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

The professor in the listening part holds that theories given by reading passage may sound plausible but actually unconvincing.

First, sea level change. Well 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 falling sea level at the end of the Triassic period 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.

First, the sea level does go down as the reading says, but the process is actually very gradually. To make a profound mass extinction like that happened in the Triassic period, the change must be very sudden. However, the fall of the sea level actually took several millions of years. So gradual a change wouldn't cause so extremely negative effect, because the coastal and shallow-ocean species were capable to deal with a gradual temperature change.

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 distinctions.

Second, the cooling effect caused by sulfur dioxide is not convincing. The sulfur dioxide, released by volcanic activities, must stay in the atmosphere long enough before it could have the cooling effect. However, this gas often combines with the water and then fails to ground as rain, which means the air is usually cleared out rather than stays in the atmosphere.

Third, very few scientists believe the asteroid theory because we haven’t found any asteroid crater the side where the asteroid hit that can be dated to the time when the mass distinction occurred. We did find a crater, but it dates to about 12 million years before the extinction. That’s just too long before the extinction to have anything to do with it.

Third, now we still lack the evidence of an asteroid strike related with the end of the Triassic period. People now do find some craters. However, these craters date back to 12 million years before the mass extinction occurs, which are too old to have anything to do with the extinction.