NARRATOR：

Now listen to part of a lecture on the topic you just read about.

FEMALE PROFESSOR：

While the theories given in the reading may sound plausible, none of them is a good explanation for the mass extinction at the end of the Triassic period.

First, sea level change. While scientists agree that the sea level fluctuated at the end of the Triassic period, often going down, this isn’t a good explanation for the extinctions.

Coastal and shallow-water ecosystems are usually capable of adapting to environmental changes that happen gradually.

The fall in sea level at the end of the Triassic was quite gradual, taking place over several million years.

The change would have to be much more sudden to have a widespread negative impact on the species in those ecosystems.

Second, global cooling. It’s true that sulfur dioxide can lower global temperatures.

But that can only happen during a relatively short period when the sulfur dioxide that’s been released by volcanoes is actually still present in the atmosphere.

In a matter of a few years, the excess SO2 is usually cleared out of the atmosphere.

Basically, the SO2 combines with water in the atmosphere and falls back on Earth as rain.

It doesn’t seem likely, therefore, that even if there was a lot of volcanic SO2 released at the end of the Triassic, it stayed in the atmosphere long enough to cause mass extinctions.

Third, very few scientists believe the asteroid theory because we haven’t found any asteroid crater—the site where the asteroid hit—that can be dated to the time when the mass extinction occurred.

We did find a crater, but it dates to about twelve million years before the extinction.

That’s just too long before the extinction to have anything to do with it.