Whether the dinosaurs that dominated the earth for 125 million years were warm-blooded or cold-blooded is a continuing source of debate, as is the question of whether their descendants live among us today as birds. Also hotly debated is the cause of their sudden and mysterious disappearance at the end of the Cretaceous period, 65 million years ago.
Today it's accepted by most scientists that the impact of a large asteroid caused the demise of the dinosaurs. But they don't know what specifically caused the die-off or how quickly it happened. Some have posited a major change in climate, arguing that if the weather got very cold very quickly the dinosaurs would have perished.
But geologist Paul Markwick, a 31-year-old PhD candidate at the University of Chicago, says that theory fails to take into account the fate of other species living at the same time--species equally if not more temperature sensitive, species that didn't die off when the dinosaurs did. Their survival, he says, illustrates the dangers of becoming too narrowly focused on one small corner of the picture.
Markwick's main interest is paleoclimate--how warm or cold the earth was in different eras--and as part of his research he began studying the fossil record of crocodilians: crocodiles, alligators, and gavials. Eventually he concluded that, whatever the cause of the dinosaur die-off, it couldn't have been a major climate change--because the crocodilians weren't affected. He presented his research last October at a meeting of the Geological Society of America and suddenly found himself and his theories at the center of a controversy.
Blond and boyish, Markwick is a native of Worthing, on the south coast of England. He earned his undergraduate degree at Oxford University, spending his summers doing research for British Petroleum, which is what got him interested in paleoclimatology. After finishing at Oxford, he worked for BP for two more years. "I was looking for a PhD, basically because the only way to advance in the company was to have a piece of paper," he says. So he started a program in the Department of Geophysical Sciences at U. of C. But he says he likes teaching and would like to stay in academia.
We talked in his cubbyhole on the second floor of the University of Chicago's geophysical sciences building, surrounded by rock samples, piles of manila folders, a huge map of geological formations in Great Britain, a Dinotopia calendar, and color maps of different epochs that demonstrate where crocodilian and other types of fossils have been found in North America.
Sarah Bryan Miller: Why geology?
Paul Markwick: I was always interested in getting outside. I also found it interesting, finding fossils in rocks and understanding why they were there. But really it allowed me to make a career of traveling around the world. It seemed an ideal combination.
SBM: Nice work if you can get it.
PM: If you can get it!
SBM: Your work is concerned with paleogeography and paleoclimatology. Can you explain their importance?
PM: Paleogeography is the reconstruction of past geographies, which includes the positions of the continents--plate tectonics--and past topography. The continents and pieces of the continents are on plates, which gradually move over millions of years; tectonics is the study of the earth's structural features. Our understandings of plate positions are generally based on paleomagnetism.
SBM: How does that work?
PM: Since the earth's magnetic field is assumed not to have changed through time, iron minerals preserve the magnetic field at the place where they were formed. By reconstructing the magnetic field preserved in the rock we can reconstruct the latitude at which it was formed.
Paleogeography is based on the distribution of sediments and the reconstruction of the environments in which they were laid down. For instance, if you found channel sands preserved, that would suggest a river environment. Or if you found a pebble deposit you might say it was a beach.
SBM: Can you explain your theory about dinosaurs, crocodilians, extinctions, and climate changes? And why crocodiles?
PM: It came through the work I was doing on paleoclimate. I was trying to find a way of reconstructing or mapping out climate through the last 100 million years. The most common way of doing this is to use fossils. I was originally going to use plants, but that includes so much information that it's unmanageable. Instead I started a pilot study using crocodiles, which is a manageable data set, and that took over and became the subject of my dissertation. I looked at the distribution of fossil crocodiles and tried to reconstruct how that reflected climate at the time. I also looked at their diversity, how many different species or genera there are at any particular time.
What I found was that modern crocodilians began to diversify sometime in the Cretaceous and just continued to diversify all the way into the Paleocene--and it seemed to cross the Cretaceous-Tertiary boundary without any sign of a major extinction. You get to the Oligocene--about 30 or 35 million years ago, when we know that there is antarctic ice forming, we know the climate is changing from other evidence--and we suddenly see the diversity of crocodilians drop. Then they recover a little bit, until you get to about three million years ago, when we start getting the formation of more arctic ice. The climate cools, and again the diversity drops.
The crocodiles are clearly following some sort of climate signal, but there's no major extinction, no major change in diversity at the Cretaceous-Tertiary boundary. This suggests that whatever caused the KT extinctions, it wasn't climate--or at least climate wasn't a major player.
SBM: Define KT.
PM: KT is an abbreviation of Cretaceous-Tertiary; the line between them is called the KT boundary. K is used because C was already used for Carboniferous. The KT boundary is 65 million years ago, separating the Mesozoic, generally referred to as the Age of Reptiles--even though mammals were around, it was only with the demise of the dinosaurs that they were able to expand--from the Cenozoic, which is the period in which we are now living, the Age of Mammals. You get past that boundary when the dinosaurs were dominant, and lo and behold the mammals sit up and say, "Hey, everything is great now--we've got all these nice niches to move into." And they seize power, although they actually take five to ten million years to do it.
SBM: Are you assuming cold-blooded or warm-blooded dinosaurs?
PM: This makes no comment about whether dinosaurs were warm- or cold-blooded. All it's saying is, look, the crocodiles don't seem to be doing anything at the KT boundary. Therefore it's suggested that it wasn't a major, long-term climate change at the time.
Some crocodilians actually do go extinct at that point. What's odd is that they all seem to be marine crocodiles.
SBM: The die-off at the KT boundary wasn't just the dinosaurs. You've got the ichthyosaurs, which are not really dinosaurs, and the pterosaurs, which are not really dinosaurs, also dying off.
PM: Exactly, although pterosaurs are fairly close to dinosaurs. One of the things you'll find is that the position of a lot of these groups relative to each other has changed over the last few years. But, yes, a lot of other things go, especially in the oceans--a lot of plankton go extinct, most marine reptiles go extinct.
About ten years ago a guy called Howard Hutchison--out of Berkeley, who was working in the American west, in Wyoming, Montana--noticed that the fossil crocodiles there don't do much at the KT boundary and neither do the turtles. My study looked at the global distribution and found the same pattern. Crocs, turtles, some mammals--they're not doing a lot. With birds it's very difficult to tell, simply because we don't have a very good fossil record prior to the Tertiary. Things like amphibians don't seem to do much, although again their fossil record is spotty. The major group that gets wiped out at the KT boundary is the dinosaurs. This is clearly a major extinction.
SBM: Could we hypothesize a sharp temperature drop that didn't last that long, that would allow amphibians and crocodilians and smaller mammals to hibernate, but that would kill off the dinosaurs?
PM: You can get a sudden, short climate change. The only thing is that crocs don't hibernate as such. There's always been a big debate as to whether reptiles hibernate at all. It's a subtle distinction, but if you think of a mammal, like a bear, it's insulated from the temperature outside. It builds up fat reserves, it can go to sleep, and it just shuts down--but it's feeding off that fat. Reptiles can't do that.
SBM: What about the toads in Australia that--
PM: That hibernate for 20 years until the rains come? That's a unique reaction.
SBM: And tortoises that disappear for the winter and come back in the spring?
PM: Things like tortoises still have to keep relatively warm during that period. There are some reptiles and amphibians that actually freeze solid. They're very, very rare.
When I was in England we'd have tortoises as pets, and in the winter we'd stick them in a box of straw. As long as they were kept warm in the garage they were fine. If you let them out in the snow, even if they quasi-hibernate in a loose sense, once it gets past a critical low temperature they're dead. With most reptiles, as the temperature decreases they shut down automatically, but if it gets too low they can't recover. Whereas with a mammal, hibernation is not only brought on by cold but also by a lack of food.
Crocodiles are too large to shut down completely. It takes them longer to heat up and cool down, and it's difficult to isolate their entire bodies from environmental changes. If they were in the arctic they could not separate their body temperature from the external temperature at all. What they tend to do in the Carolinas, for instance, when it gets cold is head for the nearest body of water and just sit there, because the water stays warmer than the surrounding land. They shut down. As long as the water temperature doesn't go below two or three degrees [Celsius], they can survive a few weeks. But at that temperature they're much more susceptible to disease, they're much more susceptible to shock, and if it stays cold they'll die.
In terms of a short change at the KT boundary--yeah, they could survive a short change. But if it was a drastic short-time change that wiped the dinosaurs out, regardless of whether they were warm- or cold-blooded, that surely would wipe out the crocs as well.
SBM: What if it were a question of dust blotting out the sun?
PM: In terms of the asteroid impact, you get the dust in the atmosphere. If I were to put my money on any of the explanations, that would be the one. Yes, the impact probably led to minor cooling, but that wasn't what killed off the organisms. You throw all this dust into the atmosphere; you block out the sun for a while, a few months, perhaps; you temporarily stop photosynthesis--and as soon as you do that anything that's endothermic, that needs to constantly eat, especially something the size of a tyrannosaurus, is going to be in big trouble. Whereas a croc is just going to sit there, quite happy, waiting for the food to reappear.
That's true for any of the reptiles. One advantage of being an ectotherm is that you don't have to eat much, so you could survive for two or three months with no food around. So that certainly has possibilities. The only problem with that is that you have to do it the whole world over. Think of all food resources shutting down for two months over the whole world.
SBM: But we know that volcanoes throwing dust into the atmosphere can have an effect across the planet. Think of the effect of Krakatoa's eruption in 1883. And if it were something orders of magnitude greater than that, could that do it long enough to have an effect?
PM: Yes, all of the theories based on the impact say there could have been dust up there. There are other things that can also affect plant life. If you put that much dust and other material into the atmosphere you can also change its chemistry. You might end up with sulfur compounds, you might have acid rain resulting from that. Various people have looked at that in terms of amphibians, frogs and things, which seem to be very susceptible to acid rain. There's no real sign of a major extinction among amphibians at that time to suggest that acid rain was responsible. The only problem is that we have so little in terms of fossil remains of amphibians that it's impossible to tell.
SBM: Too small a sample?
PM: Right. But certainly all those things combined--if you can temporarily shut down photosynthesis in plants, that might explain the extinctions. That's where I would look.
SBM: So your best bet is a food problem.
PM: Yes, you shut down the whole food chain. That seems to be the most likely and, to me, the most believable idea.
SBM: Some of the news stories about your findings seemed to imply that we could forget the impact as a part of the cause of the extinctions.
PM: I got interviewed on Australian television--it was a live interview, the first time I'd done one. All I could hear was the lead-in. The guys were saying, "Most people believe in the impact; however, this is not believed by everyone." I'm listening and I'm thinking, "Oh, God!" [He laughs.] Because of course we hadn't talked before the interview. So at the end they asked, "What caused the extinction?" I said, "The impact." They've never contacted me since.
One of the big problems with the KT is the fact that everyone's been concentrating on particular groups. Most people concentrate on the dinosaurs, but you can't explain that extinction in isolation, without looking at everything else. With any of these things you have to look at the bigger picture.
SBM: What's been the response of paleontologists to your work?
PM: Most that I've talked to seem pretty happy with it. At the meeting in Seattle people came up afterward and said, "Of course! I've always wondered about that!" Nobody came up to me and said, "That's not right."
Scott Wing, a paleobotanist at the Smithsonian, came up and said, "Think about palms--palms don't do anything across the KT boundary either, and palms are also very temperature sensitive." One of the things you find, because people generally work in isolation or just look at a single side of paleontology, is that when somebody gets something and says, "Hey, look at this, this is obvious," a lot of people will say, "Why yes, it is." And that changes how they think about the whole problem.
SBM: There were a couple of earlier big die-offs that we know about. Do you think those are climate related, or do we not know enough about them?
PM: The biggest one of all, the one that was most extensive in terms of the number of things that went extinct, was the Permian-Triassic boundary, which is about 245 million years ago. There you're losing something on the order of 95 percent of all species, at least in the oceans. As far as what caused it--goodness knows. As far as I'm aware, there's no evidence of an impact, but the further back you go in time the less we know.
Jack Sepkoski, here at the University of Chicago, has done work on species diversity, and he's identified five major extinctions based on his very large database of marine invertebrates. The Permian-Triassic is the biggest, and then there are others in the late Ordovician, in the Devonian, and also in the late Triassic. There's been a tendency to look at an extinction and then try to find an explanation for all of them. That needn't be the case. To suggest one cause for all of these extinctions doesn't make any sense. [For example] at the KT boundary you find an iridium anomaly.
SBM: That's the layer of iridium, a very rare element, which shouldn't be there but is.
PM: That's right. Iridium has two sources: one is from deep inside the earth, and the other is from extraterrestrial sources. The people who originally found this were the Alvarezes, father and son. They were simply trying to find a way of working out the rate of sedimentation at the KT boundary; they weren't actually looking for extinction causes. They simply thought that iridium could be a useful marker. Of course they got to the KT boundary, and suddenly they found that they had a lot more than could be explained just by a gradual influx from space. The only way they could explain that was by having some very large body--a few kilometers across--coming from outer space and colliding with the earth.
There are people who say the iridium's actually volcanic, that it actually comes from the Deccan Traps in south-central India--this big volcanic unit at the KT boundary, where large volumes of basaltic rocks were erupting at that time. But most of the evidence points to an impact.
SBM: Haven't they located the impact crater on the Yucatan peninsula?
SBM: Could something like that set off volcanoes in India?
PM: There have been suggestions that if you impact one side of the earth the other side will have something happen to it. All the geophysicists I've talked to say this is absurd. It's been picked up by the media, but it's not something that I would follow. You certainly have large volcanic activity at various times, but it's due to tectonics.
The other thing that you get at the KT, which is more consistent with an impact, is shocked quartz. When you look at shocked quartz under a microscope you see these--not so much fractures, but imperfections in the crystal, which are due to shock. And the only places we find those features in quartz crystals are at impact structures, such as the crater near Winslow, Arizona, or in nuclear test sites. Since we know that the dinosaurs weren't experimenting with technology, it has to be impact. And you find that the amount of shocked quartz decreases as you go away from the crater--again, that's consistent with impact.
SBM: Let's talk about things like tectonics and attempts to map earlier landforms and climate. How much do we know? How much do you think we can find out?
PM: It's one of those subjects which has been of interest to geologists since the very early days of geology. The British in the 1820s were very interested in climate. Most of their work was in northwest Europe, and they would go out to the London Clay--which is Eocene-laid, about 50 million years old, underneath London. And they'd be pulling out turtles and crocodiles and palm trees. The geologists said, Where do we find these today? In the lower latitudes, in the equatorial regions. Therefore the only explanation we have is that the climate was different in London. It was actually a damn sight warmer.
So they went through and started looking at all of these different fossils. The Cretaceous must have been warmer, because we have all of these reefs in England and northwest Europe.
Then during the 1820s a few of them went up to the arctic, started looking at the rocks there, and found exactly the same thing. They were finding tree ferns, very big ferns which you only find in the tropics today, crocodiles, and so on. And all of this pointed to the fact that the earth was much warmer in the past.
During the 19th century they found more and more evidence for this, and then they tried to explain what caused it. Indeed, most of the ideas that we play around with today in the late 20th century were originally suggested by geologists working in the 1830s and '40s, which is quite sobering.
Today we've got a lot more information, but people still tend to think of a climate change in two dimensions, in terms of overall global temperatures. You think the Cretaceous was warm, the Tertiary was warm, and then it cooled off to the present day. And if you go further back to the Carboniferous, when you have the coals forming here in the U.S. and in England--although the tectonic plates were in very different positions--you have ice at the poles again.
So you can pick up the general trends of climate. You can say that the Cretaceous is warm. But nobody's gone out and said, "What was the distribution of precipitation? What was the distribution of temperature?" Up until the last 20 years that wasn't particularly important. People had a lot of other things to do, such as get grants. Now they're interested, and the reason is the changes we're looking at today--the potential effect of carbon dioxide in the atmosphere. What happens if you warm the earth by five degrees? Will that melt the ice sheets? And what does this do for Florida?
One result of that interest has been the use of computer models which compare weather patterns. These are the things that are used to predict the weather on the six o'clock news; those same models, or adaptations of them, are used to look at the past as well. You put in the paleogeography, and you make a best guess of what you think the temperature gradient of the ocean was and how it was different from today. Then you run your climate model for, say, 50 million years ago and see what the distribution of surface temperatures was.
That doesn't give you facts. It just gives you a hypothesis to go out and test. The only way to test that is with the geological record. What we've started to do here--and what my dissertation is about--is to establish climate based on that geological data and ask, "How far can you believe this? How much cold can you have when you're using frogs, or you're using turtles, or you're using plants? And what's the climate they suggest?" And then going out and comparing that with the climate models.
Through that we're starting to get a better idea of the climate of the past, although the scale is different. We're looking at the Eocene, which was 21 million years long, and saying "Here's the climate for the Eocene." So you have to be a little careful.
SBM: If you look just at the last thousand years we know that things were so warm in North America when the Vikings discovered it that there were grapes growing in Nova Scotia, and then you have the period in late Elizabethan times when they could skate on the Thames. It seems as though there's a pretty wide fluctuation.
PM: Exactly. With things like the Little Ice Age we have fluctuations on a short scale, and then in between we have the Milankovitch cycles--
SBM: Milankovitch cycles?
PM: They're named for Milutin Milankovitch, a Serbian mathematician who looked at the earth's orbital changes, and whether orbits are more elliptical or less elliptical over 100,000 years. What he found was that they change through time on a regular basis, and one of the things that came out of that was that those changes seem to correspond to climatic changes on the earth. People have suggested, for instance, that the interglacial-glacial oscillations in the Pleistocene are due to these changes in orbit.
Then you get to the scale of tectonic changes over millions of years, due in part to changing land-sea distributions, which is really what the geological record is showing.
These differences in scale are why you can't just say, "The Eocene was warm, the present is cold." What the past does give you is an extreme possibility. We know that in the Eocene there was no ice at either pole. We know that if you went into the interior of the U.S. it was a lot warmer during the winters than it is now. So given that, how could we explain it using the climate model?
We have to remember that the finer-scale phenomenon, which is weather, is piggybacking on top of that. With the extreme example of the iceless Eocene we could ask whether that condition was due to carbon dioxide or something else. If it was carbon dioxide, how much do we need to create such a world?
The models, however, just do not reconstruct the past very well. Models don't like continental interiors; they can't predict the weather for the present day. Models make Siberia far too cold in winter and warm in summer. When was the last time the model predicted the weather right for the next week?
But we have found that in the past the distribution of moisture was very different. The continental interiors weren't as arid. One of the things we're working on at the moment is running the geological models together to look at that issue and ask, "If you increase the moisture content of the interiors, could you warm them up?" If you change the distribution of moisture over the earth that affects things quite drastically, such that you can have crocs living in Wyoming.
That has a big implication for future climate. If we have global warming today that will affect moisture, could you then change the temperature in a similar way?
We have a pretty good idea of the general trend of climate. We're now starting to look at the details. But that brings up more of these questions.
SBM: You've got maps of where various kinds of fossils were found. But have we explored enough in certain areas--places where there's political turmoil, for example--to really say what's out there? Could we be missing anything under the oceans?
PM: Most of the ocean floor is basaltic rock. It was never land, so there's nothing there to find. It's hard to look under the rain forest, so we could be missing something there. Africa is obviously difficult to work in in the best of times. There's still a lot of work to be done on those very low latitudes.
We also have a poor record of the last 100 million years in high latitudes. For instance, in Siberia there is next to no information through the Cretaceous until the Miocene. We have plants, but it's very odd that there's no vertebrate record. You would expect that after 100 years of exploration the Russians would have found something, but there's nothing.
The simplest answer is not necessarily the true one.
Art accompanying story in printed newspaper (not available in this archive): photos/Paul L. Merideth.