Now we're going to begin thinking about a different, even more catastrophic mass extinction event.
Here is a screencast to discuss the location of Permian/Triassic extinction event on the geologic timescale . This extinction event marked the end of the Paleozoic era and the beginning of the Mesozoic era, which, in turn, was ended by the K/T mass extinction we just finished reading about. Read a transcript of my discussion of the P/Tr extinction .
Approximately 250 million years ago, the biggest extinction event in the history of the Earth (in terms of the number of species that disappeared) took place at the end of the Permian period. This event marks the end of the Paleozoic era and the beginning of the Mesozoic era. The rise of reptiles, such as the dinosaurs, is most probably a direct outcome of these species flourishing in the ecological niches left by the end-Permian extinction event. Several theories have been proposed to explain the "Great Dying," but many of these lack global evidence to prove or disprove them, or they do not provide a kill mechanism that is quick enough or extensive enough. One of the difficulties in pinning down a kill mechanism is the dearth of well-preserved outcrops that record this time period in geologic history.
As you read the papers in our next activity that deal with the Permian/Triassic extinction, I want you to keep in mind the major difference between what the scientific community thinks about this extinction, as opposed to the Cretaceous/Tertiary extinction that we read about previously. In the case of the Cretaceous/Tertiary extinction, the impact hypothesis is widely believed and supported and has been for at least a decade. I can verify this because when I teach large-enrollment residential courses for non-science majors here at Penn State, the great majority of the class knows that "a meteor killed the dinosaurs." Usually none of these same students have even heard of the Permian/Triassic extinction event, despite the fact that whatever happened on Earth at the end of the Permian was quite a bit more catastrophic. At the end of this section of the lesson, we will have a summary discussion so we can compare the story of two different extinction events and discuss why the explanation for one of them is apparently much less controversial than the other.
To see some 250-million-year old rocks, check out this video clip from NOVA  in which Neil deGrasse Tyson interviews several geologists who are hunting for answers about the end-Permian extinction out in the field and through computer models.