Time, like beauty, exists in the beholder's eye. When the historian talks about bygone days he means the past twelve thousand or so years, the era of human civilization. To the archaeologist, the immediate past extends back a million and a half years, when hominids, or man-apes, first appeared. And when a paleontologist says that such-and-such happened just yesterday, he might be referring to last Tuesday or a Tuesday hundreds of millions of years ago. I'm exaggerating, of course, but only a little, and to make an important point: that one's sense of time—how old things are and how fast events take place—can vary greatly, depending on all sorts of factors. An everyday example is the profound change most of us undergo in our perception of time as we move further away from birth and closer to death. To the average teenager, days, weeks, and months unfold at a leisurely pace and years come and go so slowly that it seems like one will live forever. But that same person, once he is well into his middle years, is likely to experience days ticking by like minutes and years turning over like months. At sixty-five, life appears alarmingly shorter and time a great deal faster than they did at fifteen.

Why exactly this is so is a mystery philosophers have been trying without success to figure out ever since man became conscious of the passage of time, capable of remembering the past and anticipating the future. I won't try to improve on their efforts here. Besides, for the paleontologist it's enough to know that however long or short his own life may seem, it's but an instant, or less, when compared with the entire duration of life on Earth—about two billion years. To get a clearer sense of the comparison, imagine that that two-billion-year period is equivalent to one year, in the same way that a full-size Ultrasaurus, a sauropod that stood about six stories tall and weighed as much as 150 tons, is equivalent to a one-foot-high scale model. If that were the case, each month of the model year would represent about 165 million years; each day, about 5.5 million years; and each minute, about 3,800 years. My life so far—I just turned fifty-one—represents %t of a minute. Even if I'm lucky and reach old age, my entire existence, from birth to death, will last somewhere in the neighborhood of one second. Think of that: one second in the year that life has existed on Earth. Come and gone in the blink of an eye.

Now try thinking of the scale model of time this way: During the year that stands for two billion years, dinosaurs emerged on about November 18 and went extinct almost exactly a month later, on December 18. Consider, by contrast, human beings. As I said earlier, hominids have been around for at least a million and a half years. That's equivalent to about six and a half hours on the scale year. To the best of our knowledge, the group of hominids to which we belong, Homo sapiens sapiens, or modern man, appeared about forty thousand years ago, which is equivalent to about ten and a half minutes. In other words, human beings made their first appearance on the evolutionary stage ten minutes before midnight on the last day of the last month of the year. Whenever we get carried away with notions of our special status among all of the creatures that have made this planet their home, it would be well to remind ourselves of this fact—that we are newcomers, and that we have a long way to go before we can say that we are one of life's success stories.

What does this have to do with dinosaurs? For one thing, it helps to show that when measured against any human-based scale—from an individual life span to the entire duration of the species—dinosaurs lived for an extraordinarily long time and, what's more, they did so an extraordinarily long time ago. For another, and I can't emphasize this enough, it helps one appreciate the breadth and potential fertility of evolution. Evolution is nothing more, and nothing less, than change through time, but to grasp the extent of possible change, as well as the mechanisms responsible, you have to cultivate a much-expanded sense of history. Only within a historical context, against the backdrop of their life stories and generational sagas, do dinosaurs reveal their full significance. To make what I'm saying more concrete, picture what paleontolog-ical fieldwork would be like in the absence of historical awareness, if it were merely a matter of finding and sorting fossil bones.

Last summer, for example, I conducted a fascinating excavation outside Malta, in northeastern Montana. Nate Murphy, an amateur collector from the area, had been exploring the eroded benches along one side of a broad, grassy drainage when he spied a dinosaur tailbone protruding from a wall of exposed sandstone. Since the fossil was located on public property, administered by the Bureau of Land Management, he couldn't excavate it without a BLM permit, and he didn't qualify for that because he's not a professional paleontologist. So he came to me and asked if he might work the site under my permit. I consented, but that made me ultimately responsible for the dig, so I drove to Malta to take a look. By that time Nate had uncovered the entire tail, and it was immediately clear to me that he had probably stumbled upon a museum-quality specimen—complete, intact, undisturbed—of an adult duck-billed dinosaur, which does not happen often. Since the sandstone was soft and crumbly, my crew and I were able to expose the entire

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Excavation of the Malta Brachylophosaurus. The nearly complete skeleton is lying on its right side. The tail is to the left, the head to the right. {Bruce Selyem, reproduced courtesy of the Museum of the Rockies.)

skeleton in less than a week. And what a skeleton it was—a beautifully preserved, twenty-foot-long Brachylophosaurus that looked as if it had lain down on its right side and gone to sleep, never to rise again. Almost every bone was in place, even the fingers. The rib cage, which is crushed and flattened during the burial of most specimens, was inflated, bowed, just as if the animal still contained its internal organs. All of the tendons that kept its tail erect were present and in place.

Yes, a beautiful specimen. But what did it tell us? What was the story behind this particular brachylophosaur? By studying its anatomy, the shape of the bones and how they are joined together, we could get a pretty good idea of how the dinosaur moved. And it is true that by examining the makeup and internal structure of the bones under a microscope we could probably determine how fast it had grown prior to death. But where did the brachylophosaur come from? What was it doing here? What was its place in the larger scheme of things? Regarding these questions the skeleton was stubbornly silent. If we had to rely on the fossils alone, we would find ourselves in much the same position as the nomadic people who had discovered protoceratopsian skeletons in the desert of Outer Mongolia thousands of years ago. Like them, we could only guess at the origin of the unfamiliar animal, its relationship to other creatures, its current whereabouts. And like them, we'd probably invent an interesting tale for which there is little or no evidence—that the reason we don't see brachylophosaurs running around Malta today, for instance, is that they live deep underground.

If that scenario strikes you as something Jules Verne might have dreamed up, it's only because during the past two centuries paleontologists, historical geologists, and evolutionary biologists have been shedding light on the least understood dimension of life on Earth—time. Because of their efforts we now know that the sandstone surrounding the brachylophosaur is a physical record of the passage of time. To be precise, the outcrop from which we removed the duck-billed dinosaur skeleton belongs to a section of sedimentary rock called the Judith River Formation, and judging from the age of the outcrop we know further that the brachylophosaur died about 76 million years ago. And that's not all. The Judith River Formation consists of terrestrial sediments deposited while an inland sea shrank, its westernmost shore retreating steadily to the east, away from the Rocky Mountains, while it expanded again. In other words, the brachylophosaur inhabited the plains during a time when the plains had grown significantly wider, opening up new habitat for dinosaurs and other organisms, and thus making possible a great diversification and dissemination of life along the Rocky Mountain Front.

This isn't all I see when I contemplate the Brachylophosaurus skeleton in context—that is, against a historical backdrop—but it is enough to demonstrate why we sometimes spend as much time exploring the rock in which fossils are found as we do studying the fossils themselves. It should also convey some notion of the total search image paleontologists employ in the field. When we hunt for dinosaur bones we picture more than the rock in which we're likely to find them; we picture the world the dinosaurs inhabited when they were alive. In the largest possible sense the world we have in mind—that anyone interested in dinosaurs should have in mind—is the world of the Mesozoic era, from 230 million years ago to 65 million years ago, a time when enormous changes occurred on the surface of the planet, affecting all plants and animals, including where they lived, how they existed, and the overall course of evolution.

From the standpoint of the planet as a whole, the most significant geological event of the Mesozoic era was the breakup of Pangaea, the landmass into which all of the major continents had merged—a single colossal island in a single global ocean. Although exact dates are impossible to come by, geologists now generally believe that Pangaea remained intact until at least 220 million years ago, well into the Triassic, the earliest of the three periods that make up the Mesozoic era. This is important because it means that the first dinosaurs, not to mention the first mammals, appeared when, in principle, at least, animals could migrate from one "continent" to another without having to skirt large bodies of water. On this basis it would seem reasonable to assume that dinosaur fossils should be found throughout the world today, and in fact they are. When, at the outset of the Jurassic period, 195 million years ago, Pangaea began disintegrating, the early dinosaurs separated as well, some inhabiting Laurasia, the northern complex of continents, others inhabiting Gondwana, the southern complex.

The Jurassic period lasted about sixty million years. What was left of Pangaea continued to drift apart, with Gondwana starting to split into South America, Africa, India, and Australia-Antarctica, and a division beginning to show in Laurasia, the first sign of what would become Europe and North America-Greenland. Along that division the Atlantic Ocean eventually formed. During the Jurassic the continents were relatively low-lying. Worldwide the climate was warm and humid. Large expanses of Europe and, later, North America soon became submerged beneath shallow inland seas.

Gymnosperms, or nonflowering plants, were plentiful everywhere, with conifer forests occupying the uplands and ferns, giant horsetails, and large palmlike plants called cycads growing in the wetter, more tropical lowlands. Also well established by this time were the two great orders of dinosaurs: the Saurischia, or lizard-hipped dinosaurs, and the Ornithischia, or bird-hipped dinosaurs. Prominent saurischians included the sauropods, the largest and tallest land animals ever to have lived, and the theropods, the group of bipedal, flesh-eating dinosaurs to which Tyrannosaurus belongs.

The plate-backed stegosaur and such primitive ornithopods as Camptosaurus were among the ornithischians that flourished during the Jurassic period.

Although dinosaurs became the dominant land animals during the Jurassic, and for that reason the period is considered the zenith of dinosaur evolution (a perception reinforced by Crichton's books and Spielberg's movies), the Cretaceous period, beginning 136 million years ago and ending 65 million years ago, has proved to be a great deal more interesting—for my purposes, at any rate. Three crucial features define the last period of the Mesozoic era. First, the continents continued to drift away from one another, creating increasingly larger oceans between them, while vast inland seas expanded and contracted in slow, rhythmic pulses in Europe and North America (where the sea was contracting when our Brachylophosaurus was alive). Second, extensive mountain ranges rose along the western coasts of both Americas, accompanied by violent, often long-lasting volcanic eruptions. Of particular interest are the Rockies of North America, which profoundly altered the climate of that continent by preventing rain from reaching its interior regions. Third, flowering plants, the angiosperms, came into their own, dispersing across the continents and diversifying into all manner of environmental niches. For the dinosaurs that ate plants (and most of them did), this represented not only a new and prodigious source of food, but a source of food that perpetually renewed itself through the annual replacement of leaves.

That is how the Mesozoic era looks when viewed all at once,

Diagram showing the origination and duration of the best-known dinosaur groups. Co = "Coelurosaurs"; 0 = ornithomimosaurs; Ov = oviraptors; S = saurnithoides; D = dromaeosaurids; M = megalosaurids; T = tyrannosaurids; Pr = prosauropods; D = diplodocids; C = Camarasaurids; B = Brachiosaurids; F = fabrosaurids; Hy = hadrosaurids; Pt = ptsittacosaurids; Ct = Ceratopsids; P = pachycephalosaurids; S = stegosaurids; N = nodosaurids; A = ankylosaurids. (Based in part on data from Norman, 1985)

165 million years of geological and meteorological history as seen through a wide-angle lens, so to speak. But when we increase the resolution, trying to capture the age of dinosaurs in finer detail, the picture that emerges is a great deal more complex and problematic. Most problematic of all, from the viewpoint of paleontology, is that the fossil record is incomplete. Large parts of the Mesozoic world have disappeared without leaving behind a clear trace, not so much as one iota of direct evidence to indicate which, if any, dinosaurs lived in certain places at certain times.

As you probably know, fossilization can occur only if a number of strict requirements are met. For one thing, the organism has to die where burial will occur rapidly enough to prevent decay and weathering. Even teeth, horns, and bone, when exposed to wind, rain, and temperature extremes, will decompose beyond all recognition. For another, the means of burial must be gentle enough to avoid crushing and disintegration. This is why most fossils are found in marine sedimentary rocks. When aquatic organisms die they sink to the bottom, where they are soon covered in a protective layer of mud or other fine-grained sediment settling out of the water. Over time, under their own weight, the cumulative layers of sediment will become compacted and cemented together to form sedimentary rocks, shale and limestone primarily, while the hard parts of the organisms buried within the layers are either preserved more or less as they are (shells, typically) or are replaced by dissolved minerals (a more likely outcome in the case of bones). Throughout the entire process—death, burial, compaction, and, should it occur, remineralization—the sediments cannot be disturbed. No doubt lots of plants and animals have been buried in the geological environments that are home to igneous and metamorphic rocks, but exceedingly few fossils have survived the violent processes by which such rocks are formed.

What the selectivity of the geological record means for those of us who study fossils will be easier to grasp if you look closely at a typical present-day depositional environment—the Gallatin Valley, for example, in southwestern Montana, where the Museum of the Rockies is located. Sediment is being deposited in the valley all the time, as gravel along streams, sand in floodplains, silt and clay at the bottom of ponds, reservoirs, and lakes; and most of the sediment comes from the mountains that surround the valley or are located upstream of it—the Gallatin, Bridger, and Madison ranges, in particular. Now, jump ahead in time ten million years. What has become of the depositional environment that we once knew as Gallatin Valley?

Most notably, the mountains are gone, completely and irreversibly eroded away. Large-scale geological structures, you see, do not show up in the geological record as large-scale structures but instead as strata of sediment or sedimentary rock. Small variations in the original landscape, streambanks and shorelines, for instance, might be preserved, but certainly nothing larger. And if the hills and mountains are erased, so too are any plants or animals that might have lived in the hills and mountains. True, bones are sometimes carried long distances by streams and deposited in sand or mud, where they are buried and fossilized, but like characters without stories, such displaced specimens can tell us little more about their lives than that they once existed somewhere, sometime, somehow. Actually, the situation is worse than that, because whole skeletons cannot survive movement of any kind, much less a long, rough journey by water; when we find fossils that have been transported from another location after death they are always fragments, jumbled together at random, and often damaged.

The paleontologist who ten million years hence excavates the area that used to be the Gallatin Valley will, if she is very lucky, find fossils of only those animals that lived and died in the valley. No amount of luck will turn up the remains of a mountain goat, however, the goat having gone the way of the mountain. But, you might argue, we can infer that there once was a mountain, so why can't we infer that there was a mountain goat? A good question but, all the same, based on a misunderstanding. The only reason we know there once was a mountain is because we have witnessed erosion; we have seen with our own eyes where sedimentary deposits come from— mountains—allowing us to reason backward, using current sediments, along with other clues, to reconstruct a physical environment that existed a long time ago. One of the cardinal principles of geology is something called uniformitarianism, which means simply that the geological processes of the past are the same processes we witness today. And remember, too, that the goat in the example rep resents an animal for which we have no other evidence than what might show up as a fossil. The biological processes of the past are the very same processes that operate today, but if the dinosaurs teach us anything, it is that the organisms that arise from those processes can vary greatly from one period to another. If there existed high-altitude dinosaurs during the Mesozoic era, dinosaurs that never left the mountains, our chances of finding their remains today are slim, and of being able to understand such rare remains even slimmer.

Okay. Let's complicate things a little further, not to be mischievous but to suggest something of the actual difficulties one encounters when trying to plumb the mysteries of the natural world. Let's say that a paleontologist returns to the area once called the Gallatin Valley a hundred million years from now. What will she find? Among the many entirely normal events that might have occurred in the meantime is an overall depression of the central part of North America, causing eastern Montana to tilt downward. This, in turn, would have accelerated stream flow and the rate of erosion east of the Continental Divide, where the Gallatin Valley is located. All along the Missouri River today there are cascades and waterfalls. If the river ran faster, the cascades would grow more pronounced and the waterfalls would migrate upstream as the water, rushing at a furious pace, ate away the bottom. Cascades of various size would also form in the tributaries of the Missouri—the Gallatin, Jefferson, and Madison rivers—and they, too, would edge ever farther upstream, toward their headwaters, eventually eroding out all of the sediments that were deposited in the Gallatin Valley region, erasing even the pulverized vestiges of the mountains that once encircled the valley. Anyone visiting the region a hundred million years from now would find no record of what occurred immediately prior to ero-sion—that is, no sign of life as it is now, neither the original sedimentary rocks nor the fossils they might have contained.

Piecing together stories a hundred million years old or older on the basis of incomplete and altered geological records is exactly what occupies paleontologists today. And what I've described so far represents only a few of the factors that must be taken into consideration when trying to do so. As in the case of fossilization in marine sediments, some depositional environments are more likely to preserve the remains of animals than others. Even where the terrain is comparatively flat, the composition of sedimentary rock can vary significantly from one region to another. Upland areas, for example, which are those that lie closest to mountains, are usually well drained and thus relatively dry. They comprise an abundance of mudstones but very little sandstone, and tend to be green or red in color, which indicates that they contain large quantities of sodium, potassium, and other alkaline chemicals. And as it happens, upland alkaline sediments preserve calcium-based bone better than they do carbon-based plant material. Lowlands, by contrast, being located near seas and lakes, are poorly drained. They tend to be swampy and acidic, rich in hydrogen and generally made up of more sandstone than mudstone. Tan, gray, and sometimes black, acidic lowland sediments preserve plants better than bone. Coal, for example, which is composed largely of carbonized leaves and stems, is a typical lowland sedimentary rock. A good example of a contemporary upland area is the eastern two-thirds of Montana, which, because of its proximity to the Rockies, is high and arid, whereas Louisiana, along the Gulf of Mexico, and, say, New Jersey, near the Atlantic Ocean, possess all of the characteristics of lowland environments.

Why is it important to think about this? I can tell you why it's important to me. I want to find dinosaur eggs and baby dinosaur bones, among other things, and I want to find them as quickly and efficiently as possible. I could devote years to searching ancient swamp environments without finding a single one, not because dinosaurs never inhabited such areas but because the acidic sediments might have long ago dissolved whatever bones and teeth and horns the dinosaurs left behind, obliterating their remains forever. Finding dinosaurs, in other words, is as much a matter of knowing where evidence is likely to have been erased as where it might still be preserved. This is one of the reasons why I have spent most of my professional life hunting for fossils in central Montana. A great many of the geological formations that happen to be exposed there represent upland regions from the age of dinosaurs, more precisely, the Cretaceous period, when the two groups that have interested me the greatest during the past ten years—the duck-billed and horned dinosaurs—inhabited the plains along the Rocky Mountain Front from Alaska to Mexico.

It's time, I think, to focus our historical lens on North America between 136 and 65 million years ago and increase the magnification, bringing Montana into sharper view. Across the world, remember, continents are being driven apart from each other and new oceans are forming in the rift zones widening between them. The global climate is considerably warmer and more humid than today. There is no ice on Earth, not even at the South Pole, where, then as now, Antarctica is located. Remember, too, that large parts of North America lie at low elevations relative to the newly formed oceans. A vast body of water, the Western Interior Cretaceous Seaway, has flooded the central and southern reaches of the continent and now stretches from the Gulf of Mexico to the Arctic, isolating the Rockies from the rest of North America. The seaway is fed by tropical waters from the Gulf, and it's shallow, which means that it's also solar-heated. Crocodiles live comfortably along its shores in what is now northern Alberta. But most important, the seaway is dynamic, rising and falling at least three times during the Cretaceous, on each occasion expanding its boundary significantly westward, toward the Rockies.

Let's increase the magnification again. Uplifted during the Triassic period, the Appalachians in the east are pretty old and worn down now; little terrestrial sediment is being deposited anywhere in that region. On the other side of the continent, however, the Rocky Mountains are newly formed. They thrust above the continental floor and, significantly, they thrust eastward, which has created a broad depression, similar to a rumpled rug, which geologists call a foredeep, all along the front. Immense quantities of ter-

Western Interior Seaway

Cretaceous Seaway


East America

North America, showing the state of Montana and the position of the Interior Seaway during the Cretaceous period.

restrial sediments formed during the erosion of the mountains are not only carried eastward by creeks and rivers and, to a lesser extent, wind, they tend to be deposited in the foredeep, migrating no farther. In succeeding chapters I'll fill in the details of this picture, especially as they concern the evolution of duck-billed and horned dinosaurs in the late Cretaceous, but for the time being it is sufficient to appreciate in very general terms how the erosion of the Rockies and the rise and fall of the Western Interior Seaway interacted to produce the sedimentary formations found in central Montana today. The two depositional processes in effect dovetailed, three fingers of marine sediments, representing three expansions of the seaway, interleaved with four fingers of terrestrial sediments, representing the seaway's contractions.

As you can see on the stylized cross-section chart on the following page, each distinct marine and terrestrial formation bears

This cross section of the dinosaur-bearing sediments of central Montana shows how the marine sediments interfinger from the east. The numbers are the relative age-dates of the different strata.

its own name, most of which you needn't be concerned about right now. But I want to call your attention to the Judith River Formation, and especially to the time between 75.4 million years ago, when the seaway had regressed to its easternmost point, and 74 million years ago, when the last transgression onto the plains came to a halt and the seaway started to retreat again. It is in this suite of terrestrial sedimentary rocks, you may recall, that we found the splendid Brachylophosaurus skeleton. When earlier I said that I was seeing that particular duck-billed dinosaur in its actual historical context, in terms of the environment in which it lived, I was referring in part to the information represented in this chart, information that resulted from geologists painstakingly dating the sedimentary rocks of Montana and mapping their extent, location, and strati-graphic relationship to one another. In the chapters to come you will be hearing more about the Judith River Formation, but the group of sedimentary rocks that has intrigued me most lies to the west—the Two Medicine Formation. The Willow Creek anticline, where we first discovered eggs, nests, and babies, is located in the Two Medicine, as are many of the sites where we have excavated dinosaur fossils during the ten or so years that have passed since leaving the anticline and Egg Mountain. (Strictly speaking, we haven't left the anticline; the Museum of the Rockies still operates a field school there and every summer students find more eggs, nests, and baby dinosaurs.)

Why focus on the Two Medicine Formation? The fossil record, we know, is both biased and incomplete. Dinosaurs lived all over the world, but certain of their native environments have disappeared, as mountains do during erosion, along with any remains that might otherwise have survived, or they possess some characteristic that reduced the chances of preservation, such as the acidic quality of lowland swamps. The Two Medicine Formation, however, represents the upland reaches of a coastal plain that at its widest point, when the Western Interior Cretaceous Seaway had fully receded, was four hundred or more miles across. Over a period of about 12 million years, from 84 to 72 million years ago, streams poured out of the young Rocky Mountains, routinely and extensively flooding the plain and leaving in their wake layer upon layer of sand, mud, and other sediment, which eventually turned into rock. Today, in north-central Montana, the cumulative layers of sedimentary rock that make up the Two Medicine Formation are two thousand feet thick. The surface of the formation covers thirty-six hundred square miles and extends from extreme southern Alberta to Augusta, Montana.

The formation needn't have been that thick or extensive. If the original deposition had occurred more slowly, that same 12 million years might be represented by one thousand feet, or five hundred, or fifty. Even the most comprehensive of geological columns contains gaps, places in the strata of rock where deposition ceased altogether or some kind of secondary erosion took place, erasing sediments that had already been deposited. Such gaps are particularly-troublesome because there is no telling how long the pause in deposition or the erosion event lasted, whether a hundred years, a thousand, or a million. A gap in the geological record reveals only that something is missing; it says nothing whatsoever about what is missing, or how long a period the gap represents. And in truth, comprehensive records are rare, the intervals in most geological columns far outnumbering the surviving sediments. Deposition is simply too variable and erosion too universal to permit much else.

You can now better appreciate why I like north-central Montana so much. The sedimentary rock there has been uplifted, folded, eroded away in places, even, at times, twisted completely out of shape, but the strata as a whole remain close enough to their original orientation and condition to be deciphered and compared with other strata, within the Two Medicine Formation as well as those of other formations. What's more, the thickness of the Two Medicine Formation allows me to look at a relatively long period of time in the natural history of dinosaurs in some detail. In other words, the "resolving power" afforded by the sedimentary strata is strong.

Critical to any understanding of evolution is the ability to see relationships among organisms that lived at the same time—in what ways they are similar or dissimilar—as well as relationships among organisms that lived at different periods, that is, how certain characteristics may have changed over time. In many instances, the tools, both physical and conceptual, that we have used in the past to study evolution have lacked sufficient resolving power to bring these relationships into view. Were I to concentrate my fieldwork in a formation that is only five hundred feet thick, for example, the behavioral and evolutionary information I seek would remain fuzzy, out of focus, if they could be detected to any useful degree at all. The best dinosaur stories—that is, the least ambiguous and most clearly defined stories—come from the thickest, most complete, least disturbed sediments. In the section of the Two Medicine Formation that surfaces in north-central Montana, the same amount of time—twelve million years—is represented by four times as much rock as my hypothetical five-hundred-foot formation. This means that if a dinosaur died in a floodplain along the Rocky Mountain Front, it was four times more likely to be buried in sediment and thus fossilized. The geological column captures more detail.

Allow me one final observation about the Two Medicine Formation, and about dinosaur hunting in Montana overall, before I describe recent excavations. My primary interest is the Cretaceous period, especially the late Cretaceous, from about 80 million years ago onward, because that is when the ornithopods, or duck-billed dinosaurs, such as Maiasaura and Brachylophosaurus, flourished, as did the ceratopsians, or horned dinosaurs, the group that includes Protoceratops and Triceratops. Both groups appeared late on the dinosaurian tree, the ceratopsians very late, and they were among the last of the dinosaurs to walk the earth. More important from my standpoint is the fact that both groups formed herds. Other dinosaurs may have gathered together or acted in concert for one reason or another, but the only groups we can be certain did so are the duckbills and their closest ancestors, as well as the horned dinosaurs. And the questions that drive most of my research, regarding social behavior and the evolution of species from one generation to another, can only be addressed by comparing large numbers of the same animal. The Two Medicine Formation of north-central Montana fulfills that requirement. Indeed, with its detailed and comprehensive record of the comings and goings of immense populations, it is the Serengeti of dinosaur deposits.

My first interests are the ornithopods and ceratopsians, but they are not my only interests. In the fieldwork section you'll find that the cast of characters includes as well many other kinds of dinosaurs, most prominent among them certain key theropods {Tyrannosaurus, Allosaurus, Deinonychus) and sauropods (Apatosaurus, previously called Brontosaurus). You'll also see that we have not confined our explorations to the Two Medicine Formation or to upland environments or even to the Cretaceous period. Montana as a whole is fossil country, especially the eastern two-thirds of the state, from the Rockies well into the Great Plains, and I try to take advantage of all of the opportunities it offers. In this I am truly walking in the footsteps of the pioneers of paleontology, who found in Montana a trove of dinosaur fossils. In 1856, Ferdinand Vandiveer Hayden, a geologist, was the first to discover dinosaur remains in North America— a variety of teeth—near the confluence of the Judith and Missouri rivers, in the central part of the state. Some of the teeth belonged to a duckbill from the late Cretaceous that came to be known as Trachodon; others to the man-size theropod Troddon; still others to unspecified horned dinosaurs and a tyrannosaur named Deinodon.

Almost anywhere you search for dinosaurs in Montana you find yourself immersed not only in geological history but the history of American scientific exploration. Near Billings, at the turn of the century, Earl Douglass found numerous duckbill fossils. Some of the specimens, I learned when I examined them at Princeton University in the late 1970s, were juveniles, and that was the revelation that inspired my search for baby dinosaurs. Over the course of several expeditions to Montana between 1902 and 1916, one of the most successful dinosaur hunters of all time, Barnum Brown of the American Museum of Natural History, collected untold numbers of specimens, including Triceratops bones and the world's first Tyrannosaurus rex skeleton near Jordan, in the eastern part of the state, and duckbill skeletons from an area near the Two Medicine River, in the north-central part. And as we saw in chapter 2, John Ostrom's analysis of Deinonychus, from south-central Montana, was the first serious challenge to the conventional idea that all dinosaurs were cold-blooded, sluggish reptiles.

But in the spring of 1985, when I was casting about for new places to dig up dinosaurs, the region that most appealed to me was the Landslide Butte badlands, in extreme north-central Montana. In Gilmore's field diary from 1928, twelve years after he had used what he mistakenly thought was a layer of clamshell fragments as a datum, or benchmark, in the local sedimentary column, he described collecting dinosaur eggshell fragments. By that time he knew what he was looking at and he said so in his notes, clearly and unequivocally, but for reasons I cannot fathom he never published the find, despite the worldwide excitement the discovery of eggs in Mongolia had stirred up just a few years earlier. Be that as it may, I wanted to study more eggs and more juveniles, and Gilmore had found both. In addition, he had found them in the Two Medicine Formation, the same rock in which the Willow Creek anticline was located. Reluctant as I was to leave the anticline, Landslide Butte seemed like a promising alternative. What I didn't realize at the time, however, is that it was more than that, much more.

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