Paleontology

Paleontology is the branch of science that deals with the fossilized remains of life. Fossils may be the body parts or direct traces of activities left behind by animals, plants, fungi, and microorganisms. Thus they might include a mammalian tooth, a clam shell, a leaf, or the entire skeleton of a dinosaur, as well as dinosaur footprints, feces (coprolites), or clam burrows.

Neanderthals knew about fossils; they used, for example, the beadlike plates of the stems of ancient sea lilies to make necklaces and for exchange in trade. The ancient Greek historian Herodotus correctly deduced that the fossil shells he found in Egypt must be the remains of organisms that had once lived in a sea where now only desert stands. Many years later, the great Leonardo da Vinci saw sharks' teeth high in the mountains of Italy—and came to the same conclusion. But, for the most part, people had no idea of the true nature of fossils; one sage of the Middle Ages pronounced them to be "thunderbolts" frozen into the ground. And when the ideas of organic evolution came along, with the added possibility that fossils might actually be what they seem to be—the remains of ancient creatures, most of which no longer inhabit the earth—those who resisted the idea of evolution tended to see fossils as tricks of the devil, placed in rocks to deceive the minds of human beings. Another theory simply saw them as the remains of creatures that were excluded from Noah's Ark—and so perished in the Great Flood recounted in the Bible; indeed, some creationists still take that position.

But fossils are real, and truly are the remains of long-dead organisms. Some body parts— such as the woody tissues of plants, vertebrate teeth, or the calcareous shells of many marine invertebrates—are hard enough in life that they may be preserved for millions of years with little or no chemical alteration. Most fossils, however, are "permineralized": ground water bearing silica and other chemical compounds often perfuses the natural cavities and cracks in shells, bones, and teeth, hardening them considerably. For example, dinosaur bones from the Jurassic Morrison Formation in the western United States often have the natural cavities of the "spongy" part of their bones filled with carnotite and other uranium min

Iron Pyrite Exhibition
A curator from the Division of Paleontology, United States National Museum, Smithsonian Institution, preparing the skeleton of a dinosaur for exhibition in 1921 (From the collections of the Library of Congress)

erals—making them discoverable with Geiger counters! Silicon dioxide (quartz, in the form of chalcedony or other "crypto-crystalline" minerals) often fills the empty cells of fossilized tree trunks, as in the famous Triassic specimens that still litter the ground in Petrified Forest National Monument in Arizona.

In still other fossils, silica and minerals such as iron pyrite (iron sulphide, commonly known as "fool's gold") may replace the bone, shell, or wood completely. Sometimes this chemical replacement happens molecule-by-molecule, but in other instances, the shell or bone is dissolved, leaving a cavity in the sediments that is later filled by the replacement mineral—forming a natural "cast." Most often, however, when remains are dis solved to leave a "mold," the fossils can still be studied because the impressions of the inner and outer portions of, for example, a clam shell are often preserved in exquisite detail. Paleontologists can then pour liquid rubber or other compounds into the mold, producing an artificial cast that is an exact replica of the animal.

Fossils are most commonly found in sedimentary rocks, which are most often the hardened ("lithified") deposits of sand ("sandstone"), clay ("shale"), or particles of lime ("limestone")—or mixtures of two or all three. Metamorphic rocks—which are formed from other rocks by intense pressures and heat, may also reveal traces of fossils, if the metamorphic rock had been formed from a fossiliferous sed imentary rock; but fossils in metamorphic rocks are comparatively rare and most often highly distorted. Igneous rocks, which are formed from a liquid melt (such as lava), almost never have fossils, though there are rare exceptions: a cavity in a lava flow in Washington state preserves the outline of a wooly rhinoceros that had become trapped and died during the eruption.

Fossils usually form as dead organisms become buried by layers of sediment. Most dead organisms, of course, are completely consumed by bacteria and fungi in the normal decay process, which still goes on even after burial in muds or sands. But the harder tissues, such as wood, shells, teeth, and bones, are the last to decay, and they often escape full decomposition until they are buried so deeply that decay stops. Sometimes, though, the decay process stops before it has really begun, especially when organisms are buried suddenly in environments with little or no free oxygen (anaerobic environments); truly remarkable preservation of soft tissues such as skin and internal organs in such rare instances sheds amazing insight into long-dead worlds—such as the famous Burgess Shale deposits of the Middle Cambrian of British Columbia, where soft-bodied worms and arthropods are fossilized in intricate detail alongside the trilobites and other marine invertebrates that normally are the only kinds of fossils to be found in rocks of that age.

If at least some parts of the dead organism survive decay, then the potential fossil must be spared further destruction by chemicals and pressures in the earth's crust. To be discovered, the fossil must be uplifted as part of a rock mass exposed to the air (except for the case of deep-sea marine microfossils, which are discovered through drilling on ocean floors). As soon as rocks are exposed, they begin to erode, and many fossils that have survived millions of years are destroyed as they weather and crumble apart at the foot of some isolated cliff. With luck, though, they will be discovered by paleontologists (or knowledgeable amateurs), brought back to homes or laboratories in universities and museums, studied, and eventually named and described in the scientific literature. The study of all the processes from death to complete fossilization forms a branch of paleontology known as taphonomy.

Historically, people have studied fossils for two separate though related reasons: they have been interested, of course, in the history of life. But fossils also help geologists to "tell time": geologists discovered nearly 200 years ago that fossils occur in a definite sequence through the rock record (a sequence now known to have been produced by the evolutionary process). The same fossils found in different places are roughly the same age—a discovery that allowed geologists to unravel the pages of earth history and to divide up geological time and produce the "geological time scale." This geological side of paleontology has proven indispensable in the search for oil reserves, as oil occurs in traps in sedimentary rocks that must be studied in detail—in terms of age, as well as understanding ancient environments crucial to oil formation. The study of ancient environments through fossils is known as paleoecology.

But paleontology is perhaps best known as our only direct means of understanding the history of life. And though the fossil record is incomplete—as the remains of soft-bodied organisms only rarely are fossilized—the record is good enough for many groups not only to give us an outline of the sequential events of life's evolutionary history but also to help us understand how the evolutionary process works. Perhaps most important, the fossil record is our only means of knowing about organisms that have become extinct—thus filling in huge gaps of our knowledge of life's evolutionary diversity.

Within the past thirty years, there has been a veritable explosion of knowledge about the fossil record of the most ancient forms of life on earth. We now know that the oldest fossils are of bacteria—simple rod shapes discovered in sediments in Australia that are approximately 3.5 billion years old. The oldest eukary-otic cells (micro-organisms with complex cell structures, including a nucleus housing most of the cell's DNA) are now known as far back as some 2.2 billion years.

But it is the diversity of complex, multi-cellular, macroscopic animal and plant life, beginning some 540 million years ago, that forms the bulk of the known fossil record, and houses some truly remarkable, extinct kinds of organisms that have been very well studied by paleontologists over the past two centuries. Two examples of important, entirely extinct groups of animals are given below.

Trilobites, for example, are among the most primitive and oldest known arthropods. They first appeared in the Lower Cambrian Period— and lasted until the very end of the Paleozoic Era, nearly 300 million years later. The term trilobite refers to the three-lobe cross-sectional profile of these animals: a central, prominent axis seen on the middle region (the glabella) of the head (the cephalon), and running down the body's midsection (the thorax) and into the tail piece (the pygidium). The three front-to-back divisions of a trilobite's body (that is, cephalon, thorax, and pygidium) are unlike the anatomy of any modern arthropod; the head housed a pair of eyes. Although usually not preserved, there was a pair of antennae protruding forward from under the cephalon, as well as three pairs of legs that served the multi-purposes of walking, eating, and breathing (through an upper gill branch). The central region of the body consisted of from two to more than forty separate segments, each with a pair of legs underneath; these thoracic segments were attached to one another along a joint system, allowing trilobites to roll up into a ball.

Trilobites are among the most common fossils in the remains of Cambrian oceans. After a major extinction episode wiped out many of the Cambrian families, trilobites redi-versified and were major elements of Middle Paleozoic marine communities. The phacopid trilobites were prominent among them, possessing large eyes that are typically so well preserved that one paleontologist was able to take photographs through them. Study of the evolution, over an interval of approximately 7 million years, of the eye in one species, Pha-cops rana of the Middle Devonian of North America, led to development of the evolutionary theory of "punctuated equilibria."

Ammonoids if anything had an even longer history than trilobites. Ammonoids are coiled, shelled molluscan relatives of modern-day squid and octopi—and of the similarly coiled pearly nautilus of the southeastern Pacific Ocean. Like these other cephalopods, ammonoids swam by means of a form of jet propulsion, whereby water is taken into a body chamber (the mantle cavity) and then forcefully expelled through a nozzle (the siphon), propelling the animal in the opposite direction. With their fast speeds, cephalopods are very efficient predators.

Like the nautilus, ammonoids have partitions (septa) that are formed behind the body; as the animal grows, the body moves forward and a new partition is formed. The line where the partition meets the outer shell is called the suture—and it is the evolution of this suture pattern that is most distinctive about ammonoid history.

Evolving from nautilid ancestors in the Devonian Period, ammonoids diversified into so many species that geologists have long used them to correlate rocks and subdivide geological time. These earliest ammonites (goni-atites) had very simple, wavy suture patterns. They were cut back by the great mass extinction in the Late Permian some 245 million years ago, and only a few species survived. But the ammonoids soon bounced back and proliferated once again—this time with crinkles in some parts of their sutures (the cer-atites). Then the mass extinction at the end of the Triassic once again drove all but a very few of these ceratite ammonoids extinct. The few that survived managed once again to spring back, into the last great flowering of ammonoids—the "ammonites proper." These last ammonoids were abundant in the seaways of the Jurassic and Cretaceous, during the heyday of the dinosaurs. Theirs were the most complex sutures of all—a dense array of crinkles all over the suture. To geologists, the ammonites are the most important fossils for subdividing marine Mesozoic rocks.

Thus ammonoid history has much to tell us about the evolutionary process—including how important extinction is in eliminating some groups and spurring evolutionary bursts among surviving lineages. And ammonoid suture patterns also show that complexity sometimes increases during evolutionary history.

—Niles Eldredge

See also: Arthropods, Marine; Evolution; Evolutionary Biodiversity; Geological Time Scale; Mass Extinction; Mollusca; Punctuated Equilibria

Bibliography

Clarkson, Euan N. K. 1998. Invertebrate Palaeontology and Evolution. Malden, MA: Blackwell Science; Eldredge, Niles. 1987. Life Pulse: Episodes in the History of Life. New York: Facts On File; Fortey, Richard. 1999. Life: A Natural History of the First Four Billion Years. New York: Vintage; Fortey, Richard. 2000. Trilo-bite: Eyewitness to Evolution. New York: Knopf; Gould, Stephen Jay. 1989. Wonderful Life: The Burgess Shale and the Nature of History. New York: W. W. Norton.

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