Biology: Paleontology

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Biology: Paleontology

Introduction

Over 99.99% of all the organisms that have ever lived are now extinct. This fact, combined with the deep genealogical history that all organisms share, frames the study of the history of life. Paleontologists study the history of life based on the traces of life—fossils that are locked within the rock record—to test hypotheses about the evolution of life on earth. Before the nineteenth century, paleontology (like most sciences) did not exist as a discipline. Fossils were simply oddities of the natural world collected by scholars and naturalists. After the French Revolution, paleontology emerged as a scientific discipline in which fossils were treated as evidence for the true antiquity of Earth and the chronicle of life through time. Paleontologists did not initially offer a perspective on evolution, even though Darwin cited the fossil record to bolster his arguments. Paleontology remained disenfranchised from mainstream evolutionary thought through the eclipse of Darwinism (1875–1925), but, following the reunification of Darwin's views in modern biology in the Modern Synthesis (1940), paleontology became a unique discipline combining approaches in both geology and biology. Currently, paleontology sits at the interface of earth and biological sciences and paleontologists continue to change our understanding about evolution, mass extinctions, the tree of life, and mechanisms that generate the diversity of life on Earth.

Historical Background and Scientific Foundations

Early Developments—Paleontology from 1500–1800

Long before paleontology became codified as a scientific discipline, fossils were puzzling anomalies of the natural world. In the sixteenth and seventeenth century, a “fossil” was anything that could be found in the ground—minerals, shells, stone artifacts, and bones were all considered fossils. Conrad Gesner (1516–1565) produced the first known account of fossils, On Fossil Objects (1565), which illustrated all the known fossils and described their relative classification in the natural world. Gesner's work followed the Aristotelian tradition of encyclopedic accounts of the natural world. In some cases, Gesner could readily attribute a fossil to its living counterpart (for example, fossil sea shells obviously belonged in the same class as living ones). Other fossils were more difficult to place: Were ammonites, an extinct type of coiled mollusk, more like a snake or a worm? How did fossils get into the rock? Gesner thought that fossils were imitations of living creatures. Nonetheless, Gesner's fossils were troubling at the time, because the existence of bygone organisms directly conflicted with the Aristotelian paradigm of an eternal and static universe. Extinction was a novel concept in this tradition of thought.

Nicolaus Steno (1638–1686), a Danish naturalist working in Italy in the seventeenth century, pushed Gesner's contributions further. One day, while employed as an anatomist in Florence, Steno received the head of a large shark caught by fishermen on the coast. Steno noticed how similar the shark's teeth were with fossil ones (at the time called glossopetrae, or tongue stones, based on their shape) found in marine rocks nearby, and he established, for the first time, that the glossopetrae were indeed the remains of sharks, not mere imitations of teeth as Gesner thought. Steno could offer two possibilities regarding the origin of tongue stones: either they had been deposited when the surrounding rock was softer or fossils grew in rock like minerals and crystals. Steno's contemporaries debated these two competing hypotheses, but they did not distinguish between inorganic origins of fossils (i.e., fossils grew in rock as minerals) and organic ones (i.e., fossils as the remains of organisms). In 1669, while still pondering the origin of fossils, Steno observed that, in any given sequence of under-formed sedimentary rock, younger rock layers always overlay older ones. This simple statement, now called the law of superposition, is a fundamental concept in geology. Steno's observations were based on Tertiary strata in Tuscany that contained fossils, which made Steno think that fossils were not aberrations but the true traces of life. At a time when Western science considered natural and biblical histories complementary accounts of the past, Steno's arguments marked a significant turning point in understanding the history of life on Earth. Although Steno could not prove false either the organic or inorganic origin hypothesis for fossils, he did establish that fossils represented traces of Earth's history dating back several thousand years.

Steno's ideas, along with the counterpoints made by his contemporaries, were eventually summarized and synthesized by Georges-Louis Leclerc, Comte de Buffon (1707–1788), in his impressive, multivolume work Natural History (1749). Buffon was the first to place fossils in the great context of Earth history and favored a theory positing that Earth was tens of thousands of years old and had slowly been cooling since its molten origins. Buffon clearly understood that fossils were the factual remains of ancient organisms—some of which did not have living relatives. But his macroscopic, though largely speculative, history of Earth was met with disdain by his successors, most notably Baron Georges Léopold Chrétien Frédéric Dagobert Cuvier (1769–1832), the father of comparative anatomy and vertebrate paleontology, who thought that Buffon's account of the world was too radical and undisciplined and who demanded an account of the history of life based on evidence.

Cuvier was the first rigorous empiricist for biology, and he badly wanted comparative anatomy to become as clear and precise as the Enlightenment's physical sciences. Aiding Cuvier in this quest were the biological treasures of conquest from foreign lands that quickly filled up Cuvier's natural history museum in Paris (now the Muséum National d'Histoire Naturelle). Cuvier saw organisms as integrated wholes in which form and function were deeply related. For Cuvier, an organism's function dictated its form, and because organisms were integrated wholes, one could often reconstruct entire organisms from certain dissociated parts. Cuvier used this principle, which he called the correspondence of parts, to initiate the entire discipline of vertebrate paleontology. With the early mammal fossils found in the Paris Basin, Cuvier used the correspondence of parts (and his vast knowledge of comparative vertebrate anatomy) to reconstruct fossilized hoofed mammals that shared only vague similarities with living ones. Cuvier especially enjoyed demonstrating the predictive power of his principles in his anatomy lectures: upon discovering a fossil marsupial from the gypsum of Montmartre, Cuvier placed the incompletely prepared fossil before the lecture stand and announced that if the animal were truly a marsupial, he predicted that the fossil would exhibit specific features associated with pouched, and not placental, mammals. With deft use of a chisel, Cuvier then revealed the epipubic bones of his fossil marsupial, much to his audience's amazement and approval.

IN CONTEXT: GEORGES CUVIER COMMENTS ON THE IMPORTANCE OF FOSSILS IN GEOLOGY, 1812

“…How was it not seen that birth of the theory of earth is due to fossils alone; and that without them we would perhaps never have dreamt that there had been successive epochs, and a series of different operations, in the formation of the globe? In effect, they [fossils] alone provide the certainty that the globe has not always had the same crust, because it is certain that they would have had to live at the surface before they were buried at depth… [It] is through fossils—slight though knowledge of them has remained—that we have recognized the little that we do not know about the nature of the revolutions of the globe. They have taught us that the formations, or at least those that contain them, were deposited tranquilly in a liquid, that their variations corresponded to those of the liquid; that their exposure was occasioned by the removal of that liquid; and that that exposure took place more than once. None of all that would have been certain without fossils.”

SOURCE: Cuvier, G. Recherches surles ossemens fossiles de quadrupeds, où l'on rétablit les caracterès de plusieurs éspèces d'animaux que les revolution du globe paroissent avoic détruites , Vol. 4. Paris, 1812.Facsimile reprint, translated by Martin J.S. Rudwick, in Georges Cuvier, Fossil bones, and Geological Catastrophes. Chicago: University of Chicago Press, Chicago, 1997.

Cuvier's deep knowledge of fossil vertebrates also allowed him to develop a strong case for the concept of extinction. When Cuvier described the remains of a species of fossilized elephant (later recognized as mammoths), Cuvier relied on the distinctiveness of the mammoth from the living Asian and African elephants to demonstrate that the mammoth clearly belonged to a separate, extinct species. Cuvier coupled the idea of extinction with the stratigraphic studies of the Paris Basin he conducted with Alexandre Brongniart (1770–1847). Cuvier and Brongniart had recognized distinct cycles in the rock sequences of the Paris Basin, and they suggested that the sudden transitions between layers of strata represented cataclysmic events. In light of his paleontological and geological studies, Cuvier reasoned that life on earth underwent revolutions in which whole living assemblages must have gone extinct.

Nineteenth Century Advancements

In the early nineteenth century, debates in natural history focused less on ideas about origins and more on the study of the shape of life. The growth of natural history museums generated an interest in the study of organism design, and, with it, two perspectives to explain the diversity of life. Formalism viewed organism function as a consequence of form; and the opposing view argued that form was determined by usage and active function. Cuvier maintained that the integration of form and function in an organism prevented the tinkering of any of the individual components, and he adamantly supported the primacy of function. Étienne Geoffroy Saint-Hilaire (1772–1844), on the other hand, viewed function as a secondary outcome of form, which was centered on the unity of body plans. Geoffroy even argued that he could turn an arthropod into a vertebrate by inverting the body plan inside out, much to the consternation of Cuvier, who strongly denied transmutability, or the evolution of one species from another.

Cuvier envisioned the organization of life as four embranchements that were separate from one another, with structural differences between each branch so dissimilar that an organism could not possibly be transformed from one branch to another in a coherent manner. Geoffroy instead saw singular unity in the four embranchements and thought determining such unity of plan would show the existence of biological laws, an aspiration that Cuvier shared. These arguments about the shape and classification of life were notablypre-Darwinian. Evolution, as the concept exists today, was not a feature of these debates.

During this same time, modern geology was developing in England. The emerging modern discipline built on the work of the early geologists who argued that Earth's history was not cyclically catastrophic but instead uniform in rate and process during a history that was interminably old. Charles Darwin (1809–1882) relied heavily on these arguments, because they provided millions of years for natural selection to act upon variation, as he argued in the Origin of Species (1859). Although Darwin lamented at length about the fossil record's fragmentary and incomplete nature, he also found spectacular support for his theory of evolution in paleontology. The 1861 discovery of Archaeopteryx, an amazing fossil sporting both avian and reptilian features, was a critical test of Darwin's ideas. Darwin had predicted that the fossil record would yield such transitional forms.

In nineteenth-century England, paleontology captured the attention of naturalists and the public alike. Fossil collecting became a popular leisure hobby. Many fossil discoveries at the time were made by amateurs—often known as “gentlemen naturalists”—who described and named their discoveries. Most prized fossil collections were privately owned by wealthy hobbyists, but many of the gigantic Mesozoic marine reptiles that would adorn natural history museums in England were recovered from the Lyme Regis in Dorset by a young girl named Mary Anning (1799–1847).

In 1842, English naturalist Sir Richard Owen (1804–1892) named paleontology's most enduringly well-known subjects—dinosaurs. Owen's coined the term dinosaurs, meaning “fearfully great lizards”, from Greek.

The Bone Wars of the American West

By the middle of the nineteenth century, Europe had universities with paleontological legacies stretching back decades. Paleontology in the United States was a fledgling enterprise by comparison. The outbreak of the American Civil War (1861–1865), shortly after Darwin's ideas were published, eclipsed academic thought.

The Swiss-born Louis Agassiz (1807–1873) was among the first cadre of prominent paleontologists in the United States. Agassiz had studied under Cuvier in Paris and achieved prestige through his extensive studies on fossil fish. Harvard University invited Agassiz to the United States in 1848, and by 1859 Agassiz had secured enough money to establish the Museum of Comparative Zoology.

Agassiz was an important figure in American science and education in the mid-nineteenth century, but the first intellectual center of American paleontology developed in Philadephia, under Joseph Leidy (1823–1891). At the age of 22, Leidy had become a member of the two most prestigious gentlemen naturalist clubs in America; two years later he was elected chair of curators at the Academy of Natural Sciences. In 1848, Leidy published a definitive account of fossil horses in North America, which settled a long-standing question over the presence of horses before the Spanish conquest of the New World. Leidy also achieved high notoriety for American paleontology in Europe by describing the most complete dinosaur at the time, Hadrosaurus foulkii, which was unearthed in Haddonfield, New Jersey, in 1858. Leidy's bipedal interpretation of the dinosaur's posture ran against the prevailing quadrupedal reconstruction favored in Europe, where authorities like Owen argued that dinosaurs, being reptiles, must have sprawled.

By the 1860s, Leidy had traveled throughout the American West firsthand and published extensively on the early mammal and dinosaur fossils he discovered. Leidy later became embroiled in a scientific rivalry between two of his disciples, Edward Drinker Cope (1840–1897) and Othniel C. Marsh (1831–1899) that would rage from the dinosaur quarries of Colorado to the floor of the U.S. Congress.

Cope and Marsh were among the most prolific paleontologists in the history of the discipline. Their scientific rivalry, dubbed the Bone Wars, spurred rapid growth in academic paleontology and geology. Cope was a self-educated paleontologist; Marsh earned an M.A. from Yale in 1862 and later convinced his step-uncle, George Peabody (1795–1869), to establish a natural history museum at Yale (which would later bear Peabody's name). At Yale in 1866 Marsh secured the first professorship in paleontology in America.

By 1869, Cope and Marsh literally followed the train tracks of progress that had been laid by the exponentially growing railroad companies. The American West lay largely unexplored, and geologic survey reports of abundant and promising rock exposures fired both Cope and Marsh's imaginations. Marsh, with geological hammer in one holster and pistol in another, secured travel permits from the U.S. War Department and took advantage of sprawling railroad lines and army outposts to organize his expeditions. Marsh's expeditions sometimes numbered over 70 men. By comparison, Cope largely self-funded his expeditions and occupied an unpaid government position as chief paleontologist that garnered him meager supplies and military protection in the American West.

As their expeditions quickly discovered the wealth of fossil mammals abundantly found in rocks of Wyoming and the Dakotas, both Cope and Marsh raced to publish and announce their discoveries upon returning to the East Coast. Because publication date establishes taxonomic priority in naming species, Cope and Marsh both recognized the prestige in naming their species first, and they liberally sought its value, sometimes naming different species from the same specimen

EDWARD DRINKER COPE (1840–1897)

Edward Drinker Cope (1840–1897) was born in Fairfield, Pennsylvania, to a wealthy Quaker family in 1840. Cope attended Quaker schools until the age of 15, when he decided to pursue farming on his friends and relative's property, at his father's urging. Cope did not complete any formal education past the age of 16. As a twenty-year-old, Cope's curiosity eventually led him to attend Joseph Leidy's anatomy lectures at the University of Pennsylvania's Medical School, which he found riveting. One year later, Cope had already published 8 scientific papers based on the herpetological collections from the Academy of Natural Sciences, foreshadowing a career of that would culminate with over 1400 scientific publications.

Cope first met Othniel C. Marsh (1831–1899), the man who would become his lifelong antagonist, on a long visit to Europe in 1863. Both remained cordial shortly thereafter, even naming fossil species after one another. In contrast to Marsh's wealth and prestigious position at Yale, Cope had many problems securing a paying job. He worked mainly out of his home, 2102 Pine Street, in downtown Philadelphia, and maintained a long-standing association with the Academy of Natural Sciences. In 1869, when Cope and Marsh were still on speaking terms, Cope invited Marsh to Philadelphia to see Cope's newly-minted, marine reptile fossil from Kansas, Elasmosaurus platyrus. After Marsh carefully inspected the long, serpentine fossil, Marsh duly mentioned to Cope that the skull was mounted on the tail and that the whole animal was anatomically reversed. Cope and Marsh debated the point to no resolution, and eventually they sought Joseph Leidy (1823–1891) to arbitrate. Leidy concurred with Marsh that the skull was indeed mounted at the end of the tail, much to Marsh's satisfaction and Cope's dismay. Panic stricken, Cope spent the following days racing through Philadelphia to buy up as many copies of his original Elasmosaurus publication with the incorrect reconstruction and hope that no one would be the wiser.

During the Bone Wars of the 1880s, Cope went to great expense to keep pushing the game of scientific brinkmanship with Marsh. During the field seasons, he would move out West and bring his wife and young daughter along, although more than once his wife ended up saving him from death and disaster. His residence at 2102 Pine Street quickly became a storage facility for his many finds; his professional life consumed his inheritance, and he flwith bankruptcy. Even at the height of the Bone Wars, Cope published more than 40 papers a year. His master work, The Vertebrata of the Tertiary Formations of the West, was nearly a foot thick and weighed 15 pounds in its original format.

Cope died in 1897, leaving his family in financial ruin, but he could command a list of over 1,400 scientific titles, a figure nearly unmatched to this day. In his will, Cope stipulated that his body be donated to the Anthropometric Society (now the Wistar Institute) in Philadelphia and that his brain be removed for study and preservation alongside Leidy's.

in the same scientific report. Accusations of scientific impropriety ensued in the pages of scientific journals; Cope even obtained affidavits from printers to establish publication priority to push his cases. Marsh used his connections at high government offices to publish weighty tomes about his discoveries (volumes of hundreds of pages, lithographs, and woodcuts), although Marsh, like Cope, would usually fund publications out of his own pocket.

In the 1880s, Cope and Marsh's expeditions crisscrossed the American West, and both Cope and Marsh employed valuable field experts as fossil finders. Cope and Marsh also used spies to keep tabs on the other camp's progress in the field or mislead the other's expeditions, and neither Cope nor Marsh was beyond destroying specimens to avoid them falling into the other's hands. Marsh maintained cordial relationships with such influential people in the American West as Sioux chief Sitting Bull (1831–1890), General George Custer (1839–1876), and explorer (and later popular showman) William Frederick “Buffalo Bill” Cody (1846–1917), who accompanied Marsh on several expeditions. Cope had far fewer connections in the American West, but his fieldwork was characterized by intensive collecting in remote areas.

In 1886, Thomas H. Huxley (1825–1895), Darwin's foremost supporter, visited the United States, and Marsh quickly courted the famous Huxley to visit his museum at Yale. Marsh was a Darwinian, and he saw ample evidence for evolution in his ever-increasing fossil collections from the American West. Marsh's fossil collection astounded Huxley, a knowledgeable paleontologist in his own right. Huxley was especially impressed by Marsh's collection of fossil horses, which Marsh had collected in successively older rock formations throughout the American West. Nowhere else in the world had such a fine sequence of evolutionary transformation as illustrated by using the fossil record. Huxley was so taken with Marsh's collection of fossil horses that he later changed his sold-out lectures to showcase Marsh's finds as superlative examples of evolution.

As Marsh vaulted to high fame in paleontological circles in Europe after Huxley's glowing endorsements, Cope remained in the intellectual shadows, despite publishing extensively on Darwinian theory throughout the Bone Wars of the 1880s and 1890s. Cope thought carefully about Darwin's arguments in Origin, and he found Darwin's theory incomplete—if variation was the ultimate fuel for evolutionary change under natural selection, where did variation originate? This questionstill remains unanswered; continuing to drive many research programs in evolutionary biology and theory. Cope's name also persists in modern research programs in the form of “Cope's Rule,” which describes the evolutionary trend of some groups of organisms to increase in net size over time.

The Turn of the Twentieth Century

Cope and Marsh both died shortly before the turn of the century, and in the wake of their Bone Wars, paleontology had become a professional scientific discipline. Cope and Marsh demonstrated that paleontologists could also be professional scientists and institution builders as well. By the turn of the twentieth century, the great era of Victorian “fossilists” had passed. Much of the history of twentieth-century paleontology follows the academic institutions and collections that were supported by universities and private corporations.

The Modern Synthesis

By the late 1930s and early 1940s, older views about species, evolution, and heredity gave way to a more integrative view of organism diversity. Through the early decades of the twentieth century, an overarching evolutionary perspective on biology did not exist. In the early 1940s, the leaders in the fields of population biology, genetics, and paleontology (with the notable absence of development and embryology) convened a series of meetings to codify their separate research aims under one tent that explained disparate aspects of biology in wholly Darwinian terms. The Modern Synthesis, as Julian Huxley (1887–1975) coined it, sought to explain biology at multiple scales through the explicit agency of natural selection. George Gaylord Simpson (1902–1984), the dean of vertebrate paleontology at the time, was a key member of The Modern Synthesis. Paleontology was a placeholder for the results of selection operating above the level of populations and species.

The Modern Synthesis included paleontology as the grand game of evolution, but the achievements of paleontology were left to languish on the shelves of museums and the pages of journals and textbooks for many decades. In academic institutions, paleontology became increasingly allied with geology departments, because paleontologists provided the tools of biostrati-graphic correlation—the ability to determine rock age by correlating the presence of specific fossils, just as early geologists had done over a hundred years before. Through biostratigraphy, paleontologists proved enormously useful at locating fossil fuels for the burgeoning oil industry and hydrocarbon economy of postwar America. Because their study groups are abundantly preserved in the fossil record, invertebrate paleontologists especially benefited from the growth of “applied” paleontology.

New discoveries in paleontology still remained a driving force for changing fundamental ideas about the history of life, even in the mid-twentieth century. Previously, the only known fossil human remains were Neanderthal bones from caves in Europe and the famous Taung Child, a juvenile partial skull of Australopithecus africanus, described by Australian anthropologist Raymond Dart (1893–1988) from cave deposits in South Africa in 1925. Based on the distributions of modern great apes, Darwin strongly suspected that the clues to human origins were located somewhere in Africa. In the 1950s and ‘60s, a string of breakthrough discoveries in Kenya and Tanzania strongly affirmed the Darwinian approach to the study of human origins. This period of discovery marked the beginning of numerous discoveries of fossil relatives from different time periods along the African Rift Valley, including those with associations to stone tools, which would revolutionize our understanding of human evolution and unequivocally demonstrate our shared ancestry with modern apes.

IN CONTEXT: DISPUTES OVER THE MECHANISMS OF EVOLUTION

By the 1920s, Mendel's theory of heredity had been rediscovered, but many scientists still opposed Darwin's theory of gradual evolutionary change. Some geneticists favored the position that evolution proceeds in large jumps, or macro-mutations, rather than the gradual, incremental changes proposed by Darwin. The dispute between the two theories was reconciled by important work published in the early 1930s by independent population geneticists Ronald A. Fisher (1890–1962), J.B.S. Haldane (1892–1964), and Sewall Wright (1889–1988). Their classic work showed that natural selection was compatible with the laws of Mendelian inheritance.

This reconciliation gave rise to a period of intense research in genetics as the basis of evolution. Theodosius Dobzhansky (1900–1975), a Russian emigrant to America, began his classic investigations of evolution in fruit fly populations, and in 1937 published a book entitled Genetics and the Origin of Species, which became among the most influential books in modern genetics.

There was also a push to reconcile data from genetics studies with other proposed mechanisms and evidence of evolution. Scientists such as E.B. Ford (1901–1988) and H.B.D. Kettlewell (1901–1979) helped pioneer the subject Ford called “ecological genetics.” This was a fundamental concept because it brought together two formerly separate areas of investigation: ecological variation (the influence of different environmental conditions) and genetic variation (differences in genetic make-up) in natural populations.

Modern paleontologists, geneticists, and anatomists (those studying changes in form) still bring their own perspectives, evidence, and questions to the evolutionary biology. Although all agree on the existent and unarguable fact of evolution, arguments about the exact mechanisms still exist.

For example, some geneticists still conduct research (often with generations of fruit flies) to discover the exact mechanisms of genetic change and insist that those mechanisms fully explain all evolutionary change. Some non-geneticists dispute that all the mechanisms of evolution can be discovered from what they sometimes chidingly call studies of fruit flyvials. As paleontologists, Stephen J. Gould (1941–2002) and his colleague Niles Eldredge's (1943–) hypothesis of “punctuated equilibrium” dealt with much greater spans of time than could easily be replicated in the genetics laboratory. Proponents of this theory insist that punctuated equilibrium best explains the existing fossil record and that the fossil record should be taken into account just as fully as mechanisms derived from laboratory studies.

Revolutions in Geology and Evolutionary Classification and the Emergence of Paleobiology

Paleontology underwent an enormous transformation during the 1970s as several sweeping advancements in adjacent fields changed how paleontologists looked at the history of continents, evolutionary patterns and processes, and even dinosaurs, the most cherished icons in paleontology. By 1971, a revolution had been completed in geology as continental drift and the theory of plate tectonics became accepted by the majority of the field. Paleontologists provided crucial data weighing in favor of the non-static view of continents by showing that the same genera of plants and animals were found in rock formations on continents that had formerly been connected millions of years ago, now separated by entire ocean basins.

In 1972, Stephen J. Gould (1941–2002) and his colleague Niles Eldredge (1943–) published the first paper articulating a novel view of patterns in the history of life called “punctuated equilibrium.” Using data on the stratigraphic ranges and evolutionary relationships of different invertebrate genera in the fossil record, Gould and Eldredge observed that fossil genera remained morphologically static for the near entirety of their geological lifetimes—speciation and morphologic change were limited to exceedingly brief periods barely detected in the fossil record. Gould and Eldredge also saw similar patterns in other groups in the fossil record, prompting them to wonder if the entire mode of evolutionary change was not as gradual as Darwinians always thought.

To better account for this evidence, Gould and Eldredge proposed that “punctuated equilibria” were the dominant patterns of the fossil record, and stasis was the primary attribute of a species' lifetime in the fossil record. Gould and Eldredge's critics described the phenomenon as “evolution by jerks,” but the great predominance of punctuated equilibrium in the fossil record represents a major achievement for paleontology. Gould and Eldredge suggested an expanded role for paleontology beyond the circumscribed confines of the Modern Synthesis, which essentially asserted that the fossil record was merely selection in a fruit fly vial written across geological time (i.e. that the fossil record was a complete and accurate record of the cumulative evidence of slow genetic selection over time).

In the 1970s, paleontologists also weighed in on an incipient revolution that changed how evolutionary biologists classified the diversity of life. After the work of Carl Linnaeus (1707–1778; also known as Carolus Linnaeus or Carl Linné), biologists classified animal and plant species in a hierarchical system of ranks: kingdom, phylum, order, in increasing specificity down to genus and species. The Linnaean system provided a place for

every living organism, but its foundation was essentially pre-evolutionary: why recognize higher groupings of organisms based on unique features? If all birds have feathers, what to do when an animal that is not a bird has feathers (like some theropod dinosaurs)? In the 1950s, a German entomologist named Willi Hennig (1913–1976) devised a solution to this very problem by developing a method called cladistics, which studies evolutionary relationships and determines ancestry based on shared characteristics. Hennig would have been lost in obscurity if translations of his work had not reached the hands of paleontologists in the United States and England, who first applied cladistics to problems in the evolution of bony fishes. As other biologists relied on cladistics to resolve relationships among both living and extinct lineages of organisms, cladistics moved out of paleontology and was adopted by other evolutionary biologists in the early 1980s. After much dispute and controversy, cladistics has now become the standard method to resolve evolutionary relationships, succeeding in fields from epidemiology to linguistics.

Dinosaurs were also not immune to the changes that swept through paleontology in the 1970s, which marks the beginning of the “dinosaur renaissance” in vertebrate paleontology. Spearheaded by paleontologists like John Ostrom (1928–2005) and Robert T. Bakker (1945–), a new generation of paleontologists marshaled evidence to challenge the traditional view of dinosaurs as sluggish and dim-witted denizens of a bygone era, synonymous with obsolescence. Instead, the “hot-blooded” dinosaur proponents argued that dinosaurs were dynamic, active, and complex organisms. This theory was based on anatomical grounds; comparative work with modern relatives like birds and crocodiles; and on inferences about ecology and behavior. The discovery of fossilized dinosaur eggs and nests in Montana by Jack Horner (1946–) and field crews from the Museum of the Rockies provided crucial evidence suggesting that aspects of dinosaur social behavior and life history were radically different from the stereotyped reptilian depictions. The “hot blooded” versus “cold blooded” dinosaur debate continued through the 1980s, and more evidence accumulated that dinosaurs shared many more features with birds (their direct descendents, as cladistic analyses demonstrated) than with crocodiles and crocodiles' fossil relatives. During the late 1990s and early 2000s, the publication of work detailing exceptionally

preserved feathered theropod dinosaurs from Cretaceous deposits in China put the debate to rest: all living birds are descended from dinosaurs.

Views about the demise of the dinosaurs have also changed radically. Paleontologists use the origination and extinction of many groups to determine stratigraphic position of different rock units; but across some intervals of rock, the entire composition of the biota in the rock record changes completely, where fossil groups in older sequences leave no descendants across the boundary. Paleontologists call the wholesale extinction of the major groups in the fossil record “mass extinctions,” and paleontologists recognize five major mass extinctions over the past 500 million years. The causes of nearly all of these mass extinctions remain highly debated. The discovery of a giant impact crater off the shore of the Yucatán Penninsula furthered for many a crucial test of a meteor impact hypothesis.

Dinosaur Exhibits Become Popular

During the late 1800s, the American Museum of Natural History in New York City quickly gained in prestige, collection size, and popularity. By 1900, it was one of the great centers of American paleontology, under the leadership of Henry Fairfield Osborn (1857–1935). Osborn successfully linked the American Museum to Columbia University and then courted the interests of J.P.

Morgan (1837–1913), allowing the American Museum to launch large-scale collecting expeditions and public exhibits. Andrew Carnegie (1835–1919) later funded similar ventures for his own museum of natural history in Pittsburgh. Field excavators for the American and Carnegie Museums unearthed giants in the American West, including iconic titans like Diplodocus carnegiei in 1901 and Tyrannosaurus rex in 1905.

With great financial backing, institutions like the American Museum, the Field Museum in Chicago, and even academic ones like the University of California, sent collecting expeditions all around the world. These paleontological expeditions reflected the increasing desire for institutions to gain prestige not only in volume of collections, but also to find missing links in the history of life. The American Museum's expeditions to Outer Mongolia, led by Roy Chapman Andrews (1884–1960), epitomized these goals. Andrews mounted an expedition and used the news media to his advantage in his search for human origins, which, as Osborn had theorized, were to be discovered in Asia. Andrews failed to find human fossils, but he did return with a trove of dinosaur fossils. Paleontology captured the public imagination. Stories of expeditions were printed in newspapers and magazines. Fossil exhibits—especially those featuring large dinosaurs—remained enormously popular.

STEPHEN J. GOULD (1941–2002)

At the time of his death in 2002, Stephen J. Gould was the most prominent paleontologist, if not scientist, in the United States. Gould achieved popularity and worldwide recognition for his popular essays and respect among his peers for his contributions to the conceptual foundations and history of evolutionary theory. Gould even attained the late twentieth-century equivalent of immortality with an appearance (as himself) on the television show The Simpsons.

Born in Queens, New York, in 1941, Gould was raised in a Jewish family with strong Marxist views, which influenced his intellectual development. Gould, however, mainly credited a visit to the “Hall of Dinosaurs” at the American Museum for inspiring, at the age of 5, his decision to become a paleontologist. He attended Antioch College, and, following a brief stint abroad at Leeds University in England, he enrolled in graduate school at Columbia University in 1963. Gould studied land snails from Bermuda in graduate school, but he spent most of his time at the American Museum (through the joint studies program with Columbia), where he worked in a milieu among great paleontologists, such as George Gaylord Simpson (1902–1984) and others. By the 1970s, he landed a position at Harvard University, where he stayed for the rest of his career.

A chronicle of Gould's scientific output reveals a long series of influential publications that either created subdisciplines or revolutionized existing ones. While still a graduate student, Gould published a thorough review and critique of the biology of organismal scaling (generally called “allometry”), and Gould's 1966 paper remains required reading on the subject. In 1972, Gould and a fellow American Museum graduate, Niles Eldredge (1943–), published the concept of “punctuated equilibria,” a notion which Darwinian paleontologists harshly criticized at first but later prompted a full-scale revision of macroevolutionary patterns in paleontology. In 1977, the same year his first collected essays were published, Gould published Ontogeny and Phylogeny, the first synthesis of evolutionary and developmental biology in nearly half a century.

Throughout the 1980s and 1990s, Gould championed a rigorous and provocative approach to studying paleontology and the history of science that engendered as many critics as fans. He adamantly opposed the gene-based arguments from evolutionary psychologists and sociobiologists, and won wide acclaim for Mis-measure of Man (1981), a critique of I.Q. studies, which earned vociferous rebukes from the field's proponents. After surviving a rare stomach cancer, in the early 1980s Gould wrote Wonderful Life (1989) a much-celebrated account of the discovery of fossils from the Burgess Shale, which popularized the Cambrian “explosion” and the origin of major animal body plans. Gould enjoyed a high-profile status that few scientists in the latter half of the twentieth century attained. Gould was a prolific essayist, and his pieces appeared in newspapers and magazines throughout the world. Gould also became an expert witness who willingly testified in favor of teaching evolution in the classroom in a case that sought to undermine such teaching. Just before his death in 2002, Gould brought his popular writing to a planned end and finished a career like no other by publishing a massive opus on his favorite subject, The Structure of Evolutionary Theory (2002).

Modern Cultural Connections

In some ways, the spectacularly successful film Jurassic Park (1993) demarcates a turning point in the “dinosaur renaissance” in portraying dinosaurs as intelligent, social, and “hot blooded.” The revolution in dinosaur paleontology also has generated an immense groundswell of interest from amateur paleontologists as well, generating massive interest and activities by amateurs not only in the United States, but also in Japan, China, Australia, and elsewhere.

Paleontological discoveries by amateurs still drive some of the research in professional paleontology. Contrasted with other scientific disciplines, paleontology is one of the few fields in which amateurs can make revolutionary new discoveries (astronomy shares this distinction). In recent decades, however, the economic value of fossils to serious collectors has skyrocketed, which has generated an enormous market for the trade and sale of fossils. Professional paleontologists currently debate the ethical and practical issues associated with scientifically important and unique specimens that remain in private citizen's hands. As a measure of protection, the journals of most professional paleontological societies refuse to publish specimens that are not curated in public institutions. Even governments have taken action. Several nations regulate the export of fossils from their borders, but a black market persists nonetheless.

Through the 1990s and to the present, paleontology continues to thrive at the intersection of biology and geology. Using the pioneering database of the University of Chicago paleontologist John J. Sepkowski (1948–1999), the development of global databases on the World Wide Web with electronic data on the geographic and stratigraphic distributions of extinct organisms allows paleontologists to understand the macroevolutionary and macroecological changes in the diversity of life with strong statistical precision. The tabulations of species lifetimes in Sepkowski's database provided a geological vista on the structure of the diversity of life

that marked a turning point in our understanding of species diversity through time. Also, paleontologists are increasingly collaborating with molecular biologists to resolve a cohesive picture of the genealogical history of life, using the wide array of molecular sequence data (DNA, gene, and protein sequences) from living groups in conjunction with morphological information for both living and extinct organisms. In recent years, paleontologists have also found common ground with developmental biologists as questions about how genes build body plans have found unique examples in fossils from the Cambrian-era rocks (about 490–540 million years old) that capture a time when all the major animal body plans first evolved.

Paleontology is now an integrated discipline, utilizing a varied scientific tool-kit that includes earth and physical sciences, field and lab work. Nineteenth-century paleontologists once prized the largest fossils, while some paleontologists today use powerful microscopes and imaging technology to examine microfossils. Whereas the fossil itself used to be the only object of the paleontologists attention, current paleontological work considers the whole geological context surrounding the fossil as essential: Is the fossil related to surrounding fossils? What clues does the adjacent rock give about the condition of the earth at the time of deposit? Is this fossil site similar to other sites? Though only a fraction of modern paleontological research occurs in the field, fieldwork remains an essential—and often captivating—aspect of paleontology.

The discipline of paleontology has changed dramatically over two hundred years. From humble but promising origins, paleontology has risen to become a rigorous scientific discipline that integrates both biological and geological data to address questions about the evolution of life on earth. The description and discovery of fossils was once the pursuit of gentleman naturalists, but now discoveries by both amateur and professional paleontologists enhance and change our knowledge of life in the past. In the past decade, the discovery of early terrestrial whales from Indo-Pakistan, feathered dinosaurs from China, or seven million year-old hominid fossils from Chad all demonstrate that advancements in our understanding of evolution require data from the history of life. Paleontology is uniquely poised to contribute to this endeavor for years to come.

bibliography

Books

Bakker, R.T. The Dinosaur Heresies. New York: William Morrow, 1986.

Bowler, P. Evolution: the History of an Idea, 3rd ed. University of California Press, 2003.

Desmond, A.J. Archetypes and Ancestors: Paleontology in Victorian London, 1850–1875. Chicago: University of Chicago Press, 1986.

Desmond, A.J. Mismeasure of Man. W.W. Norton & Company, 1981.

Desmond, A.J. Ontogeny and Phylogeny. Cambridge: Harvard University Press, 1977.

Desmond, A.J. The Structure of Evolutionary Theory. Cambridge: Harvard University Press, 2002.

Desmond, A.J. Wonderful Life. W.W. Norton, 1989.

Novacek, M. Time Traveler: In Search of Dinosaurs and Other Fossils from Montana to Mongolia. Farrar, Straus and Giroux, 2002.

Rudwick, Martin J.S. The Meaning of Fossils: Episodes in the History of Palaeontology, 2nd rev. ed. New York: Science History Publications, 1976.

Web Sites

Palaeontological Association. http://www.palass.org/ (accessed April 16, 2008).

Paleobiology Database. http://paleodb.org/cgi-bin/bridge.pl (accessed April 16, 2008).

The Paleontological Society. http://www.paleosoc.org/ (accessed April 16, 2008).

IN CONTEXT: K-T EXTINCTION AND THE IMPACT HYPOTHESIS

In the past 30 years, few scientific debates have generated as much attention or polarized as many disciplines as the K-T extinction. Before the 1970s, paleontologists widely acknowledged a mass extinction evidenced in rocks at the Cretaceous-Tertiary (K-T) boundary. Strata below this boundary contained the remains of dinosaurs, marine reptiles, pterosaurs, and many other Mesozoic groups, while all the strata above contained none of these animals. Enter the father and son duo of physicist Luis Alvarez (1911–1988) and geologist Walter Alvarez (1940–). While studying the K-T boundary in limestone sequences of Italy, the Alvarezes discovered an anomalously high proportion of iridium in layers at the K-T boundary. Iridium is a highly unusual element on Earth, but it is abundantly found in meteorites. The Alvarezes sampled the K-T boundary from other sites across the globe and consistently recovered the same signal from the iridium layer, which indicated that the Italian site was no fluke. The Alvarezes then proposed a startling idea to explain their observations: could the iridium layer be the result of an extraterrestrial asteroid impact—literally, a mechanism from out of this world? If so, could such an impact have caused a global-scale catastrophe, like the K-T extinction? Their hypothesis, published in 1980, immediately touched off a huge controversy. Paleontologists could not support the Alvarez hypothesis for a sudden, catastrophic extinction at the K-T boundary—dinosaurs, for example, seemed to be on their way out several million years before the asteroid impact 65 million years ago. The Alvarez camp countered that the fossil record could not have captured the destruction wrought by an asteroid impact, but they needed more evidence. In 1996, evidence mounted that the “smoking gun” for the impact hypothesis had been found off the Yucatán Penninsula below the seafloor: an enormous crater hundreds of kilometers wide that dated to precisely 65 million years ago. The timing and size of this piece of evidence counters other less tenable hypotheses about the K-T extinction in dramatic fashion, although evidence about the details of the mode and scale of the K-T extinction remains hotly debated.

Society of Vertebrate Paleontology. http://www.vertpaleo.org/ (accessed April 16, 2008).

Understanding Evolution. http://evolution.berkeley.edu/ (accessed April 16, 2008).

Nick Pyenson

Scientific Thought: In Context