Looking For Babies

In the winter of 1978, I was working as a preparator at Princeton University. Preparators aren't professors or curators, and under normal circumstances they have next to no chance of becoming professors or curators. Usually you find them in the dusty, windowless basements that more often than not pass for laboratories in paleontology. In the summer they do go along on expeditions, hunting for fossils in the badlands of Montana, or Colorado, or Mongolia. But then, come September, somebody has to clean up the discoveries and make them look interesting. That's what preparators do. Confronted by chunks of rock with bits of fossil bone in them, or fragmented fossil bones, they chip away the rock or dissolve it with acid to liberate the fossils. Then they try to put the fragmented and broken fossil bones together. The result of their work may be something as small (but important) as the rebuilt jaw of a new variety of dinosaur, or as large and impressive as a full skeleton of Tyrannosaurus rex in a museum display.

A preparator's lot can vary greatly depending on his boss. He may be only a backroom technician, a hired hand with no say in the research. Or he may work as a junior partner in the research, collaborating with his boss rather than just scrubbing clean the summer's haul of femurs and tibias. My situation was probably the best a preparator could hope for. My boss, Don Baird, and I worked together on several research projects. And it was Don who introduced me to the fossil collections of the great museums. Traveling up and down the East Coast, we inspected the collections at the American Museum of Natural History in New York, the Philadelphia Academy of Natural Sciences, the Smithsonian Institution, and the Carnegie Museum in Pittsburgh. We spent our time not so much in the public halls, where the impressive skeletons are displayed, but in the back rooms and basements, looking at shelves of fossil bones that had been dug up years before and were kept there as a resource for paleontologists to study.

We examined these old fossils partly to pursue research interests of Don's and partly to continue my education. Before 1975, when I arrived at Princeton, my career in paleontology had been somewhat checkered. I'd been collecting dinosaur bones since I was seven years old, and I'd taken every undergraduate and graduate course in biology and geology that the University of Montana had to offer, but academically I'd had a little trouble holding to the straight and narrow. I'd ignored a few of the humanities and had never fulfilled the degree requirements. For a while, after leaving the University of Montana, I stayed in Shelby, my hometown, running the family sand-and-gravel plant with my brother. But crushing rocks held no interest for me. It paid better than paleontology, but I couldn't manage to corral the enthusiasm I had for dinosaurs and apply it to this way of making a living. I wrote letters to all the natural history museums in the English-speaking world, twice, inquiring about work. I also haunted the meetings of the Society of Vertebrate Paleontology, which serve as a job market (such as there is) in the field, and through this process I found the job at Princeton.

Don also encouraged me to do my own research. Until the winter of 1978, I'd had no particular interest in baby dinosaurs. I was interested in what are commonly called the duckbills. I'd done quite a bit of prospecting for dinosaur fossils in Montana and Alberta, Canada, on my own and with my friend Bob Makela. Duckbills were what I found, so duckbills were what I looked for. In some ways, scientific research is like taking a tangled ball of twine and trying to unravel it. You look for loose ends. When you find one, you tug on it to see if it leads to the heart of the tangle. Sometimes the loose end leads nowhere; sometimes it leads you deeper into the ball, to unexpected and intriguing knots. I guess you could say that dinosaurs were my ball of string. Duckbills were the only loose end that I had been able to find, so I had been tugging on them for all they were worth.

In our own collection at the small Princeton museum, Don and I found some duckbill specimens that had been overlooked in the scientific literature. They had been collected around 1900 by Earl Douglass in Montana in a rock deposit called the Bear Paw shale. This was a marine sediment, meaning that the rock was formed from the bottom muck of a sea. Fossils found there came from animals that had died and sunk into the muck millions of years ago. Some of these animals were sea creatures, but not all of them. The dinosaurs, at least, were land animals. There may have been dinosaurs that paddled about in lagoons, looking for aquatic vegetation, but no dinosaur was seagoing. The plesiosaurs and other great marine lizards that swam in the seas were an entirely different sort of reptile.

The dinosaurs whose bones Douglass found in the Bear Paw shale might have died on a beach or while foraging for food in shallow water; in this case, they would have been washed out to sea and their bodies would have settled into the bottom muck. Or they might have died farther inland and their bones might have been washed to the sea

In the Cretaceous period, much of the North American continent was underwater. The shaded areas on the map represent two land masses divided by a vast interior seaway stretching from the Arctic to the Gulf of Mexico.

by rivers and streams. This sea no longer exists, but at the time these dinosaurs lived, a little more than 70 million years ago, it covered the center of North America between the then emerging Rocky Mountains and the already old, eroding Appalachians. Geologists call it the Western Interior Cretaceous Seaway. The dinosaurs that are preserved in the Bear Paw shale spent their lives on a coastal plain, at its largest something like 200 miles wide, between this seaway and the Rockies.

In New Jersey and Delaware, fossils of dinosaurs had also been found in marine sediments. (In fact, this was the origin of all the dinosaur fossils I had seen from these two states.) These sediments had been deposited not by the inland sea but by the Atlantic Ocean, which, millions of years ago, covered much of the land that now makes up New Jersey and Delaware. The dinosaurs preserved here had lived on the coastal plain between the Appalachians and the Atlantic.

I became curious about dinosaur fossils from marine sediments. What did they have in common? Did they share any traits that would explain why they had been found in these old sea bottoms? I made a catalog of all the known dinosaur fossils from marine sediments and looked for similarities. What I discovered was that most of them were duck-billed dinosaurs, which was not particularly informative, and that 50 percent of the animals found in marine sediments were juveniles. That latter bit of information was no small discovery. Of all dinosaur fossils, those of young dinosaurs were the rarest of all.

IT'S HARD TO OVEREMPHASIZE how scarce juveniles were before 1978. You saw full-grown dinosaurs striking familiar poses in every natural history museum in the world, but you saw very few young. Over the 150 years that paleontologists had been hunting and finding dinosaur bones, fossils of juveniles had been found so rarely that their scarcity had become a major scientific puzzle. Other than one spectacular find in Mongolia, which I'll come to shortly, there had been some finds in New Mexico, isolated bones in France, footprints in the Peace River in British Columbia, some misidentified juveniles found in Montana and neighboring Alberta, and a few others here and there.1 If I were to make a simple list of fossils of adult dinosaurs that had been found, it would fill a book-length catalog, probably several such catalogs. The fossils of juveniles could be listed in a short pamphlet.

The fossils of baby dinosaurs could have been put on an index card. In 1978 these were all the known fossils of baby dinosaurs: some coelurosaurs from the Ghost Ranch in New Mexico; footprints of baby duckbills in shale in a coal mine in Utah; a dozen or two isolated bones of babies found in Canada and Montana; a baby found in the Judith River formation in Canada and described in 1956; a well-preserved baby skeleton from Argentina; and one tremendously impressive collection of fossils from the Mongolia find that I mentioned, including adults, eggs, and juveniles of all ages from hatchling on up. Finds of dinosaur eggs were just as rare. There were those from Mongolia, quite a number from a different sort of dinosaur in the south of France, isolated finds from India and Africa, and eggshell fragments from North and South America.2

The Mongolian group, on display at the American Museum of Natural History in New York City, constituted the one and only major find of baby dinosaurs and eggs in the history of paleontology. It was a fantastic discovery. The fossils were found by the American Museum's Third Asiatic expedition, which had gone to Mongolia in 1922 to look for the origin of man. During the trip home across the desert, when the jeep caravan stopped to rest, a photographer took a walk and stumbled upon what turned out to be a dinosaur skull and eggshell. The next year the expedition returned to mine the site fully and found the first dinosaur eggs the world had seen. Until then scientists had suspected that dinosaurs (like many other reptiles) reproduced by laying eggs, but they had never had the evidence to prove this point of view. Here that evidence was—in abundance. Fifty eggs were discovered, as well as countless fragments of eggshell. The eggs were found both as individual specimens, lying on the ground, and in one of about four clutches, or nests. In that one single season in Mongolia, the expedition dug up, in addition to the eggs, several skeletons and more than 50 skulls of Protoceratops andrewsi, the dinosaur that laid and hatched from the eggs. The skulls were of animals ranging in age from hatchling to adult, with all stages in between. The researchers ceased work after five weeks of digging, even though they were still discovering new specimens. They carried home 60 cases of fossils packed in camel hair and weighing 10,000 pounds.3

These Mongolian fossils came from red sandstone, the preserved remnants of a dry interior plain. And their discovery raised the question of why these so-called "red beds" were chock-full of baby dinosaurs when these fossils were scarce everywhere else. As succeeding years yielded no other major finds of baby dinosaurs, the question grew in importance. If you think about it (as I began to do once I had noticed the presence of juvenile fossils in the Bear Paw shale), more dinosaurs should have died young than died old; that's what happens with most animals. And that high infant mortality should have produced a lot of fossils over the course of 140 million years—a lot of fossils that had never been found.

Several explanations had been offered for the scarcity of fossil remnants of juvenile dinosaurs. Perhaps the bones of the young were not as sturdy and thus did not become fossils as easily as the bigger, harder adult bones. But, if that were the case, why had paleontologists frequently found fossilized bones of lizards and other small reptiles? Another suggestion was that the young lived in a different area from the adults, the way hatchling sea turtles disappear for a year. It had also been suggested that, to lay eggs, adult dinosaurs might have migrated to drier areas, those sections of coastal plain farthest from the various oceans on whose borders we know the dinosaurs lived. The Mongolian red beds were just such an upper coastal plain.4

If the dinosaurs laid their eggs primarily in the upper sections of coastal plains, then it would make perfect sense that so few eggs, babies and older juveniles had been found. As a coastal plain reaches farther from the sea toward the mountains, the pitch of the land increases so that the upper sections are constantly being stripped of soil by wind and water. The steeper pitch means that the streams run faster, and dust and dirt are pulled, by gravity, inexorably downward. In this situation, the bones of animals that die are likely to be washed or blown away. Even when the streams that run through these upper coastal plains do leave deposits of soil or silt, the accumulation is not as great as it is in the lower coastal plains and not as effective in preserving fossils.

The upper coastal plains (at left) were subject to erosion caused by small, fast streams; in the lower plains (right), the deposition from slow rivers built up silty deltas.

The lower sections of a coastal plain, nearer whatever sea the plain borders, are flatter. Here one finds slow, meandering streams and rivers, which deposit heavy burdens of silt on their banks during floods. They may also build up large deltas. Deposition is always occurring, and bones are thus likely to be buried and preserved. In consequence, modern paleontologists usually explore the preserved remains of ancient lower coastal plains, where fossil deposits are predictably rich. From the 1920s to the 1970s, plenty of fossils of adult dinosaurs were found in these lowland deposits. On the infrequent occasions when paleontologists did explore preserved upper coastal plains, they sometimes did find fossils of young dinosaurs but in many cases failed to recognize them for what they were.

The Mongolian red beds were the exception. The American Museum paleontologists suggested that the reason for the abundance of well-preserved fossils in this upper coastal plain was that the sandstones there were aeolian, or wind, deposits. That is to say, this was a dry, almost desert-like area when the dinosaurs lived there and the winds carried fine sand and deposited it much the way a river carries and deposits silt.

It was in the context of this dearth of juvenile dinosaurs that I came upon the preponderance of such fossils in Douglass' finds from the Bear Paw shale. I had no idea, in the winter of 1978, why there

should be so many in that shale, why nobody had noticed the numbers before, or why other shale deposits hadn't yielded similar numbers. All I knew was that the shale contained juvenile fossils, perhaps even babies, that baby dinosaurs were a coveted rarity—and that if I got myself out to Montana the next summer to take another look, I might find some baby dinosaurs myself. So, to prepare for the search, I inspected the museum collections in the East and studied all the fossils of young dinosaurs I could locate. I wanted to have a clear idea in my mind of what, precisely, I would be looking for when I got out into the field.

MY FIELD SEASON THAT YEAR was my vacation. In 1978 I had neither the position nor the funding to go off hunting dinosaurs for the whole summer at the university's expense, the way an established, degreed paleontologist would. I did, however, have four weeks off in July.

I went straight to Rudyard, Montana, where Bob Makela was waiting for me. Rudyard is on Montana's high line, the string of towns near the Canadian border, about 150 miles from the Rocky Mountains. It's not far from Shelby, where I grew up. Farming and ranching country spreads out all around these towns, flat and sparsely populated as most of Montana is east of the Rockies. You drive for miles between towns, and most of them are just a flicker of buildings as you drive through. The land is dry, and it's usually farmed that way, without irrigation. It produces wheat and cattle, and yields numerous fossils of Cretaceous shellfish.

Bob had been teaching high school science in Rudyard since he graduated from the University of Montana, where we first met. He and I were both interested in dinosaurs, and we'd both gone out on university-sponsored field trips in geology and paleontology. We became good friends. Bob was a remarkably forceful man. He was energetic, outgoing and optimistic. He loved hard work and nothing intimidated him. In fact, some things that should have intimidated him didn't. The first time I saw him, he walked into a herpetology class wearing a T-shirt and cradling a gila monster in his arms. Gila monsters are quite poisonous, and most people don't carry them around unless they've got some kind of protective clothing—gloves, or a heavy shirt at the least. But Bob didn't seem to mind the gila monster, and the gila monster didn't seem to mind him.

I don't think of myself as a milquetoast when it comes to reptiles, but Bob far surpassed me in that department. He worked in the herpetology laboratory, and sometimes when Carolina, the seven-foot Eastern diamondback rattlesnake, got loose in the lab, he'd go in and poke around under the desks and chairs until he found her and caught her. I gave him moral support. (The other thing that escaped in that lab was a bunch of baby black widow spiders. When they hatched, they were too small to be deterred by the screen that kept their mother in her cage. Sometimes the herpetology professor, who had his office in the lab, would be sitting at his typewriter and across the paper, on all eight legs, would walk a baby black widow.)

Bob and I began collecting dinosaur fossils in the mid-sixties, and we kept at it. I think we were ideally suited to each other. Both of us treated the explorations as an adventure; both of us were willing to do it on our own, whether or not we had the support of a university; and neither of us minded the physical demands of hiking, digging and hauling. In later years, when we had a big dig going, Bob's knack for dealing with people balanced my reticence. He was the person the volunteers on a dig would gather around naturally in the evening to listen to stories of Montana, or dinosaurs, or grizzly bears—whatever stories Bob wanted to tell. I think Bob loved conversation as much as he loved paleontology. It was his teepee that had the fire in the middle where everybody would get together on a cold night for beers. I was usually there, but I wasn't the host. Bob was the social, emotional and

Augusta Choteau Fossils

The large box indicates area of the Two Medicine formation in Montana (see page 37); small square, the Willow Creek anticline (page 95).

organizational anchor of the dig. If I provided the scientific direction, the paleontological planning, what you might call the head, Bob provided the heart.

From Rudyard, we drove southeast through the wheat fields and cattle ranches. We were headed to the central part of the state, near Billings. The land there is also flat. The horizon is wide. And underfoot is the Bear Paw shale. As I said before, shale is a marine sediment. And we find it in Montana because during the Cretaceous period the center of North America was occupied not by grasslands but by the interior seaway I described earlier. Over millions of years, this sea would rise and fall, expanding and contracting, leaving bottom muck that eventually turned into shale. At its largest, the seaway extended all the way from the Arctic Ocean to the Gulf of Mexico. It was shallow and filled with seagoing lizards, sharks and other fish. It probably looked something like Florida Bay, the island-dotted, biologically rich finger of the Gulf of Mexico that stretches between the southern tip of mainland Florida and the archipelago of the Florida Keys. At the end of the Cretaceous period, 65 million years ago, this sea finally dried up for good, and left behind the old sea bottom that has become the Great Plains.

The last shale deposit left by this seaway in Montana and Alberta is the Bear Paw shale, patches of which are found throughout the area. The patch near Billings was where Earl Douglass had found the juvenile dinosaur fossils. And it was the place where I hoped Bob and I would dig up a baby or two. We might have, too, if the weather had been with us. But when we got to the site, the sun disappeared, the temperature dropped, and it poured rain for three days. Wherever we walked, we sank above our ankles. Shale is an odd rock when it comes to rain. It suffers a kind of identity crisis and turns back to mud. Each time I pulled my foot out of the muck, I brought a pound or two of the old Cretaceous sea bed with it. It was like walking in a vat of warm licorice, and that's no way to look for fossils. We got in the van and drove away, with no dry shoes, and no baby dinosaurs. I hadn't given up. I planned to go back. But I was going to wait for the kind of summer weather more common to the Montana plains—100 degrees with 10 percent humidity.

We ended up at the Milk River badlands near Rudyard. Like a lot of the American West, Montana has plenty of land that's been scoured clean, and in the summer it's practically crawling with paleontologists. You see, the whole of a paleontologist's professional life is connected to either deposition or erosion. You want deposition to preserve the fossils and then erosion to expose them for you. You don't hunt fossils in a lush forest; you want something to have removed the trees, brush, topsoil and a good bit of the rock to get it ready for you.

The reason we went to the Milk River was that Bob and another science teacher, Larry French, had found some mammal fossils there and we wanted to show them to Bill Clemens, a specialist in early mammals. In a sense, being a paleontologist is like being a member of a club, and club members tend to help each other out. We also help each other because in modern science everybody has a narrow area of specialty; if you find something in somebody else's specialty, you send it on to them or show them where you got it from. As it turned out, Bill and his crew (from the University of California at Berkeley) were quite pleased with the find. It proved to be a very rich deposit, and in a kind of turnabout Bill mentioned to us that a rock shop in Bynum had a dinosaur fossil that the owner wanted identified. He was going to do it, but we were the ones who were really interested in dinosaurs and there was always the chance it might be something good. When the time came for us to leave Rudyard and the Milk River to go to work for a few days on yet another paleontological dig farther south in Montana (on this one a friend was collecting fossil fish), we took a detour through Bynum.5

Bynum is as small as a town can get before it just becomes somebody's house on the road. It had, in 1978, a gas station/store, a few buildings, and an old faded church that housed a rock shop. It was Sunday when we got there, but the rock shop was open because the owners were Seventh-Day Adventists and their sabbath is Saturday. The shop was like a lot of other little rock shops in the West: cluttered and dusty, with rock samples, gems, geodes and fossils, all for sale. The fossil in question turned out to be common enough. It was part of the backbone of a duck-billed dinosaur. We were in no hurry, so we wandered around the shop, picking out all the fossils that had been misidentified and giving them the correct identification.

The owners of the shop were Marion and John Brandvold, who now have a small tourist museum and gift shop in Choteau, Montana, a much better location. That Sunday morning, Mrs. Brandvold was running the shop. She's a dark-haired, sharp-featured woman given to fringed jackets and other Western garb. She was delighted that we were identifying everything, and when we told her there was no charge for our service, she asked us if we could identify some bones that she had in her house. She and her family had collected them earlier that year, in the spring. She went back to the house and brought out two specimens. They weren't much to look at, just two dusty pieces of gray bone, but it was immediately obvious to me that they were the hip end of a duckbill thighbone and a bit of a rib—except that they were the wrong size. The femur, or thighbone, of a typical duckbill might be four feet long and as thick as a fencepost. The femur that Mrs. Brandvold handed me, if the bone had been whole, would have been the size of my thumb. It was broken, and all that remained was a piece an inch long.

What I had in my hand was a bone from a baby dinosaur, a duckbill—exactly what I wanted, in a place I never expected to find it. And it wasn't the only one. Mrs. Brandvold took us into her house, and there, spread out on a card table, were numerous small bones. The first thing Bob and I noticed was a jawbone about two inches long. Paleontologists love jaws. They have so much detail to them, and you can learn a great deal about a jaw's owner—what he ate, for instance. Jaws are also completely unmistakable. It requires a practiced eye to see what a piece of a femur is, but a jawbone is a jawbone. An adult duckbill jaw would run about three feet long. This one was two inches long. Bob had been skeptical about the femur, but the jaw convinced him.

We told Marion Brandvold how important the fossils looked, and she agreed to give them to us. She filled up a coffee can with them and presented it to us. When we sorted out the bones later, we could tell by counting the legbones that we had the remains of at least four baby duckbills.

That was enough, in itself, to qualify as a big paleontological find. Certainly for a preparator and a high school teacher, it was a terrific find. But, in paleontology as in life, enough is never really enough. There had to be more where those fossils came from, and the

question that was foremost in our minds, the question we asked Marion Brandvold as soon as we realized what we were looking at, was: Where did they come from? The answer was most definitely not Douglass' site near Billings. Marion Brandvold had found those ba bies somewhere else, in an area that I knew well but not, apparently, well enough.

THE FOSSILS HAD COME from the Two Medicine formation, a

2,000-foot-thick wedge of sandstones, shales and mudstones stretching over a huge, ragged patch of Montana east of the Rockies.6 A formation is the basic unit used by geologists to map and catalog the earth's layers of rock. Its vertical and horizontal boundaries are determined by the characteristics of its rock beds. A given formation might, for instance, have an identifiable sequence of shale and sandstone layers derived from a particular sea. For the rock beds to constitute a formation they must, in the nature of the rock itself and in the way they fit together, stand as a unit, apart from their surroundings. There are no size limits on formations, but they are usually pretty large affairs. You could safely say that a formation is bigger than a bread box and smaller than a planet.

The surface of the Two Medicine formation covers 3,600 square miles, running from the Canadian border in the north to Augusta, Montana, in the south and from the Rocky Mountains in the west to the town of Choteau in the east. For the most part the exposed sections of the formation, where one can get at the rock, are on privately owned range land; one big section lies in the Blackfoot Indian reservation just east of Glacier National Park. The surface, however, is perhaps the least important dimension in geology and paleontology. More important is the vertical dimension. In a rock formation, depth is really a measure of time. Vertical feet or meters are a kind of record of the centuries and millennia that had to pass in order for stream sands, sea mud and other sediments to be deposited, buried under other sediments and eventually transformed by pressure into rock. This understanding of sedimentation, that it marks the passage of enormous stretches of time, is necessary for paleontology to exist as a science.

Two Medicine Formation

The Two Medicine formation, shaded light gray, lies just east of the Rocky Mountains in northern Montana. Covering 3,600 square miles on the surface, the formation comprises 2,000 vertical feet of rock laid down over a period of 12 million years.

It's also necessary to realize that rock is deposited in chronological sequence. One may have to deal with disturbances, earth movements that bend, twist and overturn perfectly laid down rock layers, but in undisturbed sedimentary rock what's on the bottom is the oldest and what's on the top is the youngest. The 2,000 vertical feet of the Two Medicine formation document the passage of roughly 12 million years, which is a big enough portion in the history of dinosaurs to track the processes of evolution—to watch new species come and go.

The formation is made up of Cretaceous rock. That is to say, the sediments originated in the Cretaceous period, which began about 140 million years ago and ended 65 million years ago. This span of 75 million

years occurs quite late in the history of the planet, which is probably four and a half billion years old, and late in the history of life, which may have begun more than two billion years ago; it's even fairly late in the history of dinosaurs, which first appeared a little more than 200 million years ago.

The time span of the Two Medicine formation (laid down in the late Cretaceous, from about 84 to 72 million years ago) is best measured not by years, but by the rise and fall of the inland sea. This is the body of water that I described earlier as expanding and contracting. The Two Medicine formation began during a contraction, or recession, that is called, in its northern reaches, the Colorado Sea. It had reached all the way to the Rocky Mountains and had begun to shrink. It's hard to say whether the Colorado Sea actually reached into the mountains and filled valleys there, because we don't know precisely where the mountains were at the time. The Rockies were then very young and in the process of growing, which means there were frequent volcanic eruptions and earthquakes as the planet shuddered and cracked and thrust the mountains up. What we can say is that they were somewhere between where they are now and 50 miles west of that line. Certainly the Colorado Sea went deep into where the mountains are now. Then, as it receded, the sea left deposits of muddy shale and of beach sand that turned into sandstone.

This beach sand deposit, called the Virgelle Sandstone, marks the bottom boundary of the Two Medicine formation. As the sea receded, a long coastal plain opened up—extending from the mountains to the sea, eventually a distance of 200 miles or more. This plain was richly populated by many varieties of dinosaurs. As the mountains thrust and bulged, they shed enormous amounts of dust and rubble that were carried by streams and rivers down to the plain. The streams and rivers overflowed frequently, leaving sand and mud on their floodplains. Over millions of years, such floods deposited enormous amounts of sediment. In addition, the thrusting of the mountains

A vertical diagram delineates the 2,000-foot wedge of the Two Medicine formation.

pushed down the land between them and the sea into what is called a foredeep. If, while you're sitting in bed, you scrunch up the blanket to push up a hill, you'll notice a dip in the blanket just on the other side of

The upthrusting Rocky Mountains (at left) created the foredeep in which the rock deposits of the Two Medicine formation accumulated.

the hill. This is more or less what happened on a grander scale with the Rockies and the Two Medicine formation. The Rockies were the hill and in front of them was a dip, or foredeep, that became the Two Medicine formation. Onto this foredeep was deposited first the muck and sand of the receding sea, and then huge amounts of silt and sediment from the rising mountains. With all this sediment, the foredeep sank even more.

Eventually, the sea stopped receding, and after a time it began a new stage of expansion called, in Montana and Alberta, the Bear Paw Sea. As it expanded, the sea inundated the coastal plain. The sea bottom muck fell on top of river and stream silt deposited earlier, and over millions of years that muck turned into the Bear Paw shale. Today the Bear Paw shale does not exist everywhere the sea once existed. Erosion has occurred in some spots, and in other spots the level of the shale is so far beneath the surface of the land that we don't know

In the late Cretaceous, the dry upper coastal plains of western North America duckbills (background) to the large, fearsome Albertosaurus {middle distance)

were dominated by dinosaurs; from Euoplocephalus (foreground) and the and fast, graceful Ornithomimus, or "bird mimic" (far distance).

whether it's still there or not. So today in Montana you find patches of this shale. The portion of the Bear Paw shale that Bob Makela and I had gone to explore, looking for babies, is one such patch. Another patch forms the upper boundary of the Two Medicine formation.

What the Two Medicine formation preserves, then, is the record of 12 million years of life on a coastal plain. During that time the plain grew and shrank as the size of the sea changed, but we know pretty much what the circumstances of life were for the dinosaurs that lived there. On the upper part of the plain, nearer the mountains, the land was dry. To the east, the sea was probably 200 miles away. To the west, I would guess the mountains were 60 miles away. There was little rain; the land was semiarid. There was no grass; it hadn't yet evolved. But, from the remains of pollen left in rock, we know there were flowering trees (dogwoods), evergreens, berry bushes and huge, palmlike plants called cycads. There were numerous small streams with heavy vegetation on their banks—mostly dogwoods and evergreen trees, I suspect. Today, in this kind of landscape, we would expect grass to fill the large, flat expanses between the streams. Then, there were fruited plains of berry bushes. In spots (and at certain time periods during the 12-million-year span) there were shallow lakes that dried up each year to leave a dry, crusted surface, or hardpan.

If, during the time when the plain extended a full 200 miles, you were to have walked east from the mountains, you would have noticed a very gradual change in the land. As you crossed the plain, you would have felt yourself to be on flat ground—perhaps not quite as flat as Indiana or Kansas, but as flat as it is today in Montana near the Rockies. Slowly the narrow streams would have widened and joined with other streams to form meandering rivers bordered by swamps. The land would have become greener and dotted with ponds. The vegetation and the dinosaurs would have become more various. And after you had walked about 200 miles, you would have reached the sea itself.

It is the upper part of this plain, the dry land and small streams, the dogwoods, evergreens and berry bushes, that is preserved in the Two Medicine formation. Several kinds of dinosaurs lived on that plain, in that vegetation. The predominant ones were duck-billed dinosaurs, feeding on the evergreens and bushes. There were also small, graceful dinosaurs—the hypsilophodontids. And there were the ceratopsian or horned dinosaurs, as well as large carnivores that looked something like Tyrannosaurus rex.

The lower, greener, swampier part of the plain, from exactly the same time and also preserved in rock and fossils, is called the Judith River formation. Similar sorts of dinosaurs would have lived here— duckbills, ceratopsians, carnivores—but the more varied and abundant plant life would have meant a greater diversity of animal life as well. For the dinosaurs, that meant more species and much larger populations. The reason this lower plain is called by a different name and considered separate from the Two Medicine formation is that between the two formations is a dome, a bulge in the skin of the earth. Like a bubble in a viscous liquid coming to a boil, this bulge has been rising up intermittently since before the Cretaceous. It probably bulged most vigorously during times when the mountains were thrusting up. The result is that the middle of the plain we've been talking about is not preserved. Instead we find this bulge, known as the Sweetgrass Arch. The deposits that built up the coastal plain may have drifted off the dome as soon as they landed on it, or they may have been shed during a later thrusting up of the dome. They may have been washed away, or blown away, or scraped off by the glaciers—the bulldozers of the Pleistocene—that scoured all of Montana. Whatever happened to them, these deposits are gone, and what sits on top of the dome is the old bottom of the Colorado Sea, complete with fossil clams and snails.

Today you can find the eastern end of the Two Medicine formation and the beginning of the Sweetgrass Arch in the town of Choteau. The arch extends east about 60 miles, all of it flat. At the end of that stretch you come to the remnant of the coastal plain—this time the lower coastal plain—known as the Judith River formation. And again, instead of fossils of marine shellfish, there are dinosaur bones.

IN HINDSIGHT THERE WERE numerous clues to suggest that the Two Medicine formation would have been a good place to look for baby dinosaurs—in fact, a much better place than the Bear Paw shale. Certainly, everybody knew that early paleontologists had taken a lot of dinosaurs out of the Two Medicine formation. And there had been reports that some of these dinosaurs, originally identified as adults, were actually juveniles. In 1976, for example, Peter Dodson of the University of Pennsylvania published a paper on fossils found by several paleontologists in the Two Medicine and Judith River formations and previously identified as adults; he had studied the fossils himself and had determined that they were really juveniles of a completely different species.7

That was one clue. Another came from C. M. Sternberg, one of the great dinosaur field scientists, who had written a paper on fragments of young dinosaurs recovered from a formation in Alberta. This formation preserved an upper coastal plain of the same age as the Two Medicine formation. Sternberg argued that just such uplands might be the place to look for young dinosaurs. Referring to the fragments of young he had found as well as the rich deposits of young from Mongolia, he wrote:

Many fine dinosaurs and other vertebrate fossils have been collected from the delta deposits of the [Judith River formation] along Red Deer River in Alberta, but very few fossils ofjuvenile dinosaurs have been reported. With the great number of experienced collectors who have examined these beds, surely some eggs and many remains of juveniles would have been reported if the dinosaurs had hatched and spent their whole lives on the deltas or in the swamps.

After describing the lowland deposits, Sternberg pointed out:

... no skeleton or skull of a very young dinosaur has been reported from these beds. This coupled with the fact that the upland deposits of Mongolia yielded so many dinosaur eggs and juveniles, leads one to believe that the dinosaurs laid their eggs on the upland and only the more or less mature animals of certain forms inhabited the deltas and flood plains."

Both of these are, of course, isolated bits of information that there would have been no reason for me to connect, or even notice, back in the winter of 1978 when I was cataloging fossils from marine sediments. There was, however, another clue that surfaced just the summer before Bob Makela and I stumbled on the fossils in Marion Brandvold's rock shop. That piece of evidence was an intact dinosaur egg, the first found in the Western Hemisphere. I found it myself. And I found it in the Two Medicine formation.

My father and I had gone exploring for fossils. Our trip was something of a replay of one we had taken when I was a child; it was in the same formation that I had found my first dinosaur fossil, with my father, when I was seven years old. In 1977 we were again walking the ridges, looking for bones. I picked up what appeared to be a crushed lump of fossil bone and took it with me. I had no idea then what it was. Over that winter, however, I realized that it was a dinosaur egg, although I didn't know what kind of dinosaur it had come from.

Now, it might seem that if one found the first dinosaur egg in the Western Hemisphere, one would certainly go back and look for more.

And perhaps I should have. But one of the odd things about paleontology is that you can find one of anything almost anywhere. On that field trip when I found the egg, the Two Medicine formation appeared to me as it always had before—bare. Fossils were very, very hard to find, and there were no indications of substantial deposits. I had found one egg, but I might well have gone back and spent summer after summer walking over it and never have found another. What paleontologists look for is a pattern—not single fossils, but hints of widespread fossil deposits. To me the egg was an anomaly, whereas I had found a clear pattern in the fossils from marine sediments. The Bear Paw shale, obviously rich in fossils ofjuvenile dinosaurs, seemed to me the place to go. After all, I had not come up with the idea of looking for fossils of juvenile dinosaurs and then set about to see where I might find them. I had stumbled on a predominance of such fossils in the Bear Paw shale, and it was that predominance that gave me the idea of looking for babies in the first place.

Only much later did I see the pattern that tipped off the presence of these rare fossils in the Two Medicine formation. Only much later did I realize how likely a spot for babies the Two Medicine formation was. During the winter of 1979, after the trip to Marion Brandvold's rock shop and our follow-up of that trip, I checked all the fossils from the Two Medicine formation in the collections of Princeton University, the American Museum of Natural History and the Smithsonian Institution. Then I found out what I hadn't realized before—and what I think nobody else had realized, either. Eighty percent of those fossils were of juvenile dinosaurs, many of them unrecognized for what they were. When the fossils had been found originally, many had been identified as adults of new genera and species, or not identified at all, and shipped back to museum cellars. For example, in the 1930s the paleontologist Charles Gilmore found seven or eight small duckbills in one pit in the Two Medicine formation. He abandoned the hole, even though he thought he might find more, because the skeletons were all the same size and of the same kind of dinosaur.9 Gilmore treated the skeletons like trout for the frying pan. He had enough, so he quit. It did not occur to him that he might have found some kind of social grouping of young; he didn't think they were young at all. He reported them as small adult dinosaurs of the genus Procheneosaurus, which had been described a few years before. Not so. They were young dinosaurs, though of what genus I'm not sure.

One can indulge in hindsight forever. There is no end of previously undiscovered clues and patterns. But the simple fact is that those first duckbill babies were found, not by painstaking analysis on my part, but by sheer luck. Marion Brandvold was lucky to find them. Bob and I were lucky to find her. And we were perfectly happy to give up our plan to go back to the Bear Paw shale. In paleontology, as in all sciences, as in all of life, you don't argue with luck. Marion Brandvold had discovered a lovely little window on the late Cretaceous. What we did was to open that window and climb through it.

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