Our attempt to drain a portion of Four Horns Lake to collect a mosasaur skeleton

covered that the mosasaur skeleton was encased in large nodules of marine limestone that had formed within the layers of shale. Many of the bones were encased in their own nodules, however. Excavating them was simply a matter of mapping their location and carrying them to shore. Other, more fragile sections we covered with plaster jackets, a practice we usually follow to prevent breakage when we remove fossils from the ground and carry them elsewhere. It took several long, wet days to transfer the specimen from the lake to the back of my truck.

The mosasaur now rests on a shelf in the basement of the Museum of the Rockies, awaiting preparation, along with more jacketed specimens than I care to contemplate at the moment. What I couldn't have anticipated back when we excavated that primitive seagoing reptile, and what I sometimes marvel at now, is the profound effect it eventually had on the direction my research took during the rest of the 1980s. I certainly believe in the adage that chance favors the prepared mind, but even the best prepared are unlikely to go far in the complete absence of such favors, especially in a non-mathematical, historical science like paleontology, where fieldwork still serves as the foundation of all that we do. The paleontologist never knows what awaits him over the next hill or during the next season, nor toward what new horizons those discoveries may show the way. In 1978, Bob and I stopped at a rock shop in the tiny town of Bynum, Montana, north of Choteau, motivated by nothing more than curiosity and a tip from our Berkeley paleontology friends. Seeing that we knew something about fossils, the owner, Marion Brandvold, asked us to identify two bones she had stored away. She then handed me the thighbone and jaw of a baby duck-billed dinosaur, which she said she had collected on Jim Peebles's cattle ranch near Choteau—the Willow Creek anticline. I had found what I wanted most to find in the last place I expected to find it. In 1984, another happy accident happened: Marvin Weatherwax's friend chanced upon a funny looking skeleton in the bottom of Four Horns Lake. And promiscuous curiosity outweighing hard-earned skepticism, as always, Bob and I visited the lake, an event that would be the key to gaining access to an area I had long wanted to excavate. Not right away, though. Fieldwork prospects were in fact looking bleak that inclement spring.

Just about the time that we were excavating the mosasaur, I received another call, this time from Jim Peebles, who informed me that, as much as they hated to do do it, his family could no longer allow my crews onto the Willow Creek anticline. I was shocked. Fourteen students and volunteers had already signed up for the 1985 summer season. Supplies and equipment had been readied. Worst of all, I didn't know where else I might turn to find more eggs, nests, and babies. At the same time, however, I appreciated the Peebles family's predicament. Word of our discoveries had spread. Amateur collectors routinely trespassed on their land, growing ever bolder in their unauthorized search for the fossil treasures of the now-famous site that Bob had named Egg Mountain. The situation had grown unmanageable, the probability of an accident or unfor tunate encounter increasing daily. After five years of putting up with me, Bob, and our crews traipsing all over their land, they simply didn't have the patience to deal with the crowds arriving at their front door. Who can blame them?

That said, my concerns now lay elsewhere: I had less than three months to find another site. The most reasonable approach, given the little time available, was to identify areas that met the criteria we had determined were critical to our success at the Willow Creek anticline, which had taught us much about the habitats of the two ornithopods we had found there—the new duck-billed dinosaur, Maiasaura, and the smaller, fleet-footed Orodromeus, a new hyp-silophodon—and the theropod Troddon. The most promising new locations would be the sedimentary remnants of upland coastal plains from the Cretaceous period. The sediments would consist primarily of red and green mudstone, like those at the anticline, and contain caliche, limestone nodules that form beneath the surface as groundwater fluctuates, depositing calcium carbonate each time it rises. The location of caliche nodules indicates how wet a climate is: the more rainfall, the nearer to the surface the nodules of calcium carbonate accrete. The size of the nodule, on the other hand, reflects the length of time that the soil above it remains stable. If the surface level changes (through erosion, say), the elevation of underlying groundwater shifts and a new caliche layer forms. Caliche layers provide a surprisingly precise way to determine whether different groups of fossils were deposited on the same soil surface, or hori-zon—that is, at the same time, a crucial element in the study of the social behavior of dinosaurs.

A few weeks later, in May, Mick Hager, who was director of the Museum of the Rockies at the time, and I were invited to a site called Devil's Pocket near Harlowton, in central Montana, that met my criteria. Though from the air Devil's Pocket looks for all the world like a meteor crater, it is in fact a different though relatively common structure called a dome—a rock formation, in this case, layers of sedimentary rock, that has bubbled up on the surface under pressure from rising volcanic magma deep within the earth's crust. The dome at Devil's Pocket formed so long ago that its center has been eroded away, producing a bowl-like feature about a half-mile wide. Exposed along the rim of the bowl are walls of red and green mudstone containing caliche. The sediments are no million years old, deposited early in the Cretaceous period, and part of an upland area of the coastal plains called the Cloverly Formation.

John Ostrom had explored Devil's Pocket in the 1960s and found nothing, having arrived, evidently, at the wrong time in the erosion cycle, a slow process in central Montana, where precipitation is scarce and decades pass before new fossils are exposed at the surface. But thirty years earlier Barnum Brown, whom chance always seemed to favor, discovered and excavated the remains of a Sauropelta, an ankylosaur, or armored dinosaur. Sauropelta lived at the same time and in the same general environment as Deinonychus and Tenontosaurus, a primitive, plant-eating ornitho-pod upon which Deinonychus preyed. That Tenontosaurus belongs to the same suborder as Maiasaura gave me reason to believe that it could have nested in colonies and cared for its young, just as Maiasaura did. Devil's Pocket, I thought, might contain eggs and babies.

On a scouting trip late in May, our host, Don Rassmussen, an oil geologist from Denver who had grown up in Harlowton, happened upon a microsite, which is a collection of small fossils, usually sorted according to size but representing different animals, that were washed together by a stream after the animals died. Weathering out of the mudstone at Don's microsite were numerous teeth—from Deinonychus, a sauropod of unknown identity, a mammal, and a lungfish—along with a variety of snail shells. Mick, for his part, reenacted Brown's discovery, finding a partial Sauropelta skeleton. I had the good fortune of collecting a single Tenontosaurus toe bone. Mildly encouraged, I sent Bob and his crew to Devil's Pocket on June 12. During the next two weeks they dug up Mick's Sauropelta, part of a sauropod that Don had found during a previous visit, another sauropod's tail, and the partial skeleton of a juvenile Tenontosaurus. I collected some eggshell shards, the first ever from the Cloverly Formation, but too small and too few to identify. There was little else to be found. As tanta-lyzing as the Cloverly was proving to be, erosion would have to scour the rim of Devil's Pocket for a long time to come before additional fossils were exposed. I still hadn't found a suitable alternative to the Willow Creek anticline.

The previous summer Jill Peterson, a student at the University of Colorado, and I had conducted a preliminary investigation of a site south of Choteau called Red Rocks. Like the sediments of the Willow Creek anticline, which are 76.7 million years old, Red Rocks is part of the Two Medicine Formation but from older, lower strata in the geological column, dating to about 80 million years ago. Jill and I had found a protoceratopsian skeleton, baby bones, and a handful of eggs. Perhaps there was more. Toward the end of June, while Bob and his crew finished their work at Devil's Pocket, I sent another, smaller crew to Red Rocks. They found lots of unidentifiable eggshells and baby bones, as well as a duck-billed dinosaur and a horned dinosaur. But Red Rocks, I soon realized, resembled Devil's Pocket, in that it was a relatively small outcrop in which few fossils had been exposed. I was glad to learn, however, that eggs and babies could be found in the lower half of the Two Medicine Formation. Charles Gilmore and others had insisted that only the upper half contained dinosaur remains.

Just like that, June was over. Frustration soon set in, since it was beginning to look as if we were going to spend the entire 1985 season wandering from one disappointing dig to another. My thoughts returned to Gilmore's field diaries from Landslide Butte. In addition to the two new species of horned dinosaur, Brachyceratops mon-tanus and Styracosaurus ovatus, he had collected juvenile dinosaurs and eggshell fragments. He even mentioned having quit excavating one quarry because it contained several skeletons of a single species, all of them the same size—small, which he interpreted as the adult form of a small duck-billed dinosaur. But I'd seen the skeletons; they are instead the juvenile form of a duck-billed dinosaur of average size—in other words, a group of young dinosaurs, exactly what interested me most. Apparently Gilmore, like his contemporaries and many collectors today, would have considered further digging worthwhile only if it promised to yield a single skeleton of a large individual. As for me, I'd much rather devote a week to collecting dozens of specimens that fit into Ziploc bags and provide clues to dinosaur social behavior and evolution than a year to excavating a single specimen that tells me little more than that I now have in my possession a skeleton too large to house at the Museum of the Rockies.

Convinced by Gilmore's discoveries and intriguing misinterpretations—remember, paleontology often thrives on both—that the Landslide Butte badlands held much promise, I had approached the Blackfeet Indian Tribal Council on several occasions, starting as early as 1982, seeking permission to collect fossils in the extreme northern region of the reservation. Each time the council had turned down my request, in the belief that our activities, the comings and goings of field crews, the exploration trips, and the excavations themselves, would damage the land or disrupt the lives of reservation residents. Early in July, desperate to salvage what was left of the 1985 season, I went before the tribal council once again. This time, however, I found support—from Marvin Weatherwax. He had witnessed the excavation Bob and I had completed at Four Horns Lake only a few months earlier. He had seen how little land we actually disturbed during the operation. And he persuaded his fellow council members that we could be trusted. They then granted us permission to explore and collect not only in the area near Landslide Butte, to which I had limited my request, but on the entire reservation. Like a 200-million-year-old marine version of a good luck charm, the mosasaur had unexpectedly opened the door to one of the richest fossil deposits in the world.

The six-mile expanse of badlands that lies between Landslide Butte and the Canadian border represents the top three hundred feet of the Two Medicine Formation, the uppermost layers of which are about 74 million years old. The strata consist of, among other things, silty, caliche-laden mudstone, red beds as well as green beds, all of which are the remnants of extensive floodplain deposits along the eastern front of the Rockies. Resting immediately above the Two Medicine Formation is the Bearpaw Shale, the same unit of marine sediments from which Bob and I had recovered the mosasaur. The shale is widespread, too, allowing us to make precise measurements of the distance between the top of the Two Medicine Formation, which the shale always defines and which we therefore adopted as our datum layer, and any dinosaur remains located at lower, older levels of the strata. And with that information we were able to determine the age of particular dinosaur specimens, and whether dinosaurs removed from different sites lived at the same time.

As you can see from the cross section back on page 47, the Bearpaw Shale was deposited during the last transgression of the Western Interior Cretaceous Seaway. Try to bring into focus again the implications of this event. About 75.4 million years ago, the coastal plain is as wide as it has ever been—maybe as much as four hundred miles from the foot of the still-growing mountains to the edge of the sea. The water starts to rise, the plain starts to narrow. It is a slow process, exceedingly slow by any human reckoning, but immensely consequential for the plants and animals that live on land, including the dinosaurs—ornithopods, ceratopsians, theropods, ankylosaurs—that now dominate most terrestrial habitats. By the time the sediments of the upper Two Medicine Formation are deposited, 74 million years ago, the coastal plain has shrunk to one-eighth or less of its original size. It is only thirty to fifty miles wide now, and into that much smaller space all of the dinosaurs are cor-ralled—predator and prey, enormous herds of duck-billed dinosaurs and horned dinosaurs, and the cousins of Tyrannosaurus, which, we can be certain, never wanted for a carcass to scavenge. Etched into the badlands of Landslide Butte is a record of this critical time in the evolutionary saga of dinosaurs.

Another picture that you might keep in mind, and which will help you make sense of the chart, is that as the seaway advanced westward, toward the Rockies, silt and other fine-grained material settled to the bottom, directly atop the sand and mud that had been deposited on land by streams and rivers flowing out of the mountains, and in time those marine deposits turned to rock—the Bearpaw Shale—as did the terrestrial deposits, which, of course, are what we now call the Two Medicine Formation. As time passed and the seaway continued expanding, more and more terrestrial deposits were buried beneath marine deposits. All of this occurred tens of millions of years ago, and geological processes being what they are, what we actually see today is not nearly this simple. Indeed, if you think about it, were it only a matter of the orderly deposition of sediments, what we would see today would consist solely of the most recent deposits—the rock we call the St. Mary River Formation along what was the upper part of the coastal plain and the Hell Creek Formation along what was the lower. Any marine deposits would be hidden deep underground.

Land surfaces, of course, are subject to a range of forces, some extremely violent, that alter their structure and composition. The sedimentary rock along the Rocky Mountain Front was uplifted and tilted, folded and fractured. Erosion occurred at different rates in different places and exercised different effects on different types of sediment. Some areas were buried in volcanic ash. Others were carved by glaciers. As a consequence, traveling west to east today or, for that matter, north to south, you may encounter rocks of vastly different age and makeup sitting side by side. In some cases, the rocks have been so jumbled up that it's nearly impossible to tell one from the other. Sometimes an outcrop of one formation will rest like an island within a sea comprised of the rock of an entirely different formation. A good example is the section of Bearpaw Shale that forms the bottom of Four Horns Lake. Among the most prominent geological features of north-central Montana today is the Sweetgrass Arch—a dome, like the one at Devil's Pocket but larger and more pronounced, and which extends east from Choteau about sixty miles. Along this bulge all the terrestrial sediments of the coastal plain have been eroded away, exposing the Colorado Shale, which was deposited during the first expansion of the seaway and thus contains countless marine fossils. Because the Sweetgrass Arch divides the plain in two, the lowlands have been given a different name, the Judith River Formation, than the uplands, the Two Medicine.

After the tribal council granted us access to the Blackfeet Reservation, we next had to secure the permission of the individuals who live in the areas where we wished to collect. One of these was Ricky Reagan, an Indian who ranches along the Milk River in the middle of the Landslide Butte badlands. Ricky is a rodeo cowboy and likes to brand calves the old-fashioned way—by chasing them down on horseback and lassoing them with a rope. He also puts off branding a few weeks longer than most Montana ranchers, allowing the calves to grow larger, stronger, which makes them more fun to wrestle. That's what Ricky and his neighbor Arne Johnson were doing when I arrived to talk to him about hunting for dinosaur fossils on his property. Although clearly valuing the ranching life as much for the privacy it offers as anything else, Ricky generously consented, opening his land to a scouting party of that ragtag band known as paleontologists, which was a relief to me. At long last, I would be able to explore the area that no member of my profession had seen since Gilmore visited decades before.

The expedition began on a bright note. Early in August, while scouting for a place along the Milk River to set up camp, Bob came across a promising bone bed, although, at the time preoccupied with matters of access and comfort, he promptly forgot its exact location. Several days later, Jill Peterson, my son Jason, who was thirteen at the time, and I rediscovered the site—a layer of sandstone about three feet thick that capped a small butte. Dino Ridge Quarry, as we called the area, contained the remains of what turned out to be a new species of horned dinosaur that would eventually be named Einiosaurus. A late Cretaceous ceratopsian that grew to eighteen feet in length, Einiosaurus weighed about two and a half tons and possessed a single, imposing nose horn. From its neck shield two long spikes were arrayed. Our efforts to excavate the site were stymied, however, when the weather turned foul, then stayed that way—for weeks. My field notes covering the rest of August read like an advertisement for an umbrella company. Rain followed by relentless drizzle interrupted now and again by downpours. The sun abandoned us altogether. Sky and earth alike turned to mud, distinguishable from each other only by their color: wet-cement gray and saturated brown. Chill, damp winds howled through the gullies and outcrops of the badlands. If there was a dry piece of clothing in camp I didn't see it, and I certainly wasn't wearing it.

In the midst of these discouraging conditions, however, we managed to discover a second important bone bed. On an afternoon when I simply couldn't sit still any longer, I trudged out into the rain looking for something, anything, that might justify our continued presence at Landslide Butte. The muddy ground rendered hillsides inaccessible, so my search was confined to the flats near camp. To my amazement, only two hundred yards from where I had been stewing all day, I found, among other remains, an adult thighbone of a new species of Hypacrosaurus, a crested duck-billed dinosaur that in its adult form might have been as long as thirty feet from head to tail and weighed up to six thousand pounds. Crested duckbills are known as lambeosaurs, to differentiate them from such noncrested duckbills as Maiasaura, collectively referred to as hadrosaurs, so I named the bone bed Lambeosite. The end of the season was then fast approaching and in the little time that remained the crew could remove few bones from the two new sites. All the same, I drove back to the Museum of the Rockies certain where I would be digging the following year and more optimistic about the likely results than I'd felt about any excavation I'd started since leaving the Willow Creek anticline.

Anticipation was high when we returned to the Blackfeet Indian Reservation on June 20, 1986. There were sixteen of us—Bob, me, and fourteen students and volunteers, all itching to see what the teases of the previous season would lead to. We set up camp on the edge of the Milk River immediately north of Landslide Butte. Half of the crew then reopened Dino Ridge Quarry while the other half reopened Lambeosite, removing the dirt cover we always spread across our excavations as protection against the elements. At Dino Ridge Quarry it didn't take long to realize that we had unearthed an unusually rich bone bed. Before us lay an astounding concentration of fossils—vertebrae, ribs, arm and leg bones, teeth, skull fragments, as many as forty per square meter in some places, all mixed together haphazardly. With the exception of a few bones belonging to an unidentified juvenile duck-billed dinosaur and numerous tyrannosaur teeth, Dino Ridge Quarry was monospecific—that is, every bone came from the same kind of dinosaur, the new ceratop-sian. An entire einiosaur herd, numbering in the dozens and perhaps more, had died in the same place at the same time.

What had killed them? Years ago, the presence of tyrannosaur teeth would have led some paleontologists to conclude that the herd of horned dinosaurs had been attacked by the relentlessly predaceous theropod of popular imagination. But as I explained in chapter i, the anatomical evidence overwhelmingly suggests that Tyrannosaurus was a scavenger that possessed an exceptionally keen sense of smell and jaws well suited for tearing flesh but none of the other qualities required to chase down, subdue, and kill prey, especially a herd of two-and-a-half-ton behemoths bearing nose horns and shield spikes as long as a man's arm. No, what the teeth at Dino Ridge Quarry told me is that a number of very lucky tyrannosaurs had fed on the carcasses of a number of einiosaurs whose luck had run out.

The manner in which their luck ran out was later determined by Raymond Rogers, a graduate student in geology at the University of Montana, in a clever bit of reasoning that involved reconstructing the climatic and environmental conditions in which the einiosaurs lived and comparing them with similar conditions today. After analyzing the placement of the bones and studying the sediments and caliche layers in the area, Ray concluded that Dino Ridge Quarry represents a water-hole environment, a place where the herbivorous horned dinosaurs had taken refuge during an extended dry period along the upland coastal plain. As the drought worsened, vegetation dwindled further. Streams and shallow lakes dried up. More animals converged on the water hole, not only to quench their thirst but to forage on the plants surviving there. As those sources of food in turn became exhausted, the dinosaurs, now severely weakened, died of opportunistic disease, starvation, maybe even sunstroke.

Also undernourished because of the drought, various carnivores, including tyrannosaurs, found their way to the water hole to scavenge the many dead and dying animals. A large percentage of the fossil bones at Dino Ridge Quarry are scratched and broken, as if they had been trampled, adding credence to this idea. A virtually identical scenario can be observed during droughts in East Africa today: among large groups of elephants, for instance, that at first are attracted to a water hole because it offers both food and water, then linger too long, eating everything in sight and starving to death rather than leaving to find more food and thereby risking dying of thirst. Lambeosite resembled Dino Ridge Quarry in many important ways. It also was monospecific, containing only the bones of the crested duckbill, a new Hypacrosaurus, with the exception, once again, of an abundance of Tyrannosaurus teeth. And as was the case at Dino Ridge Quarry, the bones had been moved by water, but not very far, before they were buried. Finally, the overall sedimentary environment was very similar. Taken together, the evidence at Lambeosite suggests that there, too, a drought-related mass death had occurred.

As the two crews excavated Dino Ridge Quarry and Lambeosite, I surveyed outcrops throughout the badlands. Within two days I had discovered an ankylosaur skull and skeleton and the leg and foot of an Ornithomimid, an ostrichlike theropod unique to the late Cretaceous that stood about eight feet tall. In addition, halfway up the side of a small bluff I came across a narrow, lens-shaped deposit of mudstone that contained the remains of another duck-billed dinosaur. Judging from the skull parts at Westside Quarry (the name I gave this bone bed), which turned out to be monospecific as well, the animal appeared to be a previously unknown species of i.

Was this article helpful?

0 0

Post a comment