The gigantic dinosaur skeletons found at the Hell Creek Formation tend to get all the press, but they don't tell the full story. So Horner and his team are carefully unearthing smaller fossils that re-create the environment in which the dinosaurs at Hell Creek lived. The photo at left includes (clockwise from the top) fish vertebrae and skull pieces, lizard limb bones, ganoid scales (from a garfish), and Champsosaurus vertebrae. In the center are dinosaur toe bones. The largest is three inches long.That has long been the accepted scenario. But no one had studied the sediment record in enough detail to determine whether the animals and plants actually endured without variation prior to the extraterrestrial impact. Everyone simply assumed it. So when Horner was casting about for a potentially fruitful location to reconstruct an ancient ecosystem, he recognized that Hell Creek offered the opportunity to pursue a number of related lines of inquiry, including reexamining the factors that led to mass extinction. ""I looked all over the world for something compact and relatively simple,"" Horner says. A 3-million-year section would fit the bill, because it encompassed enough time for significant change, including evolution, to take place, yet would still be manageable from a research standpoint. In addition, Horner wanted sedimentary rock containing fossils of sufficient number, variety, and distribution to allow comparisons to be made between different levels—that is to say, different ages. Hell Creek had that going for it, and more. ""Another reason it's so good,"" Horner says, ""is because the Berkeley group had already collected from the top.""
By ""Berkeley group,"" Horner means William Clemens, a paleontologist at the University of California at Berkeley and one of the principal members of the Hell Creek Project. Last summer marked the 29th year that Clemens and his crews have worked in the area, searching for vertebrates, especially mammals, the group of animals that prospered most after the dinosaurs' demise. ""I'm really interested in mechanisms of recovery,"" Clemens says. That's why he has focused on the first million years of Tertiary Period deposits above the K-T boundary as well as the part of the Hell Creek Formation that lies immediately below it. The extinction pattern that has emerged was not expected. Whereas the last of the nonavian dinosaurs certainly perished 64.5 million years ago, many other animals survived—crocodiles, for example, along with almost all the mammals, which at the time were small, some with skulls the size of a thimble. The top of the Hell Creek Formation, just before the asteroid impact, contains fossils of some 16 turtle species, representing diversity comparable to what is found in Florida today. And all but one of them, a large galapago-like tortoise, survived. ""The evidence really makes you pause,"" Clemens says.
Clemens also points to a recent discovery made by Thomas Stidham, one of his graduate students. While working in a Late Cretaceous site in Wyoming, Stidham uncovered what appears to be the beak of a primitive parrot. That discovery, combined with other fossil finds in North America that are related to chickens, turkeys, flamingos, and loons, among others, suggests that most major groups of modern birds originated and began to diversify before the K-T boundary. Yet they, too, survived the extinction event. Finally, all the frogs and salamanders found in the Hell Creek Formation also show up in the more recent Tertiary deposits. ""When you think about global disturbances today,"" Clemens asks, ""what are our icons?"" Because of their vulnerability, frogs and salamanders are frequently the first animals to succumb to environmental degradation, and thus are now looked upon as the ecological equivalent of the proverbial canary in the mine shaft. Still, they, like crocodiles, turtles, mammals, and birds, survived the asteroid impact, suggesting that the mass extinction at the time may not have been as massive as previously thought.
Clemens's findings have captured the imagination of Nan Crystal Arens, a colleague at Berkeley who recently transferred to Hobart and William Smith Colleges in Geneva, New York. Over coffee one day a few years ago, Clemens described the host of animals—a veritable Noah's ark—that mysteriously survived one of the most traumatic events in the planet's history. For Arens, a plant paleoecologist who believes that the fossil record should be viewed through a modern biological lens, the findings could be exploited to investigate a long-puzzling question. ""Present-day alligators and crocodiles are very sensitive to cold temperatures,"" she says. ""And acidification disrupts reproduction among frogs and salamanders."" If their ancient predecessors somehow survived, but such animals as Triceratops and T. rex did not, maybe some form of environmental stress was acting disproportionately upon dinosaurs.
(A) During the field season at Hell Creek, teams head out around 7:30 a.m., each to a different site. (B) Horner, who has been excavating dinosaurs in Montana for 30 years, supervises research for the project. The teams work independently, he says: ""We don't want to influence the interpretations of the other teams."" (C) Lee Hall records and maps the position of the bones of a T. rex skeleton. (D) This particular site, called C. Rex, contained a 45-foot-long T. rex skeleton—the world's largest. The site was discovered by Jack Horner's wife, Celeste, in 2000. Sites are named after the person who found them.
Recognizing an opportunity to test that hypothesis, Arens eagerly accepted Horner's invitation to participate in the effort at Hell Creek. And after five seasons of research at the site, she has come to believe that she, like Clemens, may devote the rest of her career to the dig at Hell Creek. What she and her students have already uncovered provides an intriguing clue to why the only member of the dinosaur family alive today possesses wings and feathers. First, fossil leaves and pollen excavated from the layers immediately below the K-T boundary show that the diversity of flowering plants substantially decreased in patches throughout the region before the extraterrestrial impact. Second, not all plants and trees were affected: Ferns, conifers, and the palmlike plants known as cycads lived to see another geological era. But the angiosperms—the flowering plants—suffered devastating losses. ""Three meters [10 feet] below the asteroid event,"" Arens says, ""we see upwards of 100 species. A meter and a half to two meters [5 to 6.5 feet] later, the number drops to 10."" Among the plants and trees that exist in our contemporary landscape, angiosperms include almost everything except conifers, ferns, cycads, and ginkgoes. Think of the chain of consequences that would ensue if 90 percent of all plant species disappeared.
Now imagine this occurring 65 million or so years ago. According to Arens, one of the many insights that conservation biologists have gleaned from their study of living ecosystems is that different organisms respond differently to environmental stress. It is also known that dinosaurs experienced a great deal of turnover on the species level. The best explanation for this turnover is that whenever environmental pressure increased dramatically, certain fauna went extinct while others evolved into new forms. Consequently, general lineages remained intact, but the animals that survived differed from their ancestors. If, as Arens's research suggests, vegetation suddenly became patchier, herbivores like Triceratops and Edmontosaurus, as well as the carnivores that ate them, like T. rex, would have found day-to-day survival much more difficult. ""They would've been set up for extinction,"" she concludes.
At the very least, the research supports a picture of Hell Creek more eventful than the conventional perspective. The climate almost certainly changed. Near the top of the section, Arens has found leaves with toothed edges, which in contemporary plants is associated with cooler temperatures. And if two petrified tree-trunk segments recently unearthed from the lower, earlier part of the formation are shown to possess rings, then seasonality will also have been established. That would probably mean they experienced a wet-dry cycle, because it is extremely unlikely that the temperature fell below freezing.
None of this evidence of environmental variation surprises Joseph Hartman, another key member of the project. Hartman, a paleontologist at the University of North Dakota in Grand Forks, specializes in fossil mollusks—clams and snails. He tracks them ""as proxies for changes to the climate and landscape."" And what the humble mollusks have revealed is that Hell Creek was very dynamic indeed.
Hartman bases his interpretation on the location and movement of water, both freshwater and salt water. Recall the widely accepted belief that the Western Interior Seaway had receded by the time the Hell Creek Formation took shape. Recall, too, that sediment deposits in the region clearly indicate that freshwater rivers, originating in the mountains to the west, were present throughout the period of Hell Creek deposition. And wherever streams run for a sufficiently long time, freshwater clams and snails colonize in abundance. Yet at the bottom of Hell Creek, very few such fossils are found. Moving up, or forward in time, however, the mollusks rapidly diversify. Then the trend reverses direction again, with the number of species dwindling in the layers of rock below the K-T boundary. As was the case with flowering plants, a staggering 90 percent of all freshwater mollusks apparently went extinct before the impact event. Moreover, the fossils that do appear at the end of the Late Cretaceous represent a distinctive fauna not found in the Tertiary rock above the K-T boundary.
Excavations at Hell Creek have turned up T. rex skeletons at eight different sites. This specimen turned out to be a duck-billed dinosaur.What does Hartman make of this record? In the past, he notes, the number and diversity of mollusks diminished whenever the seaway rose and extended its reach into the west. The pattern of mollusk loss—species disappearing, then diversifying, followed again by decline—tells him that the sea continued to transgress periodically during the Hell Creek time, altering drainages and other aspects of the landscape and making it impossible for freshwater clams and snails to thrive. How exactly all the project findings—the decline of angiosperms, mollusks, and dinosaurs; the rising and falling sea; the cooling climate—fit together isn't clear yet. But they nonetheless indicate that the Hell Creek Formation represents a time of widespread ecological disturbance. ""What we're seeing,"" Hartman says, ""is major environmental destabilization before the planet got hammered with an extraterrestrial object.""
Once again, imagine the badlands of eastern Montana. But in this picture no paleontologist appears. An extremely distant cousin, the largest mammal in existence at the time, is about the size of a field mouse, and it isn't likely to be observed because it comes out of hiding only at night. You are gazing north, and before you lies a broad, flat coastal plain. To the east is the shallow, winding shoreline of a continental sea, to the west a mammoth river system that in breadth and complexity and intensity rivals the network of tributaries in the Amazon basin today, all of which drains eastward, into the sea. Some of the larger streams follow roughly the same course for long periods. But other channels, and there are hundreds of them, often change direction, intersecting, interrupting, continually reshaping the landscape and depositing huge volumes of sediment. All manner of ephemeral features—oxbow lakes, ponds, dams, swamps—appear and disappear. Along the banks of the more stable rivers stand dawn redwood, water pine, and a variety of angiosperms, whereas farther away, claiming higher, drier ground, are forests of cycads, ferns, and conifers. Over the course of 2 to 3 million years, the sea rises and falls. It occasionally inundates the land, expanding miles and miles westward and covering everything, including the forests. When that happens, the climate changes. The temperature rises and the amount of moisture increases. During the final seaway transgression, some 65 million years ago, certain organisms undergo a great amount of environmental stress. The diversity of flowering plants plummets, and entire populations of freshwater snails and clams vanish.
It's upon this final scene that the asteroid collision was superimposed, and superimposed is the operative word. In their attempt to reconstruct this ancient ecosystem, the Hell Creek team has developed a new way to view mass extinction. ""Maybe,"" Arens explains, ""you need two things to coincide geologically."" The first is changes to the environment that significantly alter habitat, in effect ""altering the rules of the game,"" as Arens puts it, so that physical characteristics and behavior that once proved adaptive for certain organisms—dinosaurs, for instance, that relied on flowering plants for food—no longer work as well. If on top of that pressure, a large, instantaneous ecological disturbance is added, the flora and fauna already struggling to survive are likely to die. That in fact is what Horner hypothesizes happened to Triceratops, Edmontosaurus, T. rex, and the rest of their diminishing kin at the end of the Hell Creek period. The asteroid may well have been the knockout blow in a fight the dinosaurs had already lost.
Two years of prospecting remain in the Hell Creek Project, followed by an unknown amount of time for specimen preparation. Horner is already planning a sweeping new exhibit at the Museum of the Rockies, in Bozeman, Montana, where he is curator of paleontology. Called Dinosaurs Under the Big Sky, it will recount through fossil displays the whole history of dinosaurs in Montana, the final chapter of which brings to life the plants and animals of the Hell Creek Formation. The initial draw, of course, will be the dinosaurs whose names cannot be pronounced without a superlative attached—the biggest, youngest, and oldest tyrannosaurs; the horned dinosaur skull that dwarfs all others; and the edmontosaur, which at about 48 feet long is the largest duck-billed dinosaur in existence. But whether re-creating the miniature world of a micro-site or the complex features of an entire region, the aim of the exhibit will be to further acquaint the public with the principles of evolution and ecology. And it will be much more than a history lesson. For if the Hell Creek team's two-punch theory of mass extinction is correct, we may be on the brink of our own boundary event.
""We could very well be in an interglacial period that's part of a larger pattern of climate fluctuation,"" Arens warns—a time of severe environmental stress. Superimpose upon that the worldwide disturbances that human beings are causing, and another sudden, catastrophic die-out could be in the offing. Were that the case, we would be the contemporary equivalent of the asteroid that collided with Earth all those millions of years ago.