The pattern of evolution, diversification, and extinction of species on Earth is a topic worthy of an entire course (but so are the other topics in Earth 501, as you have no doubt realized at this point). Here we will focus on two major extinction events in geologic history. One of them happened approximately 250 million years ago. It marks the end of the Permian period of the Proterozoic Era and marks the beginning of the Triassic period of the Mesozoic Era. This extinction event was the most catastrophic in geologic history in terms of the number of species that disappeared at this time. The other event we'll study is better known. It happened 65 million years ago, ending the Mesozoic Era and the Cretaceous period, and beginning the Tertiary period of the Cenozoic Era. This is the event that killed the dinosaurs and is often called the "K/T" extinction (K is the abbreviation used for Cretaceous; T is the abbreviation for Tertiary).
These two extinction events have some characteristics in common: extraterrestrial impacts as kill mechanisms have been proposed for both, and in both cases, scientists continue to debate other possibilities as well. In this lesson we will examine different hypotheses proposed for each of the two events, we will examine a database of known impact sites on the planet, and we will try to wrap our minds around the concept of deep geologic time.
Read the following article, available through Canvas:
This article gives a brief overview of some of the most recent debates surrounding the causes of mass extinction events that happened during the Mesozoic Era. Read this now as an introduction to some of the topics we will pursue more deeply as this lesson progresses.
When you read this article, think about the following:
This article does not answer all the questions above, and I have italicized the questions that you may or may not know the answer to depending on your background knowledge of the geologic timescale.
Please post any comments or questions to the Questions? discussion forum, especially if you want more background pertaining to the italicized questions (or other questions). If you have the answers to help out a fellow student, please post!