The reading passage talks about possible explanations that explain the massive extinction took place at the end of the Triassic period 200 million years ago. But the professors thinks that though these theories sound plausible, none of these explanations is convincing.
Admittedly, scientists agree that sea levels were fluctuating by the end of the Triassic period. However, the ocean populations that live in the shallows and coastlines have enough time to adapt to such changes gradually. This is because sea levels changes were very gradual, rather than with a sudden.
It's true that large amounts of released sulfur dioxide (SO2) into the atmosphere could lower the global temperature, but this impact only retained in a short period. Till now, there are still many volcanic activities, so that the SO2 are not clear out in the atmosphere. And SO2 fall back to the Earth surface by rainfall.
Very few scientists believe the asteroid strike theory. No asteroid crater has been found on the Earth by the asteroid hit. Though there occurred asteroid strike 200 million years ago, which was too long before the massive extinction and had nothing to do with the massive extinction on earth.