the world practically devoid of species, thereby paving the way for the ascen-
sion of entirely new life-forms.
In the aftermath of the Permian extinction, many once minor groups,
including active predatory relatives of modern fish, squids, snails, and crabs,
were able to expand. For example, sea urchins, which were relatively uncom-
mon during the Permian, are widespread in today’s oceans. Since the great
Permian catastrophe, eight significant extinction events have occurred. These
defined the boundaries of the geologic time scale, with many of the strongest
peaks coinciding with the demarcations between geologic periods.
After discussing the reptiles in the Permian, the next chapter will exam-
ine the evolution of the dinosaurs and their geologic environment in the Tri-
assic period.Historical Geology