Dinosaurs in the Attic (24 page)

These fossils are rare—exceedingly rare. "To get this little box of jaws and teeth," McKenna says, "a field crew of eight people had to process five tons of matrix every day for a week." McKenna spends most of his summers in the field collecting fossils such as these. Over the years, he has developed a collecting method that differs radically from the way Barnum Brown, for example, collected fossils. He doesn't collect
individual
fossils at all; instead, he collects entire fossil communities—fossil ecologies, so to speak. To accomplish this, McKenna has to extract almost every fossil larger than a small pebble from a particular deposit.

McKenna seeks a picture of the life of one entire community at a particular point in time. Since bones are usually the only things that survive into future eras, he must of necessity study them. Bones can provide us with an extraordinary amount of information, especially when combined with research in other areas such as astronomy and geology.

"My research," he explains, "requires the highest-resolution data—the most detailed and complete picture possible." To accomplish this, McKenna needs thousands of bones, not just a few. To obtain such large quantities, he has his field crew dig up the fossil-bearing matrix, which at Lance Creek is made up of ancient channel sands cross-bedded with layers of clay, and dump it in water. (The highest concentrations of the fossils McKenna is after can be found in what he calls "fossil garbage dumps" in ancient streambeds, where flash floods and other processes have deposited a great assortment of bones, many of them originally contained in carnivore scat.) When the matrix has loosened in the water, it is sifted through a fine wire-mesh screen. The screen catches thousands of pebbles—and, if McKenna and his crew are lucky, a fossil tooth or two. In one day, working in assembly-line fashion, the crew can process 550 twenty-pound boxes of matrix. "It's very tedious work," McKenna says. "You have to go through a lot of beer and Cutty Sark just to find one fossil." Indeed, one of McKenna's colleagues named a toothy new species
Cuttysarkus mcnallyi
because the discoverer, a member named McNally, had been promised a bottle of the Scotch if he found a mammal jaw with six or more teeth in it.

McKenna's collecting method is tedious but wonderfully efficient. "The first day we spent at Lance Creek in 1956," McKenna says, "we collected more fossils than everyone else since the fossil locality was discovered." Yet the fossils are so rare that, even using McKenna's method, an entire summer's work could fit in a small piece of carry-on luggage for the return flight to New York.

McKenna pokes his finger around in the box of fossils. "This box represents," he says, "a picture of the animal life of one particular area at one particular moment in time—in this case, about 66 million years ago, around the time of the mass extinction at the end of the Cretaceous." Lance Creek has fossils of about eighty kinds of animals, including fishes, lizards, tiny and large dinosaurs, frogs, salamanders, birds, and early mammals. "I keep the mammal bones," McKenna says, "and spin off the rest of the fossils to other scientists."

He then explains what we can learn by looking at little bits of bone and tooth. "What can these bones tell us? An extraordinary amount, it turns out. There are many levels of 'boniness.' Bones have an exterior shape, which tells us about the anatomy of the animal and what it did for a living. Bones have an interior structure, which can provide clues about whether or not the animal was warm-blooded. Some bones have growth rings, which indicate whether the animal lived in a variable climate and had to slow down for a period every year. Bones also have a microscopic structure, which can show such things as the metabolism of the animal."

Since teeth, being extremely hard, survive better than bones, they make up most of the contents of McKenna's box. Fortunately, mammal teeth are almost a fingerprint of the animal—they are highly complicated structures with ridges, valleys, spikes, ribs, ripples, pits, and points. Few species have teeth that look exactly like another. Consequently, a paleontologist can sometimes identify an animal fairly closely by looking at a single tooth, and can usually identify it with certainty from a jaw with two or three teeth. Equally helpful is the fact that mammalian teeth don't get any bigger as the animal grows. Except for normal wear, mammalian teeth remain constant for a given species.
^*27

Once McKenna has identified the animals living in a specific community, he can start looking at some of the larger questions. Bones can reveal something about the behavior of the animal, for example. "Take foot bones," he says. "What was the animal doing with its feet? Was it hopping, swimming, galloping, climbing in trees?" Since bones have marks where ligaments and muscles were attached, the paleontologist can actually determine an extinct animal's musculature and reconstruct the form of the animal itself. (This landmark advance in paleontology was made at the Museum in the 1920s.)

Once paleontologists know what the animal looked like, they can start reconstructing its habitat. If the animal was a hopper, for example, it may have lived in an arid, sandy area. If the animal had tree-climbing limbs, its habitat must have been arboreal—even though, of course, no tree fossils may be found today. Bones, then, can shed light on past landscapes, climates, and vegetations.

"Much of this information can be found by studying this little box of fossils," McKenna continues. "You can actually work out the 'census structure' of the community with these fossils." The census structure includes not only what animals were living when, but also the relative proportions of one species to another at that time. "How did the animals interact? For example"—he pauses to pick out a tooth from the box—"this mammal tooth passed through the alimentary tract of a big reptilian meat-eater, such as a crocodile. How do I know? The tooth has been etched by stomach acid—see, it looks frosted. I suspect it was a crocodile, because crocodiles have very acidic stomachs. So we even know what animals were eating what." Thus, bones can help reconstruct ecologies that may have been extinct for many millions of years.

And yet this is just the beginning, according to McKenna. "If you want to answer the largest questions in science—and this is the goal of most scientists—you have to study the highest-resolution data possible. You can spin theories with a bone here and a bone there; but what you really need are many, many bones of many kinds from one locality at one time period.

"By looking at fossil assemblages in a number of areas, you can correlate assemblages to within about one percent of their real age. Then, by studying successive layers, a paleontologist can learn how the community and the climate evolved over time. If a group of species suddenly went extinct, the paleontologist can look for the reason. Thus, even a small area can yield information about the earth as a whole." The study of bones can help date past events, and can show how evolution proceeded, including the evolution of entire ecosystems.

As with many scientific questions, there really is no end to how far one can go. By comparing two sets of layers in different sites, paleontologists can gain data that will help draw a hypothetical map of the earth as it appeared at some time in the distant past. "Let's say you're studying two localities, one in France and one in Wyoming," McKenna says. "In layers about 50 million years old in France and Wyoming, most of the animals are normally quite different. But about 60 million years ago, European and North American deposits had many animals in common. What do you conclude? Possibly that some sort of connection—a land bridge—was present between Europe and North America for a while."

The paleontologist, by looking at
other
scientific data, may recall that huge lava flows occurred when Greenland was closer to Scotland, possibly forming a land bridge. Europe and North America were much closer together 60 million years ago. Five million years later, the two faunas—in Wyoming and France—were different again. Thus he might hypothesize that a land bridge developed between 60 million and 55 million years ago across the North Atlantic, a bridge that lasted approximately 5 million years. When the sea encroached upon the bridge, it cut off that particular route of interchange, after which the faunas of the separated areas evolved independently. The above scenario is not imaginary; it is a theory that McKenna has developed through his research.

The amount of information that bones can provide is limited only by the questions we ask. Another question McKenna has been working on is whether the dinosaurs died out suddenly or gradually. Dating the exact time of the extinction, and how rapidly it occurred, is central to any extinction theory. A significant reason why McKenna collected at Lance Creek was that the fossil-bearing strata date from the very end of the Cretaceous, just when the dinosaurs were becoming extinct.

The results of McKenna's research are a bit startling. Up to now, one of the prime candidates for dubious distinction of the extinction of the dinosaurs is the theory of asteroid impact. In this theory, a large asteroid struck the earth about 65 million years ago, causing sudden atmospheric and climatic changes that in time caused the dinosaurs to vanish from the earth. The prime bit of evidence for this theory is a 65-million-year-old layer of iridium, and indications of ancient soot particles that appear in strata in various areas around the world. Iridium is scarce on earth, but rather more common in meteorites. Such a layer could thus most plausibly come from a meteorite impact, which—if large enough—could have spread a layer of iridium-enriched dust around the world. But McKenna's research contradicts this theory. His study of bones, combined with the research of many of his colleagues, indicates that the dinosaurs became extinct
before
the asteroid impact.
^*28

"There is a small but significant thickness of rock without dinosaur fossils underneath the iridium layer," McKenna explains. "That dinosaurs could have been around but not found is possible, but the chance of that is about three percent or even less.

"Now, this is a fascinating observation. Could the various extinctions and the bolide
^†29
impact have been caused by the same thing? Here's where paleontology and other scientific disciplines can come together and provide important information. If we combine our data with astronomical data, we can tell something about the history of the earth in relation to the solar system.

"Paleontologists have noted a periodicity in mass extinctions. Astronomers have noted a periodicity in the path of the solar system as it passes back and forth through the galactic plane. Geologists have noted a periodicity in the formation of ancient meteorite craters on the earth. Geophysicists have suggested a periodicity in magnetic reversals. Could all these things be related?" McKenna noted that a number of theories do in fact relate some of these observations. One theory, now largely discarded, hypothesizes the existence of a dark companion star to the sun, named Nemesis, that periodically swings close to our solar system and sends comets plunging into earth-crossing orbits. Some of these comets, it is hypothesized, actually strike the earth.

"The point here is that all the various scientific disciplines come together and test each other's theories. What we are doing—in the Museum and elsewhere—is putting together a general picture of the earth. We want to construct the entire history of our planet, not just the history of animal life. This is
real
natural history—not just biology. We're not the American Museum of Biology. What we are working on here in the Museum has consequences and implications throughout science, from astronomy to geophysics.

"The reason we collect bones is to build up a library of
facts.
It's not postage-stamp collecting by any means. Bones, and indeed all scientific collections, differ from books in that books are opinion and interpretation, while specimens are facts. In this way, museums are different from universities. Bones are one of our greatest links with the past—they are our record of extinct vertebrate life on this planet."

GETTING BONES

It is one thing to study bones, but quite another to get them. Paleontologists (as we have seen in several of the expeditions described earlier in this book) find their bones as ready-made fossils. In this respect, mammalogists are not so fortunate. They must collect from the living—and this means finding bones that are inconveniently encased in flesh and skin. At one time, before 1930, it was an easy matter for the Museum to send scientists out to Kenya or Tibet to shoot mammals for study. Today, with many animals becoming endangered, and amid a growing awareness of conservation, many of the Museum's specimens come from carefully monitored and licensed collecting forays—as well as from zoos.

Four flights down from McKenna's office, and at the end of a cul-de-sac in the Museum's African Hall, we come to a locked door with no knob. Through this door, down a twisting corridor, up one flight in a freight elevator, and down a quick right and a left, there is a locked steel door with a tiny window. This door is outside the mammalogy preparation area. Next to the door hangs a heavy coat, which one must put on before entering the room. The room's interior looks like nothing else on earth; it is like an exotic meat locker filled with rare animals. This room—the Museum's "freezer"—is often the first stop a dead animal makes upon entering the Museum. Lying on the floor at the time of our visit, arms outstretched, is the body of a female gorilla—frozen solid. Stretched alongside her, also frozen, rests a male leopard. On the far side of the room, a number of shelves hold stacks of elephant hides and other skins; some of these hides are from elephants shot by Teddy and Kermit Roosevelt. Assorted animal remains and skinned carcasses in plastic bags are stored about the room, and in a far corner, two mounted Siberian tigers stare at the scene with fierce but sightless eyes.