Q: How did you become interested in the Gobi Desert?
A: Well, [it] started a long time ago -- I was seven years old, actually. I'd read about the Gobi Desert in books, in popular books about dinosaurs. There was this very dramatic explorer, Roy Chapman Andrews, who wanted to go to Central Asia to look for early humans and ended up finding a lot of dinosaurs. That was many decades ago, but he wrote kids about it, and kids loved those books, and I was one of those kids. So that's when I first started thinking about an expedition to the Gobi Desert.
It wasn't, of course, til many years later -- I was already a paleontologist, but not particularly hopeful that I'd ever get to the Gobi Desert-- when suddenly, out of the clear blue, suddenly this opportunity arose. [It] came about because the Mongolian people decided to ... declare a democracy and break away from the Soviet Union, and they invited us to go back.
In 1990, I led a team from the American Museum of Natural History to Mongolia on our first expedition, to look for dinosaurs and mammals and other vertebrates, with the hopes of coming up with enough that would allow us to launch a whole series of expeditions, just like Roy Chapman Andrews did in the 1920s.
Q: Tell us about Roy Chapman Andrews.
A: Well, Roy Chapman Andrews is a very interesting figure in the annals of scientific exploration. He was not really that well trained as an expert in paleontology; he was more of a zoologist. And when he came ... to the American Museum, he really worked his way up through the ranks, and received the attention, just for his sheer drive and energy.
He was a great explorer, though, and he was surely capable of organizing big expeditions. The Mongolian expeditions that he led were the first, for example, to use automobiles -- he used eight Dodge motorcars, along with 100 camels. It was a big, a very big -- and rather expensive -- enterprise. Very dramatic. He really had the feeling for what major exploration required.
He was not only very passionate about the expeditions themselves, and doggedly persistent about organizing and continuing on under some very difficult conditions, but he was a good writer. He could really describe not only the science, but the drama and the adventure and the beauty of exploring in the Gobi Desert.
Q: What did he find in the Gobi?
A: It's very interesting that the Andrews expeditions were launched on a premise that never really came true: They were hoping to find the remains of early humans. There was a popular notion, at that time, that the human lineages may actually be traceable to Central Asia rather than Africa.
But they didn't succeed at that, obviously. In the course of stumbling around, looking for remains of ancient humans, they stumbled into one of the world's most dramatic places for dinosaurs, mammals, and other vertebrates from the age of the dinosaurs.
Q: So he found mammals as well?
A: Yes. Mammals were part of the dramatic finds that the Andrews expedition uncovered. They weren't the biggest things; a lot of these mammals are little, nugget-sized creatures, but they were very, very important to science, because at that time we knew virtually nothing about mammals that old -- mammals so old that they lived alongside of the dinosaurs, nearly 100 million years ago.
So this was a very dramatic find -- from a scientific standpoint, as dramatic as the finding of the dinosaur eggs, which of course received a lot more attention in the media.
Q: When you led your team back to the Gobi in 1990, what were you hoping?
A: When I led the team back to the Gobi in 1990, we had great expectations. We knew that the Gobi was incredibly rich for these kinds of fossils, but we knew that ... we might be even more focused on some of the things that hadn't been collected to as great an extent: on the small dinosaurs, the birdlike dinosaurs, on nesting sites, and certainly on lots of mammals and lizards, some of the tinier creatures, but in many ways scientifically more dramatic ones.
So we were particularly interested in localities or sites that preserved those kinds of fossils.
Q: And what did you find?
A: During the first three years of the expedition, we were very successful at turning up some excellent dinosaurs and mammals and other vertebrates, but we hadn't really struck it rich at a new site ... Oddly enough, for three years running, we were within 10 miles of the site that we discovered in 1993. The site that is in many ways the richest site from the age of the dinosaurs in terms of its preservation of so many mammals and lizards, as well as dinosaurs and dinosaur eggs and dinosaur embryos.
But we missed the site entirely, for several years running. It just didn't look very good. We could look up in the distance with our binoculars and see these little red bumps on the hillside, and they just didn't look dramatic, like big canyon lands we were used to. We finally decided, in '93, Well, let's go up there and take a look. And it's a good thing we did.
The co-leader of our expedition, a Mongolian scientist named Demorolian Dashaveg, he and I talked about possibly actually going up to the spot. We were on our way out to a more western locality, and we said, Let's just take a very brief look. Because we've never looked there before, [and] maybe there's a possibility.
We were actually heading a little west of the site, but en route our gas tanker got stuck in the sand, which happens all the time -- these tankers and these trucks are always being mired in something. And so we had to dig it out. We decided to camp there and, as the truck was being excavated, in the morning a few of us took a couple of jeeps up to a little hill, just a knob of red, probably no higher than 20 feet, and some flat, red rock line, exposed horizontally on the surface.
We took a little drive up to this spot, and as soon as we jumped out of the car, we started finding skeletons. Within a couple of hours we probably had 40 dinosaur skeletons. By lunchtime, we had found the first nest of a meat-eating dinosaur, with the embryos of that meat-eating dinosaur in the nest. So it was a nice morning of discovery.
I could not believe that morning. ... [W]e started walking around, and there I saw a mammal skull just lying on the ground, the surface, and then Mark Norell, my colleague, would find one. And about every 15 minutes, it seemed, someone said, I got a mammal, and then I'd say, I got one, too.
We had about 50 mammal skulls by lunchtime.
It's hard to describe the thrill and excitement of finding ... several dozen ... mammal skulls in one morning at this site in the Gobi. We are very interested in these mammals that lived during the time of the dinosaurs, but, on most continents, all the evidence we have of these are what we call "spare parts" -- they're just little pieces of teeth and jaws. And most of what's written in scientific research on these mammals concern those kinds of bits and pieces, those fragments.
The Gobi is remarkable for preserving skulls and skeletons, but this locality has far, far more skulls and skeletons than have ever been found in all the other Gobi localities put together. In that one morning, with these 50 skulls or so, we had already matched the amount that's been recovered from the Gobi entirely, over a period of seven decades.
We've worked this locality for several years now, and we've collected nearly 1,000 skulls and skeletons. That's an extraordinary bounty, an extraordinary richness of mammals, in one locality, and for anywhere.
Q: How old were the fossils you found?
A: The locality we stumbled on that morning is about 80 million years old; the fossils from that locality date back to the late age of the dinosaurs, about 15 million years before the K-T extinction event.
Q: What can you tell us about mammalian evolution, based on these fossils? What do they look like?
A: Mammals today are very dramatic animals. They range from things like the blue whale to rhinos to elephants, all kinds of marvelous creatures. Many of them are very big. But you'd have to say that during the Mesozoic [about 120-140 million years ago], many of these mammals seemed to be rather inauspicious creatures. They were all rather small, somewhat shrew- or mouselike, somewhat limited in their range of habits.
The human forelimb is long, slender and mobile and, unlike that of other mammals,
does not bear weight in locomotion. The ball and socket shoulder joint enables a 360°
range of motion, and slender finger bones and a prominent thumb enable the hand to
carry out fine manipulations.
But even at that early stage, we get to pick up on an interesting diversity of their life habits. We can tell things from the skeletons about their use of their eyes, or the sensory systems, their sense of smell, their locomotion, and even their reproductive behavior.
Q: So mammals have been around for a long time?
A: Mammals go way back; they're as old as the dinosaurs. They go back over 200 million years. But, for about the first 140 million of those years, they're not very dramatic animals. They really lived in the shadow of the dinosaurs, mostly small, possibly many nocturnal creatures, possibly animals that more emphasized a sense of smell than a keen sense of vision. Not very large brains, as we can tell, in some fossils.
There's a sort of stark contrast between mammal evolution and dinosaur evolution during this early phase. Dinosaurs, of course, get big, they're baroquely diverse, with all kinds of weird adaptations, with armor, with predatory animals, with birdlike animals, long-necked things -- just a tremendous array, the dominant animal features of the landscape.
Mammals just scurry around in the shadows. They're small, shrewlike, or ratlike, in many ways. They look like some of the least dramatic things we have today.
Q: So what do the fossils tell us about mammalian evolution?
A: These fossils are very interesting, because they give us some window into the transitions in mammalian evolution. For example, they show mammals that are beginning to get larger brains, better developed brains. The eyes are probably enlarged or becoming larger. The blood systems that feed the brains become more complex, in ways that are more modern.
And even in the skeleton, behind the skull, we see a number of very interesting transitional features. Perhaps the most interesting really is in the pelvic region, where there's evidence of splintlike bones that suggest support for the abdominal cavity, ... and this probably supported a pouch, very much like living marsupials like opossums and kangaroos. And it represents a transition between a more primitive, egg-laying behavior and a more advanced behavior or a more advanced reproduction that we see in today's placental mammals, like us.
Q: And why is that important?
A: It's important to know what adaptations may be related to the success or diversity of different mammalian groups. And perhaps the most dramatic trends, the most dramatic changes we see in mammals, aren't necessarily in things like teeth or jaws or limbs; they're really in the reproductive system.
There's a great advantage to protecting the young inside the mother in prolonged pregnancy. That's a better environment to control around a developing young than leaving the womb and literally going out into the world at a very early stage.
So it made some sense that developing a reproductive system like this, with prolonged pregnancy and other features, confers some advantages to mammals ... Look at placental mammals today. They range from bats to whales to primates to carnivores of many kind, to rhinos -- it's a tremendously diverse and successful group. And reproduction may have played a key role in the diversification of this group.
Q: So how has placental reproduction helped mammals today?
A: We think this kind of internal protection and this prolonged pregnancy conferred some advantages on these kinds of animals. And look at placental mammals today; they're extremely diverse, and successful in many adaptations. They range from bats to whales to rhinos, to lions and tigers and bears -- a tremendously diverse group. And reproduction may have played a key role in launching this group and framing the success of that group.
Q: What was the K-T extinction and how did it affect mammals?
A: The K-T extinction event was a catastrophe for a lot of animals and other kinds of life. Of course, dinosaurs were one of the famous victims of this event. Technically, of course, not all dinosaurs went extinct. In fact, there was a tremendous radiation of one kind of dinosaur, the birds, even after this extinction event.
Nevertheless, a lot of the adaptive space in habitats that was formerly occupied by big dinosaurs was suddenly empty. And, as far as we can tell, it makes sense that mammals took advantages of these opportunities, on land certainly, and even in the sea, in the form of great whales.
So the fossil record, I think, shows a fairly dynamic pattern: that following the Cretaceous extinction event, the K-T extinction event, there's a little lag time for a million or two years, but then we get a tremendous radiation of forms, and those forms become more specialized, they get bigger, so that within about a period of 10 million years, most of the modern groups of mammals have emerged.
It's not unusual, after mass extinction events, to see what we call adaptive radiations, a great deal of diversity of animals or other organisms. And we generally attribute this to some kind of opportunity -- space, or availability of food, [or other] resources.
It's possible that the game of evolution has changed its rules a little bit when one of these massive extinction events takes place. That, in general, perhaps leading up to these big events, there's more crowding of resources, there are more animals and plants and other organisms squeezing in there to compete for those resources. And suddenly you've leveled the playing field, in a sense, and opened up these opportunities.
In a way, you can think of mammals, after the K-T event, as colonizers that first landed and made a toehold on a new land where there are lots of tremendous advantages to them, and they're not competing with these large, big, plant-eating or meat-eating dinosaurs.
Q: So very little happens, and then all of a sudden, over a very short period of time, we get this huge radiation of mammals?
A: With the K-T extinction event, you can visualize a situation where most of the large land animals have been wiped out -- namely, the dinosaurs. There are a few crocodiles and things hanging around, but in general, [they're all] wiped out. Mammals, meanwhile, that survive this event stay small -- they're shrew-sized, maybe for a couple of million years. They start to get slightly larger and [increase in] diversity, but it isn't until several million years after the K-T extinction event that they really explode -- they get big, they become diverse, they become very broad in their adaptations.
So we have a sort of a lag effect: this K-T event, a recovery of the ecosystem, and then the mammals really take off.
The 65-million-year history of mammals, after the K-T extinction event, is marked by invasions. There are lots of mammals moving from one continent to another, or [from] one land form to another. You can almost visualize these as armies marching across the continent, and indeed, in a somewhat militaristic fashion, some of these mammals that enter into new continents show a high degree of success in dominance, soon after the invasion. They tend to be vigorous, and dominant, as they arrive.
Q: So are biological invasions a natural process in evolution?
A: Biological invasions are clearly part of the natural process of evolutionary history, just like extinction is, and just like speciation is. You have these opportunities for plants and animals and other groups to move from one place to another.
Q: In your research you use computerized tomography, or a CT scan. What does this tell you that a microscope can't?
A: These fossils are very tiny, but the CT scan gives us an image, like a slice, through the skull, and each of those slices is very finely divided. And from those slices we can put those images together and get an idea of the internal anatomy of the skull. So a little skull like this is projected on the screen ... and you can see the external features of the skull, but now we can cruise inside of that skull, layer by layer, slice by slice, and pick up anatomical details.
Q: What are some of the things you're looking for in the internal structure?
A: We see some important examples of structures here that are important for questions that we have about mammal evolution. For example, the number of complex teeth and their root systems differentiate major groups of mammals. We know, for example, in the back of the skull, the complexity of the ear region, the number of turns in the inner ear, in the hearing organ. These can be picked out in more detail because we can see the structure of the internal chambers. That not only tells us a lot about hearing [and] the evolution of hearing, but it places these mammals in an evolutionary pattern.
This intricate anatomy shows an improvement in hearing ... at both high pitches and low pitches. And it suggests that hearing evolved fairly rapidly at this early stage in mammalian evolution.
[The CT scan technology is] really important to us because, first of all, we don't want to destroy these skulls just to get at their internal structure. On the other hand, the internal structure means a lot to us, in terms of the evolutionary questions. And also, the refinement of these images are so convenient, we begin to see things in a way we haven't seen them before.