"Biology Class" What aspect of the fossil record is this lecture mainly about?

Narrator: Listen to part of a lecture in a biology class.
Professor:
    By studying the fossil record we can read the history of life on Earth. Interestingly enough, it appears that there are long periods in which not very much change occurs; then sporadic brief periods in which there are mass extinctions of species followed by diversification of the groups that survived. How does this happen? Well, sometimes a habitat is destroyed or the environment changes. Did you know that if the temperature of the ocean falls by even a few degrees, many species will die? Incredible, isn’t it? Or, even when the environment is relatively stable, biological conditions can change when other species evolve in different directions. For example, let’s see, when a similar species evolves by developing a
shell, then the related species without shells may be more vulnerable to predators and could become
extinct as a result of changes in the other species. So you can see that extinction is a natural consequence  Q12
of history. It’s , well, inevitable. But sometimes mass extinctions occur and most of the known
species are lost, And this is very different.
    Let me mention two such mass extinctions. First, the Permian mass extinction, which occurred
about 250 million years ago. According to fossil records, more than 90 percent of the marine species
and about 30 percent of the orders of insects.., perished. Then about 65 million years ago, the Cretaceous
mass extinction claimed more than half of the marine species and many terrestrial species of
plants and animals, including the dinosaurs.
    So what causes mass extinction? This isn’t an easy question to answer. You see, it’s obvious from  Q14
the fossil records that species exist during a certain geological time period, and then, they disappear,
and we have solid evidence for that But why they disappear s, well, more speculative. In the Permian,
several extreme conditions may have converged, including the merging of the continents into one large
land mass. As you can imagine, such a radical change in the distribution of land and water would have
disturbed habitats and caused the climate to change. There’s also evidence that volcanic activity during
this period may have produced enough carbon dioxide to cause global warming, which in turn would
have affected the temperature and depth of the oceans, and it, and I’m referring here to global warming,
so it probably also caused the oxygen levels in the oceans to decrease. All of these conditions could
have converged to extinguish an enormous number of species at the same time. That’s mass extinction.
    And, a similar set of conditions may also have contributed to the mass extinction in the Cretaceous
period as well. We can gather data that convinces us about continental drift.., that it occurred along
with receding seas along the continental coastlines. In addition, we know that cooler climate was probably
the result, at least in part, of... increased volcanic eruptions, and these eruptions probably
released enough material into the atmosphere to block the sunlight. Having said all of that, many scientists  Q15
now favor a very different hypothesis, They theorize that maybe a large asteroid collided with the
Earth. Advocates of the so-called impact hypothesis speculate that there were two events that caused
the mass extinction. First, the impact probably caused a fire storm of such proportion that most of the life
in North America would have been decimated within minutes. Second, they postulate that an enormous
cloud of fallout could have blocked out the sunlight and.., that the impact was, in fact, large enough to
... darken the Earth... and we’re talking about months or even years. So the result.., of the darkness,
I mean.., that would have caused a reduction in photosynthesis, which, in turn, would have created a
disruption in the food chain. Now, such a disruption would have affected many species.
    So the advocates of the impact hypothesis…they put forward evidence that a thin layer of clay,  Q16
rich in iridium deposits, uh, can be found in the geologic material that separates the Mesozoic and the
Cenozoic eras-precisely the t me period for the Cretaceous mass extinction. So what’s special about
this clay? Well, iridium is a very rare element on Earth, but it’s quite common in meteorites and other
extraterrestrial debris that’s been analyzed. So, it’s possible that this sediment is the remains of the
impact. The fact that there was more serious damage to the species in the Western Hemisphere could
also be explained by the point of impact, and the fact that the dust cloud could have caused more acidic
precipitation nearer the area of impact. Or, there may have been a number of calamities that converged
simultaneously, disrupting planetary balances.
    But whatever the cause or causes, the fact remains that the mass extinctions occurred, and they  Q17
influenced the biological diversity of our planet in profound ways. The species that survived, whether
because they had genetic advantages or because they were fortunate enough to be farther from the
catastrophes.., these species became the ancestors of the species that have played important roles in
biological evolutionary history.