Feature: The Fourth Extinction

The Rise of Dinosaurs—and the Age of Humans

The Triassic and Today: Hinge Points in Earth’s History

by | 8.17.2012 at 4:34pm
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Paleontologist Paul Olsen has been investigating the causes of Triassic-Jurassic extinction–a turning point in earth’s history that wiped out many life forms and started the reign of dinosaurs. More than 200 million years separate us from this catastrophe (also called the End-Triassic Extinction), but it could contain some lessons for us today, says Olsen.  For one, it may have been caused in part by massive volcanism that pumped large amounts of carbon dioxide into the air, and in turn brought on rapid warming of the global climate. This might be compared in some ways to manmade increases in CO2 today. Recently, Olsen was chiseling out Triassic rocks that lie along what is now the low-tide line on the coast of southern Wales. He took a break to speak with Earth Institute science journalist Kevin Krajick about life and death in those ancient times. LISTEN TO THE INTERVIEW:

Along the western coast of Great Britain, paleontologist Paul Olsen chisels out rocks from the time of the fourth great extinction of life on earth, 200 million years ago. (Kevin Krajick/Earth Institute)

What was the upshot of the End-Triassic Extinction?

 In a very basic way, we want to know why about 50 percent of life forms died out, both in the marine realm and in continental environments. And here in Wales and in other places in the UK, the marine successions allow us to look very precisely at that transition. And there are other features such as earthquake-deformed sediments that we think can tell us what caused that mass extinction–whether it was volcanic eruptions, or it was a combination of volcanic eruptions, and perhaps an asteroid impact.

What sorts of life forms existed before, and what kinds existed afterward?

In the marine realm, there was a large diversity of molluscs, particularly cephalopods called ammonites, with coiled shells, very well ornamented, very easy to identify. They almost go entirely extinct, with only maybe one or two forms making it across the extinction level. On the continents, you had lots of crocodile relatives–terrestrial forms that were both plant-eaters, some of them, and carnivores [and] others, [and] were the dominant continental forms, with dinosaurs comprising a relatively small part. The crocodilian relatives almost all become extinct, except for the ancestors of true crocodilians, which of course prospered later on. Dinosaurs became far more abundant after the extinction, where they were relatively rare beforehand. So this is one of the great turning points in earth’s history, where you have the establishment of the real age of dinosaurs after this gigantic mass extinction.

So why did the dinosaurs take over–what was it that allowed them to become dominant?

Something akin to luck probably allowed the dinosaurs to survive and take over. Crocodilian relatives, they may have been living at higher latitudes and were susceptible to the environmental changes that occurred at the end of the Triassic. Especially extreme global warming and maybe short but important episodes of cold as well.

So you think it was the heat that did them in? Heat and/or cold?

My prejudice is that the heat was probably the more important, but certainly if volcanic eruptions or an asteroid [produced] short periods of cold followed by longer periods of extreme warmth, that combination is sort of like a one-two punch that would make it very difficult for creatures that weren’t able to [take] large swings in climate to survive.

How hot did it get, and how cold might it have gotten?

We don’t have any real way of directly measuring temperatures. We do know that plants suffered extinctions in groups that tended to have large leaves. And that’s consistent with extreme heat, lethal heat perhaps, in the 50 degrees Centigrade [120 degrees Fahrenheit] range. Cold, we don’t have any direct way of measuring that either. We do know that crocodilians and their relatives were probably very sensitive to cold, but we don’t know how cold it got. There’s no evidence of ice, for example, but that doesn’t mean it couldn’t have existed for short periods of time.

The heat was caused by what you believe was rising carbon dioxide, right? Let’s talk about that briefly.

Sure, There’s pretty convincing evidence, both from soil carbonates and plant leaves and cells that CO2 was double to triple during the extinction levels as it was beforehand, and that that was probably the origin of extreme heat.Now, there’s a lot more work to be done in that direction. We have to make sure that we understand the absolute as well as the relative magnitudes of the changes. But certainly, I think you can make a convincing case that extreme heat was very, very important in that mass extinction.

I think you’re talking about a time [when carbon dioxide] was already very high, much higher than today, and then it went even higher.

Yes, CO2 was very high to begin with, before the mass extinction. Thousands of parts per million, as opposed to hundreds now. And there’s no evidence of ice at the poles during that time at all. Zero. It’s one of the few times in history where there’s absolutely no evidence of extreme cold at the poles. So, that’s consistent with elevated temperatures already. You can imagine [in] the tropics, things are already living close to a lethal level. If you bump it up even a few degrees, it could be lethal, especially during extreme climate events during that time.

OK, last question. We’re living in a time when people are worried about rising CO2 levels. Obviously, not anything like you’re talking about [back then]–but is there any kind of scientific relevance, or even moral lesson to be drawn from this time?

Well, we’re looking at doubling CO2 concentrations now, during our lives. And it was a doubling or a tripling of CO2 in the past that looks like it was associated with these extinctions. Because we’re not starting at such a high temperature level as we were [then], the consequences of that doubling are hard to compare. But we do know, or think we know, that the time scale on which these changes occurred–the time scale over which the CO2 went up–[is] fully comparable to the human time scales, [on which] we are looking at for doubling of CO2 now. So there probably are very significant lessons to be learned about processes in the doubling of CO2. We have to get, however, the pattern, and the basic chronology and the basic history right first, before we try to learn great lessons about what lies ahead for us in the future.






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