolutionary history of life stretches back to over 3,000 million years ago, possibly as far as 3,800 million years ago. Earth formed about 4,570 million years ago and, after a collision that formed the Moon about 40 million years later, may have cooled quickly enough to have oceans and an atmosphere about 4,440 million years ago.^[44] However there is evidence on the Moon of a Late Heavy Bombardment from 4,000 to 3,800 million years ago. If, as seem likely, such a bombardment struck Earth at the same time, the first atmosphere and oceans may have been stripped away.^[45] The oldest clear evidence of life on Earth dates to 3,000 million years ago, although there have been reports, often disputed, of fossil bacteria from 3,400 million years ago and of geochemical evidence for the presence of life 3,800 million years ago.^[8][46] Some scientists have proposed that life on Earth was "seeded" from elsewhere,^[47] but most research concentrates on various explanations of how life could have arisen independently on Earth.^[48]

This wrinkled "elephant skin" texture is a trace fossil of a non-stromatolite microbial mat.
The image shows the location, in the Burgsvik beds of Sweden, where the texture was first identified as evidence of a microbial mat.^[49]

For about 2,000 million years microbial mats, multi-layered colonies of different types of bacteria, were the dominant life on Earth.^[50] The evolution of oxygenic photosynthesis enabled them to play the major role in the oxygenation of the atmosphere^[51] from about 2,400 million years ago. This change in the atmosphere increased their effectiveness as nurseries of evolution.^[52] While eukaryotes, cells with complex internal structures, may have been present earlier, their evolution speeded up when they acquired the ability to transform oxygen from a poison to a powerful source of energy in their metabolism. This innovation may have come from primitive eukaryotes capturing oxygen-powered bacteria as endosymbionts and transforming them into organelles called mitochondria.^[53] The earliest evidence of complex eukaryotes with organelles such as mitochondria, dates from 1,850 million years ago.^[19]

Multicellular life is composed only of eukaryotic cells, and the earliest evidence for it is the Francevillian Group Fossils from 2,100 million years ago,^[54] although specialization of cells for different functions first appears between 1,430 million years ago (a possible fungus) and 1,200 million years ago (a probable red alga). Sexual reproduction may be a prerequisite for specialization of cells, as an asexual multicellular organism might be at risk of being taken over by rogue cells that retain the ability to reproduce.^[55][56]

Opabinia made the largest single contribution to modern interest in the Cambrian explosion.

The earliest known animals are cnidarians from about 580 million years ago, but these are so modern-looking that the earliest animals must have appeared before then.^[57] Early fossils of animals are rare because they did not develop mineralized hard parts that fossilize easily until about 548 million years ago.^[58] The earliest modern-looking bilaterian animals appear in the Early Cambrian, along with several "weird wonders" that bear little obvious resemblance to any modern animals. There is a long-running debate about whether this Cambrian explosion was truly a very rapid period of evolutionary experimentation; alternative views are that modern-looking animals began evolving earlier but fossils of their precursors have not yet been found, or that the "weird wonders" are evolutionary "aunts" and "cousins" of modern groups.^[59] Vertebrates remained an obscure group until the first fish with jaws appeared in the Late Ordovician.^[60][61]

The spread of life from water to land required organisms to solve several problems, including protection against drying out and supporting themselves against gravity.^[62][63] The earliest evidence of land plants and land invertebrates date back to about 476 million years ago and 490 million years ago respectively.^[63][64] The lineage that produced land vertebrates evolved later but very rapidly between 370 million years ago and 360 million years ago;^[65] recent discoveries have overturned earlier ideas about the history and driving forces behind their evolution.^[66] Land plants were so successful that they caused an ecological crisis in the Late Devonian, until the evolution and spread of fungi that could digest dead wood.^[21]

At about 13 centimetres (5.1 in) the Early Cretaceous Yanoconodon was longer than the average mammal of the time.^[67]

Birds are the last surviving dinosaurs.^[68]

During the Permian period synapsids, including the ancestors of mammals, may have dominated land environments,^[69] but the Permian–Triassic extinction event 251 million years ago came very close to wiping out complex life.^[70] During the slow recovery from this catastrophe a previously obscure group, archosaurs, became the most abundant and diverse terrestrial vertebrates. One archosaur group, the dinosaurs, were the dominant land vertebrates for the rest of the Mesozoic,^[71] and birds evolved from one group of dinosaurs.^[68] During this time mammals' ancestors survived only as small, mainly nocturnal insectivores, but this apparent set-back may have accelerated the development of mammalian traits such as endothermy and hair.^[72] After the Cretaceous–Paleogene extinction event 65 million years ago killed off the non-avian dinosaurs – birds are the only surviving dinosaurs – mammals increased rapidly in size and diversity, and some took to the air and the sea.^[73][74][75]

A modern social insect collects pollen from a modern flowering plant.

Fossil evidence indicates that flowering plants appeared and rapidly diversified in the Early Cretaceous, between 130 million years ago and 90 million years ago.^[76] Their rapid rise to dominance of terrestrial ecosystems is thought to have been propelled by coevolution with pollinating insects.^[77] Social insects appeared around the same time and, although they account for only small parts of the insect "family tree", now form over 50% of the total mass of all insects.^[78]

Humans evolved from a lineage of upright-walking apes whose earliest fossils date from over 6 million years ago.^[79] Although early members of this lineage had chimp-sized brains, about 25% as big as modern humans', there are signs of a steady increase in brain size after about 3 million years ago.^[80] There is a long-running debate about whether modern humans are descendants of a single small population in Africa, which then migrated all over the world less than 200,000 years ago and replaced previous hominine species, or arose worldwide at the same time as a result of interbreeding.^[81]

Mass extinctions

K–Pg

Tr–J

P–Tr

Late D

O–S

Millions of years ago

Apparent extinction intensity, i.e. the fraction of genera going extinct at any given time, as reconstructed from the fossil record (graph not meant to include recent epoch of Holocene extinction event)

Main article: Mass extinction

Life on earth has suffered occasional mass extinctions at least since 542 million years ago. Although they are disasters at the time, mass extinctions have sometimes accelerated the evolution of life on earth. When dominance of particular ecological niches passes from one group of organisms to another, it is rarely because the new dominant group is "superior" to the old and usually because an extinction event eliminates the old dominant group and makes way for the new one.^[82][83]

The fossil record appears to show that the rate of extinction is slowing down, with both the gaps between mass extinctions becoming longer and the average and background rates of extinction decreasing. However, it is not certain whether the actual rate of extinction has altered, since both of these observations could be explained in several ways:^[84]

• The oceans may have become more hospitable to life over the last 500 million years and less vulnerable to mass extinctions: dissolved oxygen became more widespread and penetrated to greater depths; the development of life on land reduced the run-off of nutrients and hence the risk of eutrophication and anoxic events; marine ecosystems became more diversified so that food chains were less likely to be disrupted.^[85][86]

• Reasonably complete fossils are very rare, most extinct organisms are represented only by partial fossils, and complete fossils are rarest in the oldest rocks. So palaeontologists have mistakenly assigned parts of the same organism to different genera, which were often defined solely to accommodate these finds – the story of Anomalocaris is an example of this.^[87] The risk of this mistake is higher for older fossils because these are often unlike parts of any living organism. Many "superfluous" genera are represented by fragments that are not found again, and these "superfluous" genera appear to become extinct very quickly.^[84]

All genera

"Well-defined" genera

Trend line

"Big Five" mass extinctions

Other mass extinctions

Million years ago

Thousands of genera

Phanerozoic biodiversity as shown by the fossil record

Biodiversity in the fossil record, which is

"the number of distinct genera alive at any given time; that is, those whose first occurrence predates and whose last occurrence postdates that time"^[88]

shows a different trend: a fairly swift rise from 542 to 400 million years ago, a slight decline from 400 to 200 million years ago, in which the devastating Permian–Triassic extinction event is an important factor, and a swift rise from 200 million years ago to the present.^[88]

History of palaeontology

Main article: History of paleontology

Further information: Timeline of paleontology

This illustration of an Indian elephant jaw and a mammoth jaw (top) is from Cuvier's 1796 paper on living and fossil elephants.

Although palaeontology became established around 1800, earlier thinkers had noticed aspects of the fossil record. The ancient Greek philosopher Xenophanes (570–480 BC) concluded from fossil sea shells that some areas of land were once under water.^[89] During the Middle Ages the Persian naturalist Ibn Sina, known as Avicenna in Europe, discussed fossils and proposed a theory of petrifying fluids on which Albert of Saxony elaborated in the 14th century.^[90] The Chinese naturalist Shen Kuo (1031–1095) proposed a theory of climate change based on the presence of petrified bamboo in regions that in his time were too dry for bamboo.^[91]

In early modern Europe, the systematic study of fossils emerged as an integral part of the changes in natural philosophy that occurred during the Age of Reason. At the end of the 18th century Georges Cuvier's work established comparative anatomy as a scientific discipline and, by proving that some fossil animals resembled no living ones, demonstrated that animals could become extinct, leading to the emergence of paleontology.^[92] The expanding knowledge of the fossil record also played an increasing role in the development of geology, particularly stratigraphy.^[93]

The first half of the 19th century saw geological and palaeontological activity become increasingly well organized with the growth of geologic societies and museums^[94][95] and an increasing number of professional geologists and fossil specialists. Interest increased for reasons that were not purely scientific, as geology and palaeontology helped industrialists to find and exploit natural resources such as coal.^[96]

This contributed to a rapid increase in knowledge about the history of life on Earth and to progress in the definition of the geologic time scale, largely based on fossil evidence. In 1822 Henri Marie Ducrotay de Blanville, editor of Journal de Phisique, coined the word "palaeontology" to refer to the study of ancient living organisms through fossils.^[97] As knowledge of life's history continued to improve, it became increasingly obvious that there had been some kind of successive order to the development of life. This encouraged early evolutionary theories on the transmutation of species.^[98] After Charles Darwin published Origin of Species in 1859, much of the focus of paleontology shifted to understanding evolutionary paths, including human evolution, and evolutionary theory.^[98]

Haikouichthys, from about 518 million years ago in China, may be the earliest known fish.^[99]

The last half of the 19th century saw a tremendous expansion in palaeontological activity, especially in North America.^[100] The trend continued in the 20th century with additional regions of the Earth being opened to systematic fossil collection. Fossils found in China near the end of the 20th century have been particularly important as they have provided new information about the earliest evolution of animals, early fish, dinosaurs and the evolution of birds.^[101] The last few decades of the 20th century saw a renewed interest in mass extinctions and their role in the evolution of life on Earth.^[102] There was also a renewed interest in the Cambrian explosion that apparently saw the development of the body plans of most animal phyla. The discovery of fossils of the Ediacaran biota and developments in paleobiology extended knowledge about the history of life back far before the Cambrian.^[59]

Increasing awareness of Gregor Mendel's pioneering work in genetics led first to the development of population genetics and then in the mid-20th century to the modern evolutionary synthesis, which explains evolution as the outcome of events such as mutations and horizontal gene transfer, which provide genetic variation, with genetic drift and natural selection driving changes in this variation over time.^[103] Within the next few years the role and operation of DNA in genetic inheritance were discovered, leading to what is now known as the "Central Dogma" of molecular biology.^[104] In the 1960s molecular phylogenetics, the investigation of evolutionary "family trees" by techniques derived from biochemistry, began to make an impact, particularly when it was proposed that the human lineage had diverged from apes much more recently than was generally thought at the time.^[105] Although this early study compared proteins from apes and humans, most molecular phylogenetics research is now based on comparisons of RNA and DNA.^[106]

See also

Paleontology portal

• Archaeobiology
• Dinosaurs
• Fossil collecting
• Geology
• List of fossil sites (with link directory)
• List of notable fossils
• List of transitional fossils
• Radiometric dating
• Taxonomy of commonly fossilised invertebrates
• Treatise on Invertebrate Paleontology

Notes

References

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• Smithsonian's Paleobiology website: a good introduction
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Dansk (Danish)
n. - studiet af fossiler

Français (French)
n. - paléontologie

Deutsch (German)
n. - Paläontologie

Ελληνική (Greek)
n. - παλαιοντολογία

Italiano (Italian)
paleontologia

Português (Portuguese)
n. - paleontologia (f)

Русский (Russian)
палеонтология

Español (Spanish)
n. - paleontología

Svenska (Swedish)
n. - paleontologi

中文（简体）(Chinese (Simplified))
古生物学

中文（繁體）(Chinese (Traditional))
n. - 古生物學

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n. - 고생물학, 화석학

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n. - ‮תורת המאובנים‬

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