The evidence for this event was first published in 1980. Walter Alvarez, a geologist, had been in Italy studying the rate of accumulation of cosmic dust in geological strata as a way of dating them independent of fossils or other methods, when he found that right at the K/T (Cretaceous-Tertiary) boundary, there was much more iridium than in any of the other strata. He was near the town of Gubbio, and in the rocks he was studying, the K/T boundary was marked by a layer of clay. Below it were fossils of microorganisms of the Cretaceous. And above it were fossils of different microorganisms, from the Tertiary.
This boundary is visible in other formations around the world and was long known to mark a great extinction of the dinosaurs and many other forms of life. There are at least two other mass extinctions, one at the end of the Triassic, about 205 million years ago, and another one, the most significant known so far, at the end of the Permian, 250 million years ago. Ninety-five percent of all species in existence then died out.
Only in recent years have paleontologists and evolutionary biologists come to recognize the importance of mass extinctions in evolution. These events brought chaos and destruction to the planet, and opportunity. New forms of life evolved rapidly to occupy niches in the environment left open by the disaster.
But explanations for these extinctions have been hard to come by. There has been no end of argument about the extinction of the dinosaurs. So the discovery of this highly unusual concentration of iridium exactly at the time of a mass extinction was intriguing. One of the events that would produce such a spike would be the impact of an asteroid hitting the earth. Iridium is common in asteroids but not in the earth’s crust and an asteroid of sufficient size—about ten kilometers in diameter, Alvarez estimated—would do the trick. He worked with his father, Luis Alvarez, a Nobel Prize- winning physicist, and two geochemists, Helen Michel and Frank Asaro, all at the University of California at Berkeley. Michel and Asaro had found iridium spikes at two other sites marking the end of the Cretaceous—in Denmark and New Zealand.
A great scientific debate continued as many more iridium spikes in a clay layer of the same age were found, including in some parts of the Hell Creek Formation. But there were, and are, many puzzles about why some animals went extinct and others did not. And a question remained, for a time, about where the evidence of such a collision was. This was the kind of impact that would leave a mark.
It wasn’t until 1990 that seafloor cores drilled in the Gulf of Mexico showed quartz that had been transformed by an impact of the sort an asteroid would cause. And it lay underwater in an area near the town of Chicxulub that had been spotted a decade earlier as a potential impact crater. That first claim did not attract scientific attention, but the “shocked quartz” did. Other evidence accumulated—glass deposits in Haiti and sand in Montana, blown from the crater.
What exactly the asteroid did to the global environment is not known, and explanations still abound for the mechanisms of extinction, but what is clear is that there was a massive and immediate global extinction after an asteroid impact of literally unimaginable proportions. And the best place to see the fossil killing field, or iridium layer, and the terrestrial life before and after it is the Hell Creek Formation.
A year and a half ago astronomers identified what may have been the source of the Chicxulub meteorite, a collision in the asteroid belt that occurred 160 million years ago in a group of asteroids known as the Baptistina family. As was pointed out several times in the news of the discovery, this would mean that the Chicxulub asteroid was set on its course about 100 million years before it hit, in the middle of the Jurassic age.
Any modern human with even a hint of pessimism about the future of the human race has to have some sympathy for the creatures of the Hell Creek ecosystem, completely unaware that many of them were about to disappear forever. Of course, some of them were about to get their big chance—mammals, for instance. They were small, perhaps able to survive on a variety of foods, including leftovers from the extinctions, and not dependent only on photosynthetic plants, which seem to have had a hard time.
Evolution driven by catastrophe is not what Darwin had envisioned. He, and many of those who came after him, saw natural selection acting gradually, preserving, or selecting for, the traits of animals that left more offspring. That would still happen in a catastrophe, of course, but it would not be a honing of traits best suited to particular niches. For a time chaos would serve those organisms that could thrive in a wide range of environmental conditions, and at first there would be little selection of the sort Darwin imagined, because as the damaged planet recovered, there would be plenty of room for the fit and unfit to prosper, as long as they weren’t too ecologically picky.
Think of the Permian extinction, for instance. Before the extinction, in which 95 percent of species were wiped out, many changes in behavior or form prompted by mutations in genes would have been lost, because organisms had particular niches that they had evolved to exploit and too much deviation would probably diminish their fitness. After the extinction, however, refinements in exploiting one kind of environment might be nothing compared to a fast reproductive rate and an ability to eat anything and everything. If every niche were opened by the extinctions, the world would be welcoming to all sorts of mutations.
Nature had suddenly become a kind of Wild West, far less picky about whom it welcomed. It was a new frontier of sorts, something like western North America when it was being taken over by European-Americans. In the West manners were much more varied than in the East or in England. Behavior that would not be tolerated in a stratified society in which all the niches were filled was tolerated, or accepted, in a land that offered all sorts of opportunities, once the original inhabitants were gotten rid of. In the American West social mobility was great. After a mass extinction evolutionary mobility was greatly enhanced.
About 35 percent of the existing species were wiped out in the K/T extinction, a mere interruption compared to the catastrophe at the end of the Permian. The consensus in science seems to be that the asteroid impact was the primary cause of the extinction. And the meteor crash is so astonishing in its destructive power that it tends to obscure the time before and after it landed off the Yucatán.
But the period before the crash is fascinating. If we go back ten million years before the meteor hit, dinosaur diversity was at a peak. This is a time when the Judith River Formation in Montana was laid down. And this formation is rich in the numbers of different species. But then, when we turn to the Hell Creek Formation, ten million years later, we find many fossils, but far fewer different species. And the more we study the fossils, the fewer species we find.
Recently, some of the species of the Hell Creek Formation have gone extinct. In this case, the cause is paleontological. As we understand more about the growth of dinosaurs, we find that some specimens that we thought were different species are just different ages of the same species. We used to think that
Dracorex, Pachycephalosaurus,
and
Stygimoloch
were three different species of dome-headed dinosaurs that were found in the Hell Creek Formation. Now we have found that Dracorex and Stygimoloch are juvenile stages of
Pachycephalosaurus
.
Many other species actually disappeared, rather than being reidentified. In fact, the biggest drop in diversity in the 140-million-year history of the dinosaurs occurred in the 10 million years before that meteor crashed. Something took out a lot of dinosaur species before the meteor finished the job. That is what interests me more than the mass extinction of the meteor crash, perhaps because the mechanism is so unknown and hard to understand. A meteor crash is, at heart, simple. Not that it isn’t a challenge to figure out exactly what kind of havoc the meteor wreaked, but it’s pretty clear that it caused a world of trouble. We don’t know anything about why dinosaur species disappeared at a rapid rate in 10 million years before the extinction.
The unanswered questions may only increase the sense of awe that comes from standing in the Hell Creek Formation and seeing the coal that marks the end of the Cretaceous. All around you are elements of a fossil snapshot of the world just before a catastrophic event. Prospecting for fossils doesn’t just produce the discovery of new species. Each fossil is a pixel in the increasingly detailed image we have of the moment before the extinction. That is the time frame you occupy in Garfield County: the moment before the end.
Or you can occupy the present, at least the present of Garfield County, which is a bit of a dislocation from, say, the present of Berkeley, or New York, or Washington. Fortunately, the ageless feeling of the rangeland in Garfield, the sparse human population, the quiet at night and the wide open sky provide a cushion against the gap in time, so that it does not feel so odd to be straddling the eons. But one thing that the rocks of Garfield County do not offer is any sense of what happened between then and now. Life did not cease on this patch of the planet’s surface with the extinction. Sixty-five percent of species survived, some prospered, and many new ones emerged. But no record of these events was preserved here. The deposits continue for a few more million years after the K/T boundary. Some of that time is preserved in deposits in and around Jordan that look much like the Hell Creek deposits except that they are tan rather than gray, there is more coal, and it seems to have been swampier than before the extinction.
The Paleocene lasted for another ten million years, and some of that can be seen in Garfield County badlands. But not much later. Life continued in the area, but we have no sedimentary rock from that time in that location. The planet’s surface is a patchwork of different time exposures, like a canvas painted over many times, with different works showing through at different spots where the paint is thicker or more has been scraped off by curious art historians.
We have to turn to other fossil records, of which there are many, for an idea of how this part of the planet made it from then to now. If only for the sake of context, it is worth stopping to fill in a few of the blanks.
SINCE THE DINOSAURS
What has happened to North America since that time? The answer is: almost everything. The continent, and the world, went through geological and climactic upheavals. Mammals began to radiate into forms that seem outlandish today. In their range of shape and behavior they challenge the dinosaurs, although the dinosaurs get most of the press, perhaps because there are so many mammals around now, such as humans.
If one is tempted to think of the mammals as a poor sequel to the dinosaurs, it’s worth remembering that they lived through the entire age of the dinosaurs as well. What happened after the extinction of the nonavian dinosaurs was simply that they became the dominant land animals, as they are today. The impact of the meteorite was a crisis, but a manageable one for life on the planet. As for the rock we all live on, the Rockies continued to thrust upward, other mountain ranges of the West emerged, seas disappeared and reappeared in the center of North America.
In what is now the High Plains of eastern Montana, the conditions were junglelike as mammals began to radiate. The emergence of these creatures and their radiation into so many forms is as vivid an indication of how evolution proceeds as was the era of the dinosaurs, or the evolution of birds, which coincided with the mammalian explosion. Every shape and size emerged among mammals, in configurations that we are familiar with—extremely large herbivores, like the uintatheres; predators with flesh-cutting teeth; small mouse- and shrew-like animals; the swift (deer- and antelope-like creatures); and the slow (great sloths).
All of these widely divergent creatures were variations on the standard mammalian model. All were furry tetrapods with five-digit hands and feet, hearts, lungs, and brains. Those brains were protected by the ancient skull structure that had been around before anyone had thought of fur. Changes in size are easy to understand occurring quickly. A different regulation of growth, a few genes turned on and off at different times, or producing more or less of the regulatory proteins, and the tiny mammal would roar, or bellow. Shape inevitably changed with size. Teeth changed with diet. Depending on food availability, stomachs and metabolisms changed. But over the course of sixty-five million years the fundamental mammal has stayed visible.
It is too long a time to track every change that occurred in the continental interior, or worldwide, where one branch of mammals, the primates, were developing bigger and bigger brains and new behaviors. Hominins first appeared about six million years ago, according to current thinking, shortly after the last common ancestor of humans and chimpanzees, probably six to eight million years ago. The succession of hominins that led to humans is long and not entirely clear. But we do know that not until about fifty thousand years ago did physically and behaviorally modern humans leave Africa. They quickly spread around the world.
When humans first came to the Americas is a matter of some dispute. The first undisputed evidence of their arrival puts the colonization of the open continent at a little over thirteen thousand years ago. These were the so-called Clovis people, who used distinctive stone points to hunt mammoth and other large animals. Stone points of this type were first found at Clovis, New Mexico. Humans may have arrived earlier, however, as some sites, like one in Monte Verde, Chile, have materials that date to more than fourteen thousand years ago and show evidence of settled rather than nomadic life. That would seem to suggest that these people came to the New World from Asia earlier than the Clovis hunters.
The first culture to be widely represented in North America, however, is that of the Clovis hunters, who quickly spread across the continent. They came at a time when the last glaciers were receding, and a passable overland route from Asia existed where the Bering Strait is now. If other humans had come to North and South America earlier, they did not leave a mark on the environment that we have found.